Canine hip dysplasia: Differences in pectineal muscles of healthy and dysplastic German Shepherd Dogs when two months old

Abstract

SUMMARY One pectineal muscle was removed from each of 23 purebred German Shepherd Dogs when 2 months old. Transverse frozen sections of each muscle were prepared and stained for the histochemical demonstration of myofibrillar adenosine triphosphatase activity which permitted differentiation of the constituent myofibers into type I, type II, and type intermediate myofibers. The mean size and total number of myofibers per section, as well as the mean size and total number of each myofiber type per section, were determined. From those measurements, the absolute and relative myofiber and nonmyofiber compositions of each section were estimated. When the dogs were 24 months old, their hip joints were evaluated and graded for the presence of hip dysplasia by radiographic and pathoanatomic examinations. Subsequently, the pectineal muscle measurements were statistically analyzed: (i) to determine whether they were correlated with the status of the coxofemoral joints when the dogs were 24 months old and (ii) to determine whether differences existed between the muscles of dogs which developed normal hips and those which developed hip dysplasia. The pectineal muscles of dogs which developed hip dysplasia had significantly smaller type I and type II myofibers than did the pectineal muscles of dogs which developed normal hips. The pectineal muscles of dysplastic dogs were composed of a smaller myofiber component and a larger nonmyofiber component. These measurements also were correlated with the graded status of the coxofemoral joints. The total number of myofibers and the number of each myofiber type per section did not differ significantly. Critical value analyses indicated that the determination of the myofiber and nonmyofiber compositions of pectineal muscles in 2-month-old dogs may be potentially useful in predicting the subsequent development of normal or dysplastic hips. Results of these observations indicate that the composition of pectineal muscles in young dogs predisposed to hip dysplasia is an important associative or causal factor related to the development of hip dysplasia in the dog.