In vitro stability of FRA spacers with integrated crossed screws for anterior lumbar interbody fusion

Kinematics Following 3-Screw Integrated Interbody Spacers in the Lumbar Spine

BACKGROUND

"Stand-alone" fusion implants attempt to alleviate the need for supplemental posterior instrumentation.

OBJECTIVE

A biomechanical study was conducted to assess the stability of an integrated 3- screw interbody cage with, and without, supplemental posterior fixation.

METHODS

Nondestructive biomechanical testing was performed on 19 healthy cadaver spine segments. Specimens were tested in 6 degrees of motion and a maximum pure bending moment of 10 Nm was applied. Specimens were evaluated in the following sequence: Intact, cage, cage ± facet bolts, and cage ± pedicle screws. Nonconstrained motion was measured at both the index and adjacent levels.

RESULTS

The index levels were L2-L3 and L5-S1. The cage alone provided a significant decrease in motion at the L2-L3 level but not at L5-S1. At L2-L3, cage + pedicle screws decreased motion more effectively than cage + facet bolts, however, both the supplemented constructs outperformed intact (P < .05). At L5-S1, both posterior fixation systems appeared to have smaller degree of displacement compared to intact; however, no significant differences were observed at L5-S1 among the various constructs. Furthermore, the adjacent segments for each level (L1-L2 and L4-L5) had no significantly increased motion, compared to intact, for all 6 degrees of motion tested.

CONCLUSION

The stand-alone cage was more effective at L2-L3, than at L5-S1, in limiting motion. At L5-S1, supplemental fixation may need to be considered. No abnormal motion was identified at the adjacent, normal segments, for the stand-alone, or the circumferential constructs at either level tested.

Stand-alone anterior interbody fusion for substitution of iliac fixation in long spinal fixation constructs

INTRODUCTION

The use of distal sacral anchorage solely, in long spinal fusions, may lead to substantial complications. Extending the fixation down to the ilium and the addition of anterior column support are both used to facilitate construct stability and improve fusion rates. In the current study, we aimed to determine whether supplementation of long thoracolumbar fixation constructs with stand-alone anterior interbody fusion (ALIF) cage with embedded screws can eliminate the biomechanical need for iliac screws fixation biomechanically.

METHODS

Seven lumbopelvic human cadavers (L1-full pelvis) were used. All specimens were tested with the following fixation constructs: bilateral L1-S1, bilateral L1-S1 with unilateral iliac screw, and bilateral L1-S1 with bilateral iliac screw. The three constructs were tested with and without the addition of stand-alone ALIF cage. We evaluated the multidirectional rigidity and the axial S1 screw strain.

RESULTS

The addition of an ALIF cage solely did not affect rigidity and resulted in mixed S1 screw strain results. One iliac screw was superior to ALIF in rigidity and inferior in S1 screws strain. Bilateral iliac fixation produced similar rigidity and lower S1 screws strain than unilateral iliac fixation. When ALIF was combined with bilateral iliac screws, it resulted in equal rigidity and lower S1 screws strain.

CONCLUSION

Our results do not support stand-alone ALIF cage as a substitute for iliac fixation in in long posterior lumbosacral fusion. They do support the use of stand-alone ALIF for the supplementation of bilateral iliac fixation in long lumbosacral fusions.

Evaluation of Two Novel Integrated Stand-Alone Spacer Designs Compared with Anterior and Anterior-Posterior Single-Level Lumbar Fusion Techniques: An In Vitro Biomechanical Investigation

Study Design

In vitro biomechanical investigation.

Purpose

To compare the biomechanics of integrated three-screw and four-screw anterior interbody spacer devices and traditional techniques for treatment of degenerative disc disease.

Overview of Literature

Biomechanical literature describes investigations of operative techniques and integrated devices with four dual-stacked, diverging interbody screws; four alternating, converging screws through a polyether-ether-ketone (PEEK) spacer; and four converging screws threaded within the PEEK spacer. Conflicting reports on the stability of stand-alone devices and the influence of device design on biomechanics warrant investigation.

Methods

Fourteen cadaveric lumbar spines were divided randomly into two equal groups (n=7). Each spine was tested intact, after discectomy (injured), and with PEEK interbody spacer alone (S), anterior lumbar plate and spacer (AP+S), bilateral pedicle screws and spacer (BPS+S), circumferential fixation with spacer and anterior lumbar plate supplemented with BPS, and three-screw (SA3s) or four-screw (SA4s) integrated spacers. Constructs were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Researchers performed one-way analysis of variance and independent t-testing (p≤0.05).

