Background: The position of the acetabular cup is important to the outcome of total hip athroplasty (THA). We devised an instrument that uses the level indicator application of smartphone together with a mechanical alignment guide to improve the precision of cup placement. This study aims to determine the percentage of acetabular cups positioned in the Lewinnek safe zone comparing between the conventional technique (using a mechanical alignment guide alone) and the smartphone technique (using a mechanical alignment guide combined with the devised instrument and smartphone). Methods: A historical controlled trial was conducted among 82 patients who underwent primary THAs through a posterolateral approach. In the conventional group, 41 cups were placed during January 2013 and December 2014, whereas 41 cups in the smartphone group were placed during January 2015 and March 2016. Inclination and anteversion angles were measured in standardized pelvic radiographs. The cup orientation was compared between groups. Results: The inclination angle in the smartphone group was significantly lower than in the conventional group (40.9 (SD 3.8) vs. 46.3 (SD 6.7), p < 0.001), but the anteversion angle was higher (19.6 (SD 4.4) vs. 16.5 (SD 6.1), p ¼ 0.010). The smartphone group had more cups positioned in the Lewinnek safe zone (90.2% vs. 56.1%, p ¼ 0.001) and longer operative times (136 (SD 27) vs. 119 (SD 23) min, p ¼ 0.011). No significant difference was found for blood loss (p=¼ 0.384) or dislocation rate (p ¼ 0.494). Conclusion: Using the computerized function of smartphone could improve the precision of cup positioning. Most cups were placed within a narrow margin inside the Lewinnek safe zone.

Background: Malposition of the acetabular component mainly results from intraoperative pelvic motion and manual errors during cup placement. We investigated the outcomes of a device that uses a level indicator application of a smartphone to function as a pelvic tilt goniometer to assess intraoperative motion. The cup positions outside Lewinnek’s safe zone were compared between the smartphone-assisted technique and the conventional method that uses a mechanical alignment guide. Methods: A randomised controlled trial was conducted among 64 patients receiving primary cementless total hip arthroplasties (THAs) via a posterolateral approach from June 2015 to February 2017. Acetabular components were implanted either by conventional technique ( n = 32) or using smartphone technique ( n = 32). Inclination and anteversion angles were measured in supine pelvic radiographs. Results: The inclination angle in the smartphone group was not significantly different from the conventional group (41.2° ± 3.9° vs. 40.3° ± 7.9°, p = 0.567). The anteversion angle was also similar (19.3 ± 3.8° vs 19.1° ± 5.9°, p = 0.856). However, the standard deviation of the angle in the smartphone group was significantly lower for inclination ( p < 0.001) and anteversion ( p = 0.016). There were 3 outliers (9.4%) in the smartphone group, but 13 (40.6%) in the conventional group ( p = 0.008). The risk ratio was 0.23 (95% CI, 0.07–0.73). The risk difference was −0.31 (95% CI, –0.51– –0.11). Conclusion: The smartphone-assisted technique in THA improves the precision of cup placement and decreases the percentage of safe zone outliers. Thai Clinical Trials Registry (ID: TCTR20151123002).