The natural history of kyphosis in postmenopausal women.
A cross-sectional study of vertebral morphometry in 449 unscreened postmenopausal women, from the ages of 40 to 80, is reported. The wedge angles of thoracic vertebrae T4-12 were found to increase exponentially as a function of age, up to 70 years. In addition to age, the wedging phenomenon was found to be accentuated by increased bone turnover due to low calcium intake, reduced physical activity, each successive delivery, and breast feeding. Most of these variables were not correlated with isolated vertebral wedge angles, but rather with the sum of them (Sigma, sigma), assumed to assess the impact of those variables on thoracic kyphosis. In a subset of women, sigma was found to be inversely correlated with low spinal mineral density at L2-4. T-11 and T-12 were the vertebrae most frequently deformed (wedge angle exceeding mean +/- 3 SD in a group of 50 young healthy women, 25-45 years old). The distribution of deformed vertebrae was found to be significantly different from those qualified as "fractured" according to Kleerekoper et al.'s (1984) and Melton et al.'s (1989) criteria. The overall information afforded by past and present data indicates that in postmenopausal women, vertebral deformation may occur with the help of mechanical solicitations plus high bone remodeling rates, as well as by structural collapse (fracture). The information obtained does not allow one to quantify the relative contribution of each set of factors to the wedging phenomenon.