The J-sign and the body mass index determine the disease-specific quality of life in patients with lateral patellar instability.

To determine which risk factors for patellar instability contribute most relevantly to patients’ subjective disease-specific quality of life, aiming to provide implications on the overall treatment decision-making process. A total of 182 consecutive patients (male/female 70/112; mean age 23.6 ± 7.3 years) with a history of patellar instability were prospectively enrolled in this study. Patient age, body mass index (BMI), number of dislocations, reversed dynamic patellar apprehension test (ReDPAT), J-sign severity, and pathoanatomic risk factors of patellar instability were assessed. The statistical analysis evaluated the relationships among those variables and determined their ability to predict the Banff Patellofemoral Instability Instrument 2.0 (BPII 2.0) as a disease-specific quality of life measure. Using Spearman correlation, ANOVA and Fisher’s exact test, all variables with ANOVA p ≤ 0.1 or Spearman’s abs (rho) > 0.1 were entered into a multivariate linear model using backward-stepwise selection. Analysis of the individual variables’ ability to predict BPII 2.0 score values revealed ‘age’, ‘BMI’, ‘ReDPAT’, ‘high grade of trochlear dysplasia’, and ‘high-grade J-Sign’ as possible relevant factors. Backward-stepwise multivariate regression analysis yielded a final parsimonious model that included the factors ‘BMI’ and ‘J-Sign (Grade II and III)’ as the most relevant parameters influencing BPII 2.0 score values (adjusted R2 = 0.418; p < 0.001), with a cutoff value for BMI found at 28 kg/m2 (p = 0.01). The results of this study indicate that in patients with lateral patellar instability, a high-grade J-sign and an increased BMI significantly impact subjective disease-specific quality of life. Level IV.