Recent gains in knowledge reveal that the ideal acetabular cup position is in a narrower range than previously appreciated by APP software with its wide range of Lewinnek safe zone. For that reason more accurate acetabular cup positioning techniques will be important for contemporary total hip arthroplasty (THA). In contrast to APP software, the THA ACA (ACA: Acetabular Center Axis) software is patient-specific, independent of variations in anatomy or pelvic position. It gives the surgeon the exact orientation of reamer in relation to the acetabular center axis of rotation and therefore determines precisely the ideal cup position. The cup center should preferably coincide within 4 mm of acetabular center axis to maximize stability and to minimize impingement that may result in accelerated wear, reduced range of motion, dislocation or unexplained pain. The system relies on readily accessible anatomic landmarks of the acetabulum, making it significantly attractive for surgeons who use CT-free planning.
Leg length and offset length information offered by THAplus are achieved by palpating and relocating specific points in the leg without a femoral transmitter and is especially suited for less invasive surgical technique.