Fibula: The forgotten bone.

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

Simposio; VIII BLACK FOREST FORUM; 2015

International Society of Musculoskeletal & Neuronal Interactions (ISMNI)

In a previous study we had shown a different response of human tibia and fibula cortical structure to disuse. This finding led us to analyze the cortical structure of the normal human fibula from a biomechanical point of view. With that purpose, we took serial pQCT scans at every 5% of the fibula length comprised between the tibio-tarsal joint line and the proximal articular surface of the tibia (19 scans per individual) of 10/10 healthy, sedentary men and women aged 20-40 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) were determined. All cross-sectional shapes and geometrical/strength indicators suggested a sequence of five different regions along the bone, which would be successively adapted to 1. transmit loads from the articular surface to the cortical shell (close to fibula-tibia upper joint), 2. favor lateral bending (central part of upper half), 3. resist lateral bending (mid-diaphysis), 4. favor lateral bending again (central part of the lower half), and 5. resist bending/torsion (distal end). Cortical BMC and the cortical/total CSA ratio were higher at the midshaft than at both bone ends (p<0.001). However, all MIs, BSIs and SSI values and the endocortical perimeter/cortical CSA ratio (indicator of the mechanostat?s ability to re-distribute the 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). The correlations between MIs (y) and cortical vBMD (x) at each bone site ("distribution/quality" curves that describe the efficiency of distribution of the cortical tissue as a function of the local tissue stiffness) showed higher r coefficients at proximal than distal bone regions (p<0.001). Results show that 1. human fibula is primarily adapted to resist bending and torsion rather than compression, and 2. fibula?s bending strength is lower at the center of its proximal and distal halves and higher at the mid-shaft and at both bone´s ends. This would a. favor, proximally, the elastic absorption of energy by the attached muscles that rotate/evert the foot, and distally, the widening of the heel joint and the resistance to excessive lateral bending, and b. suggest that biomechanical control of structural stiffness differs between proximal and distal fibula.