Testing criteria for spinal implants: recommendations for the standardization of in vitro stability testing of spinal implants

The intradiscal pressure of the lumbar spine is affected by intervertebral disc degeneration, age, level, and motion direction: evaluation of an in vitro database comprising 107 specimens

BACKGROUND CONTEXT

Intradiscal pressure (IDP) is a fundamental parameter for the estimation of loads and muscle forces acting on the spine and a major biomechanical indicator for various spinal pathologies.

PURPOSE

To investigate primary effects of intervertebral disc degeneration, age, sex, segmental level, and motion direction on lumbar IDP using a large in vitro data collective.

STUDY DESIGN

Evaluation of an internal database comprising 107 human functional spinal units of L2-L3, L3-L4, and L4-L5 from 68 donors (19-74 years, mean 50±12 years, 42% female).

METHODS

All specimens had been loaded with pure moments of 7.5 Nm in flexion/extension, lateral bending, and axial rotation and IDP had been measured using flexible pressure sensors. Disc degeneration was assessed from radiographs using a validated classification system.

RESULTS

IDP was significantly (p<.05) reduced for degeneration grades 1 (mild degeneration) and 2 (moderate degeneration) compared to grade 0 (no degeneration) in all motion directions and for the intrinsic pressure (INTP) without any loading (moment of 0 Nm). IDP significantly (p<.05) negatively (-0.69≤r≤-0.45) correlated with age and was significantly (p<.05) reduced for an age >40 years in all motion directions and for the INTP. Sex did not significantly (p<.05) affect the IDP. The IDP at L4-L5 level was significantly (p<.05) reduced compared to the IDP at L2-L3 level in all motion directions and for the INTP and significantly (p<.05) lower in axial rotation and for the INTP compared to flexion/extension and lateral bending.

CONCLUSIONS

This study revealed that more degenerated discs and discs from elderly donors exhibit low or even negative intradiscal pressure, overall questioning in vitro and in vivo IDP measurements which disregard the degenerative condition of the intervertebral discs and the age of the donors and participants.

CLINICAL SIGNIFICANCE

Increasing disc degeneration and age as well as more distal lumbar level are associated with decreased IDP of the lumbar spine, possibly less maintaining the load sharing capacity and thus representing risk factors for spinal pathologies.

Adjacent Segment Motion of Stand-Alone ALIF Versus TLIF in the Degenerative Spine: A Biomechanical Study

Study DesignBiomechanical human cadaveric study.ObjectivesTransforaminal lumbar interbody fusion (TLIF) is a well-established procedure for treating degenerative lumbar spine pathologies. However, posterior fixation has been reported to accelerate adjacent segment degeneration (ASD). Posterior fixation can be omitted in screw-anchored stand-alone anterior lumbar interbody fusion (ALIF). The present study aimed to compare the cranial adjacent segment motion of ALIF vs TLIF in specimens with reduced bone mineral density (BMD).MethodsSixteen fresh-frozen lumbosacral spines with reduced BMD (donors' age 71 ± 13years, BMD 95.7 ± 34.5 mg HA/cm3) were used. Range of motion (ROM) and Neutral Zone (NZ) of the cranial adjacent segment were analyzed in flexion-extension, lateral bending, and axial rotation in native state and after TLIF or stand-alone screwed ALIF instrumentation.ResultsNo significant differences between TLIF and stand-alone screwed ALIF were observed for both absolute ROM and NZ of the cranial adjacent segment in instrumented state across all tested motion directions (P  ≥ .267). Decreased relative ROM of the fused segment - normalized to the corresponding segmental ROM in native state - resulted in compensatory increased relative ROM of the cranial adjacent segment after instrumentation. However, the relative adjacent segment ROM did not differ significantly between TLIF and stand-alone screwed ALIF (P ≥ .172).ConclusionsThis study found no clinically significant difference in adjacent segment motion when comparing TLIF with stand-alone screwed ALIF. Hence, both techniques appear to have a negligible impact on adjacent segment motion in poor bone quality. This suggests that neither TLIF nor stand-alone screwed ALIF increase the risk of ASD due to compensatory motion resulting from an operated adjacent segment.

Biomechanical effects of the cephalad extent of laminotomy

PURPOSE

The superior aspect of the unilateral laminotomy for bilateral decompression (ULb) typically corresponds with the superior ligamentum flavum attachment. Unlike lateral expansion, cranial expansion is considered a viable option to ensure sufficient decompression. The aim of this study was to investigate how cranial expansion affects the biomechanical stability in the lumbar spine.

METHODS

Range of motion of eight fresh-frozen human cadaveric L1-L5 specimens was assessed in flexion-extension, lateral bending (LB), and axial rotation (AR). The workflow comprised testing in the intact state and after L3-4 ULb with sequential increase of the cephalad extent over 4 steps: (1)25% of the lamina height = insertion of Ligamentum Flavum, (2)50%, (3)75%, and (4)100%. Throughout all steps, constant mediolateral and caudal dimensions were maintained.

RESULTS

Even though within the tested sequences of the workflow an overall significant change was eminent for extension (p = 0.002), right LB (p = 0.030), and left AR (p = 0.009), no significant differences was detected when comparing their five different states pairwise. At L2-3, no overall significant changes were detected for all six motion directions (p ≥ 0.107).

CONCLUSION

ULb induced minor instabilities to the operated segment and no instability to the cranial adjacent segment. However, the absolute increase remained small under the tested conditions, suggesting that unilateral laminotomy is a safe technique at all cranial extents. With a bone-sparing laminotomy preferred, extending cranially appears to be a viable option to achieve sufficient decompression.

Load Changes on a Short-Segment Posterior Instrumentation After Transosseous Disruption of L3 Vertebra - A Biomechanical Human Cadaveric Study

Study DesignBiomechanical Cadaveric Study.ObjectivesFollowing the successful use of a novel implantable sensor (Monitor) in evaluating the progression of fracture healing in long bones and posterolateral fusion of the spine based on implant load monitoring, the aim of this study was to investigate its potential to assess healing of transosseous fractures of a lumbar vertebra stabilized with a pedicle-screw-rod construct.MethodsSix human cadaveric spines were instrumented with pedicle screws and rods spanning L3 vertebra. The spine was loaded in Flexion-Extension (FE), Lateral-Bending (LB) and Axial-Rotation (AR) with an intact L3 vertebra and after its transosseous disruption, creating an AO B1 type fracture. The implant load was measured on the one rod using the Monitor and on the contralateral rod by strain gauges to validate the Monitor's measurements. In parallel, the range of motion (ROM) was assessed.ResultsROM increased significantly in all directions in the fractured model (P ≤ 0.049). The Monitor measured a significant increase in implant load in FE (P = 0.002) and LB (P = 0.045), however, not in AR. The strain gauge - aligned with the rod axis and glued onto its posterior side - detected an increased implant load not only in FE (P = 0.001) and LB (P = 0.016) but also in AR (P = 0.047).ConclusionAfter a complete transosseous disruption of L3 vertebra, the implant load on the rods was considerably higher vs the state with an intact vertebral body. Innovative implantable sensors could monitor those changes, allowing assessment of the healing progression based on quantifiable data.

Impact of transforaminal lumbar interbody fusion on rod load: a comparative biomechanical analysis between a cadaveric instrumentation and simulated bone fusion

BACKGROUND

Recent research has demonstrated the potential of implant load monitoring to assess posterolateral spinal fusion in a sheep model. This study investigated whether such a system could monitor bone fusion after interbody fusion surgery by biomechanically testing of human cadaveric lumbar spines in two states: following a transforaminal lumbar interbody fusion (TLIF) procedure and after simulating bone fusion.

METHODS

Eight human cadaveric spines underwent a TLIF procedure at L4-L5. An implantable sensor system was attached to one rod, while two strain gauges were attached to the contralateral rod (dorsally and ventrally) to derive implant load changes during unconstrained flexion-extension (FE), lateral bending (LB) and axial rotation (AR) motion. The specimens were retested after simulating bone fusion at L4-L5. Range of motion (ROM) of L4-L5 was measured during each loading mode.

RESULTS

ROM decreased in the simulated bone fusion state in all loading directions (p ≤ 0.002). Compared to the TLIF motion, the remnant motion after simulated fusion was 53 ± 21 % in FE, 40 ± 12 % in LB, and 49 ± 16 % in AR. In both states, measured strain on the posterior instrumentation was highest during LB motion. All sensors detected a significant decrease in load-induced rod strain after simulated bone fusion in LB (p ≤ 0.002). The strain measured by the implantable strain sensor, the dorsal strain gauge, and the ventral strain gauge decreased to 49 ± 12 %, 49 ± 17 %, and 54 ± 17 %, respectively.

CONCLUSION

Rod load measured via strain sensors can monitor fusion progression after a TLIF procedure when measured during isolated LB of the lumbar spine. This study provides the basis for further development and understanding of in vivo implant load data.

Loosening of stand-alone ALIF versus TLIF in degenerated lumbar human spines: an in vitro biomechanical study

PURPOSE

Biomechanical investigation is needed to determine whether stand-alone anterior lumbar interbody fusion with integrated screws (ALIF) is advisable for use in elderly patients. This study aimed to test three null hypotheses: (1) cyclic loading does not cause loosening of ALIF in degenerated lumbar specimens, (2) cyclic loading does not cause loosening in transforaminal lumbar interbody fusion (TLIF) in degenerated lumbar specimens, and (3) Neutral Zone (NZ) and Range of motion (ROM) of ALIF and TLIF do not differ after cyclic loading.

METHODS

Twelve fresh-frozen human cadaveric lumbar motion segments (L1-5, donors' age 76.1 ± 6.7 years; trabecular BMD 97.1 ± 36.9 mgHA/cm3) were utilized. NZ and ROM were assessed after ALIF or TLIF and following cyclic loading for flexion-extension (Flex-Ext), lateral bending (LB), and axial rotation (AR). Axial compression of 0-1150 N was applied over 2000 cycles. Loosening was defined as significant increase in NZ.

RESULTS

Cyclic loading significantly increased NZ of ALIF specimens in Flex-Ext (1.69 ± 1.81°, p =.048) and LB (1.84 ± 1.81°, p =.036), and showed a trend to significance in AR (0.46 ± 0.54°, p =.065). NZ of TLIF specimens did not increase significantly in any motion direction (p ≥.112). ROM and NZ did not differ significantly between ALIF and TLIF in post-cyclic states(p ≥.556).

CONCLUSION

Axial compression loading caused significant loosening of ALIF in Flex-Ext and LB, but not of TLIF in degenerated lumbar human cadaveric specimens. Hence, standalone ALIF cannot be recommended without reservation for the use in elderly patients with degenerative lumbar spines. However, the absolute differences between pre- and post-cyclic states were small, and ROM and NZ of ALIF after cyclic loading were comparable to TLIF.

