Biomechanical stability of five stand-alone anterior lumbar interbody fusion constructs

Endplate Trauma During Implant Insertion Affects the Expulsion Risk of Anterior Lumbar Interbody Fusion Devices

Background Anterior cage migration in anterior lumbar interbody fusion is a serious complication. To address this risk, cage designs are now available with integrated screw or blade fixation or specially designed surface geometries with large teeth or ridges. However, the implantation technique itself has not yet been addressed as a potential risk factor for cage migration. This study aimed to investigate whether a cage that is implantable without gouging the vertebral endplates has improved resistance to anterior migration. Methodology A novel three-piece modular cage was inserted between two vertebral body replacements (polyurethane (PU) foam grade 15 pcf) in two ways. In group 1 (modular), the cage was inserted in a wedge within a wedge fashion according to the manufacturer's instructions such that damage to the PU foam was minimized. In group 2 (mono-bloc), the modular cage was inserted pre-assembled as a one-piece, mono-bloc device. This insertion method required impaction and increased the potential of gouging the PU surfaces. Then, an axial preload was applied to the PU test blocks to simulate the preload on the spine in vivo and an anteriorly direct expulsion force was applied to the cages. Results The mean expulsion yield load in the test group with modular implantation was 392 ± 19 N compared to 287 ± 16 N in the test group where the mono-bloc implants were inserted and endplate gouging occurred. This difference was statistically significant (p < 0.05). Thus, the onset of cage migration occurred at significantly higher loads in the test group with modular insertion without endplate gouging compared to one-piece impaction with gouging taking place. In contrast, the stiffness and the ultimate load were similar in both test groups (p > 0.05). Conclusions This study showed that the cage insertion technique may have a significant effect on the cage migration risk. Prevention of endplate gouging during cage implantation has the potential to improve the primary stability of the cage.

Comparison of the Postoperative Motion Stabilization Between Anterior Cervical Decompression and Fusion with a Zero-Profile Implant System and a Plate-Cage Construct

OBJECTIVE

Due to the lack of an additional anterior plate, the motion stability of a zero-profile device with an anchored cage (AC) may be inferior to that of a traditional plate-cage construct (PCC). However, the impact of this difference in motion stability on various outcomes has not been fully explored. Therefore, the aim of this study was to compare the motion stabilization features of an AC and a PCC and analyze their impact on postoperative outcomes and complications.

METHODS

A retrospective study of patients treated with single-level anterior cervical discectomy and fusion from January 2008 to May 2016 was performed. First, clinical and radiological outcomes, postoperative complications and time to achieve motion stabilization were compared between the AC and PCC groups. Then, based on the time to achieve motion stabilization, all patients were divided into group A (time to achieve motion stabilization <3 months), group B (time to achieve motion stabilization from 3-6 months), and group C (time to achieve motion stabilization >6 months). The early postoperative complications were compared across the 3 groups. Motion stabilization was measured on dynamic cervical radiographs using the interspinous process method and Cobb angle method according to previously published methods.

RESULTS

A total of 160 patients met the inclusion criteria, including 90 patients in the AC group and 70 patients in the PCC group. There were no significant differences between the AC and PCC groups in the clinical outcomes, C2-7 angle change, segmental angle change, final fusion rate or adjacent-level degeneration (P > 0.05). The disc height loss was 2.26 ± 1.00 mm in the AC group and 1.76 ± 1.13 mm in the PCC group (P = 0.004), and the incidence of implant subsidence was 24.44% in the AC group and 11.43% in the PCC group (P = 0.036). In addition, the PCC was more dynamically stable than the AC at 3 months post-surgery (P < 0.001), and at this time, the disc height loss and implant subsidence in motion-stable patients were significantly lower than those in motion-unstable patients (P < 0.05). Furthermore, our results also showed that when the arrival time of motion stabilization was prolonged, the loss of disc height and occurrence of subsidence gradually increased.

CONCLUSIONS

More attention should be given to minimizing the adverse impact of poor motion stability in the design and development of future zero-profile cervical implants, although this has little impact on clinical efficacy.

Direct reduction of high-grade lumbosacral spondylolisthesis with anterior cantilever technique - surgical technique note and preliminary results

Backgrounds

Surgical reduction for high-grade spondylolisthesis is beneficial for restoring sagittal balance and improving the biomechanical environment for arthrodesis. Compared to posterior total laminectomy and long instrumentation, anterior lumbar inter-body fusion (ALIF) is less invasive and has the biomechanical advantage of restoring the original disk height and increasing lumbar lordosis, thus improving sagittal balance. However, the application of ALIF is still limited in treating low-grade spondylolisthesis. In this study, we developed a new technique termed anterior cantilever procedure to directly reduce the slippage of high-grade lumbosacral spondylolisthesis. The purpose of our study was to investigate the surgical outcomes of the anterior cantilever procedure followed by ALIF and posterior mono-segment instrumented fixation in high-grade spondylolisthesis.

Methods

All patients with high-grade spondylolisthesis who underwent anterior cantilever procedure followed by anterior lumbar inter-body fusion (ALIF) and posterior mono-segment instrumented fixation between November 2006 and July 2017 were enrolled in our study. The slip percentage, Dubousset’s lumbosacral angle, pelvic tilt, sacral slope, pelvic incidence, and sagittal alignment were measured pre-operatively and postoperatively at the last follow-up. Surgery time, blood loss, complications, and hospital stay were also collected and analysed.

Results

A total of 11 consecutive patients with high-grade spondylolisthesis patients were included and analysed. All of the high-grade spondylolisthesis in our series occurred at the L5-S1 level. The median age was 37 years, and the median follow-up duration was 36 months. The average slip reduction was 30% (60 to 30%, P < 0.01), and the average correction of Dubousset’s lumbosacral angle was 13.8° (84.1° to 97.9°, P < 0.01). The median intra-operative blood loss was 300 mL. All patients attained improved sagittal balance after the operation and achieved solid fusion within 9 months after surgery. No incidences of implant failure, permanent neurological deficit, or pseudarthrosis were recorded at the last follow-up.

Conclusions

Anterior cantilever procedure followed by ALIF and posterior mono-segment instrumented fixation is a valid procedure for treating high-grade spondylolisthesis. It achieved a high fusion rate, partially reduced slippage, and significantly improved lumbosacral angle, while minimizing common complications, such as pseudarthrosis, nerve traction injury, excessive soft tissue dissection, and blood loss in posterior reduction procedures. However, posterior instrumentation is still required to the structural stability in the ALIF procedure.

Level of evidence

IV

Anterior lumbar fusion techniques: ALIF, OLIF, DLIF, LLIF, IXLIF

An anterior approach to the lumbar spine is increasingly used in performing fusion. Depending on the level to be treated, several approaches have been developed to deal with the anatomic obstacles encountered: pure anterior, oblique anterior or lateral, and trans- or pre-psoas. Conventional techniques incur risk of muscle lesion and severe bleeding, and have been replaced by minimally invasive approaches, often with video assistance after rapid closure of laparoscopic approaches with gas insufflation. There has, in parallel, been great progress in anterior spinal instrumentation systems. Non-existent when these techniques were first developed, they have become increasingly sophisticated, and now employ a variety of stand-alone or not cages and anterior screwed plate that can be associated together or to posterior fixation. Each approach and type of fixation has its specific technical requirements that need to be fully mastered so as to minimize risk, especially regarding vessels, and to enable the patient to enjoy the benefit of their very low morbidity. We shall therefore detail here each step of the pure anterior approach, which is most often used for L5S1, the oblique and lateral approaches, mainly used for L2L5, and also the preparation of the lumbar spine for anterior interbody fusion, with the respective instrumentations. We shall then consider the pros, cons and risks, and also spinal or general contraindications that may sometimes preclude this option. From this, we shall derive the specific optimal and wrong indications for anterior lumbar surgery, to help decision-making when fusion is indicated.

Biomechanical investigation of extragraft bone formation influences on the operated motion segment after anterior cervical spinal discectomy and fusion

Although the clinical importance of extragraft bone formation (ExGBF) and bridging (ExGBB) has been reported, few studies have investigated the biomechanical influences of ExGBF on the motion segment. In this study, ExGBF was simulated at the C5-C6 motion segment after anterior cervical discectomy and fusion using a developed finite element model and a sequential bone-remodelling algorithm in flexion and extension. The computer simulation results showed that extragraft bone was primarily formed in the extension motion and grew to form ExGBB. A stepwise decrease in the intersegmental rotation angle, maximum von Mises stress and strain energy density on the trabecular bone with ExGBF were predicted in extension. When ExGBB was formed in the trabecular bone region, the intersegmental rotation angle slightly decreased with additional bone formation. However, the stress and strain energy density on the trabecular bone region decreased until ExGBB reached the peripheral cortical margin. The results offer a rationale supporting the hypothesis that mechanical stimuli influence ExGBF. ExGBF was helpful in increasing the stability of the motion segment and decreasing the fracture risk of trabecular bones, even in cases in which ExGBB was not formed. ExGBB can be classified as either soft or hard bridging based on a biomechanical point of view.

