Objective: To identify morphometric characteristics of the pelvic limb musculature associated with the development of cranial cruciate ligament (CCL) deficiency in Labrador Retrievers. Study Design: Cross-sectional clinical study. Animals: Pure-bred female (n=18) and male (n=12) Labrador Retrievers with (n=16) and without (n=14) CCL deficiency. Methods: Muscle conformation of pelvic limbs was evaluated by physical examination, radiography (widths of quadriceps, hamstring, and gastrocnemius were expressed relative to tibial length and to each other), and dual-energy X-ray absorptiometry (DEXA, lean contents of quadriceps, hamstring, and gastrocnemius were expressed relative to tibial length and to each other). Pelvic limbs of dogs without CCL deficiency were classified as normal (n=28 limbs), whereas those with CCL deficiency were considered diseased (n=18 limbs) or sound contralateral to CCL deficiency (n=10 limbs). Variables were compared between groups using mixed models analysis of variance, with P<05 considered significant. Results: The ratios of quadriceps width to tibial length (P=008), hamstring width (P=013), and gastrocnemius width (P=005) on lateral radiographs were lower in diseased limbs than controls. The mass of hamstring muscles in CCL deficient limbs was similar to that of normal limbs. The ratio of the lean content of gastrocnemius to hamstring muscles was greater in diseased (P=007) and sound contralateral (P=013) limbs than in normal limbs. Conclusions: Atrophy associated with CCL deficiency may predominantly affect the quadriceps muscle. Dominance of the gastrocnemius muscle over active restraints to the cranial tibial thrust may be associated with predisposition to CCL deficiency in Labrador Retrievers. Clinical Relevance: If confirmed, this dynamic imbalance between muscle groups of the rear limbs could serve as a basis for screening programs and preventive rehabilitation.
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