Comparison of eight published static finite element models of the intact lumbar spine: predictive power of models improves when combined together

Finite element (FE) model studies have made important contributions to our understanding of functional biomechanics of the lumbar spine. However, if a model is used to answer clinical and biomechanical questions over a certain population, their inherently large inter-subject variability has to be considered. Current FE model studies, however, generally account only for a single distinct spinal geometry with one set of material properties. This raises questions concerning their predictive power, their range of results and on their agreement with in vitro and in vivo values. Eight well-established FE models of the lumbar spine (L1-5) of different research centers around the globe were subjected to pure and combined loading modes and compared to in vitro and in vivo measurements for intervertebral rotations, disc pressures and facet joint forces. Under pure moment loading, the predicted L1-5 rotations of almost all models fell within the reported in vitro ranges, and their median values differed on average by only 2° for flexion-extension, 1° for lateral bending and 5° for axial rotation. Predicted median facet joint forces and disc pressures were also in good agreement with published median in vitro values. However, the ranges of predictions were larger and exceeded those reported in vitro, especially for the facet joint forces. For all combined loading modes, except for flexion, predicted median segmental intervertebral rotations and disc pressures were in good agreement with measured in vivo values. In light of high inter-subject variability, the generalization of results of a single model to a population remains a concern. This study demonstrated that the pooled median of individual model results, similar to a probabilistic approach, can be used as an improved predictive tool in order to estimate the response of the lumbar spine.

A finite element investigation of upper cervical instrumentation

STUDY DESIGN

The finite element technique was used to predict changes in biomechanics that accompany the application of a novel instrumentation system designed for use in the upper cervical spine.

OBJECTIVE

To determine alterations in joint loading, kinematics, and instrumentation stresses in the craniovertebral junction after application of a novel instrumentation system. Specifically, this design was used to assess the changes in these parameters brought about by two different cervical anchor types: C2 pedicle versus C2-C1 transarticular screws, and unilateral versus bilateral instrumentation.

SUMMARY OF BACKGROUND DATA

Arthrodesis procedures can be difficult to obtain in the highly mobile craniovertebral junction. Solid fusion is most likely achieved when motion is eliminated. Biomechanical studies have shown that C1-C2 transarticular screws provide good stability in craniovertebral constructs; however, implantation of these screws is accompanied by risk of vertebral artery injury. A novel instrumentation system that can be used with transarticular screws or with C2 pedicle screws has been developed. This design also allows for unilateral or bilateral implantation. However, the authors are unaware of any reports to date on the changes in joint loading or instrumentation stresses that are associated with the choice of C2 anchor or unilateral/bilateral use.

METHODS

A ligamentous, nonlinear, sliding contact, three-dimensional finite element model of the C0-C1-C2 complex and a novel instrumentation system was developed. Validation of the model has been previously reported. Finite element models representing combinations of cervical anchor type (C1-C2 transarticular screws vs. C2 pedicle screws) and unilateral versus bilateral instrumentation were evaluated. All models were subjected to compression with pure moments in either flexion, extension, or lateral bending. Kinematic reductions with respect to the intact (uninjured and without instrumentation) case caused by instrumentation use were reported. Changes in loading profiles through the right and left C0-C1 and C1-C2 facets, transverse ligament-dens, and dens-anterior ring of C1 articulations were calculated by the finite element model. Maximum von Mises stresses within the instrumentation were predicted for each model variant and loading scenario.

RESULTS

Bilateral instrumentation provided greater motion reductions than the unilateral instrumentation. When used bilaterally, C2 pedicle screws approximate the kinematic reductions and instrumentation stresses (except in lateral bending) that are seen with C1-C2 transarticular screws. The finite element model predicted that the maximum stress was always in the region in which the plate transformed into the rod.

CONCLUSIONS

To the best of the authors' knowledge, this is the first report of predicting changes in loading in the upper cervical spine caused by instrumentation. The most significant conclusion that can be drawn from the finite element model predictions is that C2 pedicle screw fixation provides the same relative stability and instrumentation stresses as C1-C2 transarticular screw use. C2 pedicle screws can be a good alternative to C2-C1 transarticular screws when bilateral instrumentation is applied.

