Rotation affects apparent radiographic positioning of femoral components in total hip arthroplasty.

The authors have noted a change in the varus/valgus positioning of femoral components on anteroposterior (AP) radiographs that is dependent on limb rotation after total hip arthroplasty (THA). This effect, called pseudoposition, was demonstrated in six cadaver femurs implanted with an uncemented prosthesis that is noncanal-filling in the AP dimension. The distal tip of the prosthesis was positioned posteriorly in all specimens due to the bow of the femur. External rotation causes pseudovalgus positioning, and internal rotation causes pseudovarus positioning. Awareness of this phenomenon is important in the longitudinal analysis of femoral components after THA and emphasizes the need for standardized radiographic technique.