Comparative analysis of posterior fusion constructs as treatments for middle and posterior column injuries: an in vitro biomechanical investigation

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.

Is it a requirement or a preference to use cross-links in lumbar instrumentation?

Background/Aim: The use of cross-links (CL) is controversial due to reasons such as cost increases and instrument redundancy. While there are many biomechanical studies, the clinical data is limited. The aim of this study is to present the clinical effects of CL by putting forward postoperative clinical outcomes and long-term results of patients with (CL+) and without (CL-) CL augmentation. Methods: In this retrospective cohort study, patients who underwent lumbar posterior instrumentation with CL+ (n = 164) and without CL- (n = 111) augmentation were evaluated. Demographic data, surgical results, preoperative and postoperative visual analogue scale (VAS), the Oswestry Disability Index (ODI) differences, and pseudoarthrosis and adjacent segment disease (ASD)-related recurrence for more than three years of follow-up were determined. Data of CL+ and CL- groups were compared. Results: CL+ and CL- groups were similar in terms of age and gender (P = 0.319 and P = 0.777, respectively) There was no difference between the two groups in terms of bleeding amount, duration of surgery, and duration of hospitalization (P = 0.931, P = 0.669 and P = 0.518, respectively). Groups were similar in terms of VAS and ODI differences (P = 0.915 and P = 0.983, respectively), yet there was one case of infection in the CL+ group and two cases of infection detected in the CL- group. There were 13 ASDs in the CL+ group, and eight ASDs in the CL- group. Pseudoarthrosis was seen seven times in the CL+ group, while it was four in the CL- group. Conclusion: It was observed that adding CL in patients who underwent lumbar instrumentation did not change the early period surgical results. The prevalence of complications was compatible with the scientific literature. In our study, there was no preventive advantage in terms of clinical or postoperative complications found in the use of CL.

Biomechanical Evaluation of the Cross-link Usage and Position in the Single and Multiple Segment Posterior Lumbar Interbody Fusion

OBJECTIVE

Previous studies have neither explored the usage of cross-links nor investigated the optimal position of the cross-links in posterior lumbar interbody fusion (PLIF). This study evaluates biomechanical properties of cross-links in terms of different fixation segments and optimal position in single- and multi-segment posterior lumbar interbody fusion.

METHODS

Two finite element (FE) models of instrumented lumbosacral spine with single-(L4/5) and multi-segment (L3-S1) PLIF surgery were simulated. On the basis of the two models, the benefits of the usage of cross-links were assessed and compared with the status of no application of cross-links. Moreover, the effects of position of cross-links on multi-segment PLIF surgery were studied in Upper, Middle, and Lower positions.

RESULTS

No significant difference was found in the range of motion (ROM), intersegmental rotational angle (IRA) of adjacent segments, and intradiscal pressure (IDP) regardless of the usage of cross-links in the single-segment PLIF surgery, while the cross-link increased the maximum von Mises stress in the fixation (MSF) under the axial rotation (53.65 MPa vs 41.42 MPa). In the multi-segment PLIF surgery, the usage of cross-links showed anti-rotational advantages indicated by ROM (Without Cross-link 2.35^o , Upper, 2.24^o ; Middle, 2.26^o ; Lower, 2.30^o ) and IRA (Without Cross-link 1.19^o , Upper, 1.08^o ; Middle, 1.09^o ; Lower, 1.13^o ). The greatest values of MSF were found in without cross-link case under the flexion, lateral bending, and axial rotation (37.48, 62.61, and 86.73 MPa). The application of cross-links at the Middle and Lower positions had lower values of MSF (48.79 and 69.62 MPa) under the lateral bending and axial rotation, respectively.

CONCLUSION

The application of cross-links was not beneficial for the single-segment PLIF, while it was found highly advantageous for the multi-segment PLIF. Moreover, the usage of cross-links at the Middle or Lower positions resulted in a better biomechanical stability.

Motion capture evaluation of sagittal spino-pelvic biomechanics after lumbar spinal fusion

PURPOSE

The spine and pelvis coexist as a dynamic linked system in which spinal and pelvic parameters are correlated. Investigation of this system can inform the understanding and treatment of spinal deformity. Here, we demonstrate the use of motion capture technology to measure spine biomechanical parameters using a novel testing apparatus.

