Sex Differences in Spinal Osteoarthritis in Humans and Rhesus Monkeys (Macaca mulatta)

Study Design. Retrospective study of male and female spinal osteoarthritis, characterized by lateral spine thoracolumbar radiographs, in humans and nonhuman primates. Objective. To characterize differences in prevalence and vertebral distribution of spinal osteoarthritis between men and women, between male and female macaques, and between the 2 phylogenetically related genera. Summary of Background Data. Naturally occurring spinal osteoarthritis manifests similarly in humans and rhesus macaques. Other types of osteoarthritis particularly of the knee and hip have revealed sex differences in humans. In regard to spinal osteoarthritis, sex differences have been noted but without consistent results. Sex differences in macaques have not been examined. Methods. Radiographical evidence of disc space narrowing and osteophytosis was assessed using an atlas-scoring method. Prevalence was determined according to sex, age, body mass (for macaques only), and spinal location (human T4-L5; macaque T8-L7). Results. Average scores in macaques differed between the sexes, but they did not differ between men and women. The pattern of involvement along the spine was the same in male and female monkeys but differed between men and women: women had more thoracic involvement and men had more lumbar involvement. Overall, monkeys had a significantly higher prevalence of osteoarthritis than humans. Conclusion. The appearance of sex differences in the prevalence of osteoarthritis is most likely a proxy measure for the effect of body mass. Sex differences were apparent in monkeys due to the fact that males are significantly heavier than females. No sex difference in prevalence was apparent in humans, and there is substantial overlap in body mass between men and women. Differences in the location of osteoarthritic involvement along the spine between men and women were obscured when only average scores were examined.