Magnetic Resonance Imaging Findings After Acute Patellar Dislocation in Children.

BACKGROUND: Acute patellar dislocation (APD) is a common knee injury in children. The pattern and frequency of injury to the medial patellofemoral ligament (MPFL) is different in pediatric compared with adult populations.

PURPOSE: To report on injury patterns sustained to the MPFL after APD in children and to determine whether predisposing factors for APD cited in adults hold true in the pediatric population.

STUDY DESIGN: Case series; Level of evidence, 4.

METHODS: Magnetic resonance imaging (MRI) studies were reviewed for 36 children sustaining APD. Evidence of injury to the MPFL was documented, and when the MPFL was torn, the location of tear was determined. Presence of trochlear dysplasia, patella alta, tibial tubercle-trochlear groove (TTTG) distance, and thickness of the lateral patellofemoral retinaculum (LPR) were recorded and correlated with MPFL tear.

RESULTS: Of the 36 patients sustaining APD, only 16 tore the MPFL. The location of MPFL tear was equally divided between the origin, the insertion, or both, with no case of midsubstance tear. There was a significant correlation identified between MPFL rupture and both LPR thickness greater than 3 mm and TTTG distance greater than 19 mm.

CONCLUSION: The MPFL does not always tear in children who sustain APD, and the tear location is variable. A thickened LPR and increased TTTG distance predispose to MPFL tear.