Relationship Between the Accuracy of Acetabular Cup Angle and Body Mass Index in Posterolateral Total Hip Arthroplasty With CT-Based Navigation: A Retrospective Case-Control Study

Background: Precise acetabular cup placement is essential for successful total hip arthroplasty (THA). In obese patients, its accuracy is often difficult to achieve because of the thickness of the soft tissues. This study aimed to determine the relationship between the accuracy of acetabular cup angle and body mass index (BMI) in posterolateral THA using the computed tomography-based navigation (CT-navi) system.Methods: We retrospectively reviewed 145 consecutive primary THAs using the CT-navi system between January 2015 and January 2018. All surgeries were performed using cementless cups employing the posterolateral approach with the patient in the decubitus position. We compared the radiographic inclination and anteversion obtained from the angle displayed on the CT-navi screen with those measured by the postoperative CT using the three-dimensional templating software. We evaluated the relationship between the extent of errors and correlation with BMI. Statistical analyses were performed using the Student’s t-test and Spearman’s rank coefficient test.Results: In non-overweight patients (BMI < 25, 88 hips), the mean navigation errors for inclination were 2.8 ± 2.2° and for anteversion were 2.6 ± 2.3°. Meanwhile, in overweight patients (BMI ≥ 25, 57 hips), the mean navigation errors were 2.6 ± 2.4° for inclination and 2.4 ± 2.4° for anteversion. We found no significant difference between overweight and non-overweight patients in both inclination and anteversion. The Spearman’s rank correlation coefficients were -0.04 for inclination and -0.11 for anteversion, showing no correlation between the extent of errors and BMI.Conclusions: In posterolateral THA, CT-navi can aid the precise placement of the acetabular cup irrespective of a patient’s BMI.Trial registration: This trial was retrospectively registered and approved by the institutional ethics committee of Teikyo University. The registration number is 17-190, and the date of approval was March 1, 2018. URL of trial registry is: https://www.teikyo-u.ac.jp/application/files/7015/8432/1341/2016_all_syounin_1.pdf