Evaluation of a lumbar intervertebral spacer with integrated screws as a stand-alone fixation device.

STUDY DESIGN: An in vitro cadaveric biomechanics study. OBJECTIVE: To evaluate the biomechanical contribution of the integration of screws into a polyether-ether-ketone (PEEK) spacer, and to compare the resulting construct to standard anterior lumbar interbody fusion constructs. SUMMARY OF BACKGROUND DATA: Pedicular fixation is often performed to increase the rigidity of anterior lumbar interbody fusion constructs but also increases the risk of morbidity. Integration of screws into a PEEK spacer (investigational device, ID) may increase construct rigidity and prevent migration without the drawbacks associated with supplementary fixation. METHODS: Twenty cadaveric motion segments were potted and tested under pure moments of + or - 7.5 Nm in flexion-extension, lateral bending, and axial torsion. Discectomies were performed and specimens were instrumented with the ID + or - screws, dual threaded cages, structural graft+anterior plate, and graft+pedicle screws. The ID+screws and threaded cage constructs were then pulled out at a constant rate. RESULTS: All constructs reduced the mean range of motion (ROM) below the intact level in flexion-extension and lateral bending (P<0.001) and for all devices except cages in torsion (P < or = 0.05). The median flexion-extension/bending/torsion ROM was 9.5/9.4/4.1 degrees for the intact segments, 6.1/5.1/1.8 degrees with the ID+screws, 4.9/5.2/2.4 degrees with threaded cages, 3.3/4.4/1.1 degrees with plates and 1.4/1.6/1.7 degrees with pedicle screws, respectively. The addition of the integrated screws decreased the mean ROM of the ID constructs by 0.9 degrees in flexion-extension, 1.8 degrees in bending, and 2.8 degrees in flexion-extension. The peak pullout force was 962 N for the ID and 337 N for threaded cages. CONCLUSIONS: The incorporation of screws into a PEEK interbody device was evaluated alongside traditional constructs in terms of construct rigidity and resistance to pullout. The greatest contributions of the integrated screws are in limiting torsional motion and in the containment of the interbody device. Pedicular fixation produces the most rigid constructs, but integrating screws into a PEEK spacer produces a stand-alone construct that is similar to threaded cages and provides more resistance to anterior displacement.

Biomechanical evaluation of sacroiliac joint fixation with decortication.

BACKGROUND CONTEXT: Fusion typically consists of joint preparation, grafting, and rigid fixation. Fusion has been successfully used to treat symptomatic disruptions of the sacroiliac joint (SIJ) and degenerative sacroiliitis using purpose-specific, threaded implants. The biomechanical performance of these systems is important but has not been studied. PURPOSE: The objective of this study was to compare two techniques for placing primary (12.5 mm) and secondary (8.5 mm) implants across the SIJ. STUDY DESIGN: This is a human cadaveric biomechanical study of SIJ fixation. MATERIALS AND METHODS: Pure-moment testing was performed on 14 human SIJs in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) with motion measured across the SIJ. Specimens were tested intact, after destabilization (cutting the pubic symphysis), after decortication and implantation of a primary 12.5-mm implant at S1 plus an 8.5-mm secondary implant at either S1 (S1-S1, n=8) or S2 (S1-S2, n=8), after cyclic loading, and after removal of the secondary implant. Ranges of motion (ROMs) were calculated for each test. Bone density was assessed on computed tomography and correlated with age and ROM. This study was funded by Zyga Technology but was run at an independent biomechanics laboratory. RESULTS: The mean±standard deviation intact ROM was 3.0±1.6° in FE, 1.5±1.0° in LB, and 2.0±1.0° in AR. Destabilization significantly increased the ROM by a mean 60%-150%. Implantation, in turn, significantly decreased ROM by 65%-71%, below the intact ROM. Cyclic loading did not impact ROM. Removing the secondary implant increased ROM by 46%-88% (non-significant). There was no difference between S1-S1 and S1-S2 constructs. Bone density was inversely correlated with age (R=0.69) and ROM (R=0.36-0.58). CONCLUSIONS: Fixation with two threaded rods significantly reduces SIJ motion even in the presence of joint preparation and after initial loading. The location of the secondary 8.5-mm implant does not affect construct performance. Low bone density significantly affects fixation and should be considered when planning fusion constructs. Findings should be interpreted in the context of ongoing clinical studies.

