Summary

The utility of Examination under anesthesia (EUA) for patellar instability is such that a negative EUA or inability to dislocate patella under anesthesia cannot rule out patellar instability but a positive EUA or ability to dislocate the patella under anesthesia can confirm the diagnosis of patellar instability.

Abstract

Introduction

Examination under anesthesia (EUA) has been helpful for evaluation of stability of various joints, including elbow, hip, knee and shoulder. However, the utility of EUA as a diagnostic test for patellar instability has not been evaluated. The first purpose of our study was to evaluate the utility of patellar dislocation during EUA as a diagnostic test for assessment of patellar instability. The second purpose of our study was to evaluate demographic and clinical factors associated with the ability to dislocate patella under anesthesia.

Methods

A prospective, multicenter cohort study (JUPITER: Justifying Patellar Instability Treatment by Results) database was queried for patients who underwent a surgical intervention for unilateral patellofemoral instability from a period January 2017 through July 2022. All patients had at least one documented patellar dislocation prior to surgery. Demographic and clinical variables were collected including age, sex, BMI, number of dislocations prior to surgery, J-sign and Beighton score. Prior to surgery, all patients underwent EUA in supine position for the involved and the uninvolved knee. During EUA, a submaximal manual force was applied to dislocate the patella laterally with the knee at 0° and 30˚ flexion. If the patella could be completely dislocated, then the EUA was considered positive. Results were analyzed descriptively, compared between positive and negative EUA cohorts and using multiple logistic regression analysis.

Results

478 patients who underwent unilateral patellar stabilization formed the study cohort. The mean age of the cohort was 15.8 years. 287 were females. 137 patients underwent surgery after the first dislocation, while 340 had surgery for recurrent patellar instability. Mean BMI of the cohort was 24.4.
At 0° knee extension, the sensitivity and specificity of EUA as a diagnostic test for patellar instability were 53.4% and 93.3% respectively. The positive and negative predictive values were 90.7% and 61.9% respectively. At 30° knee flexion, the sensitivity and specificity of EUA for diagnosing patellar instability were 37.8% and 96.3%, respectively. The positive and negative predictive values were 92.5% and 55.9% respectively.
Compared to negative EUA, the cohort with positive EUA at 0˚ and 30° was older (p = 0.0312), had more females (p=0.0021), had >5 patellar dislocations (p = 0.0199), had severe J-sign (p = 0.0018) and a higher Beighton score (p = 0.007). There was no difference in BMI between the cohorts. Multiple regression analysis showed that older age and severe J-sign were factors that would increase the odds of positive EUA at 30˚ knee flexion. There were no factors associated with positive EUA for contralateral knee.

Conclusions

About half (53.4%) of all knees with confirmed patellar instability had a dislocatable patella during EUA with knee in extension; this decreased to 37.8% with the knee at 30° flexion. Due to low sensitivity, patellar dislocation during EUA cannot be considered a diagnostic test for patellar instability or a prerequisite for surgical treatment. However, the specificity of 93.3% and 96.3% at 0˚ and 30˚, respectively, suggests that a positive EUA can help to confirm patellar instability.