Background Navigation systems are an effective tool to improve the installation accuracy of the cup in primary total hip arthroplasty. This study aimed to evaluate the efficacy of a computed tomography-based navigation system in achieving optimal installation accuracy of implants in revision total hip arthroplasty and to clarify the usefulness of the navigation system. Methods We conducted a retrospective study of 23 hips in 23 patients who underwent revision total hip arthroplasty using a computed tomography-based navigation system; the control group comprised 33 hips in 33 patients who underwent revision total hip arthroplasty without a navigation system. Results The average cup position with the navigation system was 40.0° ± 3.7° in radiographic abduction angle, 18.8° ± 4.8° in radiographic anteversion, and 41.2° ± 8.9° in combined anteversion; without the navigation system, the average cup position was 38.7° ± 6.1°, 19.0° ± 9.1°, and 33.6° ± 20.5°, respectively. The achievement rate of cup positioning within the Lewinnek safe zone was not significantly different between the navigation group (82.6%) and control group (63.6%). In contrast, the achievement rate of cup positioning within the Widmer combined anteversion guidelines was significantly greater in the navigation group (78.3%) than in the control group (48.0%, p = 0.029). Furthermore, outlier cases in the navigation group had a smaller variance of deviation from the optimal cup position than those in the control group did. Conclusions The results show that the use of navigation for revision total hip arthroplasty improved cup positioning and reduced the range of outliers. Improvement of cup placement accuracy influenced the installation of the stem and also improved the achievement rate of combined anteversion. Thus, a computed tomography-based navigation system is very useful for surgeons when placing the cup within the target angle in revision total hip arthroplasty.

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Unlike in cemented total hip arthroplasty (THA), several types of polyethylene liners can be used in cementless THA. Selection of the appropriate polyethylene liner affects the long-term stability after THA. 1,2) One of the main causes for revision surgery is postoperative dislocation, and the use of elevated polyethylene liners can effectively correct the inclination of metal cups, thereby preventing future dislocations. In cementless THA revision, there are some cases where the metal cup has no looseness, but the polyethylene liner is worn down and progressive osteolysis is seen. Significant malalignment of the metal cup requires removal of the metal cup, adjustment of the angle, and reinstallation. However, in cases of mild malalignment of the metal cup, the polyethylene insert may be changed from flat to elevated, with the metal cup preserved, to reduce polyethylene wear and prevent postoperative dislocation; this procedure is also less invasive for patients. 3-5) Nevertheless, no studies have compared flat and elevated-rim polyethylene liners in terms of stress distributions on the bearing surface of the liners. Our hypothesis was that the high stress area on the bearing surface would decrease when an elevated-rim polyethylene liner is used rather than a flat liner. By identifying the differences in stress distributions of the two types of liners, it may be possible to determine whether the elevated-rim liner is useful for reducing polyethylene wear. Furthermore, this could reveal other valuable practices for the use of elevated-rim liners.

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