2D measurements of cup orientation are less reliable than 3D measurements

I read with interest the paper by Davda et al. (2015) which described an assessment of the reliability of measure- ment of metal-on-metal (MoM) acetabular cup orientation using 3-D CT and EBRA. I would like to make the following comments. 1. It is generally appreciated that 3D-CT is inappropri- ate for routine cup orientation measurements because of the increased cost, radiation level and procedure/analysis time. Nevertheless, the authors of this paper would seem to encourage the belief that it is the “gold standard” for measuring cup orientation. They state: “A critical advan- tage of 3D-CT is that the pelvis is corrected to the APP (anterior pelvic plane), eliminating the variability in patient positioning at the time of scanning, thus allowing objective measurements of cup placement to be made between differ- ent subjects”. This view is not uncommon: but is it correct? The APP provides the basis for a local 3D co-ordinate axis system and is a convenient reference plane which can be repeatedly identifi ed. However, its relationship to other aspects of the pelvis may vary amongst different patients due to variations in pelvis size and morphology (Rousseau et al. 2009).