One draws the line along of the wall of the trochlear groove underneath the cartilage layer. Then a line is drawn at the back of the two condyles - from it a parallel line can be extrapolated to meet the first one, to make it easier to see the angle. Here the angle is 22 degrees, which is normal.
Interpreting the lateral trochlear inclination angle (LTI)
If the LTI is 20-22 degrees the slope of the wall of the trochlea is considered normal. However, if it is below 11 degrees it is considered dysplastic. For some severely affected people it can be even zero or less. A flattened dysplastic trochlea makes it easier for a patella to sublux or to dislocate, and thus this angle is important to calculate when assessing the causes of such patellar instability.
Patellofemoral instability: classification and imaging Berruto M, Ferrua P, Carimati G, Uboldi F and Gala L. Joints. 2013 Apr-Jun; 1(2): 7–14.
The significance of magnetic resonance imaging in severe femoral trochlear dysplasia assessment. Shen J, Qin L, Yao W-W and Li M. Exp Ther Med. 2017 Dec; 14(6): 5438–5444.