Impingement
This is again a condition unique to the shoulder and is mostly caused by the structure of the shoulder. The tendons of the rotator cuff, especially supraspinatus and infraspinatus muscles have to negotiate the subacromial space. During abduction with various degrees of internal and external rotation the tendons are trapped between the unyielding bone of the acromion above and the humeral head below. As a result the patient develops impingement pain, which occurs typically in the middle third of the arc of abduction. In addition the subacromial space is further narrowed in its cross section by a spur that develops on the under-surface of the acromion at its front edge and osteo-arthritis of the acromio-clavicular joint. These are removed during the surgical procedure known as arthroscopic subacromial decompression . A secondary phenomenon of subacromial impingement is the development of calcific tendinitis, which occurs when insoluble calcium sulphate is deposited in the supraspinatus tendon stimulated by the impingement process. This can also be removed during the arthroscopic subacromial decompression procedure.