Results

Instrumented constructs showed significantly decreased motion compared with intact except the spacer-alone construct in FE and AR (p≤0.05). SA3s showed significantly decreased range of motion (ROM) compared with AP+S in LB (p≤0.05) and comparable ROM in FE and AR. The three-screw design increased stability in FE and LB with no significant differences between integrated spacers or between integrated spacers and BPS+S in all loading modes.

Conclusions

Integrated spacers provided fixation statistically equivalent to traditional techniques. Comparison of three-screw and four-screw integrated anterior lumbar interbody fusion spacers revealed no significant differences, but the longer, larger-diameter interbody spacer with three-screw design increased stabilization in FE and LB; the diverging four-screw design showed marginal improvement during AR.

Biomechanical evaluation of an integrated fixation cage during fatigue loading: a human cadaver study

OBJECTIVE

Lumbar cages with integrated fixation screws offer a low-profile alternative to a standard cage with anterior supplemental fixation. However, the mechanical stability of integrated fixation cages (IFCs) compared with a cage with anterior plate fixation under fatigue loading has not been investigated. The purpose of this study was to compare the biomechanical stability of a screw-based IFC with a standard cage coupled with that of an anterior plate under fatigue loading.

METHODS

Eighteen functional spinal units were implanted with either a 4-screw IFC or an anterior plate and cage (AP+C) without integrated fixation. Flexibility testing was conducted in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) on intact spines, immediately after device implantation, and post-fatigue up to 20,000 cycles of FE loading. Stability parameters such as range of motion (ROM) and lax zone (LZ) for each loading mode were compared between the 2 constructs at multiple stages of testing. In addition, construct loosening was quantified by subtracting post-instrumentation ROM from post-fatigue ROM.

RESULTS

IFC and AP+C configurations exhibited similar stability (ROM and LZ) at every stage of testing in FE (p ≥ 0.33) and LB (p ≥ 0.23) motions. In AR, however, IFCs had decreased ROM compared with AP+C constructs at pre-fatigue (p = 0.07) and at all post-fatigue time points (p ≤ 0.05). LZ followed a trend similar to that of ROM in AR. ROM increased toward intact motion during fatigue cycling for AP+C and IFC implants. IFC specimens remained significantly (p < 0.01) more rigid than specimens in the intact condition during fatigue for each loading mode, whereas AP+C construct motion did not differ significantly (p ≥ 0.37) in FE and LB and was significantly greater (p < 0.01) in AR motion compared with intact specimens after fatigue. Weak to moderate correlations (R2 ≤ 56%) were observed between T-scores and construct loosening, with lower T-scores leading to decreased stability after fatigue testing.

CONCLUSIONS

These data indicate that a 4-screw IFC design provides fixation similar to that provided by an AP+C construct in FE and LB during fatigue testing and better stability in AR motion.

Computed tomography-based three-dimensional visualisation of bone corridors and trajectories for screws in open reduction and internal fixation of symphysis diastasis: a retrospective radiological study

INTRODUCTION

Typical stabilisation of pelvic open book injuries consists of plate fixation of the symphysis. No previous literature has been published about the evaluation of screw placement and their trajectory with four oblique 4.5 mm screws using a four-hole plate in symphysis diastasis. The aim of this study was to define insertion points and angles of trajectory for crossed screw placement regardless of any plate design based on an analysis of three-dimensional computed tomography data sets.

METHODS

One hundred human pelvic CT data sets were collected. Unilateral and bilateral placements of crossed 4.5 mm screws were simulated. Primary outcome measure was successful simulated screw placement without cortical breach. Secondary outcome measures included the anatomical measurements of the screw positions.

RESULTS

Simulated screw placement of two oblique screws on each side of the pubic symphysis without cortical breach was achieved in all (100 %) cases. There were a total of 400 screw simulations. Medial screws were longer, lateral screws had higher coronal angles, and the distance between both screws was higher on the right side (p < 0.001 each). The lengths of the right lateral, right medial, left lateral, and left medial screws were 44.9, 65.8, 45.4, and 67.4 mm, respectively. The sagittal angles to the dorsal surface area of the pubic rami were 10.5°, 11.1°, 9.0°, and 11.0°. The coronal angles to the vertical axis of the symphysis measured 39.5°, 16.0°, 33.8°, and 16.8°. The distances between these screws and the medial edge of the pubic crest were 33.5, 8.6, 29.5, and 7.3 mm. Furthermore, certain sex- and side-related differences were noted.

