Effect of heterotopic ossification on hip range of motion and clinical outcome.

The utility of heterotopic ossification (HO) classification systems is debatable. The range of motion and Harris hip score (HHS) were calculated in 104 patients with known HO after total hip arthroplasty and 208 matched controls without HO. The patients with HO were radiographically divided into high and low grade HO groups. There was no statistically significant association of HHS with high or low grade HO. High grade HO had a statistically significant 6° loss of terminal hip flexion, 4° loss of abduction, and 6° loss of internal rotation at the hip. The small changes in terminal hip range of motion and lack of association with HHS may be the result of false radiographic continuity resulting in an overestimation of the disability in high grade HO.