A Novel Technique for Diagnosing Posterior Implant-Implant Impingement After Total Hip Arthroplasty

Introduction: – Implant-implant impingement in total hip arthroplasty remains a primarily clinical diagnosis, with little technology available to demonstrate impingement radiographically. The purpose of this study was to determine the effectiveness of external rotation stress CT scans in evaluating for posterior impingement in patients with painful total hip prostheses.Methods: – Sixty-seven patients presenting with CT IMA (implant movement analysis) scans previously used for evaluation of potential aseptic loosening were identified between May 2021 and May 2024. Plain radiographs were evaluated to assess for acetabular cup abduction. CT IMA scans in external rotation IMA protocol position were evaluated to measure acetabular implant version and the distance between the femoral and acetabular components. Patients were separated into impingement and nonimpingement groups based on whether a clinical or intraoperative diagnosis of impingement had been made. Cup version, abduction, and implant-implant distances on the external rotation CT were compared between the groups. A receiver operating characteristic curve was created, and the area under the curve was calculated to determine an optimal implant-implant distance for diagnosing impingement.Results: – Overall, 29 patients were diagnosed with impingement and 38 patients were diagnosed with other sources of hip pain. Linear regression showed a notable negative correlation (P = 0.004) of measured cup version versus implant-implant distance. Implant-implant distance was markedly smaller in the impingement group versus the non-impingement group (P < 0.0001). The receiver operating characteristic curve demonstrated 100% sensitivity and 89% specificity impingement when the implant-implant distance was <5 mm, with an area under the curve of 0.98 ± 0.02.Conclusion: – External rotation stress CT scans used by the IMA protocol were 100% sensitive and 89% specific in identifying patients with posterior impingement when an implant-implant distance of <5 mm was considered. This technology may help clinicians objectively define a diagnosis of impingement in total hip prostheses in the absence of other clinically identifiable sources of pain.