The superficial digital flexor tendon forms the most superficial part of the calcaneal (Achilles) tendon and travels over the tuber calcanei as a flat tendon and then splits into branches for each toe. The SDFT and its muscle of origin act to flex the toes. Luxation is the medical term for dislocation. The tendon is held in place by soft tissue structures (retinacular attachments). If these structures are damaged, it can allow the SDFT to slip off the calcaneus (point of the hock), usually in a lateral direction or towards the outside of the leg.

What is a SDFT injury and how does it happen in dogs or cats?

SDFT luxation occurs when there is a rupture to the medial or lateral retinaculem. It is most commonly associated with vigorous activity and a rotational force applied to the calcaneous. The medial retinaculem is less fibrous and therefore more prone to tear, although the injury can occur on either side.

The injury is usually associated with vigorous activity and may be caused by a twisting force applied to the tendon’s insertion on the calcaneus. The medial (inside) retinacular attachments seem to be less well defined than the lateral insertion and may rupture more easily causing the SDFT to slip towards the outside. However, an abnormal shape of the end of the calcaneus bone may play a role in some cases of dislocation.

What are the signs that my dog has injured it’s superficial digital flexor tendon or SDFT?

Sudden onset of hind limb lameness is a common presentation. Intermittent severe lameness may be interposed with periods of mild lameness. An intermittent popping sound may be heard each time the tendon dislocates. Swelling over the point of the hock is common.

How is superficial digital flexor tendon (SDFT) injury diagnosed in dogs?

Dogs will usually present with an acute onset of a weight bearing hind limb lameness. There is swelling at the tarsus (ankle) most commonly at the proximal aspect of the calcaneus. With the tarsus in extension, the SDFT can be luxated to the side opposite the tear. Often there is a popping that can be palpated and/or heard when the tendon is luxated. Radiographic findings are soft tissue swelling at the tuber calcanei.

On examination by an experienced orthopaedic surgeon a popping sensation may be felt as the hock is flexed and extended. The tendon can sometimes be palpated in the luxated (dislocated) position and then reduced as the hock is extended. Flexion then results in reluxation. Radiographs (x-rays) may be recommended to rule out fractures or other problems with the bones. Often the only finding is swelling over the Achilles’s tendon.

What is the recommended treatment for a dog with superficial digital flexor tendon (SDFT) injury?

Surgical repair of the tear in the retinaculum is advised. An incision is made along the hock on the side opposite the direction of the luxation. The bursa is opened and fibrous tissue removed. The tendon is reduced to its normal position. Sutures are placed re-joining the tendon to its torn soft tissue attachments. If there is limited soft tissue for the tendon to be reattached to, it may be necessary to use screws or bone tunnels as another method of attachment. The SDFT is reduced and stabilized by using non absorbable monofilament interrupted sutures placed from the edge of the tendon to the adjoining retinacular tissue. If the injury is long standing or there is not enough tissue to imbricate, sutures can be placed through bone tunnels drilled into the calcaneus.

Post‐operative care / rehab for dogs after SDFT surgery

Following surgery, the lower limb may be supported with a lateral side splint with the tarsus in slight flexion for the next 6‐8 weeks. Strict exercise restrictions are required while the recovering dog or cat is in a splint and for two weeks after splint removal. Pain management is provided as indicated. Return to normal function can be routinely expected.