Injury to the origin of the gastrocnemius muscle as a possible cause of lameness in four horses.

Four horses were evaluated because of hind limb lameness. Two had a distinct gait abnormality at the walk characterized by lateral rotation of the point of the calcaneus and medial rotation of the toe during the stance phase. Nuclear scintigraphy revealed a focal area of intense radioisotope uptake in the caudodistal aspect of the femur in all 4 horses, and 3 of the horses had a corresponding proliferative reaction on the caudodistal cortex of the femur on radiographs. In all 4 horses, a diagnosis of injury to the origin of the gastrocnemius muscle was made. Three of the horses returned to athletic endeavors after an extended period of rest and a gradual return to exercise. The fourth improved after 5 months of rest but became lame again when returned to exercise and was euthanatized.