Noninvasive assessment of ulnar bending stiffness in women

Abstract
The load-carrying capacity of cortical bone is closely related to its geometry and to its fundamental material properties, including mineral content (BMC). Together these determine the bending stiffness EI, where I is the cross-sectional moment of inertia and E is Young's modulus of elasticity. To assess the relationship of BMC and bone width (BW) to EI in healthy women, we used mechanical response tissue analysis (MRTA), a noninvasive method that involves analysis of tissue responses to ulnar vibration. A total of 48 healthy women were enrolled into an older (64 ± 1y, n = 25) and a younger (25 ± 0.6y, n = 23) group. BMC and BW of the dominant ulna were measured by single-photon absorptiometry (SPA). EI was determined by MRTA. BMC (0.75 ± 0.02 versus 0.63 ± 0.02 g/cm), BMC/BW (0.75 ± 0.02 versus 0.63 ± 0.02 g/cm2), and EI (21.7 ± 1.3 versus 21.3 ± 1.1 N ṁ m2) were significantly greater (p < 0.005) in the young subjects. BW did not change with age (1.00 ± 0.01 versus 1.01 ± 0.01 cm). In young women, simple correlations of BMC and BW with EI were both significant. By multiple regression analysis only BW independently predicted EI (EI = −0.35 + 39.1 × BMC, R2 = 0.52). In older women BMC and BW correlated with EI, but in multiple regression only BMC was significant (EI = −34.5 + 62.1 × BW; R2 = 0.45). When this analysis of older women included only those whose BMC values were within 2 SD of the young mean, BMC remained the only significant predictor of EI. We conclude that ulnar dimensions make important contributions to axial loading strength in healthy young women, whereas changes in internal architecture, porosity, or bone material properties increase the importance of BMC as a determinant of ulnar stiffness for older women.
Funding Information
  • Research Service of the Veterans Administration
  • Stanford University School of Medicine