Meeting Abstract
The mechanical strength of bones is influenced both by their shape and by collagen fiber orientation (CFO) within the bone matrix. Birds and bats have independently evolved wing bones that are thin-walled and circular in cross-section, a shape that maximizes resistance to torsional stress for a given bone area. Variability in CFO presents a mechanism for tissue-level optimization of stiffness. We tested whether bird and bat forelimbs displayed CFO better aligned to resist torsion than non-volant outgroups using quantitative polarized light microscopy (qPLM) with a novel set of image analysis functions for the R statistical package. Mid-cortical bone of both volant and non-volant taxa showed similar distributions of longitudinal CFO. In agreement with prior work, we found that bats and birds display a distinct region of endosteal bone along the medullary cavity with more transverse CFO. Our results show that endosteal bone is composed of single-sense helically-oriented collagen fibers at ~45º pitch, a near optimal arrangement for resisting torsion. Endosteal bone displayed evidence of remodeling, whereas the mid-cortical region was most often composed of primary bone. Simple biomechanical models of strain in torsion show that despite its location closer to the cross-section centroid, endosteal bone is expected to make a major contribution to torsional stiffness. We propose that the endosteal rim of helically-oriented bone represents a compromise between tissue-level adaptation to torsional loading and the structural consequences of bone remodeling—while the endosteal position is not structurally ideal for torsion resistance, it is structurally the ‘safest’ and most accessible place from which to remove fatigue-damaged bone from the thin cortices of bird and bat long bones during remodeling.
