Randall B. Fitch, DVM, MS; Ronald D. Montgomery, DVM, MS; Michael H. Jaffe, DVM

Injuries and disorders of muscle are a diagnostic challenge requiring careful, thorough orthopedic examination. Traumatic muscle injury includes contusion, strain, contractures, myositis ossificans and compartment syndrome (pressure injury). Wider recognition and treatment of these conditions will benefit many of these patients.

Reference: Fitch RB, Montgomery RD. Jaffe MH. Muscle Injuries in Dogs. The Compendium on Continuing Education. 19(8): 947-958, 1997

Key Points

Muscle strain results in disruption of muscle fibers which most frequently occurs near the muscle-tendon junction.

Muscle heals by a combination of regeneration and fibrous scar tissue formation.

Muscle contracture results in irreversible degenerative changes to muscle including atrophy, fibrosis, and adhesion.

Myositis ossificans results in mineralization within muscle producing discomfort and lameness.

Compartment syndrome is pressure-induced injury to muscle resulting from swelling or hemorrhage within a confined osteofascial space.

Introduction

Mechanisms of Muscle Injury and Repair

• The major mechanisms of muscle injury include: contusion, laceration, rupture, strain, compartment syndrome, ischemia, and denervation.

• Muscle heals by a combination of regeneration and scar formation. The type and severity of injury determines whether the muscle heals predominately by regeneration of functional myofibrils or by scar formation. Although fibrous scar tissue provides tensile strength and plays a part in normal muscle healing, excessive scar tissue impedes muscle fiber regeneration and interferes with muscle contraction. Healing by scar tissue may decrease muscle’s functional ability to produce tension by 50%.

• Muscle heals by the same mechanisms as all tissues, undergoing stages of inflammation, debridement, repair and remodeling. The extracellular matrix that surrounds and organizes muscle fibers, also protects and organizes healing muscle by providing a scaffolding for regenerating myofibers, and the building blocks (mucopolysaccharides, proteoglycans and collagen) required for repair. The orientation and structure maintained by the extracellular matrix is essential for muscle contraction.

• Mobility across the healing muscle should not begin until the remodeling stage of healing is reached. Premature mobilization of muscle following injury or surgery increases granulation and scar tissue production, resulting in poor penetration of regenerating muscle fibers and possible disruption of the repair. However, prolonged immobilization results in irregular orientation of muscle fibers, decreased tensile strength, and excessive scar contraction. Controlled motion and stress during later stages of healing are required to promote functional parallel orientation of regenerating muscle.

• For optimal function, complete immobilization for the first 3 weeks following repair or injury followed by controlled mobilization (restricted activity with cage confinement or soft support bandage, controlled leash walks and passive range of motion) for an additional 3-6 weeks.

• Occurs from trauma.
• Palpable discomfort, swelling, and elevated creatine phosphokinase activity are frequent findings in dogs presented with traumatic injuries.
• Produces hemorrhage, edema, inflammation and pain, and in severe cases, results in necrosis and hematoma formation. Substantial muscle recovery occurs during the first week, inflammation and hematoma significantly regress and muscle regeneration is initiated through infiltration by granulation tissue and new myotubules.
• The healing process continues over several weeks, but is rarely complete. Irregularly oriented muscle fibers and scar tissue usually persist in previously injured areas of muscle.
• Rest and immobilization are believed facilitate healing.

• Involves both muscle stretch and muscle activation; most often occurring during eccentric contraction.
• Muscle is viscoelastic and therefore undergoes mechanical creep, in which muscle elongates with repeated cycles of stretching. These viscoelastic properties are influenced by temperature. Preconditioning muscle through cyclic stretching and increasing muscle temperature, decreases muscle stiffness and the likelihood of strain injury.
• Occurs usually during athletic activities from over-stretch and overuse, there is disruption of muscle fibers, most commonly near the muscle-tendon junction. These injuries are characterized by initial inflammation, followed by healing with marked fibrosis.

