Biomedical subjects

Vijay K Goel

Publications and source records attributed to Vijay K Goel.

At least 19 recordsLinked to original sources

PubMed2006

Biomechanical rationale for using polyetheretherketone (PEEK) spacers for lumbar interbody fusion-A finite element study.

STUDY DESIGN: To determine the effect of cage/spacer stiffness on the stresses in the bone graft and cage subsidence. OBJECTIVE: To investigate the effect of cage stiffness on the biomechanics of the fused segment in the lumbar region using finite element analysis. SUMMARY OF BACKGROUND DATA: There are a wide variety of cage/spacer designs available for lumbar interbody fusion surgery. These range from circular, tapered, rectangular with and without curvature, and were initially manufactured using titanium alloy. Recent advances in the medical implant industry have resulted in using medical grade polyetheretherketone (PEEK). The biomechanical advantages of using different cage material in terms of stability, subsidence, and stresses in bone graft are not fully understood. METHODS: A previously validated 3-dimensional, nonlinear finite element model of an intact L3-L5 segment was modified to simulate posterior interbody fusion spacers made of PEEK ("E" = 3.6 GPa) and titanium ("E" = 110 GPa) at the L4/5 disc with posterior instrumentation. Bone graft ("E" = 12 GPa) packed between the spacers in the intervertebral space was also simulated. The posterior lumbar interbody fusion spacer with instrumentation and graft represent a simulation of the condition present immediately after surgery. RESULTS: The peak centroidal Von Mises stresses in the graft bone increased by at least 9-fold with PEEK spacers as compared to titanium spacer. The peak centroidal Von Mises stresses in the endplates increased by at least 2.4-fold with titanium spacers over the PEEK spacers. These stresses were concentrated at places where the spacer interfaced with the endplate. The stiffness of the spacer did not affect the relative motion (stability) across the instrumented (L4/5) segment. CONCLUSIONS: Spacers less stiff than the graft will: (1) provide stability similar to titanium cages in the presence of posterior instrumentation, (2) reduce the stresses in endplates adjacent to the spacers, and (3) increase the load transfer through the graft, as evident from the increase in stresses in graft.

Benzophenones↗

PubMed2006

Biomechanics of two-level Charité artificial disc placement in comparison to fusion plus single-level disc placement combination.

BACKGROUND CONTEXT: Biomechanical studies of artificial discs that quantify parameters such as load sharing and stresses have been reported in literature for single-level disc placements. However, literature on the effects of using the Charité artificial disc (ChD) at two levels (2LChD) as compared with one-level fusion (using a cage [CG] and a pedicle screw system) plus one-level artificial disc combination (CGChD) is sparse. PURPOSE: To determine the effects of the 2LChD and CGChD across the implanted and adjacent segments. STUDY DESIGN: A finite element model of a L3-S1 segment was used to compare the biomechanical effects of the ChD placed at two lower levels (2LChD model) with L5-S1 fusion (using a CG and a pedicle screw system) plus L4-L5 level ChD placement combination (CGChD model). METHODS: We used our recently published and experimentally validated L3-S1 finite element model for the present study. The intact model was subjected to 400 N axial compression and 10.6 Nm of flexion/extension moments. The experimental constructs described above were then subjected to 400 N axial compression and a moment that produced overall motion equal to the intact model predictions (hybrid testing protocol). Resultant motion, loads across facets, and other parameters were analyzed at the experimental and adjacent levels. RESULTS: In flexion, the bending moments for the CGChD and 2LChD models were 15.4 Nm (fusion effect) and 7.3 Nm (increase in flexibility effect), respectively in comparison to 10.6 Nm for the intact model. The corresponding values in the extension mode were 11.2 Nm and 7.2 Nm. The predicted flexion rotations across the L5-S1 segment for the CGChD decreased by 76% (fusion effect), and increased at the L4-L5 and the L3-L4 levels by 68.5% and 28%, respectively. In the extension mode, motion across the L5-S1 segment decreased by 96.4% whereas it increased 74.6% and 18.2% across the L4-L5 and L3-L4 levels, respectively. For the 2LChD model, the flexion rotation across the L5-S1 segment increased by 28.2%. The motions across the L4-L5 and L3-L4 segments decreased by 12% and 24%, respectively. In extension, the corresponding changes were 10% increase, 10% increase, and 21% decrease at the L5-S1, L4-L5, and L3-L4 levels, respectively. The facet loads were in line with the changes in motion, except for the 2LChD case. CONCLUSIONS: The changes at L3-L4 level for both of the cases were of similar magnitude (approximately 25%), although in the CGChD model it increased and in the 2LChD model it decreased. The changes in motion at the L4-L5 level were large for the CGChD model as compared with the 2LChD model predictions (approximately 70% increase vs. 10% increase). It is difficult to speculate if an increase in motion across a segment, as compared with the intact case, is more harmful than a decrease in motion.

