Human Cadaveric Spines Research Articles

The Influence of Proximal Anchors on the Risk of Proximal Junctional Fracture in the Osteoporotic Spine

Biomechanical comparison of the risk of proximal junctional fracture (PJF) after multilevel spinal instrumentation using pedicle screws or transverse process hooks on the top of a pedicle screw construct. To compare the loads leading to PJF when using pedicle screws or transverse process hooks at the proximal level after multilevel spinal instrumentation using pedicle screws. With stronger spinal fixation techniques, there is increased risk of PJF, especially in the osteoporotic spine. The use of transverse process hooks over pedicle screws at the proximal level of multilevel pedicle screw constructs has been proposed to decrease the incidence of PJF. However, there is no biomechanical study evaluating this concept. Twenty-four segments of 4 vertebrae from 6 human cadaveric spines were evaluated after instrumentation of the distal 3 vertebrae using pedicle screws, except at the top of the construct where either pedicle screws (AP group) or transverse process hooks (PTPH group) were placed. The proximal vertebra was left uninstrumented. Quantitative computed tomography scan was used before instrumentation in order to assess the bone density for each specimen. Cyclic compression loading between 50 and 1000 N at 1 Hz was applied to each of 24 instrumented spinal segments until obtaining a PJF. Bone density was similar between the AP and PTPH groups. A PJF occurred in 22 of 24 tested specimens. The number of cycles required to produce the PJF ranged between 2 and 1002. The number of cycles required to produce the PJF was similar between the AP construct group (median: 3; interquartile range: 3-7) and the PTPH construct group (median: 4; interquartile range: 3-5). The current study failed to observe a significant impact of using transverse process hooks over pedicle screws on top of multilevel pedicle screw construct to decrease the risk of PJF.

Biomechanics of the lower thoracic spine after decompression and fusion: a cadaveric analysis

Background contextFew studies have evaluated the extent of biomechanical destabilization of thoracic decompression on the upper and lower thoracic spine. The present study evaluates lower thoracic spinal stability after laminectomy, unilateral facetectomy, and unilateral costotransversectomy in thoracic spines with intact sternocostovertebral articulations. PurposeTo assess the biomechanical impact of decompression and fixation procedures on lower thoracic spine stability. Study designBiomechanical cadaveric study. MethodsSequential surgical decompression (laminectomy, unilateral facetectomy, unilateral costotransversectomy) and dorsal fixation were performed on the lower thoracic spine (T8–T9) of human cadaveric spine specimens with intact rib cages (n=10). An industrial robot was used to apply pure moments to simulate flexion-extension (FE), lateral bending (LB), and axial rotation (AR) in the intact specimens and after decompression and fixation. Global range of motion (ROM) between T1–T12 and intrinsic ROM between T7–T11 were measured for each specimen. ResultsThe decompression procedures caused no statistically significant change in either global or intrinsic ROM compared with the intact state. Instrumentation, however, reduced global motion for AR (45° vs. 30°, p=.0001), FE (24° vs. 19°, p=.02), and LB (47° vs. 36°, p=.0001) and for intrinsic motion for AR (17° vs. 4°, p=.0001), FE (8° vs. 1°, p=.0001), and LB (12° vs. 1°, p=.0001). No significant differences were identified between decompression of the upper versus lower thoracic spine, with trends toward significantly greater ROM for AR and lower ROM for LB in the lower thoracic spine. ConclusionsThe lower thoracic spine was not destabilized by sequential unilateral decompression procedures. Addition of dorsal fixation increased segment rigidity at intrinsic levels and also reduced overall ROM of the lower thoracic spine to a greater extent than did fusing the upper thoracic spine (level of the true ribs). Despite the lack of true ribs, the lower thoracic spine was not significantly different compared with the upper thoracic spine in FE and LB after decompression, although there were trends toward significance for greater AR after decompression. In certain patients, instrumentation may not be needed after unilateral decompression of the lower thoracic spine; further validation and additional clinical studies are warranted.

In Vitro Characterization of a Stem-Cell-Seeded Triple-Interpenetrating-Network Hydrogel for Functional Regeneration of the Nucleus Pulposus

