Loading conditions and cortical bone construction of an artiodactyl calcaneus

Customary nonuniform distributions of physiological bone strains are though t to evoke heterogeneous material adaptation in diaphyseal cortices of some limb bones. Recent studies of artiodactyl calcanei have suggested that the regional prevalence of specific mechanical strain features such as mode an d magnitude correlate with specific variations in cortical bone ultrastruct ure, microstructure and mineralization, These data are also consistent with predictions of current algorithms of mechanically induced bone adaptation, However, detailed characterization of the customary functional strain envi ronment of these bones is needed to understand better the mechanisms of the se adaptations. An in vitro loading method and rosette strain gauges were used to record pr incipal strains, maximum shear strains and principal strain angles at multi ple locations on ten calcanei of adult male mule deer (Odocoileus hemionus hemionus). Each hind limb was fixed in an apparatus to mimic the mid-suppor t phase of the gait and loaded via the Achilles tendon over a broad range o f functional loads (0 to 2943 N). Strains were recorded on the craniolatera l, craniomedial, caudal, medial and lateral cortices at mid-diaphysis, Load ing variations included the progressive elimination of the ligament and ten don along the caudal calcaneus. The results showed that the cranial cortex experiences longitudinal compres sive strains that are nearly equal to the principal minimum strains and tha t the caudal cortex receives longitudinal tensile strains that are nearly e qual to the principal maximum strains. With a 981 N load, the mean principa l compressive strain on the cranial cortex was -636+/-344 mu epsilon (mean +/- S.D., N=9) and the mean principal tensile strain on the caudal cortex w as 1112+/-68 mu epsilon (N=9), In contrast to the cranial and caudal cortic es, principal strains in the medial and lateral cortices displayed relative ly large deviations from the longitudinal axis (medial, 24 degrees cranial; lateral, 27 degrees caudal). Although shear strains predominated at all ga uge sites, variations in maximum shear strains showed no apparent regional pattern or consistent regional predominance. The plantar ligament and tendo n of the superficial digital flexor muscle were shown to have important loa d-sharing functions. These results demonstrate that the functionally loaded artiodactyl calcaneu s generally behaves like a cantilevered beam with longitudinal compression and tension strains predominating in opposing cranial and caudal cortices, respectively. Differences in osteon remodeling rates, osteon morphology and mineral content reported previously between the cranial and caudal cortice s correlate, in part, with the magnitudes of the principal compressive and tensile strains, respectively. However, material differences that distingui sh the medial and lateral cortices from the cranial and caudal cortices cou ld not be primarily attributed to locally increased shear strains as previo usly suggested. Variations in osteon and/or collagen fiber orientation may correlate more strongly with principal strain direction.