This is an article by Pfirrmann et al from Zurich, describing the differences in the Cam and Pincer types of femoro-acetabular impingement (FAI) in the September issue of Radiology.
In Cam type of FAI, there are abnormal labral changes along the anterior and antero-superior aspects with an abnormal alpha angle due to non-sphericity and an abnormal osseous bump at the head-neck junction. In the Pincer type of FAI, there are abnormal labral changes postero-inferiorly, with over-riding of the anterior acetabulum or an increased acetabular depth.
The article nicely mentions these differences and findings and describes the various related issues.
This patient with a Cam type FAI shows acetabular ossicles on the plain radiograph (Fig. 1), which is otherwise normal. The MRI arthrogram shows an antero-superior labral tear (Figs. 2, 4 - red arrows). An osseous bump is noted at the head-neck junction on the oblique axial image (Fig. 3).