EFFECTS OF ETHNICITY AND AGE OR MENOPAUSE ON THE REMODELING AND TURNOVER OF ILIAC BONE - IMPLICATIONS FOR MECHANISMS OF BONE LOSS

We measured histologic indices of bone remodeling and turnover separat ely on the cancellous, endocortical, and intracortical subdivisions of the endosteal envelope, and on the combined total surface, in transil iac bone biopsies obtained after double tetracycline labeling in 142 h ealthy women, aged 20-74 years, 34 black and 108 white, 61 premenopaus al and 81 postmenopausal. The data were analyzed by two-way analysis o f variance of the four groups defined by age/menopause and ethnicity a nd by linear regression of the major variables on age. None of the int eraction terms was significant and none of the regression slopes on ag e differed between blacks and whites, indicating that, as for the prev iously reported structural indices, the effects of ethnicity and of ag e/menopause are independent. Accordingly, the data were also analyzed separately for the effect of ethnicity (pre- and postmenopausal combin ed) and age/menopause (blacks and whites combined). The analyses led t o the following conclusions. (1) The geometric mean bone formation rat e on the combined total surface was 25% lower in blacks than in whites ; other histologic differences between ethnic groups were inconsistent between surfaces. (2) Serum osteocalcin (OC) but not bone-specific al kaline phosphatase (BSAP) was lower by about 15% in blacks than in whi tes. (3) The lower bone turnover in blacks is most likely in the direc ted rather than in the stochastic component because of a higher bone m ass and consequent reduced susceptibility to fatigue damage. (4) All C lass 1 bone formation variables and the three resorption indices were significantly higher in the postmenopausal compared with the premenopa usal subjects, reflecting a 33% increase in activation frequency. (5) BSAP, but not OC, was increased relatively more (66%) than the bone fo rmation rate (BFR). Consequently, BSAP is more sensitive to the effect s of menopause than OC, but OC is more sensitive to the effects of eth nicity than BSAP. (6) There were highly significant differences betwee n the three subdivisions of the endosteal envelope for every non-cell- related variable, All Class 1 formation variables were highest on the endocortical surface, but the magnitude and pattern of the differences otherwise was inconsistent between variables. The contributions of th e different subdivisions to the total bone formation rate were cancell ous 54%, endocortical 13%, and intracortical 33%. (7) The previously r eported changes in bone surface location, together with the presently reported changes in activation frequency and wall thickness indicated that there was no significant effect of age/menopause on erosion depth on the cancellous and intracortical surfaces but a large increase in erosion depth on the endocortical surface. (8) The increase in bone tu rnover that results from hormonal changes is most likely in the stocha stic rather than in the directed component because it serves no purpos e but has harmful effects on skeletal integrity.