METHODS

Three complete cadaveric spines with skull and pelvis were mounted into a biomechanical testing apparatus. Each lumbar vertebra was monitored by motion capture cameras as the spines underwent maximal anterior and posterior pelvic tilts about two sagittal axes at a controlled speed and applied force. These axes were defined as the sacral axis which passes transversely through the ilium and S1, and the acetabular axis which passes transversely through both acetabula. The experiments were repeated after L4-L5 fusion, and then, after both L4-L5 and T12-S1 fusion with pedicle screw instrumentation. Data were collected for total range of motion and for coupled translation at each functional spinal unit (FSU).

RESULTS

Total range of motion and coupled translation within functional spinal units (FSUs) was decreased after spinal fusion. The displacement of each individual FSU was captured and summarized along with the observed patterns under each experimental condition.

CONCLUSION

Lumbar fusion decreases spinal motion in the sagittal plane in both overall ROM and individual coupled translations of lumbar vertebrae. This was demonstrated using motion capture technology which is useful for quantifying the translations of individual FSUs in a multisegmental spinal model.

Posterior spinal instrumentation and decompression with or without cross-link?

Background

Posterior lumbar instrumentation requires sufficient primary stiffness to ensure bony fusion and to avoid pseudarthrosis, screw loosening, or implant failure. To enhance primary construct stiffness, transverse cross-link (CL) connectors attached to the vertical rods can be used. Their effect on the stability of a spinal instrumentation with simultaneous decompression is yet not clear. This study aimed to evaluate the impact of CL augmentation on single-level lumbar instrumentation stiffness after gradual decompression procedures.

Methods

Seventeen vertebral segments (6 L1/2, 6 L3/4, 5 L5/S1) of 12 fresh-frozen human cadavers were instrumented with a transpedicular screw–rod construct following the traditional pedicle screw trajectory. Range of motion (ROM) of the segments was sequentially recorded before and after four procedures: (A) instrumented before decompression, (B) instrumented after unilateral laminotomy, (C) instrumented after midline bilateral laminotomy, and (D) instrumented after unilateral facetectomy (with transforaminal lumbar interbody fusion [TLIF]). Each test was performed with and without CL augmentation. The motion between the cranial and caudal vertebrae was evaluated in all six major loading directions: flexion/extension (FE), lateral bending (LB), lateral shear (LS), anterior shear (AS), axial rotation (AR), and axial compression/distraction (AC).

Results

ROM was significantly reduced with CL augmentation in AR by Δ0.03–0.18° (7–12%) with a significantly higher ROM reduction after more extensive decompression. Furthermore, slight reductions in FE and LB were observed; these reached statistical significance for FE after facetectomy and TLIF insertion only (Δ0.15; 3%). The instrumentation levels did not reveal any subgroup differences.

Conclusion

CL augmentation reduces AR-ROM by 7–12% in single-level instrumentation of the lumbar spine, with the effect increasing along with the extensiveness of the decompression technique. In light of the discrete absolute changes, CL augmentation may be warranted for highly unstable vertebral segments rather than for standard single-level posterior spinal fusion and decompression.

Is a cross-connector beneficial for single level traditional or cortical bone trajectory pedicle screw instrumentation?

The cortical bone trajectory (CBT) has been introduced with the aim of better screw hold, however, screw-rod constructs with this trajectory might provide less rigidity in lateral bending (LB) and axial rotation (AR) compared to the constructs with the traditional trajectory (TT). Therefore, the addition of a horizontal cross-connector could be beneficial in counteracting this possible inferiority. The aim of this study was to compare the primary rigidity of TT with CBT screw-rod constructs and to quantify the effect of cross-connector-augmentation in both. Spines of four human cadavers (T9 –L5) were cropped into 15 functional spine units (FSU). Eight FSUs were instrumented with TT and seven FSUs with CBT pedicle screws. The segments were tested in six loading directions in three configurations: uninstrumented, instrumented with and without cross-connector. The motion between the cranial and caudal vertebra was recorded. The range of motion (ROM) between the CBT and the TT group did not differ significantly in either configuration. Cross-connector -augmentation did reduce the ROM in AR (16.3%, 0.27°, p = 0.02), LB (2.9%, 0.07°, p = 0.03) and flexion-extension FE (2.3%, 0.04°, p = 0.02) for the TT group and in AR (20.6%, 0.31°, p = 0.01) for the CBT-group. The primary rigidity of TT and CBT single level screw-rod constructs did not show significant difference. The minimal reduction of ROM due to cross-connector-augmentation seems clinically not relevant. Based on the findings of these study there is no increased necessity to use a cross-connector in a CBT-construct.