Biomechanical Evaluation of a C1-C2 Posterior Arch Screw Construct

Study DesignIn-vitro biomechanical study.ObjectivesInjuries or degenerative conditions can lead to atlantoaxial instability requiring fixation. We aim to assess and compare the biomechanics of a C1-C2 posterior arch and translaminar screw construct against the Harms procedure for posterior atlantoaxial fixation on a human cadaveric model.MethodsNine human cadaveric cervical specimens from occiput to C3 (C0-C3) were used for range of motion (ROM) testing. Each specimen was tested for 4 configurations: 1. Intact, 2. Destabilized, 3. Harms construct, 4. C1-C2 posterior arch screw (PAS) construct. A pure moment of 1.5 Nm was applied, and ROM of the C1-C2 segment was measured in flexion-extension, lateral bending, and axial rotation.ResultsThe Harms group showed a decrease in ROM in all modes (P < 0.021), and the PAS group showed a decrease in ROM in flexion-extension and lateral bending (P < 0.002), but not in lateral bending (P = 0.176). Compared to the intact condition, Harms showed increased ROM for flexion-extension (P = 0.012), and PAS did not (P = 0.258). In lateral bending, both constructs did not significantly reduce ROM (P > 0.058). In axial rotation, both constructs showed a significant increase in ROM (P < 0.002). There was no significant difference in ROM when comparing Harms with PAS in flexion-extension (P = 1.000), lateral bending (P = 0.163), or axial rotation (P = 1.000).ConclusionsThe study demonstrates that a C1-C2 PAS construct restores or increases biomechanical stability compared to the intact condition. C1-C2 PAS offers similar biomechanical stability compared to the Harms construct.

Biomechanical performance of a novel zero-profile interbody cage: A cadaveric study

Zero-profile cage (ZPC) products have been widely used in anterior cervical decompression and fusion (ACDF) surgery. To develop a ZPC that meets the biomechanical requirements of the Chinese population, we designed a novel zero-profile cage (NZ) by analyzing the critical anatomical parameters of the cervical spine in healthy Chinese people. This study aims to investigate and assess whether the biomechanical properties of the newly designed NZ could satisfy the criteria for clinical application. The biomechanical properties of the NZ were evaluated by being implanted into cervical cadaveric specimens, measuring and analyzing the range of motion (ROM) of surgical segments. The experimental group in this study consisted of the NZ. As the control group, the gold standard product combination of ACDF surgery, anterior fixation plate combined with cage (P + C), and the FDA-approved ZPC product (Zero-P) were utilized. The experiment utilized six cadaveric specimens of human cervical vertebrae subjected to identical testing conditions. Following the completion of the test under intact conditions, fusion products were implanted into each specimen in segment C4-C5 in the following order: Zero-P, NZ, P + C. Biomechanical results revealed that the ROM of the surgical segment had decreased significantly under six basic working conditions following NZ implantation. Statistically significant differences were observed in the left bending (LB), right bending (RB), and left rotation (LR) conditions when compared to the intact conditions. The remaining working conditions did not exhibit a significant difference. However, the observed decreasing trend was consistent with previously documented research. In terms of the ROM of surgical segments, there was no statistically significant difference between the NZ group, the Zero-P group, and the P + C group. The biomechanical properties of the newly designed NZ in this study were superior, comparable to the fusion effect observed in conventional products of the Zero-P group and the P + C group. Furthermore, the biomechanical properties exhibited further improvement when subjected to LB and RB conditions. In the future, the newly designed NZ has great potential as a competitive choice for clinical applications.

Prediction of screw loosening by measuring the insertion torque in non-osteoporotic patients: an in vitro study

BACKGROUND

Pedicle screws are commonly used in spinal surgeries, but screw loosening remains a major concern, even in non-osteoporotic patients. Predicting pedicle screw stability via the insertion torque is a controversial topic, mainly studied on osteoporotic cadavers. Whether the insertion torque is suitable for patients with healthy bone mineral density (BMD) remains unknown. The aim was to investigate the influencing factors, namely insertion torque, BMD, screw diameter, length, surface area, volume, screw-in rotations, vertebral level, on the screw loosening stability during distractions and to understand if intra-operative predictions are possible.

METHODS

Non-osteoporotic thoraco-lumbar vertebrae (n = 50) were used to implant five different pedicle screws (n = 100) while measuring the insertion torque. After embedding the endplates, the force needed to distract the screw head by 1 mm was tested.

RESULTS

The insertion toque (2.3 ± 0.9 Nm) showed the highest influence on the distraction force (324.8 ± 84.4 N) followed by the screw size and vertebral level. BMD did not show any effects.

CONCLUSIONS

The linear correlation of insertion torque and the bending force suggests an alternative prediction metric for screw loosening which could improve the outcome of surgeries and patients' safety. This is potentially a simple, intra-operative method, which can be used in future.

A new sensing paradigm for the vibroacoustic detection of pedicle screw loosening

The current clinical gold standard to assess the condition and detect loosening of pedicle screw implants is radiation-emitting medical imaging. However, solely based on medical imaging, clinicians are not able to reliably identify loose implants in a substantial amount of cases. To complement medical imaging for pedicle screw loosening detection, we propose a new methodology and paradigm for the radiation-free, non-destructive, and easy-to-integrate loosening detection based on vibroacoustic sensing. For the detection of a loose implant, we excite the vertebra of interest with a sine sweep vibration at the spinous process and use a custom highly sensitive piezo vibration sensor attached directly at the screw head to capture the propagated vibration characteristics which are analyzed using a detection pipeline based on spectrogram features and a SE-ResNet-18. To validate the proposed approach, we propose a novel, biomechanically validated simulation technique for pedicle screw loosening, conduct experiments using four human cadaveric lumbar spine specimens, and evaluate our algorithm in a cross-validation experiment. The proposed method reaches a sensitivity of 91.50 ± 6.58 % and a specificity of 91.10 ± 2.27 % for pedicle screw loosening detection.

Topping-Off a Long Thoracic Stabilization With Semi-Rigid Constructs May Have Favorable Biomechanical Effects to Prevent Proximal Junctional Kyphosis: A Biomechanical Comparison

Study DesignIn-vitro cadaveric biomechanical study.ObjectivesLong posterior spinal fusion is a standard treatment for adult spinal deformity. However, these rigid constructs are known to alter motion and stress to the adjacent non-instrumented vertebrae, increasing the risk of proximal junctional kyphosis (PJK). This study aimed to biomechanically compare a standard rigid construct vs constructs "topped off" with a semi-rigid construct. By understanding semi-rigid constructs' effect on motion and overall construct stiffness, surgeons and researchers could better optimize fusion constructs to potentially decrease the risk of PJK and the need for revision surgery.MethodsNine human cadaveric spines (T1-T12) underwent non-destructive biomechanical range of motion tests in pure bending or torsion and were instrumented with an all-pedicle-screw (APS) construct from T6-T9. The specimens were sequentially instrumented with semi-rigid constructs at T5: (i) APS plus sublaminar bands; (ii) APS plus supralaminar hooks; (iii) APS plus transverse process hooks; and (iv) APS plus short pedicle screws.ResultsAPS plus transverse process hooks had a range of motion (ie, relative angle) for T4-T5 and T5-T6, as well as an overall mechanical stiffness for T1-T12, that was more favourable, as it reduced motion at adjacent levels without a stark increase in stiffness. Moreover, APS plus transverse process hooks had the most linear change for range of motion across the entire T3-T7 range.ConclusionsPresent findings suggest that APS plus transverse process hooks has a favourable biomechanical effect that may reduce PJK for long spinal fusions compared to the other constructs examined.

In vitro biomechanical evaluation of a strutted intradiscal spacer for lumbar discectomy

BACKGROUND

Discectomy plus implantation of a strutted intradiscal spacer has been shown to reduce reoperations and reherniations versus discectomy alone following lumbar disc herniation. This study explored the underlying biomechanics of the intradiscal spacer.

METHODS

Six fresh-frozen cadaveric lumbar spine specimens (L2 to L5) from three donors were used. Following box annulotomy to simulate disc herniation, a discectomy was performed. One segment from each donor was randomly assigned to either an untreated control group or the test group where an intradiscal spacer was implanted. A six degree of freedom universal spine tester assessed range of motion (RoM) in flexion/extension, lateral bending and axial rotation in the native state in load controlled [±7.5 Nm] and intervals up to 60,000 cycles. Disc height was measured on fluoroscopy for multiple timepoints. The segments were also analyzed to detect possible reherniation during the cycling loading.

FINDINGS

Following 60,000 cycles, the mean percentage RoM increase versus the intact state was less for discectomy plus the intradiscal spacer versus discectomy alone for lateral bending (170.7 ± 10.0 vs. 222.5 ± 33.3 %), flexion/extension (178.5 ± 6.1 vs. 204.6 ± 44.3 %) and axial rotation (284.4 ± 127.2 vs. 362.3 % ± 240.4 %). Mean overall disc height loss versus the annulotomy state was also less with the intradiscal spacer versus discectomy alone (-19.3 ± 3.7 vs. -29.1 ± 6.1 %). There was no evidence of device subsidence or migration.

INTERPRETATION

This study helps to explain the clinical observation that insertion of a strutted intradiscal spacer following discectomy reduces reherniation rate by mechanically limiting the increase in RoM and disc height loss following lumbar discectomy.

Motion preservation for hyperextension injuries of the cervical spine-an alternative to spondylodesis? A biomechanical cadaver study

INTRODUCTION

Currently, the gold standard for the treatment of AO type B3 cervical spine injuries is anterior cervical discectomy and fusion (ACDF), leading to an iatrogenic spondylodesis of the affected segment and ultimately bearing the risk of long-term morbidity. This study evaluates the biomechanical properties of a combination of a cervical total disc replacement (CTDR) with anterior fiber tape augmentation for the treatment of AO type B3 injuries in comparison to ACDF.

METHODS

14 human cadaveric cervical spine specimens (C5/6) were biomechanically tested under four different conditions: native, after simulation of an AO type B3 injury, after ACDF and CTDR + FiberTape®. All conditions were tested in the sagittal, frontal, and transversal plane with a load of 2.25Nm and the dislocation recorded. The mean value of range of motion (ROM) was calculated and analysed to identify differences in ROM and the neutral zone.

