Effect of rotation and knee flexion on radiographic alignment in total knee arthroplasties

A synthetic femur and tibia were used to create a model resurfacing total knee arthroplasty. The femoral component was placed in 7 degrees valgus; the tibial component was placed in 2 degrees varus with a 5 degrees posterior slope. The overall anatomic alignment was 5 degrees valgus. A series of radiographs were taken on 14 inch x 17 inch plates, in full extension and 10 degrees flexion, with the limb rotated, in 5 degree increments, from 20 degrees external rotation to 25 degrees internal rotation. Seven orthopaedic surgeons independently measured the tibiofemoral angle and tibial alignment for each series of radiographs; interobserver variability was insignificant. Average radiographic anatomic alignment ranged from 2.29 degrees valgus in 20 degrees external rotation and 10 degrees flexion, to 6.73 degrees valgus in 25 degrees internal rotation and 10 degrees flexion. Limb rotation and knee flexion of 10 degrees, either alone or in combination, had a highly statistically significant effect on measured values of the anatomic alignment. Tibial alignment ranged from 5 degrees varus in 20 degrees external rotation to 3 degrees valgus in 25 degrees internal rotation, with the knee flexed 10 degrees. The variability associated with changes in rotation was statistically significant. Changes associated with rotation, when the knee was flexed 10 degrees, were not significantly different than those measured with the knee fully extended. Even in a well aligned total knee arthroplasty, limb positioning at the time of radiographic assessment will alter the apparent alignment indices, making objective evaluation difficult.