Fibrotic myopathy of the gracilis and semitendinosus

What it is

Fibrotic myopathy is a chronic disorder in which muscle fibres of the caudal thigh muscles, most often the gracilis and less often the semitendinosus, are replaced by dense collagenous tissue. The fibrotic muscle cannot stretch, so it restricts the movement of the hip, stifle and hock during the swing phase of the stride. The cause is unknown; repetitive strain or trauma in active dogs, immune-mediated mechanisms and neurogenic causes have been proposed.

Who it affects

Mainly German shepherds and German shepherd crosses, usually active or working adult males; it has been reported in other breeds such as the Belgian shepherd, Rottweiler and boxer.

Anatomy involved

The gracilis (m. gracilis) arises from the pelvic symphysis by a broad aponeurosis, covers the caudomedial surface of the thigh and inserts on the cranial border of the tibia through the crural fascia, with a contribution to the common calcaneal tendon. The semitendinosus (m. semitendinosus) runs from the ischiatic tuberosity to the medial surface of the tibia and the calcaneal tuber. Both extend the hip, flex the stifle and help extend the hock. When they become fibrous, a firm band can be palpated on the caudomedial aspect of the thigh, most obvious with the hip and stifle extended.

Clinical signs

A characteristic gait: a shortened stride of the pelvic limb with, at the end of the swing phase, a rapid, elastic medial rotation of the paw, external rotation of the hock and internal rotation of the stifle. The condition is usually not painful and progresses slowly. It is often unilateral but may become bilateral.

Diagnosis

• Gait observation and palpation of a taut, non-painful band in the gracilis or semitendinosus.
• Ultrasound or MRI to show the fibrotic segment; biopsy confirms the replacement of muscle by connective tissue.
• Rule out other causes of pelvic limb lameness, especially in German shepherds (hip dysplasia, degenerative lumbosacral stenosis, degenerative myelopathy).

Treatment

Surgical excision or release of the fibrotic band improves the gait at first, but the gait abnormality usually recurs within weeks to a few months, so surgery is generally not recommended. Physiotherapy and stretching have been tried with limited effect. Because the condition is not painful, many dogs live with it and continue moderate activity; the long-term outlook for normal gait is poor.