Comparison of polyetheretherketone versus silicon nitride intervertebral spinal spacers in a caprine model

Polyetheretherketone (PEEK) is commonly used as a spinal spacer for intervertebral fusion surgery. Unfortunately, PEEK is bioinert and does not effectively osseointegrate into living bone. In contrast, comparable spacers made of silicon nitride (Si₃ N₄ ) possess a surface nanostructure and chemistry that encourage appositional bone healing. This observational study was designed to compare the outcomes of these two biomaterials when implanted as spacers in an adult caprine model. Lumbar interbody fusion surgeries were performed at two adjacent levels in eight adult goats using implants of PEEK and Si₃ N₄ . At six-months after surgery, the operative and adjacent spinal segments were extracted and measured for bone fusion, bone volume, bone-implant contact (BIC) and soft-tissue implant contact (SIC) ratios, and biodynamic stability. The null hypothesis was that no differences in these parameters would be apparent between the two groups. Fusion was observed in seven of eight implants in each group with greater bone formation in the Si₃ N₄ group (52.6%) versus PEEK (27.9%; p = 0.2). There were no significant differences in BIC ratios between PEEK and Si₃ N₄ , and the biodynamic stability of the two groups was also comparable. The results suggest that Si₃ N₄ spacers are not inferior to PEEK and they may be more effective in promoting arthrodesis. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 00B: 000-000, 2018. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 688-699, 2019.

A single center retrospective clinical evaluation of anterior cervical discectomy and fusion comparing allograft spacers to silicon nitride cages

BACKGROUND

Iliac crest autograft or allograft spacers have been traditionally utilized in anterior cervical discectomy and fusion (ACDF) to provide vertebral stabilization and enhanced osteogenesis. However, abiotic cages have largely replaced these allogenic sources due to host-site morbidities and disease transmission risks, respectively. Although devices made of polyetheretherketone (PEEK) or titanium-alloys (Ti) have gained wide popularity, they lack osteoinductive or conductive capabilities. In contrast, silicon nitride (Si3N4) is a relatively new implant material that also provides structural stability and yet purportedly offers osteopromotive and antimicrobial behavior. This study compared radiographic outcomes at ≥12 months of follow-up for osseous integration, fusion rate, time to fusion, and subsidence in ACDF patients with differing intervertebral spacers.

METHODS

Fifty-eight ACDF patients (108 segments) implanted with Si3N4 cages were compared to thirty-four similar ACDF patients (61 segments) implanted with fibular allograft spacers. Lateral radiographs (normal, flexion, and extension) were obtained at 3, 6, 12, and 24 months to assess osseous integration, the presence of bridging bone, the absence of peri-implant radiolucencies, subsidence, and fusion using both interspinous distance (ISD) and Cobb angle methods.

RESULTS

In patients with ≥12 months of follow-up, fusion for the allograft spacers and Si3N4 cages was 86.84% and 96.83%, respectively (ISD method, P=0.10), and 67.65% and 84.13%, respectively (Cobb angle method P=0.07), while osseointegration was 76.32% and 93.65%, respectively (P=0.02). The time to fusion significantly favored the Si3N4 cages (4.08 vs. 8.64 months (ISD method, P=0.01), and 6.76 vs. 11.74 months (Cobb angle method, P=0.04). The assessed time for full osseointegration was 7.83 and 19.24 months for Si3N4 and allograft, respectively (P=0.00). Average subsidence at 1-year follow-up was 0.51 and 2.71 mm for the Si3N4 and allograft cohorts, respectively (P=0.00).

CONCLUSIONS

In comparison to fibular allograft spacers, Si3N4 cages showed earlier osseointegration and fusion, higher fusion rates, and less subsidence.

Biomechanical Analysis of Lateral Lumbar Interbody Fusion Constructs with Various Fixation Options: Based on a Validated Finite Element Model

BACKGROUND

Lateral lumbar interbody fusion using cage supplemented with fixation has been used widely in the treatment of lumbar disease. A combined fixation (CF) of lateral plate and spinous process plate may provide multiplanar stability similar to that of bilateral pedicle screws (BPS) and may reduce morbidity. The biomechanical influence of the CF on cage subsidence and facet joint stress has not been well described. The aim of this study was to compare biomechanics of various fixation options and to verify biomechanical effects of the CF.

METHODS

The surgical finite element models with various fixation options were constructed based on computed tomography images. The lateral plate and posterior spinous process plate were applied (CF). The 6 motion modes were simulated. Range of motion (ROM), cage stress, endplate stress, and facet joint stress were compared.

RESULTS

For the CF model, ROM, cage stress, and endplate stress were the minimum in almost all motion modes. Compared with BPS, the CF reduced ROM, cage stress, and endplate stress in all motion modes. The ROM was reduced by more than 10% in all motion modes except for flexion; cage stress and endplate stress were reduced more than 10% in all motion modes except for rotation-left. After interbody fusion, facet joint stress was reduced substantially compared with the intact conditions in all motion modes except for flexion.

CONCLUSIONS

The combined plate fixation may offer an alternative to BPS fixation in lateral lumbar interbody fusion.

Biomechanical Analysis of Porous Additive Manufactured Cages for Lateral Lumbar Interbody Fusion: A Finite Element Analysis

BACKGROUND

A porous additive manufactured (AM) cage may provide stability similar to that of traditional solid cages and may be beneficial to bone ingrowth. The biomechanical influence of various porous cages on stability, subsidence, stresses in cage, and facet contact force has not been fully described. The purpose of this study was to verify biomechanical effects of porous AM cages.

METHODS

The surgical finite element models with various cages were constructed. The partially porous titanium (PPT) cages and fully porous titanium (FPT) cages were applied. The mechanical parameters of porous materials were obtained by mechanical test. Then the porous AM cages were compared with solid titanium (TI) cage and solid polyetheretherketone (PEEK) cage. The 4 motion modes were simulated. Range of motion (ROM), cage stress, end plate stress, and facet joint force (FJF) were compared.

RESULTS

For all the surgical models, ROM decreased by >90%. Compared with TI and PPT cages, PEEK and FPT cages substantially reduced the maximum stresses in cage and end plate in all motion modes. Compared with PEEK cages, the stresses in cage and end plate for FPT cages decreased, whereas the ROM increased. Comparing FPT cages, the stresses in cage and end plate decreased with increasing porosity, whereas ROM increased with increasing porosity. After interbody fusion, FJF was substantially reduced in all motion modes except for flexion.

CONCLUSIONS

Fully porous cages may offer an alternative to solid PEEK cages in lateral lumbar interbody fusion. However, it may be prudent to further increase the porosity of the cage.

Evaluating Outcomes of Stand-Alone Anterior Lumbar Interbody Fusion: A Systematic Review

BACKGROUND

Stand-alone anterior lumbar interbody fusion (ALIF) is an effective surgical approach for selected spinal pathologies. It avoids the morbidity and complications associated with instrumented ALIF, such as plate fixation and the traditionally used posterior approach. Despite improved disc space visualization and clearance, the associated posterior instability and increased risk of nonfusion present major challenges to this approach. The integral cage design aims to address these challenges by providing the necessary stabilization through intracorporeal screws. However, there is limited and controversial data available for stand-alone ALIF and integral cage fixation. To our knowledge, this is the first systematic review to evaluate recent findings on outcomes of stand-alone ALIF devices to explore areas of controversy and identify directions for future research.

METHODS

Two reviewers conducted independent, systematic literature searches for appropriate studies in 5 electronic databases as per Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines. Studies were filtered by the use of specified selection criteria, particularly exclusion of studies with supplementary fixation to ALIF and studies published before the year 2000. A total of 17 studies met the criteria, and their data were comprehensively extracted and analyzed.

RESULTS

The current literature is supportive of stand-alone ALIF due to acceptable clinical outcomes, promising fusion rates and disc height restoration. However, data and outcomes remain preliminary, and there are numerous areas of controversy.

CONCLUSIONS

There is evidence for the efficacy and safety of stand-alone ALIF. However, the extent of improvement based on specific indications for surgery remains unclear. Further investigation utilizing more methodologically rigorous studies of long-term outcomes is necessary to address these issues.

Anterior Lumbar Interbody Fusion Integrated Screw Cages: Intrinsic Load Generation, Subsidence, and Torsional Stability

OBJECTIVE

To perform a repeatable idealized in vitro model to evaluate the effects of key design features and integrated screw fixation on unloaded surface engagement, subsidence, and torsional stability.

METHODS

We evaluated four different stand-alone anterior lumbar interbody fusion (ALIF) cages with two, three, and four screw designs. Polyurethane (saw-bone) foam blocks were used to simulate the vertebral bone. Fuji Film was used to measure the contact footprint, average pressure, and load generated by fixating the cages with screws. Subsidence was tested by axially loading the constructs at 10 N/s to 400 N and torsional load was applied +/-1 Nm for 10 cycles to assess stability. Outcome measures included total subsidence and maximal torsional angle range.

RESULTS

Cages 1, 2, and 4 were symmetrical and produced similar results in terms of contact footprint, average pressure, and load. The addition of integrated screws into the cage-bone block construct demonstrated a clear trend towards decreased subsidence. Cage 2 with surface titanium angled ridges and a keel produced the greatest subsidence with and without screws, significantly more than all other cages ( P < 0.05). Angular rotation was not significantly affected by the addition of screws ( P < 0.066). A statistically significant correlation existed between subsidence and reduced angular rotation across all cage constructs ( P = 0.018).

CONCLUSION

Each stand-alone cage featured unique surface characteristics, which resulted in differing cage-foam interface engagement, influencing the subsidence and torsional angle. Increased subsidence significantly reduced the torsional angle across all cage constructs.

