1832 Current Concepts Review Impingement with Total Hip Replacement

he principles regarding impingement in the natural osseous (anatomic) hip put forth by Ganz et al. are similar in concept to what can occur in the prosthetic hip. To understand impingement, it is helpful to recognize the common mechanisms that cause mechanical abutment in both anatomic and prosthetic hips. In the anatomic hip joint, impingement is a mechanical abutment conflict between the bone of the femur and the pelvis; in a total hip replacement, it is contact between the metal femoral neck and the cup liner or bone-to-bone contact such as between the greater trochanter and the pelvis. The femoral head-neck ratio, which is the relationship between the diameter of the femoral head and the diameter of the femoral neck, influences impingement. Cam impingement is caused by a reduced femoral head-neck ratio. An example is the pistol-grip deformity that is created by a decreased offset of the femoral head-neck junction (Fig. 1). Cam impingement in a prosthetic hip is caused by any implant feature that reduces the head-neck ratio. A skirt on the metal femoral head or a large circular femoral neck can cause mechanical abutment in a prosthetic hip through this mechanism (Fig. 1). Pincer impingement in the anatomic hip is a mechanical abutment caused by acetabular retroversion, protrusio, or coxa profunda. Pincer impingement in the prosthetic hip is caused by hooded and constrained liners or by placement of a small femoral head in a big acetabular cup. Failure to remove acetabular osteophytes so that the metal neck or the femoral bone abuts on the osteophytes is another cause of pincer impingement (Fig. 1). Because impingement is a dynamic process, it has been difficult to identify it and to define its prevalence on the basis of clinical evaluations or plain radiographs. In the clinical setting, some causes of failure such as wear or dislocation are inferre to be related to impingement, but a direct relationship with impingement has been difficult to document. There are no radiographic techniques with which to validate the occurrence of impingement. Retrieval studies are performed to examine implants that have failed, but we are not aware of any autopsy retrieval studies of well-functioning prosthetic hips that have shown the true prevalence of impingement. The purpose of this review is to discuss the current understanding of the mechanisms of impingement in total hip replacements, the clinical consequences of impingement, and new developments. T