The sulcus angle in trochlear dysplasia
The sulcus angle measurement would be made in patients being investigated for the cause of instability of the patella.
"....a sulcus angle measuring more than 145° in a Merchant view knee radiograph with the knee in 45° knee flexion and the beam at 30° inclination caudal down the patellar femoral joint is characteristic of trochlear dysplasia"