Objective:To investigate the clinical and radiological outcomes of ultrasound guided injection of platelet-rich plasma (PRP) in the treatment of intratendinous rotator cuff tear.Methods:A retrospective study was conducted to analyze the clinical data of 43 patients (46 shoulders) who had been treated for intratendinous partial-thickness rotator cuff tear by ultrasound guided injection of PRP consecutively from July 2021 to March 2022 at Department of Sports Medicine, Peking University Third Hospital. There were 23 males and 20 females, with an age of (47.8±13.5) years and a course of disease of 6 (4, 18) months, involving 22 left shoulders and 24 right shoulders. The visual analog scale (VAS) pain score, the University of California at Los Angeles (UCLA) rating scale, and the shoulder index of the American Shoulder and Elbow Surgeons (ASES) were determined before injection and at the last follow-up. The changes in tear size were also evaluated by magnetic resonance imaging (MRI) before PRP injection and 3 to 5 months after PRP injection.Results:The 43 patients were followed up for 15 (12, 17) months after treatment. Of this cohort, 7 shoulders (15.2%, 7/46) were recovered to complete normal and very satisfied with the injection effects while 19 shoulders(41.3%, 19/46) satisfied with the effects after injection, yielding an overall satisfaction rate of 56.5% (26/46). At the last follow-up, the VAS score [3.0 (2.0, 4.0) points], ASES score [80.0 (65.0, 88.8) points], and UCLA score [29.0 (20.0, 32.0) points] were significantly improved compared with those before injection [5.5 (4.0, 8.0) points, 55.0 (39.2, 65.0) points, and 16.0 (12.0, 20.3) points] ( P < 0.05). MRI evaluation showed the tear volume was significantly reduced after PRP injection [46.1 (20.9, 77.5) mm 3 before injection versus 28.2 (12.5, 63.6) mm 3 after injection] ( P<0.05), and a >50% tear volume diminution was observed in 13 shoulders (34.2%,13/38). There were no complications during or after injection. Conclusion:As the ultrasound guided injection of PRP into intratendinous lesions is effective and safe for patients with intratendinous partial-thickness rotator cuff tear, it can be an alternative treatment for the patients or professional athletes who are unwilling to undergo surgery.