Comments on: Acetabular component navigation in lateral decubitus based on EOS imaging: A preliminary study of 13 cases of A. Billaud, N. Verdier, R. de Bartolo, N. Lavoine, D. Chauveaux, T. Fabre. Published in Orthop Traumatol Surg Res 2015;101(3):271–5

We read with interest the preliminary report by A. Billaud on omputer-assisted total hip replacement based on EOS imaging ata. As the authors state, hip navigation techniques have improved he reproducibility of implant positioning in hip replacement [1–3]. owever, navigation is at present founded on landmarking in the nterior pelvic plane, for which access in lateral decubitus is diffiult. The authors therefore developed a new and more accessible liac plane (IP), defined by the center of the acetabulum and the nterior superior and posterior superior iliac spines. They use a athematical transformation to pass from the anterior pelvic plane APP) to the IP, considering the APP to be reliable. However, in n article published in the same journal [4], we showed the APP o be unreliable for acetabular component positioning, especially s regards anteversion. Although long confused with the coronal lane, the APP is in fact subject to considerable individual variaion and varies during change of position. It is moreover difficult o determine intraoperatively and, taken together, these factors xplain its poor reliability. These findings were confirmed by Blonel et al. and Pinoit et al. [5,6]. The challenge is therefore to nticipate this dynamic variation by calculating the variation in IP rientation with respect to the APP between preand intraopertive EOS imaging. Could the authors specify whether they found correlation between preand intraoperative IP orientation with espect to the APP on EOS? This variation could then be applied or implant positioning, making the IP the intraoperative reference lane by two-fold integration: intraoperative IP = f (preoperative P) = f’ (APP). Wolf et al. already suggested using the APP as refernce, weighting it by the pelvic tilt value measured in standing osition [7]. All this, however, is still based on the assumption hat pelvic parameters are stable before and after THR, whereas e recently reported that pelvic incidence was altered after THR in 3% of cases [8]. EOS thus provides a new functional and dynamic pproach to the pelvis and hip, essential to improving implant surival and reducing dislocation risk.