Influence of Pain on Rotator Cuff Muscle Size and Function: A Case-Control Study with Ultrasound Assessment of Cross-Sectional Area and Tendon Thickness.

Background: Rotator cuff weakness is common in patients with shoulder pain, but the underlying causes are unclear when tendons are structurally intact. Atrophy is well established after full-thickness tears, but it is uncertain whether rotator cuff muscle cross-sectional area (CSA) differs in painful shoulders without rupture. Distinguishing structural atrophy from pain-related inhibition has important implications for rehabilitation. Purpose: The purpose of this study was to compare supraspinatus (SS) and infraspinatus (IS) CSA and SS tendon thickness between painful and asymptomatic shoulders. Secondary aims were to evaluate the influence of limb dominance on CSA, strength, and endurance, and to examine relationships between CSA, strength, and endurance. Study Design: Cross-sectional case-control study. Methods: Twenty-four participants (12 with unilateral shoulder pain recruited from an outpatient orthopedic clinic, 12 age- and sex-matched asymptomatic community-based controls) were enrolled. Participants in the shoulder pain group presented with chronic symptoms and MRI-confirmed intact rotator cuff tendons. Exclusion criteria included prior surgery or MRI-confirmed full-thickness tears. Demographics and patient-reported outcomes were collected. Ultrasound was used to measure the CSA of the SS and IS tendons and the SS tendon thickness. Isometric strength of the external rotators (ER) and full can was assessed with handheld dynamometry, and endurance with the Posterior Shoulder Endurance Test (PSET). Mixed-model ANOVAs examined group × dominance effects on CSA, tendon thickness, strength, and endurance outcomes; ANCOVA and regression explored the contributions of pain and CSA. Results: CSA and tendon thickness did not differ between groups or sides (all p >0.19). Painful dominant shoulders showed significantly lower ER (p = 0.003), full can strength (p = 0.003), and endurance (p = 0.021) compared with contralateral and control shoulders. Adjusting for pain eliminated group differences in strength, with pain accounting for 36-56% of the variance. Regression showed that SS CSA predicted full can strength and IS CSA predicted ER strength, but neither predicted endurance. PSET deficits persisted after adjusting for pain. Conclusion: Rotator cuff CSA and tendon thickness were preserved in individuals with shoulder pain, yet strength and endurance deficits were evident. Weakness was more closely related to pain-mediated inhibition than structural atrophy, while endurance appeared to be influenced by scapular stabilizer function. These findings suggest that early rehabilitation may emphasize pain reduction and neuromuscular control before progressive strengthening. Level of Evidence: 3.