Objective— To describe the spatial orientation of the cemented acetabular component in cemented total hip arthroplasty, based on a ventrodorsal and lateral radiographic projection of the pelvis.

Methods— Equations were derived by using trigonometric relationships that describe the radiographic rotation about the longitudinal pelvic axis (α), transverse pelvic axis (β), acetabular inclination (φ), acetabular inclination corrected for longitudinal pelvic rotation, version (φC), acetabular version (θ), acetabular version corrected for longitudinal pelvic rotation (θC), acetabular inclination corrected for transverse pelvic rotation (φβ), and acetabular version corrected for transverse pelvic rotation (θβ)

Results— α was calculated by using the equation α= sin−1(x/y) where x is the transverse distance between the dorsal spinous processes and the center of the pubis on a ventrodorsal radiograph and y is the distance from the pubis to the dorsal aspect of the first coccygeal vertebra perpendicular to the long axis of the pelvis on a lateral radiograph. φ was calculated from the long axis (LA) and short axis (SA) of the ellipse formed by the radiopaque acetabular marker ring by using the equation φ= sin−1(SA/LA). φC was calculated by using the equation φC=φ± (α - tan−1(tan α cos θC)). θ was determined as previously described. θC was calculated by using the equation θC= tan−1(tan θ cos α). φβ and θβ were calculated with the equations φβ= tan−1 (tan φ cos β) and θβ=θ− tan−1(sin β), respectively. β could not be accurately determined from ventrodorsal and lateral pelvic radiographs.

Conclusions and Clinical Relevance— These techniques allow for more accurate postoperative radiographic assessment of acetabular component positioning. This information can then be used in retrospective or prospective analyses examining that effects of implant positioning on clinical outcome.