Anatomy: The Achilles tendon also known as the calcanean tendon consists of all structures that attach to the tuber calcanei of the fibular tarsal bone. The tendon consists of three independent portions. The bulk of the tendon is made-up of the common tendon of the gastrocnemius muscles and inserts onto the proximal end of the tuber calcanei. The tendon of the superficial digital flexor muscle is the next largest part. Although at its proximal end it lies deep to the gastrocnemius, at about the level of the mid-tibia the tendon winds medially around the gastrocnemius tendon to lie on its caudal surface. On the tuber calcanei it broadens to form a cap, which is attached to the bone by way of collateral ligaments (retinacular). At this level it also unites with the crural fascia and the tendon of the biceps femoris, semitendinosus and gracilis muscles. Separating the tendon from the underlying gastrocnemius tendon there is an extensive calcaneal bursa.
The third portion of the calcaneal tendon is made up of a fusion of tendons from the semitendinosus, biceps femoris and gracilis muscles and inserts on the medial aspect of the tuber calcanei.
Classification: Achilles tendon injuries are common and they have been the subject of renewed interest because of the increased incidence of chronic injuries diagnosed in large breeds of dogs, particularly Dobermans.
Meutstege (1991) classified Achilles tendon injuries. In Type 1 injuries there is complete disruption of the entire tendon usually as the result of trauma. The outcome is maximal flexion of the hock when the stifle is extended. The separated tendon ends are palpable through the skin. In some cases a skin laceration may be evident, indicative of external trauma. Type 2 injuries show a variable degree of increased hock flexion with stifle extension and are the result of only partial disruption of the tendon. Type 2 injuries can be further sub-classified into Type 2a, where there is incomplete separation between the gastrocnemius muscle and the tendon. Type 2b where there is total disruption of the tendon, but the paratenon is still intact. Type 2c where the tendons of the gastrocnemius muscle and the semitendinosus, biceps femoris and gracilis muscles are torn from the attachment to the tuber calcanei, leaving the superficial flexor tendon component in tact.
Type 2c injuries result in a characteristic hyperflexion of the toes during weight bearing. The increased amount of hock flexion caused by the disrupted gastrocnemius tendon results in increased in tension within the superficial flexor tendon.
Type 3: these injuries may represent an earlier stage of type 2c. The distal end of the gastrocnemius tendon is thickened, as is the fibular tarsal bone. These changes may represent small tears in the tendon and enthesitis. Radiographically there maybe evidence of soft tissue swelling, with calcification or bone fragmentation and changes to the size and shape of the tuber calcanei. It is the type 2c and type 3 injuries that are recognise commonly in Dobermans.
A type of Achilles tendon injury not covered in this classification is associated with luxation of the superficial digital flexor tendon. The tendon usually luxated laterally and is associated a characteristic lameness. There appears to be an increased incidence in Shetland sheep dogs and collies.
Principles of treating tendon injuries: Damaged tendons heal as one wound by the invasion of fibroblasts and blood vessels from the surrounding paratenon. In a sheathed tendon the resultant scar tissue may result in adhesions between the tendon and the sheath which will limit the normal gliding action of the tendon. Primary wound healing of the tendon within the sheath is possible by collagenization, but this process relies on the preservation of the delicate local circulation and the absence of damage to the tendon sheath. In practical terms this situation is very unlikely.
Following tendon trauma collagen synthesis starts on about the third day and by day 14 it has increased dramatically. The vascular reaction peaks at about the 14-28 day period. Secondary remodeling of the initially random orientation of the collagen fibres starts at about day 21. This very importatant process goes on for months to increase the strength of the repair and to decrease the mass of the scar. Secondary remodeling is aided by stress or loading which increases the cross-linking between the collagen fibres and ensures that they orientated parallel with the lines of force. Clinically the strength of a repaired tendon relies on three facets:
1. Correct choice of suture pattern and material,
2. Adequate immobilization for sufficient time,
3. Controlled period of rehabilitation.
It is recommended that the suture material of choice should be strong and inert. Stainless steel has been widely used in the past, but it has a tendency to fatigue failure and is probably best avoided. Monofilament nylon, polypropylene,PDS or Maxon are the materials of choice. The gauge is selected according to the size of the tendon to be repaired, but normally a heavy gauge is preferable e.g., between 2/0 and 2. A suture pattern should be selected that minimizes ischaemia of the tendon, resists gapping and will not pull through the parallel collagen fibres. The locking-loop or Kessler suture has been shown to be superior to the traditional Bunnell woven-style pattern. To achieve the desired affect with the locking-loop suture it is important to ensure that the transverse bites are made superficial to the longitudinal bites to prevent the suture material pulling through the tissue.
