To investigate if glenohumeral arthroscopy and arthroscopic subacromial decompression is more effective than glenohumeral arthroscopy alone in improving patient-reported outcome at 12 months in patients with subacromial impingement syndrome (SIS) who are non-responders to non-operative treatment.

The trial is designed as a patient and assessor-blinded, randomized, controlled trial with two parallel groups. Participants are randomized 1:1 to either ASAD and glenohumeral arthroscopy (intervention group) or glenohumeral arthroscopy alone (control group). The two groups are subscribed identical postoperative rehabilitation consisting of 3 months physiotherapy in a municipally setting. The primary endpoint is the Shoulder Pain and Disability Index (SPADI) 12 months after surgical intervention. The study will adhere to the CONSORT guidelines.

All participants will be recruited consecutively from the outpatient clinic, Arthroscopic Center, Copenhagen University Hospital, Hvidovre. Consultant orthopedic surgeons will conduct the initial eligibility screening as part of their consultation. Eligible participants will be provided with thorough verbal and written information about the trial and their right to withdraw at any time. Patients given their written, informed consent to participate in the trial, will receive scheduled appointments for the baseline examination.

Inclusion Criteria:

• SIS diagnosis (Consultant's clinical diagnosis of SIS + at least 3 out of 5 positive tests from the following: Hawkin's, Neer's, Jobe's, Painful arc and external rotation resistance test)
• Positive subacromial injection test
• Insidious onset of shoulder pain
• Considered a surgical candidate by an orthopedic shoulder specialist.
• Symptoms for at least 6 months
• Completion of at least 3 months supervised shoulder training
• No improvement in symptoms for at least 3 months
• The patient must be expected to be able to attend rehabilitation and post-examinations.

Exclusion Criteria:

• Terminal illness or severe medical illness (ASA score higher than or equal to 4), systemic musculoskeletal disease, inflammatory joint disease (e.g. rheumatoid arthritis), symptomatic cervical spine pathology or thoracic outlet syndrome.
• Full-thickness rotator cuff tear, calcified tendonitis, labral tear, frozen shoulder, biceps tendon pathology, acromioclavicular osteoarthrosis, glenohumeral osteoarthrosis or other concomitant shoulder pathology.
• Previous surgery or radiotherapy on the affected shoulder.
• Pregnancy
• Ongoing workers compensation case or job rehabilitation process