RESULTS

In flexion/extension, native testing showed a mean deflection of 11.2° ± 3.3°, the AO type B3 injury of 13.7° ± 2.9°, the ACDF of 6.7° ± 3.8° and the CTDR + tape of 9.3° ± 2.9°. Comparing both the injured specimens to the ACDF group (p < 0.001) and the injured to the tape group (p = 0.005) as well as the native to the ACDF group (p = 0.004), the mean values revealed to be significant. Lateral bending revealed a ROM of 6.8° ± 2.7° in the native, 7.7° ± 2.4° in the injured group, 4.7° ± 2.8° after ACDF, and 5.6° ± 2.4° after CTDR + tape, whereby the injured group values were significantly higher than those after ACDF (p = 0.018). The rotation showed a mean ROM of 5.6° ± 2.8° in the native and 5.8° ± 2.6° in the injured group, 4.0° ± 2.1° after ACDF and 6.3° ± 2.8° after CTDR + tape, without significant differences.

CONCLUSION

The combination of a CTDR + FiberTape proved to stabilize AO type B3 cervical spine injury adequately in the most compromised sagittal plane while maintaining micro-mobility and approaching physiological segment mobility.

A Muscle-Driven Spine Model for Predictive Simulations in the Design of Spinal Implants and Lumbar Orthoses

Knowledge of realistic loads is crucial in the engineering design process of medical devices and for assessing their interaction with the spinal system. Depending on the type of modeling, current numerical spine models generally either neglect the active musculature or oversimplify the passive structural function of the spine. However, the internal loading conditions of the spine are complex and greatly influenced by muscle forces. It is often unclear whether the assumptions made provide realistic results. To improve the prediction of realistic loading conditions in both conservative and surgical treatments, we modified a previously validated forward dynamic musculoskeletal model of the intact lumbosacral spine with a muscle-driven approach in three scenarios. These exploratory treatment scenarios included an extensible lumbar orthosis and spinal instrumentations. The latter comprised bisegmental internal spinal fixation, as well as monosegmental lumbar fusion using an expandable interbody cage with supplementary posterior fixation. The biomechanical model responses, including internal loads on spinal instrumentation, influences on adjacent segments, and effects on abdominal soft tissue, correlated closely with available in vivo data. The muscle forces contributing to spinal movement and stabilization were also reliably predicted. This new type of modeling enables the biomechanical study of the interactions between active and passive spinal structures and technical systems. It is, therefore, preferable in the design of medical devices and for more realistically assessing treatment outcomes.

Increasing the precision of intramedullary nailing in femoral derotation osteotomies by larger core locking bolts. A biomechanical study

BACKGROUND

To correct increased femoral anteversion, surgeons perform femoral derotational osteotomies in symptomatic adolescents. Using an intramedullary nail as fixation in this setting, undersized locking screws reduce rotational precision by allowing nail toggling. However, the extent to which better-fitting locking bolts improve rotational precision in femoral derotational osteotomies remains unclear. Accordingly, we tested the hypothesis that adequately sized locking bolts enhance rotational stiffness and limit displacement, thereby decreasing nail toggling in femoral derotational osteotomies in vitro.

METHODS

We evaluated rotational stiffness, angular displacement, and laxity at zero-loading in 12 synthetic femurs with a transverse gap osteotomy to the shaft. After inserting a pediatric intramedullary nail, femurs were fixed with either conventional 4.5 mm locking screws or locking bolts with a 0.3 mm larger core diameter. Non-destructive quasi-static rotational testing of 4 Nm external and internal torque was performed according to a predefined protocol.

FINDINGS

We found significantly higher mean rotational stiffness with locking bolts than with locking screws, demonstrating a 150 % increase (0.4 Nm/degree vs. 1.0 Nm/degree, P < 0.001). Mean angular displacement was significantly lower with locking bolts than with locking screws, exhibiting a 61 % decrease (21.9 vs. 8.6 degrees, P < 0.001). Additionally, laxity with locking bolts was 69 % lower than with locking screws (3.2 degrees vs. 10.4 degrees, P = 0.0027).

INTERPRETATION

Locking bolts with a larger core diameter enhances rotational stability and fixation precision, making them a valuable advancement in intramedullary nailing for femoral derotational osteotomies. These findings may also have implications for fracture treatment.

Quantification of Internal Disc Strain Under Dynamic Loading Via High-Frequency Ultrasound

Measurement of internal intervertebral disc strain is paramount for understanding the underlying mechanisms of injury and validating computational models. Although advancements in noninvasive imaging and image processing have made it possible to quantify strain, they often rely on visual markers that alter tissue mechanics and are limited to static testing that is not reflective of physiologic loading conditions. The purpose of this study was to integrate high-frequency ultrasound and texture correlation to quantify disc strain during dynamic loading. We acquired ultrasound images of the posterior side of bovine discs in the transverse plane throughout 0-0.5 mm of assigned axial compression at 0.3-0.5 Hz. Internal Green-Lagrangian strains were quantified across time using direct deformation estimation (DDE), a texture correlation method. Median principal strain at maximal compression was 0.038±0.011 for E1 and -0.042±0.012 for E2. Strain distributions were heterogeneous throughout the discs, with higher strains noted near the disc endplates. This methodological report shows that high-frequency ultrasound can be a valuable tool for quantification of disc strain under dynamic loading conditions. Further work will be needed to determine if diseased or damaged discs reveal similar strain patterns, opening the possibility of clinical use in patients with disc disease.

Which spinal fixation technique achieves which degree of stability after thoracolumbar trauma? A systematic quantitative review

BACKGROUND CONTEXT

Unstable traumatic spinal injuries require surgical fixation to restore biomechanical stability.

PURPOSE

The purpose of this review was to summarize and quantify three-dimensional spinal stability after surgical fixation of traumatic thoracolumbar spinal injuries using different treatment strategies derived from experimental studies.

STUDY DESIGN/SETTING

Systematic literature review.

METHODS

Keyword-based search was performed in PubMed and Web of Science databases to identify all in vitro studies investigating stabilizing effects of different surgical fixation strategies for the treatment of traumatic spinal injuries of the thoracolumbar spine. Biomechanical stability parameters such as range of motion, neutral zone, and translation, as well as the experimental design were extracted, collected, and evaluated with respect to the type and level of injury and treatment strategy.

RESULTS

A total of 66 studies with human specimens were included in this review, of which 16 studies examined the treatment of incomplete (AOSpine A3) and 34 studies the treatment of complete burst fractures (AOSpine A4). Fixations of wedge fractures (AOSpine A1, n=5 studies), ligament injuries (AOSpine B, n=7 studies), and three-column injuries (AOSpine C, n=7 studies) were investigated less frequently. Treatment approaches could be divided into 5 subgroups: Posterior fixation, eg, posterior pedicle screw systems, anterior fixation, eg, anterolateral plate fixation, combined anterior-posterior fixation, vertebral body replacement with additional instrumentation, and augmentation techniques, eg, vertebroplasty and kyphoplasty. Minor injuries were generally treated with less invasive surgical methods such as augmentative and posterior approaches. Bisegmental posterior pedicle screw fixation led to stabilization of minor compression injuries, whereas in more severe injuries, eg, AOSpine A4 or AOSpine C, instability remained in at least one motion plane. More invasive fixation techniques such as long segment posterior fixation, circumferential fixation, or vertebral body replacements with circumferential fixation provided total stabilization in terms of range of motion reduction even in more severe injuries. Pure augmentative treatment did not restore multidirectional stability. Neutral zone, which was reported in 25 studies, generally exhibited higher remaining increase than range of motion, which was reported in all 66 studies. Instability characteristics after treatment differed with respect to the spinal region, as thoracic injuries were more likely to remain unstable in flexion/extension, while thoracolumbar and lumbar injuries exhibited remaining instability primarily in axial rotation.

CONCLUSIONS

The stabilizing effect of surgical treatment depends on the type, severity, and location of injury, as well as the fixation strategy. There is an enormous range of surgical approaches and instrumentation strategies available. Pure augmentative techniques have not been able to restore complex multidimensional stability in traumatic spinal injuries. More invasive fixation approaches such as circumferential instrumentation or vertebral body replacement constructs together with posterior or anterior-posterior fixation offer more stability even in severe spinal injuries. Future studies are required to expand the knowledge especially regarding the stabilization of minor compression injuries, ligament injuries, and rotational injuries.

A New Concept for Cervical Expansion Screws Using Shape Memory Alloy: A Feasibility Study

BACKGROUND

 In general, sufficient anchoring of screws in the bone material ensures the intended primary stability.

METHODS

 Shape memory materials offer the option of using temperature-associated deformation energy in a targeted manner to compensate the special situation of osteoporotic bones or the potential lack of anchoring. An expansion screw was developed for these purposes. Using finite element analysis (FEA), the variability of screw configuration and actuator was assessed from shape memory. In particular, the dimensioning of the screw slot, the actuator length, and the actuator diameter as well as the angle of attack in relation to the intended force development were considered.

RESULTS

 As a result of the FEA, a special configuration of expansion screw and shape memory element could be found. Accordingly, with an optimal screw diameter of 4 mm, an actuator diameter of 0.8 mm, a screw slot of 7.8 mm in length, and an angle of attack of 25 degrees, the best compromise between individual components and high efficiency in favor of maximum strength can be predicted.

CONCLUSION

 Shape memory material offers the possibility of using completely new forms of power development. By skillfully modifying the mechanical and shape memory elements, their interaction results in a calculated development of force in favor of a high primary stability of the screw material used. Activation by means of body temperature is a very elegant way of initializing the intended locking and screw strength.

Comparative biomechanical analysis of monocortical and bicortical polyaxial screw rod fixation in canine lumbar vertebral stabilization

Objective

This study aims to evaluate the biomechanical properties of polyaxial screws-rod fixation (PSR) in stabilizing a single vertebral motion unit (VMU) fracture model and to compare the effectiveness of different stabilization techniques such as monocortical and bicortical.

Methods

A total of 12 thoracolumbar vertebral column specimens were harvested from canine cadavers. These specimens were divided into two groups based on the stabilization technique applied: a monocortical group and a bicortical group. Each group underwent biomechanical testing to assess flexion/extension and lateral bending motions. The range of motion (ROM), neutral zone (NZ), and stiffness were measured for each lumbar VMU in three conditions: intact, fractured with unilateral stabilization, and fractured with bilateral stabilization.

Results

In the 3-column fracture model, PSR was unable to restore the ROM of an intact spine in flexion/extension. In lateral bending, only bilateral PSR successfully approached the ROM of the intact spine. Notably, PSR failures were observed in four specimens when applied as monocortical and unilateral stabilization.

Conclusion

The findings indicate that even bilateral PSR does not fully restore the intact spine's ROM in canine fracture models, highlighting the need for further research to optimize stabilization techniques. The current study demonstrates that a single 3-column lumbar fracture model VMU cannot be adequately stabilized using PSR in a canine model, suggesting potential limitations in both monocortical and bicortical approaches.

