Patellar tracking should be taken into account when measuring radiographic parameters for recurrent patellar instability

Purpose To date, many radiographic parameters on patellar instability have their measurements taken statically, and have not been studied in various degrees of flexion according to the patellar tracking. There are also limited data regarding the use of these parameters in predicting recurrent patellar dislocation. The current study aims to review the radiographic parameters of the patellofemoral joint in different degrees of knee flexion and to correlate them with the presence of recurrent instability. Methods A 10-year retrospective study was conducted on all patients who had computed tomography patellar-tracking scan done for patellar instability when aged 18 years or younger. The computed tomography patellar-tracking scans were performed with the knee in extension, 10° flexion, and 20° flexion. The axial radiographic parameters were evaluated at the patellar equator, roman arch, and distal patellar pole. Sagittal and coronal parameters were noted. Radiographic parameters were then correlated with recurrent patellar instability. Results The femoral sulcus angle and trochlear groove depth at the distal patellar pole in 10° knee flexion ( p value 0.04 and 0.03, respectively) and patellar equator in 20° knee flexion ( p value 0.02 and 0.03, respectively) had the most significant clinical correlations with recurrent instability on multivariate analysis. Other radiographic parameters found to have significant clinical correlation on univariate analysis include the patellar tilt angle, congruence angle, femoral sulcus angle, trochlear groove depth, and Wiberg’s classification. Conclusions As per the knee dynamics, axial radiographic parameters had the most significant correlation with recurrent patellar instability when measured at the distal patellar pole in 10° knee flexion and at the patellar equator in 20° knee flexion. Future axial radiographic evaluation of patellofemoral instability should then be performed at these degrees of knee flexion and axial cuts. Trochlear dysplasia, as measured by the femoral sulcus angle and trochlear groove depth, was the most significant predictor of recurrent patellar instability in the skeletally immature. Wiberg’s classification was also a novel factor found to have clinical correlation with patellofemoral instability. Level of evidence III.