Stand-alone anterior lumbar interbody fusion: indications, techniques, surgical outcomes and complications

INTRODUCTION

Anterior lumbar interbody fusion (ALIF) is a well-established technique to achieve lumbar spine fusion with various indications including degenerative disk disease, spondylolisthesis, recurrent disk herniation, adjacent level disease, pseudoarthrosis, as well as being used as part of the overall strategy to restore sagittal balance. ALIF can be an extremely useful tool in any spine surgeon's armamentarium. However, like any surgical procedure, proper patient selection is key to success. A solid understanding of the biomechanics, careful surgical planning, along with clear knowledge of the advantages and disadvantages of stand-alone ALIF will ensure optimal clinical outcome. Stand-alone ALIF may be a suitable surgical option in carefully selected patients that can provide good clinical results and adequate fusion rates without the need for posterior instrumentation. Areas covered: A brief overview of the indications, techniques, biomechanics, surgical outcome and complications of stand-alone ALIF is provided in this article with a review of the pertinent literature. Expert commentary: In this review we discuss the clinical evidence of using a stand-alone ALIF compared to other fusion techniques of the lumbar spine. The development of interbody cages with integrated screws has increased the arthrodesis rate and improved clinical outcomes while decreasing morbidity and operative time.

Biomechanical evaluation of lateral lumbar interbody fusion with secondary augmentation

OBJECTIVE Lateral lumbar interbody fusion (LLIF) has emerged as a popular method for lumbar fusion. In this study the authors aimed to quantify the biomechanical stability of an interbody implant inserted using the LLIF approach with and without various supplemental fixation methods, including an interspinous plate (IP). METHODS Seven human cadaveric L2-5 specimens were tested intact and in 6 instrumented conditions. The interbody implant was intended to be used with supplemental fixation. In this study, however, the interbody was also tested without supplemental fixation for a relative comparison of these conditions. The instrumented conditions were as follows: 1) interbody implant without supplemental fixation (LLIF construct); and interbody implant with supplemental fixation performed using 2) unilateral pedicle screws (UPS) and rod (LLIF + UPS construct); 3) bilateral pedicle screws (BPS) and rods (LLIF + BPS construct); 4) lateral screws and lateral plate (LP) (LLIF + LP construct); 5) interbody LP and IP (LLIF + LP + IP construct); and 6) IP (LLIF + IP construct). Nondestructive, nonconstraining torque (7.5 Nm maximum) induced flexion, extension, lateral bending, and axial rotation, whereas 3D specimen range of motion (ROM) was determined optoelectronically. RESULTS The LLIF construct reduced ROM by 67% in flexion, 52% in extension, 51% in lateral bending, and 44% in axial rotation relative to intact specimens (p < 0.001). Adding BPS to the LLIF construct caused ROM to decrease by 91% in flexion, 82% in extension and lateral bending, and 74% in axial rotation compared with intact specimens (p < 0.001), providing the greatest stability among the constructs. Adding UPS to the LLIF construct imparted approximately one-half the stability provided by LLIF + BPS constructs, demonstrating significantly smaller ROM than the LLIF construct in all directions (flexion, p = 0.037; extension, p < 0.001; lateral bending, p = 0.012) except axial rotation (p = 0.07). Compared with the LLIF construct, the LLIF + LP had a significant reduction in lateral bending (p = 0.012), a moderate reduction in axial rotation (p = 0.18), and almost no benefit to stability in flexion-extension (p = 0.86). The LLIF + LP + IP construct provided stability comparable to that of the LLIF + BPS. The LLIF + IP construct provided a significant decrease in ROM compared with that of the LLIF construct alone in flexion and extension (p = 0.002), but not in lateral bending (p = 0.80) and axial rotation (p = 0.24). No significant difference was seen in flexion, extension, or axial rotation between LLIF + BPS and LLIF + IP constructs. CONCLUSIONS The LLIF construct that was tested significantly decreased ROM in all directions of loading, which indicated a measure of inherent stability. The LP significantly improved the stability of the LLIF construct in lateral bending only. Adding an IP device to the LLIF construct significantly improves stability in sagittal plane rotation. The LLIF + LP + IP construct demonstrated stability comparable to that of the gold standard 360° fixation (LLIF + BPS).

Anterior lumbar interbody fusion with integrated fixation and adjunctive posterior stabilization: A comparative biomechanical analysis

BACKGROUND

Interbody fusion cages with integrated fixation components have become of interest due to their ability to provide enhanced post-operative stability and mitigate device migration. A recently approved anterior lumbar interbody fusion cage with integrated fixation anchors has yet to be compared in vitro to a standard polyetheretherketone cage when used in combination with an interspinous process clamp.

METHODS

Twelve human cadaveric lumbar segments were implanted at L4-L5 with a Solus interbody cage (n=6) or standard polyetheretherketone cage (n=6) following Intact testing and discectomy. Each cage was subsequently evaluated in all primary modes of loading after supplementation with the following posterior constructs: interspinous process clamp, bilateral transfacet screws, unilateral transfacet screw with contralateral pedicle screws, and bilateral pedicle screws. Range of motion results were normalized to Intact, and a two-way mixed analysis of variance was utilized to detect statistical differences.

FINDINGS

The Solus cage in combination with all posterior constructs provided significant fixation compared to Intact in all loading conditions. The polyetheretherketone cage also provided significant fixation when combined with all screw based treatments, however when used with the interspinous process clamp a significant reduction was not observed in lateral bending or axial torsion.

INTERPRETATION

Interbody cages with integrated fixation components enhance post-operative stability within the intervertebral space, thus affording clinicians the potential to utilize less invasive methods of posterior stabilization when seeking circumferential fusion. Interspinous process clamps, in particular, may reduce peri-operative and post-operative comorbidities compared to screw based constructs. Further study is necessary to corroborate their effectiveness in vivo.

Polyetheretherketone (PEEK) cages in cervical applications: a systematic review

BACKGROUND CONTEXT

Polyetheretherketone (PEEK) cages have been widely used during the past decade in patients with degenerative disorders of the cervical spine. Their radiolucency and low elastic modulus make them attractive attributes for spinal fusion compared with titanium and bone graft. Still, limitations are seen such as pseudoarthrosis, subsidence, and migration of the cages. Limited evidence on the clinical outcome of PEEK cages is found in the literature other than noncomparative cohort studies with only a few randomized controlled trials.

PURPOSE

To assess the clinical and radiographic outcome of PEEK cages in the treatment of degenerative disc disorders and/or spondylolisthesis in the cervical spine.

STUDY DESIGN

Systematic review of all randomized controlled trials and prospective and retrospective nonrandomized comparative studies with a minimum follow-up of 6 months and all noncomparative cohort studies with a long-term follow-up of more than 5 years.

OUTCOME MEASURES

The primary outcome variable was clinical performance. Secondary outcome variables consisted of radiographic scores.

METHODS

The MEDLINE, EMBASE, and Cochrane Library databases were searched according to the Preferred Reporting Items of Systematic reviews and Meta-Analyses statement and Meta-analysis Of Observational Studies in Epidemiology guidelines. No conflict of interest reported. No funding received.

RESULTS

A total of 223 studies were identified, of which 10 studies were included. These comprised two randomized controlled trials, five prospective comparative trials, and three retrospective comparative trials.

CONCLUSIONS

Minimal evidence for better clinical and radiographic outcome is found for PEEK cages compared with bone grafts in the cervical spine. No differences were found between PEEK, titanium, and carbon fiber cages. Future studies are needed to improve methodology to minimize bias. Publication of lumbar interbody fusion studies needs to be promoted because differences in clinical and/or radiographic scores are more likely to be demonstrated in this part of the spine.

A novel lateral lumbar integrated plate-spacer interbody implant: in vitro biomechanical analysis

BACKGROUND CONTEXT

Lateral spacers (LSs) are the standard of care for a lateral lumbar interbody fusion. However, various types of fixation, such as bilateral pedicle screws (BPSs), unilateral pedicle screws (UPSs), bilateral facet screws (BFSs), and lateral plates (LPs) have been reported to increase the stability of LSs. The biomechanics of a novel lateral interbody implant, which is an interbody spacer with an integrated plate and two bone screws (lateral integrated plate-spacer [IPS-L]), has not been investigated yet.

PURPOSE

To compare the biomechanical stability of IPS-L and LS with and without supplemental instrumentation.

STUDY DESIGN

Human lumbar cadaveric study evaluating the biomechanical stability of an IPS-L.

METHODS

Each of the six (L2-L5) spines was sequentially tested in intact; IPS-L; IPS-L+UPS; IPS-L+BPS; IPS-L+BFS; LS+BFS; LS+UPS; LS+BPS; LS; and LS+LP, using a load-control protocol in which a ±8 Nm moment was applied, for three cycles each, in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Data results were obtained from the third cycle.

RESULTS

The IPS-L construct significantly reduced the range of motion (ROM) by 75% in FE, 70% in LB, and 57% in AR, compared with intact. Lateral integrated plate-spacer demonstrated similar biomechanical stability as LS+LP, and higher stability than the LS-alone construct, but the difference was not statistically significant.

CONCLUSIONS

The IPS-L evaluated in the present study demonstrated equivalent biomechanical stability compared with standard lateral interbody fusion constructs. The addition of BPSs to the IPS-L showed significant reduction in ROM in FE, and the addition of BFSs showed significant reduction in ROM in FE and AR, compared with the integrated plate-spacer alone construct. The IPS-L with supplemental fixation may be a viable option for lateral interbody fusion. Long-term clinical studies are further required to confirm these results.

