Subacromial (shoulder) impingement syndrome

INTRODUCTION — Subacromial impingement syndrome (SIS; commonly referred to as shoulder impingement) refers to a combination of shoulder symptoms, examination findings, and radiologic signs attributable to the compression of structures around the glenohumeral joint that occurs with shoulder elevation. Such compression causes persistent pain and dysfunction. Shoulder pain is a common presenting complaint in primary care clinics, and SIS is likely the most common cause of shoulder pain in this setting [1,2].

Much has changed in our understanding of shoulder function and dysfunction since Neer's original classification of these disorders decades ago [3]. The diagnosis of SIS implies a spectrum of clinical findings, not injury to a specific structure.

The pathophysiology, diagnosis, and management of SIS is reviewed here. The approach to patients with shoulder pain, the shoulder examination, and conditions that may stem from SIS are discussed separately. (See "Evaluation of the adult with shoulder complaints" and "Physical examination of the shoulder" and "Rotator cuff tendinopathy" and "Presentation and diagnosis of rotator cuff tears".)

EPIDEMIOLOGY AND RISK FACTORS — Shoulder pain is highly prevalent within the general population, second only to lower back pain. Studies suggest that SIS is the most common cause of shoulder pain, accounting for approximately 30 to 35 percent of shoulder disorders [4-8]. However, epidemiologic calculations can vary depending upon how SIS is defined.

Risk factors — Repetitive activity at or above the shoulder during work or sports represents the main risk factor for SIS. As with many shoulder disorders, increasing age also predisposes to SIS [7,9]. SIS is common among athletes who participate in overhead sports [10-15]. These sports may include swimming, throwing, tennis, weightlifting, golf, volleyball, and gymnastics [16]. Overhead work activities that can increase risk for developing SIS include painting, stocking shelves, and mechanical repair [6,17]. (See "Throwing injuries: Biomechanics and mechanism of injury".)

Instability of the glenohumeral joint can lead to impingement. Such instability allows increased translation of the humeral head and predisposes patients to SIS, particularly if they engage in repetitive overhead activity [10,15].

Scapular instability and dyskinesis, in addition to glenohumeral joint laxity, also predisposes to impingement [18]. Other risk factors may include upper extremity inflexibility, particular acromion anatomy, and acromioclavicular joint pathology [16]. Scapular posture and orientation are thought to contribute to SIS; however, some researchers question this hypothesis [19]. In the authors' experience, poor work and home ergonomics contribute to SIS (table 1). (See 'Pathophysiology' below.)

Preliminary genomic studies suggest that genetic predisposition and biologic factors are associated with rotator cuff disease [20].

CLINICAL ANATOMY — Understanding the pathophysiology of SIS depends upon knowledge of shoulder anatomy (figure 1A-C and figure 2 and figure 3 and figure 4 and figure 5). The anatomy of the shoulder is discussed in detail elsewhere, but aspects of particular importance to SIS are reviewed here. (See "Evaluation of the adult with shoulder complaints", section on 'Anatomy and biomechanics'.)

Movement of the humeral head within the glenoid and of the scapulothoracic articulation achieves motion in multiple planes (ie, flexion, extension, internal rotation, external rotation, abduction, adduction) (figure 6 and figure 7 and picture 1 and table 2 and figure 8). This impressive range of motion can entail compression of structures within the shoulder, including the four rotator cuff muscles (ie, supraspinatus, infraspinatus, teres minor, and subscapularis), subacromial bursa, labrum, and biceps tendon (long head). (See 'Pathophysiology' below.)

Compression can occur against the acromion, osteoarthritic change on the undersurface of the acromioclavicular joint, and the coracoacromial arch. The acromion is the lateral projection of the posterior scapular spine, and its morphology may play a role in impingement syndrome (figure 9). The acromion forms an articulation with the lateral clavicle, and this articulation is stabilized by the acromioclavicular, coracoacromial, and coracoclavicular ligaments (figure 10 and figure 11). The coracoacromial arch is composed of the coracoid, anterior acromion, and coracoacromial ligament [21].