Biomechanical Phenomena↗

PubMed2006

Buck's direct repair of lumbar spondylolysis restores disc stresses at the involved and adjacent levels.

BACKGROUND: Lumbar spondylolysis was reported to cause disc degeneration at both caudal and cranial adjacent level. However, basic disc biomechanics in the spondylolytic spine is not fully understood. Purpose of this study was to analyze the disc stresses at cranial and caudal adjacent level of lumbar spondylolysis. Also, the biomechanical effects of Buck's technique on disc stresses at these two segments were evaluated. METHODS: An experimentally validated three-dimensional non-linear finite element model of the intact ligamentous L3-S1 segment was used. Bilateral lumbar spondylolysis was simulated by creating bilateral pars defects with 1.0 mm gap at L5. Buck's direct repair model was simulated with 4.0 mm cannulated Titanium screws, placed bilaterally across the defect. Von Mises stresses in the annulus fibrosus and nucleus pulposus at L4/5 (cranial adjacent) and L5/S (caudal adjacent) disc levels were analyzed in flexion, extension, lateral bending and axial rotation in response to 400 N of axial compression and 10.6 Nm moment. The highest values were compared among the three models, intact, spondylolysis and Buck's technique. FINDINGS: After spondylolysis occurred at L5, annulus fibrosus and nucleus pulposus stresses at L4/5 increased to 111% and 120%, respectively. After the Buck's technique it recovered to 102% and 105%, correspondingly. On the other hand, at L5/S, annulus fibrosus stress increased to 168%, and nucleus pulposus, 155%, which was much higher when compared to the stresses at L4/5. After the Buck's technique the stresses were decreased to 125% and 120%, correspondingly. During rotation motion, especially, the operation normalized the disc stress completely. INTERPRETATION: Spondylolysis increases disc stresses at the affected as well as cranial adjacent level, and it may lead to disc degeneration at both levels. However, the increase in stresses is higher at the affected caudal level, when compared to the cranial level. Buck's technique may restore the disc stresses back to normal at both disc levels. Thus, this technique may be beneficial from a biomechanical perspective as well.

Bone Screws↗

PubMed2006

Effect of lumbar interbody cage geometry on construct stability: a cadaveric study.

STUDY DESIGN: Biomechanical study to investigate three-dimensional motion behavior of cadaveric spines in various surgical simulations. OBJECTIVES: To determine the effect of cage geometry on the construct stability. SUMMARY OF BACKGROUND DATA: There is a wide variety of cage/spacer designs available for lumbar interbody fusion surgery. These range from circular, tapered, and rectangular with and without curvature. However, the effectiveness of cages with different designs and materials to stabilize a decompressed intervertebral space has not been fully studied. METHODS: Six fresh ligamentous lumbar spine specimens (L1-S2) were subjected to pure moments in the six loading directions. The resulting spatial orientations of the vertebrae were recorded using Optotrak Motion Measurement System. Measurements were made sequentially for intact, bilateral spacer placements across L4-L5 using a posterior approach, supplemented with pedicle screw-rod system fixation, and after the cyclic loading in flexion-extension mode. RESULTS: The stability tended to decrease after the bilateral cage placement as compared with the intact for all loading cases except flexion. In flexion, the angular displacement decreased to 80% of the intact. However, there was no significant statistical difference seen in stability between intact and after bilateral spacer placement. Following the addition of posterior fixation using pedicle screw-rod system, the stability significantly increased in all directions. Cyclic loading did not have any significant effect on the stability. CONCLUSIONS: Stand-alone cages restore motion to near-intact levels at best, and supplement instrumentation is essential for significantly increasing the stability of the decompressed segment. The effects of cage geometry and Young's modulus of the cage material do not seem to influence the stability, as compared with the other cagedesigns, especially after supplemental fixation with a posterior system.

