Automatic extraction of 2D patellofemoral bone morphology and alignment measures from 3D models in a young adolescent population.

OBJECTIVE: To determine the concurrent validity of automatically extracted alignment variables and morphology measures and to establish reference values and evaluate sex-based differences among a young adolescent population. DESIGN: Data from 1912 participants who underwent knee MRI within a large prospective population cohort study were included. Patellar and femoral landmarks were automatically extracted from two previously created statistical shape models (SSMs). Landmarks were extracted manually in 2D, manually in 3D and using the automated method of landmark extraction in 3D. Using these landmarks, four patellofemoral alignment variables and thirteen morphology measures were calculated. Concurrent validity of alignment variables and morphology measures calculated using automatically extracted landmarks compared to manual annotation on MRI was tested using a two-way, mixed-effects model with multiple raters and absolute agreement ICC. Differences between boys and girls were determined using an independent samples two-tailed t-test. RESULTS: Two alignment variables and seven morphology measures demonstrated good validity with the automated SSM-based method, with an ICC>0.75 for patellar tilt angle (ICC=0.83), bisect offset (ICC=0.93), epicondylar width (ICC=0.95), patellar width (ICC=0.86), patellar length (ICC=0.83), femoral notch width (ICC=0.90), femoral notch depth (ICC=0.83) medial (ICC=0.94) and lateral condyle length to epicondylar width ratio (ICC=0.92). Reference values of the two alignment and seven morphology measures all differed between boys and girls. CONCLUSION: Alignment variables and morphology measures can be accurately determined in 3D. All alignment variables and morphology measures, with at least a good ICC, differed between boys and girls.