Y.-H. Kim, Y. Choi, and J.-S. Kim reply:
We thank Dr. Scuderi and colleagues for their interest in our article. In response to their question concerning how we determined the appropriate size of the femoral component and how many components were upsized, downsized, or a perfect fit, we clearly described in the article our method for determining the appropriate size. After the cut of the distal end of the femur, the size of the femoral component was determined with a sizer. At the end of the implantation of the selected standard or gender-specific femoral component, the aspect ratios of the distal end of the femur (the transepicondylar width divided by the average anteroposterior dimension of the lateral and medial condyles) and of the standard or gender-specific femoral component (the distance from the medial to the lateral margin at the transepicondylar line divided by the average anteroposterior dimension of the lateral and medial condyles of the component) were compared. We defined the component fit as a close fit (perfect fit) when the aspect ratios of the distal end of the femur and the femoral component were equal. We defined the component fit as overhang (upsized) when the aspect ratio of the distal end of the femur was smaller than that of the femoral component. We defined the fit as underhang (downsized) if the aspect ratio of the distal end of the femur was larger than that of the femoral component. As mentioned in Table IV, fifty-one (60%) of eighty-five standard knee implants and fourteen (16%) of eighty-five gender-specific knee implants were close fit (perfect fit). Ten standard knee implants (12%) and no gender-specific knee implant (0%) had an overhang (upsized). Twenty-four standard knee implants (28%) and seventy-one gender-specific knee implants (84%) had an underhang (downsized).
We performed bilateral simultaneous total knee arthroplasty in the same patients. We found that the morphological data of the distal end of the femur in each knee were similar. Therefore, seventy-one knees (84%) cannot be moved into the overhang (upsized) group. These seventy-one knees (84%) consistently belong to the underhang (downsized) group. We had similar results in another study1.
We have performed more than 20,000 total knee arthroplasties using standard knee prostheses designed in North America or Europe, and fortunately we have not encountered any specific problem because of the design or size of the standard total knee prosthesis. Rarely, we have needed a very small component for patients with childhood pyogenic or tuberculous arthritis with severe hypoplasia of the knees. Also, we have not had any problem with the patella because of the thickness or angle of the anterior femoral condyle.
We agree with Dr. Scuderi and colleagues that, since the cohort in our study had good functional scores preoperatively, the knee scoring systems used in the study may not be sensitive enough to discern subtle differences between the two groups postoperatively. Certainly, we need a longer duration of follow-up to clarify the benefit of the gender-specific total knee prosthesis.