Aged↗

PubMed2006

How the increase of the cervical disc space height affects the facet joint: an anatomy study.

STUDY DESIGN: In vitro study on the effect of increasing the height of the cervical disc space on the facet joint. OBJECTIVES: To demonstrate how facet joint articulation is affected by increasing the cervical disc space height. SUMMARY OF BACKGROUND DATA: A surgeon attempts to increase the disc space and inserts a larger artificial disc than normal in order to keep the intervertebral foramen open and the prosthesis stable. However, it is hypothesized by the current authors that this procedure could have an adverse effect on the facet joints. METHODS: Computerized tomography images passing through the disc space and the center of the C4-C7 facet joints (sagittal plane) were obtained from 15 cadaveric cervical spine specimens. A 1-mm incremental increase to a total 5 mm in disc space height was performed to simulate the changes seen in disc replacement. The change in the facet joint articulation overlap and space in the sagittal plane at normal and each displacement was measured. RESULTS: Each 1-mm incremental increase in disc space at C4-C5 translated to a decrease in the facet joint articulation overlap in the sagittal plane by approximately 8%. The mean facet joint space increased approximately 0.8 mm. At the C5-C6 and the C6-C7 levels, the articulation overlap decreased by approximately 7% and the facet joint space increased approximately 0.8 mm. CONCLUSIONS: There is a significant decrease of the facet joint articulation overlap in the sagittal plane and an increase in the facet joint space following an increase in the cervical disc space. The inappropriate increase of the disc space height may result in facet joint subluxation and could lead to the accelerated failure of the artificial disc.

Cadaver↗

PubMed2006

Three-dimensional finite element analysis of the pediatric lumbar spine. Part I: pathomechanism of apophyseal bony ring fracture.

The purpose of this study was to (1) develop a three-dimensional, nonlinear pediatric lumbar spine finite element model (FEM), and (2) identify the mechanical reasons for the posterior apophyseal bony ring fracture in the pediatric patients. The pediatric spine FE model was created from an experimentally validated three-dimensional adult lumbar spine FEM. The size of the FEM was reduced to 96% taking into account of the ratio of the sitting height of an average 14-years-old children to that of an adult. The pediatric spine was created with anatomically specific features like the growth plate and the apophyseal bony ring. For the stress analyses, a 10-N m moment was applied in all the six directions of motion for the lumbar spine. A preload of 351 N was applied which corresponds to the mean body weight of the 14-years-old group. The stresses at the apophyseal bony ring, growth plate and endplate were calculated. The results indicate that the structures surrounding the growth plate including apophyseal bony ring and osseous endplate were highly stressed, as compared to other structures. Furthermore, posterior structures in extension were in compression whereas in flexion they were in tension, with magnitude of stresses higher in extension than in flexion. Over time, the higher compression stresses along with tension stresses in flexion may contribute to the apophyseal ring fracture (fatigue phenomena).

Adult↗

PubMed2006

Three dimensional finite element analysis of the pediatric lumbar spine. Part II: biomechanical change as the initiating factor for pediatric isthmic spondylolisthesis at the growth plate.

A non-linear 3-dimensional finite element pediatric lumbar spine model with vertebral growth plate and apophyseal bony ring was developed. Lumbar spondylolysis was simulated in the model. The Von Mises stresses in the structures surrounding the vertebral growth plate, including apophyseal bony ring and osseous endplate were calculated in various loading modes. Instantaneous axis of rotation (IAR) path from flexion to extension was also analyzed. The results were compared with those of the intact model and the literature. The IAR path was at the posterior disc-endplate space of the lower vertebra in the intact spine, and moved cranially towards the upper-posterior disc space in the lytic spine. This was in agreement with in vivo radiological data by Sakamaki et al. [19]. During various loading modes, stresses in the spondylolytic pediatric model were higher than that of the intact model; ranging from 1.1 to 6.0 times, with the highest value in extension at the growth plate. In conclusion, FE models indicate that stress concentrations in the lytic model increase at the growth plate which may lead to physis stress fracture leading to spondylolisthesis.

Biomechanical Phenomena↗

PubMed2006

Effect of the increase in the height of lumbar disc space on facet joint articulation area in sagittal plane.

