Hind limb sprains and strains (Proceedings)

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Hind limb sprains and strains (Proceedings)

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Apr 01, 2010

Gracilis and semitendinosus myopathy/contracture

Muscle contracture is defined as, 'The abnormal shortening of muscle tissue, rendering the muscle highly resistant to stretching. A contracture can lead to permanent disability. There is a significant difference between muscle contraction and muscle contracture. Contraction refers to the normal physiological process of muscle shortening, resulting in work. Contracture refers to the abnormal pathologic process resulting in fibrosis and permanent damage to a muscle. It is important to remember that a muscle can have a significant amount of scar tissue from prior injury and still not be considered a contracture based on its ability to function. The pathological process of muscle contracture is characterized by replacement of most or all of the entire muscle and/or associated tendon with fibrous connective tissue. This process can take weeks to months and leads to shortening of the affected muscle or muscle group. A muscle contracture can also severely affect the associated joints by fixing them in flexion or extension. In general, muscle contractures of the forelimb respond better to treatment and carry a better prognosis than muscle contractures of the hind limb.

Canine muscle contracture is reported to affect several different muscles, is associated with a number of predisposing factors, and a varying prognosis depending upon which muscle is affected. Most patients suffer some form of trauma weeks to months before the contracture is present. The clinical signs include: lameness, pain, weakness, decreased range of motion, a firmness noted throughout the entire muscle, and usually a characteristic gait. Pre-disposing factors for muscle contracture include: compartment syndrome, infection, trauma, repetitive strains, fractures, infectious diseases, immune-mediated diseases, neoplasia, and ischemia. Contractures of the gracilis and semitendinosus may be concurrent or found individually. History associated with a contracture of the gracilis and/or semitendinosus typically consists of a single or repeated strain injury with a gait abnormality developing afterwards. Gait abnormalities usually start abruptly and progress over a period of six weeks to months, at which time it becomes static. With gracilis contracture, during locomotion, the affected leg is raised in a jerky fashion with the hock hyperflexed and rotated laterally, the metatarsus is rotated medially, and the overall stride is shortened. A shortened stride with a rapid medial rotation of the paw, external rotation of the hock and internal rotation of the stifle mid stride is noted with contracture of the semitendinosus. There does appear to be some breed and age predilection, however, the sex of the animal does not have an appreciable influence. Gracilis muscle and semitendinosus contracture most often affect highly active German Shepherd Dogs and Shepherd related breeds between the ages of three and seven years. The suspected cause in the working and performing German shepherd is repetitive strain injury leading to secondary contracture.

Anatomy

The gracilis in the dog forms an extensive broad muscular sheet that is found in the superficial layer of the caudal portion of the inner surface of the thigh. The muscle arises from the pelvic symphysis and ends along the entire length of the cranial border of the tibia. An aponeurosis also spreads out into the crural fascia and from its caudal border sends a well developed reinforcing band to the calcanean tendon and attach to the tuber calcanei. The gracilis is responsible for adduction of the thigh, extension of the hip and extension of the hock.

The semitendinosus arises from the tuber ischiadicum and inserts distally on the medial surface of the tibia; it also inserts by means of a fascial attachment on the tuber calcanei. It is responsible for extension of the hip and tarsal joints and flexion of the stifle joint in the free non-weight bearing limb.

Cause

Skeletal muscle tissue cells have a very limited ability for regeneration, and differ somewhat from cardiac muscle cells which are classified as nondividing cells that cannot undergo further mitotic division in the postnatal life, and differing greatly from smooth muscle cells which are classified as quiescent cells that have a moderate capacity for regeneration and can be driven into the G1 mitotic phase of regeneration after damage. Repair to damaged muscle tissue begins early in the inflammatory cascade and involves two processes: regeneration of injured tissue by parenchymal cells of the same type, and replacement by connective tissue (fibroplasia). The balance of the two dictates how well new skeletal muscle tissue is produced and how much scar tissue is formed. It appears to be this limitation for skeletal muscle cell regeneration that drives the formation of scar tissue and inevitable fibrosis after skeletal muscle trauma instead of regenerating new cells and replacing the damaged.

