Morphology and Anatomic Patellar Instability Risk Factors in First-Time Traumatic Lateral Patellar Dislocations: A Prospective Magnetic Resonance Imaging Study in Skeletally Immature Children.
The incidence of primary lateral patellar dislocation (LPD) in children aged 9 to 14 years is 0.6 to 1.2 per 1000. Causation is assumed to be multifactorial, including anatomic variants of the patellofemoral (PF) joint that result in a higher risk of LPD. No publication has compared the morphology of the PF joint and anatomic patellar instability risk factors (APIFs) in a primary LPD population versus controls, defining children by skeletal maturity.



To characterize the PF morphology and APIFs (trochlear dysplasia, abnormal lateral patellar tilt, elevated tibial tubercle-trochlear groove [TT-TG] distance, patella alta) through magnetic resonance imaging (MRI) measurements in skeletally immature children with and without a primary LPD and to identify (potential) distinctive differences between these 2 groups.



Cross-sectional study; Level of evidence, 3.



A prospective series of 103 skeletally immature children aged 9 to 14 years with an MRI-confirmed primary LPD were matched with a control group of 69 children. The PF morphology and APIFs were assessed during a 2.5-year period with standardized MRI using sagittal and axial views.



In the LPD group, 79% had 2 to 4 APIFs compared with 7% in the control group. All major measurements of trochlear dysplasia were significantly different between the 2 groups. The mean central condylar height was significantly higher in the LPD group compared with the control group, resulting in a lower trochlear depth (2.3 vs 4.5 mm, respectively) and higher sulcus angle (156.7° vs 141.1°, respectively). The LPD group had significantly higher values of patellar height, Caton-Deschamps index (1.33 vs 1.15, respectively), lateral patellar tilt (21.1° vs 8.5°, respectively), and TT-TG distance (13.9 vs 9.8 mm, respectively) compared with the control group. The main divergent APIF was trochlear dysplasia (defined as trochlear depth <3 mm), seen in 74% of the LPD group compared with 4% of the control group. Elevated TT-TG distance as a single APIF was never present in the LPD group; patellar tilt was only seen in the LPD group. The most common APIF in the control group was patella alta (36%).



There was a significant difference in mean values of all established APIFs between the children with a first-time LPD and the controls. Trochlear dysplasia was the main APIF, and together with lateral patellar tilt (≥20°), they had the strongest association with LPD.
Department of Women's and Children's Health, Karolinska Institutet, Stockholm, Sweden.|Section of Pediatric Orthopaedic Surgery, Karolinska University Hospital Solna, Stockholm, Sweden.
Marie
M
Askenberger
Department of Women's and Children's Health, Karolinska Institutet, Stockholm, Sweden.|Capio Artro Clinic, Stockholm, Sweden.
Per-Mats
PM
Janarv
Department of Women's and Children's Health, Karolinska Institutet, Stockholm, Sweden.|Section of Pediatric Radiology, Karolinska University Hospital Solna, Stockholm, Sweden.
Throstur
T
Finnbogason
Department of Orthopaedic Surgery, University of Minnesota Twin Cities, Minneapolis, Minnesota, USA.
Elizabeth A
EA
Arendt
D000293:Adolescent; D002648:Child; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000000981:diagnostic imaging / Q000473:pathology*; D008279:Magnetic Resonance Imaging; D008297:Male; D031222:Patellar Dislocation / Q000000981:diagnostic imaging / Q000473:pathology*; D057071:Patellofemoral Joint / Q000033:anatomy & histology / Q000000981:diagnostic imaging / Q000473:pathology*; D011446:Prospective Studies; D012307:Risk Factors; D013548:Sweden
children; knee; patellar dislocation; patellofemoral anatomy; pediatric sports medicine