As the knee bends, the patella engages with the groove and is contained by its steep walls.
Towards the top of the trochlear groove the walls are flatter. When the knee is in extension (straight) and the patella in the upper part of the groove, it is more easily able to disengage (sublux).
An abnormal trochlear groove
Sometimes the groove is abnormal - trochlear dysplasia - and this may result in patellar instability.
The sulcus line of the trochlear groove is more accurate than Whiteside’s Line in determining femoral component rotation. Talbot S, Dimitriou P, Radic R, Zordan R and Bartlett J. Knee Surg Sports Traumatol Arthrosc. 2015; 23(11): 3306–3316.