Brooker grading:
Class I: Islands of bones
Class II: more than 1 cm between two bone surfaces
Class III: less than 1 cm between two bone surfaces
Class IV: Bony ankylosis
References:
Brooker AF et al. Ectopic ossification following total hip replacement. Incidence and method of a classification. J Bone Joint Surg Am 55: 1629, 1973
