Tai, Ta-Wei

Aims Heterotopic ossification (HO) is a recognized complication following periacetabular osteotomy (PAO), but its patterns, incidence, and clinical relevance remain poorly defined. The commonly used Brooker classification, developed for total hip arthroplasty, is not fully applicable to HO after PAO. This study aimed to propose a new classification system specific to HO after PAO, evaluate its clinical relevance, and report the cumulative probabilities of radiological and symptomatic HO. Methods This retrospective study included 643 patients who underwent PAO between January 2006 and July 2024. A total of 308 patients (47.9%) had concomitant arthroscopic procedures. Radiographs were analyzed to identify HO using both the Brooker classification and a newly developed system. The cumulative probabilities of radiological and symptomatic HO were calculated using Kaplan-Meier analysis. Difference of variables between patients with and without HO was analyzed. Receiver operating characteristic (ROC) curve analysis was used to compare the predictive performance of the two classification systems. Results The cumulative probabilities of radiological and symptomatic HO at five years post-PAO were 30.4% and 3.8%, respectively. The probabilities were 41.9% in males and 29.2% in females. Four patients (2.8%) required surgical excision of HO, all classified as type 3 or 4 under the new system. The ROC analysis demonstrated superior predictive performance for the new classification system (AUC = 0.928) compared to the Brooker classification (AUC = 0.697; p = 0.007). No significant differences were found between the patients with and without HO in terms of age, sex, concomitant arthroscopic procedures, or receiving prophylaxis in addition to low-dose aspirin. Conclusion This study highlighted the cumulative probabilities of HO following PAO and introduced a new classification system that improved clinical relevance and predictive accuracy. These findings have provided insights into the incidence and management of HO after PAO, and laid the groundwork for standardized reporting and future research. Cite this article: Bone Jt Open 2025;6(11):1436–1445.