Fig. 2.

a Preoperative planning of the acetabular cup position on the CT scan allows restoration of the COR of the hip with the cup. Any difference between the COR of the cup (green dot) and the native hip (magenta dot) can be factored into the reconstruction of the femur. The depth of reaming is determined to create the COR position. The acetabular anteversion can be adjusted after the femoral anteversion is known (femur is prepared first). The CT planned cup position is then superimposed on the x-ray as seen in the lower left corner. The acetabular bony geometry of the CT scan is confirmed intraoperatively by registration as seen in the lower right hand corner. b The femoral neck cut is determined by restoring the femoral head center for the stem implant used. Differences between the stem COR and the bony femoral COR can be factored into the leg length and offset. The modular head desired can be selected according to any differences needed to correct leg length and offset. A defined level of neck cut (green line), with the stageline of the stem implanted at this level, will restore the leg length and offset the surgeon chooses during preoperative planning