History:
An 11-year-old Quarter Horse mare presented to the Hospital for Large Animals for a lameness evaluation. The lameness exam revealed a grade 3/5 right hind lameness with firm swelling around the right tarsus (see upper left photo in the video below). The lower left photo is of a different horse but shows the position of the probe along the plantaromedial aspect of the tarsus. What do you think is causing the swelling in the tarsal region? What structures and abnormalities do you see in the ultrasound video?
Notice the mildly to moderately hypoechoic area with the deep digital flexor tendon and corresponding irregular fiber pattern. There was also moderate anechoic effusion and moderate synovial proliferation within the tarsal sheath. The final diagnosis was deep digital flexor tendonitis and associated tarsal sheath tenosynovitis caused by bony irregularity along the sustentaculum tali.
Additional imaging included tarsal radiographs which revealed osseous proliferation and sclerosis along the plantaromedial aspect of the sustentaculum tali. Tenoscopy of the tarsal sheath was performed followed by intrathecal injection of steroids and Atropine at a later visit. 