Intervertebral disc degeneration is implicated as a major cause of low-back pain. There is a pressing need for new regenerative therapies for disc degeneration that restore native tissue structure and mechanical function. To that end we investigated the therapeutic potential of an injectable, triple-interpenetrating-network hydrogel comprised of dextran, chitosan, and teleostean, for functional regeneration of the nucleus pulposus (NP) of the intervertebral disc in a series of biomechanical, cytotoxicity, and tissue engineering studies. Biomechanical properties were evaluated as a function of gelation time, with the hydrogel reaching ∼90% of steady-state aggregate modulus within 10 h. Hydrogel mechanical properties evaluated in confined and unconfined compression were comparable to native human NP properties. To confirm containment within the disc under physiological loading, toluidine-blue-labeled hydrogel was injected into human cadaveric spine segments after creation of a nucleotomy defect, and the segments were subjected to 10,000 cycles of loading. Gross analysis demonstrated no implant extrusion, and further, that the hydrogel interdigitated well with native NP. Constructs were next surface-seeded with NP cells and cultured for 14 days, confirming lack of hydrogel cytotoxicity, with the hydrogel maintaining NP cell viability and promoting proliferation. Next, to evaluate the potential of the hydrogel to support cell-mediated matrix production, constructs were seeded with mesenchymal stem cells (MSCs) and cultured under prochondrogenic conditions for up to 42 days. Importantly, the hydrogel maintained MSC viability and promoted proliferation, as evidenced by increasing DNA content with culture duration. MSCs differentiated along a chondrogenic lineage, evidenced by upregulation of aggrecan and collagen II mRNA, and increased GAG and collagen content, and mechanical properties with increasing culture duration. Collectively, these results establish the therapeutic potential of this novel hydrogel for functional regeneration of the NP. Future work will confirm the ability of this hydrogel to normalize the mechanical stability of cadaveric human motion segments, and advance the material toward human translation using preclinical large-animal models.

Biomechanical and anatomical considerations in lumbar spinous process fixation—an in vitro human cadaveric model

Background contextAlthough multiple mechanisms of device attachment to the spinous processes exist, there is a paucity of data regarding lumbar spinous process morphology and peak failure loads. PurposeUsing an in vitro human cadaveric spine model, the primary objective of the present study was to compare the peak load and mechanisms of lumbar spinous process failure with variation in spinous process hole location and pullout direction. A secondary objective was to provide an in-depth characterization of spinous process morphology. Study designBiomechanical and anatomical considerations in lumbar spinous process fixation using an in vitro human cadaveric model. MethodsA total of 12 intact lumbar spines were used in the current investigation. The vertebral segments (L1–L5) were randomly assigned to one of five treatment groups with variation in spinous process hole placement and pullout direction: (1) central hole placement with superior pullout (n=10), (2) central hole placement with inferior pullout (n=10), (3) inferior hole placement with inferior pullout (n=10), (4) superior hole placement with superior pullout (n=10), and (5) intact spinous process with superior pullout (n=14). A 4-mm diameter pin was placed through the hole followed by pullout testing using a material testing system. As well, the bone mineral density (BMD) (g/cm3) was measured for each segment. Data were quantified in terms of anatomical dimensions (mm), peak failure loads (newtons [N]), and fracture mechanisms, with linear regression analysis to identify relationships between anatomical and biomechanical data. ResultsBased on anatomical comparisons, there were significant differences between the anteroposterior and cephalocaudal dimensions of the L5 spinous process versus L1–L4 (p<.05). Statistical analysis of peak load at failure of the four reconstruction treatments and intact condition demonstrated no significant differences between treatments (range, 350–500 N) (p>.05). However, a significant linear correlation was observed between peak failure load and anteroposterior and cephalocaudal dimensions (p<.05). Correlation between BMD and peak spinous processes failure load was approaching statistical significance (p=.08). 30 of 54 specimens failed via direct pullout (plow through), whereas 8 of 54 specimens demonstrated spinous process fracture. The remaining cases failed via plow through followed by fracture of the spinous process (16 of 54; 29%). ConclusionsThe present study demonstrated that variation in spinous process hole placement did not significantly influence failure load. However, there was a strong linear correlation between peak failure load and the anteroposterior and cephalocaudal anatomical dimensions. From a clinical standpoint, the findings of the present study indicate that attachment through the spinous process provides a viable alternative to attachment around the spinous processes. In addition, the anatomical dimensions of the lumbar spinous processes have a greater influence on biomechanical fixation than either hole location or BMD.

Kinetic analysis of anterior cervical discectomy and fusion supplemented with transarticular facet screws

OBJECT The clinical success rates of anterior cervical discectomy and fusion (ACDF) procedures are substantially reduced as more cervical levels are included in the fusion procedure. One method that has been proposed as an adjunctive technique for multilevel ACDF is the placement of screws across the facet joints ("transfacet screws"). However, the biomechanical stability imparted by transfacet screw placement (either unilaterally or bilaterally) has not been reported. Therefore, the purpose of this study was to determine the acute stability conferred by implementation of unilateral and bilateral transfacet screws to an ACDF construct. METHODS Eight C2-T1 fresh-frozen human cadaveric spines (3 female and 5 male; mean age 50 years) were tested. Three different instrumentation variants were performed on cadaveric cervical spines across C4-7: 1) ACDF with an intervertebral spacer and standard plate/screw instrumentation; 2) ACDF with an intervertebral spacer and standard plate/screw instrumentation with unilateral facet screw placement; and 3) ACDF with an intervertebral spacer and standard plate/screw instrumentation with bilateral facet screw placement. Kinetic ranges of motion in flexion-extension, lateral bending, and axial rotation at 1.5 Nm were captured after each of these procedures and were statistically analyzed for significance. RESULTS All 3 fixation scenarios produced statistically significant reductions (p < 0.05) in all 3 bending planes compared with the intact condition. The addition of a unilateral facet screw to the ACDF construct produced significant reductions at the C4-5 and C6-7 levels in lateral bending and axial rotation but not in flexion-extension motion. Bilateral facet screw fixation did not produce any statistically significant decreases in flexion-extension motion compared with unilateral facet screw fixation. However, in lateral bending, significant reductions at the C4-5 and C5-6 levels were observed with the addition of a second facet screw. The untreated, adjacent levels (C2-3, C3-4, and C7-1) did not demonstrate significant differences in range of motion. CONCLUSIONS The data demonstrated that adjunctive unilateral facet screw fixation to an ACDF construct provides significant gains in stability and should be considered a potential option for increasing the likelihood for obtaining a successful arthrodesis for multilevel ACDF procedures.

