Prediction of osteoporotic spinal deformity.

Abstract

A biomechanical model was developed from full-spine lateral radiographs to predict osteoporotic spinal deformity in elderly subjects. To investigate the biomechanics of age-related spinal deformity and concomitant height loss associated with vertebral osteoporosis. Vertebral bone loss and disc degeneration associated with aging causes bone and disc structures to weaken and deform as a result of gravity and postural stresses. An anatomically accurate sagittal-plane, upright-posture biomechanical model of the anterior spinal column (C2-S1) was created by digitizing lateral full-spine radiographs of 20 human subjects with a mean height of 176.8 cm and a mean body weight of 76.6 kg. Body weight loads were applied to the model, after which intervertebral disc and vertebral body forces and deformation were computed and the new spine geometry was calculated. The strength and stiffness of the vertebral bodies were reduced according to an osteopenic aging model and modulus reduction algorithm, respectively. The most osteopenic model (L3 F(ult) = 750 N) produced gross deformities of the spine, including anterior wedge-like fracture deformities at T7 and T8. In this model, increases in thoracic kyphosis and decreases in vertebral body height resulted in a 25.2% decrease in spinal height (C2-S1), an 8.6% decrease in total body height, and a 15.1-cm anterior translation of the C2 spine segment centroid. The resulting deformity qualitatively resembled deformities observed in elderly individuals with osteoporotic compression fractures. These predictions suggest that postural forces are responsible for initiation of osteoporotic spinal deformity in elderly subjects. Vertebral deformities are exacerbated by anterior translation of the upper spinal column, which increases compressive loads in the thoracolumbar region of the spine.