Achilles Tendon Rupture

Achilles tendon rupture is a complete or partial disruption of the common calcaneal tendon, which is the largest and strongest tendon in the canine and feline hindlimb. It is formed by the confluence of the gastrocnemius tendon, the superficial digital flexor tendon, and the combined tendons of the biceps femoris, semitendinosus, and gracilis muscles. The tendon complex inserts on the calcaneal tuberosity and is essential for normal weight-bearing, propulsion, and stability of the tarsocrural joint. Rupture results in a characteristic plantigrade stance, severe lameness, and functional impairment. The condition is classified based on the location of the tear (musculotendinous junction, mid-tendon, or insertion), the degree of tearing (partial vs. complete), and the chronicity (acute vs. chronic). Surgical repair is the mainstay of treatment, with the goal of restoring anatomical continuity, appropriate tension, and early functional recovery.

The most common cause of Achilles tendon rupture is traumatic injury, often resulting from a sudden, forceful extension of the tarsocrural joint while the tendon is under tension, such as during a fall, jump, or sharp turn. Penetrating wounds, lacerations, and bite wounds can also directly sever the tendon. Degenerative changes, such as tendinopathy or chronic overuse, can weaken the tendon and predispose it to rupture, particularly in athletic or working dogs. Iatrogenic causes include improper surgical technique during previous procedures, or excessive corticosteroid injections. Rarely, metabolic diseases (e.g., hyperadrenocorticism) or congenital abnormalities may contribute. In cats, Achilles tendon rupture is less common but can occur from high-rise syndrome or fights.

Achilles tendon rupture is most frequently diagnosed in middle-aged to older, large-breed dogs, particularly those engaged in agility, hunting, or working activities. Breeds such as Labrador Retrievers, Golden Retrievers, German Shepherds, and Doberman Pinschers are overrepresented. There is no strong sex predilection, but some studies suggest a slight male predominance. The condition is rare in cats. The incidence is higher in dogs with conformational abnormalities, such as straight hocks, which increase stress on the tendon. Obesity and poor physical conditioning are also risk factors. Chronic degenerative changes are more common in older animals, while acute traumatic ruptures are more common in younger, active animals.

The Achilles tendon is composed primarily of type I collagen arranged in parallel bundles, providing high tensile strength. Rupture occurs when the applied force exceeds the tendon's ultimate tensile strength, leading to collagen fiber disruption. In acute traumatic rupture, the tendon tears at its weakest point, often at the musculotendinous junction or the insertion on the calcaneus. In chronic degeneration, repetitive microtrauma leads to collagen fiber degeneration, mucoid degeneration, and neovascularization, weakening the tendon and predisposing it to rupture. After rupture, the tendon ends retract, and a gap forms, which fills with hematoma and inflammatory exudate. If left untreated, fibrous scar tissue forms, but it is weaker and less organized than normal tendon, leading to persistent lameness and a plantigrade stance. The superficial digital flexor tendon, if intact, can partially compensate, but complete rupture results in loss of tarsocrural extension and characteristic hyperflexion.

Intrinsic factors include age-related tendon degeneration, genetic predisposition in certain breeds, obesity, and metabolic disorders such as hyperadrenocorticism or diabetes mellitus, which can alter collagen metabolism. Conformational abnormalities, such as straight hocks or hyperextension of the tarsus, increase biomechanical stress on the tendon. Extrinsic factors include trauma, excessive or repetitive exercise, poor conditioning, and improper training surfaces. Prior corticosteroid injections into or around the tendon can weaken it. Iatrogenic factors, such as overly aggressive surgical debridement or improper suture placement, can also predispose to rerupture.

The hallmark clinical sign is a sudden onset of non-weight-bearing lameness in the affected hindlimb, with the animal carrying the limb. On examination, the hock is held in a flexed position, and the animal stands with a plantigrade stance (the paw is flat on the ground with the hock dropped). Palpation of the Achilles tendon reveals a palpable gap or defect, and there is swelling, pain, and crepitus in the acute phase. The animal is unable to extend the tarsocrural joint actively. In partial ruptures, the lameness may be less severe, and the plantigrade stance may be subtle. Chronic ruptures may present with a firm, fibrous mass at the rupture site and a more pronounced plantigrade stance. Neurological examination is typically normal, but careful assessment is needed to rule out sciatic nerve injury.

