Fig. 3.

The measurements of the tibial tubercle-trochlear groove (TT-TG) distance. a The prevailing “two-slice measurement” method. The levels of the tibial tubercle and the trochlear groove are superimposed onto one image; the TT-TG distance (l) is measured as the distance between the two perpendiculars to the tangent of the posterior femoral condyles which also pass through the tip of the tibial tubercle and the deepest point of the trochlear groove, respectively. b, c The vector calculation used in the current study. The tip of the tibial tubercle was termed T, the bottom of the trochlear groove was termed G, and the points of tangency of the posterior femoral condyles were termed M and N. The TT-TG distance can be calculated upon the vectors of $\overrightarrow{GT}$ and $\overrightarrow{MN}$, by $\left|\frac{\overrightarrow{GT}\cdot \overrightarrow{MN}}{\left|\overrightarrow{MN}\right|}\right|$