STUDY DESIGN: Computerized tomography (CT) of the lumbar spine cadaveric specimens was used to evaluate the effect of increasing the height of the disc space in the lumbar spine to the facet joint articulation in the sagittal plane. OBJECTIVE: To show how the facet joint articulation is affected by increasing the height of the disc space in the lumbar spine. SUMMARY OF BACKGROUND DATA: The Charité Artificial Disc (DePuy Spine, Inc., Raynham, MA) was successful in relieving low back pain in the majority of patients, yet there was still a significant number of patients who did not obtain pain relief, or their pain even worsened. The etiology of their pain is still not known. To our knowledge, no study has addressed the effect on the facet joints when the disc height is increased. METHODS: CT images passing through the center of the L3-S1 facet joints (sagittal plane) were obtained from 15 cadaveric lumbar spine specimens. The articulation overlap of facet joints in sagittal plane from the L3 to S1 was measured. A 1-mm incremental increase to a total 5 mm in disc space height was performed to simulate the changes seen in disc replacement. The change in the facet joint articulation overlap in sagittal plane at normal and each displacement was then measured. There were 5 lumbar spine specimens dissected to validate the technique and standardize the measurements. Mean, percentages, and standard deviation values were calculated for all measured dimensions. RESULTS: No significant difference was found between the measurements on CT and gross specimens (P > 0.05). In 15 specimens, the mean facet joint articulation overlap on the sagittal plane was: 16.29 +/- 1.20 mm (left) and 16.22 +/- 1.16 (right) at the L3-L4 level; 17.81 +/- 1.18 mm (left) and 17.74 +/- 1.18 mm (right) at the L4-L5 level; and 18.18 +/- 1.18 mm (left) and 18.23 +/- 1.15 mm (right) at the L5-S1 level. There is no significant difference between the measured values on left and right sides (P > 0.05). Each 1-mm incremental increase in disc space at the L3-L4 level translated to a decrease in the facet joint articulation overlap in the sagittal plane by 6%, and the mean facet joint space increased 0.4 mm. At the L4-L5 level, the articulation overlap decreased by 6%, and the facet joint space increased 0.5 mm. At the L5-S1 level, the articulation overlap decreased by 4%, and the facet joint space increased 0.7 mm. CONCLUSIONS: There is a significant decrease of the facet joint articulation overlap in sagittal plane and an increase in the facet joint space following an increase in the lumbar disc space. The inappropriate increase of the height of disc space will result in facet joint subluxation.

Humans↗

PubMed2006

Comparison of two interbody fusion cages for posterior lumbar interbody fusion in a cadaveric model.

Although the Brantigan cage and Bagby and Kuslich (BAK) cage have different geometrical characteristics, clinical observations suggest that they are equally effective in restoring disc height and stability across the involved spinal segments. This study was designed to compare their performance as posterior lumbar interbody fusion devices at two levels in fresh ligamentous cadaver lumbar spines (L2-S1). After mounting in a testing frame, the three-dimensional load-displacement behaviour of each vertebra was quantified using the Selspot II Motion Measurement System for; the intact state, posterior decompression, and stabilisation, using a pair of Brantigan or BAK cages across L4-S1, additional stabilisation using Isola spinal instrumentation across L4-S1, and cyclic loading in flexion/extension. In the "cage-only" state, the Brantigan cage did not restore the stability in right axial rotation, whereas the BAK cage not only restored stability in all six directions but also improved lateral bending. After implanting the posterior instrumentation, both groups exhibited similar stability, and cyclic loading did not alter this. Although the Brantigan cage appears less effective than the BAK cage, implantation of posterior instrumentation significantly improves stability and reduces the differences between them. This underscores the need to use posterior instrumentation to achieve a higher initial stability.

Aged↗

PubMed2006

Lumbar spinal disorders in patients with athetoid cerebral palsy: a clinical and biomechanical study.