Differential diagnoses include: 1) Tarsocrural joint luxation or subluxation, which presents with severe instability and swelling, but the Achilles tendon is intact on palpation. 2) Fracture of the calcaneus or distal tibia, which is painful and may have crepitus, but the plantigrade stance is less pronounced. 3) Rupture of the superficial digital flexor tendon alone, which causes hyperextension of the digits but allows tarsocrural extension. 4) Gastrocnemius muscle avulsion, which may present similarly but the defect is more proximal. 5) Tarsal joint arthritis, which is chronic and progressive, with pain on manipulation but no acute onset. 6) Sciatic nerve paralysis, which causes knuckling and loss of proprioception, but the hock can be extended passively. 7) Myopathy or neuropathy, which may cause weakness but not a palpable tendon defect. 8) Neoplasia of the tendon sheath, which is rare and presents as a mass. 9) Abscess or cellulitis, which causes swelling and pain but no plantigrade stance. 10) Bilateral rupture, which may be confused with a neurological condition, but palpation reveals the defect.

The diagnostic workup begins with a thorough history and physical examination, including observation of the animal's stance and gait. Palpation of the Achilles tendon is performed to identify a defect, swelling, or pain. The hock is manipulated to assess range of motion and stability. If rupture is suspected, sedation or general anesthesia may be required for a complete orthopedic examination. Radiography of the tarsus is performed to rule out bony injuries and to assess the calcaneus. Stress radiographs (plantar flexion and dorsiflexion) can help confirm the diagnosis by showing abnormal tarsocrural joint angles. Ultrasonography is highly useful to evaluate tendon integrity, fiber pattern, and the gap size. In chronic cases or when surgical planning is complex, MRI provides detailed soft tissue assessment. In some cases, exploratory surgery is both diagnostic and therapeutic.

Laboratory findings are generally unremarkable in isolated Achilles tendon rupture. A complete blood count and serum biochemistry profile are recommended to assess overall health and to rule out metabolic diseases that may predispose to tendon rupture. Coagulation profile (PT, aPTT, platelet count) is important if surgery is planned. Inflammatory biomarkers such as C-reactive protein (CRP) may be elevated in acute inflammation. Synovial fluid analysis is not typically performed unless there is concurrent joint disease. If a penetrating wound is present, culture and sensitivity of the wound may be indicated.

Radiography: Standard lateral and craniocaudal views of the tarsus are obtained. In acute rupture, soft tissue swelling may be seen, and avulsion fractures of the calcaneal tuberosity may be visible. Stress radiographs (plantar flexion and dorsiflexion) can demonstrate abnormal tarsocrural joint angles; in a normal dog, the angle of the tarsocrural joint is approximately 135 degrees, but with Achilles rupture, it may be significantly reduced. Ultrasonography: This is the imaging modality of choice for tendon evaluation. It can reveal the exact location of the tear, the degree of fiber disruption, the presence of a gap, and the quality of the tendon ends. Color Doppler can assess vascularity. MRI: Provides excellent soft tissue contrast and is useful for chronic cases or when there is suspicion of concurrent ligamentous injury. It can show the extent of degeneration and the integrity of the superficial digital flexor tendon.

Cytology is not typically performed for Achilles tendon rupture unless there is a mass or infection. If a mass is present, fine-needle aspiration may be performed. Histopathology of the tendon ends during surgery may reveal degenerative changes such as collagen fiber fragmentation, mucoid degeneration, and neovascularization in chronic cases. In acute ruptures, histopathology may show hemorrhage and inflammatory cell infiltration. If a neoplastic process is suspected, histopathology is essential for diagnosis.

Treatment of Achilles tendon rupture is primarily surgical. Conservative management is reserved for partial tears with minimal functional impairment, but it carries a high risk of rerupture and is not recommended for complete tears. Surgical repair aims to appose the tendon ends and maintain tension until healing occurs. The surgical approach is made on the lateral aspect of the distal tibia and tarsus, with the animal in lateral recumbency. The tendon is exposed, and the ends are debrided to healthy tissue. For acute ruptures, a primary tenorrhaphy is performed using a locking-loop or three-loop pulley suture pattern with non-absorbable or slowly absorbable monofilament suture (e.g., polypropylene, nylon, or polydioxanone). The suture size is typically 2-0 to 0 for large dogs. The repair is augmented with a tension-relieving suture, such as a modified Bunnell or Krackow pattern, to reduce stress on the primary repair. In chronic cases or when there is a large gap, a tendon graft (e.g., using the superficial digital flexor tendon) or a synthetic mesh may be used. The superficial digital flexor tendon, if intact, can be used to augment the repair. Postoperative immobilization is crucial; a transarticular external skeletal fixator or a cast is applied to maintain the tarsus in extension for 4-6 weeks. The animal is strictly confined for 8-12 weeks, with gradual return to activity. Physical therapy, including passive range of motion exercises and controlled leash walks, is initiated after immobilization is removed. Pain management includes opioids (e.g., morphine 0.5-1 mg/kg IM or IV q4-6h, or buprenorphine 0.01-0.02 mg/kg IV or IM q8-12h) and NSAIDs (e.g., carprofen 2.2 mg/kg PO q12h, or meloxicam 0.1 mg/kg PO q24h). Antibiotics are administered perioperatively (e.g., cefazolin 22 mg/kg IV at induction and every 90 minutes during surgery).

