A computer-aided decision system (CADS) based on ontology in pediatric orthopedics was developed to assess, without assumptions performed, the abnormalities of the musculoskeletal system of lower limbs. The CADS consists of four components. The first component is a diagnosis-based ontology, called Ontologie du Système Musculosquelettique des Membres Inférieurs (OSMMI). The second component is a database for collecting clinical observations, e.g., the birth classification of the clubfeet deformities. The third component uses statistical methods (principal component analysis and decision tree) for constructing an approach to evaluate new issues. The last component is an interactive module for managing the interaction between patients, experts, and the due CADS. Our system has been validated clinically with the real patient data obtained from the Infant Surgery Service in the hospital of Robert Debré in Paris. Our CADS is a good solution to compare the studies of the clubfeet deformities before and after the treatment using a universally scoring system. The assessment, conservative treatment, and monitoring were set up. Our system was developed to allow a better assessment for improving the knowledge and thus the evaluation and treatment of the musculoskeletal pathologies, e.g., the clubfeet deformities.