Molecular events in tendinopathy: a role for metalloproteases.

The smallest unit of tendon is the fibril, which is primarily made up of collagen [2]. The fibrils aggregate together to form fibers; several fibers make up a fascicle [2]. Fascicles are aligned predominantly with the long axis of the tendon, and are responsible for the tensile strength of the tendon. A small proportion of fibers runs transversely, and in spiral and plait-like formations [3,4]. This complex ultrastructure provides resistance against transverse, shear, and rotational forces that act on the tendon. The fascicles are arranged into bundles, and each bundle is surrounded by a thin layer of loose connective tissue, the endotenon [2]. The bundles are surrounded by a final outer covering, the epitenon [2]. The endotenon and epitenon carry blood vessels, lymphatics, and nerves [2,5]. In the fascicle, the cellular elements are laid out in the extracellular matrix (ECM), which is composed of glycosaminoglycans, proteoglycans, glycoproteins, other noncollagenous proteins and water [6,7]. Tenocytes lie in the ECM. A normal tendon is able to bear a load of 50 N/m to 100 N/m [8], and Foot Ankle Clin N Am