Common calcaneal tendon injury

What it is

The common calcaneal tendon (Achilles mechanism) can be damaged in two main ways:

• Acute traumatic injury: laceration by glass, metal or other sharp objects, or rupture from a sudden overload. Any or all of its components may be cut.
• Chronic degenerative tendinopathy: progressive degeneration and partial rupture of the gastrocnemius tendon near its insertion, without a specific traumatic event, with fibrous thickening of the tendon.

A related condition is luxation of the superficial digital flexor tendon off the calcaneal tuber, usually laterally, when the retinaculum that holds it tears; it is described mainly in Shetland sheepdogs and collies.

Who it affects

Lacerations occur in dogs of any age and breed. Chronic tendinopathy affects mainly middle-aged, medium to large breeds; the Dobermann and Labrador retriever are overrepresented, and overweight dogs are at risk.

Anatomy involved

The common calcaneal tendon (tendo calcaneus communis) inserts on the calcaneal tuber of the tarsus and has three components:

• The tendon of the gastrocnemius, the main extensor of the hock.
• The tendon of the superficial digital flexor, which runs over the calcaneal tuber, where it widens into a cap held by medial and lateral retinacula, and continues distally to the digits.
• The combined tendon formed by contributions of the biceps femoris, semitendinosus and gracilis.

Because the superficial digital flexor continues to the toes, an injury of the gastrocnemius with an intact superficial flexor makes the digits curl as the hock drops (the "crab claw" posture).

Clinical signs

• Complete rupture: plantigrade stance, with the hock and metatarsus touching the ground when the dog bears weight.
• Partial or chronic injury: partial dropping of the hock with flexed digits, mild lameness and a firm swelling of the tendon just above the calcaneus.
• Lacerations are often found as a wound on the caudal aspect of the leg.

Diagnosis

• Palpation of the tendon with the stifle extended and the hock flexed: gaps, thickening, and the posture of the digits.
• Radiographs: soft-tissue swelling, mineralisation within the tendon and enthesophytes on the calcaneal tuber; avulsion fragments.
• Ultrasound or MRI to define which components are affected and the extent of degeneration.

Treatment

Most injuries are treated surgically: debridement of degenerated tissue and suture of the tendon ends with a tendon-holding pattern (such as a locking-loop or three-loop pulley), or reattachment to the calcaneus through bone tunnels. The repair must be protected by keeping the hock in extension for several weeks, with a calcaneotibial screw, a transarticular external fixator or a splint, followed by gradual, controlled return to activity. A luxated superficial digital flexor tendon is replaced and the torn retinaculum repaired. With proper repair and immobilisation, most dogs regain good function.