CONCLUSIONS

This series provides results about the feasibility and a detailed anatomical description of crossed screw placement. This is of special interest in pelvic surgery for choosing the entry points, safe screw channel parameters, and trajectories.

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 CIBOR spine interbody fusion device

BACKGROUND

The CIBOR PEEK spinal interbody fusion device is an anterior lumbar interbody fusion construct with a hollow center designed to accommodate an osteoinductive carbon foam insert to promote bony ingrowth to induce fusion where rigid stabilization is needed.

METHODS

Three different sizes of the device were investigated. Part-I: implants were tested under axial compression and rotation using polyurethane foam blocks. Part-II: simulated 2-legged stance using cadaveric specimen using the L5-S1 lumbar spine segment. Part-III: a survey feedback form was used to investigate two orthopedic surgeons concern regarding the implant.

RESULTS

In Part-I, the subsidence hysteresis under axial compression loading was found to be statistical significant difference between these three implant sizes. It was noted that the implants had migration as rotation applied, and the amount of subsidence was a factor of the axial compression loads applied. In Part-II, a minor subsidence and carbon foam debris were observed when compared to each implant size. Poor contact surface of the implant with the end plates of the L5 or S1 vertebrae from the anterior view under maximum loads was observed; however, the implant seemed to be stable. Each surgeon has their own subjective opinion about the CIBOR implant.

DISCUSSION

Two out of the three different sizes of the device (medium and large sizes) provided appropriate rigid stabilization at the physiological loads. Neither orthopedic surgeon was 100% satisfied with overall performance of the implant, but felt potential improvement could be made.

CLINICAL RELEVANCE

This study indicates an option for operative treatment of spine interbody fusion, as the CIBOR spine interbody fusion device has a hollow center. This hollow center is designed to accommodate a carbon foam insert to promote bony ingrowth. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1157-1168, 2017.

Biomechanical characteristics of an integrated lumbar interbody fusion device

Introduction

We hypothesized that an Integrated Lumbar Interbody Fusion Device (PILLAR SA, Orthofix, Lewisville, TX) will function biomechanically similar to a traditional anterior interbody spacer (PILLAR AL, Orthofix, Lewisville, TX) plus posterior instrumentation (FIREBIRD, Orthofix, Lewisville, TX). Purpose of this study was to determine if an Integrated Interbody Fusion Device (PILLAR SA) can stabilize single motion segments as well as an anterior interbody spacer (PILLAR AL) + pedicle screw construct (FIREBIRD).

Methods

Eight cadaveric lumbar spines (age: 43.9±4.3 years) were used. Each specimen's range of motion was tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) under intact condition, after L4-L5 PILLAR SA with intervertebral screws and after L4-L5 360° fusion (PILLAR AL + Pedicle Screws and rods (FIREBIRD). Each specimen was tested in flexion (8Nm) and extension (6Nm) without preload (0 N) and under 400N of preload, in lateral bending (±6 Nm) and axial rotation (±5 Nm) without preload.

Results

Integrated fusion using the PILLAR SA device demonstrated statistically significant reductions in range of motion of the L4-L5 motion segment as compared to the intact condition for each test direction. PILLAR SA reduced ROM from 8.9±1.9 to 2.9±1.1° in FE with 400N follower preload (67.4%), 8.0±1.7 to 2.5±1.1° in LB, and 2.2±1.2 to 0.7±0.3° in AR. A comparison between the PILLAR SA integrated fusion device versus 360° fusion construct with spacer and bilateral pedicle screws was statistically significant in FE and LB. The 360° fusion yielded motion of 1.0±0.5° in FE, 1.0±0.8° in LB (p0.05).

Conclusions

The PILLAR SA resulted in motions of less than 3° in all modes of motion and was not as motion restricting as the traditional 360° using bilateral pedicle screws. The residual segmental motions compare very favorably with published biomechanical studies of other interbody integrated fusion devices.

