Fig. 1

Brooker classification of heterotopic ossification in the hip. a Class 0 – no heterotopic ossification is noted about the hip; b class 1 – small islands of soft tissue ossifications are noted about the hip (arrows); c class 2- a moderate spur (arrow) arising from the right acetabulum, leaving an osseous gap of greater than 1 cm measured from the greater trochanter; d class 3 – a very large spur (arrow) arising from the greater trochanter of the right proximal femur approaching the acetabulum, leaving an osseous gap of less than 1 cm; e class 4 – arrows show large spurs arising from the acetabulum and also greater trochanter of the right proximal femur, resulting in ankylosis