The role of mechanical loading in tendon development, maintenance, injury, and repair

Abstract

Tendon injuries often result from excessive or insufficient mechanical loading, impairing the ability of the local tendon cell population to maintain normal tendon function. The resident cell population composing tendon tissue is mechanosensitive, given that the cells are able to alter the extracellular matrix in response to modifications of the local loading environment. Natural tendon healing is insufficient, characterized by improper collagen fibril diameter formation, collagen fibril distribution, and overall fibril misalignment. Current tendon repair rehabilitation protocols focus on implementing early, well-controlled eccentric loading exercises to improve repair outcome. Tissue engineers look toward incorporating mechanical loading regimens to precondition cell populations for the creation of improved biological augmentations for tendon repair.

Fig. 1

The tendon’s hierarchical structure begins at the molecular level with tropocollagen. Approximately five tropocollagen molecules form a microfibril, which then aggregate to create a subfibril. Several subfibrils form a single fibril. Multiple fibrils form a tendon fascicle, and fascicles, separated by the endotenon, join to form the macroscopic tendon. Tendon fibroblasts, or tenocytes, are found on collagen fibers allowing for the regulation of the extracellular environment in response to chemical and mechanical cues. (Reproduced, with permission of Elsevier, from: Silver FH, Freeman JW, Seehra GP. Collagen self-assembly and the development of tendon mechanical properties. J. Biomech. 2003 Oct;36(10):1529-33, Copyright 2003; and Wang JH. Mechanobiology of tendon. J Biomech. 2006;39(9):1563-82, Copyright 2006.)