Fibula: The Forgotten Bone-May It Provide Some Insight On a Wider Scope for Bone Mechanostat Control?

The human fibula responds to its mechanical environment differently from the tibia accordingly with foot usage. Fibula structure is unaffected by disuse, and is stronger concerning lateral bending in soccer players (who evert and rotate the foot) and weaker in long-distance runners (who jump while running) with respect to untrained controls, along the insertion region of peroneus muscles. These features, strikingly associated to the abilities of the fibulae of predator and prey quadrupeds to manage uneven surfaces and to store elastic energy to jump, respectively, suggest that bone mechanostat would control bone properties with high selective connotations beyond structural strength.

Meagre effects of disuse on the human fibula are not explained by bone size or geometry.

Fibula response to disuse is unknown; we assessed fibula bone in spinal cord injury (SCI) patients and able-bodied counterparts. Group differences were smaller than in the neighbouring tibia which could not be explained by bone geometry. Differential adaptation of the shank bones may indicate previously unknown mechanoadaptive behaviours of bone. INTRODUCTION: The fibula supports only a small and highly variable proportion of shank compressive load (-8 to +19 %), and little is known about other kinds of stresses. Hence, whilst effects of habitual loading on tibia are well-known, fibula response to disuse is difficult to predict. METHODS: Therefore, we assessed fibular bone strength using peripheral quantitative computed tomography (pQCT) at 5 % increments from 5 to 90 % distal-proximal tibia length in nine participants with long-term spinal cord injury (SCI; age 39.2 ± 6.2 years, time since injury 17.8 ± 7.4 years), representing a cross-sectional model of long-term disuse and in nine able-bodied counterparts of similar age (39.6 ± 7.8 years), height and mass. RESULTS: There was no group difference in diaphyseal fibula total bone mineral content (BMC) (P = 0.22, 95 % CIs -7.4 % to -13.4 % and +10.9 % to +19.2 %). Site by group interactions (P < 0.001) revealed 27 and 22 % lower BMC in SCI at 5 and 90 % (epiphyseal) sites only. Cortical bone geometry differed at mid and distal diaphysis, with lower endocortical circumference and greater cortical thickness in SCI than able-bodied participants in this region only (interactions both P < 0.01). Tibia bone strength was also assessed; bone by group interactions showed smaller group differences in fibula than tibia for all bone parameters, with opposing effects on distal diaphysis geometry in the two bones (all Ps < 0.001). CONCLUSIONS: These results suggest that the structure of the fibula diaphysis is not heavily influenced by compressive loading, and only mid and distal diaphysis are influenced by bending and/or torsional loads. The fibula is less influenced by disuse than the tibia, which cannot satisfactorily be explained by differences in bone geometry or relative changes in habitual loading in disuse. Biomechanical study of the shank loading environment may give new information pertaining to factors influencing bone mechanoadaptation.

Analysis of the independent power of age-related, anthropometric and mechanical factors as determinants of the structure of radius and tibia in normal adults. A pQCT study.

To compare the independent influence of mechanical and non-mechanical factors on bone features, multiple regression analyses were performed between pQCT indicators of radius and tibia bone mass, mineralization, design and strength as determined variables, and age or time since menopause (TMP), body mass, bone length and regional muscles' areas as selected determinant factors, in Caucasian, physically active, untrained healthy men and pre- and post-menopausal women. In men and pre-menopausal women, the strongest influences were exerted by muscle area on radial features and by both muscle area and bone length on the tibia. Only for women, was body mass a significant factor for tibia traits. In men and pre-menopausal women, mass/design/strength indicators depended more strongly on the selected determinants than the cortical vBMD did (p<0.01-0.001 vs n.s.), regardless of age. However, TMP was an additional factor for both bones (p<0.01-0.001). The selected mechanical factors (muscle size, bone lengths) were more relevant than age/TMP or body weight to the development of allometrically-related bone properties (mass/design/strength), yet not to bone tissue 'quality' (cortical vBMD), suggesting a determinant, rather than determined role for cortical stiffness. While the mechanical impacts of muscles and bone levers on bone structure were comparable in men and pre-menopausal women, TMP exerted a stronger impact than allometric or mechanical factors on bone properties, including cortical vBMD.

