Os acetabuli and femoro-acetabular impingement: aetiology, incidence, treatment, and results.

Purpose: The purpose of this study was to investigate the pathogenesis, the incidence, and the results of arthroscopic treatment of os acetabuli (OSA) in a group of patients with diagnosis of femoro-acetabular impingement (FAI).
Methods: We retrospectively analyzed the full documentation of 294 hips in 273 patients (21 bilateral) operated for FAI through hip arthroscopy. We reviewed all radiographs and arthro-MRI in order to identify the incidence of OSA. All patients with OSA were then assessed with a modified Harris hip score (MHHS) pre-operatively and at the final follow-up.
Results: Twenty-one patients (7.7%), 20 (95%) of them were male, were diagnosed with concomitant FAI and os acetabuli. In 21 cases, OSA was excised and FAI was treated with rim trimming, femoral osteoplasty, or both. In one case, a large OSA fragment was fixed with a 4 mm screw avoiding an acetabular uncoverage if excised. The average follow-up was 31 months (range from 6 to 69 months). The MHHS showed an improvement from a pre-operative MHHS of 57.5 (range from 39 to 82) to 95 (range from 73 to 100).
Conclusions: Os acetabuli is not uncommon and certainly associated with FAI and male gender. The etiology is probably microtraumatic. The arthroscopic OSA removal or fixation and concomitant FAI treatment showed very good results. Interestingly, these outcomes seem better than FAI treatment alone. Further studies with a wider number of patients and a longer follow-up are needed to confirm these results and understand the real role of OSA in this setting.