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Shoulder impingement syndrome, also called subacromial impingement, 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.
When the arm is raised, the subacromial space (gap between the anterior edge of the acromion and the head of the humerus) narrows, through which the supraspinatus muscle tendon passes. Anything that causes further narrowing has the tendency to impinge the tendon and cause an inflammatory response, resulting 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.
SIS and the Scapula
The scapula has been found to play an important role in shoulder impingement syndrome. It is a wide, flat bone lying on the thoracic wall that provides an attachment for three different groups of muscles. The intrinsic muscles of the scapula include the muscles of the rotator cuff- the subscapularis, teres minor, supraspinatus, and infraspinatus. These muscles attach to the surface of the scapula and are responsible for the internal and external rotation of the glenohumeral joint, along with humeral abduction. The extrinsic muscles include the biceps, triceps, and deltoid muscles and attach to the coracoid process and supraglenoid tubercle of the scapula, infraglenoid tubercle of the scapula, and spine of the scapula. These muscles are responsible for several actions of the glenohumeral joint. The third group, which is mainly responsible for stabilization and rotation of the scapula, consists of the trapezius, serratus anterior, levator scapulae, and rhomboid muscles and attach to the medial, superior, and inferior borders of the scapula. Each of these muscles has their own role in proper shoulder function and must be in balance with each other in order to avoid shoulder pathology. Abnormal scapular function is called scapular dyskinesis. One action the scapula performs during a throwing or serving motion is elevation of the acromion process in order to avoid impingement of the rotator cuff tendons. If the scapula fails to properly elevate the acromion, impingement may occur during the cocking and acceleration phase of an overhead activity. The two muscles most commonly inhibited during this first part of an overhead motion are the serratus anterior and the lower trapezius. These two muscles act as a force couple within the glenohumeral joint to properly elevate the acromion process, and if a muscle imbalance exists, shoulder impingement may develop.
Signs and symptoms
The most common symptoms in impingement syndrome are pain, weakness and a loss of movement at the affected 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.
Impingement syndrome can usually be diagnosed by history and physical exam. On physical exam, the physician may twist or elevate the patient's arm to test for reproducible pain (Neer's sign and Hawkin's test). These tests help localize the pathology to the rotator cuff, however they are not specific for impingement. Neer's sign may also be seen with subacromial bursitis.
The physician may inject lidocaine (usually combined with a steroid) into the bursa, and if there is an improved range of motion and decrease in pain, this is considered a positive "Impingement Test". It not only supports the diagnosis for impingement syndrome, but it is also therapeutic.
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 calcification. However, x-rays do not allow visualization of soft tissue, thus hold a low diagnostic value. Ultrasonography, arthrography and MRI can be used to detect rotator cuff muscle pathology. MRI is the best imaging test prior to arthroscopic surgery. Due to lack of understanding of the pathoaetiology, and lack of diagnostic accuracy in the assessment process by many physicians, several opinions are recommended before intervention.
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. Physiotherapy treatments would typically focus at maintaining range of movement, improving posture, strength of the muscles of the shoulder and scapula, and reduction of pain. Physiotherapists may employ the following treatment techniques to improve pain and function: Joint Mobilization, Interferential Therapy, Acupuncture, Soft Tissue Therapy, Therapeutic Taping, Scapular and rotator cuff Strengthening, and Education regarding the cause and mechanism of the condition. 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 three 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.
Cools, Witvrouw, Mahieu, and Danneels (2007) researched isokinetic scapular muscle performance in overhead athletes with and without shoulder impingement to identify any deficits in the scapular muscles of athletes with shoulder impingement. Using 30 overhead athletes with shoulder impingement (21 males and 9 females) and 30 overhead athletes (18 males and 12 females) without shoulder pain (mean age of 23.2 years old) they measured scapular protraction and retraction of the scapula at two different velocities (12.2 cm/s and 36.6 cm/s). Using valid and reliable measures, Cools et al. found that overhead athletes with shoulder impingement had decreased force output at both velocities in the protractor muscles compared to their uninjured arm and uninjured overhead athletes. Cools et al. (2007) used the importance of scapular strengthening and observed muscle activity of the upper trapezius, middle trapezius, lower trapezius and serratus anterior during twelve rehabilitation exercises to determine which exercises are the best for scapular strengthening and scapular balance. They used forty-five healthy college-aged students (twenty men and twenty-five women with an average age of 20.7 years old) and placed surface electrodes on the four muscles during the randomized order of the rehabilitation exercises. They found five exercises that had optimal utilization of each of the four scapular muscles: horizontal abduction with external rotation, side lying external rotation, side lying forward flexion, and prone extension. Wilk, Meister, James and Andrews (2002) also discussed the importance of scapular muscle strength and stability in the rehabilitation of shoulder impingement syndrome. They found that rehabilitation techniques for restoring this strength are manual resistance to the scapula during protraction and retraction as well prone horizontal abduction.
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 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 impingement syndrome and excision of parts of the anteriorinferior acromion has become a pivotal part of the surgical treatment of the syndrome.
Subacromial impingement is not free of criticism. First, the identification of acromion type shows poor intra- and inter-observer reliability. Second, a computerized three-dimensional study failed to support impingement by any portion of the acromion on the rotator cuff tendons in different shoulder positions. Third, most partial-thickness cuff tears do not occur on bursal surface fibers, where mechanical abrasion from the acromion does occur. Fourth, it has been suggested that bursal surface cuff tears could be responsible for subacromial spurs and not the opposite. And finally, there is growing evidence that routine acromioplasty may not be required for successful rotator cuff repair, which would be an unexpected finding if acromial shape had a major role in generating tendon lesions. In summary, despite being a popular theory, the bulk of evidence suggest that subacromial impingement probably does not play a dominant role in many cases of rotator cuff disease.
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