Accurate acetabular component positioning is important in total hip arthroplasty to maximize an impingement-free range of hip motion. Premature impingement can contribute to edge-loading, altered joint-loading patterns, or a higher risk for prosthetic dislocation. While a safe zone for acetabular component placement as proposed by Lewinnek et al.has been generally accepted as a target for limiting the risk of prosthetic dislocation, its application requires the consideration of two measurements—inclination and anteversion—obtained from orthogonal radiographs, rather than a single measure of a three-dimensional component position. Some contemporary studies have raised questions about the ability of surgeons to reliably implant the acetabular component within this safe zone. Other studies have drawn attention to the impact of changing pelvic position on the radiographic appearance of acetabular inclination and anteversion with their associated potential to affect acetabular component positioning during hip arthroplasty procedures.

In their study, Kanawade et al. combined clinical and experimental observations to extend our understanding of the impact of pelvic positional change on acetabular spatial orientation. Changes in pelvic tilt and acetabular orientation angle (ante-inclination) were measured on lateral spinopelvic radiographs during transitions between standing and sitting positions. The ante-inclination angle combines the contributions of inclination, anteversion, and pelvic tilt to provide a single, composite angle for the assessment of acetabular component position. In the clinical portion of the study, the authors demonstrated a mean 25° increase in the ante-inclination angle during patients’ transition from standing to sitting. This value is slightly less than the mean change in pelvic tilt (36°) previously reported by DiGioia et al.. When comparing preoperative and postoperative changes in the ante-inclination angle during patients’ positional transition, Kanawade et al. did not identify a clinically relevant difference in acetabular spatial orientation resulting from the surgical procedure, which supports the findings of Murphy et al.. Of specific interest is the study’s identification of substantial variability in changes in pelvic tilt among individual patients. In addition to patients with normal pelvic mobility, Kanawade et al. identified two patient groups with pelvic mobility that was considered to be either stiff (19%) or excessively mobile (14%). Patients with limited pelvic mobility may have more consistent dynamic component behavior relative to intraoperative placement. The converse consideration for these patients is that they may have less tolerance for component placement with inadequate anteversion or inclination.