Load-sharing characteristics of stabilized lumbar spine segments

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.

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.

Experimental Study of Failures of the Rigid Spinal Posterior Fixation System Under Compressive Load Conditions: A Cadaver Study

Background Many studies have been conducted on the biomechanics of the spine to elucidate the fixation properties of spinal fusion surgery and the causes of instrumentation failure. Among these studies, there are some studies on load sharing in the spine and measurement using strain gauges and pressure gauges, but there is a lack of research on axial compressive loads. Methods Axial compressive load tests were performed on human cadaveric injured lumbar vertebrae fixed with pedicle screws (PS). Both the strain generated in the PS rod and the intradiscal pressure were measured. Subsequently, the stress generated in the PS rod and the load sharing of the spine and instrumentation were calculated. Results Even when only compressive load is applied, bending stress of more than 10 times the compression stress was generated in the rod, and the stress tended to concentrate on one rod. Rod deformation becomes kyphotic, in contrast to the lordotic deformation behavior of the lumbar spine. The stress shielding rate was approximately 40%, less than half. Conclusions This study obtained basic data useful for constructing and verifying numerical simulations that are effective for predicting and elucidating the causes of dislodgement and failure of spinal implants.

The application of impantable sensors in the musculoskeletal system: a review

As the population ages and the incidence of traumatic events rises, there is a growing trend toward the implantation of devices to replace damaged or degenerated tissues in the body. In orthopedic applications, some implants are equipped with sensors to measure internal data and monitor the status of the implant. In recent years, several multi-functional implants have been developed that the clinician can externally control using a smart device. Experts anticipate that these versatile implants could pave the way for the next-generation of technological advancements. This paper provides an introduction to implantable sensors and is structured into three parts. The first section categorizes existing implantable sensors based on their working principles and provides detailed illustrations with examples. The second section introduces the most common materials used in implantable sensors, divided into rigid and flexible materials according to their properties. The third section is the focal point of this article, with implantable orthopedic sensors being classified as joint, spine, or fracture, based on different practical scenarios. The aim of this review is to introduce various implantable orthopedic sensors, compare their different characteristics, and outline the future direction of their development and application.

In vivo load on knee, hip and spine during manual materials handling with two lifting techniques

It is generally accepted that the lifting technique strongly influences physical loads within the human body and, thus, the risk of musculoskeletal disorders. However, there is a lack of knowledge regarding whether particular lifting techniques are effective in reducing loads. Hence, this retrospective study quantified (partly published) in vivo loads at joints within the human body during two typical lifting techniques, stoop lifting and squat lifting. Patients who had received instrumented implants underwent in vivo load measurements at either the knee (two patients), the hip (eight patients), or the upper lumbar spine (four patients) while lifting a 10 kg weight frontally with either straight (stoop) or bent (squat) knees. Contact forces and moments and the orientation of the contact force vector were determined and examined using the paired t test of Statistical Parametric Mapping. The two lifting techniques did not differ in terms of load magnitudes but did differ in terms of directions: (i) at the hip joint, the load vector varied significantly (p < 0.05) in the frontal and sagittal planes, (ii) at the knee joint, the load vector differed significantly (p < 0.05) in the sagittal plane (iii) while the load vector and magnitude did not differ at the upper lumbar spine (p > 0.05). Our findings indicate that the lifting technique causes changes in the orientation rather than the magnitude of lower extremity joint contact loads. Even though this quantification could only be performed in a small group of patients, the quantification of the relevance of such lifting technique recommendations will hopefully guide future recommendations towards a more scientific interpretation.

Biomechanical and clinical research of Isobar semi-rigid stabilization devices for lumbar degenerative diseases: a systematic review

While lumbar spinal fusion using rigid rods is a prevalent surgical technique, it can lead to complications such as adjacent segment disease (ASDis). Dynamic stabilization devices serve to maintain physiological spinal motion and alleviate painful stress, yet they are accompanied by a substantial incidence of construct failure and subsequent reoperation. Compared to traditional rigid devices, Isobar TTL semi-rigid stabilization devices demonstrate equivalent stiffness and effective stabilization capabilities. Furthermore, when contrasted with dynamic stabilization techniques, semi-rigid stabilization offers improved load distribution, a broader range of motion within the fixed segment, and reduced mechanical failure rates. This paper will review and evaluate the clinical and biomechanical performance of Isobar TTL semi-rigid stabilization devices. A literature search using the PubMed, EMBASE, CNKI, Wanfang, VIP, and Cochrane Library databases identified studies that met the eligibility criteria. Twenty-eight clinical studies and nine biomechanical studies were included in this systematic review. The VAS, the ODI, and Japanese Orthopedic Association scoring improved significantly in most studies. UCLA grading scale, Pfirrmann grading, and modified Pfirrmann grading of the upper adjacent segments improved significantly in most studies. The occurrence rate of ASD was low. In biomechanical studies, Isobar TTL demonstrated a superior load sharing distribution, a larger fixed segment range of motion, and reduced stress at the rod-screw/screw-bone interfaces compared with titanium rods. While findings from mechanical studies provided promising results, the clinical studies exhibited low methodological quality. As a result, the available evidence does not possess sufficient strength to substantiate superior outcomes with Isobar semi-rigid system in comparison to titanium rods. To establish more conclusive conclusions, further investigations incorporating improved protocols, larger sample sizes, and extended follow-up durations are warranted.

Device profile of the FlareHawk interbody fusion system, an endplate-conforming multi-planar expandable lumbar interbody fusion cage

INTRODUCTION

The FlareHawk Interbody Fusion System is a family of lumbar interbody fusion devices (IBFDs) that include FlareHawk7, FlareHawk9, FlareHawk11, TiHawk7, TiHawk9, and TiHawk11. These IBFDs offer a new line of multi-planar expandable interbody devices designed to provide mechanical stability, promote arthrodesis, and allow for restoration of disc height and lordosis through a minimal insertion profile during standard open and minimally invasive posterior lumbar fusion procedures. The two-piece interbody cage design consists of a PEEK outer shell that expands in width, height, and lordosis with the insertion of a titanium shim. Once expanded, the open architecture design allows for ample graft delivery into the disc space.

AREAS COVERED

The design and unique features of the FlareHawk family of expandable fusion cages are described. The indications for their use are discussed. Early clinical and radiographic outcome studies using the FlareHawk Interbody Fusion System are reviewed, and properties of competitor products are outlined.

EXPERT OPINION

The FlareHawk multi-planar expandable interbody fusion cage is unique amongst the many lumbar fusion cages currently on the market. The multi-planar expansion, open architecture, and adaptive geometry set it apart from its competitors.

Mechanical Analysis of Posterior Pedicle Screw System Placement and Internal Fixation in the Treatment of Lumbar Fractures

Objective

Image segmentation technology is applied to separate a single vertebra from the three-dimensional model of the spine, so as to separate a single vertebra image with smaller error, higher degree of automation, and better results. The objectives are to study the biomechanical characteristics of posterior short-segment pedicle screw fixation by three-dimensional finite element method, analyze the mechanical characteristics of posterior pedicle screw rod fixation system under different factors, and demonstrate the feasibility of its application in the treatment of lumbar fracture.

Methods

The authors searched the database for articles about the treatment of lumbar spine fracture, screw rod internal fixation system, and its mechanical parameters. The threshold segmentation method based on region segmentation method was used to segment the image, and the three-dimensional finite element model was used to analyze the biomechanical characteristics of different posterior internal fixation for lumbar spine fracture.

