Tendons and ligaments--an overview.

The structure, range of functions, blood supply, nerve supply, biochemical composition and development of tendons and ligaments are reviewed. The importance of their cells is often overlooked because of the obvious role of the extracellular matrix (ECM) in determining the physical properties of tendons and ligaments. However, it is emphasised that tendon and ligament cells have elaborate cell processes that form a three dimensional network extending throughout the extracellular matrix. The cells communicate with each other via gap junctions that could form the basis of an important load sensing system allowing the tendon to modify its ECM. Tendons and ligaments have three specialised regions along their length-the myotendinous junction, the region where tendons change direction by wrapping around bony pulleys and the enthesis (bony insertion site). The myotendinous junction is a common site of muscle strains and pulls, the wrap-around region is frequently fibrocartilaginous and a common site for degenerative change, and the enthesis may be fibrous or fibrocartilaginous according to location, and is a common site for degenerative changes or 'enthesopathies'. Enthesis fibrocartilage is just one of a series of protective devices reducing wear and tear at insertion sites. Consideration is also given to the structure and function of tendon sheaths and to the dramatic effects of exercise and deprivation on tendons and ligaments-exercise strengthens, but even relatively short periods of immobilisation can dramatically weaken tendons and ligaments.