Osteocel Plus cellular allograft in anterior cervical discectomy and fusion: evaluation of clinical and radiographic outcomes from a prospective multicenter study

STUDY DESIGN

Prospective, multicenter, nonrandomized, institutional review board-approved clinical and radiographic study.

OBJECTIVE

To evaluate and summarize the 2-year outcomes of patients treated with Osteocel Plus cellular allograft as part of an anterior cervical discectomy and fusion procedure.

SUMMARY OF BACKGROUND DATA

Osteocel Plus is an allograft cellular bone matrix containing native mesenchymal stem cells and osteoprogenitor cells that is intended to mimic the performance of iliac crest autograft without the morbidity associated with its harvest.

METHODS

A total of 182 patients were treated with anterior cervical discectomy and fusion using Osteocel Plus in a polyetheretherketone cage and anterior plating at 1 or 2 consecutive levels. Clinical outcomes included visual analogue scale for neck and arm pain, neck disability index, and SF-12 physical and mental component scores. Computed tomography and plain film radiographic measures included assessment of bridging bone, disc height, disc angle, and segmental range of motion.

RESULTS

A total of 249 levels were treated in 182 patients. Mean procedure time was 100 minutes, blood loss was less than 50 mL in 93% of patients, and hospital stay was 1 day or less in 84% of patients. Significant (P<0.05) average improvements in clinical outcomes from preoperatively to 24 months included the following: neck disability index: 21.5%; visual analogue scale neck: 34 mm; visual analogue scale arm: 35 mm; SF-12 physical component score: 11.2; SF-12 mental component score: 6.8. At 24 months, 93% of patients were satisfied with their outcome. In patients treated at a single level with a minimum of 24-month follow-up, 92% (79/86) of levels achieved solid bridging and 95% of levels demonstrated range of motion of less than 3°. In combined single- and 2-level procedures, 87% (157/180) of levels achieved solid bridging and 92% (148/161) had range of motion of less than 3° at 24 months. No patient required revision for pseudarthrosis.

CONCLUSION

Improvements in clinical results at 2 years, high patient satisfaction, and high radiographic and clinical fusion rates provide confidence in Osteocel Plus as an effective alternative to structural allograft or autograft in anterior cervical discectomy and fusion procedures.

LEVEL OF EVIDENCE

4.

Primary stiffness of a modified transforaminal lumbar interbody fusion cage with integrated screw fixation: cadaveric biomechanical study

STUDY DESIGN

In vitro biomechanical study using human fresh-frozen vertebrae.

OBJECTIVE

To investigate the influence of the additional screw fixation on the stability of a noncommercially available prototype transforaminal lumbar interbody fusion (TLIF) cage, when used as a stand-alone fusion device and in combination with pedicle screws (PSs).

SUMMARY OF BACKGROUND DATA

Generally interbody fusion cages are supplemented by additional fixation devices such as PS. However, such posterior instrumented techniques are associated with additional soft-tissue trauma and potentially increased complication rate. To limit such drawbacks, a conventional posterior TLIF cage was modified to allow supplemental screw fixation to the adjacent vertebral bodies, to increase initial stiffness and possibly allow as a stand-alone posterior interbody cage.

METHODS

Six monosegmental lumbar spine segments were loaded in a spine simulator with pure bending moments of 7.5 Nm in lateral bending, flexion/extension, and axial rotation. The following paradigms were tested: intact spines; a destabilized spine (i.e., after discectomy and unilateral facetectomy); and the modified TLIF cage with (i.e., fixed TLIF cage) and without (i.e., TLIF cage) integrated screw fixation as a stand-alone model and with and without additional posterior fixation with bilateral PS. The range of motion (RoM) was recorded by a 3-dimensional motion analysis system.

RESULTS

The TLIF cage with integrated screw fixation had minimal additional stabilizing effect in all motion planes with or without supplemental PS fixation. Moreover, compared with the intact spines, the stand-alone TLIF cage with and without integrated screw fixation did not reduce the RoM in any of the 3 motion planes. Comparison of the TLIF cage with integrated screw fixation to the TLIF cage supplemented with PS showed a significantly greater RoM in all testing conditions (P < 0.05).

CONCLUSION

In several testing paradigms, the prototype TLIF cage with the integrated screw fixation had limited effect in reducing RoM and providing stability. The PS was the main contributor in reducing RoM in the destabilized spine and remains the current "gold standard" in posterolateral spinal fixation.

LEVEL OF EVIDENCE

N/A.

Biomechanical effects of cage positions and facet fixation on initial stability of the anterior lumbar interbody fusion motion segment

STUDY DESIGN

An in vitro biomechanical study using porcine lumbar segments as specimens.

OBJECTIVE

To evaluate the effects of interbody cage support and endplate strength on the stability of instrumented segments.

SUMMARY OF BACKGROUND DATA

The anterior lumbar interbody fusion (ALIF) cage is widely used to restore disc height and support the anterior column. Transpedicle or posterior spinal fusion or facet screw fixation (FSF) can improve the stability of the vertebra-instrumented segments. The cage position can affect the anterior support and initial stability of the ALIF region, but there is no consistent data on its biomechanical effects on ALIF and ALIF/FSF segments.

METHODS

Nine variations of 3 instrumentation modes (intact, ALIF, ALIF/FSF) and 3 cage positions (type I, anterolateral; type II, mediolateral; and type III, posteromedial) are tested under 5 lumbar motions. The range of motion and axial displacement are used as comparison indices for the different variations.

RESULTS

The cage placement serves as support for the intervertebral loads while the posterior fixation behaves as lever to further enhance the anterior support. At the endplate-cage interfaces, the endplate strength directly affects the cage subsidence. Type III exhibits higher stability for standing due to the greater strength of the endplate in the posterior region. Otherwise, type I consistently has higher stability for all other types of motion.

CONCLUSION

The initial stability of the ALIF region is affected by the moment arm and the mechanical strength of the engaged endplates. Type I has greater moment arm and provides more efficient support to the instrumented segments. Endplate strength provides an ability to withstand lumbar loads and suppress the cage subsidence. Bone quality at the endplate-cage interfaces must therefore be cautiously evaluated preoperatively.

LEVEL OF EVIDENCE

N/A.

Fracture of the L-4 vertebral body after use of a stand-alone interbody fusion device in degenerative spondylolisthesis for anterior L3-4 fixation

Many studies attest to the excellent results achieved using anterior lumbar interbody fusion (ALIF) for degenerative spondylolisthesis. The purpose of this report is to document a rare instance of L-4 vertebral body fracture following use of a stand-alone interbody fusion device for L3-4 ALIF. The patient, a 55-year-old man, had suffered intractable pain of the back, right buttock, and left leg for several weeks. Initial radiographs showed Grade I degenerative spondylolisthesis, with instability in the sagittal plane (upon 15° rotation) and stenosis of central and both lateral recesses at the L3-4 level. Anterior lumbar interbody fusion of the affected vertebrae was subsequently conducted using a stand-alone cage/plate system. Postoperatively, the severity of spondylolisthesis diminished, with resolution of symptoms. However, the patient returned 2 months later with both leg weakness and back pain. Plain radiographs and CT indicated device failure due to anterior fracture of the L-4 vertebral body, and the spondylolisthesis had recurred. At this point, bilateral facetectomies were performed, with reduction/fixation of L3-4 by pedicle screws. Again, degenerative spondylolisthesis improved postsurgically and symptoms eased, with eventual healing of the vertebral body fracture. This report documents a rare instance of L-4 vertebral body fracture following use of a stand-alone device for ALIF at L3-4, likely as a consequence of angular instability in degenerative spondylolisthesis. Under such conditions, additional pedicle screw fixation is advised.

The SNAP trial: a double blind multi-center randomized controlled trial of a silicon nitride versus a PEEK cage in transforaminal lumbar interbody fusion in patients with symptomatic degenerative lumbar disc disorders: study protocol

Background

Polyetheretherketone (PEEK) cages have been widely used in the treatment of lumbar degenerative disc disorders, and show good clinical results. Still, complications such as subsidence and migration of the cage are frequently seen. A lack of osteointegration and fibrous tissues surrounding PEEK cages are held responsible. Ceramic implants made of silicon nitride show better biocompatible and osteoconductive qualities, and therefore are expected to lower complication rates and allow for better fusion.

Purpose of this study is to show that fusion with the silicon nitride cage produces non-inferior results in outcome of the Roland Morris Disability Questionnaire at all follow-up time points as compared to the same procedure with PEEK cages.

Methods/Design

This study is designed as a double blind multi-center randomized controlled trial with repeated measures analysis. 100 patients (18–75 years) presenting with symptomatic lumbar degenerative disorders unresponsive to at least 6 months of conservative treatment are included. Patients will be randomly assigned to a PEEK cage or a silicon nitride cage, and will undergo a transforaminal lumbar interbody fusion with pedicle screw fixation. Primary outcome measure is the functional improvement measured by the Roland Morris Disability Questionnaire. Secondary outcome parameters are the VAS leg, VAS back, SF-36, Likert scale, neurological outcome and radiographic assessment of fusion. After 1 year the fusion rate will be measured by radiograms and CT. Follow-up will be continued for 2 years. Patients and clinical observers who will perform the follow-up visits will be blinded for type of cage used during follow-up. Analyses of radiograms and CT will be performed independently by two experienced radiologists.