Experimental investigation of pull-out performance of pedicle screws at different polyurethane (PU) foam densities

Pedicle bone screws are one of the most critical materials used in spinal orthopaedic operations. Screw loosening and pull-out (PO) are basic complications encountered during or after surgery. Pull-out Strength (POS) of the bone is one of the significant parameters to understand the mechanical behaviour of a screw fixed to poor quality or osteoporotic bone. This study investigates how the POS of a pedicle screw is affected by the factors of the screw diameter and the polyurethane (PU) foam density by experimental analysis. In the experiments, two different diameter (5.5 and 6.5 mm) of conical pedicle screws and five different density (0.08, 0.16, 0.24, 0.32 and 0.48 g·cm^-3) PU foams were used. According to the force-displacement curves obtained from experimental results, the POS increased with the increases in screw diameter and PU foam density.

Cross-links in posterior pedicle screw-rod instrumentation of the spine: a systematic review on mechanical, biomechanical, numerical and clinical studies

Purpose

Dorsal screw-rod instrumentations are used for a variety of spinal disorders. Cross-links (CL) can be added to such constructs, however, no clear recommendations exist. This study aims to provide an overview of the available evidence on the effectiveness of CL, potentially allowing to formulate recommendations on their use.

Methods

A systematic literature review was performed on PubMed and 37 original articles were included and grouped into mechanical, biomechanical, finite element and clinical studies. The change in range of motion (ROM) was analyzed in mechanical and biomechanical studies, ROM, stiffness and stress distribution were evaluated in finite element studies and clinical outcome parameters were analyzed in clinical studies.

Results

A relative consistent reduction in ROM in axial rotation with CL-augmentation was reported, while minor and less consistent effects were observed in flexion–extension and lateral bending. The use of CLs was clinical beneficial in C1/2 fusion, while the limited clinical studies on other anatomic regions show no significant benefit for CL-augmentation.

Conclusion

While CL provides some additional axial rotation stability in most situations, lateral bending and flexion–extension are less affected. Based on clinical data, CL-augmentation can only be recommended for C1/2 instrumentations, while for other cases, further clinical studies are needed to allow for evidence-based recommendations.

Impact of Electrocautery on Fatigue Life of Spinal Fusion Constructs-An In Vitro Biomechanical Study

Instrumentation failure in the context of spine surgery is attributed to cyclic loading leading to formation of fatigue cracks, which later propagate and result in rod fracture. A biomechanical analysis of the potential impact of electrocautery on the fatigue life of spinal implants has not been previously performed. The aim of this study was to assess the fatigue life of titanium (Ti) and cobalt-chrome (CoCr) rod-screw constructs after being treated with electrocautery. Twelve spinal constructs with CoCr and Ti rods were examined. Specimens were divided into four groups by rod material (Ti and CoCr) and application of monopolar electrocautery on the rods' surface (control-group and electrocautery-group). Electrocautery was applied on each rod at three locations, then constructs were cyclically tested. Outcome measures were load-to-failure, total number of cycles-to-failure, and location of rod failure. Ti-rods treated with electrocautery demonstrated a significantly decreased fatigue life compared to non-treated Ti-rods. Intergroup comparison of cycles-to-failure revealed a significant mean decrease of almost 9 × 105 cycles (p = 0.03). No CoCr-rods failed in this experiment. Electrocautery application on the surface of Ti-rods significantly reduces their fatigue life. Surgeons should exercise caution when using electrocautery in the vicinity of Ti-rods to mitigate the risk of rod failure.

Biomechanical comparison of an interspinous fusion device and bilateral pedicle screw system as additional fixation for lateral lumbar interbody fusion

BACKGROUND

This investigation compares an interspinous fusion device with posterior pedicle screw system in a lateral lumbar interbody lumbar fusion.

METHODS

We biomechanically tested six cadaveric lumbar segments (L1-L2) under an axial preload of 50N and torque of 5Nm in flexion-extension, lateral bending and axial rotation directions. We quantified range of motion, neutral zone/elastic zone stiffness in the following conditions: intact, lateral discectomy, lateral cage, cage with interspinous fusion, and cage with pedicle screws.

