Reliability and predictability of the centre-edge angle in the assessment of pincer femoroacetabular impingement

Purpose The aim of the study was to assess the use of the centre-edge (CE) angle in the assessment of pincer femoroacetabular impingement (FAI) for reliability and predictability in the diagnosis. Methods Between 2004 and 2008, 55 patients underwent surgical treatment for FAI. A control group of 30 was identified among patients attending the emergency department with normal radiographs. Radiographs were assessed by two independent observers both before and after the operation. Nine patients with trauma were excluded. The magnetic resonance arthrogram reports of the remaining 46 patients were assessed for pincer FAI. Nineteen patients were identified and underwent repeat radiographic assessment. All underwent surgical dislocation of hip (SDH), acetabular, with/without femoral osteochondroplasty. Acetabular depth and version were also assessed. The intraclass correlation (ICC) was used to assess reliability of the CE angle. The paired t test and independent groups t test were used to assess the difference between the pincer FAI group, both pre-op and post-op and against controls. Results The control and pincer groups were similar in demographics (p=0.1769). Coxa profunda was present in 14 patients with eight also having retroverted acetabuli. Of the rest two had retroverted acetabuli and one protrusio. The mean CE angle in the control group was 31.4 degrees, in the pre-op pincer group 46.2 degrees and in the post-op pincer group 38.3 degrees. The ICC for intra-observer correlation was 0.977 and 0.992 pre-op and 0.986 and 0.974 post-op. The ICC for inter-observer correlation was 0.960 and 0.957 pre-op and 0.979 and 0.967 post-op. The p value was <0.001 between the controls, the pre-op and post-op pincer groups. The test characteristics using the CE angle >= 40 is a reasonably good predictor of FAI, with a sensitivity of 84.2% and a specificity of 100%. Conclusions The pincer FAI can be reliably assessed with the CE angle and can be predicted in patients presenting with FAI.