Gastrocnemius muscle avulsion
The gastrocnemius muscle originates in two heads, one on each supracondylar tuberosity, and each contains one sesamoid, called
a fabella. The muscle tendon inserts on the calcaneal bone to extend the tarsus and flex the stifle.6
Avulsion of the gastrocnemius muscle at its head results in an acute nonweightbearing lameness, while partial avulsion in
which some of the origin of the muscle remains attached to the femur results in partial weightbearing. The hock may be hyperflexed
in some dogs when they try to bear weight on the limb because the gastrocnemius muscle functions to flex the stifle.52,53 On radiographs of the stifle, one or both of the fabellae may be distally displaced.
Conservative management of partial avulsions with therapeutic ultrasound and rest can resolve clinical signs of lameness in
some dogs.54 Surgical reattachment of the muscle head to the supracondylar femur and restriction of stifle extension for three weeks
postoperatively are recommended in athletes. The lameness will resolve, but dogs rarely return to their previous performance
level.24
Superficial digital flexor tendon luxation
Collies and Shetland sheepdogs are becoming increasingly popular in flyball and agility sports and are more prone to lateral
luxation of the superficial digital flexor tendon than are other breeds.55 The superficial digital flexor arises from the lateral supracondylar tuberosity of the femur and the lateral fabella, crosses
the stifle joint and lies proximally cranial to the gastrocnemius, and then passes medially to the caudal aspect of the gastrocnemius
tendon as it courses distally. The superficial digital flexor tendon continues distally after inserting on the tuber calcanei
to insert on the proximal caudal border of the second phalange.6
The luxation occurs at the level of the calcaneal bone where the retinaculum ruptures. The dog will exhibit pain and lameness
in the hindlimb and may have its toes elevated off the floor when standing on the affected limb. The diagnosis is made by
palpation of the area where the tendon can be replaced and luxated again, similar to a luxating patella.55
Treatment involves surgical repair of the retinaculum by suturing it over the tendon by using simple interrupted or cruciate
sutures with absorbable monofilament suture material. The tendon itself is not sutured so that it may continue to glide over
the calcaneal bone and function to flex the digits and extend the tarsus. After surgical repair, rehabilitation and a gradual
return to activity are recommended.4
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