Discussion

In this study a PEEK cage will be compared with a silicon nitride cage in the treatment of symptomatic degenerative lumbar disc disorders. To our knowledge, this is the first randomized controlled trial in which the silicon nitride cage is compared with the PEEK cage in patients with symptomatic degenerative lumbar disc disorders.

Trial registration

NCT01557829

Biomechanics of lateral lumbar interbody fusion constructs with lateral and posterior plate fixation: laboratory investigation

OBJECT

Lumbar interbody fusion is indicated in the treatment of degenerative conditions. Laterally inserted interbody cages significantly decrease range of motion (ROM) compared with other cages. Supplemental fixation options such as lateral plates or spinous process plates have been shown to provide stability and to reduce morbidity. The authors of the current study investigate the in vitro stability of the interbody cage with a combination of lateral and spinous process plate fixation and compare this method to the established bilateral pedicle screw fixation technique.

METHODS

Ten L1-5 specimens were evaluated using multidirectional nondestructive moments (± 7.5 N · m), with a custom 6 degrees-of-freedom spine simulator. Intervertebral motions (ROM) were measured optoelectronically. Each spine was evaluated under the following conditions at the L3-4 level: intact; interbody cage alone (stand-alone); cage supplemented with lateral plate; cage supplemented with ipsilateral pedicle screws; cage supplemented with bilateral pedicle screws; cage supplemented with spinous process plate; and cage supplemented with a combination of lateral plate and spinous process plate. Intervertebral rotations were calculated, and ROM data were normalized to the intact ROM data.

RESULTS

The stand-alone laterally inserted interbody cage significantly reduced ROM with respect to the intact state in flexion-extension (31.6% intact ROM, p < 0.001), lateral bending (32.5%, p < 0.001), and axial rotation (69.4%, p = 0.002). Compared with the stand-alone condition, addition of a lateral plate to the interbody cage did not significantly alter the ROM in flexion-extension (p = 0.904); however, it was significantly decreased in lateral bending and axial rotation (p < 0.001). The cage supplemented with a lateral plate was not statistically different from bilateral pedicle screws in lateral bending (p = 0.579). Supplemental fixation using a spinous process plate was not significantly different from bilateral pedicle screws in flexion-extension (p = 0.476). The combination of lateral plate and spinous process plate was not statistically different from the cage supplemented with bilateral pedicle screws in all the loading modes (p ≥ 0.365).

CONCLUSIONS

A combination of lateral and spinous process plate fixation to supplement a laterally inserted interbody cage helps achieve rigidity in all motion planes similar to that achieved with bilateral pedicle screws.

Are stand-alone cages sufficient for anterior lumbar interbody fusion?

Anterior lumbar interbody fusion (ALIF) has increased in popularity because it has advantages over posterior fusion. Because there is disagreement about the stability of stand-alone cage ALIF, some surgeons use various types of supplementary fixation, including anterior plates, pedicle screw systems and translaminar screws, to increase segmental stability. Many factors associated with both the cages and endplates influence the time of onset and extent of subsidence after use of stand-alone cage ALIF. A large round cage with an adequate central opening is recommended to facilitate maximum contact with the periphery of the endplate. With regard to the relationship between radiographic fusion and recurrence of symptoms with the development of subsidence, most researchers have reported finding no correlation. Subsidence may be due to a process of bone incorporation between cages and endplates. Does subsidence or nonfusion really matter clinically? Further prospective, randomized controlled trials are very much needed to answer these questions.

Sensing movement: microsensors for body motion measurement

Recognition of body posture and motion is an important physiological function that can keep the body in balance. Man-made motion sensors have also been widely applied for a broad array of biomedical applications including diagnosis of balance disorders and evaluation of energy expenditure. This paper reviews the state-of-the-art sensing components utilized for body motion measurement. The anatomy and working principles of a natural body motion sensor, the human vestibular system, are first described. Various man-made inertial sensors are then elaborated based on their distinctive sensing mechanisms. In particular, both the conventional solid-state motion sensors and the emerging non solid-state motion sensors are depicted. With their lower cost and increased intelligence, man-made motion sensors are expected to play an increasingly important role in biomedical systems for basic research as well as clinical diagnostics.

Biomechanical analysis and review of lateral lumbar fusion constructs

STUDY DESIGN

Biomechanical study and the review of literature on lumbar interbody fusion constructs.

OBJECTIVE

To demonstrate the comparative stabilizing effects of lateral interbody fusion with various supplemental internal fixation options.

SUMMARY OF BACKGROUND DATA

Lumbar interbody fusion procedures are regularly performed using anterior, posterior, and more recently, lateral approaches. The biomechanical profile of each is determined by the extent of resection of local supportive structures, implant size and orientation, and the type of supplemental internal fixation used.

METHODS

Pure moment flexibility testing was performed using a custom-built 6 degree-of-freedom system to apply a moment of ±7.5 Nm in each motion plane, while motion segment kinematics were evaluated using an optoelectronic motion system. Constructs tested included the intact spine, stand-alone extreme lateral interbody implant, interbody implant with lateral plate, unilateral and bilateral pedicle screw fixation. These results were evaluated against those from literature-reported biomechanical studies of other lumbar interbody constructs.

RESULTS

All conditions demonstrated a statistically significant reduction in range of motion (ROM) as a percentage of intact. In flexion-extension, ROM was 31.6% stand-alone, 32.5% lateral fixation, and 20.4% and 13.0% unilateral and bilateral pedicle screw fixation, respectively. In lateral bending, the trend was similar with greater reduction with lateral fixation than in flexion-extension; ROM was 32.5% stand-alone, 15.9% lateral fixation, and 21.6% and 14.4% unilateral and bilateral pedicle screw fixation. ROM was greatest in axial rotation; 69.4% stand-alone, 53.4% lateral fixation, and 51.3% and 41.7% unilateral and bilateral pedicle screw fixation, respectively.

CONCLUSION

The extreme lateral interbody construct provided the largest stand-alone reduction in ROM compared with literature-reported ALIF and TLIF constructs. Supplemental bilateral pedicle screw-based fixation provided the overall greatest reduction in ROM, similar among all interbody approach techniques. Lateral fixation and unilateral pedicle screw fixation provided intermediate reductions in ROM. Clinically, surgeons may evaluate these comparative results to choose fixation options commensurate with the stability requirements of individual patients.

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.

Anterior lumbar interbody fusion with stand-alone interbody cage in treatment of lumbar intervertebral foraminal stenosis : comparative study of two different types of cages

OBJECTIVE

This retrospective study was performed to evaluate the clinical and radiological results of anterior lumbar interbody fusion (ALIF) using two different stand-alone cages in the treatment of lumbar intervertebral foraminal stenosis (IFS).

METHODS

A total of 28 patients who underwent ALIF at L5-S1 using stand-alone cage were studied [Stabilis(R) (Stryker, Kalamazoo, MI, USA); 13, SynFix-LR(R) (Synthes Bettlach, Switzerland); 15]. Mean follow-up period was 27.3 +/- 4.9 months. Visual analogue pain scale (VAS) and Oswestry disability index (ODI) were assessed. Radiologically, the change of disc height, intervertebral foraminal (IVF) height and width at the operated segment were measured, and fusion status was defined.

RESULTS

Final mean VAS (back and leg) and ODI scores were significantly decreased from preoperative values (5.6 +/- 2.3 --> 2.3 +/- 2.2, 6.3 +/- 3.2 --> 1.6 +/- 1.6, and 53.7 +/- 18.6 --> 28.3 +/- 13.1, respectively), which were not different between the two devices groups. In Stabilis(R) group, postoperative immediately increased disc and IVF heights (10.09 +/- 4.15 mm --> 14.99 +/- 1.73 mm, 13.00 +/- 2.44 mm --> 16.28 +/- 2.23 mm, respectively) were gradually decreased, and finally returned to preoperative value (11.29 +/- 1.67 mm, 13.59 +/- 2.01 mm, respectively). In SynFix-LR(R) group, immediately increased disc and IVF heights (9.60 +/- 2.82 mm --> 15.61 +/- 0.62 mm, 14.01 +/- 2.53 mm --> 21.27 +/- 1.93 mm, respectively) were maintained until the last follow up (13.72 +/- 1.21 mm, 17.87 +/- 2.02 mm, respectively). The changes of IVF width of each group was minimal pre- and postoperatively. Solid arthrodesis was observed in 11 patients in Stabilis group (11/13, 84.6%) and 13 in SynFix-LR(R) group (13/15, 86.7%).

CONCLUSION

ALIF using stand-alone cage could assure good clinical results in the treatment of symptomatic lumbar IFS in the mid-term follow up. A degree of subsidence at the operated segment was different depending on the device type, which was higher in Stabilis(R) group.

Minimally invasive anterior lumbar interbody fusion followed by percutaneous translaminar facet screw fixation in elderly patients

OBJECT

The purpose of this study was to determine whether anterior lumbar interbody fusion (ALIF) followed by percutaneous translaminar facet screw fixation is effective in elderly patients with degenerative spinal disease.

METHODS

Twenty-nine patients > 60 years old who underwent ALIF with percutaneous translaminar facet screw fixation from January to June 2004 were studied. The radiological and clinical data of these patients were collected and analyzed. The mean follow-up period was 14.6 months (range 12-17 months).

