Quantifying cup overhang after total hip arthroplasty. Standardized measurement using reformatted computed tomography and association of overhang distance with iliopsoas impingement

Currently, there is no standardized measurement method for evaluating cup overhang (CO) in patients undergoing total hip arthroplasty (THA). We propose a novel, standardized method of measuring overhang distance in patients following primary total hip arthroplasty (THA) on computed tomography (CT) images after multiplanar reformation and compare it to a previously proposed measurement method on sagittal CT images. This retrospective study included patients who underwent primary THA with an anterior approach. Patients with cup overhang (CO) and iliopsoas impingement (IPI) were identified by clinical and imaging data. Uncorrected overhang distance (OD${}_u$) was evaluated on orthogonal sagittal CT images while corrected overhang distance (OD${}_c$) was evaluated on reformatted sagittal CT images corrected for pelvic rotation and aligned with the plane of the cup face. Out of 220 patients with THA, 23 patients (10.4%) with CO and 16 patients (7.3%) with IPI were identified. CO was significantly associated to IPI (p < 0.001). The inter- and intrareader agreement was almost perfect for OD${}_c$ (κ = 0.822, κ = 0.850), whereas it was fair and moderate for OD${}_u$ (κ = 0.391, κ = 0.455), respectively. The discriminative ability of OD${}_c$ was excellent (area under the curve (AUC) = 0.909 (95% confidence interval (CI) 0.784-1.000)) in the receiver operating characteristic analysis. Conversely, AUC for OD${}_u$ was poor, measuring 0.677 (95% CI 0.433-0.921). We implemented a novel measurement method for CT images reformatted at the plane of the cup face to assess overhang distance in patients with CO following THA. While further validation is necessary, the proposed method is characterized by its high reproducibility and might be used to predict the occurrence of iliopsoas impingement in patients with cup overhang following total hip arthroplasty.