Introducing the Reflection Index: A Novel Radiographic Assessment of Cam Morphology in Femoroacetabular Impingement and Predictor of Outcomes After Hip Arthroscopy With Minimum 2-Year Follow-up

BACKGROUND: Femoroacetabular impingement (FAI) is commonly treated with hip arthroscopy, and cam underresection is a common reason for failure. Cam resection is assessed intraoperatively using radiographic parameters. However, currently used parameters have had equivocal results predicting patient-reported outcomes (PROs) after hip arthroscopy for FAI. PURPOSE/HYPOTHESIS: The purpose of this study was to define a novel radiographic parameter, termed the "reflection index," which can be used to assess cam resection. The authors hypothesized that the postoperative reflection index would correlate with outcomes after hip arthroscopy. STUDY DESIGN: Case series; Level of evidence, 4. METHODS: Patients with symptomatic FAI undergoing primary hip arthroscopy between 2019 and 2021 were identified. Pre- and postoperative alpha angle, head-neck offset ratio (HNOR), and reflection index were measured. Reflection index was defined as a curvilinear area superior to the femoral neck divided by a curvilinear area inferior to the femoral neck on 45° Dunn lateral radiograph. A reflection index of 1 was defined as a perfect cam resection. PROs were obtained preoperatively and 2 years postoperatively, including modified Harris Hip Score (mHHS), PROMIS Global Physical Health, PROMIS Global Mental Health, and visual analog scale (VAS) for pain. Multivariate and logistic regression models were utilized to evaluate the predictive value of each radiographic measure on outcomes. RESULTS: = .038), respectively, for the transformed variable abs(log(reflection index)). A reflection index closer to 1 was an independent predictor of achieving the Patient Acceptable Symptom State and substantial clinical benefit for mHHS at 2 years. Alpha angle and HNOR had no association with postoperative mHHS, Global Physical Health, Global Mental Health, or VAS scores. CONCLUSION: The reflection index can be used to assess cam resection in hip arthroscopy. A postoperative reflection index closer to 1 was an independent predictor of significantly superior PROs at final follow-up. Alpha angle and HNOR failed to predict outcomes. Thus, the reflection index may be a useful radiographic assessment in hip arthroscopy to provide prognostic value postoperatively and potentially guide resection if future technology allows for intraoperative calculation.