Normal Ultrasound Thickness of the Subacromial-Subdeltoid Bursa
The normal subacromial-subdeltoid bursa measures less than 2 mm on ultrasound examination in asymptomatic individuals, with bursal fluid thickness rarely exceeding 2 mm and typically measuring 0-2 mm (mean 1.3 mm, median 2 mm). 1, 2
Established Normal Values
In asymptomatic shoulders, the subacromial-subdeltoid bursa thickness ranges from 0 to 2 mm, with the majority of normal individuals showing minimal to no fluid within the bursa. 1, 2
Only 2 out of 36 asymptomatic volunteers (5.6%) exceeded 3 mm in bursa thickness in a controlled MRI study, establishing 3 mm as a clear threshold for abnormality. 2
Approximately 36% of normal volunteers have no detectable bursal fluid, while 61% show only a small amount of fluid located lateral to the acromioclavicular joint. 2
Threshold for Pathology
Bursal thickness exceeding 2 mm should raise suspicion for subacromial pathology, and thickness greater than 3 mm is definitively abnormal. 1, 2
Increased bursa thickness in symptomatic shoulders averages 1.27 mm compared to 0.75 mm on the asymptomatic side (p < 0.0001), demonstrating that even subtle thickening below the traditional 2 mm threshold may indicate early impingement or rotator cuff pathology. 1
In patients with full-thickness rotator cuff tears, bursal thickness ranges from 0 to 8 mm (mean 3.3 mm, median 3 mm), significantly higher than normal controls. 2
Anatomic Distribution Patterns
The location of bursal fluid provides additional diagnostic information beyond simple thickness measurements.
In normal shoulders, bursal fluid is predominantly located in the posterior quarter (53%) or middle quarter (22%) of the bursa when present. 2
Pathologic fluid distribution includes fluid located medial to the acromioclavicular joint (37% of rotator cuff tear patients) or in the anterior quarter of the bursa (89% of rotator cuff tear patients), patterns rarely seen in asymptomatic individuals. 2
Only 1.7% of asymptomatic shoulders demonstrate fluid in both the glenohumeral joint and subacromial bursa simultaneously, making this combination highly specific (99%) for rotator cuff tears with a 95% positive predictive value. 3
Clinical Measurement Technique
Bursa thickness should be measured from the superficial peribursal fat to the upper margin of the supraspinatus tendon using standardized ultrasound views. 1
The short-axis view of the supraspinatus in the modified Crass position provides the most reliable measurements for subacromial-subdeltoid bursa assessment, with good to excellent intra-rater reliability (ICC 0.76-0.84). 4
Acromiohumeral distance measurements in neutral position combined with bursal thickness assessment offer complementary diagnostic information with good intra-rater reliability (ICC 0.76-0.79). 4
Critical Diagnostic Pitfalls
Dynamic ultrasound showing bursal "gathering" during arm abduction does not reliably distinguish pathologic from normal shoulders, as similar increases in bursal thickness occur in both symptomatic patients (0.39 ± 0.41 mm) and healthy controls (0.35 ± 0.32 mm) during dynamic maneuvers (p = 0.72). 5
Static measurements are more diagnostically valuable than dynamic assessment, with statistically significant differences found between patients (1.01 ± 0.48 mm) and controls (0.67 ± 0.32 mm) on short-axis static views (p = 0.006). 5
Isolated bursal fluid without joint effusion has limited diagnostic value (sensitivity 7%, specificity 96%), but when combined with glenohumeral joint effusion, specificity increases to 99% for rotator cuff tears. 3