Medial Patellofemoral Ligament Injuries in Children with First-Time Lateral Patellar Dislocations

Background: A lateral patellar dislocation (LPD) is the most common knee injury in children with traumatic knee hemarthrosis. The medial patellofemoral ligament (MPFL), the important passive stabilizer against LPDs, is injured in more than 90% of cases. The MPFL injury pattern is most often defined in adults or in mixed-age populations. The injury pattern in the skeletally immature patient may be different. Purpose: To describe MPFL injuries in the skeletally immature patient by magnetic resonance imaging (MRI), and to compare the results with the injury pattern found at arthroscopic surgery. Study Design: Case series; Level of evidence, 4. Methods: This was a prospective series of patients aged 9 to 14 years with acute, first-time traumatic LPDs in whom clinical examinations, radiographs, MRI, and arthroscopic surgery were performed within 2 weeks from the index injury. The MPFL injury was divided into 3 different groups according to the location: patellar site, femoral site, or multifocal. The MPFL injury site was confirmed on MRI by soft tissue edema. The length of the MPFL injury at the patellar site was measured at arthroscopic surgery, and those ≥2 cm were defined as total ruptures. Results: A total of 74 patients (40 girls and 34 boys; mean age, 13.1 years) were included; 73 patients (99%) had an MPFL injury according to MRI and arthroscopic surgery. The MRI scans showed an isolated MPFL injury at the patellar attachment site in 44 of 74 patients (60%), a multifocal injury in 26 patients (35%), an injury at the femoral site in 3 patients (4%), and no injury in 1 patient (1%). Arthroscopic surgery disclosed an isolated MPFL injury at the patellar site in 60 of 74 patients (81%) and a multifocal injury in 13 patients (18%); the MPFL injury at the patellar site was a total rupture in 49 patients (66%). Edema at the patellar attachment site on MRI was proven to be an MPFL rupture at the same site at arthroscopic surgery in 99% of the patients. A patellar-based injury, isolated or as part of a multifocal injury, was present on MRI in 95% (n = 70) of the patients, with a false-negative rate of 5% (n = 4) of patients compared with arthroscopic surgery. Conclusion: Skeletally immature children are more prone to sustaining an MPFL injury at the patellar attachment site. Arthroscopic surgery and MRI complement each other in the investigation of MPFL injuries.