Compressive Preload Reduces Segmental Flexion Instability After Progressive Destabilization of the Lumbar Spine

Biomechanical human cadaveric study. We hypothesized that increasing compressive preload will reduce the segmental instability after nucleotomy, posterior ligament resection, and decompressive surgery. The human spine experiences significant compressive preloads in vivo due to spinal musculature and gravity. Although the effect of destabilization procedures on spinal motion has been studied, the effect of compressive preload on the motion response of destabilized, multisegment lumbar spines has not been reported. Eight human cadaveric spines (L1-sacrum, 51.4 ± 14.1 yr) were tested intact, after L4-L5 nucleotomy, after interspinous and supraspinous ligaments transection, and after midline decompression (bilateral laminotomy, partial medial facetectomy, and foraminotomy). Specimens were loaded in flexion (8 Nm) and extension (6 Nm) under 0-N, 200-N, and 400-N compressive follower preload. L4-L5 range of motion (ROM) and flexion stiffness in the high-flexibility zone were analyzed using repeated-measures analysis of variance and multiple comparisons with the Bonferroni correction. With a fixed set of loading conditions, a progressive increase in segmental ROM along with expansion of the high-flexibility zone (decrease of flexion stiffness) was noted with serial destabilizations. Application of increasing compressive preload did not substantially change segmental ROM, but did significantly increase the segmental stiffness in the high-flexibility zone. In the most destabilized condition, 400-N preload did not return the segmental stiffness to intact levels. Anatomical alterations representing degenerative and iatrogenic instabilities are associated with significant increases in segmental ROM and decreased segmental stiffness. Although application of compressive preload, mimicking the effect of increased axial muscular activity, significantly increased the segmental stiffness, it was not restored to intact levels; thereby suggesting that core strengthening alone may not compensate for the loss of structural stability associated with midline surgical decompression. This suggests that there may be a role for surgical implants or interventions that specifically increase flexion stiffness and limit flexion ROM to counteract the iatrogenic instability resulting from surgical decompression. N/A.

Modified Decancellation Posterior Closing-Wedge Osteotomy for Correction of Traumatic Thoracolumbar Kyphotic Deformity: A Cadaveric and Preliminary Clinical Study

Study design: Cadaveric and clinical studies. Objectives: To investigate the safety and efficiency of modified decancellation posterior closing-wedge osteotomy for traumatic fixed kyphotic deformity of thoracolumbar spine. Methods: Single-level vertebral osteotomy was performed on two groups of fresh-frozen human cadaveric lumbar spines. Group I underwent conventional decancellation posterior closing-wedge osteotomy, and Group II underwent our modified cancellation posterior closing-wedge osteotomy. Sagittal plane angulation as well as anterior height and distance between the most cephalad and caudal endplate were measured before and after osteotomy. Twenty six cases of old thoracolumbar fractures with spinal cord injury were recruited in this study. The mean age was 35.6 years. The mean time from injury to operation was 25 months ranging from 3 months to 11 years. Prior to the index surgery, 9 patients received conservative treatment, and 17 patients underwent surgical treatment. There was complete paraplegia in 10 cases and incomplete paraplegia in 14 (Frankel B in 2 cases, C 10 and D 2). Two patients had no neurological deficit. All patients suffered from low back pain, the average score of Visual Analog Scale (VAS) was 4.5 (range 2.5-6.0). The patients were found to have a mean remaining kyphotic deformity of 35° (range 20°-75°). According to the deformity angles conventional or modified decancellation posterior closing-wedge osteotomy was performed. Results: The mean correction was 36° ± 3.6° for Group I and 49° ± 2.0° for Group II. The mean change in anterior height was only 2–4 mm for Group I and II. All cases were followed up for 10 months to 6 years with a mean of 12.5 months. Successful decompression and satisfied correction of kyphosis was noticed. The post-operatively mean angle of kyphosis deformity was 10.8°, ranging from 0° to 40°. Neurological function recovery was noted in 50% of cases. For complete spinal cord injury, 30% of cases had partial recovery (sensation) whereas recovery was observed in 64.3% of cases with incomplete spinal cord injury. The statistical difference between the groups was p<0.01. The mean score of visual analog scale (VAS) was 2.3 (range 1.0-3.5) at last follow-up. Conclusion: The fixed kyphotic deformity of thoracolumbar spine in traumatic spinal cord injury could be treated with conventional or modified decancellation posterior closing-wedge osteotomy. Neurological function recovery and alleviation of low back pain is expected.

