A method for assessing knee joint position after surgery using the preoperative long-leg radiograph and the postoperative knee radiograph is described. Assessment of the formula has shown a near perfect correlation between the calculated position on the long-leg radiograph compared with the measured position for 44 knees. Three hundred eighteen knees after total joint arthroplasty were retrospectively reviewed and the postoperative position was determined. The preoperative position of the mechanical axis was 14.5 +/- 37.3 mm medial to the knee joint center. Using the standing knee radiograph the postoperative position of the mechanical axis was 3.07 +/- 9.2 mm lateral to the knee joint center, while the portable radiograph placed the mechanical axis 4.5 +/- 12.4 mm medial to the knee joint center. There was a highly significant difference in the position of the knee joint center depending on the radiograph used for calculation (standing or portable). The difference between the two means was not due to opening of the knee joint, but likely due to change in the rotation of the knee in the presence of knee flexion. This series of the knee arthroplasties has a low projected rate of aseptic failure.