Association Between MRI-Defined Acromioclavicular Joint Degeneration and Supraspinatus Fatty Infiltration in Shoulders Without MRI-Visible Supraspinatus Tear

Supraspinatus fatty infiltration can occur without MRI-visible supraspinatus tear, but its relationship with acromioclavicular joint degeneration on shoulder MRI is unclear. We evaluated whether MRI-defined acromioclavicular joint degeneration was associated with supraspinatus fatty infiltration in shoulders without MRI-visible supraspinatus tear or definite tendinopathy. This retrospective single-center cross-sectional study included 250 shoulders from 250 patients selected from 673 consecutive shoulder MRI examinations performed between 2020 and 2024. Two readers independently assessed fatty infiltration using the Goutallier classification and four prespecified acromioclavicular joint degenerative features, with disagreements resolved by consensus. Acromioclavicular joint degeneration was operationally defined as at least two of four features. Fatty infiltration was present in 92 shoulders (36.8%) and acromioclavicular joint degeneration in 173 (69.2%); fatty infiltration occurred in 49.1% with versus 9.1% without degeneration (p < 0.001). After adjustment for age, sex, and side, acromioclavicular joint degeneration remained associated with fatty infiltration (adjusted odds ratio, 5.44; 95% confidence interval, 2.32–14.40; p < 0.001). Findings were consistent across sensitivity analyses addressing alternative definitions, age, and MRI field strength. In this referral-based cohort, MRI-defined acromioclavicular joint degeneration and supraspinatus fatty infiltration frequently coexisted despite the absence of MRI-visible supraspinatus tear or definite tendinopathy.