RESULTS

The mean preoperative, immediate postoperative, and 6- and 12-month postoperative posterior disc heights were 7.1, 11.6, 9.8, and 9.8 mm, respectively. Subsidences of posterior disc height > 20% developed in 9 patients (30%). The significant risk factor for subsidence was found to be 2-level operations (p = 0.023). The mean preoperative Oswestry Disability Index score and visual analog scale scores for the back and leg were 24.4, 6.6, and 7.5, respectively, and improved postoperatively to 14.2, 1.5, and 1.8, respectively.

CONCLUSIONS

Minimally invasive ALIF followed by percutaneous translaminar facet screw fixation was performed as a minimally invasive surgical technique in elderly patients. However, in certain circumstances such as multilevel operations or in patients with severe osteoporosis, significant cage subsidence can develop.

Interbody device endplate engagement effects on motion segment biomechanics

BACKGROUND CONTEXT

Stand-alone nonbiologic interbody fusion devices for the lumbar spine have been used for interbody fusion since the early 1990s. However, most devices lack the stability found in clinically successful circumferential fusion constructs. Stability results from cage geometry and device/vertebral endplate interface integrity. To date, there has not been a published comparative biomechanical study specifically evaluating the effects of endplate engagement of interbody devices.

PURPOSE

Lumbar motion segments implanted with three different interbody devices were tested biomechanically to compare the effects of endplate engagement on motion segment rigidity. The degree of additional effect of supplemental posterior and anterior fixation was also investigated.

STUDY DESIGN/SETTING

A cadaveric study of interbody fusion devices with varying degrees of endplate interdigitation.

OUTCOME MEASURES

Implanted motion segment range of motion (ROM), neutral zone (NZ), stiffness, and disc height.

METHODS

Eighteen human L23 and L45 motion segments were distributed into three interbody groups (n=6 each) receiving a polymeric (polyetheretherketone) interbody spacer with small ridges; a modular interbody device with endplate spikes (InFix, Abbott Spine, Austin, TX, USA); or dual tapered threaded interbody cages (LT [Lordotic tapered] cage; Medtronic, Memphis, TN, USA). Specimens were tested intact using a 7.5-Nm flexion-extension, lateral bending, and axial torsion flexibility protocol. Testing was repeated after implantation of the interbody device, anterior plate fixation, and posterior interpedicular fixation. Radiographic measurements determined changes in disc height and intervertebral lordosis. ROM and NZ were calculated and compared using analysis of variance.

RESULTS

The interbody cages with endplate spikes or threads provided a statistically greater increase in disc height versus the polymer spacer (p=.01). Relative to intact, all stand-alone devices significantly reduced ROM in lateral bending by a mean 37% to 61% (p< or =.001). The cages with endplate spikes or threads reduced ROM by approximately 50% and NZ by approximately 60% in flexion-extension (p< or =.02). Only the cage with endplate spikes provided a statistically significant reduction in axial torsion ROM compared with the intact state (50% decrease, p<.001). Posterior fixation provided a significant reduction in ROM in all directions versus the interbody device alone (p<.001). Anterior plating decreased ROM over interbody device alone in flexion-extension and torsion but did not have additional effect on lateral bending ROM.

CONCLUSION

The cages with endplate spikes or threads provide substantial motion segment rigidity compared with intact in bending modes. Only the cages with endplate spikes were more rigid than intact in torsion. All devices experienced increased rigidity with anterior plating and even greater rigidity with posterior fixation. It appears that the endplate engagement with spikes may be beneficial in limiting torsion, which is generally difficult with other "stand-alone" devices tested in the current and prior reports.

Effects of facetectomy and crosslink augmentation on motion segment flexibility in posterior lumbar interbody fusion

STUDY DESIGN

Biomechanical assessment using calf lumbar motion segments.

OBJECTIVE

To determine whether facetectomy affects the primary stability of posterior lumbar interbody fusion.

SUMMARY OF BACKGROUND DATA

To improve visualization and access to the disc space, the facet joints frequently are removed. Previous biomechanical studies have indicated a fundamental role for the facet joints in maintaining spinal segment stability.

METHODS

Single motion segments from calf lumbar spines were tested for pure-moment flexibility in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). After testing intact, an interbody cage and pedicle screw system were implanted. Next, a bilateral facetectomy was performed, and finally a crosslink was added. Flexibility testing was repeated at each stage of implantation. Data are reported for range of motion (ROM), neutral zone (NZ), and a new compliance parameter (COM), based on the slopes of the moment-angle curve in the neutral and elastic regions.

RESULTS

With posterior lumbar interbody fusion implantation, ROM in FE was reduced 82% +/- 4% (mean +/- standard deviation) and NZ 78% +/- 7% over intact (P < 0.015: Wilcoxon). Reduction in LB was slightly more, whereas reduction in AR was considerably less and did not achieve statistical significance for NZ. After facetectomy, ROM in FE increased 0.3 degrees (P < 0.05), on average, and NZ did not change. In LB neither changed significantly. In AR, ROM increased 0.6 degrees (P < 0.05), and NZ increased 0.2 degrees (P < 0.05). The addition of a crosslink changed ROM and NZ less than 0.1 degrees in FE and LB, whereas in AR it restored half of the stability lost due to facetectomy in ROM (P < 0.05), and had a similar trendwise effect on NZ. The new compliance measure, COM, was found to agree with the direction of change in ROM more consistently than did NZ.

CONCLUSION

Facetectomy causes a nominal increase in ROM and NZ in FE and LB, which are not affected by the addition of a crosslink. Although the effect of facetectomy is greater in AR-and crosslink has a measurable restoring effect-all differences are within a few tenths of a degree under this loading paradigm. Thus, the clinical utility of adding a crosslink may not be justified based on these small biomechanical changes. COM can serve as a complement to ROM and NZ, or even as a surrogate when its 2 components are reported together, as it shows strong agreement with ROM, effectively distinguishes between lax and elastic region behaviors, and provides a measure of flexibility independent of the load range.

[Influence of disc height on outcome of posterolateral fusion]

PURPOSE OF THE STUDY

Experimentally, posterolateral fusion only provides incomplete control of flexion-extension, rotation and lateral inclination forces. The stability deficit increases with increasing height of the anterior intervertebral space, which for some warrants the adjunction of an intersomatic arthrodesis in addition to the posterolateral graft. Few studies have been devoted to the impact of disc height on the outcome of posterolateral fusion. The purpose of this work was to investigate the spinal segment immobilized by the posterolateral fusion: height of the anterior intervertebral space, the clinical and radiographic impact of changes in disc height, and the short- and long-term impact of disc height measured preoperatively on clinical and radiographic outcome.

MATERIALS AND METHODS

In order to obtain a homogeneous group of patients, the series was limited to patients undergoing posterolateral arthrodesis for degenerative spondylolisthesis, in combination with radicular release. This was a retrospective analysis of a consecutive series of 66 patients with mean 52 months follow-up (range 3-63 months). A dedicated self-administered questionnaire was used to collect data on pre- and postoperative function, the SF-36 quality of life score, and patient satisfaction. Pre- and postoperative (early, one year, last follow-up) radiographic data were recorded: olisthesic level, disc height, intervertebral angle, intervertebral mobility (angular, anteroposterior), and global measures of sagittal balance (thoracic kyphosis, lumbar lordosis, T9 sagittal tilt, pelvic version, pelvic incidence, sacral slope). SpineView was used for all measures. Univariate analysis searched for correlations between variation in disc height and early postoperative function and quality of fusion at last follow-up. Multivariate analysis was applied to the following preoperative parameters: intervertebral angle, disc height, intervertebral mobility, sagittal balance parameters, use of osteosynthesis or not.

RESULTS

At the olisthesic level, there was a 30% mean decrease in disc height and intervertebral angle. These variations were not correlated with functional outcome or quality of fusion observed at last follow-up. Disc height preoperatively did not affect these variations. The only factor correlated with decreased disc height was T9 sagittal tilt: disc height decreased more when T9 sagittal tilt approached 0 degrees .

DISCUSSION

In this very restricted context (retrospective study, short arthrodesis for degenerative spondylolisthesis), we were unable to find any evidence supporting the notion that high disc height is an argument which should favor complementary intersomatic arthrodesis in combination with posterolateral fusion. Analysis of the spinal balance in the sagittal plane would probably allow a more pertinent assessment of the specific needs of individual patients.

[Biomechanical study of a ventral stand-alone cage for the lumbar spine with and without additional posterior fixation]

AIM

Biomechanical study to examine the stability of a stand-alone cage (SAC; Topaz, Ulrich Medizintechnik, Ulm) incorporating integrated anterior fixation with and without additional posterior fixation.

METHOD

Six human spinal specimens (L4/5) were loaded in a spine tester in the three main motion planes in the following states: (a) intact, (b) defect (nucleotomy), (c) SAC, (d) SAC+internal fixator (IF), and (e) SAC+translaminar facet screws (TFS). Facet joint translation (FJT) and range of motion (ROM) were measured and used to evaluate the stability of the tested states.

RESULTS

The SAC stabilized the segment in comparison to the intact (a) and defect-containing (b) segments. The most rigid fixation was found for SAC+IF compared with the other states (a, b, c, e). The ROM and FJT of the SAC with the additional IF (d) showed a significant higher stability in all three motion planes. All differences in ROM and FJT between the tested states were statistically significant (p<0.05) except for the FJT SAC and SAC+TFS.

