Failure of the common calcanean tendon mechanism (Achilles injury) leads to an inability to weight bear through the hock extensor mechanism and a plantigrade stance. Dogs injure their common calcanean tendon in two ways: acute strain injury without external injury or laceration of the back of the leg by a sharp object. The former is more common in working dogs. Athletic breeds are capable of exceeding the elastic limit of the tendon, resulting in either tearing of the muscular-tendinous junction, avulsion from the calcanean tuber, or mid-substance rupture. A fourth injury, failure of the femorofabellar ligament (diagnosed by x-ray), is rare and may lead to less dramatic clinical signs.

Dogs may present acutely lame or suffer more chronic signs of tearing finally leading to failure. Incomplete rupture refers to rupture of gastroc/common tendons but an intact superficial digital flexor. Such dogs have digits which appear to curl due to a greater path length for the SDF. Complete injury, when the SDF is ruptured, more commonly results from laceration. Diagnosis is primarily by physical examination. Weight bearing can be induced by lifting the other hind leg and noting the plantigrade posture. Palpation of the tendon at its insertion may reveal swelling or a palpable defect indicating lesion location. Radiographs may show avulsion fragments or osteophytes in chronic cases. Ultrasound examination can be used to confirm the location and extent of tendon rupture and rule out musculotendinous rupture.

Surgical repair can restore function and should be recommended in working dogs. This is despite surgical intervention being controversial in humans. The digitigrade stance of dogs and our difficulty enforcing controlled rehabilitation, make rigid immobilisation a better option for our patients. Casting alone (in extension) should only be attempted in certain partial or chronic strain cases. Surgical management involves open reapposition of ruptured tendons and sustained immobilisation for 6 weeks to protect the repair. The surgical procedure has been well described. Controversy surrounds the desirability of tendon debridement. Mid tendon ruptures are reapposed with 3-loop-pulley or locking-loop sutures of 0 to 2 USP Prolene or Ethilon. Avulsions are reattached to the calcanean tuber via bone tunnels. The author prefers 2 tunnels for a locking-loop per tendon or a single larger tunnel for a 3-loop-pulley in chronic cases with one tendon mass. The 3-loop pulley has been shown to resist gap formation better than any other pattern. There are several options for post-op immobilisation including transarticular ESF, modified ring fixation with tension wires (olive) and calcaneotibial screws. I prefer a calcaneotibial screw (protected by a cast) in working dogs due to its simplicity and low cost with few revisits. The talocrural joint is locked in extension with the aid of self-locking bone reduction forceps in preparation for a calcaneotibial screw. This takes tension off the repair and indeed can be performed before reapposition of the tendon elements. The cast can be bivalved to allow dressing changes with minimal sedation and reuse of the halved cast. Alternatively the patient can be sedated for bivalving at 10–14 days. Inspect the incision site for complications at 10 days, then redress for a further 5 weeks with weekly checks. At 8 weeks, the screw is removed. A cranial or caudal splint is placed for 1–2 weeks. After bandage removal, the dog is slowly returned to light training over one month. Full work should not resume till three months after surgery. Ultimate return of good strength will require several more months.

Worth AJ, Danielsson F, Bray JP, Burbidge HM, Bruce WJ. Ability to work and owner satisfaction following surgical repair of common calcanean tendon injuries in working dogs in New Zealand. New Zealand Veterinary Journal. 2004;52(3):109–116.

Ten New Zealand Huntaway or Heading dogs (working Collies) with complete or partial tears of the common calcanean tendon, were treated by locking-loop suturing and casting, with (7) or without (3), a calcaneotibial screw. All dogs were actively in work on sheep or cattle farms at the time of injury, and return to work was the desired outcome. Ability to work and owner satisfaction were investigated by a telephone questionnaire at a mean follow-up interval of 14.6 months.

Results

Overall, 7 dogs returned to full or substantial levels of work. Postoperative complications occurred in 2 dogs that did not return to full or substantial levels of work. Moderate persistent lameness (3/5) was present in 2 of the 7 dogs that returned to full or substantial levels of work, equating to a 71% good to excellent functional outcome within this group. Seven owners felt the financial investment in opting for surgical repair was worthwhile. A screw and cast method of rigid immobilisation was superior (7/7 returned to work) to casting alone (0/3).

