Older horses can be at risk of sustaining an uncommon injury: acute rupture of the proximal (upper) superficial digital flexor tendon (SDFT) where the cannon bone meets the carpus (knee). This is because as horses age, the SDFT stiffens and becomes less elastic, decreasing its resistance to cyclic loading to the point that it can potentially tear.

At the 2014 American Association of Equine Practitioners Convention, held Dec. 6-10 in Salt Lake City, Utah, Betsy Vaughan, DVM, associate professor at the University of California, Davis, School of Veterinary Medicine, described the characteristics of this condition based on her review of records from the university’s Large Animal Ultrasound Service from 2003 to2013.

First, she said, know what to look for: Clinical signs of SDFT rupture include visible swelling over the back of the knee or cannon bone and a reluctance to straighten the knee. Sometimes the veterinarian can see or palpate a notch on the back of the limb, just below the carpus. Pain on palpation varies, and veterinarians rely primarily on ultrasound for diagnosis.

In Vaughan’s retrospective study of 1,317 metacarpal ultrasound exams, she determined that 171 horses (13%) had a primary SDFT injury; however, only 13 horses and one mule, ranging in age from 15 to 30 years, had completely ruptured the SDFT. Ten of these were retired or used for light riding, three were show jumpers, and one was a breeding stallion. About half sustained the injury during turnout, three following jumping competition, and one during transport. The mule was affected bilaterally (on both sides). Four horses had a previous history of superficial flexor tendinitis (inflammation of the structure)