Morphologic and biomechanical comparison of spinous processes and ligaments from scoliotic and kyphotic patients.

The spinous processes and supraspinous and interspinous ligaments (SSL and ISL, respectively) limit flexion and may relate to spinal curvature. Spinous process angles and mechanical properties of explanted human thoracic posterior SSL/ISL complexes were compared for scoliosis (n=14) vs. kyphosis (n=8) patients. The median thoracic coronal Cobb angle for scoliosis patients was 48°, and sagittal angles for kyphosis patients was 78°. Spinous processes were gripped and four strain steps of 4% were applied and held. Percent relaxation was calculated over each step, equilibrium load data were fit to an exponential equation, and a Kelvin model was fit to the load from all four curves. Failure testing was also performed. Median ligament complex dimensions from scoliosis and kyphosis patients were, respectively: ISL width=16.5mm and 16.0mm; SSL width=4.3mm and 3.8mm; ISL+SSL area=17.2mm and 25.7mm; these differences were not significant. Significant differences did exist in terms of spinous process angle vs. spine axis (47° for scoliosis and 32° for kyphosis) and SSL thickness (2.1mm for scoliosis and 3.0mm for kyphosis). Fourth-step median relaxation was 42% for scoliosis and 49% for kyphosis. Median linear region stiffness was 42N/mm for scoliosis and 51N/mm for kyphosis. Median failure load was 191N for scoliotic and 175N for kyphotic ligaments. Differences in loading, relaxation, viscoelastic and failure parameters were not statistically significant, except for a trend for greater initial rate of relaxation (T1) for scoliosis ligaments. However, we found significant morphological differences related to the spinous processes, which suggests a need for future biomechanical studies related to the musculoskeletal aspects of spinal alignment and posture.

Stiffness of prosthetic nucleus determines stiffness of reconstructed lumbar calf disc.

BACKGROUND CONTEXT: Currently, artificial spinal discs require transection or partial removal of the annulus fibrosis in order to excise the nucleus and implant a prosthetic nucleus or implant a total disc device, respectively. Preservation of the annulus for prosthetic disc replacement maintains the function of the annulus and may improve annulus load sharing with the prosthesis. PURPOSE: To quantify the biomechanical characteristics of an annular sparing intervertebral prosthetic disc (IPD) in a lumbar calf spine model. The aim of the study was to determine whether altering the stiffness of the elastic component of this unique prosthesis would correspond to changes of the overall reconstructed disc. STUDY DESIGN/SETTING: A biomechanical study was conducted in vitro using cadaveric calf spines such that each specimen served as its own control. Investigations were performed at the Minneapolis Medical Research Foundation, Orthopaedic Biomechanics Laboratory. METHODS: Six L45 or L56 motion segments (from which the posterior elements had been removed) were studied in axial compression, sagittal and lateral bending and torsion. These load states were applied to the intact, denucleated and prosthetically reconstructed disc using four IPDs of differing stiffness. RESULTS: Load-displacement testing demonstrated that stiffer IPDs resulted in a decreased range of motion and neutral zone, and greater stiffness of the reconstructed disc. Disc reconstruction with the stiffest IPD approximated the behavior of the intact disc. CONCLUSIONS: The overall biomechanical characteristics of a reconstructed disc are related to the stiffness of a nucleus prosthesis. The similarities in the mechanical behavior of reconstructed and intact discs suggest that additional feasibility studies for the annulus-sparing IPD are warranted.

Compressive properties of fibrous repair tissue compared to nucleus and annulus.

