Fibrocartilage in tendons and ligaments - an adaptation to compressive load

Where tendons and ligaments are subject to compression, they are frequently fibrocartilaginous. This occurs at 2 principal sites: where tendons (and s ometimes ligaments) wrap around bony or fibrous pulleys, and in the region where they attach to bone, i.e. at their entheses. Wrap-around tendons are most characteristic of the limbs and are commonly wider at their point of b ony contact so that the pressure is reduced. The most fibrocartilaginous te ndons are heavily loaded and permanently bent around their pulleys. There i s often pronounced interweaving of collagen fibres that prevents the tendon s from splaying apart under compression. The fibrocartilage can be located within fascicles, or in endo- or epitenon (where it may protect blood vesse ls from compression or allow fascicles to slide). Fibrocartilage cells are commonly packed with intermediate filaments which could be involved in tran sducing mechanical load. The ECM often contains aggrecan which allows the t endon to imbibe water and withstand compression. Type II collagen may also be present, particularly in tendons that are heavily loaded. Fibrocartilage is a dynamic tissue that disappears when the tendons are rerouted surgical ly and can be maintained in vitro when discs of tendon are compressed. Fini te element analyses provide a good correlation between its distribution and levels of compressive stress, but at some locations fibrocartilage is a si gn of pathology. Enthesis fibrocartilage is most typical of tendons or liga ments that attach to the epiphyses of long bones where it may also be accom panied by sesamoid and periosteal fibrocartilages. It is characteristic of sites where the angle of attachment changes throughout the range of joint m ovement and it reduces wear and tear by dissipating stress concentration at the bony interface. There is a good correlation between the distribution o f fibrocartilage within an enthesis and the levels of compressive stress. T he complex interlocking between calcified fibrocartilage and bone contribut es to the mechanical strength of the enthesis and cartilage-like molecules (e.g. aggrecan and type II collagen) in the ECM contribute to its ability t o withstand compression. Pathological changes are common and are known as e nthesopathies.