Triangular Index Ratio as an Alternative Method to the Alpha Angle for Defining the Presence of Cam Morphology: A...

Cam morphology, which is 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, this study compares the predictive performance of cam morphology as defined by the TIR with AA for RHOA development.

The Cohort Hip and Cohort Knee study included 1,002 participants recruited from the Dutch population aged 45 to 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 and 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 of the two measurements (TIR ≥1.05 and AA ≥ 60°) for predicting RHOA were evaluated, respectively.

TIR-defined cam morphology was associated with the development of RHOA at follow-up with an adjusted odds ratio of 2.48 (95% confidence interval [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).

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, whereas AA may avoid losing too many target cases. Both methods can therefore be used to define cam morphology.