Introduction. : Measurements of overall vertebral bone mineral density (BMD_v) do not adequately explain the observed patterns of osteoporotic vertebral fracture. Perhaps bone loss from specific regions of the vertebra has a more important effect on vertebral strength, and risk of fracture, than overall bone loss? We hypothesise that ‘stress shielding’ of the anterior vertebral body by the neural arch in erect standing postures can reduce BMD_v in the anterior vertebral body and thereby reduce vertebral compressive strength.

Materials and Methods: A compressive force of 1.5kN was applied to lumbar ‘motion segments’. positioned to simulate erect standing posture. Compressive stresses within the intervertebral disc were measured by pulling a miniature pressure transducer through it. ‘Stress profiles’ were integrated over area to calculate the total compressive force on the disc¹. This was subtracted from the 1.5kN to calculate the force resisted by the neural arch. Motion segments were then compressed to failure in moderate flexion (to simulate heavy lifting) and their compressive strength obtained. After disarticulation, the BMD_v, of the whole and the anterior half of each vertebral body was measured by dual energy x-ray absorptiometry (DXA). We report preliminary results from 9 specimens, aged 72–92 yrs.

Results: Vertebral strength (in flexion) was inversely related to load-bearing by the neural arch in erect posture (r²=0.42, p=0.05). Strength was directly related to the BMD_v of the whole (r²=0.65, p=0.06) and the anterior (r²=0.8, p=0.005) vertebral body.

Conclusions: These results suggest that habitual load-bearing by the neural arch in erect postures can lead to stress shielding of the anterior vertebral body so that the latter losesBMD_v, and the vertebra is weakened in the anterior vertebral body appears to be a BMD_v better predictor of vertebral strength than BMD_v, of the whole vertebra.