Biomechanical impact of C2 pedicle screw length in an atlantoaxial fusion construct.

Background:Posterior, atlantoaxial (AA) fusions of the cervical spine may include either standard (26 mm) or short (16 mm) C2 pedicle screws. This manuscript focused on an in vitro biomechanical comparison of standard versus short C2 pedicle screws to perform posterior C1-C2 AA fusions.Methods:Twelve human cadaveric spines underwent C1 lateral mass screw and standard C2 pedicle screw (n = 6) versus short C2 pedicle screw (n = 6) fixation. Six additional controls were not instrumented. The peak torque, peak rotational interval, and peak stiffness of the constructs were analyzed to failure levels.Results:The peak torque to construct failure was not statistically significantly different among the control spine (12.2 Nm), short pedicle fixation (15.5 Nm), or the standard pedicle fixation (11.6 Nm), P = 0.79. While the angle at the peak rotation statistically significantly differed between the control specimens (47.7° of relative motion) and the overall instrumented specimens (P < 0.001), the 20.7° of relative rotation in the short C2 pedicle screw specimens was not statistically significantly higher than the 13.7° of relative rotation in the standard C2 pedicle screw specimens (P = 0.39). Similarly, although the average stiffness was statistically significantly lower in control group (0.026 Nm/degree) versus the overall instrumented specimens (P = 0.001), the standard C2 pedicle screws (2.54 Nm/degree) did not differ from the short C2 pedicle screwsConclusions:Both standard and short C2 pedicle screws allow for equally rigid fixation of C1 lateral mass-C2 AA fusions. Usage of a short C2 pedicle screw may be an acceptable method of stabilization in carefully selected patient populations.

Are Deer and Boar Spines a Valid Biomechanical Model for Human Spines?

Objective: To examine the validity of using cadaveric spines of deer or boars for biomechanical experiments as substitutes for the cadaveric spine of humans. Materials and Methods: Five specimens of the L3-4 functional spinal unit of human cadavers, mature deer and mature boars were prepared according to 3 models: 1) normal model, 2) injured model and 3) pedicle screw fixation model and they were evaluated in 8-direction bending and 2-direction rotation tests. The mean ROM in bending and rotation tests of each specimen and the rate of relative change of ROM were calculated. Results: Flexibility of cadaveric spine of deer and boars was slightly higher than that of cadaveric spine of humans in the bending and rotation tests, but the rates of relative change of ROM in the rotational and bending tests were similar across species. Conclusions: It is reasonable to use cadaveric spines of deer and boars as a model of the human cadaveric spine in biomechanical experiments.

Biomechanical Comparison of One-and Two-Level Cervical Arthroplasty Versus Fusion

PurposeTo analyze the biomechanics of cervical spine after one-and two-level Total Disc Replacement (TDR) and two-level Anterior Cervical Discectomy and Fusion (ACDF).MethodsSeven adult human cadaveric cervical spines were biomechanically evaluated under eccentric displacement control in six mechanical modes, including flexion (Flex), extension (Ext), left bending (LB), right rending (RB), left rotation (LR) and right rotation (RR).ResultsIn fusion-treated specimens, range of motion (ROM) at instrumented level decreased as much as 81.78%, and other levels also demonstrated big difference in ROM. In arthroplasty-treated specimens, ROM showed little difference from that of the intact state. Large motion variation happened in LB, RB and Ext after both fusion and nonfusion surgical treatments.ConclusionsTDR had a more reasonable motion sharing than ACDF, especially in Flex, Ext, LR and RR. No evident influence of motion change was observed after adding an extra level of TDR.

Biomechanical analysis of the upper thoracic spine after decompressive procedures

