Differential effects of knee flexion on Caton-Deschamps, patellotrochlear and Insall-Salvati indices: A dynamic magnetic resonance imaging-controlled study.

Abstract

To evaluate the influence of knee flexion on patellar height measurements during active muscle movement using dynamic magnetic resonance imaging (MRI). Dynamic magnetic resonance (MR) images from 48 healthy participants were analyzed. Images were acquired over 30 s using a 3-Tesla MRI unit during active knee motion between 40° of flexion and full extension. Sagittal images were used to determine the femorotibial flexion angle (FTA), and patellar height was assessed at increasing flexion angles (5°-10° increments) using the Caton-Deschamps index (CDI), Insall-Salvati ratio (ISR) and patellotrochlear index (PTI). Measurements were performed independently by two raters on two separate occasions. Statistical analysis was performed using mixed-effects models and regression analyses. Eighty knees from 48 individuals (24 males, 24 females; age 26.7 ± 5.0 years; height 176 ± 9.5 cm) were included. Within a mean active range of motion from 4.3° ± 7.8° to 38.5° ± 10.4°, patellar height changed differentially across all parameters. CDI (mean in extension 1.19 ± 0.16, 95% confidence interval [CI]: 1.10-1.19) decreased by 0.004 per degree of flexion (<i>p</i> < 0.001), ISR (mean in extension 1.19 ± 0.16, 95% CI: 1.15-1.24) decreased by 0.001 per degree of flexion (<i>p</i> < 0.001), and PTI (mean in extension 0.26 ± 0.20) increased by 0.02 per degree of flexion (<i>p</i> < 0.001). Inter- and intra-observer reliability were excellent for CDI (ICC 0.91/0.86) and PTI (ICC 0.97/0.98), and good for ISR (ICC 0.76/0.75). Patellar height measurements strongly depend on knee position, particularly at low flexion angles. CDI and PTI demonstrated significant changes, whereas changes in ISR were minimal. Patellar height should therefore be interpreted with careful consideration of knee position and the specific index used. Level III, diagnostic study.