Subacromial (SA) impingement is defined as the mechanical compression of subacromial structures between the coraco-acromial arch and the humerus during active elevation of the arm above shoulder height.
Clinically Relevant Anatomy
Structures involved in subacromial impingment:
- the coracoacromial arch composed of acromion, processus coracoideus and ligamentum coracoacromiale
- the humerus
- the tendons of the Rotator Cuff
- the long head of de m. biceps brachii
- the subacromial bursa
- shoulder capsule
The subacromial space is the margin between the superior portion of the humeral head and the inferior portion of the acromion. It contains:
- belly and tendon of the supraspinatus muscle
- long head of the biceps muscle
- subacromial bursa
The shape of the acromion plays an important role in impingment syndromes. 3 types of acromial shape can be distinguished:
- Type I acromion: flat shape
- Type II acromion: curved shape
- Type III acromion: hooked shape (most likely to contribute to impingement and irritation)
Bony spurs at the bottom aspect of the acromion can also be involved.
Impingement syndrome occurs in one out of three persons, half of whom still report pain after three years.
Subacromial impingement occurs when the subacromial space is narrowed and weak structures in that space get compressed. Several causes for subacromial impingment have been detected: 
- Anatomical variations such as narrow SA space, type II or III acromion, osteophytes.
- Rotator cuff weakness, causing the humeral head to drift more superior.
- Chronic rotator cuff irritation due to overuse.
- Posterior GH capsule tightness
- Poor posture (forward shoulder posture can cause functional narrowing of SA space)
- Abnormal muscle activation
Mentioned biomedical aspects but also psychological factors as kinesiophobia or catastrophizing can have negative influence and thus cause chronic pain and disability. 
The shoulder pain during active elevation of the arm is usually reported at the anterior or lateral side of the shoulder. Activities performed with the involved arm below shoulder level are mostly not painful. 
Onset is more of a gradual, degenerative condition rather than due to a strong external force. Therefore, patients have difficulty determining the exact time of onset.
- First stage:
Moderate pain during exercise
No loss of strength
No limitation in movement
- Second stage:
Pain during ADL-activities and especially during the night
Loss of mobility
- Third stage:
Strong restriction in movement due to calcifications
Loss of muscle strength
Diagnosis should be based on: 
- Clinical examination
History and clinical examination are imparitive. X-rays may be used as an extra test detect sclerosis and osteophyte-formation on the acromion. The size of the subacromial space can also be measured. MRI can show full or partial tears in the tendons of the rotator cuff, cracks in the capsule and inflammation to weak structures.
Following tests help to confirm or rule out subacromial impingment syndrome: 
- Neer impingement test (useful screening test to rule out SAI)
- Painful Arc (between 60° and 120°) (useful screening and helpful confirming test to rule out SAI)
- Empty can (Jobe) (helpful test to confirm SAI)
- External rotation resistance tests (useful screening and helpful confriming test to rule out SAI)
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Medical Management (current best evidence)
Several surgical techniques are available, depending on the character and severity of the injury:
- Surgical repair of torn tissues, mostly of supraspinatus muscle, long head of biceps tendon or joint capsule.
- Bursectomy or removal of the subacromial bursa.
- Subacromial decompression to increase the subacromial space by removing bony spurs or the coracoacromial ligament
- Acromioplasty to increase the subacromial space by removing a part of the acromion. Arthroscopic acromioplasty is less invasive and requires lesser rehabilitation than the open (Neer) acromioplasty.
Physical Therapy Management (current best evidence)
There is strong evidence that non-operative rehabilitation - when supervised - both decreases pain in the shoulder and increase function.  Non-operative treatment should therefor be attempted first, assuming there is no tear that requires surgery. No data guarantee that surgical treatment is more effective than conservative treatment.
Several conservative treatments are available: 
- RICE therapy in the acute phase to reduce pain and swelling
- NSAIDS or nonsteroidal anti-inflammatory drugs
- Corticosteroid injections
- Physical therapy
- Manual therapy techniques of the shoulder that have a significant effect on decreasing pain levels.
- Electrical stimulation
- Low-level laser therapy has positive effects on all symptoms except on muscle strength. 
Although exercise therapy alone has prooved efficiency, the addition of manual therapy insures further increase in muscle strength.  Exercise therapy is a vital part of treatment for subacromial impingement but results showed no significant difference between home-based exercises and clinical exercise.
