B-12 Three Dimensional Assessment of Cam Engagement in

®electromagnetic tracking system. Following three-dimensional segmentation and reconstruction, the dynamic behaviour of the cam lesion was investigated for the previously analyzed motions. Important differences in the pattern of cam engagement were noticed. In 7 hips (54%) abutment of the cam lesion and the acetabular cartilage was observed in flexion and in abduction in 11 hips (84%). Internal rotation with the hip in 90 degrees of flexion caused intrusion of the cam lesion into the joint in 10 of the investigated cases (77%). Neutral rotation did not cause any conflict between the cam lesion and acetabulum for the measured range of motion. Typically during flexion the medial side of the cam lesion impinged again the acetabular cartilage, during abduction the lateral side, and during internal rotation in 90 degrees of flexion the centromedial portion. A similar area in the anterosuperior quadrant of the acetabulum appeared to be involved in the different motions that were analyzed. The findings suggest that only a complete resection of the cam lesion can prevent progression of acetabular cartilage destruction. Incomplete medial or lateral resection will cause ongoing cam abutment with the injured acetabular cartilage during flexion or abduction, respectively. B-13