The pQCT 'Bone Strength Indices' (BSIs, SSI). Relative mechanical impact and diagnostic value of the indicators of bone tissue and design quality employed in their calculation in healthy men and pre- and post-menopausal women.

The pQCT-assessed Bone Strength Indices (BSI's, SSI) depend on the product of a 'quality' indicator, the cortical vBMD (vCtD), and a 'design' indicator, one of the cross-sectional moments of inertia or related variables (MIs) in long bones. As the MIs vary naturally much more than the vCtD and represent different properties, it could be that the variation of the indices might not reflect the relative mechanical impact of the variation of their determinant factors in different individuals or circumstances. To understand this problem, we determined the vCtD and MI's in tibia scans of 232 healthy men and pre- and post-MP women, expressed in SD of the means calculated for each group, and analyzed the independent influence of 1 SD unit of variation of each factor on that of the indices by multiple correlations. Results showed: 1. that the independent influence of the MIs on the indices was generally larger than that of the vCtD, and 2. that in post-MP women the influence of the vCtD was larger than it was in the other groups. This confirms the view that inter-individual variation of vCtD is comparatively small, and that mechanical competence of human bone is mostly determined by 'design' factors.

Mineralization- and remodeling-unrelated improvement of the post-yield properties of rat cortical bone by high doses of olpadronate.

Some pharmacologic effects on bone modeling may not be evident in studies of remodeling skeletons. This study analyzes some effects of olpadronate on cortical bone modeling and post-yield properties in femurs diaphyses (virtually only-modeling bones) of young rats by mid-diaphyseal pQCT scans and bending tests. We studied 20/22 male/female animals traetad orally with olpadronate (45-90 mg/kg/d, 3 months) and 8/9 untreated controls. Both OPD doses enhanced diaphyseal cross-sectional moments of inertia (CSMI) with no change in cortical vBMD and elastic modulus. Yield stiffness and strength were mildly increased. Post-yield strength, deflection and energy absorption were strikingly enhanced. Ultimate strength was enhanced mainly because of effects on bone mass/geometry and post-yield properties. The large improvement of post-yield properties could be explained by improvements in bone geometry. Improvements in bone mass/geometry over weight-bearing needs suggest an enhanced modeling-related response to mechanical stimuli. Effects on tissue microstructural factors (not measured) could not be excluded. Results reveal novel olpadronate effects on bone strength and toughness unrelated to tissue mineralization and stiffness, even at high doses. Further studies could establish whether this could also occur in modeling-remodeling skeletons. If so, they could counteract the negative impact of anti-remodeling effects of bisphosphonates on bone strength.

pQCT-assessed relationships between diaphyseal design and cortical bone mass and density in the tibiae of healthy sedentary and trained men and women.

In a pQCT study of running-trained and untrained men and women we had shown that bone mass distribution along the tibia was adapted to the usage-derived stress pattern. To study the possible association between the efficiency of diaphyseal design and bone material stiffness, we extend the analysis of the same sample to correlate pQCT indicators of the distribution (CSMIs), mass (BMC), and density (vBMD) of cortical bone tissue as descriptors of "distribution/mass" (d/m) or "distribution/quality" (d/q) relationships. The d/m and d/c curves followed positive (exponential) and negative (hyperbolic-like) equations, respectively. Distribution curves of r coefficients throughout the bone were all bell-shaped, reaching a maximum towards the mid-diaphysis. The CSMIs and BMC were higher, and vBMD was lower in men than women and in runners than non-runners. The d/m relationships were described by unique curves for all groups while d/q relationships were better adjusted to separate curves for men and women. Results support that: 1. diaphyseal design reflects the relative influence of bending/torsion stress along the bones, tending to minimize bone mass; 2. there is a trade-off between cortical bone "quality" and distribution; 3. d/m and d/q relationships are related to bone mechanical environment, and 4. d/q relationships are affected by sex.