Background contextDecompressive procedures such as laminectomy, facetectomy, and costotransversectomy are routinely performed for various pathologies in the thoracic spine. The thoracic spine is unique, in part, because of the sternocostovertebral articulations that provide additional strength to the region relative to the cervical and lumbar spines. During decompressive surgeries, stability is compromised at a presently unknown point. PurposeTo evaluate thoracic spinal stability after common surgical decompressive procedures in thoracic spines with intact sternocostovertebral articulations. Study designBiomechanical cadaveric study. MethodsFresh-frozen human cadaveric spine specimens with intact rib cages, C7–L1 (n=9), were used. An industrial robot tested all spines in axial rotation (AR), lateral bending (LB), and flexion-extension (FE) by applying pure moments (±5 Nm). The specimens were first tested in their intact state and then tested after each of the following sequential surgical decompressive procedures at T4–T5 consisting of laminectomy; unilateral facetectomy; unilateral costotransversectomy, and subsequently instrumented fusion from T3–T7. ResultsWe found that in all three planes of motion, the sequential decompressive procedures caused no statistically significant change in motion between T3–T7 or T1–T12 when compared with intact. In comparing between intact and instrumented specimens, our study found that instrumentation reduced global range of motion (ROM) between T1–T12 by 16.3% (p=.001), 12% (p=.002), and 18.4% (p=.0004) for AR, FE, and LB, respectively. Age showed a negative correlation with motion in FE (r=−0.78, p=.01) and AR (r=−0.7, p=.04). ConclusionsThoracic spine stability was not significantly affected by sequential decompressive procedures in thoracic segments at the level of the true ribs in all three planes of motion in intact thoracic specimens. Age appeared to negatively correlate with ROM of the specimen. Our study suggests that thoracic spinal stability is maintained immediately after unilateral decompression at the level of the true ribs. These preliminary observations, however, do not depict the long-term sequelae of such procedures and warrant further investigation.

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

In this paper the authors evaluate through in vitro biomechanical testing the performance of an interspinous fusion device as a stand-alone device, after lumbar decompression surgery, and as supplemental fixation to expandable cages in a posterior lumbar interbody fusion (PLIF) construct. Nine L3-4 human cadaveric spines were biomechanically tested under the following conditions: 1) intact/control; 2) L3-4 left hemilaminotomy with partial discectomy (injury); 3) interspinous spacer (ISS); 4) bilateral pedicle screw system (BPSS); 5) bilateral hemilaminectomy, discectomy, and expandable posterior interbody cages with ISS (PLIF-ISS); and 6) PLIF-BPSS. Each test consisted of 100 N of axial preload with ± 7.5 Nm of torque in flexion-extension, right/left lateral bending, and right/left axial rotation. Significant changes in range of motion (ROM), neutral zone stiffness (NZS), elastic zone stiffness (EZS), and energy loss (EL) were explored among conditions using nonparametric Friedman test and Wilcoxon signed-rank comparisons (p ≤ 0.05). The injury increased ROM in flexion (p = 0.01), left bending (p = 0.03), and right/left rotation (p < 0.01) and also decreased NZS in flexion (p = 0.01) and extension (p < 0.01). Both the ISS and BPSS reduced flexion-extension ROM and increased flexion-extension stiffness (NZS and EZS) with respect to the injury and intact conditions (p < 0.05), but the ISS condition provided greater resistance than BPSS in extension for ROM, NZS, and EZS (p < 0.01). The BPSS increased the rigidity (ROM, NZS, and EZS) of the intact model in lateral bending and axial rotation (p ≤ 0.01), except in EZS for left rotation (p = 0.23, Friedman test). The incorporation of posterior cages marginally increased (p = 0.05) the EZS of the BPSS construct in flexion but these interbody devices provided significant stability to the ISS construct in lateral bending and axial rotation for ROM (p = 0.02), in lateral bending for NZS (p = 0.02), and in flexion/axial rotation for EZS (p ≤ 0.03); however, both PLIF constructs demonstrated equivalent ROM and stiffness (p ≥ 0.16), except in lateral bending where the PLIF-BPSS was more stable (p = 0.02). In terms of EL, the injury increased EL in flexion-extension (p = 0.02), the ISS increased EL for lateral bending and axial rotation (p ≤ 0.03), and the BPSS decreased EL in lateral bending (p = 0.02), with respect to the intact condition. The PLIF-ISS decreased lateral bending EL with respect to the ISS condition (p = 0.02), but not enough to be smaller or, at least, equivalent, to that of the PLIF-BPSS construct (p = 0.02). The ISS may be a suitable device to provide immediate flexion-extension balance after a unilateral laminotomy, but the BPSS provides greater immediate stability in lateral bending and axial rotation motions. Both PLIF constructs performed equivalently in flexion-extension and axial rotation, but the PLIF-BPSS construct is more resistant to lateral bending motions. Further biomechanical and clinical evidence is required to strongly support the recommendation of a stand-alone interspinous fusion device or as supplemental fixation to expandable posterior interbody cages.

Biomechanics of an integrated interbody device versus ACDF anterior locking plate in a single-level cervical spine fusion construct

