VOL. 89-B, No. 6, JUNE 2007 839 A computer model of the position of the combined component in the prevention of impingement in total hip replacement W. K. Barsoum, R. W. Patterson, C. Higuera, A. K. Klika, V. E. Krebs, R. Molloy From the Cleveland Clinic, Cleveland, USA  W. K. Barsoum, MD, Orthopaedic Surgeon  R. W. Patterson, MD, MPH (Master of Public Health), Resident Physician  C. Higuera, MD, Resident Physician  A. K. Klika, MS, Research Assistant  V. E. Krebs, MD, Orthopaedic Surgeon  R. Molloy, MD, Orthopaedic Surgeon Department of Orthopaedic Surgery Cleveland Clinic, 9500 Euclid Avenue, Cleveland, Ohio 44195, USA. Correspondence should be sent to Mr W. K. Barsoum; e-mail: barsouw@ccf.org ©2007 British Editorial Society of Bone and Joint Surgery doi:10.1302/0301-620X.89B6. 18644 $2.00 J Bone Joint Surg [Br] 2007;89-B:839-45 Received 14 September 2006; Accepted after revision 14 February 2007 Dislocation remains a major concern after total hip replacement, and is often attributed to malposition of the components. The optimum position for placement of the components remains uncertain. We have attempted to identify a relatively safe zone in which movement of the hip will occur without impingement, even if one component is positioned incorrectly. A three-dimensional computer model was designed to simulate impingement and used to examine 125 combinations of positioning of the components in order to allow maximum movement without impingement. Increase in acetabular and/or femoral anteversion allowed greater internal rotation before impingement occurred, but decreases the amount of external rotation. A decrease in abduction of the acetabular components increased internal rotation while decreasing external rotation. Although some correction for malposition was allowable on the opposite side of the joint, extreme degrees could not be corrected because of bony impingement. We introduce the concept of combined component position, in which anteversion and abduction of the acetabular component, along with femoral anteversion, are all defined as critical elements for stability. Dislocation is a major complication after total hip replacement (THR), with a reported inci- dence of 0.6% to 10%. 1 Malposition of the components has been reported to be responsi- ble for 30% or more of cases of instability and dislocation. 2 The optimum position for the implants has been subject to considerable dis- cussion. That which is usually recommended is 30˚ to 50˚ of abduction and 0˚ to 30˚ of anteversion of the acetabular component and anteversion of 0˚ to 15˚ for the femoral compo- nent. 3-9 Charnley 10 recommended 45˚ of abduction, 0˚ of acetabular anteversion and 5˚ of femoral anteversion. In addition to the posi- tioning of the component, the ratio of the diameters of the head and the neck is a known contributor to stability, 11,12 with larger ratios believed to lead to better stability. However, some studies have shown no difference in sta- bility with increased head size. 13-17 Several combinations of positioning of the implants offer acceptable clinical results, but variations in positioning can lead to differences in the movement and stability of the joint. Instability in THR can be caused by impinge- ment of one component against another, of bone against bone or a combination of these. 18,19 The challenge for the orthopaedic surgeon is to ensure that components are posi- tioned in such a way that a physiological range of movement (ROM) is allowed while impingement is minimised. Malposition of the components can lead to accelerated wear of polyethylene which may subsequently lead to the generation of metal debris from the fem- oral component, osteolysis, aseptic loosening, and dislodgement of the liner. 20,21 Because of the numerous negative effects of early impingement on the stability and longevity of the hip, it is necessary to allow the maximum ROM before impingement of the components occurs. Much has been written about the opti- mum positioning of the acetabular component to achieve this goal. Utilisation of a computer model allowed Barrack et al 22 to define an acceptable range of positioning of the acetabu- lum as 45˚ ± 10˚ of abduction and 20˚ ± 10˚ of anteversion. Similarly, a safe zone of 40˚ ± 10˚ of abduction and 15˚ ± 10˚ of anteversion was defined by Lewinnek et al. 23 Implants placed out- side this zone dislocated four times more than those within it (6% vs 1.5%). 23 Using a mathe- matical model, Widmer and Zurfluh 24 recom- mended abduction of 40˚ to 45˚ and acetabular anteversion of 20˚ to 28˚. McCollum and Gray 7 suggested greater anteversion in the range of 20˚ to 40˚. Biedermann at el 25 recommended antever- sion of 15˚ and abduction of 45˚.