Results

The posterior pedicle internal fixation system for the treatment of multilevel spinal fractures is a mature surgical technique and has fewer postoperative complications. Transpedicle fixation is effective and reliable. It can effectively restore the coronal and sagittal curvature of the vertebral body and restore the stability of the spine better. But the choice of internal fixation method should be individualized based on fracture type, identification of critical and secondary injury sites, and stability assessment. Only after mastering the biomechanical characteristics of the posterior screw rod system for the treatment of lumbar fracture, selecting the appropriate method, and fixing the appropriate movement unit can the best fixation be achieved.

Conclusion

Threshold method is the most direct and simple image segmentation method. The core technology of thresholding is the selection of threshold, which will affect the final segmentation effect. The most common segmentation method is to calculate the segmentation threshold by histogram. The threshold method has less computation and good segmentation effect for the image with large contrast between background and target. Posterior pedicle screw rod system internal fixation has the advantages of less trauma, good reduction, reliable fixation, and less complications. The design, placement angle and depth of various internal fixation systems, and the number of fixed segments all show different mechanical characteristics. As long as we master the above characteristics, choose the appropriate method and fix the appropriate motor unit, and we can get the best fixation; it can be used as an effective treatment for lumbar fracture.

Thoracolumbar Burst Fracture: McCormack Load-sharing Classification: Systematic Review and Single-arm Meta-analysis

STUDY DESIGN

A systematic review and single-arm meta-analysis of randomized clinical trials.

OBJECTIVE

The aim of this study was to evaluate whether the load-sharing classification (LSC) is reliable to predict the best surgical approach for thoracolumbar burst fracture (TBF).

SUMMARY OF BACKGROUND DATA

There is no previous review evaluating the efficacy of the use of LSC as a guide in the surgical treatment of burst fractures.

METHODS

On April 19th, 2019, a broad search was performed in the following databases: EMBASE, PubMed, Cochrane, SCOPUS, Web of Science, LILACS, and gray literature. This study was registered on the International Prospective Register of Systematic Reviews. We included clinical trials involving patients with TBF undergoing posterior surgical treatment, classified by load-sharing score, and that enabled the analysis of the outcomes loss of segmental kyphosis and implant failure (IF). We performed random- or fixed-effects models meta-analyses depending on the data homogeneity. Heterogeneity between studies was estimated by I2 and τ2 statistics.

RESULTS

The search identified 189 references, out of which nine studies were eligible for this review. All articles presenting LSC up to 6 proved to be reliable in indicating that only posterior instrumentation is necessary, without screw failures or loss of kyphosis correction. For cases where the LSC was >6, only 2.5% of the individuals presented IF upon posterior approach alone. For loss of kyphosis correction, only 5% of patients had this outcome where LSC >6. For both outcomes together, we had 6% of postoperative problems (I2 = 77%, τ2 < 0.0015, P < 0.01).

CONCLUSION

Load-sharing scores up to 6 are 100% reliable, only requiring posterior instrumentation for stabilization. For scores >6, the risk of implant breakage and loss of kyphosis correction in posterior fixation alone is low. Thus, other factors should be considered to define the best surgical approach to be adopted.Level of Evidence: 1.

Spine biomechanical testing methodologies: The controversy of consensus vs scientific evidence

Biomechanical testing methodologies for the spine have developed over the past 50 years. During that time, there have been several paradigm shifts with respect to techniques. These techniques evolved by incorporating state-of-the-art engineering principles, in vivo measurements, anatomical structure-function relationships, and the scientific method. Multiple parametric studies have focused on the effects that the experimental technique has on outcomes. As a result, testing methodologies have evolved, but there are no standard testing protocols, which makes the comparison of findings between experiments difficult and conclusions about in vivo performance challenging. In 2019, the international spine research community was surveyed to determine the consensus on spine biomechanical testing and if the consensus opinion was consistent with the scientific evidence. More than 80 responses to the survey were received. The findings of this survey confirmed that while some methods have been commonly adopted, not all are consistent with the scientific evidence. This review summarizes the scientific literature, the current consensus, and the authors' recommendations on best practices based on the compendium of available evidence.

Load-sharing biomechanics of lumbar fixation and fusion with pedicle subtraction osteotomy

Pedicle subtraction osteotomy (PSO) is an invasive surgical technique allowing the restoration of a well-balanced sagittal profile, however, the risks of pseudarthrosis and instrumentation breakage are still high. Literature studied primary stability and posterior instrumentation loads, neglecting the load shared by the anterior column, which is fundamental to promote fusion early after surgery. The study aimed at quantifying the load-sharing occurring after PSO procedure across the ventral spinal structures and the posterior instrumentation, as affected by simple bilateral fixation alone, with interbody cages adjacent to PSO level and supplementary accessory rods. Lumbar spine segments were loaded in vitro under flexion–extension, lateral bending, and torsion using an established spine tester. Digital image correlation (DIC) and strain-gauge (SG) analyses measured, respectively, the full-field strain distribution on the ventral surface of the spine and the local strain on posterior primary rods. Ventral strains considerably decreased following PSO and instrumentation, confirming the effectiveness of posterior load-sharing. Supplemental accessory rods considerably reduced the posterior rod strains only with interbody cages, but the ventral strains were unaffected: this indicates that the load transfer across the osteotomy could be promoted, thus explaining the higher fusion rate with decreased rod fracture risk reported in clinical literature.

Current Concepts of Contemporary Expandable Lumbar Interbody Fusion Cage Designs, Part 1: An Editorial on Their Biomechanical Characteristics

BACKGROUND

Bidirectional expandable designs for lumbar interbody fusion cages are the latest iteration of expandable spacers employed to address some of the common problems inherent to static interbody fusion cages.

OBJECTIVE

To describe the rationales for contemporary bidirectional, multimaterial expandable lumbar interbody fusion cage designs to achieve in situ expansion for maximum anterior column support while decreasing insertion size during minimal-access surgeries.

METHODS

The authors summarize the current concepts behind expandable spinal fusion open architecture cage designs focusing on advanced minimally invasive spinal surgery techniques, such as endoscopy. A cage capable of bidirectional expansion in both height and width to address constrained surgical access problems was of particular interest to the authors while they analyzed the relationship between implant material stiffness and geometric design regarding the risk of subsidence and reduced graft loading.

CONCLUSIONS

Biomechanical advantages of new bidirectional, multimaterial expandable interbody fusion cages allow insertion through minimal surgical access and combine the advantages of proven device configurations and advanced material selection. The final construct stiffness is sufficient to provide immediate anterior column support while accommodating reduced sizes required for minimally invasive surgery applications.

LEVEL OF EVIDENCE

7.