Fibrotic myopathy or muscular contracture is a chronic, progressive disorder of severe muscle contracture and fibrosis. The exact cause is usually unknown. The fibrotic myopathy may result from acute trauma, chronic repetitive trauma, autoimmune disease, drugs reactions, infections, neurogenic disorders and vascular abnormalities. Ischemia secondary to indirect trauma may also lead to fibrosis and contracture. Histologically, muscle is replaced by dense, collagenous connective tissue. In humans, indirect muscle injuries occur subsequent to rapid acceleration during athletic activities. Muscle strains are caused by excessive force or stress on the muscle that induces tearing of muscle fibers or, most often, tearing of the musculotendinous junction. The type and severity of injury determines whether the muscle heals predominately by regeneration of functional myofibrils or by scar formation. Severe damage to a muscle is followed by fibrosis and contracture, with minimal regeneration. 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 and relaxation, resulting in varying degrees of mechanical lameness.

Muscle injuries in dogs may be underestimated because of the failure to establish a definitive diagnosis, poor recognition of muscular damage when accompanied by concurrent, more severe trauma, and problems with the classification of muscular trauma. Muscle injuries are said to account for only 5% of reported musculoskeletal disease. Lameness is usually more intense in the acute phase and improves with time. When injured muscle undergoes fibrous contraction, a mechanical lameness may remain.

Diagnosis

Presentation of gracilis and/or semitendinosus contracture is very unique and consistent and diagnosis can be effectively made with a thorough history, observation of gait and physical examination.

Most contractures have a history of acute injury or lameness weeks to months before the onset of contracture. The lameness and initial swelling usually subsides with supportive treatment, however, clinical signs of the contracture present a few weeks later. Dogs typically have a hind-limb gait abnormality characterized by a shortened stride with a rapid, elastic medial rotation of the paw, internal rotation of the hock and external rotation of the calcaneus [corrected] and internal rotation of the stifle during the mid-to-late swing phase of the stride.

Physical exam may be normal, except for a firm mass within the gracilis and/or semitendinosus muscle(s). Typically, a taut, firm band is palpable in the caudo-medial aspect of the thigh.

A complete chemistry panel and CBC are usually within normal limits, however, creatinine phosphokinase (CK) may be elevated. Radiographs of the hind limb are typically normal, however an increased radiopacity may be observed between muscle and the lower tendon junction in some cases. Ultrasound may be a useful diagnostic tool and may reveal a slightly echogenic muscle. MRI may also be useful for diagnosing this condition in both the acute and chronic phases. Histopathology of the contracted muscle typically shows replacement of degenerating myofibers with connective tissue. The findings are consistent with primary myopathy due to polyphasic muscular damage (degeneration, segmental necrosis, phagocitosis, and fibrosis).

Treatment

Medical management prior to or in lieu of surgery was attempted in a previous report with no apparent response. Fifteen dogs had one or multiple surgical procedures. Although transection, partial excision, or complete resection of the affected muscle resulted in resolution of lameness following surgery, lameness recurred six weeks to five months (mean, 2.5 months; median, two months) following surgery. Adjunctive medical treatment did not prevent recurrence. Variable replacement of the affected muscle with fibrous connective tissue (predominantly along the caudolateral border of the muscle) was evident grossly, and replacement of myofibers with fibrous connective tissue was confirmed histologically. A definitive etiology could not be established. Myectomy of the entire gracilis muscle still has a poor prognosis because the gait abnormality will still return within three to five months due to semitendinosus involvement.

Rehabilitation therapy is currently the treatment of choice. Rehabilitation therapy may consist of continuous ultrasound therapy, manual therapy and a home therapeutic exercise program including massage and stretching. While rehabilitation therapy may significantly improve the gait abnormality and function of the hind limb in chronic cases it rarely leads to complete resolution of clinical signs. Working Shepherds with this condition may continue to remain on active duty, however a continued rehabilitation and home maintenance program is crucial.

Education for all German Shepherd and Shepherd related breed owners and handlers is an important component to prevention. Appropriate warm-ups, active stretching followed by cool-downs and passive stretching and massage should be performed whenever possible before working and events.

Conclusion

Performance dogs can have a wide range of sport-related conditions and injuries. A sound knowledge base of structure and biomechanics and of the sport involved aids in successful rehabilitative management. In addition, rehabilitative needs may change as the patient progresses through therapy and returns to activity, therefore, appropriate and successful rehabilitative management requires constant reevaluation of the patient and modification of the rehabilitation therapy program to address those changing needs.