Supraspinatus Tendon Thickness Is Associated With Collagen Disorganization in Symptomatic and Asymptomatic Shoulders of Individuals With Unilateral Rotator Cuff Tendinopathy

Rotator cuff (RC) tendinopathy is a common source of shoulder pain and is frequently linked to structural changes in the supraspinatus tendon of greater thickness and disrupted collagen organization. This study aimed to characterize differences in tendon thickness and collagen organization between individuals with unilateral symptomatic RC tendinopathy, their contralateral asymptomatic shoulder, and asymptomatic controls, and to examine the relationship between thickness (macro-morphology) and collagen organization (micro-morphology). This is a cross-sectional observational study of 64 participants diagnosed with RC tendinopathy compared to 64 asymptomatic controls. On ultrasound imaging of the tendon, thickness was measured, and collagen organization was quantified via spatial frequency analysis of peak spatial frequency radius (PSFR). Compared to controls, individuals with RC tendinopathy showed significantly greater tendon thickness on the involved side (4.3 ± 0.7 vs. 3.9 ± 0.5 mm, p = 0.0097), but no differences in PSFR. Participants with RC tendinopathy showed no significant differences between sides in tendon thickness or PSFR. In the RC tendinopathy group, tendon thickness and PSFR were inversely correlated on both the involved (r = -0.24, p = 0.060) and uninvolved (r = -0.32, p = 0.01) sides, indicating that increased tendon thickness was associated with decreased collagen organization-an association not observed in controls. Findings suggest tendon thickness reflects maladaptive structure in RC tendinopathy, and collagen organization alone may not sufficiently identify tendon pathology. The relationship between tendon thickness and collagen disorganization in RC tendinopathy indicates that tendon maladaptation differs from adaptive hypertrophy in healthy shoulders. Statement of Clinical Significance: Combined evaluation of tendon hypertrophy and collagen organization can enhance structural characterization in those with tendinopathy.