The mechanism of primary patellar dislocation

BACKGROUND AND PURPOSE Several mechanisms are responsible for patellar dislocation. We investigated how the primary pathomechanism relates to patient characteristics and the outcome. METHODS 126 patients (81 females) with primary patellar dislocation reported the knee position before the episode, the movement during it, and whether the patella was locked in dislocation. The median age was 20 (9-47) years. The subjective outcome and Kujala, Hughston VAS, and Tegner scores were evaluated after an average of 7 years. RESULTS 102 patients moved to flexion during the dislocation, 98 from a straight start and 4 from a well-bent start. 10 extended the knee from a well-bent start; they were older (mean 25 vs. 19 years) and more often had low trauma energy (5/10 vs. 15/102) and a locked dislocation (10/10 vs. 50/102). 4 had a direct hit to the knee and 1 only rotated it while stretching. 24 of 60 patients with open growth line of the tibial tubercle and 43 of 66 with closed tubercle had locked primary dislocation (p = 0.005). 33% of girls, 52% of boys, 57% of women, and 71% of men had locked primary dislocation. There was no correlation between trauma mechanism and outcome. INTERPRETATION Movement to flexion occurred in 84% of primary patellar dislocations and movement to extension in 8%. Spontaneous patellar relocation is common in skeletally immature girls and locked dislocation is common in skeletally mature men.