Nail-patella syndrome (NPS) is a rare hereditary patellar dysostosis characterized by nail hypoplasia or aplasia, aplastic or hypoplastic patellae, elbow dysplasia, and the presence of iliac horns as well as renal and ocular anomalies.

A flexion contracture is a bent (flexed) joint that cannot be straightened actively or passively. It is thus a chronic loss of joint motion due to structural changes in muscle, tendons, ligaments, or skin that prevents normal movement of joints.