Radiological evaluation of anterior lumbar fusion using PEEK cages with adjacent vertebral autograft in spinal deformity long fusion surgeries

PURPOSE

The aim of this study was to evaluate the radiographic characteristics of polyetheretherketone (PEEK) cages packed with adjacent vertebral autograft material in lumbar anterior lumbar interbody fusion (ALIF) in spinal deformity long fusion surgeries.

METHODS

This is a retrospective radiographic study. From April 2008 to April 2012, 40 patients (5 males and 35 females, mean age 67 ± 9 years) with coronal and/or sagittal spine deformities underwent staged corrective surgery combined with lumbar ALIF using PEEK cages at the L3-L4, L4-L5 or L5-S1 segment with posterior long (≥ 4 levels) instrumentation. The mean follow-up time was 27.5 months (13-49 months). We examined the interbody fusion rate and cage subsidence at 3 months postoperatively and final follow-up. Additionally, we evaluated the distance of cage migration at final follow-up and the improvement in lumbar lordosis. The rate of "collapse" of the adjacent vertebra where the autograft was harvested was assessed at the final follow-up. Finally, we examined the cage-related postoperative complications in this series.

RESULTS

Solid interbody fusion was achieved in 96.4 % (81/84) of the levels at the final follow-up. A mild forward cage migration was observed, and the mean migration distance at final follow-up was 0.83 mm in L3/4, 0.36 mm in L4/5 and 0.55 mm in L5/S1. There was cage subsidence observed in 8.3 % (7/84) of the levels. In all patients, the PEEK cage maintained a significant increase in segmental lordosis at all postoperative visits. However, a mild reduction in segmental lordosis still occurred with time. The adjacent lumbar vertebral bodies where the autografts were harvested appeared to be intact in height radiologically at the final follow-up. There were no postoperative complications due to bone harvesting or cage insertion. Proximal junctional kyphosis was found in one patient who underwent a subsequent revision surgery.

CONCLUSIONS

The use of lumbar ALIF with PEEK cages and adjacent vertebral autografts in spinal deformity long fusion surgeries is an effective and safe procedure. The allograft filler is safe and effective in maintaining the shape of harvested vertebrae. Additional long-term follow-up studies are needed to further justify its use.

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

Background

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

Methods

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

Results

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

Conclusions

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

Biomechanical evaluation of stand-alone lumbar polyether-ether-ketone interbody cage with integrated screws

BACKGROUND CONTEXT

Stand-alone interbody cages with integrated screws potentially provide a biomechanically stable solution for anterior lumbar interbody fusion (ALIF) that alleviates the need for additional exposure for supplemental fixation, thereby reducing the chance of additional complications and morbidity.

PURPOSE

To compare the stability of a stand-alone anterior interbody fusion system with integrated fixation screws against traditional supplemental fixation methods and to evaluate the difference between three and four fixation screws in the stand-alone cage.

STUDY DESIGN

In vitro cadaveric biomechanical study.

METHODS

Eight cadaveric lumbar spines (L2-sacrum) were tested using a flexibility protocol consisting of three cycles to ±7.5 Nm in flexion-extension, lateral bending, and axial rotation. The conditions evaluated were intact spine; polyether-ether-ketone cage (zero integrated screws) at L4-L5; cage (zero screws)+bilateral pedicle screws (PS); cage (three screws); cage (four screws); cage (zero screws)+anterior plate; and cage (three screws)+spinous process plate. Motion at the index level was assessed using an optoelectronic system.

RESULTS

The cage without integrated screws reduced the motion in flexion-extension and lateral bending (p<.001) compared with that in the intact spine. In axial rotation, mean range of motion (ROM) was 8% greater than in intact spine (p>.962). The addition of three integrated screws reduced ROM significantly compared with the cage without screws in all motion planes (p<.001). A fourth screw had no statistically significant effect on the ROM, although there was a trend toward less motion with four screws compared with three. In flexion-extension, the cage with three integrated screws and the spinous process plate was the most rigid condition. There was no significant difference from the bilateral PS (p=.537); however, this was more rigid than all other conditions (p<.024). The most stable condition in lateral bending and axial rotation was the cage with bilateral PS. In lateral bending, the cage (three or four screws) was not significantly different from the cage with anterior plate or the cage (three screws) with spinous process plate fixation; however, only the latter condition was statistically comparable with bilateral PS. In axial rotation, there were no significant differences between the conditions that included integrated screws or supplemental fixation (p>.081).