The importance of the posterior osteoligamentous complex of the lumbar spine: dogma changing biomechanical insights

BACKGROUND

During full flexion of the spine, the paraspinal muscles are largely inactive. This suggests that passive structures like the posterior osteoligamentous complex (POLC), consisting of interspinous and supraspinous ligaments and the spinous processes, play a key role in spinal stability and protection of the spinal column. The POLC, however, is often resected or damaged during spinal decompression surgeries, whereas the biomechanical implications of this resection or damage are not yet fully understood.

METHODS

A stepwise reduction study was performed on three fresh frozen cadaveric torsi (aged 30-78 years) using a custom setup which only allows sagittal plane motion. After preloading and locking in full flexion, the posterior lumbar structures were gradually resected in the following order: Skin, fascia, musculature, facet joints, ligamentum flavum, posterior ligamentous complex, and posterior longitudinal ligaments. Load cells measured force increase on the fixation frame after each resection step.

RESULTS

The load increased sequentially with each resection, demonstrating load transfer from the passive structures onto the fixation frame. The POLC, including the supraspinous and interspinous ligaments at L2-L5, accounted for 69 - 74% of the measured passive load resistance in full flexion, representing the largest contribution. Facet joints with their capsules contributed 10-18%, while muscular contributions were negligible (< 2%).

CONCLUSION

The experiment indicates that the POLC is the primary passive stabilizer of the fully flexed lumbar spine. Surgical resection of this structure can redistribute loads and increase stresses on remaining spinal tissues, potentially leading to spinal instability, accelerated degeneration, and poor clinical long-term outcomes.

Biomechanical Comparison of Anterior Cervical Plate Fixation Versus Integrated Fixation Cage for Anterior Cervical Discectomy and Fusion

STUDY DESIGN

Cadaveric, biomechanic study.

OBJECTIVE

To compare the range of motion profiles of the cervical spine following one-level anterior cervical discectomy and fusion (ACDF) constructs instrumented with either an interbody cage and anterior plate or integrated fixation cage in a cadaveric model.

SUMMARY OF BACKGROUND DATA

While anterior plates with interbody cages are the most common construct of fixation in ACDF, newer integrated cage-plate devices seek to provide similar stability with a decreased implant profile. However, differences in postoperative cervical range of motion between the 2 constructs remain unclear.

METHODS

Six cadaveric spines were segmented into 2 functional spine units (FSUs): C2-C5 and C6-T2. Each FSU was nondestructively bent in flexion-extension (FE), right-left lateral bending (LB), and right-left axial rotation (AR) at a rate of 0.5°/s under a constant axial load until a limit of 2-Nm was reached to evaluate baseline range of motion (ROM). Matched pairs were then randomly assigned to undergo instrumentation with either the standard anterior cage and plate (CP) or the integrated fixation cage (IF). Following instrumentation, ROM was then remeasured as previously described.

RESULTS

For CP fixation, ROM increased by 61.2±31.7% for FE, 36.3±20.4% for LB, and 31.7±19.1% for AR. For IF fixation, ROM increased by 64.2±15.5% for FE, 56.7±39.8% for LB, and 94.5±65.1% for AR. There was no significant difference in motion between each group across FE, LB, and AR.

CONCLUSION

This biomechanical study demonstrated increased motion in both the CP and IF groups relative to the intact, un-instrumented state. However, our model showed no differences in ROM between CP and IF constructs in any direction of motion. These results suggest that either method of instrumentation is a suitable option for ACDF with respect to constructing stiffness at time zero.

Maximizing screw length in expandable lateral lumbar interbody spacers with integrated fixation may obviate the need for supplemental pedicle screws

BACKGROUND CONTEXT

Lateral lumbar interbody fusion (LLIF) is a minimally invasive surgical technique that provides a wide footprint interbody cage for correction of lumbar coronal and sagittal deformity. Traditional spinal interbody fusion procedures utilize pedicle screws and rods for additional stability. An expandable lateral titanium interbody cage with an integrated lateral fixation (eLLIFp) device provides a stand-alone LLIF that is intended to function autonomously. This may reduce the complexity of the surgery and the potential risks associated with supplemental posterior instrumentation. The minimum-acceptable screw length to promote adequate biomechanical fixation and stability for a stand-alone eLLIFp has not been determined.

PURPOSE

To investigate the effective ratio (of screw length/cage length) of a stand-alone eLLIFp construct that provides adequate biomechanical fixation and stability as compared to the eLLIFp with supplemental bilateral pedicle screw-rod fixation.

STUDY DESIGN/SETTING

In vitro cadaveric biomechanical testing and finite element modeling.

PATIENT SAMPLE

Eight fresh-frozen human cadaveric lumbar spine specimens (L2-5) were used.

OUTCOME MEASURES

Range-of-motion (ROM) measurements of intact and treated specimens with simulated stresses within the construct and surrounding bone during flexion-extension (FE), lateral bending (LB), and axial rotation (AR).

METHODS

Specimens with similar age and DEXA scores were selected. ROM of intact specimens was measured before treatment with LLIF at L3-4. Specimens were treated with expandable lateral cages with integrated fixation (stand-alone eLLIFp) or eLLIFp with supplemental posterior fixation using bilateral pedicle screws and rods (eLLIFp + BPS). ROM was measured using a custom-built 6-degrees-of-freedom motion simulator (±7.5Nm) and normalized as a percentage of intact. Four patient-specific lumbar functional spinal unit finite element models (FEMs) were developed, validated, and then instrumented with eLLIFp stand-alone devices. The integrated screw lengths were varied to achieve screw-to-cage length ratios of 0.6, 0.75 and 0.9. Stresses were compared among the constructs under a 7.5Nm pure moment load in FE, LB, and AR.

RESULTS

The stand-alone and posteriorly supplemented eLLIFp constructs were not sensitive to the ratio during FE and LB (with only a 4%-9% change in motion trends from low-to-high ratios, relative to intact). Independent of ratio, these constructs had minimal differences in FE and LB motion. However, during AR both constructs were sensitive to the ratio showing greater stability and less variability in performance with higher ratios (≥0.65). Regression analysis revealed that posteriorly supplemented eLLIFp constructs had a linear 13% reduction in AR motion as the ratio increased from low-to-high (p<.05). AR also imposed the highest stresses on the eLLIFp and these stresses increased with higher ratios (maximum stress 259MPa for ratio 0.9 during AR), yet implant failure was improbable because of the material properties of the titanium alloy used. Similarly, surrounding bone stresses were higher during AR and longer screws reduced these stresses (63MPa with a 0.6 ratio compared to 38MPa with a 0.9 ratio).

CONCLUSIONS

Independent of screw-to-cage length ratio, eLLIFp had comparable reduction of FE and LB motion with or without posterior fixation. For eLLIFp with and without posterior fixation, torsional performance and repeatability increased with screw-to-cage length ratio. Torsional stability was comparable between stand-alone eLLIFp with high ratios (≥0.65) and posteriorly supplemented eLLIFp with low ratios (0.55). However, a threshold screw-to-cage length ratio for optimizing the clinical performance of eLLIFp cannot be prescribed. Implant stress findings reinforced torsion as the critical loading condition. Surrounding bone stress decreased as the screw length increased, indicating the benefit of using longer screws. Surgeons using eLLIFp should consider longer screw lengths based on anatomical considerations.

CLINICAL SIGNIFICANCE

eLLIFp cages can be used as a stand-alone device in appropriately selected patients. This avoids the morbidity and cost associated with futher supplemental posterior fixation. Surgeons using eLLIFp should consider using longer screws to optimize fixation.

Development and biomechanical evaluation of a 3D printed analogue of the human lumbar spine

BACKGROUND

There exists a need for validated lumbar spine models in spine biomechanics research. Although cadaveric testing is the current gold standard for spinal implant development, it poses significant issues related to reliability and repeatability due to the wide variability in cadaveric physiologies. Moreover, there are increasing ethical concerns with human dissection practices. Analogue models can act as cost saving alternatives to human tissue with better repeatability. The current study proposes a new methodology of spinal biomechanics testing using 3D printable surrogates and characterized its multi-dimensional stiffness in displacement-controlled loading scenarios.

METHODS

The model consisted of L1 to S1 vertebrae, intervertebral discs (IVD), intertransverse, interspinous, anterior and posterior longitudinal ligaments. The vertebrae and the IVDs were derived from an open-source 3D MRI anatomography database, while the ligaments were modeled based on literature incorporating mounting points on the spinous and transverse processes. Stereolithography 3D printing along with a combination of stiff and soft photopolymer resins were used to manufacture the vertebrae and the soft tissues in the model. Thereafter, displacement-controlled pure moments were applied in the range of ± 15° at 0.5°/sec in all bending modes using a torsion testing machine and a custom pure bending jig. Model rotation and resisting moment under loading were recorded to quantify the rotational stiffness and hysteresis in the model.

RESULTS

The model reached a maximum of 5.66Nm and 3.53Nm at 15° flexion-extension, 3.84Nm and 3.93Nm at 15° right and left lateral bending, and 2.45Nm and 2.59Nm at 15° right and left axial rotation respectively. Model RMS error against ex vivo human response was estimated to be 1.57°, 1.64°, 0.82° in flexion-extension, lateral bending and axial rotation respectively. Bilateral symmetry in model stiffness was observed in lateral bending and axial rotation directions.

CONCLUSIONS

This study presents a reproducible 3D printable L1-S1 lumbar spine and validated it in all three orthogonal bending modes in the range of ± 15° against ex vivo and in silico data. The 3D printed analogue spine model described herein shows promising results, suggesting this model, with further validation, could have potential as a human cadaveric tissue substitute within the explored contexts of use.

Efficacy and Safety of Low-Density Pedicle Screw versus High-Density Screw in Lenke I Scoliosis: A Systematic Review and Meta-Analysis

OBJECTIVE

To evaluate the efficacy and safety of low-density versus high-density pedicle screw in patients with Lenke I adolescent idiopathic scoliosis through systematic review and meta-analysis.

METHODS

A comprehensive literature search was conducted in PubMed, Web of Science, and Embase databases. Studies comparing low-density and high-density pedicle screw in Lenke I adolescent idiopathic scoliosis were included. Two authors independently selected studies, assessed risk of bias, and extracted data. Meta-analysis was performed using systematic review software.

RESULTS

The meta-analysis included 11 studies comprising 697 patients (397 in low-density group and 300 in high-density group). No significant differences were found between low-density and high-density groups in terms of blood loss, operative time, complication rates, or revision rates. Radiographic outcomes, including major Cobb angle, curve correction, thoracic kyphosis, and coronal and sagittal balance, were also similar between the groups. However, low-density pedicle screw was associated with significantly lower costs.

