The Ultrasound-Assisted Patellar Glide Test: A Novel Examination Method for Quantifying Patellar Instability

Patellofemoral instability is a difficult problem to assess because of its dynamic nature, which is not easily quantified using physical examination techniques. This study aimed to describe and evaluate a novel examination method using stress ultrasonography to quantify patellar instability. The secondary aim was to assess the relationship of stress ultrasonography measurements with clinical and morphologic risk factors for patellar instability.

Knees with symptomatic patellar instability underwent the ultrasound-assisted patellar glide test. In this test, the patella was translated laterally from its resting position during ultrasound visualization until an endpoint was reached. The medial patellofemoral distance (MPFD) was used to quantify the gap between the medial boundary of the patella and the trochlea, and the difference in MPFD between the resting (unloaded) and loaded conditions was defined as the delta MPFD. Measurements were compared with those in asymptomatic contralateral knees of patients with unilateral instability, asymptomatic knees after patellar stabilization surgery, and control knees. Regression analyses assessed for relationships of MPFD with morphological and clinical risk factors. Receiver operating characteristic (ROC) curve analysis assessed the ability of stress ultrasonography measurements to discriminate between knees with and without symptomatic patellar instability.

Four hundred and seventy-seven knees in 277 patients were included in this study; 173 of the knees had patellar instability (patient age, 24 ± 8 years; 72% female). Delta MPFD was 3 times greater in knees with patellar instability (median, 3.3 mm [95% confidence interval, 3.1 to 3.4 mm]) than in the contralateral asymptomatic (1.1 mm [0.9 to 1.3 mm]), postoperative (1.0 mm [0.8 to 1.2 mm]), and control knees (1.4 mm [1.1 to 1.6 mm]). ROC analysis demonstrated an optimal threshold value for delta MPFD of 2.0 mm, which had an area under the curve (AUC) of 0.97 (0.94 to 0.99), demonstrating excellent discrimination in identifying knees with patellar instability. No relationships of clinical or morphologic risk factors with delta MPFD were found.

A delta MPFD of ≥2 mm on the ultrasound-assisted patellar glide test had an AUC of 0.97 for identifying knees with symptomatic patellar instability, indicating excellent discriminatory ability. Additional studies utilizing this method are recommended to standardize and quantify assessments of patellar instability.

Diagnostic Level III. See Instructions for Authors for a complete description of levels of evidence.