Etiology and clinical trends in hip osteoarthritis in Japan: Insights from a multicenter cross-sectional study.

Background: Epidemiological characteristics of Hip osteoarthritis (OA) continue to change due to population aging and emerging disease concepts, such as femoroacetabular impingement (FAI) and subchondral insufficiency fracture of the femoral head (SIF). This study aimed to analyze the current etiologies of hip OA in Japan and identify characteristics associated with each etiology.

Methods: A multicenter cross-sectional study was conducted in 2022 at 12 major Japanese hospitals, including 1197 patients (1515 hips) newly diagnosed with hip OA. Etiologies were classified into nine categories based on patient background and imaging findings: hip dysplasia, primary OA, FAI, SIF, rapidly destructive coxopathy (RDC), trauma, Legg-Calvé-Perthes disease (LCPD), slipped capital femoral epiphysis (SCFE), and skeletal dysplasia.

Results: The etiologies included hip dysplasia (74.4 %), primary OA (13.4 %), FAI (4.8 %), SIF (3.4 %), RDC (1.3 %), trauma (1.3 %), LCPD (0.9 %), SCFE (0.1 %), and skeletal dysplasia (0.1 %). Patients in their 70s comprised the largest group, with a predominance of female patients across all ages. The proportion of hip dysplasia decreased in the elderly patients, whereas primary OA and SIF increased. Hip dysplasia was more common in female than male (78.2 % vs. 55.2 %), whereas FAI was more frequent in male (17.9 % vs. 2.2 %). The Japanese Orthopaedic Association hip scores were highest for FAI and lowest for SIF. Kellgren-Lawrence (KL) grade ≥3 was more common in primary OA (82.0 %) and hip dysplasia (76.7 %) than in FAI (57.7 %) and SIF (66.7 %).

Conclusions: Hip dysplasia remains the predominant cause of hip OA in Japan. Age-specific analyses revealed that FAI was more common in younger patients, while primary OA and SIF were prevalent in older patients. Patients with hip dysplasia and primary OA often presented with advanced KL grades, while SIF was associated with the poorest clinical scores. These findings may inform diagnostic and treatment approaches based on predominant etiologies.