Dislocation of the patella is when the kneecap (patella) comes right out of the groove in the thighbone (femoral groove) in which it normally rides.
Normal situation where the patella is well located in the underlying groove of the femur.
Patellar dislocation, where the patella has jumped completely out of the groove.
What allows the kneecap to dislocate?
Dislocation may follow a traumatic event, like a blow on the knee. Frequently, however, there is an underlying anatomical problem that predisposes the knee to patellar instability. In either case it is common to tear the medial patellofemoral ligament (MPFL) which then makes it easier for the patella to dislocate again. Most early surgery would be focused on addressing this problem and reconstructing the ligament. We have a Q&A looking into all the issues in more detail.
Primary traumatic patellar dislocation. Tsai C-H, Hsu C-J, Hung C-H and Hsu H-C. J Orthop Surg Res. 2012; 7: 21.
Current Concepts for Patellar Dislocation. Petri M, Ettinger M, Stuebig T, Brand S, Krettek C, Jagodzinski M and Omar M. Arch Trauma Res. 2015 Sep; 4(3): e29301.