Adhesive capsulitis (AC) is a disease in which the range of motion (ROM) of the shoulder joint is limited due to chronic inflammation and fibrosis of the joint capsule. It is usually manifested as thickening of the glenohumeral ligament or axillary capsule, joint capsule contracture, and reduced joint volume. The axillary capsule is the most relaxed part of the joint capsule, allowing for sliding when the shoulder joint is abducted and rotated. When chronic inflammation and fibrosis of the joint capsule cause contracture and thickening, the abduction, external rotation (ER), and internal rotation (IR) angles of the shoulder joint will also be restricted. For AC patients, the current imaging examination methods mainly include ultrasound and magnetic resonance imaging. Relevant studies have shown that the normal thickness of the axillary sac under ultrasound is approximately 1.3-2.8 mm, and the threshold for diagnosing AC is about 3.2-3.34 mm. The normal axillary sac thickness under magnetic resonance imaging is approximately 2.9 mm, and the threshold for diagnosing AC is about 4.0-5.0 mm. Ultrasound has the advantages of being non-invasive, convenient, and real-time. It can also dynamically observe the morphological changes of the axillary sac during joint movement, making it more suitable for pre-treatment assessment and post-treatment follow-up of AC. Magnetic resonance imaging (MRI) can simultaneously assess lesions in the axillary sac, rotator cuff tendons, and surrounding soft tissues, making it more suitable for preoperative evaluation. However, to date, most scholars have focused mainly on the threshold for diagnosing AC, the therapeutic effect of AC treatment, and the consistency of axillary capsule measurement by ultrasound and MRI. However, few scholars have compared ultrasound-measured axillary capsule thickness with shoulder joint ROM.

To verify the correlation between axillary capsule thickness and physical examination angle, as well as the correlation between the measurement of axillary capsule thickness by ultrasound and magnetic resonance imaging.

In this study, 30 healthy volunteers were included for ultrasound and physical examination, and 36 patients diagnosed with unilateral AC in the outpatient department of our hospital were included for ultrasound, MRI, and physical examination. Personal information, axillary sac thickness, and physical examination angles (including abduction, external rotation, and internal rotation) were collected for correlation analysis and measurement consistency analysis.

Data show that, in healthy shoulder joints, there is no statistically significant difference in parameters such as age, BMI, and axillary sac thickness between healthy volunteers and patients; however, there is a statistically significant difference in internal rotation (P=0.042). The axillary sac on the affected side was significantly negatively correlated with external rotation (r=-0.360, P=0.031) and internal rotation (r=-0.497, P=0.002), while significantly positively correlated with height (r=0.429, P=0.009) and weight (r=0.361, P=0.030). After controlling for the healthy side, the difference in axillary capsule thickness was significantly negatively correlated with the difference in external rotation (r= -0.373, P = 0.025), and the correlation improved. It was significantly negatively correlated with the difference in internal rotation (r = -0.383, P = 0.021), and the correlation was weakened. In addition, this study found that the thickness of the axillary capsule measured by ultrasound (4.39±1.19) mm was significantly lower than that measured by MRI (9.08±2.03) mm, and there was a significant positive correlation between ultrasound and MRI measurements (r=0.676, P<0.001) .

This cross-sectional study fully confirmed a significant negative correlation between axillary cyst thickness and physical examination angles (external and internal rotation). The consistency of axillary cyst measurement by ultrasound and magnetic resonance imaging was moderate, and the thickness measured by magnetic resonance imaging was significantly greater than that measured by ultrasound. Meanwhile, comparing healthy volunteers with those with limited IR revealed that limited IR might be an early warning factor for AC.