Figure 2. First, the PTAA was determined by identifying the anteroposterior diameter of the tibia at 5cm distal to the tibial tubercle and 15cm distal to the joint line, and dividing these distances by two. Next, a line connecting these two half-way reference points was extended proximally to the level of the tibial plateau, then posteriorly at a 90-degree angle. Secondly, a line connecting the highest-most points of the anterior medial plateau and posterior medial plateau was drawn. The posterior tibial slope was then measured, and represented by the “theta” in the above image.