Differential adaptation of the tibia and fibula to exercise and disuse cannot be explained by bone geometry or habitual loading.

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

Colonia

Workshop; I HUMAN PHYSIOLOGY WORKSHOP.; 2017

Institute of Aerospace Medicine, German Aerospace Center (DLR)

Ina previous study we observed a differential response of human tibia and fibulacortical structure to disuse. This finding led us to analyze the corticalstructure of the normal human fibula from a biomechanical point of view. Withthat purpose, we took serial pQCT scans at 5% increments of the fibula length (measuredfrom the tibio-tarsal joint line to the proximal articular surface of the tibia,giving 19 scans per individual) of 10/10 healthy, sedentary men and women aged20-40 years. Indicators of fibula cortical mass (BMC, cross-sectional area-CSA-), mineralization (vBMD), design (perimeters, moments of inertia -MIs-)and strength (Bone Strength Indices, BSIs; Strength-Strain Index, SSI) weredetermined. Allcross-sectional shapes and geometrical/strength indicators suggested a sequenceof five different regions along the bone, which would be successively adaptedto 1. transmit loads from the articular surface to the cortical shell (close tofibula-tibia upper joint), 2. favor lateral bending (central part of upperhalf), 3. resist lateral bending (mid-diaphysis), 4. favor lateral bendingagain (central part of the lower half), and 5. resist bending/torsion (distalend). Cortical BMC and the cortical/total CSA ratio were higher at the midshaftthan at both bone ends (p<0.001). However, all MIs, BSIs and SSI values andthe endocortical perimeter/cortical CSA ratio (indicator of the mechanostat?s ability to re-distributethe available cortical mass) showed a ?W-shaped? distribution along the bone,with maximums at the mid-shaft and at both bone?s ends (p<0.001). Thecorrelations between MIs (y) and cortical vBMD (x) at each bone site(?distribution/quality? curves that describe the efficiency of distribution ofthe cortical tissue as a function of the local tissue stiffness) showed higher rcoefficients at proximal than distal bone regions (p<0.001). Resultsshow that 1. the human fibula is primarily adapted to resist bending andtorsion rather than compression, and 2. the fibula?s bending strength is lowerat the center of its proximal and distal halves and higher at the mid-shaft andat both bone´s ends. This would a. favor, proximally, the elastic absorption ofenergy by the attached muscles that rotate/evert the foot, and distally, thewidening of the heel joint and the resistance to excessive lateral bending, andb. suggest that biomechanical control of structural stiffness differs betweenproximal and distal fibula.