Introduction

Imaging of patellofemoral instability is limited by the inability to dynamically load the joint during assessment. The Glide test assesses lateral patellar translation during physical examination, however, precise quantification with this technique remains difficult. This study aimed to quantify patellar position using ultrasound imaging under dynamic loading conditions to distinguish between knees with and without injury to the medial patellofemoral complex (MPFC).

Methods

In 10 cadaveric knees, a portable ultrasound device was used to assess patellar position. The medial patellofemoral distance was measured to quantify patellar position from 0-40 degrees of knee flexion at 10-degree increments. Knees were evaluated at each flexion angle in unloaded and loaded conditions with 20N of laterally directed force on the patella. Images were obtained before and after transection of the MPFC, and measurements of patellar position were compared for significant differences. Area under the receiver operating characteristic (ROC) curve analysis and Delong test were used, and optimal cutoff values to distinguish between intact vs MPFC-deficient state was determined using Youden's J statistic.

Results

At all flexion angles, medial patellofemoral distance increased after the MPFC fibers were transected (p values .005 to <.001). The greatest increase in medial patellofemoral distance was by 32.8 percent (6mm) at 20 degrees of knee flexion under 20N load. The optimal threshold for identifying MPFC injury was 19.2 mm of medial patellofemoral distance at 20 degrees of knee flexion under 20N load (AUC 0.93; sensitivity 77.8%; specificity 100%; accuracy 88.9%).

Conclusion

Using dynamic ultrasonographic assessment, we found that medial patellofemoral distance significantly increases with disruption of the MPFC, and this measurement can be used to identify the presence of MPFC injury with high accuracy. As dynamic ultrasound allows for quantitative assessment of patellar instability in multiple clinical settings. Future studies are recommended to assess the utility of this measurement technique and thresholds in the diagnosis and treatment of patellar instability.