We have investigated both mechanisms of femoroacetabular impingement. We hypothesised that cam impingement results in a pattern of articular damage which differs from that seen in impingement resulting from acetabular overcover. This mechanism has been termed pincer impingement. 2, 6 In these hips the femoral neck that is normal impinges against an over-covering acetabulum, resulting in linear contact between the neck and the acetabular rim. 2, 3, 6Ībnormalities of the acetabulum associated with impingement include excessive cover of the head as is seen in retroversion 7, 8 or in a deep acetabulum. 2 They cause a cam-type impingement in which the abnormal junction is driven into the acetabulum producing damage to the cartilage in the area of the anterosuperior rim. 10 They are also seen in slipped capital femoral epiphysis 3 or in any hip in which the femoral neck is too large. These changes have been named the pistol-grip 9 or tilt deformity. This appears as an aspherical junction between the head and neck, with an increased radius of the femoral epiphysis as it joins the neck. 2, 7, 8 The femoral abnormalities are characterised by decreased or absent waisting of the junction of the femoral neck and head. 1 – 6 It reflects abutment of the acetabular rim and the femoral neck and is associated with abnormalities of the proximal femur 1, 2, 5 and of the acetabulum. Labral damage indicates ongoing impingement and rarely occurs alone.įemoroacetabular impingement as a cause of early degenerative changes in the hips of young adults has been recognised and described only recently. During movement the labrum is crushed between the acetabular rim and the femoral neck causing degeneration and ossification.īoth cam and pincer impingement lead to osteoarthritis of the hip. In pincer impingement, the cartilage damage was located circumferentially and included only a narrow strip. During flexion, the cartilage was sheared off the bone by the non-spherical femoral head while the labrum remained untouched. Cam impingement caused damage to the anterosuperior acetabular cartilage with separation between the labrum and cartilage. Of 302 analysed hips only 26 had an isolated cam and 16 an isolated pincer impingement. We hypothesised that both mechanisms result in different patterns of articular damage. There are two mechanisms of impingement: 1) cam impingement caused by a non-spherical head and 2) pincer impingement caused by excessive acetabular cover. Recently, femoroacetabular impingement has been recognised as a cause of early osteoarthritis.
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