Effect of Rotation and Knee Flexion on Radiographic Alignment in Total Knee Arthroplasties

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Abstract
A synthetic femur and tibia were used to create a model resurfacing total knee arthroplasty. The femoral component was placed in 7 ° valgus; the tibial component was placed in 2 ° varus with a 5 ° posterior slope. The overall anatomic alignment was 5 ° valgus. A series of radiographs were taken on 14 inch × 17 inch plates, in full extension and 10 ° flexion, with the limb rotated, in 5 ° increments, from 20 ° external rotation to 25 ° 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 ° valgus in 20 ° external rotation and 10 ° flexion, to 6.73 ° valgus in 25 ° internal rotation and 10 ° flexion. Limb rotation and knee flexion of 10 °, either alone or in combination, had a highly statistically significant effect on measured values of the anatomic alignment. Tibial alignment ranged from 5 ° varus in 20 ° external rotation to 3 ° valgus in 25 ° internal rotation, with the knee flexed 10 °. The variability associated with changes in rotation was statistically significant. Changes associated with rotation, when the knee was flexed 10 °, 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.