A near-far type of suture pattern (pulley-suture) has been demonstrated to provide better resistance to separation, but it is more difficult to insert, especially in small tendons.
A 6-8 week period of complete immobilization in extension is recommended to protect the repair from separation during the critical period of collagen synthesis.
This period of immobilization should then be followed by a period of controlled exercise designed to gradually increase the load on the repair to assist secondary remodeling. Research has confirmed the value of early mobilization versus prolonged immobilization. For example a standardized repair immobilized for 5 weeks was many hundreds of grams weaker than the repair immobilized for 3 weeks and then allowed freedom of movement for 3 weeks. This research may have more relevance to the management of non-weight bearing tendon repairs.
Treatment of type 2c Achilles tendon injuries: This consists of reattaching the ruptured end of the gastrocnemius tendon to the fibular tarsal bone using tendon sutures placed through parallel drill holes in the bone. In long-standing cases it maybe more appropriate to resect a portion of the thickened tendon and then to perform an end to end anastomoses. Immobilization for 6-8 weeks is important and is achieved by using a cast, Robert Jones (RJ) bandage, Type 2 external fixation device or a transosseous positional screw between the fibular tarsal bone and the distal tibia.
In my hands I prefer to use the fixation screw and to insert the screw first before repairing the tendon. I believe that this gives me adequate immobilization (when combined with a RJ bandage postoperatively) and it also allows me to work on the tendon repair in a stable environment. The patient is prepared for surgery and positioned in ventral recumbency with the affected leg extended backwards and supported on a sandbag. A caudal approached is employed. The tendon of the superficial digital flexor tendon is identified and its medial collateral cut to allow lateral displacement of the tendon. This gives good access to the caudal surface of the calcaneus. The fixation screw is placed so that the threads engage in all cortices. It is not used as a compression or lag screw. The drill hole for the screw is placed just below the tuber calcanei and at right angles to the long axis of the bone. Care is taken to ensure that the drill hole engages both cortices of the tibia. The leg must be kept fully extended and absolutely still during this phase of the operation.
The tendon injury is then explored. The anatomy can be quite confusing due to the presence of abundant scar tissue. Bleeding is also usually quite brisk. The correct tendon length is restored by debridement of some the scar tissue. It doesn't appear to be necessary or desirable to resect back to normal tendon. Fine drill holes are then placed across the tuber calcanei for attachment of the tendon sutures.
The gastrocnemius tendon repair is then repaired using a series of tendon sutures. The superficial digital tendon is then replaced and secured and the wound closed routinely. The screw fixation is further supported with a RJ bandage or a half-cast. After regular rechecks and assuming the repair feels secure, the fixation screw is removed after 6-8 weeks and once again the limb is supported with a RJ bandage which will, as it becomes soft and more pliable, allow progressively more load to be taken on the repair. The bandage is usually removed after 2 weeks. The results I have achieved using this protocol have been good. In the early days I had some failures due to inappropriate screw placement and I have had one breakdown which was attributed to poor owner compliance. It has also been shown that in the Doberman with the chronic Type 2c and Type 3 injuries the rate of collagen remodeling is slow and longer periods of immobilization may be required.
I have had limited experience in treating these case by immobilization alone. It is my understanding that success can be achieved solely by temporary screw fixation, especially in the Type 3 cases.
Type 1 injuries should be treated by immediate exploration of the wound and if possible primary repair of the various elements of the Achilles tendon with individual locking-loop sutures. If the wound is very contaminated it maybe preferable to tag the ends of the tendon with sutures and then treat the wound primarily. Anastomoses should then be conducted as soon as the wound is healthy. The postoperative treatment is the same as described for type 2c injuries. Care should be taken in performing the screw fixation technique because in these acute injuries as the fibular tarsal bone is quite thin and using a large screw can weaken the bone and lead to secondary fractures. This is less likely to occur with type 2c injuries because the bone is much thicker.
Superficial digital tendon luxation is repaired by placing a series of simple interrupted nonabsorbable sutures in the torn collateral (opposite side to the direction of the luxation). Postoperative immobilization for a month is also important.
REFERENCES
1. Meutstege,F.J.(1991).Tendon lesions of the hind limb in Small Animals: Treatment and Aftercare. Abstracted in Vet.Comp.Orth.Trauma.4,153.
2. Reinke,J.(1997).. In Current Techniques in Small Animal Surgery Fourth Edition ed.M.J.Bojrab. pp 1255-1260 Lea and Febiger, Philadelphia
3. Brinker,W, Piemattei,D and Flo,G (1997) Handbook of Small Animal Orthopaedics and Fracture Repair. Third edition. WB Saunders, Philadelphia.