STUDY DESIGN: Radiologic study for patients with athetoid cerebral palsy (CP), and a biomechanical study using the finite-element model (FEM). OBJECTIVES: To understand the lumbar disorders of athetoid CP patients and the etiology based on the results from the FEM study. SUMMARY OF BACKGROUND DATA: Cervical spondylotic myelopathy is a well-identified spinal disorder associated with cerebral palsy, especially in athetoid type. One can rationalize that the athetoid involuntary trunk movement may induce stress-related lumbar disorders. Until now, very few investigations have been conducted on lumbar disorders that could occur in patients with CP. METHODS: Sixty-one cases with athetoid cerebral palsy were clinically reviewed. Radiographs of the lumbar spine in 30 cases with low back pain were taken. From these radiographs, the incidence of lumbar spondylolysis and endplate lesion was analyzed. The biomechanical study was conducted using a three-dimensional FEM of the ligamentous lumbar spine. Axial compression of 400 N was applied to simulate preload on the spine during the standing position followed with 10.6 Nm moment to simulate extension and axial rotation. The combined motion of extension and axial rotation was also simulated. During supine position, pure extension and rotation were simulated, and no preload was applied. The von Mises stresses were computed at the pars interarticularis at the each level and compared for different motions. RESULTS: Among the 61 patients, 41 complained of symptoms such as low back and leg pain. Thirty of these 41 patients gave their consents to undergo radiographic evaluation. Twenty-eight of the 30 patients (93.3%) showed stress-related disorders such as the spondylolysis and/or vertebral endplate lesions. Spondylolysis was found in 18 of the 30 patients (60.0%). Spondylolysis was observed at the multiple levels in 4 patients involving two levels in 2 cases and three levels in the other 2 cases. Vertebral endplate lesions (deformity) were seen in 26 (86.7%) of the 30 patients. Overall, the lesions (deformity) were found in 109 of 330 endplates (33.0%) out of all lumbar endplates in the 30 patients. The FEM based results showed that stresses at pars interarticularis of L5 were the highest of all levels in all loading modes, with or without preload. However, compared with the extension or axial rotation alone modes, the combined motion of extension and rotation showed higher stresses at the pars interarticularis. CONCLUSIONS: The athetoid involuntary movements in CP patients may cause stress-related lumbar disorders. Especially, combined extension and rotation due to the involuntary trunk motion may contribute to the high incidence of spondylolysis.

Adult↗

PubMed2006

MRI signal changes of the pedicle as an indicator for early diagnosis of spondylolysis in children and adolescents: a clinical and biomechanical study.

STUDY DESIGN: Clinical review of pediatric patients with lumbar spondylolysis and biomechanical analysis using finite-element lumbar spine model. OBJECTIVES: To evaluate the usefulness of the signal changes observed on MR images of the pedicle for the early diagnosis of spondylolysis, and to investigate the pathomechanism of the signal changes based on the stresses in pedicles, as predicted using finite-element analyses. Furthermore, to evaluate the usefulness of the signal change to predict the bony healing following conservative treatment. SUMMARY OF BACKGROUND DATA: Since early-stage spondylolysis can achieve osseous healing conservatively, it is important to diagnose this disorder as early as possible. Presently, there is no well-established, noninvasive, and reliable diagnostic tool for the early diagnosis. METHODS: Thirty-seven pediatric patients with spondylolysis were included. Sixty-eight defects were examined and their stages as revealed on CT scans were recorded. High signal changes (HSC) of the pedicles on axial T2-weighted MRI were compared with the CT-based stages of the defect. Among them, 16 patients, including 15 boys and 1 girl, were treated conservatively for at least a 3-month period. Bony healing of the fracture site was evaluated on CT, and the results were compared between two groups with or without HSC at the initial consultation. Using a three-dimensional nonlinear finite-element model of the L3-L5 segment, stress distributions in the pars and pedicle regions were analyzed in response to 400 N compression and 10.6 Nm moment. RESULTS: Based on CTs, 68 pars defects were classified as follows: 8 very early, 24 late-early, 16 progressive, and 20 terminal stages. All defects in very early and late-early stages (100%) showed HSC on T2-weighted MRI at the ipsilateral pedicle. Among 16 progressive stages, eight (50%) showed HSC, while no defects of the terminal stage (0%) were found to have HSC. In total, 29 pars defects were treated conservatively out of 16 patients. In 19 of the HSC positive defects, 15 (79%) showed bony healing after the conservative treatment, whereas none of the 10 HSC negative defects (0%) showed any healing. The results were statistically significant at P < 0.05 (chi). Stress results from the finite-element model indicated that pars interarticularis showed the highest value in all loading modes, and the pedicle showed the second highest. CONCLUSIONS: The correlation between the high stresses in the pedicle and the corresponding HSC suggest that signal changes in MRI could be used as an indicator for early diagnosis of spondylolysis. The HSC of the pedicle provided useful information to diagnose early stage spondylolysis. Furthermore, the HSC may be a good indicator as to whether a bony union will result from conservative treatment.

