Static patella tilt and axial engagement in knee extension are mainly influenced by knee torsion, the tibial tubercle-trochlear groove distance (TTTG), and trochlear dysplasia but not by femoral or tibial torsion.

The aim of this study was to investigate the association of femoral (FT), tibial (TT), and knee torsion (KT) on the patella tilt (PT), the axial engagement index (AEI), and the tibial tuberosity-trochlear groove distance (TTTG). Femoral torsion, tibial torsion, knee torsion, patella tilt, the axial engagement index, the TTTG, and trochlear dysplasia were retrospectively evaluated on 59 patients suffering from recurrent patella instability or anterior knee pain with 118 torsional lower limb magnetic resonance imaging studies. FT and TT did not show any significant associations with TTTG, PT, and AEI (n.s.). KT was significantly associated with a higher TTTG, higher PT, and lower AEI (all, p < 0.001). Higher grade trochlear dysplasia was associated with a higher PT and lower AEI (both, p < 0.001). The Dejour classification showed no significant association with FT, TT, KT, and TTTG (n.s.). All measurement parameters showed an excellent interrater agreement (ICC 0.89-0.97). Static patella tilt and patellofemoral axial engagement in knee extension are mainly influenced by knee torsion, TTTG, and trochlear dysplasia but not by femoral or tibial torsion. These findings help to understand the underlying reasons for the patella position in knee extensions in CT and MRI investigations in patients suffering from patella instability and patellofemoral pain syndrome. III.