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Brooker classification of heterotopic ossification9
| Class 1 | Islands of bone within the soft tissues over the hip |
| Class2 | Bone spurs from the pelvis or proximal end of the femur, leaving at least one centimeter between opposing bone surfaces. |
| Bone spurs from the pelvis or proximal end of the femur, reducing | |
| Class 3 | the space between opposing bone surfaces to less than one centimeter. |
| Class 4 | Apparent bone ankylosis of the hip |
Della Valle classification of heterotopic ossification49
| Class 1 | Absence of HO or islands measuring <1 cm in length |
| Class 2 | Islands >1 cm or spurs leaving at least 1 cm between femur and pelvis |
| Class 3 | Spurs leaving <1 cm between opposing surfaces or bony ankylosis |
Schmidt and Hackenbroch classification of heterotopic ossification50
| Region 1 | Heterotopic ossifications strictly below tip of greater trochanter |
| Region 2 | Heterotopic ossifications below and above tip of greater trochanter |
| Region 3 | Heterotopic ossifications strictly above tip of greater trochanter |
| Grade A | Single or multiple heterotopic ossifications < 10 mm in maximal extent without contact with pelvis or femur |
| Grade B | Heterotopic ossifications > 10 mm without contact with pelvis but with possible contact with femur; no bridging from femur to proximal part of greater trochanter, with no evidence of ankylosis |
| Grade C | Ankylosis by means of firm bridging from femur to pelvis |