The wound healing process includes filling the void between implant and tissue edges by collagenous connective repair tissue. This fibrous repair tissue may load share or stabilize implants such as spinal disc replacements. The objective of this study was the biomechanical characterization of human fibrous tissue compared to annulus fibrosus and nucleus pulposus. Human lumbar discs (10 nucleus and annulus) and 10 lumbar deep wound fibrous tissue specimens were sectioned into 12mm diameter×6mm high cylindrical samples. Confined compression testing, after 2h swelling at 0.11MPa, was performed at 5%, 10% and 15% strain over 3.5h. Unconfined dynamic testing (2-0.001Hz) was performed at 5-15% strain. Semi-quantitative histology estimated the proportion of proteoglycan to collagen. Fibrous tissue exhibited a decrease in height during the swelling period whereas annulus and nucleus tissues did not. The aggregate modulus was significantly less for fibrous tissue (p<0.002). Percent stress relaxation was greatest for the fibrous tissue and similar for annulus and nucleus. Dynamic testing found the storage modulus (E') was greater than the loss modulus (E″) for all tissues. Annulus were found to have greater E' and E″ than nucleus, whereas E' and E″ were similar between annulus and fibrous tissue. Fibrous tissue had the greatest increase in both moduli at greater frequencies, but had the lowest hydration and proteoglycan content. Fibrous tissue would not be a substitute for native tissue within the disc space but if adjacent to a disc prosthesis may impart some degree of intersegmental stability during acute loading activities.

Quantification of intradiscal pressures below thoracolumbar spinal fusion constructs: is there evidence to support "saving a level"?

STUDY DESIGN: In vitro cadaveric study. OBJECTIVE: The purpose of this study was to quantify the relative biomechanical protection resulting from "saving a level" in long spinal fusions. SUMMARY OF BACKGROUND DATA: "Saving levels" in spinal deformity surgery is desirable. Constructs with lowest instrumented vertebra (LIV) in the lumbar spine may increase loads on unfused lumbar intervertebral discs, leading to accelerated disc degeneration. No study to date has quantified the relative pressure changes that occur in the unfused caudal discs with progressively longer fusions. METHODS: We used a validated in vitro cadaveric long fusion model to assess intradiscal pressures (IDPs) below simulated fusions. Eight fresh frozen T8-S1 specimens were instrumented from T8 to L5. A follower-type loading system and 7.5-N·m moments were applied in flexion and extension. IDP profiles were assessed with a pressure transducer. After acquiring IDP measurements at a given construct length, the rod was cut 1 level higher until LIV = T12. IDP data from each unfused disc were averaged and normalized to the mean value of the disc when immediately subjacent to the LIV. RESULTS: In both flexion and extension, the mean normalized IDP of the unfused discs below the LIV increased with increasing fusion length. For each 1-level increase in construct length, pressure increased by 3.2% ± 4.8% in flexion and 4.3% ± 4.5% in extension for each unfused disc. Although the differences in pressure for a given unfused disc with differing LIV were not significant, there were significant differences between unfused discs at a given LIV. With shorter fusion lengths, pressure in the disc immediately subjacent to the fusion was consistently greater than for the caudal-most discs. CONCLUSION: Unfused caudal lumbar discs experienced increased IDPs with increasing length of instrumentation, most notably at the subjacent discs closest to the LIV.

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

BACKGROUND CONTEXT: Unstable burst fractures of the thoracolumbar spine may be treated surgically. Vertebral body replacements (VBRs) give anterior column support and, when used with supplemental fixation, impart rigidity to the injured segments. Although some VBRs are expandable, device congruity to the vertebral end plates is imprecise and may lead to stress risers and device subsidence. PURPOSE: The objective of this study was to compare the rigidity of a VBR that self-adjusts to the adjacent vertebral end plates versus structural bone allograft and with an unsupported anterior column in a traumatic burst fracture reconstruction model. STUDY DESIGN: Biomechanical flexibility testing with rod strain measurement. PATIENT SAMPLE: Twelve T11-L3 human spine segments. OUTCOME MEASURES: Range of motion, neutral zone, and posterior fixation rod stress (moments). METHODS: Flexibility testing was performed to ± 6 Nm in flexion-extension, lateral bending, and axial rotation on 12 intact human T11-L3 specimens. Burst fractures were created in L1, and flexibility testing was repeated in three additional states: subtotal corpectomy with posterior instrumentation (PI) only from T12 to L2, reconstruction with a femoral strut allograft and PI, and reconstruction with a VBR (with self-adjusting end plates) and PI. The PI consisted of pedicle screws and strain gage instrumented rods that were calibrated to measure rod stress via flexion-extension bending moments. RESULTS: There was no statistical difference in range of motion or neutral zone between the strut graft and VBR constructs, which both had less motion than the PI-only construct in flexion/extension and torsion and were both less than the intact values in flexion/extension and lateral bending (p < .05). Posterior rod moments were significantly greater for the PI-only construct in flexion/extension relative to the strut graft and VBR states (p = .03). CONCLUSIONS: This study, which simulated the immediate postoperative state, suggests that a VBR with self-adjusting end plate components has rigidity similar to the standard strut graft when combined with PI. Posterior rod stress was not significantly increased with this type of VBR compared with the strut graft reconstruction. The benefits of burst fracture stabilization using a self-adjusting VBR ultimately will not be known until long-term clinical studies are performed.