Background contextNo profile, integrated interbody cages are designed to act as implants for cervical spine fusion, which obviates the need for additional internal fixation, combining the functionality of an interbody device and the stabilizing benefits of an anterior cervical plate. Biomechanical data are needed to determine if integrated interbody constructs afford similar stability to anterior plating in single-level cervical spine fusion constructs. PurposeThe purpose of this study was to biomechanically quantify the acute stabilizing effect conferred by a single low-profile device design with three integrated screws (“anchored cage”), and compare the range of motion reductions to those conferred by a standard four-hole rigid anterior plate following instrumentation at the C5–C6 level. We hypothesized that the anchored cage would confer comparable postoperative segmental rigidity to the cage and anterior plate construct. Study designBiomechanical laboratory study of human cadaveric spines. MethodsSeven human cadaveric cervical spines (C3–C7) were biomechanically evaluated using a nondestructive, nonconstraining, pure-moment loading protocol with loads applied in flexion, extension, lateral bending (right+left), and axial rotation (left+right) for the intact and instrumented conditions. Range of motion (ROM) at the instrumented level was the primary biomechanical outcome. Spines were loaded quasi-statically up to 1.5 N-m in 0.5 N-m increments and ROM at the C5–C6 index level was recorded. Each specimen was tested in the following conditions:1. Intact2. Discectomy+anchored cage (STA)3. Anchored cage (screws removed)+anterior locking plate (ALP)4. Anchored cage only, without screws or plates (CO) ResultsROM at the C5–C6 level was not statistically different in any motion plane between the STA and ALP treatment conditions (p>.407). STA demonstrated significant reductions in flexion/extension, lateral bending, and axial rotation ROM when compared with the CO condition (p<.022). ConclusionsIn this in vitro biomechanical study, the anchored cage with three integrated screws afforded biomechanical stability comparable to that of the standard interbody cage+anterior plate cervical spine fusion approach. Due to its low profile design, this anchored cage device may avoid morbidities associated with standard anterior plating, such as dysphagia.

The effect of neighboring segments on the measurement of segmental stiffness in the intact lumbar spine

Background contextDegeneration, injury, and surgical interventions may alter the mechanical properties of spinal motion segments, but the quantification of these alterations in vivo is problematic. Manual or instrumented loading of single segments in the intact spine as applied intraoperatively may overestimate the mechanical properties of this segment, because the applied load is partly sustained by the adjacent segments. PurposeThe distribution of stiffness values of individual spinal segments within and across spines was determined so as to use these data as input to a model simulation of segment stiffness tests in intact spines, to assess measurement errors. Study designBiomechanical stiffness measurements on human cadaveric spines and model simulation to assess measurement errors. MethodsSeventeen human cadaveric lumbar spines were loaded with pure moments in flexion/extension, lateral bending, and torsion. An optical system was used to measure the angular rotations of each motion segment and load-displacement curves were used to determine stiffness. With the distribution of measured stiffness data as input, a stochastic mechanical model was constructed to investigate how the stiffness of adjacent segments influences stiffness estimates obtained by loading a single segment in the intact spine. ResultsThe variance in stiffness values was high for all directions, but covaried between segments within a spine. Model simulations indicated that stiffness estimates obtained by loading a single segment in an intact spine are highly correlated with actual stiffness, but overestimate stiffness by a median of 18% with peak errors of close to 400%. ConclusionCurrent measurement devices and manual assessment substantially overestimate segmental stiffness due to the effect of adjacent spinal levels. In addition, the variance in stiffness within spines can occasionally cause large errors, which might lead to erroneous surgical decisions.

Stability of transforaminal lumbar interbody fusion in the setting of retained facets and posterior fixation using transfacet or standard pedicle screws

Background contextThe transforaminal lumbar interbody fusion (TLIF) technique supplements posterior instrumented lumbar spine fusion and has been tested with different types of screw fixation for stabilization. Transforaminal lumbar interbody fusion is usually placed through a unilateral foraminal approach after unilateral facetectomy, although extraforaminal entry allows the facet to be spared. PurposeTo characterize the biomechanics of L4–L5 lumbar motion segments instrumented with bilateral transfacet pedicle screw (TFPS) fixation versus bilateral pedicle screw-rod (PSR) fixation in the setting of intact facets and native disc or after discectomy and extraforaminal placement of a TLIF technology graft. Study designHuman cadaveric lumbar spine segments were biomechanically tested in vitro to provide a nonpaired comparison of four configurations of posterior and interbody instrumentation. MethodsFourteen human cadaveric spine specimens (T12–S1) underwent standard pure moment flexibility tests with intact L4–L5 disc and facets. Seven were studied with intact discs, after TFPS fixation, and then with TLIF and TFPS fixation. The others were studied with intact discs, after PSR fixation, and then combined with extraforaminally placed TLIF. Loads were applied about anatomic axes to induce flexion-extension, lateral bending, and axial rotation. Three-dimensional specimen motion in response to applied loads during flexibility tests was determined. A nonpaired comparison of the four configurations of posterior and interbody instrumentation was made. ResultsTransfacet pedicle screw and PSR, with or without TLIF, significantly reduced range of motion during all directions of loading. Transfacet pedicle screw provided greater stability than PSR in all directions of motion except lateral bending. In flexion, TFPS was more stable than PSR (p=.042). A TLIF device provided less stability than the intact disc in constructs with TFPS and PSR. ConclusionsThese results suggest that fixation at L4–L5 with TFPS is a promising alternative to PSR, with or without TLIF. A TLIF device was less stable than the native disc with both methods of instrumentation presumably because of a fulcrum effect from a relatively small footplate. Additional interbody support may be considered for improved biomechanics with TLIF.