Uniportal Full Endoscopic Posterolateral Transforaminal Lumbar Interbody Fusion with Endoscopic Disc Drilling Preparation Technique for Symptomatic Foraminal Stenosis Secondary to Severe Collapsed Disc Space: A Clinical and Computer Tomographic Study with Technical Note

Background: Severe collapsed disc secondary to degenerative spinal conditions leads to significant foraminal stenosis. We hypothesized that uniportal posterolateral transforaminal lumbar interbody fusion with endoscopic disc drilling technique could be safely applied to the collapsed disc space to improve patients’ pain score, restore disc height, and correct the segmental angular parameters. Methods: We included patients who met the indication criteria for lumbar fusion and underwent uniportal full endoscopic posterolateral transforaminal lumbar interbody fusion with pre-operative Computer Tomography mid disc height of less than or equal to 5 mm and MRI of Grade 3 Foraminal Stenosis. Visual analogue scale and computer tomography pre-operative and post-operative sagittal disc height in the anterior, middle and posterior part of the disc; sagittal focal segmental angle; mid coronal disc height and coronal wedge angles were evaluated. Results: 30 levels of Endo-TLIF were included, with a mean follow up of 12 months. The mean improvement in decreasing pain score was 2.5 ± 1.1, 3.2 ± 0.9 and 4.3 ± 1.0 at 1 week post operation, 3 months post operation and at final follow up, respectively, p < 0.05. There was significant increase in mid sagittal computer tomographic anterior, middle and posterior disc height of 6.99 ± 2.30, 6.28 ± 1.44, 5.12 ± 1.79 mm respectively, p < 0.05. CT mid coronal disc height showed an increase of 7.13 ± 1.90 mm, p < 0.05. There was a significant improvement in the CT coronal wedge angle of 2.35 ± 4.73 and the CT segmental focal sagittal angle of 1.98 ± 4.69, p < 0.05. Conclusion: Application of Uniportal Endoscopic Posterolateral Lumbar Interbody Fusion in patients with severe foraminal stenosis secondary to severe collapsed disc space significantly relieved patients’ pain and restored disc height without early subsidence or exiting nerve root dysesthesia in our cohort of patients.

Stress analysis of the implants in transforaminal lumbar interbody fusion under static and vibration loadings: a comparison between pedicle screw fixation system with rigid and flexible rods

The use of a pedicle screw fixation system with rods made of more compliant materials has become increasingly popular for spine fusion surgery in recent years. The aim of this study was to compare stress responses of the implants in transforaminal lumbar interbody fusion (TLIF) when using flexible and conventional rigid posterior fixation systems. A previously validated intact L1-S1 finite element model was modified to simulate single-level (L4-L5) TLIF with bilateral pedicle screw fixation using two types of connecting rod (rigid and flexible rods). The von Mises stresses in the implants (including TLIF cage, pedicle screws and rods) for the rigid and flexible fixations were analyzed under static and vibration loadings. The results showed that compared with the rigid fixation, the use of flexible fixation decreased the maximum stress in the pedicle screws, but increased the maximum stress in the cage and the ratio of maximum stress in the rods to the yield stress. It was also found that with decreasing diameter of the flexible rod (i.e. increasing flexibility of the rod), the maximum stress was decreased in the pedicle screws but increased in the cage and the rods. The findings imply that compared with the rigid rod, application of the flexible rod in the pedicle screw fixation system for the TLIF might decrease the breakage risk of pedicle screws but increase the risk of cage subsidence and rod breakage. Moreover, flexibility of the rod in the flexible fixation system should be carefully determined.

Investigation into the biomechanics of lumbar spine micro-dynamic pedicle screw

Background

Numerous reports have shown that rigid spinal fixation contributes to a series of unwanted complications in lumbar fusion procedure. This innovative micro-dynamic pedicle screw study was designed to investigate the biomechanical performance of lumbar implants using numerical simulation technique and biomechanical experiment.

Methods

Instrumented finite element models of three configurations (dynamic fixation, rigid fixation and hybrid fixation) using a functional L3-L4 lumbar unit were developed, to compare the range of motion of the lumbar spine and stress values on the endplate and implants. An in vitro experiment was simultaneously conducted using 18 intact porcine lumbar spines and segmental motion analyses were performed as well.

Results

Simulation results indicated that the dynamic fixation and the hybrid fixation models respectively increased the range of motion of the lumbar spine by 95 and 60% in flexion and by 83 and 55% in extension, compared with the rigid fixation model. The use of micro-dynamic pedicle screw led to higher stress on endplates and lower stress on pedicle screws. The outcome of the in vitro experiment demonstrated that the micro-dynamic pedicle screw could provide better range of motion at the instrumented segments than a rigid fixation.

Conclusion

The micro-dynamic pedicle screw has the advantage of providing better range of motion than conventional pedicle screw in flexion-extension, without compromising stabilization, and has the potential of bringing the load transfer behavior of fusional segment closer to normal and also lowers the stress values of pedicle screws.

Combined anteroposterior fixation using a titanium cage versus solely posterior fixation for traumatic thoracolumbar fractures: A systematic review and meta-analysis

Study Design:

Systematic review with meta-analysis.

Objective:

Additional anterior stabilization might prevent posterior implant failure, but over time, the disadvantageous of bone grafts have become evident. The objective of this systematic review was to compare risks and advantages of additional anterior stabilization with a titanium cage to solely posterior fixation for traumatic thoracolumbar fractures.

Methods:

An electronic search was performed in the literature from 1980 to March 2016. Studies comparing only posterior with anteroposterior fixation by means of a titanium cage were included in this study. Data extraction and Cochrane risk of bias assessment were done by two independent authors. In addition, the PRISMA statement was followed, and the GRADE approach was used to present results.

Results:

Of the 1584 studies, two randomized controlled trials (RCTs) and one retrospective cohort study were included in the meta-analysis. The RCTs reported evidence of high quality that anteroposterior stabilization maintained better kyphosis correction than posterior stabilization alone. However, these results were neutralized in the meta-analysis by the cohort study. Implant failure was reported by one study, in the posterior group. No differences in follow-up visual analog scale scores, neurologic improvement, and complications were found. Operation time, blood loss, and hospital stay all increased in the anteroposterior group.

Conclusions:

Patients with a highly comminuted or unstable fracture could benefit from combined anteroposterior stabilization with a titanium cage, for some evidence suggests this prevents loss of correction. However, large randomized studies still lack. There is a risk of cage subsidence, and increased perioperative risks have to be considered when choosing the optimal treatment.

Biomechanical analysis of proximal junctional failure following adult spinal instrumentation using a comprehensive hybrid modeling approach

BACKGROUND

Proximal junctional failure is a severe proximal junctional complication following adult spinal instrumentation and involving acute proximal junctional kyphotic deformity, mechanical failure at the upper instrumented vertebra or just above, and/or proximal junctional osseoligamentous disruption. Clinical studies have identified potential risk factors, but knowledge on their biomechanics is still lacking for addressing the proximal junctional failure issues. The objective of this study was to develop comprehensive computational modeling and simulation techniques to investigate proximal junctional failure.

METHODS

A 3D multibody biomechanical model based on a 47year old lumbar scoliosis surgical case that subsequently had traumatic proximal junctional failure was first developed to simulate patient-specific spinal instrumentation (from T11 to S1), compute the postoperative geometry of the instrumented spine, simulate different physiological loads and movements. Then, a highly detailed finite element model of the proximal junctional spinal segment was created using as input the geometry and displacements from the multibody model. It enabled to perform detailed stress and failure analysis across the anatomical structures.

FINDINGS

The simulated postoperative correction and traumatic failure (wedge fracture at upper instrumented vertebra) agreed well with the clinical report (within 2° difference). Simulated stresses around the screw threads (up to 4.7MPa) generated during the instrumentation and the buckling effect of post-operative functional loads on the proximal junctional spinal segment, were identified as potential mechanical proximal junctional failure risk factors.

