Biology of tendon injury: healing, modeling and remodeling

Tendon disorders are frequent, and are responsible for much morbidity both in sport and the workplace. Although the presence of degenerative changes does not always lead to symptoms, pre-existing degeneration has been implicated as a risk factor for acute tendon rupture. The term tendinopathy is a generic descriptor of the clinical conditions in and around tendons arising from overuse. The terms "tendinosis" and "tendinitis/tendonitis" should only be used after histopathological examination. Disordered healing is seen in tendinopathy, and inflammation is not typically seen. In acute injuries, the process of tendon healing is an indivisible process that can be categorized into three overlapping phases for descriptive purposes. Tendon healing can occur intrinsically, via proliferation of epitenon and endotenon tenocytes, or extrinsically, by invasion of cells from the surrounding sheath and synovium. Despite remodeling, the biochemical and mechanical properties of healed tendon tissue never match those of intact tendon. Tendon injuries account for considerable morbidity, and often prove disabling for several months, despite what is considered appropriate management. Chronic problems caused by overuse of tendons probably account for 30% of all running-related injuries, and the prevalence of elbow tendinopathy in tennis players can be as high as 40%. The basic cell biology of tendons is still not fully understood, and the management of tendon injury poses a considerable challenge for clinicians. This article describes the structure of tendons, and reviews the pathophysiology of tendon injury and healing.