Biomechanical characterization of an annulus-sparing spinal disc prosthesis.

BACKGROUND CONTEXT: Current spine arthroplasty devices require disruption of the annulus fibrosus for implantation. Preliminary studies of a unique annulus-sparing intervertebral prosthetic disc (IPD) found that preservation of the annulus resulted in load sharing of the annulus with the prosthesis. PURPOSE: Determine flexibility of the IPD versus fusion constructs in normal and degenerated human spines. STUDY DESIGN/SETTING: Biomechanical comparison of motion segments in the intact, fusion and mechanical nucleus replacement states for normal and degenerated states. PATIENT SETTING: Thirty lumbar motion segments. OUTCOMES MEASURES: Intervertebral height; motion segment range of motion, neutral zone, stiffness. METHODS: Motion segments had multidirectional flexibility testing to 7.5Nm for intact discs, discs reconstructed using the IPD (n=12), or after anterior/posterior fusions (n=18). Interbody height and axial compression stiffness changes were determined for the reconstructed discs by applying axial compression to 1,500N. Analysis included stratifying results to normal mobile versus rigid degenerated intact motion segments. RESULTS: The mean interbody height increase was 1.5mm for IPD reconstructed discs versus 3.0mm for fused segments. Axial compression stiffness was 3.0+/-0.9kN/mm for intact compared with 1.2+/-0.4kN/mm for IPD reconstructed segments. Reconstructed disc ROM was 9.0 degrees +/-3.7 degrees in flexion extension, 10.6 degrees +/-3.4 degrees in lateral bending, and 2.8 degrees +/-1.4 degrees in axial torsion that was similar to intact values and significantly greater than respective fusion values (p<.001). Mobile intact segments exhibited significantly greater rotation after fusion versus their more rigid counterparts (p<.05); however, intact motion was not related to motion after IPD reconstruction. The NZ and rotational stiffness followed similar trends. Differences in NZ between mobile and rigid intact specimens tended to decrease in the IPD reconstructed state. CONCLUSION: The annulus-sparing IPD generally reproduced the intact segment biomechanics in terms of ROM, NZ, and stiffness. Furthermore, the IPD reconstructed discs imparted stability by maintaining a small neutral zone. The IPD reconstructed discs were significantly less rigid than the fusion constructs and may be an attractive alternative for the treatment of degenerative disc disease.

Mature runt cow lumbar intradiscal pressures and motion segment biomechanics.

