Sinus tarsi synovitis (syndrome) is a common biomechanical sequela of tibialis posterior tendon deficiency. Whenever you spot sinus tarsi synovitis you should remember to look for tibialis posterior tendinopathy/tear/rupture, and vice versa.

This case emphasizes the importance of always examining each of the tendons at the ankle (using Tom, Dick and Harry mnemonic to help you medially) as a missing tendon due to complete rupture can be difficult to appreciate unless you look for each tendon specifically.