The Distribution of Elastin and Collagen Underpinning the Smart Properties of the Interosseous Membrane

The interosseous membrane (IOM), a ligament-like structure spanning the radius and ulna, reduces strain in the ulna and structurally stiffens the radio-ulnar complex of the forearm. Using two-photon and second-harmonic-imaging we measured collagen and elastin signal intensity to test the hypothesis that their spatial distributions correspond to predominant loading patterns in the IOM. Distinct spatial gradients in collagen and elastin, as well as cruciate ligament-like architectures, were observed at the submicron and the micron to mesoscopic length scales. Quantitative analysis revealed anisotropies in the elastin-collagen composite comprising the IOM, with elastin 4-6 times higher than collagen concentrations at radius/ulna - IOM interfaces, and organized in the tensile loading direction, i.e. along the major Centroidal Axis, of the IOM. Hence, the IOM exhibits a composite structure comprising elastin and collagen, with spatial distribution of elastin higher than collagen at bone-IOM interfaces and decreasing from the interface with the ulna to that of the radius. These increased concentrations of elastin at interfaces are expected to confer elasticity (spring function). In contrast, peaks in collagen concentrations represent collagens organization into fibers, parallel to the length of the IOM, bridging the radius and ulna, and conferring toughness and damping function to the IOM and forearm construct. Mapping the cross-scale elastin and collagen composition of the IOM gives unprecedented insight into its emergent properties and associated mechanical function, an understanding of which may guide future surgical treatments, implant and medical textile design and manufacture, as well as physical therapy protocols to promote healing.