The primary function of a tendon is to transmit the contraction force of its associated muscle to the bone. Consequently, the tendon needs to have sufficient tensile strength. Tendons have various shapes, such as the sheet-like aponeurosis of the latissimus dorsi or the long, pencil-like structure of the biceps brachii. They are constructed with parallel collagen fibers running the length of the tendon. The longitudinal arrangement of the collagen fibers gives the tendon its tensile strength.

Tendons are a fundamental part of the contractile unit. The tensile strength in a tendon can be more than twice that of its associated muscle.¹ As a result, they are rarely torn. Even in muscles where complete ruptures occur, such as the biceps brachii or triceps surae group, the rupture usually is at the musculotendinous junction or in the muscle fibers. The musculotendinous junction is the weak point in the entire contractile unit because it's where the two different tissue types (muscle and tendon) meet.

In some cases, the muscle fibers remain intact and the tendon tears or pulls away from its attachment site on the bone (another instance where different tissue types meet). This is known as an avulsion. More often, tendons are damaged with internal structural pathologies such as tendinosis and tenosynovitis. These conditions generally result from repetitive overuse as opposed to an acute injury.

Tendinosis/Tendinitis

The most common pathological problem involving tendons used to be referred to as tendinitis but is now more correctly known as tendinosis, which means abnormal condition of the tendon. Tendinitis implies an inflammatory condition and it previously was believed that chronic overuse lead to tendon fiber tearing and inflammation. We now know this does not occur in most overuse tendon pathologies. True tendinitis, or tendon fiber tearing with inflammation, occurs but it's a rare condition.²

Recent investigation of tendon overuse dysfunction shows most overuse tendon pathologies are devoid of inflammatory cells and instead involve a breakdown in the collagen matrix.^3,4 Because of the lack of inflammatory activity in these conditions, the term tendinosis is encouraged. The term tendinosis does not specify the pathological process, only that the tendon is dysfunctional. High levels or prolonged periods of tensile stress on the tendon can lead to collagen breakdown. While any tendon can develop tendinosis, tendons in the extremities are more susceptible. Another result of chronic load on the tendon is alteration in the tendon's vascularity (blood flow). An increase in vascularity is indicated in some studies, while other research shows decreased vascularity. Either problem contributes to chronic tendon pathology.

Even though there is significant research and evidence showing it's the pathology of tendinosis occurring, physician diagnosis and rehabilitation practitioners often call this injury tendinitis. Rehabilitation, in many cases, continues to focus on anti-inflammatory treatment strategies, rather than collagen rebuilding. In some cases, the use of anti-inflammatory medication, such as corticosteroid injections or oral anti-inflammatory medications, can be detrimental for healing collagen degeneration.⁵ Overuse tendon disorders can take a long time to heal due to the slow rebuilding of collagen. If tendon fiber tearing (tendinitis) were the primary problem, the tissue would heal rather quickly as it moves through the various stages of the inflammation and tissue repair process. Collagen rebuilding is a slow process and tendinosis can become chronic or recurrent.

Tenosynovitis

Another chronic overuse tendon problem is tenosynovitis, which is an inflammation and/or irritation between a tendon and its surrounding synovial sheath. This condition affects only those tendons enclosed within a synovial sheath. The synovial sheath is also called the epitenon. The synovial sheath surrounds tendons in the distal extremities and a few other locations, such as the biceps brachii long head tendon as it travels through the bicipital groove. The sheath reduces friction between the tendon and the retinaculum (or, infrequently, a ligament) that binds the tendon close to the joint. The tendon must be able to glide freely within the sheath.

Chronic overloading or excess friction leads to adhesion between the tendon and its sheath. The adhesions cause a roughening of the surface between the tendon and its sheath, and a subsequent inflammatory reaction results. The rough tendon surface routinely produces crepitus (grating sensations) when the muscle-tendon unit and affected joint are moved through their range of motion. The symptoms of tendinosis and tenosynovitis are similar, but one can help distinguish between the two by determining if the tendon has a synovial sheath. If it does, tenosynovitis is possible. If there is no sheath, tendinosis is probably the cause.

Avulsion

An avulsion is an acute tendon injury resulting from high tensile loads, in which a tendon is forcibly torn away from its attachment site on the bone. In a majority of tensile stress injuries of the musculotendinous unit, fiber tearing occurs at the musculotendinous junction producing a strain. In some other cases these fibers remains intact and the tendon pulls away from its bony attachment site.

Avulsion injuries occur in regions where a large muscle attaches at a relatively small site on the bone, such as the hamstring attachment.