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- Discussion:
    - glenohumeral arthritis;
          - look for the characteristic osteophyte at the inferior aspect of the humeral head;
    - nerve palsy:
          - w/ atrophy of both the supraspinatus and infraspinatus, consider obtaining an MRI of the shoulder, to evaluate for space occupying lesion causing suprascapular nerve palsy;
          - also consider EMG:
    - glenohumeral instability (either recurrent anterior or multidirectional)
          - commonly mistaken for impingement syndrome in younger patients;
          - w/ large labral detachement or tear, symptoms may be similar to impingement syndrome;
          - positive apprehension sign will confirm the diagnosis;
          - subacromial injection of lidocaine does not usually relieve symptoms relating to shoulder instability;
    - cervical spondylosis (radiculitis);
    - frozen shoulder (non outlet impingement)
          - soft tissue contractures of frozen shoulder may cause translation of humeral head w/ attempted flexion which can result in impingement despite normal acromial anatomy;
          - patients w/ contracture of the posterior capsule (and loss of internal rotation) will be most likely to demonstrate signs of impingement (despite normal acromial anatomy);
    - AC joint arthritis;
          - lidocaine injection into acromioclavicular joint distinguishes this problem from impingement;
    - paralysis of the trapezius
    - interarticular pathology, such as glenohumeral arthritis, presents w/ bony crepitus and greater limitations of passive motion;
          - true AP & axillary view of glenohumeral joint will show arthritis;
    - calcific tendinitis:
    - cuff tear arthropathy
    - biceps tendonitis
    - reflex sympathetic dystrophy
    - thoracic outlet syndrome
    - os acromiale