Biomechanics of tendon injury and repair
Introduction
Tendons are soft connective tissues consisting of parallel collagen fibers embedded within an extracellular matrix. This organized structure allows tendons to withstand and transmit large forces between muscle and bone. However, as tendons are subjected to repeated motion and degeneration over time, they are prone to both acute and chronic injuries. After injury, the healing process in tendons results in the formation of a fibrotic scar. The structural, organizational, and mechanical properties of this healed tissue are inferior to normal tendon (Frank et al (1992), Frank et al (1983)). Although these properties improve over time, they do not return to normal levels, even after long periods (Frank et al., 1997). In an attempt to better understand tendon healing mechanisms and to improve these inferior properties, researchers have investigated a broad range of factors believed to affect tendon injury and repair, such as activity level, motion after injury, various injury modalities, and different injury locations. In addition, many researchers have applied tissue engineering concepts to address this problem, from creating scaffolds and constructs out of relevant biomaterials and cells to applying cytokines exogenously to injured tissue, or using gene and cell therapy. This paper provides an overview of these studies, which demonstrate that the functional outcome of injured tendons is dependent upon many factors.
Section snippets
Normal tendon
Tendons connect muscle to bone and form a musculotendinous unit whose primary function is to transmit tensile loads generated by muscles to move and stabilize joints. Under normal loads, it has been shown that tendons maintain smooth physiological mechanics throughout range of motion. While subjected to higher loads, tendons prevent joint displacement beyond anatomical barriers, thus preventing injury and maintaining normal function (O’Brien, 1992; Dykyj and Jules, 1991). Within the tendon
Tendon healing
When tendons are injured, the body initiates a process of healing and scar formation that can be divided into phases, which are briefly described below, are distinguishable by specific peaks in a cascade of cellular and biochemical events. These phases overlap and their duration can vary greatly due to location of injury or disease (Gomez, 1995; Montgomery, 1989). Although the tensile strength of the healing tendon improves over time, it does not reach the levels of uninjured, normal tissue (
Tendon injury and repair
Because tendons heal poorly, tendon injury and repair mechanisms have been studied extensively in both the clinical and experimental setting. Clinical examples of different types of injury and widely used treatment methods are discussed, followed by a review of both in vitro and in vivo experimental models. As described below, although clinical and experimental studies have their own inherent advantages and disadvantages, they both provide important information on tendon injury and repair. The
Future directions
Over the past decade, new developments and our understanding of molecular biology have progressed and allowed investigators to apply new therapies to improve tendon healing. Cell therapies involve the delivery of mesenchymal stem cells directly to the injury site, and gene therapies allow for the introduction of genetic material into the cells participating in healing. Each of these methodologies has its own advantages, and both are very promising for treating tendon injuries.
Mesenchymal stem
Summary
The clinical treatment of tendon injuries has improved in no small part due to therapies and rehabilitation protocols developed from both clinical and experimental research. Experimental animal models, whether in vitro or in vivo, allow us to focus on and investigate a certain aspect or a combination of factors affecting tendon healing. Whether the study design incorporates activity level and mobilization or looks at different types of injuries and injury locations, the findings of each study
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