Background: Subacromial impingement is one of the most common complaints of shoulder. Treatments include avoiding of painful activities, oral anti-pain drugs, physical therapy modalities, corticosteroid injection and exercise therapy. Some studies have shown that platelet- rich plasma(PRP) is effective on tendinitis and tearing of tendons, ligaments and muscles, but evidence that has proved PRP as a conservative treatment in shoulder pathologies is very limited. This study aims to investigate the effect of PRP injection on relieving pain and improving daily function of patients with shoulder impingement syndrome.

Materials and Methods: In this clinical trial study, patients older than 40 with pain more than three months were included. If they had three of four positive diagnostic clinical tests of shoulder impingement that were confirmed by shoulder MRI, could be injected PRP twice. The time between injections was 1 month. Pain was measured by visual analog scale (VAS) and function was measured by two questionnaires named disabilities of the arm, shoulder and hand (DASH) and western Ontario rotator cuff  (WORC) index. Range of motion (ROM) of shoulder was measured in five directions by goniometry . All of these parameters were evaluated before intervention and in 1, 3, 6 months later.

Results: with due attention to a six-month folloe-up, PRR injection was effective in pain reduction and improvement of patient's function (p<0.05). Shoulder Rom increased in all directions except external rotation and the power of shoulder muscles was evidently improved statistically in flexion, abduction and internal toration.

Conclusion: PRP injection could effectively reduce pain and improve daily activities in patients with shoulder impingement syndrome.