The ultrastructure and mechanics of elastic ligaments

The major central part of elastic ligaments is constituted by longitudinally orientated elastic fibres, each surrounded by a sheath of spirally orientated collagen fibrils with an intervening thin layer of microfibrils. In the terminal region of such ligaments the orientation of the collagen fibrils is longitudinal and the elastic fibres end within the ligament without any attachment to bone. The ligament response to a first, unphysiologically high load produces a continuous stress-strain curve. On the other hand the curve becomes biphasic after repeated application of such loads. It is suggested that three factors are involved in this mechanical response, namely, frictional forces between elastic fibres and their sheaths, compressive forces exerted upon elastic fibres by their sheaths, and intre-sheath friction due to rearrangement of the constituent collagen fibrils.