CONCLUSIONS

Biomechanical testing revealed that the stand-alone cage with integrated screws provides more immediate stability than a cage alone and provides equivalent stability to ALIF constructs with supplemental fixation in lateral bending and axial rotation. Additional flexion-extension rigidity of the anterior cage maybe realized by the addition of a spinous process plate that was found to be as stable as supplemental bilateral PS.

The effect of screw insertion angle and thread type on the pullout strength of bone screws in normal and osteoporotic cancellous bone models

Screw fixation can be extremely difficult to achieve in osteoporotic (OP) bone because of its low strength. This study determined how pullout strength is affected by placing different bone screws at varying angles in normal and OP bone models. Pullout tests of screws placed axially, and at angles to the pullout axis (ranging from 10° to 40°), were performed in 0.09 g cm(-3), 0.16 g cm(-3) and 0.32 g cm(-3) polyurethane (PU) foam. Two different titanium alloy bone screws were used to test for any effect of thread type (i.e. cancellous or cortical) on the screw pullout strength. The cancellous screw required a significantly higher pullout force than the cortical screw (p<0.05). For both screws, pullout strength significantly increased with increasing PU foam density (p<0.05). For screws placed axially, and sometimes at 10°, the observed mechanism of failure was stripping of the internal screw threads generated within the PU foam by screw insertion. For screws inserted at 10°, 20°, 30° and 40°, the resistance to pullout force was observed to be by compression of the PU foam material above the angled screw; clinically, this suggests that compressed OP bone is stronger than unloaded OP bone.

The effects of screw orientation in severely osteoporotic bone: a comparison with locked plating

BACKGROUND

Techniques such as varying screw insertion angles and the use of locked plating have been shown to improve the strength of fixation in bone. The effects of these methods is less clearly understood in bone of exceedingly poor quality.

METHODS

Forty plate-bone constructs were assembled and divided into four groups of ten. Perpendicularly placed screws were placed in one group, convergently placed crossing screws were placed in a second group, an oblique end screw was placed in a third group, and a fourth group utilized perpendicularly placed locking screws in a locking plate. All test subjects were mounted and loaded in cantilever bending to the point of failure. Stiffness, initial load to failure, and maximal load tolerated were all analyzed.

FINDINGS

All four groups demonstrated evidence of failure at similar loads (21.8-26.1N). The locked group was able to tolerate significantly higher loads overall (37.3N, P=.044). All three non-locked groups demonstrated similar failure patterns and load to failure. Locking constructs demonstrated a distinctly different failure pattern. No significant differences were detected with regard to screw orientation and load to failure. The group with an oblique end screw was significantly less stiff than the other three constructs (P=.017).

INTERPRETATION

In a severely osteoporotic model, failure in cantilever bending at low forces will take place regardless of fixation methods used. The mechanism of failure is different in locked constructs compared to traditional constructs. The added benefit of oblique screw placement observed in healthy bone is not observed in osteoporotic bone.

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.

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

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

Enhancing the stability of anterior lumbar interbody fusion: a biomechanical comparison of anterior plate versus posterior transpedicular instrumentation

STUDY DESIGN

Biomechanical study using human cadaver spines.

OBJECTIVE

To assess the stabilizing effect of a supplemental anterior tension band (ATB, Synthes) plate on L5-S1 anterior lumbar interbody fusion (ALIF) using a femoral ring allograft (FRA) under physiologic compressive preloads, and to compare the results with the stability achieved using FRA with supplemental transpedicular instrumentation.

SUMMARY OF BACKGROUND DATA

Posterior instrumentation can improve the stability of ALIF cages. Anterior plates have been proposed as an alternative to avoid the additional posterior approach.

METHODS

Eight human specimens (L3 to sacrum) were tested in the following sequence: (i) intact, (ii) after anterior insertion of an FRA at L5-S1, (iii) after instrumentation with the ATB plate, and (iv) after removal of the plate and adding transpedicular instrumentation at the same level. Specimens were tested in flexion-extension, lateral bending, and axial rotation. Flexion-extension was tested under 0 N, 400 N, and 800 N compressive follower preload to simulate physiologic compressive preloads on the lumbar spine.