FINDINGS

A complete lateral discectomy and annulectomy increased motion in all directions compared to all other conditions. The lateral cage reduced motion in lateral bending and flexion/extension with respect to the intact and discectomy conditions, but had minimal effect on extension stiffness. Posterior instrumentation reduced motion, excluding interspinous augmentation in axial rotation with respect to the cage condition. Interspinous fusion significantly increased flexion and extension stiffness, while pedicle screws increased flexion/extension and lateral bending stiffness, with respect to the cage condition. Both posterior augmentations performed equivalently throughout the tests except in lateral bending stiffness where pedicle screws were stiffer in the neutral zone.

INTERPRETATION

A lateral discectomy and annulectomy generates immediate instability. Stand-alone lateral cages restore a limited amount of immediate stability, but posterior supplemental fixation increases stability. Both augmentations are similar in a single level lateral fusion in-vitro model, but pedicle screws are more equipped for coronal stability. An interspinous fusion is a less invasive alternative than pedicle screws and is potentially a conservative option for various interbody cage scenarios.

Axial rotation mechanics in a cadaveric lumbar spine model: a biomechanical analysis

BACKGROUND CONTEXT

Postoperative patient motions are difficult to directly control. Very slow quasistatic motions are intuitively believed to be safer for patients, compared with fast dynamic motions, because the torque on the spine is reduced. Therefore, the outcomes of varying axial rotation (AR) angular loading rate during in vitro testing could expand the understanding of the dynamic behavior and spine response.

PURPOSE

To observe the effects of the loading rate in AR mechanics of lumbar cadaveric spines via in vitro biomechanical testing.

STUDY DESIGN

An in vitro biomechanical study in lumbar cadaveric spines.

METHODS

Fifteen lumbar cadaveric segments (L1-S1) were tested with varying loading frequencies of AR. Five different frequencies were normalized with the base line frequency (0.125 Hz n=15) in this analysis: 0.05 Hz (n=6), 0.166 Hz (n=6), 0.2 Hz (n=10), 0.25 Hz (n=10), and 0.4 Hz (n=8).

RESULTS

The lowest frequency (0.05 Hz) revealed significant differences (p<.05) for all parameters (torque, passive angular velocity, axial velocity [AV], axial reaction force [RF], and energy loss [EL]) with respect to all other frequencies. Significant differences (p<.05) were observed in the following: torque (0.4 Hz with respect to 0.2 Hz and 0.25 Hz), passive sagittal angular velocity (SAV) (0.4 Hz with respect to all other frequencies; 0.166 Hz with respect to 0.25 Hz), axial linear velocity (0.4 Hz with respect to all other frequencies), and RF (0.4 Hz with respect to 0.2 Hz and 0.25 Hz). Strong correlations (R2>0.75, p<.05) were observed between RF with intradiscal pressure (IDP) and AR angular displacement with IDP. Intradiscal pressure (p<.05) was significantly larger in 0.2 Hz in comparison with 0.125 Hz.

CONCLUSIONS

Evidences suggest that measurements at very small frequencies (0.05 Hz) of torque, SAV, AV, RF, and EL are significantly reduced when compared with higher frequencies (0.166 Hz, 0.2 Hz, 0.25 Hz, and 0.4 Hz). Higher frequencies increase torque, RF, passive SAV, and AV. Higher frequencies induce a greater IDP in comparison with lower frequencies.

Feasibility and biomechanical performance of a novel transdiscal screw system for one level in non-spondylolisthetic lumbar fusion: an in vitro investigation

BACKGROUND CONTEXT

The bilateral pedicle screw system (BPSS) is currently the "gold standard" fusion technique for spinal instability. A new stabilization system that provides the same level of stability through a less invasive procedure will have a high impact on clinical practice. A new transdiscal screw system is investigated as a promising minimally invasive device.

PURPOSE

To evaluate the feasibility of a novel transdiscal screw in spinal fixation as an alternative to BPSS, with and without an interbody cage, in non-spondylolisthesis cases.

STUDY DESIGN

An in vitro biomechanical study in lumbar cadaveric spines.

METHODS

Twelve lumbar cadaveric segments (L4-S1) were tested under flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Six treatments were simulated as follows: (1) intact, (2) bilateral facetectomy at L4-L5, (3) transdiscal screw system, (4) BPSS, (5) BPSS with transforaminal lumbar interbody cage, and (6) transdiscal screws with transforaminal interbody cage. Specimens were randomly divided into two testing groups: Group 1 (n=6) was tested under the first five conditions, in the order presented, whereas Group 2 (n=6) was tested under the first, second, third, fourth, and sixth conditions, with the fourth condition preceding the third. Range of motion (ROM) and neutral zone stiffness (NZS) were estimated and normalized with respect to the intact condition to explore statistical differences among treatments using non-parametric approaches.

