Clayton J. Adam

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.

Variability in Cobb angle measurements using reformatted computerized tomography scans.

Study Design. Survey of intraobserver and interobserver measurement variability. Objective. To assess the use of reformatted computerized tomography (CT) images for manual measurement of coronal Cobb angles in idiopathic scoliosis. Summary of Background Data. Cobb angle measurements in idiopathic scoliosis are traditionally made from standing radiographs, whereas CT is often used for assessment of vertebral rotation. Correlating Cobb angles from standing radiographs with vertebral rotations from supine CT is problematic because the geometry of the spine changes significantly from standing to supine positions, and 2 different imaging methods are involved. Methods. We assessed the use of reformatted thoracolumbar CT images for Cobb angle measurement. Preoperative CT of 12 patients with idiopathic scoliosis were used to generate reformatted coronal images. Five observers measured coronal Cobb angles on 3 occasions from each of the images. Intraobserver and interobserver variability associated with Cobb measurement from reformatted CT scans was assessed and compared with previous studies of measurement variability using plain radiographs. Results. For major curves, 95% confidence intervals for intraobserver and interobserver variability were ±6.6° and ±7.7°, respectively. For minor curves, the intervals were ±7.5° and ±8.2°, respectively. Intraobserver and interobserver technical error of measurement was 2.4° and 2.7°, with reliability coefficients of 88% and 84%, respectively. There was no correlation between measurement variability and curve severity. Conclusions. Reformatted CT images may be used for manual measurement of coronal Cobb angles in idiopathic scoliosis with similar variability to manual measurement of plain radiographs.

Stress analysis of interbody fusion––finite element modelling of intervertebral implant and vertebral body

OBJECTIVE To investigate stresses in cage type interbody fusion systems during compressive loading.Design. The study uses finite element methods to investigate predicted stresses. Previously published experimental material properties are used as inputs to the numerical simulation. BACKGROUND Interbody spinal fusion procedures using cage style intervertebral implants often cause subsidence failure of the vertebral endplate, resulting in potential pain and mechanical instability of the fusion system. METHODS Finite element models were developed to simulate compressive load transfer between interbody implants and adjacent vertebral body. The vertebral body was modelled using separate finite element mesh regions for cancellous core and cortical shell, with the meshes tied together at the core/shell interface. Coulomb friction was implemented to model the contact between implants and vertebral endplate.Results. Simulation results predicted endplate stresses of approximately 12 times the nominal contact pressure due to differing deformation stiffnesses of the implant and endplate structures. Reduction of the cancellous core elastic modulus to simulate severely osteoporotic bone resulted in endplate stresses up to three times higher than the values for an intact cancellous core. CONCLUSIONS In this study, finite element analysis was used to investigate the stresses in interbody fusion systems. Published vertebral loads corresponding to certain activities were shown to generate endplate stresses which approach and exceed the failure stress for cortical bone. Endplate stresses are strongly dependent on the modulus of the underlying cancellous core. RELEVANCE Endplate subsidence failure can potentially occur at the corners of existing cage-type interbody implants under physiological compressive loads. Matching material properties between cortical endplate and implant does not guarantee optimal contact conditions, and overall bending stiffness should be assessed.

The effect of soft tissue properties on spinal flexibility in scoliosis: biomechanical simulation of fulcrum bending.

Study Design. Biomechanical analysis of the scoliotic thoracolumbar spine and ribcage using a three-dimensional finite element model. Objective. To explore how the mechanical properties of spinal ligaments and intervertebral discs affect coronal curve flexibility in the fulcrum bending test. Summary of Background Data. Preoperative coronal curve flexibility assessment is of key importance in the surgical planning process for scoliosis correction. The fulcrum bending radiograph is one flexibility assessment technique which has been shown to be highly predictive of potential curve correction using posterior surgery; however, little is known about the extent to which soft tissue structures govern spinal flexibility. Methods. CT-derived spinal anatomy for a 14-year-old female adolescent idiopathic scoliosis patient was used to develop the three-dimensional finite element model. Physiologic loading conditions representing the gravitational body weight forces acting on the spine when the patient lies on their side over the fulcrum bolster were simulated. Initial mechanical properties for the spinal soft tissues were derived from existing literature. In 6 separate analyses, the disc collagen fiber and ligament stiffness values were reduced by 10%, 25%, and 40% respectively, and the effects of reduced tissue stiffness on fulcrum flexibility were assessed by comparison with the initial model. Finally, the effect of discectomy on fulcrum flexibility was simulated for thoracic levels T5–T12. Results. Reducing disc collagen fiber stiffness resulted in a greater change in segmental rotations in the fulcrum bending test than reducing ligament stiffness. However, reductions of up to 40% in disc collagen fiber stiffness and ligament stiffness produced no clinically measurable increase in fulcrum flexibility. By contrast, after removal of the discs, the simulated fulcrum flexibility increased by more than 80% compared with the initial case. Conclusion. Homogeneous reduction in either the disc collagen fiber or ligament stiffness had minimal influence on scoliotic curve reducibility. However, discectomy simulation shows that the intervertebral discs are of critical importance in determining spinal flexibility.