Acute biomechanical and histological effects of intradiscal electrothermal therapy on human lumbar discs

STUDY DESIGN

Human cadaver lumbar spines were used to assess the acute effects of intradiscal electrothermal therapy in vitro.

OBJECTIVE

To determine whether intradiscal electrothermal therapy produces acute changes in disc histology and motion segment stability.

SUMMARY OF BACKGROUND DATA

Intradiscal electrothermal therapy has been introduced as an alternative for the treatment of discogenic low back pain. Several hypothesized mechanisms for the effect of intradiscal electrothermal therapy have been suggested including shrinkage of the nucleus or sealing of the anulus fibrosus by contraction of collagen fibers, and thermal ablation of sensitive nerve fibers in the outer anulus.

METHODS

Intradiscal electrothermal therapy was performed with the Spinecath by Oratec on 19 fresh, frozen human lumbar cadaver specimens. In a separate study, eight specimens were tested biomechanically and instrumented to map the thermal distribution, whereas five specimens were tested only biomechanically, both before and after intradiscal electrothermal therapy. Six additional specimens were heated with intradiscal electrothermal therapy, and the resulting canal was backfilled with a silicone rubber compound to allow colocalization of the catheter and anular architecture.

RESULTS

A consistent pattern of increased motion and decreased stiffness was observed. For the specimens in which only biomechanical measurements were taken, a 10% increase in the motion, on the average, at 5 Nm torque was observed after intradiscal electrothermal therapy. No apparent alteration of the anular architecture was observed around the catheter site in the intradiscal electrothermal therapy-treated discs.

CONCLUSION

The data from this study suggest that the temperatures developed during intradiscal electrothermal therapy are insufficient to alter collagen architecture or stiffen the treated motion segment acutely.

Pathomechanisms of failures of the odontoid

STUDY DESIGN

A finite element investigation to determine the causal mechanisms that lead to odontoid fracture.

OBJECTIVES

To elucidate which loading scenarios, including rotational moments, compression-tension, and lateral and anteroposterior shear, can result in Type I, Type II, and Type III odontoid failures.

SUMMARY OF BACKGROUND DATA

There is considerable controversy about the major loading path that causes odontoid fractures. A review of the clinical and laboratory research literature did not provide a consensus on this issue.

METHODS

A three-dimensional, nonlinear finite element model of the occipito-atlantoaxial (C0-C1-C2) complex was generated from human cadaveric data. Force loads were applied at the posterior margin of the occiput and were applied as lone entities or after the model was prepositioned in flexion, extension, or lateral-bending moments through applied rotation moments. Intraosseous stresses were reported to characterize the probability of fracture due to the applied loadings.

RESULTS

The data indicate that hyperextension can lead to failure of the odontoid at its superior tip (Type I). Finite element model predictions also demonstrated the propensity of loads that induce axial rotation to create relatively high maximum von Mises stress in the Type II fracture region. Flexion prepositioning reduced the stress response of the odontoid.

CONCLUSIONS

Force loading that puts the head in extension coupled with lateral shear or compression leads to Type I fractures, whereas axial rotation and lateral shear can produce Type II fractures. The model failed to elucidate causal mechanisms for Type III fractures. Flexion seems to provide a protective mechanism against force application that would otherwise cause a higher risk of odontoid failure.

Biomechanical rationale for the pathology of rheumatoid arthritis in the craniovertebral junction

STUDY DESIGN

A finite-element model of the craniovertebral junction was developed and used to determine whether a biomechanical mechanism, in addition to inflammatory synovitis, is involved in the progression of rheumatoid arthritis in this region of the spine.

OBJECTIVES

To determine specific structure involvement during the progression of rheumatoid arthritis and to evaluate these structures in terms of their effect on clinically observed erosive changes associated with the disease by assessing changes in loading patterns and degree of anterior atlantoaxial subluxation.

SUMMARY OF BACKGROUND DATA

Rheumatoid arthritis involvement of the occipito-atlantoaxial (C0-C1-C2) complex is commonly seen. However, the biomechanical contribution to the development and progression of the disease is neither well understood nor quantified. Although previous cadaver studies have elucidated information on kinematic motion and fusion techniques, the modeling of progressive disease states is not easily accomplished using these methods. The finite-element method is well suited for studying progressive disease states caused by the gradual changes in material properties that can be modeled.