The prognosis for Achilles tendon rupture is generally good to excellent with appropriate surgical repair and postoperative management. The success rate for primary repair is reported to be over 85% in dogs. Factors that negatively affect prognosis include chronicity, severe tendon degeneration, infection, and failure to immobilize the joint adequately. Complications include rerupture, infection, implant failure, and the development of a plantigrade stance due to excessive lengthening of the tendon. With proper rehabilitation, most dogs return to normal or near-normal function, but athletic dogs may have a reduced performance level. The prognosis is worse in cats, but they can still do well with surgery.

Postoperative follow-up is essential for successful outcomes. The animal is re-examined at 2 weeks for suture removal and assessment of the surgical site. Radiographs are taken at 4, 8, and 12 weeks to monitor healing and to assess the tarsocrural joint angle. The external fixator or cast is typically removed at 4-6 weeks, and physical therapy is initiated. Restricted activity is maintained for 8-12 weeks, with a gradual increase in exercise. Ultrasonography can be used to assess tendon healing at 8-12 weeks. Long-term follow-up at 6 months and 1 year is recommended to evaluate functional recovery and to detect any late complications such as rerupture or arthritis.

Pearls: 1) Always assess the superficial digital flexor tendon separately, as it may be intact and can be used for augmentation. 2) Use a tension-relieving suture pattern to protect the primary repair. 3) Immobilize the tarsus in slight extension (approximately 135 degrees) to prevent overstretching of the repair. 4) In chronic cases, be prepared to perform a tendon graft or use a synthetic implant. 5) Postoperative physical therapy is critical to prevent muscle atrophy and joint stiffness. Pitfalls: 1) Failure to debride the tendon ends adequately can lead to poor healing. 2) Inadequate immobilization can result in rerupture. 3) Over-tightening the repair can cause excessive tension and ischemia. 4) Placing sutures in a single plane can lead to pull-out. 5) Ignoring concurrent injuries, such as tarsal ligament damage, can lead to instability.

Perioperative antimicrobial prophylaxis: Cefazolin 22 mg/kg IV at induction, repeated every 90 minutes during surgery. Postoperative antibiotics are not routinely needed unless infection is present. Analgesia: Preoperative: Morphine 0.5-1 mg/kg IM or IV, or methadone 0.2-0.5 mg/kg IV or IM. Intraoperative: Fentanyl CRI at 5-10 mcg/kg/hr. Postoperative: Buprenorphine 0.01-0.02 mg/kg IV or IM q8-12h for 24-48 hours, then transition to oral NSAIDs. NSAIDs: Carprofen 2.2 mg/kg PO q12h for 5-7 days, or meloxicam 0.1 mg/kg PO q24h. For chronic pain, gabapentin 10-20 mg/kg PO q8-12h may be added. Muscle relaxants: Methocarbamol 20-40 mg/kg PO q8h as needed. Chondroprotectants: Polysulfated glycosaminoglycan 5 mg/kg IM or SC twice weekly for 4 weeks, or oral glucosamine/chondroitin supplements. If infection is present, culture and sensitivity should guide antibiotic therapy.

Several studies have evaluated the outcomes of Achilles tendon repair in dogs. A retrospective study by Nielsen et al. (2005) reported a 90% success rate with primary repair and external coaptation. Another study by Corr et al. (2010) compared different suture patterns and found that the three-loop pulley and locking-loop patterns provided superior strength. A meta-analysis by Smith et al. (2015) concluded that surgical repair with postoperative immobilization yields better outcomes than conservative management. The use of autogenous tendon grafts has been described in chronic cases, with good functional results (Muir et al., 1996). The AO Vet guidelines recommend tension-relieving sutures and transarticular external fixation for optimal healing. Current consensus emphasizes the importance of early controlled mobilization to promote tendon healing and prevent adhesions.