Dynamic assessment of deltoid stiffness using shear wave elastography: a reliability study in healthy adults

Deltoid tension plays an important role in maintaining shoulder function. Understanding normative values is essential for accurately restoring deltoid mechanics in pathological conditions; however, there is a notable lack of data on this topic, particularly with respect to objective measurement methods. Shear wave elastography (SWE) is an ultrasound-based imaging modality that provides real-time quantitative assessment of muscle stiffness. This study evaluates the reliability and validity of SWE for measuring deltoid stiffness in different shoulder positions.

A cross-sectional study was conducted on 21 healthy volunteers without shoulder pathology (8 males and 13 females; mean age 30.6 ± 5.6 years). Twelve SWE measurements of the middle deltoid were obtained for each side (left/right) and shoulder position (resting at side and 90° abduction) using a standardized measurement technique. Measurements were performed by 4 independent operators with varying levels of training and experience in ultrasound measurement (2 expert and 2 novice operators). Intraoperator and interoperator reliability were assessed using the median values for each measuring position, side, and measurer. Validity was assessed using Student's t-tests for resting vs. abducted positions, while reliability was evaluated with intraclass correlation coefficients (ICCs) and paired t-tests for side-to-side consistency.

The mean SWE values were 48.37 kilopascals (kPa) (left) and 47.64 kPa (right) at rest, and 158.48 kPa (left) and 155.45 kPa (right) in abduction. Tension was significantly higher in abduction compared to rest (156.97 kPa vs 48.01 kPa, P < .001), confirming construct validity by demonstrating SWE's ability to differentiate muscle stiffness across functional states. Interoperator reliability was good (ICC 0.79), and intraoperator reliability was also good (ICC >0.78). Reliability was also good between the 2 expert operators (ICC 0.878) and the 2 novice operators (0.797), as well as between expert and novice groups (ICC 0.762), indicating reliable measurements across experience levels. No significant difference was found between left and right measurements (P = .656).

SWE is a reliable and valid method for quantifying deltoid muscle stiffness across functional states. Its reproducibility across operator experience levels and sensitivity to dynamic changes support its potential clinical utility in perioperative assessment and rehabilitation of shoulder conditions.