Pattern of collagen fiber orientation in the ovine calcaneal shaft and its relation to locomotor‐induced strain

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Abstract
Background: Gebhardt (1905. Arch. Entwickl. Org., 20:187–322) originated the hypothesis that the direction of collagen fibers in bone is a structural response to the type of mechanical load to which the bone is subjected. He proposed that collagen fibers aligned parallel to the loading axis are best suited to withstand tensile strain, whereas fibers oriented perpendicular to the loading axis are best able to resist compressive strain. Research comparing load patterns with fiber alignment in bone have tended to support Gebhardt's hypothesis. The aim of the present study is to further test this hypothesis by assessing the correspondence between the distribution of strain and the distribution of collagen fiber orientation in a bone that is subjected to compound loading (i.e., both tension and compression at different phases during the loading cycle). The ovine calcaneum was selected to meet this criterion. Methods: Calcaneum surface strain distributions were obtained from experimental results reported by Lanyon (1973. J. Biomech. 6:41–49). Histological sections of the calcaneal shaft were prepared and observed using circularly polorized light (CPL) microscopy to determine the distribution of collagen fiber alignment. The observed alignment pattern was then compared with the predicted pattern based on Gebhardt's hypothesis. Results: Contrary to previous studies, our findings show no clear correspondence between the strain type of greatest magnitude and the direction of collagen fibers. Areas of bone characterized by high compression and low tension showed predominantly longitudinal collagen alignment (contra to Gebhardt). Conclusions: It is argued that even small magnitudes of tension operating on local areas of bone may be sufficient to induced collagen alignment favorable to this type of strain, even when greater magnitudes of compressive strain are acting on the same bone volume.