The tibialis posterior tendon (behind the medial malleolus) is markedly thickened (compare it to the adjacent flexor digitorum tendon), showing intrasubstance degenerative signal and partial tear (evident along its anterior fibers), its tendon sheath is distended by fluid signal (tenosynovitis).
Adjacent bone marrow edema of the medial malleolus is noted and extensive subcutaneous soft tissue edema signal.