Introduction: Osteoporotic fractures in elderly people are usually attributed to hormonal changes and inactivity. But why should the anterior vertebral body be affected so often?

Materials and Methods: Forty-one cadaveric thoraco-lumbar motion segments aged 62–94 yrs were loaded to simulate upright and flexed postures. A pressure transducer was used to measure “stress” inside the disc, and calculations showed how compressive loading was distributed between the neural arch, and the anterior and posterior halves of the vertebral body. Compressive strength was measured in flexed posture. Regional volumetric bone mineral density (BMD) and histomorpho-metric parameters were measured.

Results: Upright posture. Compressive load-bearing by the neural arch increased with grade of disc degeneration, averaging 52+25% in specimens with grade 3 or 4 discs. In these same specimens, the anterior half of the vertebral body resisted only 16+18% of the applied load. Relative unloading of the anterior vertebral body was associated with low BMD and with histomorphometric evidence of inferior bone quality. Flexed posture. Flexion always transferred loading to the anterior half of the vertebral body, so that it resisted 55+17% in specimens with grade 3/4 discs. Compressive strength measured in this posture was most closely proportional to BMD in the anterior vertebral body (r² = 0.75), and inversely proportional to neural arch load-bearing in the upright posture (r² = 0.39).

Conclusion: Disc degeneration causes the anterior vertebral body to be unloaded in habitual upright postures, reducing bone density and quality within it. This predisposes to wedge fracture when the spine is flexed.