Abstract
Entheses (insertion sites, osteotendinous junctions, osteoligamentous junctions) are sites of stress concentration at the region where tendons and ligaments attach to bone. Consequently, they are commonly subject to overuse injuries (enthesopathies) that are well documented in a number of sports. In this review, we focus on the structure–function correlations of entheses on both the hard and the soft tissue sides of the junction. Particular attention is paid to mechanical factors that influence form and function and thus to exploring the relationship between entheses and exercise. The molecular parameters indicative of adaptation to mechanical stress are evaluated, and the basis on which entheses are classified is explained. The application of the ‘enthesis organ’ concept (a collection of tissues adjacent to the enthesis itself, which jointly serve the common function of stress dissipation) to understanding enthesopathies is considered and novel roles of adipose tissue at entheses are reviewed. A distinction is made between different locations of fat at entheses, and possible functions include space-filling and proprioception. The basic anchorage role of entheses is considered in detail and comparisons are explored between entheses and other biological ‘anchorage’ sites. The ability of entheses for self-repair is emphasized and a range of enthesopathies common in sport are reviewed (e.g. tennis elbow, golfer's elbow, jumper's knee, plantar fasciitis and Achilles insertional tendinopathies). Attention is drawn to the degenerative, rather than inflammatory, nature of most enthesopathies in sport. The biomechanical factors contributing to the development of enthesopathies are reviewed and the importance of considering the muscle–tendon–bone unit as a whole is recognized. Bony spur formation is assessed in relation to other changes at entheses which parallel those in osteoarthritic synovial joints.
Keywords: biomechanical, enthesopathy, fibrocartilage, insertion sites, osteotendinous junctions
Introductory remarks
In engineering terms, tendons and ligaments can be regarded as machines with multiple moving parts (fibrils, fibres and fascicles) that perform the basic function of force transfer to and from the skeleton. They distribute the loads applied to them dynamically in order to execute movement patterns. Their complex response to loading allows for multi-axis bending, and this adds to the stress concentration in the region where they attach to bone. This attachment site will be referred to in this review as an ‘enthesis’, but it is also known as an ‘insertion site’, or an ‘osteotendinous’ or ‘osteoligamentous’ junction. It is stress concentration at the hard–soft tissue interface which makes entheses vulnerable to acute or overuse injuries in sport. Conditions such as tennis and golfer's elbow, jumper's knee and various Achilles insertional tendinopathies are well known to primary-care physicians, orthopaedic surgeons, rheumatologists and sports medicine specialists alike. However, the importance of entheses extends beyond this, as they are also the primary target organ in a collection of rheumatic conditions known as the seronegative spondyloarthropathies, the best known of which is ankylosing spondylitis. Intriguingly, not all entheses are equally targeted in these patients and one of the theories accounting for the differential effect is that the inflammatory enthesopathies that afflict these patients are most typical of entheses that are subject to mechanical trauma. Entheses are also of clinical significance to orthopaedic surgeons who may be faced with the challenge of reattaching a tendon or ligament to bone, particularly when creating an autograft for a ruptured anterior cruciate ligament (ACL). In the present review, we focus on the structure–function correlations of entheses and pay particular attention to mechanical factors that influence form and function and thus to the relationship between entheses and exercise. The concept of mechanical load influencing the structure of a musculoskeletal tissue is well recognized and is part of the growing subject of mechanobiology. It is the basis of the ‘form follows function’ principle which underpins Wolff's. However, far less attention has been paid to tendons and ligaments than to bone and much of the current interest has its roots in the pioneering work of Ploetz (1938), Kummer (1959, 1962, 1978) and Pauwels (1980).
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