Patients with first-time patellar dislocations with skeletal immaturity and MRI measurements of sulcus angle 154° or greater and a patellar height of 1.3 or greater had a high risk for lateral patellar redislocations, researchers suggest.

“The risk equation model created in this study adds to our knowledge of which patients are most likely to experience redislocation and how significant is the risk,” says Elizabeth A. Arendt, MD, in a media release from “Specifically, there is a high risk of lateral patellar redislocation when a patient presents with skeletal immaturity as well as trocheal dysplasia and patella alta, defined by MR measurements of sulcus angle 154°and patellar height as measured by Insall-Salvati ratio 1.3. A patient will have a low risk for lateral patellar redislocation with the inverse findings.”

In the study, published in the American Journal of Sports Medicine, Arendt and colleagues identified 145 patients with primary first-time lateral patellar dislocations, MRIs within 6 weeks, and a minimum 2-year follow-up. Investigators identified patients from a prospective study with a 2-year follow-up or from a prospectively identified cohort with retrospective chart review. Investigators obtained follow-up clinically or via email for patients who did not have a 2-year clinical follow-up.

Results showed 61 patients with primary lateral patellar dislocations of recurrent dislocation within 2 years.

Stepwise logistic regression analysis showed significant predictors of redislocations included skeletal immaturity, sulcus angle, and Insall-Salvati ratio. The cut-out points as defined by receiver operator characteristic curves were the sulcus angle 154° or greater and Insall-Salvati ratio of 1.3 or greater. The probability of redislocation was 5.8 with no factors present and 22.7% when one factor was present. The probability increased to 78.5% when all three factors were present, the release explains.