Biphasic behavior and site- and gender-specificity of pQCT-assessed ?distribution/quality? relationships concerning torsion strength throughout the human tibia.

GUSTAVO ROBERTO COINTRY; LAURA NOCCIOLINO; JOERN RITTWEGER; JOSÉ LUIS FERRETTI; JOSÉ LUIS FERRETTI; RICARDO FRANCISCO CAPOZZA

Congreso; XXXVII ANNUAL MEETING, AMERICAL SOCIETY FOR BONE & MINERAL RESEARCH (ASBMR).; 2015

"Distribution/quality" (d/q) curves describe the negative, hyperbolic relationships between pQCT-assessed indicators of the efficiency of cross-sectional design concerning bending or torsion (CSMI?s, y) and of the bone tissue stiffness (cortical vBMD, vCtD, x) in long bones [Capozza et al, JMNI 2013]. The slopes and significances of d/c curves would reflect the modeling-dependent ability of bone mechanostat to distribute cortical tissue as an inverse function of its stiffness. We have shown that the significance of d/q relationships for bending of the tibia, higher in men than women, is also higher toward the midshaft than at both bone ends. This should cope with tibia bending stresses (maximal at midshaft), but not with torsion stress which can be high toward the heel, where cortical mass is the lowest throughout the bone. To search for a possible homeostatic compensation of that inadequacy, this study describes the evolution of d/q relationships calculated for torsion in serial slices taken at every 5% of the tibia length from heel (S5) to knee (S95) in 17/21 men/women untrained (10/12) or chronically trained (7/9) in long-distance running. The d/q curves showed the typical hyperbolic shape from S10 to S85. The distribution of correlation coefficients r of the d/c curves throughout that tibia region was biphasic ("V"- shaped). Values were minimal at S25-S35 (-0.050 to -0.330, non-significant), increased homogeneously to maximal values toward both bone? ends (up to about -0.700 at S10 and S70, all significant) and were higher by average 0.200 in men than women. The ordinates of the curves were significantly higher in men than in women (p<0.001). Running tended to enhance pCSMI and reduce vCtD values (probably by increasing microdamage-induced remodeling), but it did not affect the d/c relationships. In this study, a higher r value of a d/q relationship does not indicate a higher bone strength, but just a higher efficiency of bone mechanostat to spatially orient the available cortical mass to resist torsion. The progressively increasing r values from S25 to the knee would have reflected what had been previously observed concerning bending, just as a parallel effect. However, the rapid distal r increase from S25 to S10 (not observed in the bending analysis), where bone section tends to be circular, would confirm the proposed homeostatic control of tibia torsion strength toward the heel as a compensation for the naturally low bone mass at that level.