Novel MRI Measurements of Extensor Mechanism Containment Can Predict Recurrent Patellar Instability

OBJECTIVES: Identifying risk factors for recurrent patellar dislocation (PD) following the first dislocation may help guide treatment. Tools for assessing the relationship of the extensor mechanism can distinguish dislocators from regular patients, but their usefulness in predicting the risk of re-dislocation has not been assessed. The purpose of this study was to determine the association of several novel MRI measures of patellar containment with recurrence in patients presenting with a first-time PD. METHODS: An IRB approved query was conducted at a tertiary care children’s hospital (2005-2014) for patients (aged 8-19 years) diagnosed with a first-time PD. Patients were classified as either non-recurrent or recurrent-dislocators based on the review of the medical record. Next, MRI measurements were made by two independent raters. Interobserver reliability was assessed for all measurements via an intraclass correlation coefficient (ICC). Only measurements with ICC > 0.8 were included for further analysis. Univariate and multivariate regression analysis was used to evaluate variables associated with recurrence. RESULTS: One hundred sixty-five patients were identified with a median follow up the length of 12.2 months. Median patients age was 14 years, and 57.6% were females. A second instability event or significant subluxation episode was documented in 98 (59.4%) patients. MRI measurements with excellent correlation (ICC>0.8) were Tibial tuberosity to trochlear groove distance (TT-TG), Tangential axial width of the patella (TAWP), Tangential axial trochlear width (TATW), Axial width of patellar tendon beyond lateral trochlear ridge (PT-LTR), and Lateral trochlear ridge to tibial tuberosity distance (LTR-TT). In univariate analysis, all mentioned MRI measurements had significant differences when comparing the group of RD to those that did not have a recurrent instability event. However, following both backward and forward stepwise regression analysis, LTR-TT was the only independent predictor of RD (p=0.003 in both). Patients with an LTR-TT value greater than -1 had a significantly higher rate of recurrent dislocation (72%) with Odds Ratio 2.4 (95% C.I. 1.2-4.7); p=0.011). CONCLUSION: Assessment of the relationship of the lateral trochlear ridge to the tibial tubercle had greater predictive value for recurrent instability than the relationship of the tibial tubercle to the trochlear groove. Patients with a tibial tubercle even with or outside of the lateral trochlear ridge have a significantly increased risk of re-dislocation compared with patients with a tibial tubercle inside the lateral trochlear ridge. This novel measurement (LTR-TT) can help predict recurrent patellar instability and may thus aid surgeons and patients when deciding on a treatment strategy.