Bilateral Ultrasonographic Findings in Patients with Unilateral Subacromial Pain Syndrome and Intact Rotator Cuff Tendons

Background: The etiology of subacromial pain syndrome (SAPS) remains enigmatic. It is theorized that the supraspinatus tendon and the subacromial bursa are the primary pain-generating structures. The supraspinatus tendon and the subacromial bursa are considered to be thickened in patients with SAPS but this assumption lacks validation. The aim of this study was to ultrasonographically measure the subacromial structures and evaluate the presence of impingement in patients with SAPS and to compare it with their asymptomatic shoulder. Methods: Patients were recruited consecutively from an orthopedic outpatient clinic using validated criteria for SAPS. Patients with contralateral shoulder pain and patients with acromioclavicular osteoarthrosis, rotator cuff tears, calcified tendinopathy, biceps tendon, or labral pathology were excluded. Validated ultrasonographical methods were used. Thickness of the supraspinatus tendon and the subacromial bursa were measured perpendicular to the tendon longitudinal axis 2.0 cm from the lateral border of the supraspinatus tendon footprint with the shoulder in slight internal rotation. Acromio-humeral distance was measured as the shortest distance from the anterolateral acromion to the humerus with the shoulder in neutral position. Ultrasonographic impingement was defined as visual bulging of the subacromial bursa during active shoulder abduction and internal rotation. Results: We examined 58 patients with unilateral SAPS and intact rotator cuff tendons. We found significantly more cases of ultrasonographic impingement in painful shoulders compared to the pain-free (45 vs. 18, Chi-Square P < .001). There were no significant differences between affected and unaffected shoulders regarding supraspinatus tendon thickness (5.4 vs. 5.5 mm), subacromial bursa thickness (1.9 vs. 1.9 mm), or the acromio-humeral distance (11.1 vs. 11.0 mm). The mean age of the included patients was 51 years, 64% were women, the median symptom duration was 18 months, and the dominant shoulder was affected in 71% of cases. Conclusion: In this cohort of patients with isolated unilateral SAPS, we found more cases of ultrasonographic impingement in affected shoulders compared to unaffected, but no significant differences in supraspinatus tendon thickness, subacromial bursa thickness, or acromio-humeral distance. These findings question ultrasonography's ability to discriminate between shoulders with and without SAPS based on measurements of subacromial structures alone.