METHODS

A ligamentous, nonlinear, sliding-contact, three-dimensional finite-element model of the C0-C1-C2 complex was generated from 0.5 mm thick serial computed tomography scans. Validation of the model was accomplished by comparing baseline kinematic predictions with experimental data. Transverse, alar, and capsular ligament stiffness were reduced sequentially by 50%, 75%, and 100% (removal) of their intact values. All models were subjected to flexion moments replicating the clinical diagnosis of rheumatoid arthritis using full flexion lateral plane radiographs. Stress profiles at the transverse ligament-odontoid process junction were monitored. Changes in loading profiles through the C0-C1 and C1-C2 lateral articulations and their associated capsular ligaments were calculated. Anterior and posterior atlantodental interval values were calculated to correlate ligamentous destruction with advancement of atlantoaxial subluxation.

RESULTS

Model predictions (at 0.3 Nm) fell within one standard deviation of experimental means, and range of motion data agreed with published in vitro and in vivo values. The model predicted that stresses at the posterior base of the odontoid process were greatly reduced with transverse ligament compromise beyond 75%. Decreases through the lateral C0-C1 and C1-C2 articulations were compensated by their capsular ligaments. Anterior and posterior atlantodental interval values indicate that the transverse ligament stiffness decreases beyond 75% had the greatest effect on atlantoaxial subluxation during the early stages of the disease (no alar and capsular ligament damage). Subsequent involvement of the alar and capsular ligaments produced advanced atlantoaxial subluxation, for which surgical intervention may be warranted.

CONCLUSIONS

To the best of the authors' knowledge, this is the first report of a validated, three-dimensional model of the C0-C1-C2 complex with application to rheumatoid arthritis. The data indicate that there may be a mechanical component (in addition to enzymatic degradation) associated with the osseous resorption observed during rheumatoid arthritis. Specifically, erosion of the odontoid base may involve Wolff's law of unloading considerations. Changes through the lateral aspects of the atlas suggest that this same mechanism may be partially responsible for the erosive changes seen during progressive rheumatoid arthritis. Anterior and posterior atlantodental interval values indicate that complete destruction of the transverse ligament coupled with alar and/or capsular ligament compromise is requisite if advanced levels of atlantoaxial subluxation are present.

Biomechanical testing sequelae relevant to spinal fusion and instrumentation

The increasing prevalence of spinal disorders and associated treatments has produced a dramatic increase in the number of available devices. The biomechanical evaluation leading to the design, development, and implementation of spinal instrumentation has resulted in a number of in vitro and in vivo testing methods. This article reviews some of the methods and associated results obtained by various evaluation techniques of spinal fusion hardware. Current work and future considerations also are presented.

Bioabsorbable pin fixation of intercarpal joints: an evaluation of fixation stiffness

OBJECTIVE: Bioabsorbable pins made of poly-p-dioxanone (Johnson and Johnson Orthopaedics, Raynham, Massachusetts) were compared to steel pins in the fixation of the scapholunate and lunotriquetral intercarpal joints of the wrist. DESIGN: An in vitro experiment was performed using fresh-frozen cadaver wrists. BACKGROUND: In the surgical treatment of wrist ligament injuries, temporary intercarpal joint fixation is required. Although steel pins are currently used, there are several clinical problems associated with their use. These complications could potentially be reduced by using bioabsorbable fixation. METHODS: The carpal bones were mounted between sets of plates which were then attached to a materials testing machine. Two parallel pins were used for joint fixation; steel pin fixation was tested first in each joint. Seven scapholunate joints and seven lunotriquetral joints were each tested in three different modes: translation in two orthogonal directions, and rotation. RESULTS: Bioabsorbable pin fixation provided an average of 46% of the stiffness of steel pin fixation. CONCLUSIONS: Although bioabsorbable pin fixation was statistically less stiff than steel pin fixation, relatively high loads were required to induce significant intercarpal joint translation and rotation. RELEVANCE: The strength retention profile of poly-p-dioxanone pins and the quality of fixation demonstrated in this study indicate that these bioabsorbable pins could provide satisfactory fixation of the scapholunate and lunotriquetral joints in the treatment of partial injuries of the intercarpal ligaments.