INTERPRETATION

Overall, we demonstrated the feasibility of the developed hybrid modeling technique, which realistically allowed the simulation of the spinal instrumentation and postoperative loads, which constitutes an effective tool to further investigate proximal junctional failure pathomechanisms.

In Vitro Comparison of Dynesys, PEEK, and Titanium Constructs in the Lumbar Spine

Introduction. Pedicle based posterior dynamic stabilization systems aim to stabilize the pathologic spine while also allowing sufficient motion to mitigate adjacent level effects. Two flexible constructs that have been proposed to act in such a manner, the Dynesys Dynamic Stabilization System and PEEK rod, have yet to be directly compared in vitro to a rigid Titanium rod. Methods. Human lumbar specimens were tested in flexion extension, lateral bending, and axial torsion to evaluate the following conditions at L4-L5: Intact, Dynesys, PEEK rod, Titanium rod, and Destabilized. Intervertebral range of motion, interpedicular travel, and interpedicular displacement metrics were evaluated from 3rd-cycle data using an optoelectric tracking system. Results. Statistically significant decreases in ROM compared to Intact and Destabilized conditions were detected for the instrumented conditions during flexion extension and lateral bending. AT ROM was significantly less than Destabilized but not the Intact condition. Similar trends were found for interpedicular displacement in all modes of loading; however, interpedicular travel trends were less consistent. More importantly, no metrics under any mode of loading revealed significant differences between Dynesys, PEEK, and Titanium. Conclusion. The results of this study support previous findings that Dynesys and PEEK constructs behave similarly to a Titanium rod in vitro.

Characterization of the behavior of a novel low-stiffness posterior spinal implant under anterior shear loading on a degenerative spinal model

PURPOSE

Dynamic implants have been developed to address potential adjacent level effects due to rigid instrumentation. Rates of revision surgeries may be reduced by using improved implants in the primary surgery. Prior to clinical use, implants should be rigorously tested ex vivo. The objective of our study was to characterize the load-sharing and kinematic behavior of a novel low-stiffness spinal implant.

METHODS

A human cadaveric model of degenerative spondylolisthesis was tested in shear. Lumbar functional spinal units (N = 15) were tested under a static 300 N axial compression force and a cyclic anterior shear force (5-250 N). Translation was tracked with a motion capture system. A novel implant was compared to three standard implants with shear stiffness ranging from low to high. All implants were instrumented with strain gauges to measure the supported shear force. Each implant was affixed to each specimen, and the specimens were tested intact and in two progressively destabilized states.

RESULTS

Specimen condition and implant type affected implant load-sharing and specimen translation (p < 0.0001). Implant load-sharing increased across all degeneration-simulating specimen conditions and decreased across the three standard implants (high- to low-stiffness). Translation increased with the three standard implants (trend). The novel implant behaved similarly to the medium-stiffness implant (p > 0.2).

CONCLUSIONS

The novel implant behaved similarly to the medium-stiffness implant in both load-sharing and translation despite having a different design and stiffness. Complex implant design and specimen-implant interaction necessitate pre-clinical testing of novel implants. Further in vitro testing in axial rotation and flexion-extension is recommended as they are highly relevant loading directions for non-rigid implants.

Characterization and prediction of rate-dependent flexibility in lumbar spine biomechanics at room and body temperature

BACKGROUND CONTEXT

The soft tissues of the spine exhibit sensitivity to strain-rate and temperature, yet current knowledge of spine biomechanics is derived from cadaveric testing conducted at room temperature at very slow, quasi-static rates.

PURPOSE

The primary objective of this study was to characterize the change in segmental flexibility of cadaveric lumbar spine segments with respect to multiple loading rates within the range of physiologic motion by using specimens at body or room temperature. The secondary objective was to develop a predictive model of spine flexibility across the voluntary range of loading rates.

STUDY DESIGN

This in vitro study examines rate- and temperature-dependent viscoelasticity of the human lumbar cadaveric spine.

METHODS

Repeated flexibility tests were performed on 21 lumbar function spinal units (FSUs) in flexion-extension with the use of 11 distinct voluntary loading rates at body or room temperature. Furthermore, six lumbar FSUs were loaded in axial rotation, flexion-extension, and lateral bending at both body and room temperature via a stepwise, quasi-static loading protocol. All FSUs were also loaded using a control loading test with a continuous-speed loading-rate of 1-deg/sec. The viscoelastic torque-rotation response for each spinal segment was recorded. A predictive model was developed to accurately estimate spine segment flexibility at any voluntary loading rate based on measured flexibility at a single loading rate.

RESULTS

Stepwise loading exhibited the greatest segmental range of motion (ROM) in all loading directions. As loading rate increased, segmental ROM decreased, whereas segmental stiffness and hysteresis both increased; however, the neutral zone remained constant. Continuous-speed tests showed that segmental stiffness and hysteresis are dependent variables to ROM at voluntary loading rates in flexion-extension. To predict the torque-rotation response at different loading rates, the model requires knowledge of the segmental flexibility at a single rate and specified temperature, and a scaling parameter. A Bland-Altman analysis showed high coefficients of determination for the predictive model.

CONCLUSIONS

The present work demonstrates significant changes in spine segment flexibility as a result of loading rate and testing temperature. Loading rate effects can be accounted for using the predictive model, which accurately estimated ROM, neutral zone, stiffness, and hysteresis within the range of voluntary motion.

A novel method of anterior lumbosacral cage reconstruction

Reconstruction of the lumbosacral junction is a considerable challenge for spinal surgeons due to the unique anatomical constraints of this region as well as the vectors of force that are applied focally in this area. The standard cages, both expandable and nonexpendable, often fail to reconstitute the appropriate anatomical alignment of the lumbosacral junction. This inadequate reconstruction may predispose the patient to continued back pain and neurological symptoms as well as possible pseudarthrosis and instrumentation failure. The authors describe their preoperative planning and the technical characteristics of their novel reconstruction technique at the lumbosacral junction using a cage with adjustable caps. Based precisely on preoperative measurements that maintain the appropriate Cobb angle, they performed reconstruction of the lumbosacral junction in a series of 3 patients. All 3 patients had excellent installation of the cages used for reconstruction. Postoperative CT scans were used to radiographically confirm the appropriate reconstruction of the lumbosacral junction. All patients had a significant reduction in pain, had neurological improvement, and experienced no instrumentation failure at the time of latest follow-up. Taking into account the inherent morphology of the lumbosacral junction and carefully planning the technical characteristics of the cage installation preoperatively and intraoperatively, the authors achieved favorable clinical and radiographic outcomes in all 3 cases. Based on this small case series, this technique for reconstruction of the lumbosacral junction appears to be a safe and appropriate method of reconstruction of the anterior spinal column in this technically challenging region of the spine.

The Comprehensive Biomechanics and Load-Sharing of Semirigid PEEK and Semirigid Posterior Dynamic Stabilization Systems

Alternatives to conventional rigid fusion have been proposed for several conditions related to degenerative disc disease when nonoperative treatment has failed. Semirigid fixation, in the form of dynamic stabilization or PEEK rods, is expected to provide compression under loading as well as an intermediate level of stabilization. This study systematically examines both the load-sharing characteristics and kinematics of these two devices compared to the standard of internal rigid fixators. Load-sharing was studied by using digital pressure films inserted between an artificially machined disc and two loading fixtures. Rigid rods, PEEK rods, and the dynamic stabilization system were inserted posteriorly for stabilization. The kinematics were quantified on ten, human, cadaver lumbosacral spines (L3-S1) which were tested under a pure bending moment, in flexion-extension, lateral bending, and axial rotation. The magnitude of load transmission through the anterior column was significantly greater with the dynamic device compared to PEEK rods and rigid rods. The contact pressures were distributed more uniformly, throughout the disc with the dynamic stabilization devices, and had smaller maximum point-loading (pressures) on any particular point within the disc. Kinematically, the motion was reduced by both semirigid devices similarly in all directions, with slight rigidity imparted by a lateral interbody device.

