What is the Achilles tendon apparatus?

The Achilles tendon apparatus is the bundle of tendons than run down the back of the hind limb to the point of the hock (tuber calcanei). It is composed of three tendons:

1. The gastrocnemius tendon, which extends from the gastrocnemius (calf) muscle (originating from the end of the back of the femur) to the point of the hock (tuber calcanei) 
2. The combined tendon of the biceps femoris, gracilis, and semitendinosus muscles (coursing down the posterior (caudal) portion of the thigh), which attaches to the point of the hock (tuber calcanei)
3. The superficial digital flexor tendon, which extends from the superficial digital flexor muscle (originating from the end of the back of the femur) over the point of the hock (tuber calcanei) and down to the foot where it branches before attaching to each toe

The gastrocnemius tendon, with minor contributions from the common tendon of the biceps femoris, gracilis, and semitendinosus, functions to extend the hock, while the superficial digital flexor tendon flexes the toes. 

What injuries can occur to the Achilles tendon apparatus?

Injuries to the Achilles tendon apparatus can be classified into three major types:

1. Type 1: Complete transection of all tendon components.  
2. Type 2a: Partial rupture of the gastrocnemius tendon only
3. Type 2b: Complete rupture of the gastrocnemius tendon only, in which the paratenon (connective tissue sheath surrounding the tendon) remains intact
4. Type 2c: Tearing of the gastrocnemius tendon and the combined tendon of the biceps femoris, gracilis, and semitendinosus muscles away from their insertion point on the hock
5. Type 3: Inflammation (enthesitis) and micro tears at the site of attachment of the gastrocnemius and biceps femoris, gracilis, and semitendinosus tendons on the point of the hock

Type one injuries are usually due to a laceration across the back of the hind leg. Type two and three injures may be due to trauma, chronic strain, or degeneration. Chronic strain is most frequently identified in highly active sporting and herding dogs, while degeneration is encountered primarily in older large breed dogs, particularly the Labrador, Doberman, and German Shepherd. The degenerative process underlying these injuries is not fully understood, and it is believed that factors such as genetics, the use of specific medications (fluoroquinolone antibiotics and steroids), obesity, diabetes mellitus, and Cushing’s disease may contribute to its occurrence. 

How are Achilles tendon apparatus injuries diagnosed?

Achilles tendon injuries often manifest with either a non-weight bearing or partially-weight bearing lameness and a plantigrade stance, in which the hock sinks closer to the ground than normal, and is sometimes even in contact with the ground. The toes may appear to be hyper-flexed, giving the appearance of walking on their toes. With type 3 injuries, the lameness may be chronic and intermittent, while with the other types of injuries, signs tend to develop suddenly and either remain constant or get worse over time. 

On physical exam, a swelling or thickening is often palpable at the site of the injury. X-rays may reveal a fractured point of the hock (usually in type 2c injuries in dogs under two years old) or tiny mineral deposits within the tendon (called dystrophic mineralization) resulting from chronic inflammation. Ultrasound can be used to directly view the damaged tendon, thus confirming the diagnosis. 

How are Achilles tendon apparatus injuries treated?

Both complete and partial tendon ruptures are best treated with surgery. At surgery, scar tissue is removed, and the torn ends of the tendon(s) are sutured back together using very strong suture placed in a special configuration to maximize stability. If the tendon has torn directly off the bone at the point of the hock, small tunnels are drilled through the heel bone to enable the placement of these sutures. In some cases, placement of a connective tissue graft or synthetic mesh may be necessary between the two ends if a gap is required to restore proper tendon length.

Stem cell therapy and/or platelet rich plasma therapy may help to expedite healing in tendon injuries, and either or both of these treatments may be applied at the time of surgery. Stem cell therapy utilizes the patient’s own stem cells (collected from a sample of bone marrow or fat), applied at the site of injury, to replace damaged cells and promote the growth of healthy new tissue. Platelet rich plasma, which is isolated from a sample of the patient’s blood, is also applied directly to the site of injury, and provides growth factors and anti-inflammatory compounds that aid in healing. 

The hock joint must be held rigidly in extension for 6-8 weeks while the tendon heals. There are several ways to achieve this, and which is used depends primarily on surgeon preference. At surgery, a bone screw can be placed through the back of the heel bone and into the bottom of the tibia (shin bone) while the hock is held in an extended position. Alternatively, an external fixator can be placed on the limb by driving surgical pins perpendicular to the long axis of the limb through the tarsal bones and bottom third of the tibia, and rigidly securing these pins to external rods to immobilize the joint. A third option is to place a cast on the limb after surgery. 

What is the aftercare following surgical repair of these injuries?

Most patients are able to go home the day after surgery. Depending on the method used to immobilize the hock during the post-operative period, they may go home with a cast, splint, or bandaged external fixator on their limb. It is essential that any bandaging or external device on the limb remain clean and dry. The best way to achieve this is to apply a waterproof plastic sleeve or bootie over the limb whenever the patient goes outside. Strict activity restriction is also extremely important to the healing process, as any stabilization method can fail if subjected to strong repetitive forces. Patients should be confined to a large crate or small room, with only brief controlled leash walks outside to use the bathroom. Absolutely no running, jumping, or playing with other pets should be permitted during the post-operative period. Weekly rechecks and bandage changes are performed to ensure there is no destabilization and to detect and treat any skin irritation or bandages sores that may develop. 

After 6-8 weeks, the rigid stabilization is removed. If the patient has an external fixator or a calcaneo-tibial screw, they will need to be briefly placed under anesthesia for surgical removal. If a cast was used, no anesthesia is necessary for removal. At this point, a splint is applied, which provides support while allowing a small amount of movement. After 1-2 weeks, this is replaced with a soft padded bandage, which provides only a small amount of support, and allows for a further increase in mobility of the hock. After 1-2 weeks, this soft bandage is removed and physical rehabilitation therapy begins. 

A therapy program consisting of once or twice weekly appointments with a physical rehabilitation technician and daily at-home exercises is designed for each patient with input from the surgeon. Exercises are geared toward rebuilding muscle mass, restoring normal joint mobility, and further strengthening the healing tendon. Activities such as walking in the underwater treadmill, active and passive stretches, and balance and strength exercises will be gradually intensified as the patient progresses. Class IV laser therapy is incorporated to combat inflammation and support ongoing healing. At home, progressively longer leash walks, along with specific exercises designed to improve strength, flexibility, and balance will be prescribed. The duration of physical rehabilitation therapy for a specific patient depends on their rate of progress, and may last from four to twelve weeks. Most patients can be expected to return to full activity within four to six months of surgery. 

What are the potential risks and complications of this surgery?

The three main complications that can be encountered following tendon repair are infection of the surgical site, bandage-related sores and infections, and failure of the tendon repair. Surgical site infections and bandage-related complications are treated with appropriate antibiotics and topical dressings. Failure of the tendon repair, while uncommon with appropriate post-operative immobilization, is a serious complication that requires additional surgery to address. 

What is the prognosis following surgery?

The prognosis for all types of Achilles apparatus injuries is good. While some sporting and herding dogs regain the ability to perform at their pre-injury level, many will experience a mild to moderate decrease in peak performance levels. Patients who do not engage in competitive athletic pursuits typically return to their pre-injury ability to comfortably walk, run, jump, and play.