Effect of rotation on the radiographic appearance of the femoral canal.

Radiographic templating is a key element of preoperative planning for cementless total hip arthroplasty, and it aids in the selection of an appropriate implant. Frequently, the radiographic projection of the proximal femur does not correspond to that of the femoral prosthesis on its template due to variations in patient positioning. This discrepancy is a potential source of error when predicting which femoral component will best fit within the femoral canal. To evaluate the effect of femoral rotation on the size and shape of its radiographic image, anteroposterior and lateral radiographs of 12 femora were prepared over a range of positions. Several medullary canal dimensions were measured for each projection. The changes in these dimensions were compared using the image at neutral rotation as a reference. Each femur was then implanted with an appropriately sized cementless prosthesis to determine its actual rotational orientation in the canal. On the anteroposterior projection, statistically significant changes in the width of the proximal canal with femoral rotation were noted. There was no statistically significant change in distal canal dimensions with rotation. On the lateral projection, the dimensions of the proximal canal changed significantly with internal rotation; however, external rotation had no effect on canal dimensions. In general, the magnitude and direction of the canal dimensions were highly variable. The final rotational orientation of the femoral component in the canal was quite variable with respect to the plane of the femoral neck. Errors in implant selection may be due to excessive reliance on preoperative templating, which can be misleading because of femoral rotation.