Objective The aim was to develop a CT-based protocol to accurately measure post-operative acetabular cup inclination and anteversion establishing which bony reference points facilitate the most accurate estimation of these variables.
Materials and methods An all-polyethylene acetabular liner was implanted into a cadaveric acetabulum. A conventional pelvic CT scan was performed and reformatted images created in both functional and anterior pelvic planes. CT images were transferred to a Freedom-Plus Graphics software package enabling an identical, virtual 3D model of the cadaveric pelvis to be created and definitive acetabular cup orientation established. Using coronal and axial slices of the CT scans, acetabular cup inclination and anteversion were measured on five occasions by ten radiographers using differing predetermined bony landmarks as reference points. The intra-and inter-observer variation in measurement of acetabular cup orientation using varying bony reference points was assessed in comparison to the elucidated definitive cup position.
Results and conclusion Virtually derived definitive acetabular cup orientation was measured showing cup inclination and anteversion as 41.0 and 22.5 degrees respectively. Mean CT-based measurement of cup inclination and anteversion by ten radiographers were 43.1 and 20.8 degrees respectively. No statistically significant difference was found in intra-and inter-observer recorded results. No statistically significant differences were found when using different bony landmarks. CT assessment of acetabular component inclination and anteversion is accurate, reliable and reproducible when measured using differing bony landmarks as reference points. We recommend measuring acetabular inclination and anteversion from the inferior acetabular wall/teardrop and posterior ischium respectively.