CONCLUSIONS

This meta-analysis suggests that low-density pedicle screw can achieve similar clinical and radiographic outcomes compared with high-density constructs in patients with Lenke I adolescent idiopathic scoliosis, while potentially reducing costs, making it a more cost-effective option without compromising patient outcomes.

The role of loading-induced convection versus diffusion on the transport of small molecules into the intervertebral disc

PURPOSE

Limited nutrient transport is hypothesized to be involved in intervertebral disc (IVD) degeneration. It is widely recognized that the dominant mode of transport of small molecules such as glucose is via diffusion, rather than convection. However, recent findings suggest a role for convection-induced by fast (motion-related) and slow (diurnal) dynamic loading in molecular transport of even such small solutes. The aim of this study was to investigate whether fluid exchange induced by simulated physiological loading (composed of both fast cyclic or slower diurnal loading) can influence the molecular transport of a small molecule through the cartilage endplate (CEP) into the nucleus pulposus (NP) of IVDs.

METHODS

The molecular transport of fluorescein through the CEP and into the NP was studied in a bovine CEP/NP explant model and loading was applied by an axial compression bioreactor. The loaded explants (convection and diffusion) were compared to unloaded explants (diffusion alone).

RESULTS

In the initial 24 h, there were no differences between loaded and unloaded explants, indicating that convection did not enhance molecular transport of small solutes over diffusion alone. Notably, after 48 h which corresponds to two complete diurnal cycles of tissue compression, fluid exudation/imbibing and redistribution, the fluorescein concentration was significantly increased in the top and bottom layer of the explant, when compared to the unloaded explant.

CONCLUSIONS

Slower diurnal cyclic compression of the IVD might enhance the transport of small molecules into the IVD although it could not be discerned whether this was due to diffusion/convection or a combination.

How does thoracic scoliosis surgery affect thoracolumbar spinal flexibility and lumbar intradiscal pressure? An in vitro study confirming the importance of the rib cage

PURPOSE

To evaluate effects of spinal and rib osteotomies on the resulting spinal flexibility for surgical correction of thoracic scoliosis and to explore effects of posterior fixation on thoracolumbar segmental range of motion and lumbar intervertebral disc loading.

METHODS

Six fresh frozen human thoracolumbar spine and rib cage specimens (26-45 years, two female / four male) without clinically relevant deformity were loaded with pure moments of 5 Nm in flexion/extension, lateral bending, and axial rotation. Optical motion tracking of all segmental levels (C7-S) and intradiscal pressure measurements of the lumbar spine (L1-L5) were performed (1) in intact condition, (2) after Schwab grade 1, (3) Schwab grade 2, and (4) left rib osteotomies at T6-T10 levels, as well as (5) after posterior spinal fixation with pedicle screw-rod instrumentation at T4-L1 levels.

RESULTS

Schwab grade 1 and 2 osteotomies did not significantly (p > 0.05) affect spinal flexibility, whereas left rib osteotomies significantly (p < 0.05) increased segmental ranges of motion at upper and lower levels in flexion/extension and at treated levels in lateral bending. Posterior fixation caused significantly (p < 0.05) increased range of motion at upper adjacent thoracic and mid-lumbar levels, as well as significantly (p < 0.05) increased intradiscal pressure at the lower adjacent level.

CONCLUSION

Low effects of Schwab grade 1 and 2 osteotomies question the impact of isolated posterior spinal releases for surgical correction maneuvers in adolescent idiopathic scoliosis, in contrast to additional concave rib osteotomies. High effects of posterior fixation potentially explain frequently reported complications such as adjacent segment disease or proximal junctional kyphosis.

Spinal instrumentation length affects adjacent segment range of motion and intradiscal pressure

Scoliosis instrumentation length depends on the type and degree of deformity and the individual preference of the surgeon. This in vitro study aimed to explore effects of increasing instrumentation length on adjacent segment mobility and intervertebral disc loading. Six fresh frozen human spine specimens (C7-sacrum) with entire rib cage from young adult donors (26-45 years) were loaded with pure moments of 5 Nm. Range of motion (ROM) of all segments was determined using optical motion tracking. Lumbar intradiscal pressure (IDP) was measured using flexible pressure sensors from L1 to L5. The specimens were tested in two groups with increasing posterior instrumentation length in proximal (group 1) and distal direction (group 2). Significant (p < 0.05) adjacent segment ROM increases compared to the condition without any instrumentation and compared to other instrumentations were primarily found proximally to the instrumentation in lateral bending. IDP significantly (p < 0.05) increased in flexion in the distal adjacent segment for T4-L1 instrumentation and by up to 550% at instrumented levels compared to the condition without instrumentation. These findings may explain clinical complications such as adjacent segment disease and associated proximal and distal junctional kyphosis. To reduce loads on adjacent segments, instrumentation should therefore be applied as short as reasonable.

Experimental ex vivo characterization of the biomechanical effects of laminectomy and posterior fixation of the lumbo-sacral spine

Laminectomy and posterior fixation are well-established surgical techniques to decompress nervous structures in case of lumbar spinal stenosis. While laminectomy is suspected to increase the instability of the spine, posterior fixation is associated with some complications such as adjacent segment degeneration. This study aimed to investigate how laminectomy and posterior fixation alter the biomechanics of the lumbar spine in terms of range of motion (ROM) and strains on the intervertebral discs. Twelve L2-S1 cadaveric spines were mechanically tested in flexion, extension, and lateral bending in the intact condition, after two-level laminectomy and after L4-S1 posterior fixation. The ROM of the spine segment was measured in each spine condition, and each loading configuration. The strain distribution on the surface of all the intervertebral discs was measured with Digital Image Correlation. Laminectomy significantly increased the ROM in flexion (p = 0.028) and lateral bending (p = 0.035). Posterior fixation decreased the ROM in all the loading configurations. Laminectomy did not significantly modify the strain distribution in the discs. Posterior fixation significantly increased the principal tensile and compressive strains in the disc adjacent the fixation both in flexion and in lateral bending. These findings can elucidate one of the clinical causes of the adjacent segment degeneration onset.

Clinical efficacy and biomechanical analysis of a novel hollow pedicle screw combined with kyphoplasty for the treatment of Kümmell disease

Background

Vertebral augmentation is the preferred treatment for Kümmell disease (KD), but there exists a risk of cement displacement resulting in severe back pain and exacerbation of kyphosis. The study aimed to investigate the efficacy and safety of a novel hollow pedicle screw combined with kyphoplasty (HPS-KP) for treating KD, effectively preventing postoperative bone cement displacement.

Methods

The prospective study included 50 KD patients with no neurological deficit detected during clinical and radiological evaluation who underwent HPS-KP (n = 25) and PKP (n = 25) surgeries. The visual analogue scale (VAS) score, Oswestry dysfunction index (ODI), anterior vertebral height (AVH), wedge-shape affected vertebral Cobb angle (WCA), bisegmental Cobb angle (BCA), and complications were evaluated and compared in both groups. Besides, a finite element (FE) model of T11-L2 was constructed. The stress distributions, maximum von Mises stresses of vertebrae and bone cement, and maximum displacement of bone cement were compared and analyzed.

Results

The VAS and ODI scores at 3 days, 3 and 6 months, and 1 year after surgery significantly improved in both groups (p < 0.05). The AVH, BCA, and WCA significantly improved initially after the surgery in both groups (p < 0.05). The displacement of M2 was larger than other models, especially in flexion, right bending, and left and right rotation, while that of M6 was the lowest under all conditions.

Conclusion

HPS-KP was a safe and effective treatment for KD, effectively relieving pain, restoring vertebral height, and correcting local kyphosis, and it had better biomechanical stability and safety than ordinary single PKP and PKP combined with pediculoplasty in avoiding cement loosening and displacement.

A novel spine tester TO GO

Background

Often after large animal experiments in spinal research, the question arises-histology or biomechanics? While biomechanics are essential for informed decisions on the functionality of the therapy being studied, scientists often choose histological analysis alone. For biomechanical testing, for example, flexibility, specimens must be shipped to institutions with special testing equipment, as spine testers are complex and immobile. The specimens must usually be shipped frozen, and, thus, biological and histological investigations are not possible anymore. To allow both biomechanical and biological investigations with the same specimen and, thus, to reduce the number of required animals, the aim of the study was to develop a spine tester that can be shipped worldwide to test on-site.

Methods

The "Spine Tester TO GO" was designed consisting of a frame with three motors that initiate pure moments and rotate the specimen in three motion planes. A load cell and an optical motion tracking system controlled the applied loads and measured range of motion (ROM) and neutral zone (NZ). As a proof of concept, the new machine was validated and compared under real experimental conditions with an existing testing machine already validated employing fresh bovine tail discs CY34 (n = 10).

Results

The new spine tester measured reasonable ROM and NZ from hysteresis curves, and the ROM of the two testing machines formed a high coefficient of determination R 2 = 0.986. However, higher ROM results of the new testing machine might be explained by the lower friction of the air bearings, which allowed more translational motion.

Conclusions

The spine tester TO GO now opens up new opportunities for on-site flexibility tests and contributes hereby to the 3R principle by limiting the number of experimental animals needed to obtain full characterization of spine units at the macroscopic, biomechanical, biochemical, and histological level.

Regional variations of mechanical responses of IVD to 7 different motions: An in vivo study combined with FEA and DFIS

The abnormal mechanical behaviour of a lumbar intervertebral disc (IVD) is commonly recognized as a direct indicator of intervertebral disc degeneration (IDD). However, current methods cannot evaluate the patient-specific mechanical performance of an IVD in vivo during movement. This study establishes a patient-specific (PS) model that combines the kinematics parameters of the lumbar spine obtained with a dual fluoroscopic imaging system (DFIS) and a finite-element (FE) method for the first time to reveal the mechanical behaviours of IVDs in vivo under seven motions. Three healthy participants were recruited for this study. CT images were obtained to create finite-element models of L3-L5 spine segments. Meanwhile, participants were required to take specific positions including upright standing, flexion, extension, left and right lateral bending, as well as left and right axial torsion in the DFIS. The in vivo kinematic parameters, calculated by registering the CT images with images obtained with DFIS, were considered as loading conditions in FE simulations. Significant differences of von Mises stresses and principal strains were found between PS model and GN model which employing a generalized moment as loading conditions, former resulting in up to 76.74 % lower strain than the GN model. Also, considerable differences were observed for five anatomical regions of the IVD (L3-L5). Under all motions, the stress in the centre region (nucleus pulposus) was the lowest, while the stress in the posterior region was the highest in extension motion. Therefore, activities such as stretching with an extension, should be avoided by patients with a herniated disc, in which the posterior region was the herniation site. The PS model combining in vivo kinematics and FE simulations shows the potential in the design and assessment of patient-specific implants.

