Magnetic resonance imaging of acute patellar dislocation in children:  patterns of injury and risk factors for recurrence.
BACKGROUND
Acute patellar dislocation is a common traumatic condition of the knee seen in the active adolescent. The patterns of injury to the ligamentous and chondral surfaces following dislocation have not been well defined in the pediatric population. The purpose of this study was to characterize the patterns of medial patellofemoral ligament (MPFL), vastus medialis obliqus (VMO), and osteochondral injury on magnetic resonance imaging (MRI) following first-time acute lateral patellar dislocation in pediatric patients.


METHODS
Following approval by the Institutional Review Board, a radiology query was performed to identify all patients between the ages of 11 and 18 years who underwent MRI following an acute first-time patellar dislocation over a 10-year period. The presence and location of injury to the MPFL, VMO, and chondral surfaces were evaluated on MRI images. A retrospective review of the patient's chart was conducted to confirm that clinical history was consistent with an acute patellar dislocation. Demographic data, including age at the time of injury, sex, knee affected, mechanism of injury, and recurrence of dislocation, were retrieved from the chart. The data were analyzed as a single cohort.


RESULTS
One hundred and eleven patients, including 56 males and 46 females with a mean age of 14.9 years (range, 11 to 18 y), were included. MRI demonstrated MPFL injury in 87 patients (78.4%). MPFL injury was present at an isolated patellar insertion in 34 patients (31%) and an isolated femoral insertion in 16 patients (14%). MPFL injury at more than one location was present in 37 patients (33%). VMO edema was present in 62 patients (56%), consistent with sprain or tear. Osteochondral fracture was identified in 38 knees (34%), with 25 from the medial patellar facet, 5 from the lateral femoral condyle, and 8 from both locations.


CONCLUSIONS
Acute patellar dislocations remain a common injury in pediatric patients. The pattern of injury to the MPFL and VMO on MRI has not been described in a pediatric population. The triad of injury to the MPFL, VMO, and chondral surfaces should be recognized and understood, particularly when surgical reconstruction is necessary.


METHODS
Level III.
Department of Orthopaedic Surgery, University of Michigan, Ann Arbor, MI, USA.
Mark
M
Seeley
Karl F
KF
Bowman
Christopher
C
Walsh
Brian J
BJ
Sabb
Kelly L
KL
Vanderhave
D000293:Adolescent; D002648:Child; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000473:pathology*; D007719:Knee Joint / Q000473:pathology; D008023:Ligaments, Articular / Q000293:injuries* / Q000473:pathology; D008279:Magnetic Resonance Imaging / Q000379:methods*; D008297:Male; D010329:Patella / Q000293:injuries* / Q000473:pathology; D031222:Patellar Dislocation / Q000175:diagnosis*; D012008:Recurrence; D012307:Risk Factors