Intraobserver reliability and interobserver agreement in radiographic classification of heterotopic ossification

The most widely used radiologic classification system for heterotopic ossification after total hip arthroplasty (THA) is the Brooker scale. In 2002, Della Valle et al proposed a modified rating system for heterotopic ossification to increase intraobserver reliability and interobserver agreement. To date, no study comparing these 2 classification systems has been conducted. Moreover, these studies were grossly underpowered. In the current study, 3 clinicians reviewed the charts of 236 patients with documented radiographic heterotopic ossification at least 2 months after THA and independently graded the amount of heterotopic ossification according to the Brooker and Della Valle classification systems. Then the intraobserver reliability and the inter-observer agreement of each classification system were calculated with Cohen's kappa (κ) coefficient of agreement. The Brooker scale showed moderate to substantial intraobserver reliability (0.43≤κ<0.71), and the Della Valle classification system showed substantial intraobserver reliability (0.65≤κ<0.77). Both classification systems showed moderate interobserver agreement (0.40≤κ<0.60). Della Valle grade C (ie, presence of bone spurs from the pelvis or femur leaving less than 1 cm between opposing surfaces and apparent bone ankylosis) and Brooker grade IV had the best interobserver agreement. The best interobserver agreement for any grade was seen with grade C of the Della Valle classification system, which showed substantial interobserver reliability (0.60≤κ<0.80). The Della Valle classification system may be slightly better in patients with large amounts of heterotopic ossification, but both classification systems lack sufficient clarity and are open to significant subjective interpretation.