BACKGROUND CONTEXT: The optimal animal model for in vivo testing of spinal implants, particularly total or partial disc replacement devices, has not yet been determined. Mechanical and morphological similarities of calf and human spines have been reported; however, limitations of the calf model include open growth plates and oversized vertebrae with growth. Mature runt cows (Corrientes breed) may avoid these limitations. PURPOSE: This study compared vertebral morphology and biomechanical properties of human and runt cow lumbar motion segments. STUDY DESIGN: In vivo disc pressure measurements were obtained in six mature runt cows at L4-L5. In vitro evaluation was performed on these same segments and repeated on 12 human motion segments. METHODS: Disc pressures were measured in vivo in runt cow (Corrientes breed) L45 discs using a percutaneous transducer with the animal performing various activities. These motion segments were then harvested and morphologic and biomechanical evaluations (disc pressure in compression, flexibility tests to 7.5Nm) were performed on both cow and male human L23 and L45 segments. RESULTS: The transverse lumbar disc dimensions were slightly smaller for (mixed gender) cow versus (male) humans, but were within the range of reported (mixed gender) human values. The mean+/-SD disc height was smaller for runt cow (7+/-1mm) versus human discs (13+/-2mm, p<.001). The vertebral bodies of the cow were approximately twice as tall as the human. In vitro testing revealed significantly greater disc pressure response to applied axial loading in the runt cow versus humans (1.27+/-0.18 vs. 0.84+/-0.15kPa/N, respectively) but similar overall stiffness (2.15+/-0.71 vs. 1.91+/-0.94kN/mm, respectively). Runt cow and human segment flexibility curves were similar with the following exceptions: runt cow stiffness was approximately 40% greater in torsion (p<.05), runt cow segment lateral bending motion was greater versus humans (range of motion by 30%, neutral zone by 100%; both p<.05), and flexion range of motion tended to be smaller in runt cow versus human specimens (by approximately 40%, p=NS). In vivo, the standing disc pressure in the runt cow was 0.80+/-0.24MPa. CONCLUSIONS: Although no animal replicates the human motion segment, the runt cow lumbar spine had a number of biomechanical and morphological measurements within the range of human values. The closed physes and temporally stable morphology of the mature runt cow may make this model more suitable versus standard calf models for human intradiscal implant studies.

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.

In vitro disc pressure profiles below scoliosis fusion constructs.

STUDY DESIGN: Biomechanical human cadaveric study comparing straight and scoliotic spines with healthy and degenerated L4/5 discs. OBJECTIVE: To describe the biomechanical environment of discs under various spinal alignments by measuring the coronal intradiscal pressure profiles. SUMMARY OF BACKGROUND DATA: Abnormal loading of the lumbar discs in the concavity of scoliotic curves may accelerate disc degeneration, which may be related to pain. METHODS: Eight intact human cadaver spines (T1-S1; mean donor age 47 years old) underwent radiographs, DEXA, and MRI and were graded for disc degeneration. Each specimen was instrumented in a normal (straight coronal) spinal alignment from T4-L4. Intradiscal pressure profiles for the L4/5 disc and resultant moments were obtained under axial follower loads up to 1500 N. Testing was repeated for bilateral 3-cm decompensation. Posterior instrumentation was used to induce scoliosis (thoracic and lumbar curve average = 25 degrees, fractional lumbosacral curve average = 5 degrees), and testing was repeated for all load states. RESULTS: MRI found 4 healthy (grade I and II) and 4 degenerated (grade III to V) L4/5 discs. Scoliosis and decompensation significantly increased coronal moments (P < 0.003). Disc pressures increased linearly with greater applied loads for all specimens. Healthy L4/5 discs exhibited uniform pressure profiles with normal spinal alignment and minimal effect with simulated scoliosis or decompensation. For degenerated discs, there was a relative pressure profile depression in the nucleus relative to the anulus region; with spinal malalignment, either due to scoliotic curvature, decompensation, or both, there was disc pressure profile asymmetry. The ratio of maximum intradiscal pressure at the concavity relative to the convexity was 1.1 (range, 1.0-1.2) for healthy discs and 3.6 (range, 2.2-4.4) for degenerated discs in the scoliotic specimens (P = 0.008). CONCLUSION: Disc pressure profilometry below long spinal constructs found asymmetric loading with the greatest loads at the concave inner anulus, especially in the presence of disc degeneration, scoliosis, and decompensation. For the degenerated cases, there was substantial disc pressure profile asymmetry despite only mildly severe scoliotic curvatures. These results suggest that scoliosis surgeons should minimize end-vertebra tilt, maximize lumbar curve, and balance correction at the time of surgical intervention. These results combined with prior animal studies suggest a compounding effect of asymmetric loading and progression of disc degeneration.

