Sex Differences in Parameters of Bone Strength in New Recruits: Beyond Bone Density

EVANS, R. K. C. NEGUS, A. J. ANTCZAK, R. YANOVICH, E. ISRAELI, and D. S. MORAN. Sex Differences in Parameters of Bone Strength in New Recruits: Beyond Bone Density. Med. Sci. Sports Exerc., Vol. 40, No. I I S, pp. S645-S653 2008. Background: Stress fracture (SF) injuries in new recruits have long been attributed to low bone mineral density (BMD). Low areal BMD assessed using two-dimensional dual-energy x-ray absorptiometry imaging, however, reflects structural density and is affected by smaller measures of bone geometry. Recent studies support a relationship between bone size and SF and indicate that slender bones are more susceptible to damage under identical loading conditions. Peripheral quantitative computed tomography (pQCT) is a three-dimensional imaging tool that provides measures of tissue density and geometry parameters of the tibia, a common site of SF. Purpose: To evaluate sex differences in parameters of volumetric BMD (vBMD), geometry, and strength of the tibia in new recruits using a novel pQCT image analysis procedure. Methods: pQCT images were obtained from 128 healthy men and women (20 male, 108 female, aged 18-21 yr) entering a 4-month gender-integrated combat training program in the Israeli Defense Forces. Tibial scans taken at sites 4% (trabecular bone), 38%, and 66% (cortical bone) from the distal end plate were analyzed using MATLAB to assess whole-bone and regional parameters. Measures included vBMD, geometry (diameter, area, cortical thickness, and canal radius), and strength (moments of inertia and bone strength and slenderness indices), Results: With the exception of normalized canal radius, which did not differ between sexes, all measures of bone geometry (P < 0.0001) and strength (P < 0,000 1 to P = 0.07) were greater in men. Women exhibited 2.7% to 3.0% greater cortical vBMD than men, whereas trabecular vBMD was 8.4% lower in women (P < 0.001). These differences remained significant after adjusting for body size. Conclusion: Sex differences in bone geometry and mineralization of the tibia may contribute to a decreased ability to withstand the demands imposed by novel, repetitive exercise in untrained individuals entering recruit training.