Longitudinal MR Quantification of the Fat Fraction within the Supraspinatus and Infraspinatus Muscles in Patients with Shoulder Pain

Rationale and Objectives: Knowing the natural history of fatty degeneration of rotator cuff muscles is important for estimating the risk and rate of progression to cuff tear arthropathy (CTA). The purpose of this study was to investigate the changes in rotator cuff muscle fatty degeneration over time quantitatively in patients treated conservatively for shoulder pain. Materials and Methods: Thirty patients with a baseline and follow-up shoulder MRI, including a 2-point Dixon sequence, which were performed at least 1 year apart, were included. We classified patients into 3 groups: “full-thickness tear” (n = 7), “partial-thickness tear” (n = 13), and “no-tear” (n = 10) groups. The fat fraction in the supra- and infraspinatus muscles, and the rate of change in the fat fraction (ΔF^fr) were calculated using the formula “fat fraction of follow-up MRI/fat fraction of initial MRI.” We investigated the difference in ΔF^fr among the 3 groups and the degree of progression to CTA. Results: Statistically significant differences in ΔF^fr within the supraspinatus and infraspinatus muscles were found among full-thickness, partial-thickness, and no-tear groups (2.54 vs 1.02 vs 0.75, p < 0.001 and 1.96 vs 1.07 vs 0.73, p = 0.021, respectively). Overall, 71.4% of the full-thickness tear group showed progression of CTA, and 28.6% of the full-thickness tear group needed reverse shoulder arthroplasty within an average follow-up period of 34 months. Conclusions: MR quantification, together with the knowledge of change in fatty degeneration over time, may be useful for the management of patients with shoulder pain.