Tibialis posterior tendinosis is wear and tear of a tendon that passes behind and around the inner ankle.
The usual cause is excessive ongoing strain because of a problem in how the ankle moves. Most often, the problem is that the person's arch is low and, when walking, the foot tends to turn inward because the person is overweight. Tendon dysfunction may further contribute to flattening of the arch. The tendon may tear completely, sometimes suddenly in a young person.
Early on, people have occasional pain behind the inner ankle. Over time, the pain becomes severe, and swelling occurs. Normal standing and walking become difficult. Standing on the toes is usually painful and may be impossible if the tendon is completely torn.
Doctors can often base the diagnosis on the person's symptoms and the results of an examination. However, sometimes magnetic resonance imaging (MRI) is necessary to confirm the diagnosis and to see the extent of tendon damage.
Devices that change the position or range of movement of the foot and relieve pressure on affected joints (orthoses) and ankle braces are usually enough. Complete rupture may require surgery for a person to regain normal function. Surgery is especially important in young active people with tears that develop suddenly.
Resource: Merck Manual Home Edition