• In humans, muscle strains are the most frequent injury in sports and they have been divided into four grades. Grade I muscle strains (the least severe) involve tearing of a few muscle fibers with pain, and local spasm. Higher grades indicate increased structural damage, pain and hemorrhage, with grade IV involving complete muscle rupture.
• Strength of muscle contraction is greatly affected by strain injury. Experimentally, muscle strained to 80% of failure produced minimal histologic muscle fiber rupture and hemorrhage. However, biomechanically, muscle contraction decreased by 30% immediately following injury, by 50% after 24 hours, recovered to 25% deficient after 48 hours and was 90% of its original strength after one week. Although muscle contractile abilities are substantially affected, recovery appears to be rapid. Some muscle fibers regenerate, but normal histology is not restored and scar tissue persists.

• Computed tomography of human athletes reveals that inflammation and edema play a large role in muscle strains. Muscle disruption and minor hemorrhage are present immediately following injury. Inflammation becomes pronounced in the days following and by one week the inflammation is replaced by fibrous tissue. Although the decremental effects on tensile strength and contractile ability recover, healing of muscle by fibrous scar tissue predisposes it to reinjury and possibly to muscle contracture.

• Dogs
• Mature, athletic dogs are most commonly affected and present with varying degrees of lameness, focal swelling, subcutaneous hemorrhage, palpable muscle displacement, and pain.

• Dogs with iliopsoas muscle strain injury present with pelvic limb lameness that can be confused with hip dysplasia. However, careful evaluation of these dogs reveal pain in the iliopsoas muscle with no associated coxofemoral pain, crepitance or radiographic evidence of hip dysplasia. Iliopsoas muscle pain can be elicited by stressing the iliopsoas muscle with simultaneous extension of the hip and internal rotation affected limb. Sonographic confirmation of this injury is possible and revealed hyperechoic and hypoechoic regions in the affected muscle of 16 dogs reported.

• Muscles at particular risk to strain injury include muscles which cross two or more joints and thus are subjected to stretching across multiple joints. These muscles include the biceps brachii, long head of the triceps, flexor carpi ulnaris, gracilis, sartorius, tensor fascia lata, rectus femoris and muscles associated with the Achilles mechanism.

• Injuries can occur in different regions of a muscle complex. Although the musculotendinous junction is most commonly injured, injuries also occur at the origin and at the tendinous insertion. Spontaneous muscle ruptures have also been reported secondary to corticosteroid injections and parasitic migration. Confirmation of muscle strains can be difficult, but may be aided by ultrasound examination, magnetic resonance imaging or surgical exploration in more severe cases.

• Treatment of muscle strain injury has varied considerably in dogs and humans. Most treatment regimes are empirically adapted from clinical practice. Few studies have been performed to evaluate the effectiveness of different forms of treatment. Reported treatment modalities include ice packs during the first 24 hours post-trauma, warm compresses after 24 hours, compressive wraps, anti-inflammatory medications, analgesic medications, muscle relaxants, rest, controlled physical therapy and surgery. Although acute, minor injuries appear to respond well to non-aggressive treatment, chronic and severe muscle injuries often do not. Severe muscle strains may require a more aggressive approach involving surgical debridement, repair or tenomyectomy. Surgical repair of acute gracilis muscle ruptures using multiple near-far-far-near absorbable sutures in (23) racing greyhounds, resulted in a greater than seventy-five percent successful return to racing.

• Contracture of muscle is believed to occur secondary to injury resulting in fibrosis and functional shortening of the muscle.
• Muscle is replaced by a non-compliant, non-functional fibrous tissue that restricts normal motion often adhering to the adjacent joint and interferes with normal limb action. The most common muscle contractures in the dog include the infraspinatus, quadriceps, gracilis, supraspinatus and semitendinosus muscles

Infraspinatus contracture

• Occurs in medium and large, active, middle-aged dogs.

• Lameness rapidly develops with progressive contracture of the infraspinatus muscle resulting in tethering of normal shoulder motion producing an unusual limb action characterized by circumduction of the leg. The forelimb is externally rotated with notable loss in shoulder extension and palpable atrophy of the infraspinatus.