Adolescent↗

PubMed2006

Frequency response of pig intervertebral disc cells subjected to dynamic hydrostatic pressure.

The pathogenesis of vibration-induced disorders of intervertebral disc at the cellular level is largely unknown. Dynamic loads with frequencies close to that of the in vivo human spine resonant frequency (4-6 Hz) have a destructive effect, which may induce extracellular disc matrix (ECM) degradation. To investigate this issue, three-dimensional (3D) alginate cultures of normal pig intervertebral disc nucleus and inner annulus cells were tested under dynamic hydrostatic loading. Alginate cultures of each region were divided into six groups; five groups were exposed to cyclic hydrostatic pressures of frequencies 1, 3, 5, 8, and 10 Hz with the same amplitude (1 MPa), and group 6 was the control group (no loading). Cultures of different groups were loaded for 3 days (30 min daily) in a hydraulic chamber. Effects of loading frequency on disc collagen and protein metabolism were investigated by measuring 3H-proline-labeled proteins associated with the cells in the extracellular matrix and release of 3H-proline-labeled molecules into culture medium. The results indicated a poor synthesis rate and more degradation near the 5 Hz frequency. The repeatability of experiments was verified by performing two experiments with the same protocol. Both experiments indicated that a threshold frequency of around 5 Hz disrupted protein metabolism.

Animals↗

PubMed2006

Test protocols for evaluation of spinal implants.

Prior to implantation, medical devices are subjected to rigorous testing to ensure safety and efficacy. A full battery of testing protocols for implantable spinal devices may include many steps. Testing for biocompatibility is a necessary first step. On selection of the material, evaluation protocols should address both the biomechanical and clinical performance of the device. Before and during mechanical testing, finite element modeling can be used to optimize the design, predict performance, and, to some extent, predict durability and efficacy of the device. Following bench-type evaluations, the biomechanical characteristics of the device (e.g., motion, load-sharing, and intradiscal pressure) can be evaluated with use of fresh human cadaveric spines. The information gained from cadaveric testing may be supplemented by the finite element model-based analyses. Upon the successful completion of these tests, studies that make use of an animal model are performed to assess the structure, function, histology, and biomechanics of the device in situ and as a final step before clinical investigations are initiated. The protocols that are presently being used for the testing of spinal devices reflect the basic and applied research experience of the last three decades in the field of orthopaedic biomechanics in general and the spine in particular. The innovation within the spinal implant industry (e.g., fusion devices in the past versus motion-preservation devices at present) suggests that test protocols represent a dynamic process that must keep pace with changing expectations. Apart from randomized clinical trials, no single test can fully evaluate all of the characteristics of a device. Due to the inherent limitations of each test, data must be viewed in a proper context. Finally, a case is made for the medical community to converge toward standardized test protocols that will enable us to compare the vast number of currently available devices, whether on the market or still under development, in a systematic, laboratory-independent manner.

Animals↗

PubMed2005

Effects of charité artificial disc on the implanted and adjacent spinal segments mechanics using a hybrid testing protocol.

