Mechanical experiments on cadaveric thoracolumbar spine specimens showed that intervertebral disc degeneration was associated with reduced loading of the anterior vertebral body in upright postures. Reduced load bearing corresponded to locally reduced BMD and inferior trabecular architecture as measured by histomorphometry. Flexed postures concentrated loading on the weakened anterior vertebral body, leading to compressive failure at reduced load.
Introduction: Osteoporotic fractures are usually attributed to age-related hormonal changes and inactivity. However, why should the anterior vertebral body be affected so often? We hypothesized that degenerative changes in the adjacent intervertebral discs can alter load bearing by the anterior vertebral body in a manner that makes it vulnerable to fracture.
Materials and methods: Forty-one thoracolumbar spine "motion segments" (two vertebrae and the intervertebral disc) were obtained from cadavers 62-94 years of age. Specimens were loaded to simulate upright standing and flexed postures. A pressure transducer was used to measure the distribution of compressive "stress" inside the disc, and stress data were used to calculate how compressive loading was distributed between the anterior and posterior halves of the vertebral body and the neural arch. The compressive strength of each specimen was measured in flexed posture. Regional volumetric BMD and histomorphometric parameters were measured.
Results: In the upright posture, compressive load bearing by the neural arch increased with disc degeneration, averaging 63 +/- 22% (SD) of applied load in specimens with severely degenerated discs. In these specimens, the anterior half of the vertebral body resisted only 10 +/- 8%. The anterior third of the vertebral body had a 20% lower trabecular volume fraction, 16% fewer trabeculae, and 28% greater intertrabecular spacing compared with the posterior third (p < 0.001). In the flexed posture, flexion transferred 53-59% of compressive load bearing to the anterior half of the vertebral body, regardless of disc degeneration. Compressive strength measured in this posture was proportional to BMD in the anterior vertebral body (r2 = 0.51, p < 0.001) and inversely proportional to neural arch load bearing in the upright posture (r2 = 0.28, p < 0.001).
Conclusions: Disc degeneration transfers compressive load bearing from the anterior vertebral body to the neural arch in upright postures, reducing BMD and trabecular architecture anteriorly. This predisposes to anterior fracture when the spine is flexed.