Introduction
The patellar J sign is a clinical examination finding that involves increased lateralization of the patella during active knee extension. Increased severity of the patellar J sign has been reported to have high sensitivity for detecting the presence of patellar instability, yet the factors that contribute to this are not fully understood. Trochlear dysplasia is a known risk factor for patellar instability and refers to loss of the osteochondral restraint of the patellofemoral joint due to decreased depth of the groove. Recent studies have highlighted the role of trochleoplasty in addressing this abnormality, but quantitative descriptions of trochlear dysplasia have not been well described. Therefore, the purpose of this study was to determine the relationship between patellar J sign and abnormalities in trochlear morphology.
Methods
Patients with patellar instability were included in this study, with analysis of the contralateral (asymptomatic) knee to isolate the effects of morphological abnormalities without instability. Knees were imaged on dynamic CT during active knee extension and kinematic 3D models were created for analysis. Trochlear morphology was described in terms of depth (mm), width (% of femoral width), and medial-lateral position on the femur (with 100% indicating the most lateral boundary of the femur). Corresponding position of the patella was assessed at 10? intervals through the transepicondylar axis (with 90? being directly anterior) and described in terms of lateralization on the femur as well as position relative to the center of the trochlear groove. Univariate regression analysis was performed to determine the relationship of trochlear depth with trochlear width and position on the femur. Stepwise multiple regression analysis was performed to assess for relationships between trochlear morphology and measurements of patellar position.
Results
17 knees were included in this study (age 23.3 ± 7.5; 5M, 12F). With increasing dysplasia (decreased depth) of the trochlea, there was significantly decreased width of the trochlea (R=0.71, R2=0.51,p<0.001) and increased lateral position of the trochlea on the femur (R=0.47, R2=0.23,p<0.001). During active knee extension, patellar lateralization relative to the femur was found to correlate with increased lateralized position of the trochlea (R=0.42, R2=0.17, p=0.002). However, there was no significant relationship between trochlear morphology and patellar displacement from the trochlear center in this group of asymptomatic knees.
Conclusion
In knees with trochlear dysplasia without patellar instability, patellar lateralization in extension was found to correlate with the position of the trochlea on the femur rather than subluxation out of the trochlear groove. Furthermore, trochlear dysplasia was associated with decreased width and increased lateralized position on the femur. These findings may further our understanding of the role of trochlear dysplasia in surgical decision-making in the treatment of patellar instability.