• Treatment of choice is infraspinatus tenotomy, which releases the infraspinatus tendon pull and restores a nearly normal gait.

• Occurs primarily in actively growing dogs less than months-of-age following fractures of the distal femur with voluntary or enforced immobilization of the limb.

• Quadriceps contracture results from prolonged hyperextension of the stifle resulting in development of adhesions between the vastus intermedius muscle and the distal femur. Stifle joint immobilization results in contracture of the joint capsule and periarticular structures with fatty infiltration and adhesions in the joint space. Degenerative joint disease, including cartilage atrophy, fibrillation, and erosion progresses. Finally, fibrous ankylosis of the stifle joint develops with disuse osteoporosis and muscle atrophy in the limb. Initially, many of these changes are reversible, but become permanent after several weeks. Early mobilization may reverse many of the articular and periarticular changes and restore some stifle flexion, but patient and client compliance with prescribed physical therapy is often less than ideal. The prognosis for advanced quadriceps contracture is poor even with treatment and commonly results in a non-functional leg.
• 
• Treatment should be directed toward prophylaxis and early recognition of this condition. Early implementation of rigid fracture fixation and encouragement of stifle joint and muscle motion provide the best chances for preservation of good joint function. Prophylactically maintaining joint flexion with application of a "90-90" flexion splint for 3-5 days following surgery effectively reduced incidence of quadriceps contracture in dogs with femoral fractures (good stifle function preserved in 22 of 26 dogs). The "90-90" splint is not effective in treating existing quadriceps contractures. Surgical treatment of existing quadriceps contracture has only limited success and is directed at restoring stifle function by freeing the adhesion between the quadriceps muscle and the femur, lengthening the quadriceps mechanism, and returning the stifle to a normal standing angle. More recently, surgery in conjunction with a dynamic flexion apparatus to aid in passive and active flexion of the stifle has been used successfully for correction of advanced quadriceps contracture in one dog.

• Reported mostly in active, middle-aged German Shepherd type dogs, but other breeds have been reported.

• Affected dogs are able to maintain normal activity and exercise freely, but have a characteristic "jerky" gait. The gait consisted of a shortened stride with external rotation of the hock, medial rotation of the foot and internal rotation of the stifle during the swing phase of the stride.

• Dogs present with acute gait changes which progressively worsened over several weeks and then remained unchanged. The gracilis muscle is markedly firm, taut and distinct on physical examination. Pain may be elicited with direct pressure on the affected muscle in some cases.

• Surgery initially alleviates the lameness, but recurrence occurred several months following surgery in all reported (19) cases. Surgeries including myotomy (transection), full-thickness complete and segmental myectomy or myotendinectomy (resection), some incorporating a fat graft.

• Medical treatments with prednisolone and azathioprine had temporary limited success as did combination of surgery with medical therapy. Both conservatively and surgically treated dogs remained active and healthy with no progression of the disorder although the abnormal gait remained unchanged. The underlying etiology is unknown, but it has been speculated to be due to repetitive stress-producing strain injuries in these very active dogs.

• Reported in the German Shepherd dog resulting in restrictive fibrotic bands in the muscle producing a characteristic gait abnormality with hyperflexion of the stifle joints. These fibrotic bands are distinctly palpable within the semitendinosus muscle and limits stifle extension on physical examination.

• Surgical resection of these fibrous bands produces only temporary relief with recurrence usually within six months.

Heterotopic bone formation in muscle. The localized form has been described in several locations in the dog including muscles of the caudal hip region, shoulder, quadriceps and the cervical musculature.

Large, middle-aged, active dogs are most commonly affected. This condition is believed to occur secondary to trauma. Doberman pinschers with von Willebrand’s factor deficiencies were over-represented in early reports, although recent reports have described other breeds of dogs.

Mineralization does not always produce lameness or palpable discomfort. Bilateral mineralization was present radiographically in nearly half of the cases presented with unilateral lameness for dogs with myositis ossificans of the supraspinatus muscle. Lameness is believed to result from mechanical interference caused by the mineralized mass, therefore the size and location of the mineralization plays an important role in the severity of lameness.

