Traumatic patellar dislocation in children : epidemiology, risk factors, the MPFL and treatment outcome

The knee joint is one of the most common injury sites in children, with the spectrum of injuries differing from those in adults. Traumatic lateral patellar dislocation (LPD) is the most common serious injury; the incidence is approximately 1/1000 in children 9-15 years of age. The risk for recurrent dislocation is high in this age group. The most important static stabilizer for lateral patellar dislocation is the medial patellofemoral ligament (MPFL). The injury to the MPFL and the anatomic patellar instability risk factors for lateral patellar dislocation are best described in the adult population. There is no consensus about the best treatment for the first-time traumatic lateral patellar dislocation in children.

The aim of this thesis was firstly to describe the current spectrum of acute knee injuries presenting with knee hemarthrosis, with a detailed description of the most common injury, the traumatic lateral patellar dislocation. Secondly, the aim was to describe the patellofemoral joint morphology, the anatomic patellar instability risk factors and the medial patellofemoral ligament injury in the skeletally immature child. Thirdly, the aim was to evaluate if an acute refixation of the medial patellofemoral ligament injury vs. non-operative treatment for firsttime traumatic lateral patellar dislocation could reduce the high recurrence rate. All studies were prospective in design. Patients were skeletally immature, 9-14 years old, and previously had healthy knees before an acute knee trauma that caused hemarthrosis. They were following an algorithm to investigate the injury with standardized radiographs and MRI within two weeks from the index injury.

In Study I, 117 patients with acute knee trauma were examined. Seventy percent had a serious knee injury that needed specific medical care. Fifty-six percent of these patients had no visible injury on their radiographs. Lateral patellar dislocation (41%), ACL injury (12%) and anterior tibial spine injury (7%) were the most common injuries and the majority were sports-related. Study II included 74 patients with first-time traumatic LPD. The medial patellofemoral injury was evaluated by MRI and arthroscopy. An injury to the MPFL at the patellar attachment site was diagnosed in 99% of patients, either as an isolated injury at the patellar site or as part of a multi-focal injury. Study III consisted of 103 patients with firsttime traumatic LPD and a control group of 69 patients with acute knee trauma without LPD. The morphology of the patellofemoral joint and anatomic patellar instability risk factors were analyzed and the two groups were compared. Central condylar height was higher in the group with lateral patellar dislocation, resulting in lower trochlear depth and higher sulcus angles. There was a significant difference in mean values of all established anatomic patellar instability risk factors between children with first-time traumatic LPD and the control group. The main divergent anatomic patellar instability risk factor was trochlear dysplasia (defined as trochlear depth < 3mm), which was seen in 74% in the LPD group compared to 4% in the control group. Trochlear dysplasia, together with lateral patellar tilt (³ 20°), had the strongest association with LPD. The 74 patients from Study II were in Study IV randomized in a RCT to either non-operative treatment (soft knee brace and physiotherapy) or operative treatment (arthroscopic assisted medial patellofemoral ligament refixation, soft cast splint and physiotherapy). The follow-up time was two years; the main outcome was redislocation and evaluation of subjective and objective knee function. A refixation of the MPFL injury in the acute phase in skeletally immature children with first-time traumatic lateral patellar dislocation significantly reduced the redislocation rate but did not improve the subjective and objective knee function compared with non-operative treated patients. The majority of the patients were satisfied with their knee function.

List of scientific papers

I. Askenberger M, Ekström W, Finnbogason T, Janarv P-M. Occult Intraarticular Knee Injuries in Children with Hemarthrosis. The American Journal of Sports Medicine. Vol. 42, No. 7 2014.
https://doi.org/10.1177/0363546514529639

II. Askenberger M, Arendt E. A, Ekström W, Voss U, Finnbogason T, Janarv P-M. Medial Patellofemoral Ligament Injuries in Children With First-Time Lateral Patellar Dislocations. The American Journal of Sports Medicine. Vol. 44, No. 1. 2016.
https://doi.org/10.1177/0363546515611661

III. Askenberger M, Janarv P-M, Finnbogason T, Arendt E. A. Morphology and Anatomic Patellar Instability Risk Factors in First-Time Traumatic Lateral Patellar Dislocations. The American Journal of Sports Medicine. Vol. XX, No. X. 2016.
https://doi.org/10.1177/0363546516663498

IV. Askenberger M, Ekström W, Bengtsson-Moström E, Arendt E. A, Hellsten A, Mikkelsen C, Janarv P-M. Operative Versus Non-Operative treatment of Acute First-Time Traumatic Lateral Patellar Dislocation in Children- A Prospective Randomized Study. [Manuscript]