LAUSR

Purpose Muscle fat content of the rotator cuff increases after a tear. In the healthy rotator cuff, the influence of age, body mass index (BMI) and critical shoulder angle (CSA) on muscle fat content is unknown. The primary aim was to correlate muscle fat content with age, BMI and CSA. The secondary aims were (1) to correlate muscle fat content in the entire muscle and slice Y (most lateral sagittal slice with scapular spine) and (2) assessed the reliability for CSA measurement in MRI. Methods In 26 healthy shoulders (17 subjects), aged 40–65 years, BMI 20–35 kg/m 2 , Goutallier grade 0, Dixon MRI was applied. The CSA was > 35° in 14 shoulders and < 30° in 12 shoulders. Muscle fat content was calculated from Dixon MRI. Results Infraspinatus muscle fat content correlates moderately with age ( r  = 0.553; p  = 0.003) and BMI ( r  = 0.517; p  = 0.007). Supraspinatus muscle fat content does not correlate with age ( r  = 0.363, p  = 0.069) and BMI ( r  = 0.342, p  = 0.087). No correlation between CSA and muscle fat content was found. Muscle fat content measurement in the entire muscle correlates strongly with measurement in slice Y (intraclass correlation coefficient supraspinatus muscle: 0.757; infraspinatus muscle: 0.794). CSA intermethod analysis between radiography and MR images shows very high reliability (intraclass correlation coefficient > 0.9) and no systematical deviation in Bland–Altman analysis. Conclusion Muscle fat content in the healthy infraspinatus muscle does correlate with age and BMI, but not with the CSA. Muscle fat content measurement in the rotator cuff using Dixon MRI showed a high reliability between slice Y and the entire muscle. Level of evidence III.