This study investigates the hypothesis that the integrity of the cartilage hinge at the distal humeral epiphysis determines the stability of fractures of the lateral humeral condyle. Sixteen patients with lateral humeral condyle fractures were studied with radiographs and magnetic resonance imaging (MRI). The clinical course of each patient was compared using these imaging studies to determine whether initial fracture displacement and the integrity of the cartilage hinge correlated with fracture stability. Radiographically, 4 fractures were considered unstable (with initial fracture displacement >3 mm) and 12 were stable (initial displacement < or =3 mm). On MRI, 6 fractures were complete (with disruption of the lateral cartilage hinge) and 10 were incomplete. All unstable fractures had complete fractures on MRI. Ten of the 12 patients with radiographically stable injuries had incomplete fractures on MRI. None of these displaced during treatment. Two patients had radiographically stable fractures and complete fractures on MRI. One of these fractures displaced, confirming the hypothesis that the stability of lateral humeral condyle fractures is related to the integrity of the cartilage hinge.