Reliability and validity of the grading of heterotopic ossification

The purpose of this study is to assess the reliability and validity of the Brooker grading of heterotopic ossification. Anteroposterior hip radiographs of 77 patients, taken 6 months following total hip arthroplasty, were blindly evaluated twice by two surgeons using the Brooker scale. Patients were also evaluated with the Harris hip scale, and range of motion of the hip was determined. All inconsistencies in heterotopic ossification grading were reviewed by the same two surgeons who developed six points of clarification to minimize ambiguities in the Brooker grading criteria. A second set of 76 radiographs were reviewed by the two surgeons using this modified Brooker index. The intraobserver reliability of the two readers was good, with agreements of 77 and 86% and weighted kappa values of 0.63 and 0.69, respectively. The two surgeons demonstrated fair interobserver reliability on the first reading (agreement of 68% and weighted kappa of 0.57) that improved to a good level of reliability (agreement of 77% and weighted kappa of 0.68) using the revised Brooker index. The grade of heterotopic ossification was correlated (r = -.25, P = .005) with the aggregate range of hip motion, but had no relationship to the patient's Harris rating. In conclusion, the grading of heterotopic ossification is reliable. Although the severity of heterotopic ossification did not correlate with the Harris scale, the relationship between heterotopic ossification and range of motion indicates that the Brooker index is also a valid measurement.