In vitro analysis of anterior and posterior fixation in an experimental unstable burst fracture model.

STUDY DESIGN: A biomechanical comparison of fixation constructs in an experimental fracture model. OBJECTIVE: To determine the relative postoperative stability of anterior graft and plating with that of posterior or combined fixation constructs in an unstable thoracolumbar burst fracture model. SUMMARY OF BACKGROUND DATA: Several treatment modalities have been proposed for unstable thoracolumbar burst fractures, but the optimal technique is unclear. Previous cadaveric biomechanical studies in unstable burst fracture models have not considered the commonly used posterior (interpedicular) and anterior (plate) constructs. METHODS: Nine human spine segments (T11-L3) were potted in epoxy and scanned using dual energy x-ray absorptiometry and computed tomography. Intact specimens had baseline flexibility testing. Unstable L1 burst fractures as verified by computed tomography were created using an impulse load and posterior surgical osteoligamentous destabilization (ie, transection of the lamina, interspinous ligaments, facet capsules, and ligamentum flavum). Specimens were instrumented posteriorly with pedicle screws and rods and tested to 6 Nm in flexion-extension, lateral bending, and torsion. Corpectomy and strut grafting were then performed, and testing was repeated in varying order with posterior fixation, anterior plating and circumferential fixation. Range of motion (ROM) and neutral zone was calculated for each test and fixation groups were compared using analysis of variance. RESULTS: All specimens had AO B1.2 (unstable burst) fractures. Mean ROM for posterior-only constructs was significantly less than that of the intact in lateral bending, flexion, and extension (P<0.001). Anterior-only constructs after corpectomy and strut grafting generally resulted in a smaller ROM versus intact in flexion (NS: P=0.1) and lateral bending (P<0.001). In contrast, all anterior-only and posterior constructs had greater ROM than intact in torsion (all at P<0.05). Circumferential fixation resulted in statistically smaller ROM compared with all other constructs (P< or =0.04), and reached that of the intact specimen in torsion. Increased ROM was correlated with greater fracture comminution for posterior-only fixation (P<0.05), and was weakly correlated with lower dual energy x-ray absorptiometry score (R=0.3) for anterior-only fixation. CONCLUSIONS: Circumferential instrumentation provided the most rigid fixation, followed by posterior fixation with anterior strut grafting, posterior fixation alone, and by anterior fixation with strut grafting. These results were dependent on bone quality and the comminution severity of the fracture. These results should aid surgical decision making in addition to other factors in the overall clinical situation.

Validation, reliability, and complications of a tethering scoliosis model in the rabbit.

This study was conducted to refine a small animal model of scoliosis, and to quantify the deformities throughout its growth period. Subcutaneous scapula-to-contralateral pelvis tethering surgery was selected due to its minimally invasive nature and potential applicability for a large animal model. The procedure was performed in 7-week-old New Zealand white rabbits. Group A animals (n=9) underwent the tethering procedure with a suture that spontaneously released. Group B animals (n=17) had the identical procedure with a robust tether and pelvic fixation, which was maintained for 2 months during growth. All animals developed immediate post-operative scoliosis with a Cobb angle of 23 degrees (range, 6-39 degrees) in group A and 59 degrees (range, 24-90 degrees) in group B animals. During the 2 month post-tethering, group A animals lost their tether and scoliosis resolved, whereas all animals in group B maintained their tether until scheduled release at which time the mean scoliosis was 62 degrees. Immediately after tether release, group B scoliosis decreased to a mean 53 degrees. Over the following 4 months of adolescent growth, the scoliosis decreased to a mean of 43 degrees at skeletal maturity; the decrease usually occurred in animals with less than 45 degrees curves at tether release. Radiographs revealed apical vertebral wedging (mean 19 degrees ) in all group B animals. Sagittal spinal alignment was also assessed, and for group B animals, the scoliotic segment developed mild to moderate kyphosis (mean 28 degrees) and torsional deformity, but the kyphosis resolved by 4 months after tether-release. Complications specific to this technique included a high rate of transient scapulothoracic dissociation and cases of cor pulmonale. In conclusion, this tethering technique in immature rabbits consistently produced scoliosis with vertebral wedging when the tether was intact through the first 2 months of the protocol. The transient exaggeration of kyphosis suggests that the production of scoliosis is not necessarily dependent on lordosis in this model. Because this technique does not violate thoracic or spinal tissues, it may be useful in the investigation of secondary physiologic effects of mechanically-induced scoliosis, and may be scalable to larger animal species.

