This is a rare but important paediatric condition where the capital femoral epiphysis slips through the hypertrophic zone of the physis leading to the appearance of the femoral head having "slipped" on x-ray. Capital femoral epiphysis to slip inferiorly and posteriorly. Patients who get this condition have and inherent weakness through the hypertrophic zone often accentuated by rapid growth or underlying endocrine conditions (hypothyroid, klienfieters). As the position of the proximal physis changes from horizontal to oblique during preadolescence and adolescence the forces through the hip are changed from compression to shear forces. These changes in force angle and the abnormally wide growth plate lead to the physeal slip.
It is more common in boys with a peak age around 13 years old. The left hip is more commonly affected than the right.
N.B. The inability to abduct the hip in a child is a very sensitive test.