How to improve the mechanical safety of a novel spinal implant while saving costs and time

Background

Spinal implant failure is associated with prolonged patient suffering, high costs for the medical device industry, and a high economic burden for the health care system. Pre-clinical mechanical testing has great potential to reduce the risk of such failure. However, there are no binding regulations for planning and interpretation of mechanical testing. Therefore, different strategies exist. Mainly for novel implants an option is to start with a structured scientific literature search that forms an objective background for the definition of an implant-specific test plan, the derivation of acceptance criteria and interpretation of the test results.

Methods

This paper describes, how a literature-based approach can look like from the initial literature search through the derivation of the test plan and the acceptance criteria, to the final test result evaluation and how this approach can support the proof that the device meets all necessary safety and performance standards.

Results

The main advantage of this literature-based approach is that testing and test result interpretation are linked with the loads acting on the individual implant in vivo. In an ideal case, testing is focused on the individual implant in a way that ensures maximum efficiency during the development and approval process combined with maximum insight in safety and effectiveness of the implant. Even comparative implant testing may become obsolete, which is a big advantage if comparative implant and related data are not available.

Conclusion

This approach to pre-clinical mechanical testing offers the potential to create a chain of arguments, from literature review through testing to the interpretation of test results. This methodology can significantly enhance testing efficiency, reduce risk of failure, and ultimately prevent unnecessary patient suffering and healthcare costs. By synthesizing scientific insights with regulatory requirements, this review aims to guide clinicians and researchers in improving patient care and advancing device technologies.

Scoliosis instrumentation alters primary and coupled motions of the spine: An in vitro study using entire thoracolumbar spine and rib cage specimens

Background

Effects of rigid posterior instrumentation on the three-dimensional post-operative spinal flexibility are widely unknown. Purpose of this in vitro study was to quantify these effects for characteristic adolescent idiopathic scoliosis instrumentations.

Methods

Six fresh frozen human thoracic and lumbar spine specimens (C7-S) with entire rib cage from young adult donors (26-45 years) without clinically relevant deformity were loaded quasi-statically with pure moments of 5 Nm in flexion/extension, lateral bending, and axial rotation. Primary and coupled motions of all segments were measured using optical motion tracking. Specimens were tested without instrumentation and with posterior rod instrumentations ranging from T2 to L1 (for Lenke Type 2) and from T8 to L3 (for Lenke Type 5) based on survey results among spinal deformity surgeons. Statistical differences were evaluated using the pairwise Friedman test.

Results

Primary ranges of motion were significantly (p < 0.05) reduced in all six motion directions in the entire thoracic spine (T1-L1) for both instrumentations, but solely in extension and axial rotation in the entire lumbar spine (L1-S) for T8-L3 instrumentation. Without instrumentation, strong ipsilateral axial rotation during primary lateral bending and strong contralateral lateral bending during primary axial rotation were detected in the thoracic spine (T1-L1) and slight inverse coupled motions in the lumbar spine (L1-S). While coupled axial rotation was significantly (p < 0.05) reduced, especially in the upper thoracic spine (T1-T5) for T2-L1 instrumentation and in the lumbar spine (L1-S) for T8-L3 instrumentation, coupled lateral bending was solely significantly (p < 0.05) reduced in the upper thoracic spine (T1-T5) for T2-L1 instrumentation. Coupled motions in primary flexion and extension were non-existent and not affected by any fixation (p > 0.05).

Conclusions

Instrumentation reduces the primary flexibility and diminishes the natural coupling behavior between lateral bending and axial rotation, primarily in the upper thoracic spine, potentially causing correction loss and junctional deformity in the long-term.

An in vitro biomechanical evaluation of integrated lateral plate combined with oblique lateral interbody fusion in different bone conditions

Oblique lateral interbody fusion (OLIF) is a minimally invasive surgery for the treatment of lumbar degenerative diseases (LDD). Under normal bone mass(NB), supplemental with lateral plate (LP) fixation has been proven to provide stability and reduce complications. However, it is unclear whether OLIF combined with LP fixation can achieve satisfactory fixation effects in cases of osteoporosis(OP) or osteopenia (OS)? In this study, Eighteen L3-5 spinal specimens from 3 to 6 months old fresh calves were equally divided into 3 groups: group A (NB), group B (OS) and group C (OP). A load control scheme was adopted and evaluated using multidirectional nondestructive moments (± 7.5 N·m). An electronic universal tester and a tensile/torsion tester were simulated to generate 6 degrees of freedom of motion, and a VIC-3D three-dimensional optical full-field strain measurement system dynamically tracked the surgical segmental displacement. Each spine was evaluated under the following conditions at the L4-5 level: intact (INT); OLIF stand-alone (SA); cage supplemented with LP, cage supplemented with unilateral pedicle screws (UPS), and cage supplemented with bilateral pedicle screws (BPS). The current data show that With NB and OS models, LP fixation significantly reduced ROM in the LB and AR directions, with slightly less stability than BPS fixation and comparable to UPS. In the case of OP, LP fixation may increase the risk of internal fixation failure, and it is more preferable to choose BPS fixation with stronger stability.

Papain Injection Creates a Nucleotomy-like Cavity for Testing Gels in Intervertebral Discs

Biomaterials, such as hydrogels, have an increasingly important role in the development of regenerative approaches for the intervertebral disc. Since animal models usually resist biomaterial injection due to high intradiscal pressure, preclinical testing of the biomechanical performance of biomaterials after implantation remains difficult. Papain reduces the intradiscal pressure, creates cavities within the disc, and allows for biomaterial injections. But papain digestion needs time, and cadaver experiments that are limited to 24 h for measuring range of motion (ROM) cannot not be combined with papain digestion just yet. In this study, we successfully demonstrate a new organ culture approach, facilitating papain digestion to create cavities in the disc and the testing of ROM, neutral zone (NZ), and disc height. Papain treatment increased the ROM by up to 109.5%, extended NZ by up to 210.9%, and decreased disc height by 1.96 ± 0.74 mm. A median volume of 0.73 mL hydrogel could be injected after papain treatment, and histology revealed a strong loss of proteoglycans in the remaining nucleus tissue. Papain has the same biomechanical effects as known from nucleotomies or herniations and thus creates a disc model to study such pathologies in vitro. This new model can now be used to test the performance of biomaterials.

The effects of setup parameters on the measured kinetic output of cervical disc prostheses

Mechanical testing machines are used to evaluate kinematics, kinetics, wear, and efficacy of spinal implants. The simulation of "physiological" spinal loading conditions necessitates the simultaneous use of multiple actuators. The challenge in achieving a desired loading profile lies in achieving close synchronization of these actuators. Errors in load application can be attributed to both the control system and the intrinsic sample response. Moreover, the presence of friction in the setup can have an impact on the measured outcome. The optimization of setup parameters can substantially improve the ability to simulate spinal loading conditions and obtain reliable data on implant performance. In this study, a reproducible kinematic test protocol was developed to evaluate the sensitivity of the kinetic response (i.e., measured loads, moments, and stiffnesses) of a cervical disc prosthesis to several testing parameters. In this context, five ceramic ball and socket sample implants were mounted in a 6 DOF material testing machine and tested with a constant axial compressive force of 100 N in two motion modes: 1) flexion-extension (±7.5°) and 2) lateral bending (±6°). Parameters including rotation rate, slider friction, friction between the samples' articulating surfaces, and moment arm were considered to determine their effects on measured kinetic parameters. The sensitivity analysis indicated that all setup parameters except friction between the samples' articulating surfaces had a substantial effect on the results. The findings were then compared to predictions from a free body diagram to determine the optimal setup parameters. Consequently, the setup with the lowest rotation rate and employing passive sliders yielded results that were consistent with the free body diagram. This study demonstrated the significance of a comprehensive setup evaluation for reliable and reproducible testing of spinal implants, also for comparison between labs.

Facet deflection and strain are dependent on axial compression and distraction in C5-C7 spinal segments under constrained flexion

Background

Facet fractures are frequently associated with clinically observed cervical facet dislocations (CFDs); however, to date there has only been one experimental study, using functional spinal units (FSUs), which has systematically produced CFD with concomitant facet fracture. The role of axial compression and distraction on the mechanical response of the cervical facets under intervertebral motions associated with CFD in FSUs has previously been shown. The same has not been demonstrated in multi-segment lower cervical spine specimens under flexion loading (postulated to be the local injury vector associated with CFD).

Methods

This study investigated the mechanical response of the bilateral inferior C6 facets of thirteen C5-C7 specimens (67±13 yr, 6 male) during non-destructive constrained flexion, superimposed with each of five axial conditions: (1) 50 N compression (simulating weight of the head); (2-4) 300, 500, and 1000 N compression (simulating the spectrum of intervertebral compression resulting from neck muscle bracing prior to head-first impact and/or externally applied compressive forces); and, (5) 2 mm of C6/C7 distraction (simulating the intervertebral distraction present during inertial loading of the cervical spine by the weight of the head). Linear mixed-effects models (α = 0.05) assessed the effect of axial condition.

Results

Increasing amounts of intervertebral compression superimposed on flexion rotations, resulted in increased facet surface strains (range of estimated mean difference relative to Neutral: maximum principal = 77 to 110 με, minimum principal = 126 to 293 με, maximum shear = 203 to 375 με) and angular deflection of the bilateral inferior C6 facets relative to the C6 vertebral body (range of estimated mean difference relative to Neutral = 0.59° to 1.47°).

Conclusions

These findings suggest increased facet engagement and higher load transfer through the facet joint, and potentially a higher likelihood of facet fracture under the compressed axial conditions.

Biomechanics of annulus fibrosus: Elastic fiber simplification and degenerative impact on damage initiation and propagation

This study addresses three primary objectives related to lumbar intervertebral disc (IVD) biomechanics under ramping quasi-static loading conditions. First, we explore the conditions justifying the simplification of axisymmetric elastic fiber families into single fiber bundles through discretized strain energy functions. Simulations reveal that a concentration factor exceeding 10 allows for a consistent deviation below 10% between simplified and non-simplified responses. Second, we investigate the impact of elastic fibers on the physiological stiffness in IVDs, revealing minimal influence on biological motions but significant effects on degeneration. Lastly, we examine the initiation and progression of annulus fibrosus (AF) damage. Our findings confirm the validity of simplifying elastic fiber families and underscore the necessity of considering elastic fiber damage in biomechanical studies of AF tissues. Elastic fibers contribute to increased biaxial stretch stiffness, and their damage significantly affects the loading capacity of the inner AF. Additionally, degeneration significantly alters the susceptibility to damage in the AF, with specific regions exhibiting higher vulnerability. Damage tends to extend circumferentially and radially, emphasizing the regional variations in collagen and elastic fiber properties. This study offers useful insights for refining biomechanical models, paving the way for a more comprehensive understanding of IVD responses and potential clinical implications.

