VOL. 100-B, No. 7, JULY 2018 831  PAPERS FROM THE INTERNATIONAL HIP SOCIETY Does acetabular coverage influence the clinical outcome of arthroscopically treated cam-type femoroacetabular impingement (FAI)? M. M. Ibrahim, S. Poitras, A. C. Bunting, E. Sandoval, P. E. Beaulé From The Ottawa Hospital, University of Ottawa, Ontario, Canada  M. M. Ibrahim, MD, PhD, Clinical Fellow Arthroplasty and Adult Reconstruction, Division of Orthopaedic Surgery, The Ottawa Hospital/l’Hôpital d’Ottawa, Ottawa, Ontario, Canada and Lecturer of Orthopaedic Surgery, Faculty of Medicine, Helwan University, Cairo, Egypt.  S. Poitras, PT, PhD, Associate Professor Rehabilitation Sciences, Faculty of Health Sciences, University of Ottawa, Ottawa, Ontario, Canada.  A. C. Bunting, MD, Resident of Orthopaedic Surgery Division of Orthopaedic Surgery, The Ottawa Hospital/ l’Hôpital d’Ottawa  E. Sandoval, MD, Clinical Fellow Arthroplasty and Adult Reconstruction, Division of Orthopaedic Surgery, The Ottawa Hospital/l’Hôpital d’Ottawa,, Ottawa, Ontario, Canada and Alai Sports Medicine Clinic, Madrid, Spain.  P. E. Beaulé, MD, FRCSC, Professor of Surgery, Head Division of Orthopaedic Surgery University of Ottawa, Ottawa, Ontario, Canada and The Ottawa Hospital/l’Hôpital d’Ottawa, Ottawa, Ontario, Canada. Correspondence should be sent to P. E. Beaulé; email: pbeaule@toh.ca ©2018 The British Editorial Society of Bone & Joint Surgery doi: 10.1302/0301-620X.100B7. BJJ-2017-1340.R2 $2.00 Bone Joint J 2018;100-B:831–8. Aims What represents clinically significant acetabular undercoverage in patients with sympto- matic cam-type femoroacetabular impingement (FAI) remains controversial. The aim of this study was to examine the influence of the degree of acetabular coverage on the functional outcome of patients treated arthroscopically for cam-type FAI. Patients and Methods Between October 2005 and June 2016, 88 patients (97 hips) underwent arthroscopic cam resection and concomitant labral debridement and/or refixation. There were 57 male and 31 female patients with a mean age of 31.0 years (17.0 to 48.5) and a mean body mass index (BMI) of 25.4 kg/m 2 (18.9 to 34.9). We used the Hip2Norm, an object-oriented-platform program, to perform 3D analysis of hip joint morphology using 2D anteroposterior pelvic radiographs. The lateral centre-edge angle, anterior coverage, posterior coverage, total femoral coverage, and alpha angle were measured for each hip. The presence or absence of crossover sign, posterior wall sign, and the value of acetabular retroversion index were identified automatically by Hip2Norm. Patient-reported outcome scores were collected preoperatively and at final follow-up with the Hip Disability and Osteoarthritis Outcome Score (HOOS). Results At a mean follow-up of 2.7 years (1 to 8, SD 1.6), all functional outcome scores significantly improved overall. Radiographically, only preoperative anterior coverage had a negative correlation with the improvement of the HOOS symptom subscale (r = -0.28, p = 0.005). No significant difference in relative change in HOOS subscale scores was found according to the presence or absence of radiographic signs of retroversion. Discussion Our study demonstrated the anterior coverage as an important modifier influencing the functional outcome of arthroscopically treated cam-type FAI. Cite this article: Bone Joint J 2018;100-B:831–8. In patients with femoroacetabular impingement (FAI), the goal of hip preservation surgery is to improve function, decrease pain, and delay or prevent progression to osteoarthritis. 1 In cam-type FAI, the aim of surgery is to restore the sphericity of the femoral head, which improves the clearance between the acetabular rim and the proximal femur. In pincer-type FAI, clearance is restored by trimming the overhanging acetabular rim (in cases of focal impingement) or by osteotomy and reorientation of the acetabulum in cases where there is retroversion. 2,3 Long-term follow-up of cases of FAI managed with arthroscopic or open surgery have demonstrated that patients who have a greater degree of arthritis at the time of surgery, and those in whom acetabular rim trimming was performed, have the worst clinical outcomes. 4-7 A substantial proportion of patients with hip dysplasia also demonstrate the radiographic fea- tures of impingement such as asphericity of the femoral head, or the acetabular crossover sign. 8-10 It is not clear where, as the acetabular coverage decreases, a hip goes from being a hip with cam impingement alone (which can be treated with arthroscopic or open reshaping of the head neck junction) to being a dysplastic hip, which requires pelvic osteotomy. By convention, the lateral centre-edge angle (LCEA) 11 has been a surrogate marker of acetabular coverage but, given the three- dimensional nature of dysplastic pathology, 12-14 it