Objective: Compare the acetabular component positioning and the operative time in two consecutive short-stem cementless THA series without and with using an imageless navigation.

Material and Method: The retrospective study consisted of 31 cases of short-stem cementless THA without navigation (NNAV) and 30 cases with navigation (NAV). CT scans were performed in all cases at two-month or later postoperatively. The abduction and anteversion angles measured on postoperative CT were compared between two groups using t-test. The percentage of cup placement (abduction, anteversion and combined) within the safe zone for each group was compared using Chi-square test at a 0.05 level of significance. The operative time was compared between two groups using t-test.

Results: The mean abduction was 43.97 (range, 33-52, SD 4.44) in NNAV group and 41.37 (range, 37-45, SD 2.01) in NAV group. This difference was significant (p = 0.004). The mean anteversion was 22.58 (range, 2-39, SD 10.68) in NNAV group and 13.57 (range, 7-18, SD 3.28) in NAV group. This difference was significant (p<0.001). According to the criteria of Lewinnek et al, 96.8% in NNAV group were placed within the safe zone for abduction, 51.6% for anteversion, and 48.4% for both abduction and anteversion. In NAV group, all 30 cups (100%) were placed within the safe zone for abduction, anteversion, and both. There were significant differences in the percentage of cup placement within the safe zone for anteversion (p<0.001), for both abduction and anteversion (p<0.001) but not significant for abduction (p = 0.32) between two groups. The mean operative time was 107.09 and 110.67 minutes for NNAV and NAV group respectively, this difference was not significant (p = 0.49).

Conclusion: The present study demonstrated a significant increase in the placement of acetabular cups within the safe zone using imageless navigation compared to freehand technique, especially at anteversion angle.

Keywords: Hip arthroplasty, Imageless navigation