RESULTS

Significant differences in FE ROM were observed in the pedicle screws-cage condition with respect to the facetectomy (p<.01), the pedicle screw (p=.03), and the transdiscal screw (p<.02) conditions. All fixation constructs significantly restricted LB and AR ROM (p<.01) with respect to facetectomy. In terms of stiffness, the pedicle screw and the transdiscal screw systems increased (p<.01) LB and AR NZS with respect to facetectomy. The pedicle screws-cage condition significantly increased flexion and extension stiffness with respect to all other conditions (p<.05). However, LB NZS for the pedicle screws-cage and the transdiscal screws-cage condition could not be explored due to a testing order bias effect. There was not enough evidence to state any difference between the pedicle and transdiscal screw conditions in terms of ROM or NZS.

CONCLUSIONS

Transdiscal and pedicle screw systems showed comparable in vitro biomechanical performance in the immediate stabilization of a complete bilateral facetectomy. The pedicle screws-cage condition was the most stable in FE motion; however, comparison with respect to the transdiscal screws-cage condition could not be investigated.

Designs and techniques that improve the pullout strength of pedicle screws in osteoporotic vertebrae: current status

Osteoporosis is a medical condition affecting men and women of different age groups and populations. The compromised bone quality caused by this disease represents an important challenge when a surgical procedure (e.g., spinal fusion) is needed after failure of conservative treatments. Different pedicle screw designs and instrumentation techniques have been explored to enhance spinal device fixation in bone of compromised quality. These include alterations of screw thread design, optimization of pilot hole size for non-self-tapping screws, modification of the implant's trajectory, and bone cement augmentation. While the true benefits and limitations of any procedure may not be realized until they are observed in a clinical setting, axial pullout tests, due in large part to their reproducibility and ease of execution, are commonly used to estimate the device's effectiveness by quantifying the change in force required to remove the screw from the body. The objective of this investigation is to provide an overview of the different pedicle screw designs and the associated surgical techniques either currently utilized or proposed to improve pullout strength in osteoporotic patients. Mechanical comparisons as well as potential advantages and disadvantages of each consideration are provided herein.

In vitro evaluation of a lateral expandable cage and its comparison with a static device for lumbar interbody fusion: a biomechanical investigation

OBJECT

Through in vitro biomechanical testing, the authors compared the performance of a vertically expandable lateral lumbar interbody cage (EC) under two different torque-controlled expansions (1.5 and 3.0 Nm) and with respect to an equivalent lateral lumbar static cage (SC) with and without pedicle screw fixation.

METHODS

Eleven cadaveric human L2-3 segments were evaluated under the following conditions: 1) intact; 2) discectomy; 3) EC under 1.50 Nm of torque expansion (EC-1.5Nm); 4) EC under 3.00 Nm of torque expansion (EC-3.0Nm); 5) SC; and 6) SC with a bilateral pedicle screw system (SC+BPSS). Load-displacement behavior was evaluated for each condition using a combination of 100 N of axial preload and 7.5 Nm of torque in flexion and extension (FE), lateral bending (LB), and axial rotation (AR). Range of motion (ROM), neutral zone stiffness (NZS), and elastic zone stiffness (EZS) were statistically compared among conditions using post hoc Wilcoxon signed-rank comparisons after Friedman tests, with a significance level of 0.05. Additionally, any cage height difference between interbody devices was evaluated. When radiographic subsidence was observed, the specimen's data were not considered for the analysis.

RESULTS

The final cage height in the EC-1.5Nm condition (12.1 ± 0.9 mm) was smaller (p < 0.001) than that in the EC-3.0Nm (13.9 ± 1.1 mm) and SC (13.4 ± 0.8 mm) conditions. All instrumentation reduced (p < 0.01) ROM with respect to the injury and increased (p ≤ 0.01) NZS in flexion, extension, and LB as well as EZS in flexion, LB, and AR. When comparing the torque expansions, the EC-3.0Nm condition had smaller (p < 0.01) FE and AR ROM and greater (p ≤ 0.04) flexion NZS, extension EZS, and AR EZS. The SC condition performed equivalently (p ≥ 0.10) to both EC conditions in terms of ROM, NZS, and EZS, except for EZS in AR, in which a marginal (p = 0.05) difference was observed with respect to the EC-3.0Nm condition. The SC+BPSS was the most rigid construct in terms of ROM and stiffness, except for 1) LB ROM, in which it was comparable (p = 0.08) with that of the EC-1.5Nm condition; 2) AR NZS, in which it was comparable (p > 0.66, Friedman test) with that of all other constructs; and 3) AR EZS, in which it was comparable with that of the EC-1.5Nm (p = 0.56) and SC (p = 0.08) conditions.

