Dr Reeve asks about the sex differences in the magnitude and timing of periosteal apposition, the role of estrogen deficiency and loading in producing these sex differences, and whether regional differences in hip fractures reflect differences in the material and structural determinants of bone strength.
Greater periosteal apposition in men than in women is reported in most but not all cross-sectional studies.(1-6) Inconsistencies may be the result of secular increases in statural height. This will underestimate periosteal apposition. If secular changes in height occur in one sex but not the other or in the axial but not appendicular skeleton, cross-sectional studies may well produce inconsistent observations.(7-9)
Prospective studies do not resolve the problem.(10-14) Age-related periosteal apposition is modest, increasing bone diameter by only a few millimeters from adulthood to old age. These small changes are difficult to detect in prospective studies accounting for many null observations in the literature.(14) If periosteal apposition is partly an adaptive response to loss of bone strength produced by endocortical bone loss, it needs only to be modest as bending strength is proportional to the fourth power of the radius; less periosteal bone formation is needed to offset the loss of bending strength produced by a given amount of endocortical bone resorption.(15)
The question of whether periosteal apposition is greater in one sex than another is difficult because extent of periosteal apposition may be independently determined by the initial bone size. For a given rate of endosteal resorption, more periosteal apposition is needed to maintain bending strength in bigger bones.(14)
Kaptoge et al.(6) report a greater percentage increase in periosteal apposition in women than men, but men have a wider femoral neck diameter than women, so the same absolute change will be less in percentage terms. It would be instructive to know whether the greater percentage increase in women than men remained when the changes were expressed as a slope, which is independent of the initial value.
The causes of periosteal apposition are difficult to define because the small changes in periosteal apposition during adulthood are difficult to measure. Differences in loading may contribute to differences in periosteal apposition at the upper and lower limbs or iliac crest. Estrogen deficiency may remove the restraint on periosteal apposition-the increase in periosteal apposition after menopause seemed to vary according to duration of menopause in the prospective study of Ahlborg et al.(16-18); therefore, there may be temporal effects at the same site, and periosteal apposition may be intermittent.
Women with hip fractures may have smaller, normal or increased femoral neck periosteal diameter than controls.(19-21) We reported that these patients and their premenopausal daughters had greater femoral neck diameter than age-matched controls.(22) There may be sampling bias because ambulant survivors of hip fracture are likely to differ from the large numbers of elderly hip fracture cases suffering from dementia or debilitating illnesses. Periosteal apposition may vary by age, sex, years since menopause, region of the skeleton studied, race, and generation to generation. This could produce differences in bone shape on anterior, posterior, medial, and lateral surfaces; however, measuring these changes is proving to be difficult.