STUDY DESIGN: Finite element model of L3-S1 segment and confirmatory cadaveric testing were used to investigate the biomechanical effects of a mobile core type artificial disc (Charité artificial disc; DePuy Spine, Raynham, MA) on the lumbar spine. OBJECTIVE: To determine the effects of the Charité artificial disc across the implanted and adjacent segments. SUMMARY OF BACKGROUND DATA: Biomechanical studies of artificial discs that quantify parameters, like the load sharing and stresses, are sparse in the literature, especially for mobile-type core artificial disc designs. In addition, there is no standard protocol for studying the adjacent segmental effects of such implants. METHODS: Human osteo-ligamentous spines (L1-S1) were tested before and after L5-S1 Charité artificial disc placement. The data were used to validate further an intact 3-dimensional (3-D) nonlinear L3-S1 finite element model. The model was subjected to 400-N axial compression and 10.6 Nm of flexion/extension pure moments (load control) or pure moments that produced the overall rotation of the L3-S1 Charité model equal to the intact case (hybrid approach). Resultant motion, load, and stress parameters were analyzed at the experimental and adjacent levels. RESULTS: Finite element model validation was achieved only with the load-controlled experiments. The hybrid approach, believed to be more clinically relevant, revealed that Charité artificial disc leads to motion increases in flexion (19%) and extension (44%) at the L5-S1 level. At the instrumented level, the decrease in the facet loads was less than at the adjacent levels; the corresponding decrease being 26% at L3-L4, 25% at L4-L5, and 13.4% at L5-S1 when compared to the intact. Intradiscal pressure changes in the L4-L5 and L3-L4 segments were minimal. Shear stresses at the Charité artificial disc-L5 endplate interface were higher than those at S1 interface. However, in the load control mode, the increase in facet loads in extension was approximately 14%, as compared to the intact case. CONCLUSIONS: The hybrid testing protocol is advocated because it better reproduces clinical observations in terms of motion following surgery, using pure moments. Using this approach, we found that the Charité artificial disc placement slightly increases motion at the implanted level, with a resultant increase in facet loading when compared to the adjacent segments, while the motions and loads decrease at the adjacent levels. However, in the load control mode that we believe is not that clinically relevant, there was a large increase in motion and a corresponding increase in facet loads, as compared to the intact.

Biomechanical Phenomena↗

PubMed2005

Athletes with unilateral spondylolysis are at risk of stress fracture at the contralateral pedicle and pars interarticularis: a clinical and biomechanical study.

BACKGROUND: Unilateral spondylolysis is common in youths; its clinical and biomechanical features, especially effects on the contralateral side, are not fully understood. HYPOTHESIS: Unilateral spondylolysis predisposes the contralateral side to stress fracture, especially in athletes actively engaged in sporting activities involving torsion of the trunk. STUDY DESIGN: Case series and descriptive laboratory study. METHODS: Thirteen athletes younger than age 20 with unilateral spondylolysis were included. The contralateral pedicle and pars of spondylolytic vertebrae were examined using computed tomography and magnetic resonance imaging. Using a finite element model of the intact ligamentous L3-S1 segment, stress distributions were analyzed in response to 400-N axial compression and 10.6-N.m moment in flexion, extension, lateral bending, and axial rotation. Unilateral spondylolysis was created in the model at L5. The stress results from the unilateral defect model were compared to the intact model predictions and correlated to the contralateral defects seen in patients. RESULTS: Among 13 patients, there were 6 early-, 2 progressive-, and 5 terminal-stage defects. Three (23.1%) showed contralateral stress fracture. Among them, 2 belonged to the progressive-stage and 1 to the terminal-stage spondylolysis group. The remaining 4 patients in the terminal defect group showed stress reactions, such as sclerosis at the contralateral pedicle. In the finite element analysis model with an L5 left spondylolysis, the stresses at the contralateral and pars interarticularis were found to increase in all loading modes, with increases as high as 12.6-fold compared to the intact spine. CONCLUSIONS: Unilateral spondylolysis could lead to stress fracture or sclerosis at the contralateral side due to an increase in stresses in the region. CLINICAL RELEVANCE: Surgeons should be aware of possibility of contralateral stress fractures in cases in which patients, especially athletes engaged in active sports, show unilateral spondylolysis and persistent low back pain complaints.

Adolescent↗

PubMed2005

Posterior instrumentation reduces differences in spine stability as a result of different cage orientations: an in vitro study.

