Triangular Index Ratio as An Alternative Method to the Alpha Angle for Defining the Presence of Cam Morphology: A Prospective Cohort Study

OBJECTIVE: Cam morphology, a significant risk factor for hip osteoarthritis, is commonly quantified by the alpha angle (AA). This study aims to explore the potential of the triangular index ratio (TIR) to quantify cam morphology on anteroposterior radiographs by assessing the association between TIR-defined cam morphology and the development of radiographic hip osteoarthritis (RHOA) at 10-year follow-up. Additionally, to compare the predictive performance of cam morphology as defined by the TIR versus AA for RHOA development. METHODS: The Cohort Hip and Cohort Knee (CHECK) study included 1,002 participants recruited from the Dutch population aged 45-65 years who presented with first onset hip or knee pain or both to the general practitioner. The associations between cam morphology hips free of RHOA at baseline (Kellgren & Lawrence (KL) grade <2) and the development of RHOA (KL ≥2) were estimated using logistic regression adjusted for age, sex, and body mass index. Sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of the two measurements (TIR ≥1.05 and AA ≥ 60°) for predicting RHOA were evaluated respectively. RESULTS: TIR-defined cam morphology was associated with the development of RHOA at follow-up with an adjusted Odds Ratio of 2.48 (95% CI 1.38-4.46). The PPVs of cam morphology defined by TIR (70.6%, 95% CI 59.0-80.0%) and AA (62.3%, 95% CI 53.8-70.1%) overlapped, as did their sensitivity (TIR: 7.8%, 95% CI 5.8-10.2%; AA: 12.3%, 95%CI 9.8-15.2%). CONCLUSIONS: Cam morphology defined by a TIR ≥1.05 at baseline is associated with the development of RHOA 10 years later. TIR may offer a definition of cam morphology more closely related to RHOA while AA may avoid losing too many target cases. Both methods can therefore be used to define cam morphology.