RESULTS

Stand-alone FRAs significantly decreased the range of motion (ROM) in all tested directions (P < 0.05); however, the resultant ROM was large in flexion-extension ranging between 6.1 +/- 3.1 degrees and 5.1 +/- 2.2 degrees under 0 N to 800 N preloads. The ATB plate resulted in a significant additional decrease in flexion-extension ROM under 400 N and 800 N preloads (P < 0.05). The flexion-extension ROM with the ATB plate was 4.1 +/- 2.3 under 0 N preload and ranged from 3.1 +/- 1.8 to 2.4 +/- 1.3 under 400 N to 800 N preloads. The plate did not significantly decrease lateral bending or axial rotation ROM compared with stand-alone FRA (P > 0.05), but the resultant ROM was 2.7 +/-1.9 degrees and 0.9 +/- 0.6 degrees , respectively. Compared with the ATB plate, the transpedicular instrumentation resulted in significantly less ROM in flexion-extension and lateral bending (P < 0.05), but not in axial rotation (P > 0.05).

CONCLUSION

The ATB plate can significantly increase the stability of the anterior FRA at L5-S1 level. Although supplemental transpedicular instrumentation results in a more stable biomechanical environment, the resultant ROM with the addition of a plate is small, especially under physiologic preload, suggesting that the plate can sufficiently resist motion. Therefore, clinical assessment of the ATB plate as an alternative to transpedicular instrumentation to enhance ALIF cage stability is considered reasonable.

Stand-alone anterior lumbar discectomy and fusion with plate: initial experience

BACKGROUND

The stability of the lumbar spine after ALIF with lateral plate fixation and/or posterior fixation has previously been investigated; however, stand-alone ALDF with plate has not. Previous clinical studies have demonstrated poor fusion rates with stand-alone anterior interbody fusion in the absence of posterior instrumentation. We review our initial experience with stand-alone ALDF with segmental plate fixation for degenerative disc disease of the lumbar spine and compare these results with our experience with traditional ALIF and supplemental posterior instrumentation.

METHODS

Forty-nine patients treated at the University of California, San Francisco between 2002 and 2005 were included in this analysis. The study was retrospective in nature. All patients presented with discogram-positive back pain and had failed conservative treatment. Twenty-four patients underwent ALDF with plate, and 25 underwent ALIF with posterior instrumentation. Patients underwent flexion/extension imaging at 6 weeks, 3 months, 6 months, and 1 year postoperatively. All patients completed ODI and VAS questionnaires at 3 months, 6 months, and 1 year postoperatively.

RESULTS

Average follow-up was 11.6 and 21.7 months in the ALDF with plate and ALIF with instrumentation groups, respectively. All patients demonstrated radiographic evidence of fusion at last follow-up. None developed instability at the fusion level, and none developed hardware failure (plate back-out, screw lucency, etc). Average subsidence at 6 months postoperatively was 2.2 and 2.5 mm, respectively. The VAS and ODI scores are presented in Tables 3 and 4.

CONCLUSIONS

Preliminary results of stand-alone ALDF with plate suggest it may be safe and effective for the surgical treatment of patients with degenerative disc disease of the lumbar spine. Long-term follow-up is clearly needed. Subsidence is diminished with ALDF and plating compared with ALIF with posterior instrumentation. It is unclear at this time which subset of patients may ultimately require posterior hardware supplementation, but those with circumferential stenosis or severe facet disease are not ideal candidates for ALDF with plate. For some patients in whom lumbar arthroplasty is not indicated, or as a salvage procedure, ALDF with plate may be a satisfactory alternative and may eliminate the need for a supplemental posterior procedure.

[Vertebral body replacement in spine surgery]

Autografts and allogeneous bone grafts as well as cages are used for the reconstruction of the anterior column after corpectomy. Recently, expandable cages for vertebral body replacement have been developed. Based on our own experience, the purpose of this study was to summarize the available biomechanical and clinical data of expandable corpectomy cages and to compare it with established fixation techniques. If used correctly, expandable cages offer several surgical advantages in comparison to non-expandable cages. However there were no significant differences between the biomechanical properties of expandable and non-expandable cages. Additionally, design variations of expandable corpectomy cages did not show any significant impact on the biomechanical stability. Currently available mid-term clinical and radiological data on the treatment of fractures, metastasis and infection of the cervical, thoracic and lumbar spine demonstrated no significant difference between expandable and non-expandable cages. However, the increased stress-shielding effect of expandable cages compared to non-expandable cages might result in a deterioration of the long-term clinical outcome.