STUDY DESIGN: A multisegmental cadaveric spine model was used to quantify the load-displacement behavior of intact spine specimens, specimens injured and stabilized using Bagby and Kuslich (BAK) cages as lumbar interbody fusion devices with or without posterior instrumentation across two levels. OBJECTIVES: To compare the stabilities imparted by the cages placed using an oblique and conventional posterior approaches and to determine the effects of supplementary posterior instrumentation. SUMMARY OF BACKGROUND DATA: The BAK cage as posterior lumbar interbody fusion (PLIF) has been used to restore disc height, reduce morbidity, provide immediate stability to the patients, and enhance fusion rates. The obliquely inserted BAK cage has the advantages of reducing exposure and precise implantation. The biomechanical efficacy of this procedure is sparse, especially in comparison to the PLIF with and without posterior instrumentation. METHODS: Nine fresh human ligamentous spines (L2-S1) were affixed within a testing frame for determining their load-displacement behaviors. Load testing in clinically relevant modes was performed sequentially for the intact and the following procedures across the L4-S1 segment: posterior destabilization, stabilization using two parallel BAK cages (CBAK group) or one oblique BAK cage (OBAK group), further stabilization with posterior instrumentation, and finally cyclic loading in flexion-extension. Spatial positions of the LEDs attached to vertebral bodies were recorded using a three-dimensional motion measurement system. RESULTS: When used alone to restore stability, the orientation of the cage affected the outcome. In flexion OBAK orientation and in extension CBAK orientation provided better stability (decrease in motion with respect to intact case), compared with the other orientation. In lateral bending, CBAK orientation was found to be better than OBAK. In axial mode, CBAK orientation was effective in both directions while OBAK was effective only in right axial rotation. With the supplementary posterior fixation, the differences in stability resulting from the orientations were not noticeable at all, both before and after cyclic tests. CONCLUSIONS: Owing to the differences in the surgical approach and the amount of dissection, the stability for the cages when used alone as a function of cage orientation was different. These subtle differences were reduced by the use of posterior fixation device, underscoring the importance of using instrumentation when cages are used as PLIFs. However, the oblique insertion may be more favorable since it requires less exposure, enables precise implantation, and is less expensive, especially when used with supplementary instrumentation.

Aged↗

PubMed2005

Biomechanical comparison of two stabilization techniques of the atlantoaxial joints: transarticular screw fixation versus screw and rod fixation.

OBJECTIVE: To compare the biomechanical stability imparted to the C1 and C2 vertebrae by either transarticular screw fixation (TSF) or screw and rod fixation (SRF) techniques in a cadaver model. METHODS: Ten fresh ligamentous human cervical spine specimens were harvested from cadavers. The specimens were tested sequentially in the intact state, after injury and stabilization (unilateral left side and bilateral), and after fatiguing to 5000 cycles (0.5 Hz) at +/-1.0 N.m of flexion and extension. The specimens were stabilized by use of TSF in 5 spines or SRF in the other 5 spines. The data were converted to angular displacements, and the stabilized cases were compared with intact states for evaluating the efficacies of the two techniques in stabilizing the C1-C2 segments. RESULTS: In the TSF group, the unilateral fixation using one screw imparted a significant stability in only the axial rotation mode. The unilateral procedure in the SRF group was effective in stabilization in all modes except in extension. The bilateral procedure in both of the groups was effective across the C1-C2 segment. However, the SRF group afforded higher stability than the corresponding TSF group in the flexion and extension modes. The degree of stability did not change after fatigue compared with the prefatigue data. CONCLUSION: In general, a surgeon should undertake a bilateral fixation to achieve sufficient stability across the atlantoaxial complex, and either technique will provide satisfactory results, although the SRF technique may be better in the flexion and extension modes. One should use the SRF procedure while trying to achieve stability with a unilateral system.

Aged↗

PubMed2004

Vertebral forward slippage in immature lumbar spine occurs following epiphyseal separation and its occurrence is unrelated to disc degeneration: is the pediatric spondylolisthesis a physis stress fracture of vertebral body?

STUDY DESIGN: Radiographic and histologic evaluation of a rat model of lumbar spine slippage. OBJECTIVES: To clarify the pathomechanism of slippage in the immature spine. SUMMARY OF BACKGROUND DATA: There are controversial hypotheses regarding the pathogenesis of slippage of the pediatric spine with pars defects. Some studies supported that disc degeneration was its cause, while others indicated the growth plate injury was the cause. METHODS: An immature lumbar spine slippage model in 4-week-old rats was used. Following posterior destabilizing surgery, the lumbar spine was radiographically and histologically examined at 1, 3, 5, and 7 days after surgery. RESULTS: Radiographically, slippage occurred about 7% in the % slip on day 7, and no slippage was observed before day 5. Histologically, epiphyseal separation also appeared on day 7; before day 5, the growth plate showed no abnormalities. Within 7 days after the operation, the anulus fibrosus did not show any sign indicating degeneration. The nucleus pulposus was also normal up to day 7. CONCLUSION: The findings of this study support the hypothesis that vertebral forward slippage of the immature spine occurs following epiphyseal separation and its occurrence is unrelated to disc degeneration.

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