The apTTTG distance decreases linearly with knee flexion: A dynamic CT-based analysis in patients with patellofemoral instability.

Abstract

The anterior-posterior tibial tubercle-trochlear groove (apTTTG) distance is a radiographic parameter describing the sagittal alignment of the patellofemoral joint and has been proposed as an indirect estimate of patellofemoral contact pressure, but its dynamic behaviour across different degrees of knee flexion remains unclear. This study aimed to evaluate flexion-dependent changes in apTTTG using dynamic kinematic computed tomography (DKCT) imaging and to compare its behaviour with other established patellofemoral alignment parameters. This study was designed as a retrospective analysis of prospectively enroled patients with unilateral patellofemoral instability who underwent bilateral DKCT imaging as part of their diagnostic evaluation. The apTTTG was measured as the antero-posterior distance from the deepest point of the trochlear groove to the central prominence of the tibial tubercle. This was measured at 0°, 30° and 60° of flexion. Changes in apTTTG were compared across flexion states using nonparametric statistics. Linear, quadratic and generalized additive models (GAMs) were used to characterize the relationship between apTTTG and knee flexion. Multivariable models included morphologic risk factors for patellar instability, including tibiofemoral rotation angle (TFRA), TT-TG, patellar tilt, bisect offset and sulcus angle. Forty-two knees from 21 patients (mean age 22.5 ± 7.2 years) were included. The apTTTG decreased from -3.8 ± 5.7 mm in extension to -8.9 ± 7.4 mm at 30° and -17.8 ± 8.4 mm at 60° of flexion (<i>p</i> < 0.001). Linear regression demonstrated a strong inverse association between knee flexion and apTTTG, corresponding to a reduction of 0.29 mm per degree of knee flexion (<i>β</i> = -0.29, <i>R</i> = 0.75, <i>p</i> < 0.001). In contrast, rotational alignment and patellar positioning parameters exhibited their most pronounced changes between extension and midflexion. No other anatomical parameter independently predicted apTTTG. The apTTTG distance decreases linearly with increasing knee flexion, whereas other patellofemoral alignment parameters change predominantly between extension and midflexion. Level IV.