Acute patellar dislocation remains a common injury in both adult and pediatric patients. Non-operative management has been advocated for patients without a history of recurrent instability. Although pathologic thresholds for consideration of operative management have previously been reported in adults, it is largely unknown in children. A retrospective review of all skeletally immature patients diagnosed with acute lateral patellar dislocation who had MRI imaging were included for analysis. An age-based control group was also identified. Six radiographic measurements were compared: lateral trochlear inclination (LTI), trochlear facet asymmetry (TFA), trochlear depth (TD), tibial tuberosity–trochlear groove (TT–TG), sulcus angle (SA) and patellar height ratio. A total of 178 patients were included for analysis (study: n
= 108, control: n
= 70). The mean age of patients in the study and control groups was 13.7 and 12.1 years respectively (p
≤ 0.001). Study group patients had significant differences in all radiographic measurements including a decreased LTI (p
< 0.001), increased TFA (p
< 0.001) and SA (p
< 0.001). The mean trochlear depth was 3.4 mm and 5.6 mm for patients in the study and control groups respectively (p
< 0.001). Study group patients had an increased patellar height ratio (p
< 0.001) and TT–TG distance (p
< 0.001). Morphologic abnormalities may predispose skeletally immature patients to an increased risk of acute lateral patellar instability.
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