Height restoration of osteoporotic vertebral compression fractures using different intravertebral reduction devices: a cadaveric study

BackgroundThe treatment of osteoporotic vertebral compression fractures using transpedicular cement augmentation has grown significantly during the past two decades. Balloon kyphoplasty was developed to restore vertebral height and improve sagittal alignment. Several studies have shown these theoretical improvements cannot be transferred universally to the clinical setting. PurposeThe aim of the current study is to evaluate two different procedures used for percutaneous augmentation of vertebral compression fractures with respect to height restoration: balloon kyphoplasty and SpineJack. Materials and methodsTwenty-four vertebral bodies of two intact, fresh human cadaveric spines (T6–L5; donor age, 70 years and 60 years; T-score −6.8 points and −6.3 points) were scanned using computed tomography (CT) and dissected into single vertebral bodies. Vertebral wedge compression fractures were created by a material testing machine (Universal testing machine, Instron 5566, Darmstadt, Germany). The axial load was increased continuously until the height of the anterior edge of the vertebral body was reduced by 40% of the initial measured values. After 15 minutes, the load was decreased manually to 100 N. After postfracture CT, the clamped vertebral bodies were placed in a custom-made loading frame with a preload of 100 N. Twelve vertebral bodies were treated using SpineJack (SJ; Vexim, Balma, France), the 12 remaining vertebral bodies were treated with balloon kyphoplasty (BKP; Kyphon, Medtronic, Sunnyvale, CA, USA). The load was maintained during the procedure until the cement set completely. Posttreatment CT was performed. Anterior, central, and posterior height as well as the Beck index were measured prefracture and postfracture as well as after treatment. ResultsFor anterior height restoration (BKP, 0.14±1.48 mm; SJ, 3.34±1.19 mm), central height restoration (BKP, 0.91±1.04 mm; SJ, 3.24±1.22 mm), and posterior restoration (BKP, 0.37±0.57 mm; SJ, 1.26±1.05), as well as the Beck index (BKP, 0.00±0.06 mm; SJ, 0.10±0.06), the values for the SpineJack group were significantly higher (p<.05) ConclusionThe protocols for creating wedge fractures and using the instrumentation under a constant preload of 100 N led to reproducible results and effects. The study showed that height restoration was significantly better in the SpineJack group compared with the balloon kyphoplasty group. The clinical implications include a better restoration of the sagittal balance of the spine and a reduction of the kyphotic deformity, which may relate to clinical outcome and the biological healing process.

Posterior instrumentation improves the stabilities of Brantigan and Bagby and Kuslich (BAK) methods of posterior lumbar interbody fusion across the L4–L5 segments in a cadaveric model

BackgroundThe Brantigan and Bagby and Kuslich (BAK) cages for posterior lumbar interbody fusion have different geometric characteristics. However, both cage designs have been demonstrated to be helpful in restoring disc space across spinal motion segments in clinical observations. This study was designed to compare the biomechanical performance of these devices at one-motion segments and to determine the effects of posterior instrumentation on their stabilities. MethodsEight intact fresh human cadaver spines (L2-S1) were affixed within a testing frame for in vitro biomechanical testing: four randomly assigned spines for the BAK cage group and four for the Brantigan cage group. For each spine, the three-dimensional load-displacement behavior of each vertebra was quantified using the Selspot II Motion measurement system during the following steps: (1) intact state; (2) destabilization after laminectomy and discectomy across L4-L5; (3) stabilization using a pair of BAK cages or Brantigan cages; and (4) additional stabilization using variable screw plates (VSP) across L4-L5. ResultsThe Brantigan cage alone did not show satisfactory results in improving the stability of one-motion segment destabilized spines in left and right axial rotation. However, the BAK cages appeared to provide significant stability in extension, flexion, left and right lateral bending, and left axial rotation. After implanting the additional posterior instrumentation, both cages provided similar and significantly improved stabilities. ConclusionAlthough the results indicate that the Brantigan cage did not provide satisfactory improvement in the stabilities as the BAK cage in the one-motion segment model, implantation with additional posterior instrumentation may significantly improve the stabilities and reduce the differences between the two cage designs.

Biomechanical comparison of a two-level anterior discectomy and a one-level corpectomy, combined with fusion and anterior plate reconstruction in the cervical spine

Common fusion techniques for cervical degenerative diseases include two-level anterior discectomy and fusion and one-level corpectomy and fusion. The aim of the study was to compare via in-vitro biomechanical testing the effects of a two-level anterior discectomy and fusion and a one-level corpectomy and fusion, with anterior plate reconstruction. Seven fresh frozen human cadaveric spines (C3-T1) were dissected from posterior musculature, preserving the integrity of ligaments and intervertebral discs. Initial biomechanical testing consisted of no-axial preload and 2Nm in flexion-extension, lateral bending and axial rotation. Thereafter, discectomies were performed at C4-5 and C5-6 levels, then two interbody cages and an anterior C4-C5-C6 plate was implanted. The flexibility tests were repeated and followed by C5 corpectomy and C4-C6 plate reconstruction. Biomechanical testing was performed again and statistical comparisons among the means of range of motion and axial rotation energy loss were investigated. The two-level cage-plate construct had significantly lower range of motion than the one-level corpectomy-plate construct (P≤0.03). Axial rotation energy loss was significantly (P≤0.03) greater for the corpectomy-plate construct than for the two-level cage-plate construct and the intact condition. A two-level cage-plate construct provides greater stability in flexion, extension and lateral bending motions when compared to a one-level corpectomy-plate construct. A two-level cage-plate is more likely to maintain axial balance by reducing the energy lost in axial rotation.

