Yup. That is exactly right!
No, you are not nutz, just observant! This rare but “obvious” physical exam finding during a lameness exam will make you look like a genius to your clients. “Well, what we have here is a Superficial Digital Flexor Tendon Luxation.” Strut those $100 words!
A superficial digital flexor tendon luxation happens acutely with a “yipe!” while a dog is playing. S/he is acutely 3-legged lame, and the point of the heel swells into a small fluctuant ball. As days/weeks pass, the lameness becomes weight bearing, and the heel becomes firm.
On exam, if carefully palpated much like palpating a patella with thumb and forefinger on either side, there feels to be a “slip” on and off midline. As day/weeks pass, it is harder and harder to appreciate due to advancing granulation and fibrosis.
On radiographs, all we see is a ball of soft tissue swelling over the proximal aspect of the calcaneus. No boney changes are appreciated, and the soft tissue shadow of the common calcaneal tendon is contiguous.
The patient signalment is typically young-middle aged and active. An over-represented breed is the Sheltie, but I’ve operated everything from Chihuahua to PittieX. (No cats yet, but I’m still young!) Sequential bilateral injury is not unheard of, unfortunately, suggesting there may be a congenital/anatomic predisposition to the injury.
The mainstay of therapy is surgical reduction and imbrication, always protected by immobilization with hock splint (or custom orthotic) until early fibrosis (6wks). Gradual return to function is introduced over the subsequent 4-6wks (mature fibrosis).
Since most of these patients are nutzo kids difficult to restrict, I really like to use the custom orthotic bracing option, rather than a medical splint. Skin health is far superior over the 6+wk wear time, and owners can manage these at home in consultation with prosthetist directly. Orthotics have come a LONG way in the last five years, with the ability to dynamize the immobilization (i.e. partially, selectively stress the injury/repair during healing period) and materials that preserve skin health proactively.
Lara Marie Rasmussen, DVM, MS
Diplomate, American College of Veterinary Surgeons
Direct Veterinary Surgery, PC
 Tendon Luxation – August 2018){kind=link}