Apparent changes in the alignment of the femoral component in hip arthroplasties associated with limb positioning.

Three femoral components (one cemented and one uncemented straight-stem Muller and one uncemented Anatomic Medullary Locking) were placed into cadaveric or commercially available plastic femora to establish that rotation of the femur can simulate changes in femoral component alignment on the anteroposterior radiograph. The change in alignment is governed by the position of the prosthesis within the femoral canal. If the stem of the prosthesis is positioned posteriorly in the canal, external rotation simulates valgus; internal rotation results in a varus orientation. If the prosthetic stem is located anteriorly in the canal, the reverse is true. This effect was noted in all specimens for values as small as 20 degrees of rotation. Because a change in the position of a prosthesis may signify loosening, it is recommended that standardized positioning techniques be used for radiographic analysis of total hip arthroplasties.