Impingement syndrome
Impingement syndrome, also called painful arc syndrome, supraspinatus syndrome, swimmer's shoulder, and thrower's shoulder, is a clinical syndrome which occurs when the tendons of the rotator cuff muscles become irritated and inflamed as they pass through the subacromial space, the passage beneath the acromion. This can result in pain, weakness and loss of movement at the shoulder. 

# Causes
The rotator cuff muscle tendons pass through a narrow space between the acromion process of the scapula and the head of the humerus. Anything which causes further narrowing of this space can result in impingement syndrome. This can be caused by bony structures such as subacromial spurs (bony projections from the acromion), osteoarthritic spurs on the acromioclavicular joint, and variations in the shape of the acromion. Thickening or calcification of the coracoacromial ligament can also cause impingement. Loss of function of the rotator cuff muscles, due to injury or loss of strength, may cause the humerus to move superiorly, resulting in impingement. Inflammation and subsequent thickening of the subacromial bursa may also cause impingement.

# Signs and symptoms
The most common symptoms in impingement syndrome are pain, weakness and a loss of movement at affected the shoulder. The pain is often worsened by shoulder overhead movement and may occur at night, especially if the patient is lying on the affected shoulder. The onset of the pain may be acute if it is due to an injury or may be insidious if it is due to a gradual process such as an osteoarthritic spur. Other symptoms can include a grinding or popping sensation during movement of the shoulder. 
The range of motion at the shoulder may be limited by pain. A painful arc of movement may be present during forward elevation of the arm from 60 to 120.  Passive movement at the shoulder will appear painful when a downwards force is applied at the acromion but the pain will ease once the downwards force is removed. 

# Diagnosis
Impingement syndrome can usually be diagnosed by history and physical exam.  Plain x-rays of the shoulder can be used to detect some joint pathology and variations in the bones, including acromioclavicular arthritis, variations in the acromion, and calcifcation.   Ultrasonography, arthrography and MRI can be used to detect rotator cuff muscle pathology.   Due to lack of understanding of the pathoaetiology, and lack of diagnostic accuracy in the assessment process by many doctors, several opinions are recommended before intervention.

# Treatment
Impingement syndrome is usually treated conservatively, but sometimes is treated with arthroscopic surgery or open surgery. Conservative treatment includes rest, cessation of painful activity, and physiotherapy focused at maintaining range of movement and avoid shoulder stiffness. NSAID's and ice packs may be used for pain relief.   
Therapeutic injections of corticosteroid and local anaesthetic may be used for persistent impingement syndrome.  The total number of injections is generally limited to 3 due to possible side effects from the corticosteroid.
Corticosteroids actually cause musculoskeletal disorders, which explains the low success rate of cortisone injections. Research has shown that over 90% of tendinopathies have no inflammation, thus the term tendinosis is more appropriate than tendinitis for most diagnoses. For tendinosis, prolotherapy injections or cross-fiber (transverse) friction massage can be very effective.
A number of surgical interventions are available, depending on the nature and location of the pathology. Surgery may be done arthroscopically or as open surgery. The impinging structures may be removed in surgery, and the subacromial space may be widened by resection of the distal clavicle and excision of osteophytes on the under-surface of the acromioclavicular joint.  Also damaged rotator cuff muscles can be surgically repaired.

# History
Impingement syndrome was reported in 1852. Impingement of the shoulder was previously thought to be precipitated by shoulder abduction and surgical intervention focused on lateral or total acromionectomy. In 1972, Charles Neer proposed that impingement of was due to the anterior third of the acromion and the coracoacromial ligament and suggested surgery should be focused on these areas. The role of anteriorinferior aspect of the acromion in impingment syndrome and excision of parts of the anteriorinferior acromion has become a pivotal part of the surgical treatment of the syndrome.