Specimen-specific Finite Element Research Articles

Effects of Intervertebral Disk Degeneration on the Flexibility of the Human Thoracolumbar Spine

The objective of this study was to investigate the effects of intervertebral disk degeneration on the flexibility of the thoracolumbar spine in flexion and extension, both experimentally and computationally. A seven-level biomechanically tested human cadaveric spine (T11-L5) and a 3D finite element model of the same thoracolumbar spine were used for this purpose. The anatomically accurate computer model was generated from detailed computed tomography images and included the vertebral shell, the trabecular centrum, cartilage endplates, intervertebral disks, seven spinal ligament groups, and the facet joints. The cadaveric spinal segment and the specimen-specific finite element model were subjected to various compressive loads ranging from 75 to 975 N using the follower load principle and an oscillating bending moment of +/-5 Nm applied in the sagittal plane. The biomechanical behavior of the finite element model of the spine was validated with the experimental mechanical test data for the corresponding physical thoracolumbar spine specimen. In addition, the effect of intervertebral disk material property variation within the thoracolumbar spinal column on the spinal flexibility was extensively studied. The results of this study provided significant insight into how mechanical properties of the intervertebral disk influence spinal flexibility along the thoracolumbar spinal column. It was found that in order to get comparable results between experimental and computed data, the material properties of the intervertebral disks had to vary along the spinal column. However, these effects are diminished with increasing axial compressive load. Because of the trend between disk properties and spinal level, we further concluded that there might be a mechanism at play that links endplate size, body weight fraction, and segmental flexibility. More studies are needed to further investigate that relationship.

Preservation of periarticular cancellous morphology and mechanical stiffness in post-traumatic experimental osteoarthritis by antiresorptive therapy

Background Bone changes in experimental post-traumatic osteoarthritis occur early after transection of the anterior cruciate. The purpose of this study was to determine whether antiresorptive bisphosphonate therapy could slow or arrest the periarticular bone architecture and mechanical properties in a post-traumatic model of knee osteoarthritis. Methods Skeletally mature, female New Zealand white rabbits were assigned to three groups ( N = 8/group). In two groups, anterior cruciate ligament transection was performed and half were left untreated, and the other half dosed with risedronate (0.01 mg/kg s.c. daily) for a six week period. A third group included age-matched normal controls. At the end of the six week period, all rabbits were euthanized and the femur was scanned by micro-computed tomography to assess morphology and density. Specimen-specific finite element analyses quantified the periarticular bone architecture and mechanical properties. Findings The untreated transected group had reduced bone volume ratio and mechanical properties compared to the controls ( P < 0.05) and risedronate-treated transected animals ( P < 0.02), suggesting bone conservation. Changes in bone volume ratio and mechanical properties of the risedronate-treated transected animals compared to the controls were not detected, indicating that risedronate did not improve these properties relative to the normal controls. The untreated transected group had reduced apparent cancellous bone mineral density and cortical thickness compared to transected animals treated with risedronate ( P < 0.05). Interpretation Bisphosphonate therapy altered the short-term progression of periarticular bone changes including micro-structure and mechanical integrity by slowing early-stage changes to the micro-architecture. These changes following joint trauma may impact the long-term outcome of post-traumatic osteoarthritis.

Evolution of vertebroplasty: a biomechanical perspective.

This paper is a collection of computational, finite element studies on vertebroplasty performed in our laboratory, which attempts to provide new biomechanical evidence and a fresh perspective into how the procedure can be implemented more effectively toward the goal of preventing osteoporosis-related fractures. The percutaneous application of a bone cement to vertebral defects associated with osteoporotic vertebral compression fracture has proven clinical successful in alleviating back pain. When the biomechanical efficacy of the procedure was examined, however, vertebroplasty was found to be limited in its ability to provide sufficient augmentation to prevent further fractures without risking complications arising from cement extravasations. The procedure may instead be more efficient biomechanically as a prophylactic treatment, to mechanically reinforce osteoporotic vertebrae at risk for fracture. Patient selection for such intervention may be reliably achieved with the more accurate fracture risk assessments based on vertebral strength, predicted using geometrically detailed, specimen-specific finite element models, rather than on bone density alone. Optimal cement volume, placement, and material properties were also recommended. The future of vertebroplasty involving biodegradable augmentation material laced with osteogenic agents that upon release will stimulate new bone growth and increase bone mass was proposed.