Subacromial space measured by ultrasound imaging in asymptomatic subjects and patients with subacromial shoulder pain: an inter-rater reliability study

Background Acromiohumeral distance (AHD) and supraspinatus tendon thickness (STT) measured by ultrasound (US) can be combined in the occupation ratio (OR). Inter-rater reliability on these subacromial measures depends on the US experience of raters and on the subject status, differing between asymptomatic or patients with subacromial shoulder pain (SSP). Objective To evaluate inter-rater reliability between two raters with different US experience (experienced examiner and novice examiner). Methods In total, 20 asymptomatic subjects (controls) and 21 patients with SSP were examined on one shoulder. Inter-rater reliability was evaluated with intraclass correlation coefficient (ICC). Results ICC for controls was good for AHD at rest and at 60 degrees (0.76-0.77), moderate for STT and AHD at 60 degrees with weights (0.53-0.72), while OR was poorly reliable (below 0.44). ICC for SSP was moderate for AHD at rest and at 60 degrees, STT, OR at rest and at 60 degrees (0.52-0.74) and poor for AHD at 60 degrees with weights and OR at 60 degrees with weights (0.33-0.36). Bland-Altman plots showed systematic bias. Conclusion Inter-rater reliability varied largely from poor to good between two examiners with different US experience. Clinicians might use the US as additional tool for detecting the subacromial structures, but a structured training including also symptomatic subjects is suggested. Researchers may further investigate the OR in matched case-control studies, and an overall agreement phase is recommended before starting the inter-rater reliability phase.