Posterior tibial tendonitis and tendon dysfunction are painful and potentially disabling conditions that affect a tendon that runs behind the inner side of the ankle and attaches on the inner side of the foot. This tendon attaches the posterior tibial muscle to the foot, and allows this muscle to curve the foot inward. This motion resists the foot's tendency to flatten out, and stabilizes the foot while walking and standing.

People with flat feet can develop strain and inflammation to this tendon as it is stretched over the side of the foot during the flattening motion. Those with normal arches can also injure this tendon when walking on uneven surfaces or during sprains. Regardless of the cause, once the tendon is injured it will enter into a continual cycle of microscopic tissue tearing and inflammation that is worsened with every step one takes. Tendonitis is the early disease, in which damage and inflammation is present in the tendon, but as a whole the tendon is still intact and relatively healthy. If untreated, this disease process continue to worsen, until the more serious posterior tibial tendon dysfunction develops. In this condition, the tendon becomes very weak and structurally unsound. As a result, the foot will begin to flatten further as the tendon loses the ability to keep the arch stable, and walking becomes extremely difficult. Eventually, the foot will become so unstable that it is functionally limited, and aggressive treatment must be performed to restore the ability to walk properly.

Early symptoms of posterior tibial tendon disease include pain, swelling, warmth, or tightness to the foot on it's inner side, towards the other foot. This pain can start around the ankle, and progress to the middle part of the foot on that side. At times, leg pain can also be present if the muscle is involved, although this is uncommon. The pain is worsened with barefoot walking, and feels better in a supportive athletic shoe. It can start suddenly, can follow an injury, or can develop gradually. If untreated, the foot will become weak on that side, and will visibly flatten further.

Treatment is centered around reducing inflammation and stabilizing the foot structure to prevent further degeneration of the tendon. Rest, ice, anti-inflammatory medications, and the use of an ankle brace are the initial treatments of choice. If these measures are not helping, physical therapy or immobilization in a pneumatic walking boot are employed to resolve the inflammation. Cases that persist beyond these treatments, or cases that enter the later stages of posterior tibial tendon dysfunction may require surgical intervention, in which the tendon is repaired and augmented with various modern techniques that can include strengthening grafts, radiofrequency wave treatment to stimulate blood flow, or platelet gel application to stimulate internal healing. Special braces that combine ankle and foot support may be needed to prevent further foot collapse, and the worst cases may require fusion of foot bones to stop the arch collapse permanently.