Patellar dislocations are common, particularly in adolescent females and athletes. Patients usually present with an inability to extend an obviously deformed knee. A sizable effusion may also be seen. This injury may be due to direct trauma to the patella or to a valgus stress combined with flexion and external rotation.
The reported incidence of patellar dislocation is 5.8 per 100,000, but it may be as high as 29 per 100,000 in the adolescent population.  There are several varieties of patellar dislocation, as follows:
Lateral - The most common type of patellar dislocation
Horizontal - A rare occurrence, in which the patella has rotated on its horizontal axis with the articular surfaces facing either proximally or distally
Vertical - Also a very uncommon event, in which the patella rotates around its vertical axis with impaction of one of the lateral surfaces in the intercondylar notch of the femur
Intercondylar - Any type of dislocation in which the patella remains in its anatomic position and may be rotated around its vertical or horizontal axis
Several anatomic factors, including a lateralized tibial tubercle,  the tibial tuberosity–trochlear groove distance, [2, 3] the tibial tuberosity–posterior cruciate ligament distance,  the shape and dimensions of the patella, [5, 6] and the width of the patellar tendon,  may increase the likelihood of lateral patellar dislocation.
Reduction of a lateral dislocation of the patella is a simple and safe procedure. Otherwise, an orthopedist should be consulted for these more uncommon types of dislocations.
Nonsurgical reduction can be attempted on any lateral or medial dislocation of the patella. An immediate attempt at reduction should be made on any dislocation associated with vascular compromise of the distal extremity, though this is exceedingly uncommon in the setting of an isolated lateral patellar dislocation and should prompt further examination into possible concurrent injuries.
Some surgeons have advocated surgical rather than nonsurgical intervention to treat patellar dislocation, out of concern for possible recurrence. A Cochrane review by Smith et al found that although there was some evidence that appeared to favor surgical management of primary patellar dislocation, the quality of the currently available evidence was too poor to allow any firm conclusions to be made. 
In a systematic review that included 2134 primary acute patellar dislocations (2086 patients) treated either conservatively or surgically, Longo et al found that whereas surgical treatment was associated with a significantly lower rate of recurrence and better results in the short-to-medium term, the results of the two approaches were not significantly different in the long term. 
A patellar dislocation associated with a fracture of the proximal tibia or distal femur should not be reduced in this manner. Osteochondral fractures may occur in the setting of patellar dislocation. Caution should be used in evaluating for a fracture, either by exam or by radiographs, prior to attempting reduction. Any superior, intercondylar, or horizontal dislocation should be examined by an orthopedic surgeon. Any dislocation with suspected locked osteophyte should be examined by an orthopedic surgeon.