Utility of Diagnostic Ultrasound in the Assessment of Patellar Instability

Abstract

Background: The use of imaging to diagnose patellofemoral instability is often limited by the inability to dynamically load the joint during assessment. Therefore, the diagnosis is typically based on physical examination using the glide test to assess and quantify lateral patellar translation. However, precise quantification with this technique remains difficult.

Purpose: To quantify patellar position using ultrasound imaging under dynamic loading conditions to distinguish between knees with and without medial patellofemoral complex (MPFC) injury.

Study design: Controlled laboratory study.

Methods: In 10 cadaveric knees, the medial patellofemoral distance was measured to quantify patellar position from 0° to 40° of knee flexion at 10° increments. Knees were evaluated at each flexion angle under unloaded conditions and with 20 N of laterally directed force on the patella to mimic the glide test. Patellar position measurements were made on ultrasound images obtained before and after MPFC transection and compared for significant differences. To determine the ability of medial patellofemoral measurements to differentiate between MPFC-intact and MPFC-deficient states, area under the receiver operating characteristic (ROC) curve analysis and the Delong test were used. The optimal cutoff value to distinguish between the deficient and intact states was determined using the Youden J statistic.

Results: A significant increase in medial patellofemoral distance was observed in the MPFC-deficient state as compared with the intact state at all flexion angles (P = .005 to P < .001). When compared with the intact state, MPFC deficiency increased medial patellofemoral distance by 32.8% (6 mm) at 20° of knee flexion under 20-N load. Based on ROC analysis and the J statistic, the optimal threshold for identifying MPFC injury was 19.2 mm of medial patellofemoral distance at 20° of flexion under dynamic loading conditions (area under the ROC curve = 0.93, sensitivity = 77.8%, specificity = 100%, accuracy = 88.9%).

Conclusion: Using dynamic ultrasound assessment, we found that medial patellofemoral distance significantly increases with disruption of the MPFC.

Clinical relevance: Dynamic ultrasound measurements can be used to accurately detect the presence of complete MPFC injury.

Keywords: dynamic imaging; knee; medial patellofemoral complex; medial patellofemoral ligament; patellar dislocation; patellar instability; patellofemoral; stress imaging; ultrasound.

Figure 1.

Experimental setup demonstrating probe placement of the portable ultrasound on a left knee. (A) The probe is positioned at the medial patella at its widest portion and oriented parallel to the joint line to visualize the medial patellar facet and the medial trochlear facet on 1 image. Ultrasound images were obtained with and without 20 N of laterally directed force. (B) Setup shown from a different angle.

Figure 2.

Measurement on ultrasound images of the medial patellofemoral distance, defined as the distance from the apex of the medial trochlea (dashed yellow lines) to the most medial aspect of the patellar articular surface (asterisks). (A) The medial patellofemoral distance between the apex of the medial trochlea and the medial facet of the patella under 0-N force. (B) Increased medial patellofemoral distance under a standardized 20-N laterally directed force. MFC, medial femoral condyle.

Figure 3.

Ultrasound images demonstrating the medial patellofemoral distance between the apex of the medial trochlea (dashed yellow lines) and the medial facet of the patella (asterisks) in MPFC-intact and MPFC-deficient states at 20° of knee flexion under a 20-N load. Ultrasound images showing (A) the medial patellofemoral distance in the intact state and (B) the increased medial patellofemoral distance in the MPFC-deficient state. MFC, medial femoral condyle; MPFC, medial patellofemoral complex.