Tendonitis (also tenonitis or tendinitis) is an inflammation of a tendon. For example, patellar tendonitis (jumper’s knee) is an inflammation of the patellar tendon, which connects the tibia to the patella.

Chronic overuse of tendons leads to microscopic tears within the collagen matrix, which gradually weakens the tissue. Swelling in a region of microdamage or partial tear can be detected visually or by palpation. Increased water content and disorganised collagen matrix in tendon lesions may be detected by ultrasonography or magnetic resonance imaging.

Due to their highly specialised ultrastructure and slow collagen turnover, tendons and ligaments are very slow to heal if injured, and rarely regain their original strength. Partial tears heal by the rapid production of disorganised type-III collagen, which is inferior in strength to normal tendon. Recurrence of injury in the damaged region of tendon is common.

Treatment of tendon injuries is largely palliative. Non-steroidal anti-inflammatory drugs combined with rest and gradual return to exercise is a common therapy. Acupuncture is an alternative treatment that can be used to reduce pain and heal injured tendon more quickly. Return to function may be accelerated by the injection of stem cells. Completely ruptured tendons may be sutured together with or without grafted material.

Achilles tendonitis is a common injury, particularly in sports that involve lunging and jumping.

A veterinary equivalent to Achilles tendonitis is bowed tendon, tendonitis of the superficial digital flexor tendon of the horse.

Symptoms can vary from an achy pain and stiffness to the local area of the tendon, to a burning that surrounds the whole joint around the inflamed tendon. With this condition, the pain is usually worse during and after activity, and the tendon and joint area can become stiffer the following day.

The most common tendon areas that become inflamed are the elbow, wrist, biceps, shoulder (including rotator cuff attachments), leg, knee (patellar), ankle, hip, and Achilles. Of course, tendonitis will vary with each person, as it strikes the areas you use most.