Biomechanical comparison of anterior lumbar interbody fusion: stand-alone interbody cage versus interbody cage with pedicle screw fixation -- a finite element analysis

Background

Anterior lumbar interbody fusion (ALIF) followed by pedicle screw fixation (PSF) is used to restore the height of the intervertebral disc and provide stability. Recently, stand-alone interbody cage with anterior fixation has been introduced, which eliminates the need for posterior surgery. We compared the biomechanics of the stand-alone interbody cage to that of the interbody cage with additional PSF in ALIF.

Methods

A three-dimensional, non-linear finite element model (FEM) of the L2-5 segment was modified to simulate ALIF in L3-4. The models were tested under the following conditions: (1) intact spine, (2) destabilized spine, (3) with the interbody cage alone (type 1), (4) with the stand-alone cage with anterior fixation (SynFix-LR®; type 2), and (5) with type 1 in addition to PSF (type 3). Range of motion (ROM) and the stiffness of the operated level, ROM of the adjacent segments, load sharing distribution, facet load, and vertebral body stress were quantified with external loading.

Results

The implanted models had decreased ROM and increased stiffness compared to those of the destabilized spine. The type 2 had differences in ROM limitation of 8%, 10%, 4%, and 6% in flexion, extension, axial rotation, and lateral bending, respectively, compared to those of type 3. Type 2 had decreased ROM of the upper and lower adjacent segments by 3-11% and 3-6%, respectively, compared to those of type 3. The greatest reduction in facet load at the operated level was observed in type 3 (71%), followed by type 2 (31%) and type 1 (23%). An increase in facet load at the adjacent level was highest in type 3, followed by type 2 and type 1. The distribution of load sharing in type 2 (anterior:posterior, 95:5) was similar to that of the intact spine (89:11), while type 3 migrated posterior (75:25) to the normal. Type 2 reduced about 15% of the stress on the lower vertebral endplate compared to that in type 1. The stress of type 2 increased two-fold compared to the stress of type 3, especially in extension.

Conclusions

The stand-alone interbody cage can provide sufficient stability, reduce stress in adjacent levels, and share the loading distribution in a manner similar to an intact spine.

Load transfer characteristics between posterior spinal implants and the lumbar spine under anterior shear loading: an in vitro investigation

STUDY DESIGN

A biomechanical human cadaveric study.

OBJECTIVE

To determine the percentage of shear force supported by posterior lumbar spinal devices of varying stiffnesses under anterior shear loading in a degenerative spondylolisthesis model.

SUMMARY OF BACKGROUND DATA

Clinical studies have demonstrated beneficial results of posterior arthrodesis for the treatment of degenerative spinal conditions with instability. Novel spinal implants are designed to correct and maintain spinal alignment, share load with the spine, and minimize adjacent level stresses. The optimal stiffness of these spinal systems is unknown. To our knowledge, low-stiffness posterior instrumentation has not been tested under an anterior shear force, a highly relevant force to be neutralized in the clinical case of degenerative spondylolisthesis.

METHODS

The effects of implant stiffness and specimen condition on implant load and intervertebral motion were assessed in a biomechanical study. Fifteen human cadaveric lumbar functional spinal units were tested under a static 300 N axial compression force and a cyclic anterior shear force (5-250 N). Implants (high-stiffness [HSI]: ø 5.5-mm titanium, medium-stiffness [MSI]: ø 6.35 × 7.2-mm oblong PEEK, and low-stiffness [LSI]: ø 5.5-mm round PEEK) instrumented with strain gauges were used to calculate loads and were tested in each of 3 specimen conditions simulating degenerative changes: intact, facet instability, and disc instability. Intervertebral motions were measured with a motion capture system.

RESULTS

As predicted, implants supported a significantly greater shear force as the specimen was progressively destabilized. Mean implant loads as a percent of the applied shear force in order of increasing specimen destabilization for the HSI were 43%, 67%, and 76%; mean implant loads for the MSI were 32%, 56%, and 77%; and mean implant loads for the LSI were 18%, 35%, and 50%. Anterior translations increased with decreasing implant stiffness and increasing specimen destabilization.

CONCLUSION

Implant shear stiffness significantly affected the load sharing between the implant and the natural spine in anterior shear ex vivo. Low-stiffness implants transferred significantly greater loads to the spine. This study supports the importance of load-sharing behavior when designing new implants.

Biomechanical evaluation of the Total Facet Arthroplasty System® (TFAS®): loading as compared to a rigid posterior instrumentation system

PURPOSE

To gain insight into a new technology, a novel facet arthroplasty device (TFAS) was compared to a rigid posterior fixation system (UCR). The axial and bending loads through the implants and at the bone-implant interfaces were evaluated using an ex vivo biomechanical study and matched finite element analysis. Kinematic behaviour has been reported for TFAS, but implant loads have not. Implant loads are important indicators of an implant's performance and safety. The rigid posterior fixation system is used for comparison due to the extensive information available about these systems.

METHODS

Unconstrained pure moments were applied to 13 L3-S1 cadaveric spine segments. Specimens were tested intact, following decompression, UCR fixation and TFAS implantation at L4-L5. UCR fixation was via standard pedicle screws and TFAS implantation was via PMMA-cemented transpedicular stems. Three-dimensional 10 Nm moments and a 600 N follower load were applied; L4-L5 disc pressures and implant loads were measured using a pressure sensor and strain gauges, respectively. A finite element model was used to calculate TFAS bone-implant interface loads.

RESULTS

UCR experienced greater implant loads in extension (p < 0.004) and lateral bending (p < 0.02). Under flexion, TFAS was subject to greater implant moments (p < 0.04). At the bone-implant interface, flexion resulted in the smallest TFAS (average = 0.20 Nm) but greatest UCR (1.18 Nm) moment and axial rotation resulted in the greatest TFAS (3.10 Nm) and smallest UCR (0.40 Nm) moments. Disc pressures were similar to intact for TFAS but not for UCR (p < 0.04).

CONCLUSIONS

These results are most applicable to the immediate post-operative period prior to remodelling of the bone-implant interface since the UCR and TFAS implants are intended for different service lives (UCR--until fusion, TFAS--indefinitely). TFAS reproduced intact-like anterior column load-sharing--as measured by disc pressure. The highest bone-implant moment of 3.1 Nm was measured in TFAS and for the same loading condition the UCR interface moment was considerably lower (0.4 Nm). For other loading conditions, the differences between TFAS and UCR were smaller, with the UCR sometimes having larger values and for others the TFAS was larger. The long-term physiological meaning of these findings is unknown and demonstrates the need for a better understanding of the relationship between spinal arthroplasty devices and the host tissue as development of next generation motion-preserving posterior devices that hope to more accurately replicate the natural functions of the native tissue continues.

