Rotator cuff tear is a common cause of shoulder pain and disability, and is among the most common conditions affecting the shoulder (1). Such injuries increase with age, reducing the quality of life due to shoulder pain and weakness with shoulder flexion and abduction. Minimally-invasive arthroscopic rotator cuff repair is increasingly popular for treatment of full-thickness rotator cuff tear following the introduction of innovative arthroscopic techniques (2). However, operative outcomes are far from perfect, with a mean re-tear rate of 26.6% at 2 years after surgery (3). Postoperative re-tears are associated with greater fatty degeneration, larger tear size, and advanced age. The purpose of this study was to quantify the pre- and post-operative muscular fatty degeneration using a 2-Point Dixon sequence in patients with rotator cuff tears treated by arthroscopic rotator cuff repair (ARCR). Further, we aim to assess the relationship of preoperative fat fraction values within rotator cuff muscles between patients who experience re-tear and those who do not. Fifty patients with full-thickness supraspinatus tears (18 men, 32 women; mean age, 67.0 ± 8.0 years; age range, 41-91 years) were recruited for this study. All MR examinations were performed on a 3.0-T unit (Magnetom Verio, Siemens AG, Erlangen, Germany) using a 4-channel flex coil. The degree of preoperative and postoperative fatty degeneration was quantified on a 2-Point Dixon MR image by measuring the mean signal intensity value on in-phase “S(In)” and fat image “S(fat)” by a board-certified radiologist on the PACS system with a region of interest (ROI) placed over supraspinatus muscle in the most lateral oblique-sagittal section, which appears as a “scapular Y-view”(Fig.1). Estimates of fatty degeneration were calculated with “fat fraction” values using the formula S(Fat)/S(In) within the supraspinatus at baseline pre-operative and post-operative 1-year follow-up MRI. In patients who had re-tear of the supraspinatus tendon, we evaluated the tear location by measurement of the distance from the footprint of the humeral head where suture anchors were placed and tear length. The Mann-Whitney U test was used to compare preoperative fat fraction values between failed repair and intact repair groups. The paired t-test was used to compare fat fraction values between baseline and 1-year follow-up MRI. Of 50 patients at baseline, 30 patients had isolated non-massive supraspinatus tears. The remaining 20 patients had massive tears. Of 50 patients, failed repair of the supraspinatus tendon was seen on postoperative MRI in 11. The preoperative fat fraction values of the supraspinatus muscle were statistically higher in the failed repair group compared with the intact repair group (37.0% vs 19.5%, p<0.01, Fig.2). The postoperative fat fraction values of the supraspinatus muscle on 1-year follow-up MRI were statistically higher in the failed repair group compared with the intact repair group (42.8% vs 19.3%, p<0.01). Fatty degeneration tended to progress at 1 year post-operatively only in the failed repair group, although there was no statistical significance (Fig.3). In patients with failed repair, the supraspinatus tear location was 12.4 ± 4.8 mm proximal to the footprint, and the average tear length was 20.4 ± 7.4 mm. We applied a 2-Point Dixon MR sequence for the evaluation of fatty degeneration between pre- and 1-year post-operative MRI in patients with supraspinatus tendon tears. The preoperative fat fraction values of the supraspinatus muscle were significantly higher in patients with failed repair compared with intact repair. Fatty degeneration of the rotator cuff muscles did not tend to progress in patients with intact repair on 1-year follow-up MRI, while it tended to progress in patients with failed repair. Measurement of preoperative fat fraction values within the rotator cuff muscles may be a viable method for predicting post-operative re-tear.