Recovery of pulmonary function following endoscopic anterior scoliosis correction: Evaluation at 3, 6, 12, and 24 months after surgery

Study Design. A series of patients with scoliosis undergoing endoscopic anterior instrumentation and fusion undertaking repeated pulmonary function assessments. Objective. To assess recovery of pulmonary function in the 2 years following endoscopic anterior scoliosis correction. Summary of Background Data. Recent studies have found that pulmonary function returns to preoperative levels 12–24 months following endoscopic anterior scoliosis correction, and a small improvement in forced expiratory volume (FEV1) has also been reported. Methods. A series of 44 patients with endoscopic anterior scoliosis correction had pulmonary function tests before surgery, and at 3, 6, 12, and 24 months after surgery. Forced vital capacity (FVC), FEV1, and total lung capacity (TLC) were measured. Nonparametric statistical analysis was used to investigate changes in pulmonary function between successive assessments. Results. Pulmonary function decreased by approximately 10% at 3 months after surgery. At 24 months after surgery, FVC and FEV1 recovered to 5% to 8% higher than preoperative levels, while TLC returned to preoperative levels. Statistically significant improvements in most pulmonary function values occurred between 3 and 6, and 6–12 months. Improvements in mean FVC, FEV1, and TLC continue between 12 and 24 months, although only the increase in absolute FVC for this time is statistically significant. Conclusions. Endoscopic anterior scoliosis surgery has no lasting negative effect on pulmonary function, and with prolonged follow-up, pulmonary capacity improves beyond preoperative levels.

Automatic measurement of vertebral rotation in idiopathic scoliosis.

Study Design. Development of an automatic measurement algorithm and comparison with manual measurement methods. Objectives. To develop a new computer-based method for automatic measurement of vertebral rotation in idiopathic scoliosis from computed tomography images and to compare the automatic method with two manual measurement techniques. Summary of Background Data. Techniques have been developed for vertebral rotation measurement in idiopathic scoliosis using plain radiographs, computed tomography, or magnetic resonance images. All of these techniques require manual selection of landmark points and are therefore subject to interobserver and intraobserver error. Methods. We developed a new method for automatic measurement of vertebral rotation in idiopathic scoliosis using a symmetry ratio algorithm. The automatic method provided values comparable with Aaro and Ho’s manual measurement methods for a set of 19 transverse computed tomography slices through apical vertebrae, and with Aaro’s method for a set of 204 reformatted computed tomography images through vertebral endplates. Results. Confidence intervals (95%) for intraobserver and interobserver variability using manual methods were in the range 5.5° to 7.2°. The mean (±SD) difference between automatic and manual rotation measurements for the 19 apical images was −0.5° ± 3.3° for Aaro’s method and 0.7° ± 3.4° for Ho’s method. The mean (±SD) difference between automatic and manual rotation measurements for the 204 endplate images was 0.25° ± 3.8°. Conclusions. The symmetry ratio algorithm allows automatic measurement of vertebral rotation in idiopathic scoliosis without intraobserver or interobserver error due to landmark point selection.

The effect of friction on indenter force and pile-up in numerical simulations of bone nanoindentation.