Posterior spinal instrumentation: biomechanical study on the role of rods on hardware response to axial load

Posterior spinal instrumentation is frequently used for the treatment of spine disorders. Importantly, different requirements have to be considered for the optimal use of these systems in various clinical scenarios. In this work, we focused on the role of rods diameter on hardware's stiffness. For this purpose, we established an in vitro model and compared the response to axial load of a posterior stabilization system, characterized by rods of different diameter (4, 5, 6 mm), with that of Dynesys®. Intuitively, the higher the stiffness of the hardware, the lower the load is transferred to the disc. However, the 4 hardware tested showed a different trend in the response to the load regimens: when increasing the load, more flexible systems display a progressive reduction in the percentage of load which is transferred to the disc while more rigid system display the opposite trend. Considering that the load which is transferred, and not by-passed by the hardware, influences the healing of a fracture; the integration of a bone graft or a cage; the fusion process, these data have a relevant impact on clinical practice and highlight features that have to be considered in the choice for the optimal posterior spinal instrumentation.

In vitro biomechanics of an expandable vertebral body replacement with self-adjusting end plates

BACKGROUND CONTEXT

Unstable burst fractures of the thoracolumbar spine may be treated surgically. Vertebral body replacements (VBRs) give anterior column support and, when used with supplemental fixation, impart rigidity to the injured segments. Although some VBRs are expandable, device congruity to the vertebral end plates is imprecise and may lead to stress risers and device subsidence.

PURPOSE

The objective of this study was to compare the rigidity of a VBR that self-adjusts to the adjacent vertebral end plates versus structural bone allograft and with an unsupported anterior column in a traumatic burst fracture reconstruction model.

STUDY DESIGN

Biomechanical flexibility testing with rod strain measurement.

PATIENT SAMPLE

Twelve T11-L3 human spine segments.

OUTCOME MEASURES

Range of motion, neutral zone, and posterior fixation rod stress (moments).

METHODS

Flexibility testing was performed to ± 6 Nm in flexion-extension, lateral bending, and axial rotation on 12 intact human T11-L3 specimens. Burst fractures were created in L1, and flexibility testing was repeated in three additional states: subtotal corpectomy with posterior instrumentation (PI) only from T12 to L2, reconstruction with a femoral strut allograft and PI, and reconstruction with a VBR (with self-adjusting end plates) and PI. The PI consisted of pedicle screws and strain gage instrumented rods that were calibrated to measure rod stress via flexion-extension bending moments.

RESULTS

There was no statistical difference in range of motion or neutral zone between the strut graft and VBR constructs, which both had less motion than the PI-only construct in flexion/extension and torsion and were both less than the intact values in flexion/extension and lateral bending (p < .05). Posterior rod moments were significantly greater for the PI-only construct in flexion/extension relative to the strut graft and VBR states (p = .03).

CONCLUSIONS

This study, which simulated the immediate postoperative state, suggests that a VBR with self-adjusting end plate components has rigidity similar to the standard strut graft when combined with PI. Posterior rod stress was not significantly increased with this type of VBR compared with the strut graft reconstruction. The benefits of burst fracture stabilization using a self-adjusting VBR ultimately will not be known until long-term clinical studies are performed.

Biomechanical evaluation of an expandable meshed bag augmented with pedicle or facet screws for percutaneous lumbar interbody fusion

OBJECTIVE

To evaluate the biomechanics of lumbar motion segments instrumented with stand-alone OptiMesh system augmented with posterior fixation using facet or pedicle screws and the efficacy of discectomy and disc distraction.

BACKGROUND CONTEXT

OptiMesh bone graft containment system has been used for vertebral compression fractures and percutaneous lumbar interbody fusion. The filled mesh bag serves as the interbody device providing structural support to the motion segment being fused. No biomechanical data of this new device are available in the literature.

METHODS

Twenty-four fresh human cadaveric lumbar motion segments were divided into two groups. In the control group, multidirectional flexibility testing was conducted after an intact condition and standard transforaminal lumbar interbody fusion (TLIF) procedure. In the OptiMesh group, testing was performed following intact, stand-alone OptiMesh procedure, OptiMesh with facet screws (placed using the transfacet approach), and OptiMesh with pedicle screws and rods. Range of motion (ROM) was calculated for each surgical treatment. The lordosis and disc height change of intact and instrumented specimens were measured in the lateral radiographs to evaluate the disc space distraction. In the OptiMesh group, cyclic loading in flexion extension (FE) was applied to measure cage subsidence or collapse (10,000 cycles at 6 Nm). After biomechanical testing, all the specimens were dissected to inspect the discectomy and end plate preparation. The area of discectomy was measured.

RESULTS

The mean ROM of the intact specimens was 2.7°, 7.4°, and 7.2° in axial torsion (AT), lateral bending (LB), and FE, respectively. There was no difference between the control group and OptiMesh group. The mean ROM of the stand-alone OptiMesh system decreased to 2.4°, 5.1°, and 4.3° in AT, LB, and FE. The ROM decreased to 0.9° in AT, 2.2° in LB, and 0.9° in FE with OptiMesh system and facet screws. On average, OptiMesh system with pedicle screws and rods reduced the ROM to 1.3° in AT, 1.6° in LB, and 1.1° in FE. Compared with the intact condition and stand-alone OptiMesh system, both posterior fixation options had significant statistical difference (p<.001). In AT, ROM of facet screws was lower than that of pedicle screws (p < .05). There was no statistical difference between the facet and pedicle screws in LB and FE (p > .05). The mean volume of bone graft packed into each bag was 8.3 ± 1.5 cc. The average increase of lordosis was 0.6° ± 1.0° after meshed bag was deployed. The average distraction achieved by the OptiMesh system was 1.0 ± 0.6 mm. The average prepared area of discectomy was 42% of the total disc. The disc height change after cyclic loading was 0.2 mm. No subsidence or collapse was noticed.

CONCLUSIONS

The OptiMesh system offers large volume of bone graft in the disc space with small access portals. The OptiMesh system had similar construct stability to that of standard TLIF procedure when posterior fixation was applied. However, the amount of distraction was limited without additional distraction tools. With the anterior support provided by the expandable meshed bag, facet screws had comparable construct stability to that of pedicle screws. Slightly higher stability was observed in facet screws in AT.

Revision of transforaminal lumbar interbody fusion using anterior lumbar interbody fusion: a biomechanical study in nonosteoporotic bone

Object

Transforaminal lumbar interbody fusion (TLIF) is a popular fusion technique for treating chronic low-back pain. In cases of interbody nonfusion, revision techniques for TLIF include anterior lumbar interbody fusion (ALIF) approaches. Biomechanical data of the revision techniques are not available. The purpose of this study was to compare the immediate construct stability, in terms of range of motion (ROM) and neutral zone (NZ), of a revision ALIF procedure for an unsuccessful TLIF. An in vitro biomechanical comparison of TLIF and its ALIF revision procedure was conducted on cadaveric nonosteoporotic human spine segments.

Methods

Twelve cadaveric lumbar motion segments with normal bone mineral density were loaded in unconstrained axial torsion, lateral bending, and flexion-extension under 0.05 Hz and ± 6-nm sinusoidal waveform. The specimens underwent TLIF (with posterior pedicle fixation) and anterior ALIF (with intact posterior fixation). Multidirectional flexibility testing was conducted following each step. The ROM and NZ data were measured and calculated for each test.

Results

Globally, the TLIF and revision ALIF procedures significantly reduced ROM and NZ compared with that of the intact condition. The revision ALIF procedures achieved similar ROM as the TLIF procedure.

