Pediatr Radiol (2006) 36: 1163–1170 DOI 10.1007/s00247-006-0293-0 ORIGINAL ARTICLE Ali Zaidi . Paul Babyn . Ivan Astori . Lawrence White . Andrea Doria . William Cole MRI of traumatic patellar dislocation in children Received: 23 March 2006 / Revised: 17 July 2006 / Accepted: 25 July 2006 / Published online: 12 September 2006 # Springer-Verlag 2006 Abstract Background: Traumatic patellar dislocations (TPD) are common injuries in children, and MRI is useful in evaluation of pediatric musculoskeletal injuries. How- ever, no pediatric studies on the MR features of TPD have been reported. Objective: To review the injuries after TPD in children. Materials and methods: Patients with clinical or radiological recognition of TPD and those with suggestive MR findings were selected. Bone, cartilage and soft-tissue injuries and patellofemoral relationships were assessed. Results: A total of 26 patients (age range 10– 18 years) were identified. The following injuries were seen: bone bruising of the inferomedial patella (81% of patients) and the lateral femoral condyle (81% of patients), cartilage injuries of the inferomedial patella (38% of patients) and the lateral femoral condyle (38% of patients), osteochondral fragments (42% of patients) and injuries of the medial patellar restraints (81% of patients). Conclusion: Pediatric manifestations of TPD seen on MRI are similar to those in adults. TPD is often occult in children. Early recognition of bone bruising of the patella and lateral femoral condyle, associated osteochondral injuries, and medial patellar stabilizer injury is important for timely diagnosis. Keywords MRI . Patellar . Dislocation . Injury . Child Introduction Acute patellar dislocations are common injuries in children, accounting for approximately 9–16% of acute knee trauma in young athletes with hemarthrosis [1–4]. In acute dislocation, there is complete, almost always lateral, displacement of the patella from the femoral trochlear groove. Spontaneous relocation occurs in the majority of patients and often the diagnosis is not recognized [1, 5, 6]. Clinical assessment and differentiation from other knee injuries is often difficult as extensive tenderness, hemar- throsis and pain are often present [7]. Indeed, it has been estimated that 50–75% of patellar dislocations are occult and not correctly diagnosed at the time of initial clinical and plain film evaluation [1, 8]. MR imaging is increasingly being applied to the evaluation of pediatric musculoskeletal injuries [9], including knee injuries, as it provides accurate and timely assessment of bone, cartilage, ligaments, menisci, and adjacent soft tissues [10–12]. MR imaging can detect occult traumatic patellar dislocation by the identification of a constellation of indirect signs of injury: hemarthrosis, osteochondral injury, bone bruising of the medial patellar facet and lateral femoral condyle, and soft-tissue damage to the medial retinaculum and medial patellofemoral ligament (MPFL) [1, 7]. In addition, abnormalities of patellar position such as lateral subluxation and lateral patellar tilt, and predisposing factors such as patella alta and shallow femoral trochlea might be present on MR imaging. However, the literature on MR characteristics of acute patellar dislocation in children has been limited to individual case reports and mixed adult and pediatric cohorts without highlighting specific pediatric findings [1, A. Zaidi (*) Diagnostic Imaging, The Hospital for Sick Children, 555 University Avenue, Toronto, ON M5G 1X8, Canada e-mail: a.zaidi@utoronto.ca Tel.: +1-416-8136026 Fax: +1-416-8137591 P. Babyn . A. Doria Diagnostic Imaging, The Hospital for Sick Children, Toronto, Canada I. Astori Paediatric Orthopaedic Surgery, Mater Children’ s Hospital, Brisbane, Australia L. White Diagnostic Imaging, Mount Sinai Hospital and the University Health Network, University of Toronto, Toronto, Canada W. Cole Orthopaedic Surgery, The Hospital for Sick Children, Toronto, Canada