Fig. 1

TT-TG distance is measured using axial slices. a First tangential line to the posterior condyles is drawn (Line 1). Next a line, perpendicular to Line 1, transecting the deepest point of the bony outline of the trochlear groove. This line is drawn to the most cranial slice with complete cartilaginous coverage of the trochlea (Line 2). b The most anterior point of the tibial tuberosity is defined as reference point. Keeping the cursor steadily in locations of the reference point, the image slab is then rolled back until the previously defined Line 2 is present. c The distance between the location of the cursor and Line 2 is measured in millimeters presenting the TT-TG distance, in this case 6 mm