Entheses--the bony attachments of tendons and ligaments

Most tendons and ligaments have fibrocartilaginous entheses where there are 4 zones of tissue at their bony attachments--dense fibrous connective tissue, uncalcified fibrocartilage, calcified fibrocartilage and bone. Such entheses leave smooth, circumscribed markings on dried bones. The uncalcified fibrocartilage dissipates the bending of collagen fibres away from the bone, ensures that a stretched tendon or ligament does not narrow too close to the bone and acts as a mini growth plate. The zone is thickest at entheses where a great deal of bending of the tendon/ligament accompanies joint movement. The calcified fibrocartilage anchors the tendon/ligament to the bone and enables it to withstand shear. Enthesis fibrocartilage may be accompanied by sesamoid and periosteal fibrocartilages that similarly protect the enthesis from wear and tear and dissipate stress. Nevertheless, each fibrocartilage can show distinctive pathological changes. A wide variety of ECM molecules has been reported in enthesis fibrocartilage, but it is best characterised by its content of type II collagen and aggrecan which account for its compression-tolerance properties.