Superficial mineralization of the supraspinatus muscle appears to cause little discomfort or lameness. In contrast, mineralization deep in the supraspinatus muscle caused greater lameness, which is believed due to mechanical interference with the biceps tendon. Biceps tenosynovitis has also been reported in association with myositis ossificans, therefore thorough evaluation of the biceps tendon is essential and may affect treatment and outcome.

Surgically debulking the mineralized mass is the preferred treatment since this lameness is not responsive to systemic antiinflammatory drugs or local steroid injections. Surgically debulking the mass resolves the discomfort and lameness in most cases. Complete excision of the mineralized mass is not necessarily required for resolution of lameness, although recurrence and continued lameness are a risk with incomplete excision. Both tendon splitting and tendon elevation (retraction) methods for approaching the mineralized mass in the supraspinatus muscle have been described, complete excision can be provided with either approach, but the tendon elevation approach may be less traumatic. Post-operative radiographs can assist in the evaluation of the completeness of the mass removal.

Compartment Syndrome

• Results when interstitial pressure is elevated in an anatomically confined osteofascial compartment. Elevated interstitial pressure impairs microvascular perfusion and produces neuromuscular injury with pain, swelling, and eventual necrosis of muscle.

• Although a frequent problem in humans, only a few reports of compartment syndrome exist in dogs. Three general mechanisms exist for increasing the compartmental pressure: (1) hemorrhage or injection into the compartment, (2) post-ischemic tissue swelling within the compartment, and (3) external pressure from a bandage or cast. Clinical signs of compartment syndrome are non-resolving swelling, pain and tense muscle.

• Four osteofascial compartments (muscle regions anatomically confined by fascia and bone) which are at potential risk to this injury have been described in dog extremities: the craniolateral crus, caudal crus, caudal antebrachium and quadriceps (cranial femur).

• Reported cases in the dog include: the antebrachium following a gunshot wound with a suspected penetrating injury to the median artery, the femoral compartment resulting from a tight surgical fascial closure in a fracture repair which resolved after releasing the involved suture line, the femoral compartment acutely following a comminuted femoral fracture injury, and the caudal crus compartment several weeks following a bite wound which produced a vascular injury and developed a pseudoaneurysm.

• Compartment syndrome can be diagnosed by measuring compartmental pressure using a pressure catheter (Wick catheter, Slit catheter, transducer type catheter, and hypodermic needle connected to a three-way stopcock and manometer). Normal intrafascial pressure is -2 to +8 mm Hg. Pressures exceeding 30 mm Hg are clinically significant and result in necrosis of skeletal muscle if the duration is greater than 8 hours.

• Alternatively, diagnosis of compartment syndrome can be done using contrast arteriography. Fasciotomy can be used to surgical decompression and relieve compartmental pressure. The actual prevalence of this condition in veterinary medicine is unknown since compartment syndrome is not widely recognized and the equipment required for diagnosis is not commonplace in veterinary practices

The ability to detect and diagnose muscle injury requires awareness of these injuries and high reliance on clinical orthopedic examination. Palpation of specific muscles combined with stressing the muscle through flexion and extension of the associated joints should improve the frequency of accurate diagnosis.

Although bilateral conditions may exist, many are unilateral allowing the veterinarian to compare normal position, size, and conformation with the unaffected side.

Palpable discomfort appears to be the most common clinical finding in muscle injuries, but swelling, muscle disruption and subcutaneous hemorrhage may be present. Confirmation of the diagnosis via radiography is limited in many of these conditions, but essential to rule-out osseous disorders.

Myopathies, which are characterized by generalized muscle changes (weakness, exercise intolerance, and gait changes), can be quite localized and confused with muscle injury.

Ultrasound, nerve and muscle biopsy, and electrodiagnostics are useful ancillary diagnostic tools.

Appropriate identification and treatment of muscle injuries can reduce scar tissue formation and help restore normal function to the muscle, with less potential for re-injury or chronic disability.