Association between pelvic muscle mass and canine hip dysplasia

Objective - To investigate the relationship between pelvic muscle mass and development and expression of canine hip dysplasia (CHD). Design - Prospective study. Animals - 5 Greyhounds with anatomically normal hip joints, 59 German Shepherd Dogs (23 with CHD, 24 with near-normal hip joints, and 12 with normal hip joints), and 18 German Shepherd Dog-Greyhound crossbreeds (7 with CHD, 6 with near-normal hip joints, and 5 with normal hip joints) between 12 and 47 months old in which pelvic muscle mass was evaluated. Pectineal muscle and hip joint development were evaluated in 25 German Shepherd Dogs at 8 and 16 or 24 weeks of age. Procedures - For evaluation of pelvic muscle mass, individual pelvic muscles were weighed and hip joints were assigned a score on the basis of severity of degenerative changes. For evaluation of pectineal muscle development, muscle sections were stained and examined. Results - Pelvic muscle mass was greatest in Greyhounds, intermediate in crossbred dogs, and smallest in German Shepherd Dogs. Differences in pelvic muscle mass among breeds were attributable to differences in weights of individual muscles. Hip score was negatively correlated with pelvic muscle mass and weights of selected pelvic muscles. Dogs with pectineal hypotrophy at 8 weeks of age had type-2 muscle fiber paucity or muscle fiber-type grouping at 16 or 24 weeks of age. At 8 weeks of age, hip joints were composed of multiple centers of ossification, and the acetabulum was largely cartilaginous. By 24 weeks of age, the pelvic bones were largely, although incompletely, fused. Clinical Implications - Diminished pelvic muscle mass in dogs with CHD and altered muscle fiber size and composition in 8-week-old dogs that subsequently develop CHD strongly suggest that abnormalities of pelvic musculature are associated with development of CHD. The complex development of the hip joint from multiple centers of ossification may make the joint susceptible to abnormal modeling forces that would result from abnormalities in pelvic muscle mass.