Patellar translation (TT-TG distance) is an analysis performed by overlapping or superimposing axial images of the femoral condyles and tibial tuberosity. The measurements are used to quantify patellar instability.
Superimpose axial images of:
- femoral condyles
- tibial tuberosity
Draw a line along the posterior femoral condyles, and then draw the following lines perpendicular to this line:
- bisecting the tibial tuberosity (TT)
- bisecting the trochlear groove sulcus (TG)
- measure the distance between TT and TG = TT-TG distance
A distance of <15 mm is considered normal, 15-20 mm is considered borderline and a distance >20 mm is considered abnormal. The distance has been shown to be slightly less when measured on MRI than CT 1.