Evaluation of Load on Cervical Disc Prosthesis by Imposing Complex Motion: Multiplanar Motion and Combined Rotational-Translational Motion

(1) Background: The kinematic characteristics of disc prosthesis undergoing complex motion are not well understood. Therefore, examining complex motion may provide an improved understanding of the post-operative behavior of spinal implants. (2) Methods: The aim of this study was to develop kinematic tests that simulate multiplanar motion and combined rotational-translational motion in a disc prosthesis. In this context, five generic zirconia-toughened alumina (BIOLOX®delta, CeramTec, Germany) ball and socket samples were tested in a 6 DOF spine simulator under displacement control with an axial compressive force of 100 N in five motion modes: (1) flexion-extension (FE = ± 7.5°), (2) lateral bending (LB = ± 6°), (3) combined FE-LB (4) combined FE and anteroposterior translation (AP = 3 mm), and (5) combined LB and lateral motion (3 mm). For combined rotational-translational motion, two scenarios were analyzed: excessive translational movement after sample rotation (scenario 1) and excessive translational movement during rotation (scenario 2). (3) Results: For combined FE-LB, the resultant forces and moments were higher compared to the unidirectional motion modes. For combined rotational-translational motion (scenario 1), subluxation occurred at FE = 7.5° with an incremental increase in AP translation = 1.49 ± 0.18 mm, and LB = 6° with an incremental increase of lateral translation = 2.22 ± 0.16 mm. At the subluxation point, the incremental increase in AP force and lateral force were 30.4 ± 3.14 N and 40.8 ± 2.56 N in FE and LB, respectively, compared to the forces at the same angles during unidirectional motion. For scenario 2, subluxation occurred at FE = 4.93° with an incremental increase in AP translation = 1.75 mm, and LB = 4.52° with an incremental increase in lateral translation = 1.99 mm. At the subluxation point, the incremental increase in AP force and lateral force were 39.17 N and 38.94 N in FE and LB, respectively, compared to the forces in the same angles during the unidirectional motion. (4) Conclusions: The new test protocols improved the understanding of in vivo-like behavior from in vitro testing. Simultaneous translation-rotation motion was shown to provoke subluxation at lower motion extents. Following further validation of the proposed complex motion testing, these new methods can be applied future development and characterization of spinal motion-preserving implants.

Primary Stability of Kyphoplasty in Incomplete Vertebral Body Burst Fractures in Osteoporosis: A Biomechanical Investigation

Background: The objective of our study was to biomechanically evaluate the use of kyphoplasty to stabilize post-traumatic segmental instability in incomplete burst fractures of the vertebrae. Methods: The study was performed on 14 osteoporotic spine postmortem samples (Th11-L3). First, acquisition of the native multisegmental kinematics in our robot-based spine tester with three-dimensional motion analysis was set as a baseline for each sample. Then, an incomplete burst fracture was generated in the vertebral body L1 with renewed kinematic testing. After subsequent kyphoplasty was performed on the fractured vertebral body, primary stability was examined again. Results: Initially, a significant increase in the range of motion after incomplete burst fracture generation in all three directions of motion (extension-flexion, lateral tilt, axial rotation) was detected as proof of post-traumatic instability. There were no significant changes to the native state in the adjacent segments. Radiologically, a significant loss of height in the fractured vertebral body was also shown. Traumatic instability was significantly reduced by kyphoplasty. However, native kinematics were not restored. Conclusions: Although post-traumatic segmental instability was significantly reduced by kyphoplasty in our in vitro model, native kinematics could not be reconstructed, and significant instability remained.

Biomechanical Comparisons between One- and Two-Compartment Devices for Reconstructing Vertebrae by Kyphoplasty

BACKGROUND

This biomechanical in vitro study compared two kyphoplasty devices for the extent of height reconstruction, load-bearing capacity, cement volume, and adjacent fracture under cyclic loading.

METHODS

Multisegmental (T11-L3) specimens were mounted into a testing machine and subjected to compression, creating an incomplete burst fracture of L1. Kyphoplasty was performed using a one- or two-compartment device. Then, the testing machine was used for a cyclic loading test of load-bearing capacity to compare the two groups for the amount of applied load until failure and subsequent adjacent fracture.

RESULTS

Vertebral body height reconstruction was effective for both groups but not statistically significantly different. After cyclic loading, refracture of vertebrae that had undergone kyphoplasty was not observed in any specimen, but fractures were observed in adjacent vertebrae. The differences between the numbers of cycles and of loads were not statistically significant. An increase in cement volume was strongly correlated with increased risks of adjacent fractures.

CONCLUSION

The two-compartment device was not substantially superior to the one-compartment device. The use of higher cement volume correlated with the occurrence of adjacent fractures.

Cyclic testing of standalone ALIF versus TLIF in lumbosacral spines of low bone mineral density: an ex vivo biomechanical study

PURPOSE

Screwed anterior lumbar interbody fusion (SALIF) alleviates the need for supplemental posterior fixation leading to reduction of perioperative morbidity. Specifically, elderly and multimorbid patients would benefit from shorter operative time and faster recovery but tend to have low bone mineral density (BMD). The current study aimed to compare loosening, defined as increase of ROM and NZ, of SALIF versus transforaminal lumbar interbody fusion (TLIF) under cyclic loading in cadaveric spines with reduced BMD.

METHODS

Twelve human spines (L4-S2; 6 male 6 female donors; age 70.6 ± 19.6; trabecular BMD of L5 84.2 ± 24.4 mgHA/cm3, range 51-119 mgHA/cm3) were assigned to two groups. SALIF or TLIF were instrumented at L5/S1. Range of motion (ROM) and neutral zone (NZ) were assessed before and after axial cyclic loading (0-1150 N, 2000 cycles, 0.5 Hz) in flexion-extension (Flex-Ext), lateral bending, (LB), axial rotation (AR).

RESULTS

ROM of the SALIF specimens increased significantly in all loading directions (p ≤ 0.041), except for left AR (p = 0.053), whereas for TLIF it increased significantly in left LB (p = 0.033) and Flex (p = 0.015). NZ of SALIF showed increase in Flex-Ext and LB, whereas NZ of TLIF did not increase significantly in any motion direction.

CONCLUSIONS

Axial compression loading caused loosening of SALIF in Flex-Ext and LB, but not TLIF at L5/S1 in low BMD specimens. Nevertheless, Post-cyclic ROM and NZ of SALIF is comparable to TLIF. This suggests that, neither construct is optimal for the use in patients with reduced BMD.

Effect of two-level decompressive procedures on the biomechanics of the lumbo-sacral spine: an ex vivo study

Hemilaminectomy and laminectomy are decompressive procedures commonly used in case of lumbar spinal stenosis, which involve the removal of the posterior elements of the spine. These procedures may compromise the stability of the spine segment and create critical strains in the intervertebral discs. Thus, this study aimed to investigate if decompressive procedures could alter the biomechanics of the lumbar spine. The focus was on the changes in the range of motion and strain distribution of the discs after two-level hemilaminectomy and laminectomy. Twelve L2-S1 cadaver specimens were prepared and mechanically tested in flexion, extension and both left and right lateral bending, in the intact condition, after a two-level hemilaminectomy on L4 and L5 vertebrae, and a full laminectomy. The range of motion (ROM) of the entire segment was assessed in all the conditions and loading configurations. In addition, Digital Image Correlation was used to measure the strain distribution on the surface of each specimen during the mechanical tests, focusing on the disc between the two decompressed vertebrae and in the two adjacent discs. Hemilaminectomy did not significantly affect the ROM, nor the strain on the discs. Laminectomy significantly increased the ROM in flexion, compared to the intact state. Laminectomy significantly increased the tensile strains on both L3-L4 and L4-L5 disc (p = 0.028 and p = 0.014) in ipsilateral bending, and the compressive strains on L4-L5 intervertebral disc, in both ipsilateral and contralateral bending (p = 0.014 and p = 0.0066), with respect to the intact condition. In conclusion, this study found out that hemilaminectomy did not significantly impact the biomechanics of the lumbar spine. Conversely, after the full laminectomy, flexion significantly increased the range of motion and lateral bending was the most critical configuration for largest principal strain.

Influence of cervical total disc replacement on motion in the target and adjacent segments

BACKGROUND CONTEXT

The motion limitation after cervical discectomy and fusion alters the spine´s kinematics. Unphysiological strains may be the result and possible explanation for adjacent segment degeneration. Alterations to cervical kinematics due to cervical total disc replacement (TDR), especially two-level, are still under investigated.

PURPOSE

To investigate cervical motion including coupled motions after one-level and two-level TDR in the treated and also the adjacent segments.

STUDY DESIGN

An in-vitro study using pure moment loading of human donor spines.

METHODS

Seven fresh frozen human cervical spine specimens (C4-T1, median age 46 with range 19-60 years, four female) were included in this study. Specimens were tested in the intact condition first, followed by one-level TDR at C5-6 which was subsequently extended one level further caudal (C5-7). Each specimen was quasistatically loaded with pure moments up to 1.5 Nm in flexion/extension (FE), lateral bending (LB), and axial rotation (AR) in a universal spine tester for 3.5 cycles at 1 °/s. During the tests three dimensional motion tracking was performed for each vertebral body individually. From that, the primary and coupled ROM of each spinal level during the third full cycle of motion were evaluated. Nonparametric statistical analysis was performed using a Friedman-test and post hoc correction with Dunn-Bonferroni-tests (p<.05). Ethics approval was obtained in advance.

RESULTS

In FE, one-level TDR (C5-6) moderately increased primary FE in all four segments, but only significantly at the cranial adjacent level C4-5. Additional TDR at C6-7 further increased the ROM at the target segment without much influence on the other levels. Increasing implant height at C6-7 partially counteracted the increased FE. Coupled motions were minimal in all test conditions at all levels. In LB, coupled AR was observed in all test conditions at all levels. One-level TDR decreased primary LB at the target segment C5-6 significantly, without much influence on the other levels. Extending TDR to C6-7 decreased ROM in the target segment but without gaining statistical significance. Increasing implant height at C6-7 further decreased primary LB at the target segment, still without significance. Notably, coupled AR was significantly decreased at the cranial adjacent segment C4-5 compared to the intact condition. In AR, coupled LB was observed in all test conditions at the levels C4-5, C5-6, and C6-7, while the transition level to the thoracic spine C7-T1 showed only little coupled LB. Both one-level and two-level TDR showed little influence on primary AR or coupled motions at any level. Only after increasing implant height at C6-7 was the motion of the caudally adjacent level C7-T1 significantly altered.

