The influence of reconstruction of the center of rotation on total hip arthroplasty instability

Introduction: The aim of this study is to determine the influence of the center of rotation (CoR) and offset reconstruction in stable and unstable THAs. Material and methods: A prospective matched case-control study included patients who underwent primary THA and had a dislocation treated with closed reduction within two years. Cases were matched 1:2 by age, gender, and ASA classification to stable THA patients with >2 years follow-up. THAs performed for indications other than hip osteoarthritis or with unknown surgical approach/implants were excluded. Coronal CoR reconstruction (vertical and horizontal shifts) and offset were measured on calibrated pre- and postoperative pelvic radiographs. Logistic regression was used to calculate Odds Ratios (OR), 95% Confidence Intervals (CI), and p-values. Confounders altering regression coefficients were adjusted for, yielding adjusted ORs (aOR). Results: 25 unstable THAs were matched to 50 controls. The maximum distance from the preoperative femoral CoR is 6.71 mm in unstable THA and 7.38 mm in stable THA. The maximum distance from the preoperative acetabular CoR is 6.85 mm in unstable THA and 7.22 mm in stable THA. No statistically significant differences between the unstable and stable THA were found for either the shifting of horizontal/vertical CoR and the femoral/total offset. Logistic regression for CoR shifting were not statistically significant, aORs were non-significant as well. Conclusion: No differences in CoR or femoral and total offset reconstruction were found between unstable and stable primary THAs.