Conclusions

Revision ALIF maintained biomechanical stability of TLIF in nonosteoporotic spines. Revision ALIF can be performed without sacrificing spinal stability in cases of intact posterior instrumentation.

Comparative biomechanical investigation of a modular dynamic lumbar stabilization system and the Dynesys system

The goal of non-fusion stabilization is to reduce the mobility of the spine segment to less than that of the intact spine specimen, while retaining some residual motion. Several in vitro studies have been conducted on a dynamic system currently available for clinical use (Dynesys). Under pure moment loading, a dependency of the biomechanical performance on spacer length has been demonstrated; this variability in implant properties is removed with a modular concept incorporating a discrete flexible element. An in vitro study was performed to compare the kinematic and stabilizing properties of a modular dynamic lumbar stabilization system with those of Dynesys, under the influence of an axial preload. Six human cadaver spine specimens (L1-S1) were tested in a spine loading apparatus. Flexibility measurements were performed by applying pure bending moments of 8 Nm, about each of the three principal anatomical axes, with a simultaneously applied axial preload of 400 N. Specimens were tested intact, and following creation of a defect at L3-L4, with the Dynesys implant, with the modular implant and, after removal of the hardware, the injury state. Segmental range of motion (ROM) was reduced for flexion-extension and lateral bending with both implants. Motion in flexion was reduced to less than 20% of the intact level, in extension to approximately 40% and in lateral bending a motion reduction to less than 40% was measured. In torsion, the total ROM was not significantly different from that of the intact level. The expectations for a flexible posterior stabilizing implant are not fulfilled. The assumption that a device which is particularly compliant in bending allows substantial intersegmental motion cannot be fully supported when one considers that such devices are placed at a location far removed from the natural rotation center of the intervertebral joint.

Non-fusion instrumentation of the lumbar spine with a hinged pedicle screw rod system: an in vitro experiment

In advanced stages of degenerative disease of the lumbar spine instrumented spondylodesis is still the golden standard treatment. However, in recent years dynamic stabilisation devices are being implanted to treat the segmental instability due to iatrogenic decompression or segmental degeneration. The purpose of the present study was to investigate the stabilising effect of a classical pedicle screw/rod combination, with a moveable hinge joint connection between the screw and rod allowing one degree of freedom (cosmicMIA). Six human lumbar spines (L2-5) were loaded in a spine tester with pure moments of +/-7.5 Nm in lateral bending, flexion/extension and axial rotation. The range of motion (ROM) and the neutral zone were determined for the following states: (1) intact, (2) monosegmental dynamic instrumentation (L4-5), (3) bisegmental dynamic instrumentation (L3-5), (4) bisegmental decompression (L3-5), (5) bisegmental dynamic instrumentation (L3-5) and (6) bisegmental rigid instrumentation (L3-5). Compared to the intact, with monosegmental instrumentation (2) the ROM of the treated segment was reduced to 47, 40 and 77% in lateral bending, flexion/extension and axial rotation, respectively. Bisegmental dynamic instrumentation (3) further reduced the ROM in L4-5 compared to monosegmental instrumentation to 25% (lateral bending), 28% (flexion/extension) and 57% (axial rotation). Bisegmental surgical decompression (4) caused an increase in ROM in both segments (L3-4 and L4-5) to approximately 125% and approximately 135% and 187-234% in lateral bending, flexion/extension and axial rotation, respectively. Compared to the intact state, bisegmental dynamic instrumentation after surgical decompression reduced the ROM of the two-bridged segments to 29-35% in lateral bending and 33-38% in flexion/extension. In axial rotation, the ROM was in the range of the intact specimen (87-117%). A rigid instrumentation (6) further reduced the ROM of the two-bridged segments to 20-30, 23-27 and 50-68% in lateral bending, flexion/extension and axial rotation, respectively. The results of the present study showed that compared to the intact specimen the investigated hinged dynamic stabilisation device reduced the ROM after bisegmental decompression in lateral bending and flexion/extension. Following bisegmental decompression and the thereby caused large rotational instability the device is capable of restoring the motion in axial rotation back to values in the range of the intact motion segments.

Transpedicular corpectomy with posterior expandable cage placement for L1 burst fracture

The surgical management of lumbar burst usually involves either a posterior or an anterior approach. Posterior-only procedures usually rely on ligamentotaxis or manual tamping of bone fragments for decompression of the spinal canal. Transpedicular corpectomies allow for circumferential surgery through a single posterior approach; however, they are rarely done for lumbar burst fractures. The presence of intervening nerve roots is one impediment to the placement of expandable cages to reconstruct the anterior column through the transpedicular approach. Using an adaptation of a technique previously described for the treatment of spinal tumors, we were able to successfully treat a traumatic lumbar burst fracture with an expandable cage from a purely posterior approach. This treatment included decompression of the neural elements, correction of the kyphosis, restoration of vertebral body height and reconstruction of the anterior column using an expandable cage with nerve root preservation. We describe our surgical procedure in this technical note.

Biomechanical comparison of two different concepts for stand alone anterior lumbar interbody fusion

Segmental instability in degenerative disc disease is often treated with anterior lumbar interbody fusion (ALIF). Current techniques require an additional posterior approach to achieve sufficient stability. The test device is an implant which consists of a PEEK-body and an integrated anterior titanium plate hosting four diverging locking screws. The test device avoids posterior fixation by enhancing stability via the locking screws. The test device was compared to an already established stand alone interbody implant in a human cadaveric three-dimensional stiffness test. In the biomechanical test, the L4/5 motion segment of 16 human cadaveric lumbar spines were isolated and divided into two test groups. Tests were performed in flexion, extension, right and left lateral bending, right and left axial rotation. Each specimen was tested in native state first, then a discectomy was performed and either of the test implants was applied. Finite element analysis (FE) was also performed to investigate load and stress distribution within the implant in several loading conditions. The FE models simulated two load cases. These were flexion and extension with a moment of 5 Nm. The biomechanical testing revealed a greater stiffness in lateral bending for the SynFix-LR compared to the established implant. Both implants showed a significantly higher stiffness in all loading directions compared to the native segment. In flexion loading, the PEEK component takes on most of the load, whereas the majority of the extension load is put on the screws and the screw-plate junction. Clinical investigation of the test device seems reasonable based on the good results reported here.

Cement augmentation of vertebral screws enhances the interface strength between interbody device and vertebral body

STUDY DESIGN

An in vitro cadaveric study comparing cage-vertebra interface strengths for 3 different screw-cement configurations.

OBJECTIVES

To determine the effects of cement augmentation of pedicle screws on cage-vertebra interface failure properties for 2 interbody device shapes (elliptical or cloverleaf); and to compare between pedicle and anterior vertebral body screws with cement augmentation.

SUMMARY OF BACKGROUND DATA

Pedicle or anterior screw fixation is commonly used with interbody device fixation. Cement has recently been shown to augment screw fixation in the osteoporotic spine by improving the screw-bone interface strength. The effect of cement augmentation of pedicle or anterior screws on cage-vertebra interface properties has not been previously studied or compared.

METHODS

An elliptical or a cloverleaf-shaped indentor covering 40% of the endplate was axially compressed against the superior endplate of 48 thoracolumbar vertebrae. Each vertebra had polymethylmethacrylate cement augmentation of 1) anterior screws, 2) pedicle screws, or 3) pedicle screws without cement. Compressive load was applied through a mechanism that allowed unconstrained rotation of the indentors.