CONCLUSION

Evaluating primary FE, LB, and AR together with the associated coupled motions revealed widespread influence of cervical TDR not only on the motion of the treated level but also at the adjacent segments. The influence of two-level TDR is more widespread and involves more levels than one-level TDR.

CLINICAL SIGNIFICANCE

The prevention of unphysiological strains due to altered kinematics after cervical fusion, which could possibly explain adjacent segment degeneration, were a driving factor in the development of TDR. These experimental findings suggest cervical TDR influences the whole cervical spine, not only the treated segment. The effect becomes more extensive, involving more levels and motion directions, after two-level than after one-level TDR.

Finite element analysis of implanted lumbar spine: Effects of open laminectomy plus PLF and open laminectomy plus TLIF surgical approaches on L3-L4 FSU

Several finite element (FE) studies reported performances of various lumbar fusion surgical approaches. However, comparative studies on the performance of Open Laminectomy plus Posterolateral Fusion (OL-PLF) and Open Laminectomy plus Transforaminal Interbody Fusion (OL-TLIF) surgical approaches are rare. In the current FE study, the variation in ranges of motions (ROM), stress-strain distributions in an implanted functional spinal unit (FSU) and caudal adjacent soft structures between OL-PLF and OL-TLIF virtual models were investigated. The implanted lumbar spine FE models were developed from subject-specific computed tomography images of an intact spine and solved for physiological loadings such as compression, flexion, extension and lateral bending. Reductions in the ROMs of L1-L5 (49 % to 59 %) and L3-L4 implanted FSUs (91 % to 96 %) were observed for both models. Under all the loading cases, the maximum von Mises strain observed in the implanted segment of both models exceeds the mean compressive yield strain for the vertebra. The maximum von Mises stress and strain observed on the caudal adjacent soft structures of both the implanted models are at least 22 % higher than the natural spine model. The findings indicate the risk of failure in the implanted FSUs and higher chances of adjacent segment degeneration for both models.

Comparison of four in vitro test methods to assess nucleus pulposus replacement device expulsion risk

Background

Nucleus replacement devices (NRDs) are not routinely used in clinic, predominantly due to the risk of device expulsion. Rigorous in vitro testing may enable failure mechanisms to be identified prior to clinical trials; however, current testing standards do not specify a particular expulsion test. Multiple methods have therefore been developed, complicating comparisons between NRD designs. Thus, this study assessed the effectiveness of four previously reported expulsion testing protocols; hula-hoop (Protocol 1), adapted hula-hoop (Protocol 2), eccentric cycling (Protocol 3), and ramp to failure (Protocol 4), applied to two NRDs, one preformed and one in situ curing.

Methods

Nucleus material was removed from 40 bovine tail intervertebral disks. A NRD was inserted posteriorly into each cavity and the disks were subjected to one of four expulsion protocols.

Results

NRD response was dependent on both the NRD design and the loading protocol. Protocol 1 resulted in higher migration and earlier failure rates compared to Protocol 2 in both NRDs. The preformed NRD was more likely to migrate when protocols incorporated rotation. The NRDs had equal migration (60%) and expulsion (60%) rates when using unilateral bending and ramp testing. Combining the results of multiple tests revealed complimentary information regarding the NRD response.

Conclusions

Adapted hula-hoop (Protocol 2) and ramp to failure (Protocol 4), combined with fluoroscopic analysis, revealed complimentary insights regarding migration and failure risk. Therefore, when adopting the surgical approach and animal model used in this study, it is recommended that NRD performance be assessed using both a cyclic and ramp loading protocol.

Lateral fenestration of lumbar intervertebral discs in rabbits: development and characterisation of an in vivo preclinical model with multi-modal endpoint analysis

PURPOSE

To evaluate the biological and biomechanical effects of fenestration/microdiscectomy in an in vivo rabbit model, and in doing so, create a preclinical animal model of IVDD.

METHODS

Lateral lumbar IVD fenestration was performed in vivo as single- (L3/4; n = 12) and multi-level (L2/3, L3/4, L4/5; n = 12) fenestration in skeletally mature 6-month-old New Zealand White rabbits. Radiographic, micro-CT, micro-MRI, non-destructive robotic range of motion, and histological evaluations were performed 6- and 12-weeks postoperatively. Independent t tests, one-way and two-way ANOVA and Kruskal-Wallis tests were used for parametric and nonparametric data, respectively. Statistical significance was set at P < 0.05.

RESULTS

All rabbits recovered uneventfully from surgery and ambulated normally. Radiographs and micro-CT demonstrated marked reactive proliferative osseous changes and endplate sclerosis at fenestrated IVDs. Range of motion at the fenestrated disc space was significantly reduced compared to intact controls at 6- and 12-weeks postoperatively (P < 0.05). Mean disc height index percentage for fenestrated IVDs was significantly lower than adjacent, non-operated IVDs for both single and multi-level groups, at 6 and 12 weeks (P < 0.001). Pfirrmann MRI IVDD and histological grading scores were significantly higher for fenestrated IVDs compared to non-operated adjacent and age-matched control IVDs for single and multi-level groups at 6 and 12 weeks (P < 0.001).

CONCLUSIONS

Fenestration, akin to microdiscectomy, demonstrated significant biological, and biomechanical effects in this in vivo rabbit model and warrants consideration by veterinary and human spine surgeons. This described model may be suitable for preclinical in vivo evaluation of therapeutic strategies for IVDD in veterinary and human patients.

Stepwise reduction of bony density in patients induces a higher risk of annular tears by deteriorating the local biomechanical environment

BACKGROUND CONTEXT

The relationship between osteoporosis and intervertebral disc degeneration (IDD) remains unclear. Considering that annular tear is the primary phenotype of IDD in the lumbar spine, the deteriorating local biomechanical environment may be the main trigger for annular tears.

PURPOSE

To investigate whether poor bone mineral density (BMD) in the vertebral bodies may increase the risk of annular tears via the degradation of the local biomechanical environment.

STUDY DESIGN

This study was a retrospective investigation with relevant numerical mechanical simulations.

PATIENT SAMPLE

A total of 64 patients with low back pain (LBP) and the most severe IDD in the L4-L5 motion segment were enrolled.

OUTCOME MEASURES

Annulus integration status was assessed using diffusion tensor fibre tractography (DTT). Hounsfield unit (HU) values of adjacent vertebral bodies were employed to determine BMD. Numerical simulations were conducted to compute stress values in the annulus of models with different BMDs and body positions.

METHODS

The clinical data of the 64 patients with low back pain were collected retrospectively. The BMD of the vertebral bodies was measured using the HU values, and the annulus integration status was determined according to DTT. The data of the patients with and without annular tears were compared, and regression analysis was used to identify the independent risk factors for annular tears. Furthermore, finite element models of the L4-L5 motion segment were constructed and validated, followed by estimating the maximum stress on the post and postlateral interfaces between the superior and inferior bony endplates (BEPs) and the annulus.

RESULTS

Patients with lower HU values in their vertebral bodies had significantly higher incidence rates of annular tears, with decreased HU values being an independent risk factor for annular tears. Moreover, increased stress on the BEP-annulus interfaces was associated with a stepwise reduction of bony density (ie, elastic modulus) in the numerical models.

CONCLUSIONS

The stepwise reduction of bony density in patients results in a higher risk of annular tears by deteriorating the local biomechanical environment. Thus, osteoporosis should be considered to be a potential risk factor for IDD biomechanically.

The Injection of Gels Through an Intact Annulus Maintains Biomechanical Performance without Extrusion Risk

For autologous-disc-derived chondrocyte transplantation (ADCT) a transglutaminase crosslinked gelatine gel and an albumin hyaluronic acid gel, crosslinked with bis-thio-polyethylene glycol, were injected through a syringe into a degenerated intervertebral disc, where they solidified in situ. This biomechanical in vitro study with lumbar bovine motion segments evaluated disc height changes, motion characteristics in a quasi-static spine loading simulators, and the potential extrusion risk of these biomaterials in a complex dynamic multi-axial loading set-up with 100,000 loading cycles. After the injection and formation of the gel in the center of the nucleus, the disc height increase was about 0.3 mm. During cyclic testing, a gradual decrease in height could be detected due to viscoelastic effects and fluid loss. No gel extrusion could be observed for all specimens during the entire test procedure. A macroscopic inspection after dissections showed an accumulation of the solidified gel in the center of the nucleus. The results demonstrate that the injection of in situ solidifying gels through the intact annulus allows for the stable maintenance of the injected gel at the target location, with high potential for use as a suitable scaffold to anchor therapeutically applied cells for disc regeneration within the treated nucleus pulposus.

An additively manufactured model for preclinical testing of cervical devices

Purpose

Composite models have become commonplace for the assessment of fixation and stability of total joint replacements; however, there are no comparable models for the cervical spine to evaluate fixation. The goal of this study was to create the framework for a tunable non-homogeneous model of cervical vertebral body by identifying the relationships between strength, in-fill density, and lattice structure and creating a final architectural framework for specific strengths to be applied to the model.

Methods

The range of material properties for cervical spine were identified from literature. Using additive manufacturing software, rectangular prints with three lattice structures, gyroid, triangle, zig-zag, and a range of in-fill densities were 3D-printed. The compressive and shear strengths for all combinations were calculated in the axial and coronal planes. Eleven unique vertebral regions were selected to represent the distribution of density. Each bone density was converted to strength and subsequently correlated to the lattice structure and in-fill density with the desired material properties. Finally, a complete cervical vertebra model was 3D-printed to ensure sufficient print quality.

Results

Materials testing identified a relationship between in-fill densities and strength for all lattice structures. The axial compressive strength of the gyroid specimens ranged from 1.5 MPa at 10% infill to 31.3 MPa at 100% infill and the triangle structure ranged from 2.7 MPa at 10% infill to 58.4 MPa at 100% infill. Based on these results, a cervical vertebra model was created utilizing cervical cancellous strength values and the corresponding in-fill density and lattice structure combination. This model was then printed with 11 different in-fill densities ranging from 33% gyroid to 84% triangle to ensure successful integration of the non-homogeneous in-fill densities and lattice structures.

Conclusions

The findings from this study introduced a framework for using additive manufacturing to create a tunable, customizable biomimetic model of a cervical vertebra.