SCIENTIFIC ARTICLE Quantifying the contribution of pincer deformity to femoro-acetabular impingement using 3D computerised tomography Wael Dandachli & Ali Najefi & Farhad Iranpour & Jonathan Lenihan & Alister Hart & Justin Cobb Received: 19 October 2011 / Revised: 1 January 2012 / Accepted: 26 February 2012 / Published online: 17 March 2012 # ISS 2012 Abstract Objective To provide a simple, reliable method for the three- dimensional quantification of pincer-type hip deformity. Materials and methods Computerised tomography scans of 16 normal female hips and 15 female hips with clinical femoro-acetabular impingement (FAI) and radiographic signs of pincer secondary to acetabular protrusio were analysed. After orientating the pelvis in the anterior pelvic plane, the acetabular centre was determined, and the ratios of its coordi- nates to the corresponding pelvic dimensions were calculated. Acetabular coverage of the femoral head and centre-edge angles were also measured for the two groups. Results In hips with a pincer, the hip was medialised by 37 % (p 0 0.03), more proximal by 5 % (p 0 0.05) and more posterior by 9 % (p 0 0.03) compared with the normal hips. Coverage of the femoral head in protrusio hips was significantly greater than normal (average 71 % vs 82 %, p 0 0.0001). Both the lateral centre-edge angle and the combined anterior–posterior centre- edge angle were greater in protrusio hips than in the normal ones (48° vs 37 °, p <0.001; and 216° vs 176°, p <0.0001 respectively). Conclusion Displacement in acetabular protrusio occurs in all planes. This CT-based method allows for the accurate and standardised quantification of the extent of displacement, as well as 3D measurement of femoral head coverage. In the adult female population, a combined centre-edge angle of over 190° suggests an acetabulum that is too deep and a potential cause of symptoms of femoro-acetabular impingement. Conversely, an acetabulum that has a combined centre-edge angle of less than 190° may be considered to be of normal depth, and therefore not contributing a pincer to FAI should it occur. Keywords Acetabular protrusio . Centre-edge angle . Femoral head coverage . Computerized tomography Introduction Femoro-acetabular impingement (FAI) was initially described as occurring as a result of either a pincer- or a cam-type deformity [1]. Subsequently, FAI was reported to occur simul- taneously in many cases [2], although this view has been challenged [3]. Pincer impingement is often caused by ace- tabular protrusio, which is defined as the intra-pelvic displace- ment of the medial wall of the acetabulum [4]. This occurs in 10–15 % of middle-aged women [5], resulting in a particular pattern of anterior FAI with central and posterior wear of the acetabulum [6]. A reliable quantification of the extent of the acetabular over-coverage and the position of the acetabulum that result in pincer-type FAI has not yet been described, and considerable debate continues regarding the relative contribu- tions of the acetabulum and the femoral head to FAI [7]. Plain radiographic measurements have been reported for diagnosing and reporting the depth of protrusion, especially in rheumatoid arthritis [8]. The three most commonly used indi- ces are the lateral centre-edge angle (CEA) of Wiberg [9], the distance between the medial wall of the acetabulum and Kohler ’ s ilio-ischial line (acetabular-ilio-ischial distance) [10], and the configuration of the tear drop [10]. The lateral CEA has been classified as being between 20 and 40º in normal patients, below 20° in dysplasia and above 46º in W. Dandachli (*) : A. Najefi : F. Iranpour : J. Lenihan : A. Hart : J. Cobb Department of Orthopaedic Surgery, Imperial College London, Charing Cross Hospital, 7th Floor, East Wing, Fulham Palace Road, London W6 8RF, UK e-mail: w.dandachli@imperial.ac.uk Skeletal Radiol (2012) 41:1295–1300 DOI 10.1007/s00256-012-1389-2