Examining the effect of sex and body mass index on rotator cuff fatty infiltration in young adults without shoulder pathology.

Rotator cuff fatty infiltration (FI) can lead to poor post-operative outcomes in rotator cuff tear patients. While higher magnitudes of FI are associated with aging and pathology, anthropometric variables may also influence FI. This research investigated the effect of sex and body mass index (BMI) on supraspinatus (SS) and infraspinatus (IS) FI among young adults without shoulder pathology. Magnetic resonance imaging (MRI) and ultrasound (US) were captured from 40 adult males and females with varying BMI. US images were captured using different participant positioning, and transducer techniques. Upper limb static strength assessments were also performed. Regression analysis revealed that both sex and BMI explained significant variance in MRI FI for SS (p < 0.0001, R2 = 0.51), and BMI for IS (p < 0.0001, R2 = 0.54). Further, US echogenicity generally showed excellent reliability, and explained significant variance in MRI Fat Fraction (FF) across techniques and participant positions, while SS had more varied results. Significant relationships between strength and FI were only demonstrated for SS MRI FI (range, r = -0.21 to -0.33). This study showed that sex and BMI are significantly related to rotator cuff FI. Future research should continue to explore FI among an aging population, with and without rotator cuff pathology.