The Role of the ‘Femur First’ Technique and Spinopelvic Characteristics in Achieving the Combined Sagittal Index in Total Hip Arthroplasty: Results from a Retrospective Tertiary-Center Clinical Study

<b>Background:</b> Emerging parameters, such as the spino-pelvic unit and the combined sagittal index (CSI), are gaining importance in evaluating and optimizing implant positioning in total hip arthroplasty (THA). Our institution adopts the "femur first" technique to achieve the desired combined anteversion (CA). The purpose of this study is to evaluate the role of the 'femur first' technique and spinopelvic characteristics in achieving the CSI_standing 'safe zone' in primary THA. <b>Methods:</b> Consecutive patients undergoing primary THA were included in the present retrospective study. All patients underwent radiographic assessments in the standing position with the EOS 2D/3D radiography system. <b>Results:</b> Forty patients (40 hips) were enrolled. Of these, 34 patients fell inside the CSI_standing "safe zone" (205-245°). When considering the restricted CSI_standing "safe zone" for patients at high risk for adverse spinopelvic characteristics (215-245°), only 16 patients fell inside the range. We demonstrated a positive linear correlation between CSI_standing and CA (<i>p</i> < 0.0001). Pelvic tilt (PT) showed a positive correlation both in standing and relaxed sitting positions, (<i>p</i> < 0.001). Sacral slope (SS) showed a significant positive correlation in the relaxed sitting (<i>p</i> = 0.003) position but not in the standing position (<i>p</i> = 0.34). The correlation analysis between CSI_{relaxed-sitting} and ΔSS showed a positive correlation (<i>p</i> = 0.003). <b>Conclusions:</b> The "femur first" technique is able to achieve the CSI "safe zone" in most patients; however, it seems insufficient in those with adverse spinopelvic characteristics, who are at higher risk of dislocation. Moreover, the CA, the position of the pelvis in space (PT), and its mobility (ΔSS) greatly influence the CSI "safe zone" in patients undergoing primary THA.