Prevalence and characteristics of femoroacetabular impingement among Japanese patients: A nationwide epidemiological study.

Background: Femoroacetabular impingement (FAI) causes hip pain and osteoarthritis (OA). However, its prevalence and characteristics in the Japanese population remain unclear. Therefore, this large-scale nationwide epidemiological study aimed to determine the current aetiologies of hip disorders in Japan, focusing on the prevalence and clinical features of primary FAI.

Methods: A total of 2402 patients presenting with groin pain were prospectively enrolled from 16 hip-specialised institutions across Japan. Diagnoses were based on standardised clinical and radiographic evaluations, including the Japanese Hip Society criteria for primary FAI. Centralised radiographic assessment was also conducted using dedicated measurement software.

Results: Secondary OA due to hip dysplasia was the most common diagnosis (52.9 %), followed by primary OA (16.8 %), osteonecrosis of the femoral head (8.5 %), and primary FAI (6.2 %). Patients with FAI were predominantly younger males. Cam-type FAI was the most frequently identified FAI subtype. Centralised radiographic assessment revealed discrepancies compared with institutional diagnoses, in which 12.8 % of patients initially diagnosed with FAI showed signs of advanced OA, the percentage of hips with a lateral center-edge angle <25° was higher in the centralised measurements (13.8 % vs. 6.4 %; P < 0.001), and the percentage of hips with an alpha angle ≥55° was reduced (40.3 % vs. 67.7 %; P = 0.0393).

Conclusion: This nationwide study identified secondary OA due to hip dysplasia as the most common (52.9 %), with primary FAI identified in 6.2 % of cases, predominantly affecting younger males with cam-type morphology. Standardised radiographic assessment revealed an underdiagnosis of hip dysplasia and OA and a potential overdiagnosis of cam-type FAI, emphasising the need for accurate diagnosis to guide appropriate treatment decisions.