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A rare congenital limb malformation characterized by permanent and manually irreducible lateral dislocation of the kneecap. It typically presents with flexion contracture of the knee, genu valgus, absent or dysplastic trochlear groove of the femur, external rotation of the tibia, and dysfunction of the extensor mechanism. The defect may be unilateral or bilateral and can occur as an isolated malformation, be associated with other malformations of the lower limb, or be part of a polymalformative syndrome.