CONCLUSION

The ventral Topaz SAC is a stable implant for the lumbar spine. Additional dorsal stabilization is an option to increase the stability.

Tapered cages in anterior lumbar interbody fusion: biomechanics of segmental reactions

Object

The aim of this study was to determine the in vitro biomechanical responses of lumbar spinal segments after implantation of tapered cages.

Methods

Range of motion (ROM)– and stiffness-related data were determined in 10 human cadaveric T12–S1 columns subjected to flexion, extension, and lateral bending modes before and after anterior lumbar interbody fusion in which stand-alone LT-CAGE devices were used. The overall column showed no significant changes in ROM or stiffness. At the instrumented level, stiffness increased significantly (p < 0.05) in flexion and lateral bending modes. Indications of instability in extension were present, but these values were not statistically significant. There was no evidence of adjacent-level instability at any level in any mode, except for the segment superior to the fixation level in flexion; here there was a significant increase in ROM (p < 0.05) and a decrease in stiffness.

Conclusions

The anatomical conformity and bilateral placement of cages provide ample stability and rigidity at the treated level, comparable to that of other cage systems. Because hypermobility is traditionally related to early degenerative changes, the present results appear to suggest that cages do not significantly contribute to such alterations.

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.

Primary spinal segment stability with a stand-alone cage: in vitro evaluation of a successful goat model

BACKGROUND

Interbody cages have been developed to restore disk height and to increase stability of the spinal segment, and thereby enhance fusion. However, they often prove inadequate as a stand-alone device. It is unknown how much primary stability is required to facilitate fusion. In various goat studies, we have obtained spinal fusion routinely with a stand-alone cage device. However, data covering the mechanical conditions under which these fusions have been obtained are lacking. In this study, we addressed the issue of primary stability.

METHODS

We used an established goat model for spinal fusion in vitro. 48 native lumbar spine segments were mechanically tested in flexion/extension, axial torsion (left/right), anterior/posterior shear, and left/right lateral bending. Then all segments were provided with a titanium cage using the exact surgical procedure of our earlier in vivo studies, and the mechanical tests were repeated. Under shear force and axial torsion, a significant loss of stiffness was seen in the operated segments as compared to nonoperated controls. No increase in stiffness was found in any of the loading directions.

INTERPRETATION

Cage implantation in a lumbar spinal segment does not increase immediate postoperative stability as compared to the native segment in this goat model. This is attributable to both the annular damage during cage implantation and the subsequent loss of segment height. Yet previous in vivo studies using this goat model have generally shown fusion. This implies that high primary segment stability is not required for fusion or, alternatively, that the tested range of motion of the spinal segment in vitro does not occur at these magnitudes in vivo.

Biomechanical assessment of anterior lumbar interbody fusion with an anterior lumbosacral fixation screw-plate: comparison to stand-alone anterior lumbar interbody fusion and anterior lumbar interbody fusion with pedicle screws in an unstable human cadaver model

STUDY DESIGN

Human lumbosacral cadaveric specimens were tested in an in vitro biomechanical flexibility experiment using physiologic loads in 5 sequential conditions.

OBJECTIVE

To determine the biomechanical differences between anterior lumbar interbody fusion (ALIF) using cylindrical threaded cages alone or supplemented with an anterior screw-plate or posterior pedicle screws-rods.

SUMMARY OF BACKGROUND DATA

Clinically and biomechanically, stand-alone ALIF performs modestly in immobilizing the unstable spine. Pedicle screws improve fixation stiffness significantly, but supplementary anterior instrumentation has not been studied.

METHODS

There were 7 specimens tested: (1) intact, (2) after discectomy and facetectomy to induce moderate rotational and translational hypermobility, (3) with 2 parallel ALIF cages, (4) with cages plus a triangular anterior screw-plate, and (5) with cages plus pedicle screws-rods. Pure moments without preload induced flexion, extension, lateral bending, and axial rotation; linear shear forces induced anteroposterior translation. Angular and linear motions were measured stereophotogrammetrically, and range of motion (ROM) and stiffness were quantified.

RESULTS

Compared to the destabilized spine, interbody cages alone reduced ROM by 77% during flexion, 53% during extension, 60% during lateral bending, 69% during axial rotation, and 71% during anteroposterior shear (P < 0.001, analysis of variance/Fisher least significant difference). Addition of an anterior plate or pedicle screws-rods, respectively, further reduced ROM by 8% or 13% during flexion (P = 0.21), 21% or 28% during extension (P = 0.15), 5% or 25% during lateral bending (P = 0.04), 11% or 18% during axial rotation (P = 0.13), and 18% or 18% during anteroposterior shear (P = 0.17). Compared to stand-alone ALIF, both the anterior screw-plate and pedicle screw-rod fixation reduced vertebral ROM to less than 1.2 degrees of rotation and less than 0.1 mm of translation.

CONCLUSIONS

The anterior screw-plate and pedicle screws-rods both substantially reduced ROM and increased stiffness compared to stand-alone interbody cages. There was no significant difference in the amount by which the supplementary fixation devices limited flexion, extension, axial rotation, or anteroposterior shear; pedicle screws-rods better restricted lateral bending.

Double-segment Wilhelm Tell technique for anterior lumbar interbody fusion in unstable isthmic spondylolisthesis and adjacent segment discopathy

The Wilhelm Tell technique is a novel instrumented anterior lumbar interbody fusion (ALIF) procedure using a specially designed composite carbon fibre cage and a single short-threaded cancellous screw that obliquely passes through the upper adjacent vertebral body, the interbody cage itself and through the lower adjacent vertebral body. This single-stage fusion method, which is in principle a combination of the Louis technique and modern cage surgery, is reported to have a lower rate of pseudoarthrosis formation than stand-alone cage techniques. In addition, it eliminates both the surgical trauma of paravertebral muscle retraction and the risk of neural damage by poorly located pedicular screws. This anterior approach allows decompression of neural structures within the anterior part of the spinal canal and the foraminal region. It is the purpose of this case report, to present the successful application of this novel technique in a 32-year-old woman who concurrently suffered from severe instability-related back pain from L4/5 isthmic spondylolisthesis and marked L5/S1 degenerative disc disease.

Interkorporelle Metallimplantate („Cages“) bei lumbalen Spondylodesen

Over the last 15 years, interbody metal implants have become commonly used worldwide for lumbar interbody fusion. The so called "cages" are made of metal or absorbable materials. By using different surgical techniques, they can be implanted either regularly or via endoscopy. The published results on surgical techniques using cages for the lumbar spine show, in most cases and with or without additional instrumentation, rates of fusion of more than 90%. It seems that the use of osteoinductive substances (especially BMP) leads to even better results. Dorsoventral fusion with internal fixation and bone show the same rate of consolidation, but the advantages of cages are primarily in the maintenance of the distraction and the possibility of a single surgical procedure without additional instrumentation (including endoscopy), and in a lower donor side morbidity.

The in vitro stabilising effect of polyetheretherketone cages versus a titanium cage of similar design for anterior lumbar interbody fusion

This biomechanical study was performed to test the primary segmental in vitro stabilising effect of a standard and large footprint radiolucent poly-ether-ether-ketone (PEEK) box cage versus a titanium box cage for anterior lumbar interbody fusion. Eighteen L2-L3 and sixteen L4-L5 cadaveric motion segments were divided into three groups and received a titanium cage or a radiolucent PEEK cage with standard or large footprint. All specimens were tested in three testing conditions: intact, stand-alone anterior cage and finally with supplemental translaminar screw fixation. Full range of motion and neutral zone measurements were determined and anterior cage pull out force was tested. The titanium design was significantly more effective in reducing the range of motion only in axial rotation. The larger footprint radiolucent cage did not increase stability as compared to the standard footprint. The titanium cage pull out force was significantly (P=0.0002) higher compared to both radiolucent cage constructs.

CLINICAL RELEVANCE

Supplemental posterior fixation is strongly recommended to increase initial stability of any anterior interbody fusion cage construct. Although the biomechanical stability necessary to achieve spinal fusion is not defined, the radiolucent designs tested in this study, with a standard footprint as well as with a larger footprint, may be insufficiently stabilised with translaminar screws as compared to the titanium implant. Supplemental pedicle screw fixation may be required to obtain adequate stabilisation in the clinical setting.

Biomechanical testing of the lumbar facet interference screw

STUDY DESIGN

An in vitro study was conducted to determine the biomechanical properties of a new simple, percutaneous, posterior fixation technique for the lumbar spine involving a new implant, the so-called Lumbar Facet Interference Screw.

OBJECTIVES

The purpose of this study was to compare the biomechanical properties of this new fixation device with translaminar and pedicle screw fixation.

SUMMARY OF BACKGROUND DATA

Several techniques were described to perform a minimal invasive posterior stabilization of the lumbar spine after an anterior lumbar interbody fusion procedure. Yet, due to the high complexity of these minimally invasive surgical procedures, currently, hardly any of these percutaneous posterior fixation techniques is carried out routinely.

METHODS

Ten human lumbar spines were tested in flexion, extension, axial rotation, and lateral bending using a nonconstrained testing method. First, all motion segments were evaluated intact (group 1). After complete discectomy of L4-L5, the following stabilization techniques were tested sequentially (n = 10/group): group 2: "stand-alone" cage; group 3: cage plus translaminar screws; group 4: cage plus Lumbar Facet Interference Screw; and group 5: cage plus pedicle screws. Stiffness, ranges of motion, and neutral and elastic zones were determined.

