Unification through disarray

摘要

Joint motion is achieved through the transfer of muscle load from tendon to bone - two tissues with dramatically different compositions, structures and mechanical properties. From the continuum perspective, the Poisson ratios for stretching in this direction can differ by nearly one order of magnitude, while the moduli of tendon and bone along the direction of muscle force differ by nearly a hundredfold. From the structural perspective, both are hierarchically organized, with nanoscale tropocollagen triple helices bundling into micrometre-diameter fibrils, which in turn bundle into fibres hundreds of micrometres in diameter and many millimetres long. In bone, the fibrils are stiffened by the cross-linking caused by the insertion and coating of bioapatite mineral crystals. The tendon-to-bone attachment would be expected to fail from Bogy- or Williams-type free-edge singularities if stress concentrations at the interface were not attenuated with a suite of cross-scale deformation mechanisms. Indeed, the rates of failure after surgical repair pose a major challenge. How, then, does the healthy tendon-to-bone attachment achieve effective load transfer from one tissue to the other, and why does the healing attachment fail so frequently? The answer lies in a unique hierarchical transitional tissue that exists at the interface between these dissimilar materials, and identifying and restoring the resilience mechanisms of this material is now a major target for tendon-to-bone repair strategies.