Innervation of pathologies in the lumbar vertebral end plate and intervertebral disc

Background contextMagnetic resonance imaging (MRI) has limited diagnostic value for chronic low back pain because of the unclear relationship between any anatomic abnormalities on MRI and pain reported by the patient. Assessing the innervation of end plate and disc pathologies—and determining the relationship between these pathologies and any abnormalities seen on MRI—could clarify the sources of back pain and help identify abnormalities with enhanced diagnostic value. PurposeTo quantify innervation in the vertebral end plate and intervertebral disc and to relate variation in innervation to the presence of pathologic features observed by histology and conventional MRI. Study design/settingA cross-sectional histology and imaging study of vertebral end plates and intervertebral discs harvested from human cadaver spines. MethodsWe collected 92 end plates and 46 intervertebral discs from seven cadaver spines (ages 51–67 years). Before dissection, the spines were scanned with MRI to grade for Modic changes and high-intensity zones (HIZ). Standard immunohistochemical techniques were used to localize the general nerve marker protein gene product 9.5. We quantified innervation in the following pathologies: fibrovascular end-plate marrow, fatty end-plate marrow, end-plate defects, and annular tears. ResultsNerves were present in the majority of end plates with fibrovascular marrow, fatty marrow, and defects. Nerve density was significantly higher in fibrovascular end-plate marrow than in normal end-plate marrow (p<.001). Of the end plates with fibrovascular and fatty marrow, less than 40% were Modic on MRI. Innervated marrow pathologies collocated with more than 75% of the end plate defects; hence, innervation was significantly higher in end plate defects than in normal end plates (p<.0001). In the disc, nerves were observed in only 35% of the annular tears; in particular, innervation in radial tears tended to be higher than in normal discs (p=.07). Of the discs with radial tears, less than 13% had HIZ on T2 MRI. Innervation was significantly less in radial tears than in fibrovascular end-plate marrow (p=.05) and end-plate defects (p=.02). ConclusionsThese findings indicate that vertebral end-plate pathologies are more innervated than intervertebral disc pathologies and that many innervated end-plate pathologies are not detectable on MRI. Taken together, these findings suggest that improved visualization of end-plate pathologies could enhance the diagnostic value of MRI for chronic low back pain.

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

Background contextThe soft tissues of the spine exhibit sensitivity to strain-rate and temperature, yet current knowledge of spine biomechanics is derived from cadaveric testing conducted at room temperature at very slow, quasi-static rates. PurposeThe primary objective of this study was to characterize the change in segmental flexibility of cadaveric lumbar spine segments with respect to multiple loading rates within the range of physiologic motion by using specimens at body or room temperature. The secondary objective was to develop a predictive model of spine flexibility across the voluntary range of loading rates. Study designThis in vitro study examines rate- and temperature-dependent viscoelasticity of the human lumbar cadaveric spine. MethodsRepeated flexibility tests were performed on 21 lumbar function spinal units (FSUs) in flexion-extension with the use of 11 distinct voluntary loading rates at body or room temperature. Furthermore, six lumbar FSUs were loaded in axial rotation, flexion-extension, and lateral bending at both body and room temperature via a stepwise, quasi-static loading protocol. All FSUs were also loaded using a control loading test with a continuous-speed loading-rate of 1-deg/sec. The viscoelastic torque-rotation response for each spinal segment was recorded. A predictive model was developed to accurately estimate spine segment flexibility at any voluntary loading rate based on measured flexibility at a single loading rate. ResultsStepwise loading exhibited the greatest segmental range of motion (ROM) in all loading directions. As loading rate increased, segmental ROM decreased, whereas segmental stiffness and hysteresis both increased; however, the neutral zone remained constant. Continuous-speed tests showed that segmental stiffness and hysteresis are dependent variables to ROM at voluntary loading rates in flexion-extension. To predict the torque-rotation response at different loading rates, the model requires knowledge of the segmental flexibility at a single rate and specified temperature, and a scaling parameter. A Bland-Altman analysis showed high coefficients of determination for the predictive model. ConclusionsThe present work demonstrates significant changes in spine segment flexibility as a result of loading rate and testing temperature. Loading rate effects can be accounted for using the predictive model, which accurately estimated ROM, neutral zone, stiffness, and hysteresis within the range of voluntary motion.