Differences in predominant collagen fiber orientation between dorsal and plantar trabecular bone tracts of adult mule deer calcanei suggest strain‐mode‐specific adaptation

Studies examining biomechanics of trabecular (cancellous) bone often focus on its architectural characteristics, including bone volume fraction, degree of preferred trabecular alignment, and morphology of individual trabeculae. Studies examining relationships of these characteristics with load history in specific regions of limb bones usually aim to identify species affiliations or how these characteristics affect mechanical properties. This focus on structural characteristics seems warranted because >70% of variance in strength tests of bulk trabecular bone specimens is explained by structural characteristics. However, biomechanically significant differences in trabecular bone material characteristics may be consequently overlooked. We examined this possibility by quantifying predominant collagen fiber orientation (CFO) in thin transverse sections from adult deer calcanei at mid-shaft. This bone has simple load history, producing net compression in its dorsal aspect and net tension in its plantar aspect. Predominant CFO was quantified in circularly polarized light images as weighted mean gray levels (WMGLs) of the (1) dorsal "compression cortex" and plantar "tension cortex," and (2) dorsal and plantar trabecular tracts (presumably also net compression- vs. tension-loaded). Brighter gray levels are expressed as higher WMGLs and reflect compression-adapted CFO; darker gray levels reflect tension-adapted CFO. Results showed more oblique-to-transverse CFO in the dorsal trabecular bone and longitudinal in the plantar trabecular bone (dorsal 126.6 ± 18.6; plantar 98.3 ± 16.2; p < 0.01). Although this difference resembles strain-mode-specific (compression vs. tension) adaptation in the dorsal/plantar cortices, adequate exploration of this hypothesis in trabecular bone will require mechanical testing coupled with micro- and nano-scales analyses in this model and in other bones in other species.