Tendons are composed of densely organized collagen fibers, proteoglycans, elastin and lipids. They are surrounded by the thin epitendon, which also contains nerves and blood vessels. Tendon structure and matrix composition vary greatly depending on functional requirements and position of the human body. Muscular force is transferred to the skeletal system at the point where the tendon inserts into the bone, which results in stabilization or movement across the joint.

This junction point is the most common site of various tendon injuries, although problems can occur throughout the tendon. Proximal to that point tendons are relatively hypovascular (i.e. containing a low number of blood vessels), which can act as a predisposing factor for hypoxic tendon degeneration. Changes in tendon composition can lead to degenerative, mechanical and overuse disease, as well as the inflammation.

Definition of tendonitis

Tendon injuries are caused by intrinsic or extrinsic factors, or even a combination of them. There are two general classifications of tendon disease. While tendinopathy is a more general term which refers to degenerative, mechanical and overuse disease, tendonitis refers to inflammation of the tendons. Inflammation represents a natural response of the human body to injury or disease, often causing swelling, pain, or irritation.

When referring to the inflammatory injuries of the tendon, we can differentiate between acute tendonitis alone, chronic tendinosus (histopathological degenerative state) with acute tendonitis and chronic tendinosus alone. Even though true inflammatory tendinopathies exist, most patients present with prolonged symptoms prior to their visit to a family physician, by which time acute inflammation is often subsided and replaced by degeneration of a normally highly arranged collagen fiber structure.

The exact role of inflammation is still a topic of research, but current animal and human studies support the notion that inflammation plays an important role in the etiology of acute tendonitis. It has also been demonstrated that an inflammatory process may be related to the development of chronic tendinopathies. The absence of inflammatory cells in the vicinity of the lesion does not automatically mean that inflammatory mediators are not implicated in different kinds of tendinopathies.

Examples of tendonitis

One of the most common pathologies of tendons is Achilles tendonitis, which causes prominent pain along the back of the leg. This condition is usually not related to a specific injury, but a common cause of this problem is a repetitive stress to the tendon. Sudden increase in the intensity or amount of exercise, tight calf muscles and extra bone growth in the junction point are additional risk factors.

Patellar tendonitis, also known as jumper's knee, can get inflamed as a result of repetitive jumping and other knee bending activities. It can also occur when quadriceps muscles are tight or weak. Patellar tendon is extremely vulnerable to injury (which is a reason why occurs in athletes in many different sports) and often difficult to manage successfully.

DeQuervain’s stenosing tenosynovitis is the most common tendonitis affecting the wrist of tennis players. It caused by impaired gliding of the two tendons at the base of the thumb. The subsequent inflammation causes the tunnel around the tendons to swell, causing tenderness and painful thumb and wrist movements.

In some instances, accumulation of calcium phosphate crystals in many locations along the tendon can lead to so called calcifying tendonitis. The most commonly affected are rotator cuff tendons, and such multifocal calcifications are localized approximately 2 centimeters from the insertion of the supraspinatus tendon, inserted into the upper facet of the greater tubercle of the humerus.