In vitro comparison of bioresorbable and titanium anterior cervical plates in the immediate postoperative condition.

Bioresorbable plates have recently been used with anterior cervical discectomy and fusion (ACDF). Compared with metallic plates, bioresorbable plates provide segmental stabilization with minimal imaging artifact, eventual resorption, and increased load sharing. The objectives of the present study were to determine whether a bioresorbable plate can withstand simulated physiologic static and cyclic loading, to compare the reduction in flexibility provided by bioresorbable and titanium plates, and to quantify load sharing between the plate and spine with graft. Sixteen human cervical motion segments were tested to +/-2.5 Nm in flexion-extension, lateral bending, and axial rotation. Range of motion (ROM) was measured (1) in the intact state, (2) with ACDF without plating, (3) after addition of either a bioresorbable or titanium plate, and (4) after 500 cycles of combined flexion-extension and axial torsion. Load sharing was evaluated by applying the same fixed rotation both without and with the plate, and was calculated as the moment resisted by the uninstrumented ACDF expressed as a percentage of the plated ACDF state. No plate failures or graft migration occurred during testing. Compared with the uninstrumented ACDF, bioresorbable plates reduced mean ROM by 49% in flexion-extension and 25% in lateral bending, with very little change in torsion. Titanium plates reduced uninstrumented ACDF ROM by 69% in flexion-extension, 45% in lateral bending, and 27% in torsion. Differences between bioresorbable and titanium plates were significant in flexion-extension and lateral bending. Cyclic loading did not significantly change ROM for either plate. More moment was shared in lateral bending by the spine/graft with bioresorbable plates (78%) compared with titanium plating (63%). Bioresorbable plates contained an intervertebral graft, provided some stabilization, remained intact throughout the simulated immediate postoperative loading, and shared more load with the graft and osteoligamentous spine than titanium plates.

In vitro, biomechanical comparison of an anterior lumbar interbody fusion with an anteriorly placed, low-profile lumbar plate and posteriorly placed pedicle screws or translaminar screws.

STUDY DESIGN: An in vitro biomechanical comparison of anteriorly placed lumbar plates, pedicle screws, and translaminar screws in the anterior lumbar interbody fusion (ALIF) setting. OBJECTIVES: To determine whether an anteriorly placed lumbar plate reduces the flexibility in terms of neutral zone and range of motion of a simulated ALIF, and to compare this reduction in flexibility to that provided by posteriorly placed pedicle screws and translaminar screws. SUMMARY OF BACKGROUND DATA: Pedicular and translaminar facet fixation add stability and increase fusion rates, compared with ALIF alone. An anteriorly placed lumbar plate has been introduced to provide stability without the need for a secondary approach. However, this plate has not been evaluated biomechanically. METHODS: Seven intact, cadaveric lumbar motion segments were tested to +/- 7.5 Nm in flexion-extension, lateral bending, and axial torsion. Specimens were retested after ALIF, and after subsequent instrumentation with pedicle screws, translaminar screws, and anterior lumbar plates. The range of motion and neutral zone were measured from resulting flexibility curves. RESULTS: Mean (+/- standard deviation) flexion-extension range of motion for intact segments (9.9 degrees +/- 3.1 degrees ) was significantly reduced to 7.7 degrees +/- 1.8 degrees after ALIF (P = 0.02), and was further reduced to 3.0 degrees +/- 0.9 degrees with lumbar plates (P < 0.001), 1.5 degrees +/- 0.6 degrees with pedicle screws (P < 0.001), and 0.9 degrees +/- 0.4 degrees with translaminar screws (P < 0.001). All 3 devices also reduced flexion-extension neutral zone and torsion neutral zone and range of motion, compared with ALIF alone (P < 0.05). Lumbar plates did not decrease lateral bending range of motion or neutral zone (P > 0.05), whereas pedicle and translaminar screws did (P < 0.05). CONCLUSIONS: Although not as rigid as pedicle or translaminar screws, anterior lumbar plating does add significant stability to an ALIF and may provide a valuable, single-approach alternative to supplemental posterior fixation.