Nanoindentation is a useful technique for probing the mechanical properties of bone, and finite element (FE) modeling of the indentation allows inverse determination of elastoplastic constitutive properties. However, all but one FE study to date have assumed frictionless contact between indenter and bone. The aim of this study was to explore the effect of friction in simulations of bone nanoindentation. Two-dimensional axisymmetric FE simulations were performed using a spheroconical indenter of tip radius 0.6 μm and angle 90°. The coefficient of friction between indenter and bone was varied between 0.0 (frictionless) and 0.3. Isotropic linear elasticity was used in all simulations, with bone elastic modulus E = 13.56 GPa and Poissons ratio of 0.3. Plasticity was incorporated using both Drucker-Prager and von Mises yield surfaces. Friction had a modest effect on the predicted force-indentation curve for both von Mises and Drucker-Prager plasticity, reducing maximum indenter displacement by 10% and 20% respectively as friction coefficient was increased from zero to 0.3 (at a maximum indenter force of 5 mN). However, friction has a much greater effect on predicted pile-up after indentation, reducing predicted pile-up from 0.27 to 0.11 μm with a von Mises model, and from 0.09 to 0.02 μm with Drucker-Prager plasticity. We conclude that it is potentially important to include friction in nanoindentation simulations of bone if pile-up is used to compare simulation results with experiment.

Perioperative aspects of endoscopic anterior scoliosis surgery: The learning curve for a consecutive series of 100 patients

The reported benefits of endoscopic versus open scoliosis surgery include improved visualization, a muscle sparing approach, reduced pulmonary morbidity, reduced pain, and improved cosmesis. Some aspects of the surgical learning curve for this technically demanding method have been previously reported; however, improvements in other factors with increasing experience have not been quantified. This paper presents a series of 100 consecutive endoscopic anterior scoliosis corrections performed between April 2000 and February 2006. We report changes in the following perioperative factors with increasing experience; operative set-up time, operative time, x-ray irradiation time, number of instrumented levels, blood loss, intercostal catheter drainage, chest drain removal time, days in intensive care, days to mobilize, days in hospital, and early complications. Statistical comparisons were made between the first 20 (1 to 20), middle 20 (41 to 60), and last 20 (81 to 100) cases. Results showed statistically significant improvements and increased consistency in operative time, operative set-up time, x-ray irradiation time, blood loss, hospital stay, and mobilization time with experience. The complication rate was comparable to other recently published endoscopic studies. In the last 20 cases of the series, operative times had reduced to 35 minutes per level, x-ray irradiation times to 15 seconds per level, and blood loss to 38 mL per level. Most perioperative surgical factors therefore improve significantly with increasing experience in endoscopic anterior scoliosis correction.

Development of a multi-scale finite element model of the osteoporotic lumbar vertebral body for the investigation of apparent level vertebra mechanics and micro-level trabecular mechanics

Osteoporotic spinal fractures are a major concern in ageing Western societies. This study develops a multi-scale finite element (FE) model of the osteoporotic lumbar vertebral body to study the mechanics of vertebral compression fracture at both the apparent (whole vertebral body) and micro-structural (internal trabecular bone core) levels. Model predictions were verified against experimental data, and found to provide a reasonably good representation of the mechanics of the osteoporotic vertebral body. This novel modelling methodology will allow detailed investigation of how trabecular bone loss in osteoporosis affects vertebral stiffness and strength in the lumbar spine.

Are coupled rotations in the lumbar spine largely due to the osseo-ligamentous anatomy?--a modeling study.

Prior studies have found that primary rotations in the lumbar spine are accompanied by coupled out-of-plane rotations. However, it is not clear whether these accompanying rotations are primarily due to passive (discs, ligaments and facet joints) or active (muscles) spinal anatomy. The aim of this study was to use a finite element (FE) model of the lumbar spine to predict three-dimensional coupled rotations between the lumbar vertebrae, due to passive spinal structures alone. The FE model was subjected to physiologically observed whole lumbar spine rotations about in vivo centres of rotation. Model predictions were validated by comparison of intra-discal pressures and primary rotations with in vivo measurements and these showed close agreement. Predicted coupled rotations matched in vivo measurements for all primary motions except lateral bending. We suggest that coupled rotations accompanying primary motions in the sagittal (flexion/extension) and transverse (axial rotation) planes are primarily due to passive spinal structures. For lateral bending the muscles most likely play a key role in the coupled rotation of the spine.