CONCLUSIONS

A 3.0-Nm torque expansion of a lateral interbody cage provides greater immediate stability in FE and AR than a 1.5-Nm torque expansion. Moreover, the expandable device provides stability comparable with that of an equivalent (in size, shape, and bone-interface material) SC. Specifically, the SC+BPSS construct was the most stable in FE motion. Even though an EC may seem a better option given the minimal tissue disruption during its implantation, there may be a greater chance of endplate collapse by over-distracting the disc space because of the minimal haptic feedback from the expansion.

Biomechanical analysis of an interspinous fusion device as a stand-alone and as supplemental fixation to posterior expandable interbody cages in the lumbar spine

OBJECT

In this paper the authors evaluate through in vitro biomechanical testing the performance of an interspinous fusion device as a stand-alone device, after lumbar decompression surgery, and as supplemental fixation to expandable cages in a posterior lumbar interbody fusion (PLIF) construct.

METHODS

Nine L3-4 human cadaveric spines were biomechanically tested under the following conditions: 1) intact/control; 2) L3-4 left hemilaminotomy with partial discectomy (injury); 3) interspinous spacer (ISS); 4) bilateral pedicle screw system (BPSS); 5) bilateral hemilaminectomy, discectomy, and expandable posterior interbody cages with ISS (PLIF-ISS); and 6) PLIF-BPSS. Each test consisted of 100 N of axial preload with ± 7.5 Nm of torque in flexion-extension, right/left lateral bending, and right/left axial rotation. Significant changes in range of motion (ROM), neutral zone stiffness (NZS), elastic zone stiffness (EZS), and energy loss (EL) were explored among conditions using nonparametric Friedman test and Wilcoxon signed-rank comparisons (p ≤ 0.05).

RESULTS

The injury increased ROM in flexion (p = 0.01), left bending (p = 0.03), and right/left rotation (p < 0.01) and also decreased NZS in flexion (p = 0.01) and extension (p < 0.01). Both the ISS and BPSS reduced flexion-extension ROM and increased flexion-extension stiffness (NZS and EZS) with respect to the injury and intact conditions (p < 0.05), but the ISS condition provided greater resistance than BPSS in extension for ROM, NZS, and EZS (p < 0.01). The BPSS increased the rigidity (ROM, NZS, and EZS) of the intact model in lateral bending and axial rotation (p ≤ 0.01), except in EZS for left rotation (p = 0.23, Friedman test). The incorporation of posterior cages marginally increased (p = 0.05) the EZS of the BPSS construct in flexion but these interbody devices provided significant stability to the ISS construct in lateral bending and axial rotation for ROM (p = 0.02), in lateral bending for NZS (p = 0.02), and in flexion/axial rotation for EZS (p ≤ 0.03); however, both PLIF constructs demonstrated equivalent ROM and stiffness (p ≥ 0.16), except in lateral bending where the PLIF-BPSS was more stable (p = 0.02). In terms of EL, the injury increased EL in flexion-extension (p = 0.02), the ISS increased EL for lateral bending and axial rotation (p ≤ 0.03), and the BPSS decreased EL in lateral bending (p = 0.02), with respect to the intact condition. The PLIF-ISS decreased lateral bending EL with respect to the ISS condition (p = 0.02), but not enough to be smaller or, at least, equivalent, to that of the PLIF-BPSS construct (p = 0.02).

CONCLUSIONS

The ISS may be a suitable device to provide immediate flexion-extension balance after a unilateral laminotomy, but the BPSS provides greater immediate stability in lateral bending and axial rotation motions. Both PLIF constructs performed equivalently in flexion-extension and axial rotation, but the PLIF-BPSS construct is more resistant to lateral bending motions. Further biomechanical and clinical evidence is required to strongly support the recommendation of a stand-alone interspinous fusion device or as supplemental fixation to expandable posterior interbody cages.