‘Tendinitis’ has been the clinical term traditionally applied to virtually all painful tendon structures, including the tendon, the synovial sheath, and the adjacent bursa. It has, however, been established that there is only a limited inflammatory response in the tendon to injury.
Injuries are most often associated with a degenerative process, which increases with age as the tendons begin to lose their elasticity. Fatigue ruptures of tendon fibers are probably a contributory cause to tendon problems and may precede degeneration and tendinosis, especially in young athletes.
Degeneration is characterized by disruption of the tendon fibrils, collagen (the protein of which the tendon is built) in the fibril splitting and fragmentation, and loss of collagen orientation. The cell metabolism is altered, and there is a formation of different collagens and proteoglycans, proliferation of capillaries, and minimal inflammatory cell infiltration. Degeneration will result in microtears and disruption of the tissue. The end result is cell hypotrophy (the cells decrease in size).
Contributory causes of tendon degeneration are inadequate oxygen supply, decreased nutrition, hormonal change, chronic inflammation, and aging. The degenerative process is secondary to tensile overuse, fatigue, weakness, and possibly vascular changes. Most spontaneous ruptures of tendons (97%) are preceded by pathologic changes in the tendon. Injuries to tendons that are associated with degeneration and not inflammation are often located in areas of poor circulation.
Achilles tendon injuries, for example, may be located 2–5 cm (1–2 in) proximal to the tendon’s attachment to the calcaneus where there is decreased vascularity. Injury to the supraspinatus tendon may occur 1–2 cm (0.5–1 in) from its attachment to the humerus, where blood supply is also poor. Owing to this poor circulation there is also little inflammatory response and repair capacity.
Based on the anatomy of the tendon, it is possible to describe four pathologic conditions. This classification emphasizes the distinction between peritenon, or synovial inflammation, and increasing involvement of the tendon substance as a likely reflection of the failure to adapt to physical load, and emphasizes the variable stress responses in the tendon structure.
These categories are:
- Peritenonitis (paratenonitis, tenosynovitis): inflammation of only the peritenon.
- Peritenonitis with tendinosis: tendon sheath inflammation associated with intratendinous degeneration.
- Tendinosis: degeneration in the tendon itself due to cellular hypotrophy.
- Tendinitis: asymptomatic degeneration (defined above) of the tendon with disruption and inflammatory repair response.
A commonly proposed name for tendon pain problems is tendinopathy.
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