Therapeutic exercise should include: 
- rotator cuff strengthening
- lower and middle trapezius strengthening
- anterior and posterior shoulder stretching.
Strengthening of the lower part of the trapezius muscle is an important part of exercise therapy. Individuals with impingement syndrome show greater ratios of upper and lower trapezius activity than asymptomatic individuals.  Soft tissue mobilization to normalize muscle spasm and other soft tissue dysfunction has been shown to be effective alongside joint mobilizations to restore motion in treatment of SAI. 
There are a variety of shoulder conditions that can initially be confused with subacromial impingement, although a thorough examination is usually sufficient for identifying this condition.
Partial and full rotator cuff tears are or rotator cuff tendinitis are often the result of subacromial impingement syndrome, but can occur without impingement and usually demonstrate some sort of lag sign upon evaluation.
Adhesive capsulitis, or "frozen shoulder", can also present with limitation of active arm elevation and significant shoulder pain; however, adhesive capsulitis usually presents with glenohumeral motion loss in a capsular pattern, meaning that external rotation and abduction are the two directions with the greatest magnitude of motion lost.
Further on, calcific tendinitis and an acute or chronic inflammation of the bursa subacromialis could be differential diagnosis, as well as a subluxating shoulder or thoracic outlet syndrome. 
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Clinical Bottom Line
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Recent Related Research (from Pubmed)
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- ↑ 1.0 1.1 Kachingwe AF, Phillips B, Sletten E, Plunkett SW. Comparison&amp;nbsp;of Manual Therapy Techniques with Therapeutic Exercise in the Treatment of Shoulder Impingement: A Randomized Controlled Pilot Clinical Trial. The Journal of Manual fckLRManipulative Therapy 2008;16(4):238-‐247
- ↑ 2.0 2.1 TATE A.R., MCCLURE P.W., YOUNG I.A., SALVATOR R., MICHENER L.A. Comprehensive impairment-based exercise and manual therapy intervention for patients with subacromial impingement syndrome: a case series. The Journal of orthopaedic and sports physical therapy. 2010 Aug; 40(8): 474-93
- ↑ 3.0 3.1 3.2 BIRRER R.B., O’CONNOR F.G. Sports medicine for the primary care physician. 3rd edition, Boca Raton: RCR PRESS, 2004.p507- 10
- ↑ 4.0 4.1 DE BIE R.A., BASTIANENEN C.H.G. Effectiveness of individualized physiotherapy on pain and functioning compared to a standard exercise protocol in patients presenting with clinical signs of subacromial impingement syndrome. A randomized controlled trial. BMC Musculoskeletal Disorders. 2010 Jun 9; 11:114
- ↑ MICHENER L.A., WALSWORTH M.K., DOUKAS W.C., MURPHY K.P. Reliability and Diagnostic Accuracy of 5 Physical Examination Tests and Combination of Tests for Subacromial Impingement. Archives of Physical Medicine and Rehabilitation. 2009 Nov; 90(11): 1898-903
- ↑ DORRESTIJN O., STEVENS M., WINTERS J.C., VAN DER MEER C.,DIERCKS R.l. Conservative or surgical treatment for subacromial impingement syndrome? A systematic review.Journal of shoulder and elbow surgery board of trustees.2009 Jul-Aug;18(4):652-60
- ↑ YELDAN I., CETIN E., OZDINCLER A.R. The effectiveness of low-level laser therapy on shoulder function in subacromial impingement syndrome. Disability and rehabilitation. 2009; 31(11): 935–940
- ↑ Bang MD, Deyle GD. Comparison of Supervised Exercise With and Without Manual Physical Therapy for Patients with Shoulder Impingement Syndrome. Journal of Orthopaedic and Sports Physical Therapy. 2000;30(3):126-137
- ↑ Kuhn JE. Exercise in the treatment of rotator cuff impingement: A systematic review and synthesized evidence-based rehabilitation protocol. Journal fo Shoulder and Elbow Surgery. 2009;18:138-160
- ↑ 10.0 10.1 Smith M, Sparkes V, Busse M, Enright S. Upper and Lower trapezius muscle activity in subjects with subacromial impingement symptoms: Is there imbalance and can taping change it? Physical Therapy in Sport. 2009:10, 45-50