ROLE OF TRABECULAR MORPHOLOGY IN THE ETIOLOGY OF AGE-RELATED VERTEBRAL FRACTURES

Osteoporotic compression fractures of the spine differ from most other age-related fractures in that they usually are associated with minima l trauma and with loads no greater than those encountered during norma l activities of daily living. With aging and osteoporosis, there is pr ogressive resorption of bone, resulting in reductions in bone density, thinning of trabeculae, and loss of trabecular contiguity. These chan ges in trabecular bone structure are associated with losses in bone st rength which are disproportionate to the reductions in bone mass alone . To explain this disproportionate loss of bone strength, the prevaili ng opinion is that density reductions in the vertebral centrum are acc ompanied by a reduction in the number of trabeculae, by preferential r esorption of horizontal trabeculae, and by hypertrophy of the remainin g vertical trabeculae. To evaluate this view of vertebral morphology, we performed three-dimensional stereological analysis of trabecular bo ne extracted from midsagittal sections of first lumbar vertebral bodie s from 12 donors spanning an age of 27-81 years. We found that both th e number (R2 = 0.63, P < 0.01) and thickness (R2 = 0.91, p < 0.01) of trabeculae decreased linearly with density (as expressed by bone volum e fraction) whereas the spacing between the trabeculae (R2 = 0.61, P < 0.01) increased reciprocally. There were more vertical trabeculae wit h transverse trabeculae at all densities, and the number of vertical t rabeculae changed with density at twice the rate of the number of tran sverse trabeculae (P < 0.001). These data do not support the prevailin g view that there is preferential resorption of horizontal trabeculae or hypertrophy of the remaining vertical trabeculae. Bone density was also a strong (R2 = 0.90, p < 0.01) power law function of the ratio of trabecular thickness to mean intertrabecular spacing. From buckling t heory, the critical buckling load of a trabecula is related to this ra tio of trabecular thickness to effective length. The changes in trabec ular morphology observed with decreasing bone density thus pose a ''tr iple threat'' to the strength and stability of vertebral trabecular bo ne, as not only are there fewer trabeculae, but the remaining trabecul ae are both thinner and longer.