Shoulder Impingement:
Effectiveness of Surgical and Postsurgical Interventions for the Subacromial Impingement Syndrome: A Systematic Review
 
This review shows that there is no evidence that surgical treatment is superior to conservative treatment or that one particular surgical technique is superior to another. Because of possibly lower risks for complications, conservative treatment may be preferred. When choosing for surgery, arthroscopic decompression may be preferred because of the less invasive character of the procedure.
 
Go here for abstracts:  http://www.archives-pmr.org/current