Effect of limb rotation on radiographic alignment in total knee arthroplasties

Even in a well-aligned total knee arthroplasty (TKA), limb rotation at the time of radiographic assessment will alter the measurement of alignment. This could influence the radiographic outcome of TKA. The purpose of this study was to evaluate the effect of limb rotation on radiographic alignment after TKA and to establish a re-calculation of this rotation by using existing radiographic landmarks.

Synthetic femur and tibia (Sawbones^®, Inc. Vashon Island, WA) were used to create a TKA of the Triathlon^® knee prosthesis system (Stryker^®, Limerick, Ireland). The femoral alignment was 6.5° valgus. The model was fixed in an upright stand. Five series of nine anteroposterior (AP) long leg radiographs were taken on a 30 cm × 120 cm plates in full extension with the limb rotated, in 5° increments, from 20° external rotation to 20° internal rotation. After digitizing each radiograph (Scanner Hewlett Packard XJ 527), an observer measured the anatomic mechanical angle of the femur [AMA (°)], the mechanical lateral proximal femur angle [mLPFA (°)], the mechanical lateral distal femur angle [mLDFA (°)], the mechanical medial proximal tibia angle [mMPTA (°)] and the mechanical lateral distal tibia angle [mLDTA (°)] using a digital measurement software (MediCAD^®, Hectec, Altfraunhofen, Germany). Besides, the observer measured the geometrical distances of the femoral component figured on the long leg radiograph. A ratio of one distance to another was measured (called femoral component distance ratio).

The average radiographic anatomic alignment ranged from 6.827° AMA (SD = 0.22°) in 20° internal rotation to 4.627° AMA (SD = 0.22°) in 20° external rotation. Average mLPFA (°) ranged from 101.63° (SD = 0.63) in 20° internal rotation to 93.60° (SD = 0.74°) in 20° external rotation. Average mLDFA (°) ranged from 90.59° (SD = 3.01°) in 20° internal rotation to 86.76° (SD = 0.36°) in 20° external rotation. Average mMPTA (°) ranged from 90.35° (SD = 0.81°) in 20° internal rotation to 88.49° (SD = 0.52°) in 20° external rotation. Average mLDTA (°) ranged from 98.89° (SD = 2.3°) in 20° internal rotation to 90.53° (SD = 3.39°) in 20° external rotation. Without an application of limb rotation, the femoral component distance ratio was measured to be 0.89 (SD = 0.01), in 20° internal rotation 0.63 (SD = 0.01) and in 20° external rotation 1.16 (SD = 0.01).

Limb rotation had a highly statistically significant effect on measured anatomic alignment and mechanical angles. A correlation between limb rotation, anatomic mechanical angle, mechanical angles measured at femur and tibia and the femoral component distance ratio was established. As the anatomic mechanical angle and the femoral component distance ratio change linearly in the range of 20° internal and external limb rotation, a calculation of the femoral component distance ratio could be used to re-calculate the limb rotation at the time of radiographic assessment to evaluate the evidence of a long leg radiograph.