RESULTS

In comparison to the intact motion segment, the "stand-alone" cage showed a significantly higher (P < 0.05) range of motion, neutral zone, and elastic zone and a significantly lower (P < 0.05) stiffness in extension and rotation. Generally, all fixation techniques using cages plus posterior stabilization decreased range of motion, neutral zone, and elastic zone and increased stiffness in comparison to the "stand-alone" cage group. There was no significant difference between the cage plus interference screw and the cage plus translaminar screw group in all test modes. In comparison to the 2 facet joint stabilization techniques, pedicle screw stabilization decreased (P < 0.01) range of motion, neutral zone, and elastic zone and increased (P < 0.01) stiffness significantly in flexion and rotation.

CONCLUSIONS

Results of this study indicate that the new Lumbar Facet Interference Screw fixation yields initial biomechanical stability similar to translaminar screw fixation, yet inferior biomechanical stability compared to pedicle screw fixation. Although these results are encouraging, additional biomechanical studies including cyclic loading tests have to evaluate the mid- and long-term stabilization capacity of this new minimally invasive fixation technique before human application.

Biomechanical comparison: stability of lateral-approach anterior lumbar interbody fusion and lateral fixation compared with anterior-approach anterior lumbar interbody fusion and posterior fixation in the lower lumbar spine

Object. The stability of lateral lumbar interbody graft—augmented fusion and supplementary lateral plate fixation in human cadavers has not been determined. The purpose of this study was to investigate the immediate biomechanical stabilities of the following: 1) femoral ring allograft (FRA)—augmented anterior lumbar interbody fusion (ALIF) after left lateral discectomy combined with additional lateral MACS HMA plate and screw fixation; and 2) ALIF combined with posterior transpedicular fixation after anterior discectomy.

Methods. Sixteen human lumbosacral spines were loaded with six modes of motion. The intervertebral motion was measured using a video-based motion-capturing system. The range of motion (ROM) and the neutral zone (NZ) in each loading mode were compared with a maximum of 7.5 Nm.

The ROM values for both stand-alone ALIF approaches were similar to those of the intact spine, whereas NZ measurements were higher in most loading modes. No significant intergroup differences were found. The ROM and NZ values for lateral fixation in all modes were significantly lower than those of intact spine, except when NZ was measured in lateral bending. All ROM and NZ values for transpedicular fixation were significantly lower than those for stand-alone anterior ALIF. Transpedicular fixation conferred better stabilization than lateral fixation in flexion, extension, and lateral bending modes.

Conclusions. Neither approach to stand-alone FRA-augmented ALIF provided sufficient stabilization, but supplementary instrumentation conferred significant stabilization. The MACS HMA plate and screw fixation system, although inferior to posterior transpedicular fixation, provided adequate stability compared with the intact spine and can serve as a sound alternative to supplementary spinal stabilization.

A two-cage reconstruction versus a single mega-cage reconstruction for lumbar interbody fusion: an experimental comparison

BACKGROUND

Interbody cages are used as an adjunct to anterior lumbar interbody fusion, but exposure and insertion of two cages can be difficult. A biomechanical study was performed to compare the stability and exposed surface for fusion obtained with interbody reconstruction using two traditional cylindrical cages (18-mm diameter) vs. a single expanded mega-cage (24-mm diameter). A single-cage technique could result in safer exposure, shorter operating time, and less cost.

METHODS

STUDY DESIGN

nondestructive testing of L5-S1 motion segments with cages compared the two configurations, and direct measure of the size of the fusion bed was made.

PATIENT SAMPLE

16 human cadaveric lumbar motion segments.

OUTCOME MEASURES

significant differences in motion segment stiffness and cancellous surface areas were compared using a Wilcoxon rank sum test. Motion segments were biomechanically tested intact, and then tested again after insertion of two interbody cages (n=8) or a single mega-cage (n=8). Nondestructive biomechanical loading was performed consisting of: (1) compression (maximum load 900 N); (2) Flexion, extension, left and right lateral bending (maximum moment 18 Nm); and (3) left and right torsion (maximum moment 10 Nm). From the load-deformation curves obtained, stiffness values were calculated to compare the two-cage and the single mega-cage reconstructions. After testing, the specimens were disarticulated and the surface area of the endplate bed created in the cancellous bone (of both vertebrae) was measured to compare the potential vascular surface for osteogenesis with both constructs.

RESULTS

The averages of the normalized values of stiffness were significantly greater for the two-cage group as compared to the mega-cage group in flexion only (1.08 vs. 0.74, p<0.05). For extension, torsion and lateral bending there was no significant difference in stiffness. In compression the two-cage group was stiffer, although not significantly (0.92 vs. 0.68, p<0.07). The average cancellous bed surface area was slightly greater for the single-cage reconstruction (1,208 mm(2) vs. 1,155 mm(2)), although this difference was not significant.

CONCLUSIONS

The stiffness with a single anterior mega-cage was significantly lower in flexion compared with two standard cages. However, in all other modes of testing the constructs were statistically equivalent, although neither construct was significantly stiffer than the intact specimen. Additionally, the single mega-cage provides an equivalent cancellous bed for fusion as compared to dual cages. While this study is not sufficient to recommend human application, these results and our previous experience with the successful in vivo use of a single cage in rhesus monkeys [4] suggest that the single expanded anterior cage may be an acceptable concept although subsidence risk needs further investigation. The potential advantages of a single mega-cage (safer for the foramen, safer for the vessels, more consistent decortication and possibly cheaper) further suggest that examination should be given to this method as an approach to anterior interbody reconstruction in selected patients.

A biomechanical comparison of supplementary posterior translaminar facet and transfacetopedicular screw fixation after anterior lumbar interbody fusion

Object. Facet screw fixation is the lowest profile lumbar stabilization method. In this study the immediate biomechanical stability provided by the two different types of fixation are compared: translaminar facet screw (TLFS) and transfacetopedicular screw (TFPS) placement after anterior lumbar interbody fusion (ALIF) using a femoral ring allograft. Both facet screw fixation types were also compared with the gold standard, transpedicular screw and rod (TSR) fixation.

Methods. Twenty-four human lumbosacral spines were tested in the following sequence: intact state, after discectomy, after ALIF, and after TLFS, TFPS, or TSR fixation. Intervertebral motions were measured by a video-based motion capture system. The range of motion (ROM) and neutral zone (NZ) were compared for each loading to a maximum of 7.5 Nm.

The ROMs for stand-alone ALIFs were less than but similar to those of the intact spine, but NZs were slightly increased in all modes. The ROMs for both TLFS and TFPS fixation were significantly decreased from those of the intact spine in all modes and those of the stand-alone ALIF in flexion and extension. The TLFS and TFPS fixations significantly reduced NZs to below that of the intact spine in all modes. Compared with NZs for ALIF, both types of fixation revealed significantly lower values, except for TLFS placement in lateral bending and TFPS fixation in lateral bending and rotation. There were no significant differences between TLFS and TFPS fixation. There were also no significant differences among both TLFS and TFPS and TSR fixations, except that TFPS was inferior to TSR in lateral bending.

Conclusions. Stand-alone ALIF may not provide sufficient stability. Both facet fixations produced significant additional stability and both are comparable to TSR fixation. Although TFPS fixation revealed a slightly inferior result, TFPSs can be placed percutaneously with the assistance of fluoroscopic guidance and it makes the posterior facet fixation minimally invasive. Therefore, the TFPS fixation can be considered as a good alternative to TLFS fixation.

Biomechanical comparison of anterolateral plate, lateral plate, and pedicle screws-rods for enhancing anterolateral lumbar interbody cage stabilization

STUDY DESIGN

A repeated measures in vitro flexibility experiment was performed in calf spines.

OBJECTIVES

To determine the biomechanical differences among three techniques for augmenting stability of an anterolateral lumbar threaded interbody cage.

BACKGROUND

Stand-alone interbody cages are known to inadequately stabilize the spine. Surgeons often add supplementary instrumentation for a more stable construct.

METHODS

Six L2-L5 calf spines (L3-L4 level instrumented) were tested: 1) intact; 2) with a single anterolateral interbody cage; 3) with cage plus anterolateral plating; 4) with cage plus lateral plating; and 5) with cage plus pedicle screw fixation. Specimens were loaded in each anatomic plane quasistatically (maximum 5.0 Nm). Angular motion was measured stereophotogrammetrically.

RESULTS

The stand-alone interbody cage allowed significantly less range of motion than normal during all loading modes except axial rotation. Addition of pedicle screws-rods, anterolateral plate, or lateral plate significantly further reduced range of motion in all planes. Pedicle screws slightly outperformed the anterolateral plate during extension and lateral bending and slightly outperformed the lateral plate during flexion, extension, and left axial rotation (range of motion differences <0.65 degrees, P < 0.05). The anterolateral plate outperformed the lateral plate during flexion and extension, whereas the lateral plate outperformed the anterolateral plate during lateral bending (range of motion difference <0.57 degrees, P < 0.05).

CONCLUSION

Anterolateral or lateral lumbar plating increases stability significantly compared to stand-alone interbody cage fixation. These findings support anterolateral or lateral plate fixation as a potential clinical alternative to pedicle screws-rods in this role and may obviate the need for combined anterior and posterior approaches when spinal instability exists.