Purpose: Femoroacetabular impingement (FAI) results from abnormal abutment between the proximal femur and acetabulum (Ganz et al., 2003). FAI occurs in three forms; cam, pincer and mixed (cam and pincer combined). The cam type has been quantified radiographically (Beall et al., 2005), but pincer FAI is poorly defined. Radiographic measures, including the center-edge angle (Wiberg, 1953), and Sharp’s angle (Sharp, 1961) have been used to define hip dysplasia, but these measures have not been used to define FAI. The purpose was to test these measurements to compare pincer patients with controls.

Method: This study is a retrospective, observational analysis of anterior-posterior pelvic radiographs for control (N=76 hips; 40 patients) and pincer (N=20 hips; 19 patients) groups. Control radiographs were obtained from injury-free pelvic x-rays from the emergency department. Lateral center-edge (CE) angle Sharp’s angle and a proposed measurement of Femoral Head Containment (FHC) were measured using PACS. FHC was defined as the percentage of the 2D area of the femoral head circle covered by the acetabulum, using chord length, height and diameter of the femur head. Non-parametric statistics with post-hoc analyses were used. Pearson’s correlations were calculated for within- and between-observer reproducibility.

Results: Mean (± SD) CE angle was significantly larger in the FAI group [37.4° (±5.2)] compared to controls [31.0° (±3.9)]. Mean Sharp’s angle was significantly less in the FAI group [37.6° (±3.9)] compared to the controls [41.2° (±3.5)]. Mean FHC was significantly larger in the FAI group [26.4% (±5.3)] compared to control group [21.5% (±5.3)]. Intra-observer r-values ranged from 0.86–0.97 and inter-observer correlations ranged from 0.93–0.96. There was significantly greater acetabular overcoverage in the pincer group based on these three measures, suggesting these may be used diagnostically.

Conclusion: Pincer FAI is a debilitating condition that has not been quantified. This study found that CE angle, Sharp’s angle and FHC measures may be useful in diagnosing pincer FAI. A new method of quantifying FHC was proposed, evaluated and appears to be a promising new measure for evaluating pincer FAI. The CE and Sharp’s angles are simple, reproducible measures that can easily be used in a clinic setting to assist with diagnosing pincer FAI.