RESULTS

Cement augmentation of pedicle screws resulted in significantly higher failure loads (54%) and failure strength (69%) for both shaped indentors when compared with uncemented pedicle screws. There was no significant difference in failure load and failure strength between pedicle and anterior screws with cement augmentation. Indentor shape was not a significant factor on failure load or failure strength.

CONCLUSIONS

Cage-vertebra interface properties were improved when cement was used to augment vertebral and pedicle screws. Cement augmentation of pedicle or anterior screws may reduce interbody device subsidence.

Revision Deformity Surgery

Revision deformity surgery may be necessary for several reasons. Symptomatic pseudarthrosis, implant failure or pull-out, or loss of correction may mandate reoperation. The keys to a successful revision procedure are a careful analysis of the problem, particularly the mode of failure and the contributing biomechanical factors, and the development of an appropriate surgical plan.

In vitro fixator rod loading after transforaminal compared to anterior lumbar interbody fusion

BACKGROUND

Cages are commonly used to assist lumbar interbody fusion. They are implanted from various approaches. In many cases internal fixators are added to provide sufficient stability. However, how the rods of these fixators are loaded and whether the kind of approach affects these loads is still unknown. The aim of this in vitro study therefore was to determine the loads acting on fixator rods and cages after anterior compared to transforaminal lumbar interbody fusion.

METHODS

Six intact human lumbar spine specimens (L1-5) were loaded in a spine tester with pure moments (+/-7.5 N m) in the frontal, sagittal and transverse plane. Loading was repeated, first, after the segments L2-3 and L4-5 were instrumented either with an anterior or a transforaminal lumbar interbody fusion cage "stand alone" and, second, after additional stabilisation with an internal fixator. The rods of the fixator and the four "corners" of the cages were instrumented with strain gauges.

FINDINGS

The loads transmitted through the rods were highest in lateral bending. In this loading direction an axial distraction force of in median up to 140 N, an axial compression force of up to 100 N, and a resultant bending moment of up to 1.1 N m were measured in each rod. These loads tended to be lower for the anterior compared to the transforaminal approach. For comparison, the load applied was +/-7.5 N m. The axial strains recorded in the four "corners" of the cages considerably varied from one specimen to the other. Differences in cage strain between the two approaches could not be detected.

INTERPRETATION

The loads acting on the rods of the fixator were small compared to the load that was applied. Thus, other structures such as the cages or the facet joints still play an important role in load transfer. The type of approach (anterior or transforaminal) had only little effect on the loading of the rods. This also applies to the local loading of the cages, which probably more depends on the fit between cage and endplates and on the local stiffness properties of the adjacent vertebral bodies.

Torsional stability of cross-link configurations: a biomechanical analysis

BACKGROUND CONTEXT

Cross-link systems have been used to augment segmental spinal instrumentation since the earliest introduction of these fixation systems. Although transverse cross-links have little impact on sagittal motion of spinal constructs, cross-linkage does affect torsional rigidity. Despite the wide variety of cross-link designs, almost all have been configured as transverse devices. The relative mechanical benefit of different cross-link configurations is not known.

PURPOSE

The purpose of this study was to compare the torsional stability of three different cross-link configurations.

STUDY DESIGN

Biomechanical analysis of segmental instrumentation constructs using porcine spines.

METHODS

Thoracic porcine spines (T4 to T10) were instrumented with 6.5-mm conical pedicle screws and 7.0-mm connecting rods from T5 to T9. Terminal vertebrae were embedded in polymethylmethacrylate (PMMA) after a T7 corpectomy. Four cross-link configurations were tested in a randomized manner: Un-cross-linked Control (CONT); Transverse Rod-Rod (RR); Transverse Screw-Screw (SS); and Diagonal Screw-Screw (DX) Cross-links. The specimens were rotated to 3 Nm at a rate of 0.2 degrees/s and cycled six times with data acquisition over the final two cycles. Stiffness, rotation, and energy data were normalized to each control. A Newman-Keuls repeated measures analysis of variance was used to infer significant differences.

RESULTS

Diagonal cross-link configurations provided the most rigid construct. Transverse cross-links did not significantly change torsional behavior compared with the unlinked control. Rotation and energy expended were not significantly greater torsional stiffness compared with other constructs tested (p<.01).

CONCLUSIONS

The diagonal cross-link configuration provided increased torsional stiffness as compared with unlinked or transverse configurations. This observation should be considered in future cross-link designs.

Pure shear properties of lumbar spinal joints and the effect of tissue sectioning on load sharing

STUDY DESIGN

An in vitro biomechanical study on lumbar intervertebral joints.

OBJECTIVES

To examine the mechanical properties of lumbar motion segments under pure shear loading and establish whether a simple model for functional differentiation between the anterior column and the posterior elements is applicable.

SUMMARY OF BACKGROUND DATA

Anteroposterior shear has been implicated as a major factor in spinal instability. There is a substantial amount of data on shear motion as a coupled part of flexion-extension; data on the pure shear properties of intervertebral joints is limited.

METHODS

Eighteen human cadaver lumbar motion segments were subject to nondestructive testing under pure shear loads (anterior shear and posterior shear). An MTS standard testing machine was used to record the load-deformation characteristics of specimens subject to deformation at a constant rate to a maximum shear load of approximately 250 N. Tissue sectioning was then performed with the specimen mounted in the testing machine. Eight specimens were sectioned through the intervertebral disc, including the anterior and posterior longitudinal ligaments, and 8 specimens were sectioned through the pedicles to remove the posterior elements. The same deformation pattern applied to the intact specimen was then reapplied to the sectioned specimen, and the load-deformation characteristics following sectioning were evaluated.

RESULTS

The shear stiffness of the intact segments were found to be higher in anterior shear (mean group A = 583.8, B = 607 N/mm) than in posterior shear (mean group A = 469, B = 438.4 N/mm). Section of the anterior column and adjacent longitudinal ligaments resulted in a mean stiffness decreased by 22.8% of the intact value under anterior shear and 23.9% under posterior shear. Much larger change in shear stiffness was seen, and the mean sectioned stiffness dropped by 77.7% in anterior shear and 79% in posterior shear after removal of the posterior elements. After the anterior column was sectioned, 12% and 18% increases in the deformation for anterior and posterior directions were seen, whereas a distinct increase in the deformations was found after posterior elements sectioned.

CONCLUSIONS

The posterior elements of the lumbar spine are more efficient in resisting anterior and posterior shear loads. However, the anterior column will exhibit similar load-displacement characteristics if subject to greater deformations. The sum of the normalized mean shear loads of the anterior column and posterior elements sustained at maximum intact deformation is significantly different from the shear load sustained by the intact spine at the same deformation. A simple concept of load sharing between the anterior column and the posterior elements may not be valid.

Revision strategies for lumbar pseudarthrosis

Management and avoidance of lumbar pseudarthrosis are among the most common and challenging tasks faced by reconstructive spine surgeons. The risks of pseudarthrosis can be broadly divided into two categories: those within a surgeon's control and those not within his/her control. These include biological factors, graft choices, site preparation, and surgical design. The authors review the biological factors that affect fusion and how they can be manipulated to avoid or manage lumbar pseudarthrosis. Surgical planning and construct design to prevent or treat pseudarthrosis will also be discussed. Additionally, the importance of restoring sagittal balance will be reviewed.