Carpal Hyperextension Injury

Severe carpal injury requiring PCA is a recognised problem in working dogs. Dogs working with stock and around machinery, often on challenging terrain, are susceptible to traumatic carpal injury. These carpal injuries can vary from ligamentous injury to complete dislocation or fracture and can be very debilitating, often resulting in degenerative OA, ongoing pain and lameness. Carpal arthrodesis is indicated when there is joint instability/luxation (and primary reconstruction is either not feasible or would have a low rate of success), intractable pain due to osteoarthrosis, or irreparable intraarticular fracture.

Categories of Carpal Hyperextension

Carpal hyperextension can be divided into three categories based on lesion location (see Fossum). Primary repair of the individual palmar ligaments is not feasible; arthrodesis is therefore the preferred technique. Accurate diagnosis is critical for planning appropriate intervention and requires careful palpation and assessment under anaesthesia. Stress radiography can be useful. Note the degree of carpal extension in the contralateral leg in the standing dog.

Category II and III lesions can theoretically be stabilised with partial carpal arthrodesis. However, this may lead to antebrachiocarpal joint OA as a result of increased stress placed on this joint, interference by implants or from misdiagnosed injury to, or subsequent breakdown of, the palmar-carpal ligaments/fibrocartilage. For these reasons, some authors recommend PCA as the initial surgical option for any carpal injury resulting from hyperextension. In my hands the results of partial carpal arthrodesis in working dogs are disappointing, and pancarpal arthrodesis is a more definitive procedure with good results.

Carpal arthrodesis plates (CAP, Veterinary Instrumentation, Sheffield, United Kingdom) are specifically designed for dorsal plating of the canine carpus, and overcome several problems associated with the standard DCP design. Carpal arthrodesis plates can be placed with minimal contouring as they provide for 5 degrees of extension by design. They take smaller screws distally for fixation of MC3 and have a tapered low-profile design to aid wound closure. A DCP suitably chosen for the patient's weight and size of radius will be oversized for the metacarpal bone. The diameter of the screw will then often exceed 30% of the third metacarpal's width, predisposing it to fracture, and necessitating very accurate screw placement. Fracture of a metacarpal bone following PCA is a known risk of PCA and in one study was related to the percentage of the 3rd metacarpal bone covered by the plate. The plate should cover at least 50% of MC3 to reduce the risk of metacarpal fracture. In a materials study, carpal arthrodesis plates failed at higher loads than DCPs, and all failures in both groups occurred at MC3. It is likely that the use of smaller screws reduces the likelihood of a MC3 stress riser. Recommended practice is to place a full cylinder cast for 6–8 weeks after PCA.

Tip - Using small-diameter cross pins in addition to the dorsal plate increases stiffness by 27% over plating alone. This can obviate the use of a cast. The author has used the carpal arthrodesis plate dorsally without a cast in pet animals with owners enforcing confinement.

Tip - Use a 1.1-mm drill bit to drill several small holes (forage) in the radius and radial carpal bone following cartilage removal. This increases blood flow to the arthrodesis site in the early phase of healing and hence speeds osteointegration.

A second custom plate, the "castless carpal arthrodesis plate," has been designed with angled distal screw holes that engage both the 3rd and 4th metacarpal bones. Application of these plates is somewhat more challenging and requires a specific sequence of screw placement. Iatrogenic fracture of the MCs is reported. The advantage of not using a cast should limit postoperative complications, and I have used them in a few working dog cases without complication.

Some authors advocate a circular ring fixator technique (CRF) to avoid complications relating to plate fixation. This technique offers the advantage of postoperative adjustment to ensure optimal alignment. In addition it allows complete removal of implants; however, it requires good hygiene and fixator care and frequent frame assessment (which the author deems impractical for rural clients living at a distance from a referral centre). I have also had a couple of nonunions using the CRF and believe that the small amount of axial micromotion seen with CRFs (normally advantageous for fracture healing) is disadvantageous for arthrodesis (compression not achieved). I reserve CRF for cases where there is an issue with preexisting infection or lack of soft tissue coverage (for instance, if the carpus was previously skin grafted).

Worth AJ, Bruce WJ. Long-term assessment of pancarpal arthrodesis performed on New Zealand working dogs. New Zealand Veterinary Journal. 2008;56(2):78–64.

Eight Heading dogs (working Collies) and four New Zealand Huntaways all actively in work on sheep or cattle farms at the time of injury. 6/12 (50%) of dogs could perform duties as before surgery. A further three (33%) dogs could perform most former duties. Overall, 83% of the dogs treated using PCA returned to full or substantial degrees of work.