Cams and pincer impingement are distinct, not mixed: the acetabular pathomorphology of femoroacetabular impingement

Abstract

Background: Many impinging hips are said to have a mix of features of femoral cam and an overcovered acetabulum causing pincer impingement. Correction of such a mixed picture by reduction of the cam lesion and the acetabular rim is the suggested treatment.

Questions/purposes: We therefore asked two questions: (1) Is the acetabulum in cam impingement easily distinguishable from the pincer acetabulum, or is there a group with features of both types of impingement? (2) Is version or depth of socket better able to distinguish cam from pincer impingement?

Methods: We analyzed the morphologic features of the acetabulum and rim profile of 20 normal, healthy hips, 20 with cams and 20 with pincers on CT. Pelvises were digitized, orientated to the best-fit acetabular plane, and a rim profile was plotted.

Results: Cam hips were shallower than normal hips, which in turn were shallower than pincer hips (84 degrees +/- 5 degrees versus 87 degrees +/- 4 degrees versus 96 degrees +/- 5 degrees, respectively). The rim planes of cam, normal, and pincer hips had similar version (23 degrees, 24 degrees, 25 degrees), but females were 4 degrees more anteverted than males.

Conclusions: We concluded cam and pincer hips are distinct pathoanatomic entities. Cam hips are slightly shallower than normal, whereas pincers are deeper.

Clinical relevance: Before performing surgery for cam-type femoroacetabular impingement, surgeons should consider measuring the acetabular depth. The cam acetabulum is shallower than normal and may be rendered pathologically shallow by acetabular rim resection leading to early joint failure.

Fig. 1

Morphologic features to diagnose FAI are measured by the center-edge angle of the acetabulum (blue) and the alpha angle of the femoral head (red). The normal hip has a normal center-edge angle and alpha angle, the cam hip has a normal center-edge angle and a large alpha angle, and the pincer hip has a large center-edge angle and a normal alpha angle.

Fig. 2A–C

The method for defining the acetabular rim is illustrated. (A) A best-fit sphere and its center (CoS) based on 34 points taken from the bony articular surface of the acetabulum are shown. (B) Two of 52 points being fitted to the acetabular rim are shown, starting anterior to the inferomedial part of the acetabular notch (IMAN) (green). (C) The acetabular rim plane (ARP) fitted to all 52 rim points around the acetabulum is shown. Also shown are the NCoS, a line normal to the plane passing through the CoS representing the axis of the acetabulum, and the subtended angle between the NCoS and two rim points.

Fig. 3

A Bland and Altman plot shows the reliability between two observers. The mean difference between observers is less than 1°, with the limits of agreement, shown by the lines at two standard deviations either side of the mean difference, being equal at 13°.

Fig. 4

Acetabular rim profiles in normal, cam, and pincer FAI with standard error bars are shown with the bony eminences highlighted. The subtended angles for 52 rim points are interpolated at every degree around the rim. Larger subtended angles indicate greater coverage.

Fig. 5

Acetabular rim profiles in normal, cam, and pincer FAI in a clock face plot, with the inferomedial part of the acetabular notch at 6 o’clock, show the absence from the impingement zone of the cam-type hip contrasted with the extensive incursion into the anterior impingement zone by the pincer-type hip.