Posterior augmentation of an anterior lumbar interbody fusion: minimally invasive fixation versus pedicle screws in vitro.

STUDY DESIGN: An in vitro biomechanical comparison of four posterior fixation techniques in the setting of an anterior lumbar interbody fusion (ALIF). OBJECTIVE: To compare the initial stability, in terms of range of motion and neutral zone, provided by pedicle screws, facet screws, translaminar facet screws, and H-graft plus interspinous cables in the presence of an anteriorly placed femoral ring allograft. SUMMARY OF BACKGROUND DATA: Pedicular fixation has been used to increase ALIF fusion rates but has also been linked with increased morbidity. Alternative posterior fixation options are available, but comprehensive biomechanical comparisons of these techniques do not exist. METHODS.: Twelve cadaveric lumbar motion segments were loaded to 5 Nm in unconstrained flexion-extension, lateral bending, and axial torsion. Specimens were tested intact, after ALIF, and after applying pedicle screws, translaminar screws, facet screws, and H-graft plus cables. The resulting neutral zones and ranges of motion were measured. RESULTS: The mean (+/-SEM) range of motion for each construct in flexion-extension was as follows: intact: 6.39 degrees (+/-0.47 degrees); ALIF alone: 3.31 degrees (+/-0.22 degrees); (ALIF+) pedicle screws: 0.6 degrees (+/-0.06 degrees); facet screws: 0.75 degrees (+/-0.12 degrees); translaminar screws: 0.61 degrees (+/-0.09 degrees); and H-graft: 1.74 degrees (+/-0.26 degrees). Pedicle, translaminar facet, and facet screws significantly decreased range of motion and neutral zone compared to ALIF alone in flexion-extension, lateral bending, and axial torsion (all at P < 0.04, except translaminar screws in torsion neutral zone where P = 0.09). H-graft decreasedflexion-extension range of motion and neutral zone only (P < 0.01) and resulted in a significantly greater neutral zone than pedicle and facet screws in torsion and lateral bending neutral zones (P < 0.03). CONCLUSIONS: In the ALIF setting, facet screw and translaminar screw techniques, which may be associated with less morbidity than pedicle screws clinically, provided initial posterior stabilization similar to pedicular fixation in this in vitro study.

Measurement of in vivo intradiscal pressure in healthy thoracic intervertebral discs.

STUDY DESIGN: In vivo pressures were measured in radiologically healthy middle and lower thoracic discs in 6 adult volunteers. OBJECTIVES: To quantify and compare intradiscal pressures from the middle and lower thoracic spine during various body positions and maneuvers, and to investigate the potential variation of these pressures with orientation of the measurement transducer. SUMMARY OF BACKGROUND DATA: In vivo intradiscal pressures have been reported for the lumbar spine; however, the authors are unaware of any studies presenting intradiscal pressures in the thoracic spine. METHODS: A specially constructed pressure-sensing needle was inserted into the nucleus pulposa, and pressures were recorded during a variety of body positions and maneuvers in middle and lower thoracic discs in 6 study participants. In three of the body positions, pressures were measured with the needle in both vertical and horizontal orientations to investigate whether the measured pressures were directionally dependent. RESULTS: Intradiscal pressure varied significantly with body position and maneuver, with pressures being greatest in positions where study participants held 10-kg weights in each hand. Disc level and orientation of the pressure needle did not significantly influence intradiscal pressure. In some body positions, thoracic intradiscal pressures were significantly different from previously reported pressures from the lumbar spine. CONCLUSIONS: Thoracic intradiscal pressure was significantly influenced by body position and maneuver but not disc level. Intradiscal pressures are useful for gaining greater insight into the biomechanics of the thoracic spine.