UCSD Musculoskeletal Radiology
Brooker classification of heterotopic bone
Class I has islands of bone within the soft tissues.
Class II has bone spurs from the pelvis or proximal end of the femur, leaving at least 1 cm between opposing surfaces.
Class III has bone spurs from the pelvis and/or proximal end of the femur, reducing the space between opposing bone surfaces to less than 1 cm.
Class IV shows apparent bone ankylosis of the hip.