The Achilles tendon is attached to the heel bone (calcaneous bone) of the hind limb and is responsible for keeping the heel bone elevated off the ground. The Achilles tendon is actually composed of five distinct tendons: the gastrocnemius (G), superficial digital flexor (SDF), and three other less significant tendons called the gracilis, semitendinosus, and the biceps femoris tendons (B). The gastrocnemius tendon is quite large and attaches onto the calcaneous bone. The superficial digital flexor widens over the calcaneous bone to form a cap that is attached to the bone on each side by fibrous tissue (called a retinaculum). As this tendon passes down the backside of the paw it splits into four separate tendons that attach onto each of the four toes (digits) of the hind limb.
Cause of tendon dislocations
If the retinaculum that keeps the superficial digital flexor tendon in place gets torn due to a traumatic injury, the tendon will pop off to the side of the calcaneous bone and cause lameness. In shetland sheepdogs and collies, the end of the calcaneous bone may be malformed, which predisposes the tendon to dislocate.
Signs and diagnosis
Most dogs that have this kind of injury are mature large-breeds. Lameness of a hind limb is the most common sign of a dislocating superficial digital flexor tendon. Sometimes the owner will hear a popping noise and intermittent lameness from the hind limb as the tendon dislocates. The diagnosis of a dislocating superficial digital flexor tendon is made with a physical examination, however, X-ray of the area may be used to rule out other problems. Prior to anesthesia and surgery, blood work is done in order to evaluate the health of the internal organs.
Surgical repair of the retinaculum of the tendon is recommended for a successful outcome. An incision is made over the site of the dislocating tendon and the torn retinaculum is sutured together. If the dislocating tendon is chronic, a fascia or collagen graft may be needed to complete the repair. The limb is supported in an extension cast or splint for a period of one month after surgery.
Surgical repair of a superficial digital flexor tendon luxation allows for the best outcome. Most patients respond very well to the treatment with resolution of lameness. Uncommon complications following surgery include infection, pressure sores from the cast and recurrence of the tendon dislocation.
For more information on this subject, speak to the veterinarian who is treating your pet.