The concept of combined anteversion for total hip arthroplasty (THA) proposes a relationship between the cup and stem components that theoretically maximises the postoperative range of motion and minimises the risk for impingement of the joint. Using computer-assisted navigation tools, an anteversion angle of the cup component can be made to be dependent on the antetorsion angle of the stem component (or vice versa). We studied how this functional concept would be different from the traditional cup placement according to the Lewinnek safe zone.
We prospectively reviewed 42 patients (42 hips) who underwent imageless, computer-assisted THA with cementless implants due to osteoarthritis between May and October 2008. Using computer navigation, we determined the cup anteversion with optimised containment and measured femoral stem antetorsion. Our goal was to implant the original implants with a combined anteversion of 37 degrees.
Mean cup anteversion was 22.5 degrees, mean combined anteversion was 35.2 degrees. Femoral antetorsion ranged from -13 to 38 degrees (mean: 18 degrees). Mean anteversion of the trial cup with optimised containment was 15.9 degrees and therefore close to the recommendation according to the Lewinnek safe zone. The total postoperative range of motion (flexion, extension, abduction, internal/external rotation) as measured with the navigation system intraoperatively was 209 degrees compared to 94 degrees measured clinically preoperatively. No THA dislocation occurred during the test.
The combined anteversion concept results in a cup position with more anteversion when compared to the traditional cup placement according to the Lewinnek safe zone. In this context, modern navigation techniques open a new frontier for an optimised component position. Placing the cup and stem in relation to the anteversion for both components allows consideration of the patient-specific biomechanics.