Effect of rotation on the axial alignment of the femur. Pitfalls in the use of femoral intramedullary guides in total knee arthroplasty.

Intramedullary alignment guides for total knee arthroplasty are currently routine instruments. However, in some cases, the intramedullary guide can be misleading because of unusual femoral bowing and the rotational position of the femur. To determine the effect of rotation, an intramedullary guide was inserted into seven normal adult anatomic specimen femurs that were then examined roentgenographically in known positions of internal and external rotation. A variation of 2.5 degrees was found between the positions of 20 degrees of internal rotation and 20 degrees of external rotation of the femur. Intramedullary instrumentation provides sufficient accuracy when the femur has a normal anatomic shape. However, when an excessive degree of femoral bowing is present, rotational attitudes can affect the roentgenographic measurements, and thus it is prudent to use the intramedullary and extramedullary alignment guides whenever the roentgenogram indicates an unusual angle of femoral resection.