Background

Lateral patellar dislocation can become a recurrent problem after a first episode. Identifying patients who are at increased risk of redislocation might help with the treatment decision process.

Aim

To identify clinical and radiological risk factors for recurrent patellar dislocation after a first episode and to create a model that may be used to predict this risk.

Methods

An observational, retrospective case-control study was conducted including patients with lateral patellar dislocation and a 1-year minimum follow-up who were seen between 2011-2018. Subjects aged 10 years and older were included. Demographic characteristics and radiographs were reviewed. The Caton-Deschamps and Insall-Salvati indexes were used to evaluate patella alta. High-grade trochlear dysplasia was defined as type B or D in the Dejour classification.

Results

A total of 139 patients with primary lateral patellar dislocation were included. Eighty-three (59.71%) had recurrent dislocation. Stepwise logistic regression analysis demonstrated that age under 21 years old (OR, 2.53; 95% CI, 1.11-5.77; P = .027), a Caton-Deschamps ratio >1.15 (OR, 2.39; 95% CI, 1.09-5.22; P = .029) and high-grade trochlear dysplasia (OR, 4.17; 95% CI, 1.90-9.17; P < .001) were significant predictors of patellar redislocation. Based on the presence of these factors, the probability of dislocation after a first lateral patellar dislocation was 31.2 % with no factors present,36.6% with any one factor present, 71.7% with any two factors present, and 86.2% if all three conditions are present.

Conclusion

Patella alta, high-grade trochlear dysplasia and age younger than 21 years old are independent risk factors for patellar redislocation after a first episode, with an additive effect when they are present together. Surgical treatment could be considered when two or more factors are present.