The small surface area of the glenoid fossa makes the glenohumeral joint relatively unstable. Stability depends largely on surrounding ligamentous, capsular, tendinous, and muscular structures. Any weakness or dysfunction of these stabilizing structures can increase glenohumeral instability, allowing increased translation of the humeral head. This increased motion makes surrounding structures susceptible to impingement [18].

PATHOPHYSIOLOGY

General — SIS consists of a spectrum of clinical findings, not injury to a specific structure (ie, rotator cuff) [22]. This spectrum of disease was first described by Charles Neer in 1972 and consists of the following stages [3,23]:

●Stage 1: Edema and hemorrhage (patient generally <25 years).

●Stage 2: Fibrosis and tendinitis (patient 25 to 40 years). Current term is "tendinopathy." (See "Rotator cuff tendinopathy".)

●Stage 3: Rotator cuff tear, biceps tendon rupture, bony change (patient generally >40 years). (See "Presentation and diagnosis of rotator cuff tears" and "Biceps tendinopathy and tendon rupture".)

The underlying mechanism of injury occurs when the rotator cuff, subacromial bursa, and other soft tissues (eg, long biceps tendon) are compressed between the humeral head and the undersurface of the acromion, acromioclavicular joint, or coracoacromial arch [16,21,24]. A number of anatomic and mechanistic factors play a role in this mechanism, including:

●Increased translation of the humeral head

●Acromion morphology that predisposes to impingement (figure 9)

●Decreased distance between undersurface of acromion and humeral head

●Osteophytic change of the acromioclavicular joint (image 1)

Weakness or dysfunction of the structures that stabilize the glenohumeral joint (eg, rotator cuff muscles) can lead to increased superior translation of the humeral head [18]. Increased translation can lead to compression of the subacromial bursa and rotator cuff tendons, causing injury. This extrinsic compression is one of several injuries that contribute to the development of SIS and rotator cuff tendinopathy. In addition, a decrease in the distance between the undersurface of acromion and humeral head appears to correlate with clinical symptoms in patients with impingement syndrome [25].

The relationship between the anterior third of the acromion and subacromial structures accounts in part for compression and the development of SIS [3]. Three acromion types have been described (figure 9):

●Type I: Flat

●Type II: Curved

●Type III: Hooked

These acromion morphologies were originally defined by their relationship to rotator cuff tear, with type III acromions having the highest association [18]. (See "Presentation and diagnosis of rotator cuff tears".)

Anatomic factors other than superior translation of the humeral head and acromion morphology can also contribute to the development of SIS. Osteophytic change of the acromioclavicular joint can cause compression and mechanical irritation of underlying soft tissues [18,21]. The lateral band of the coracoacromial ligament has been implicated in impingement of the rotator cuff [24,26,27]. Scapulothoracic dysfunction may play a role in SIS, but it remains unclear if such dysfunction is causative or secondary [18,28].

Throwing athletes — Throwing athletes suffer from a unique form of subacromial impingement. Impingement of the superior and posterior labrum and rotator cuff occurs with external rotation, extension, and abduction of the shoulder (ie, the cocking phase of throwing). This motion together with anterior translation of the humeral head causes impingement. Glenohumeral instability accentuates the anterior translation and subsequent impingement [10,18]. Repetitive use of the shoulder in extremes of rotation with throwing activities, combined with weakness of the rotator cuff and laxity of the glenohumeral ligaments, places athletes at risk for this form of SIS [10].

This form of impingement, sometimes referred to as posterior SIS, does not occur in all overhead athletes but specifically in throwing athletes whose motion involves a cocking phase (figure 12). Such activities include baseball pitching primarily but also tennis serves, American football throws, and javelin throws. (See "Throwing injuries: Biomechanics and mechanism of injury" and "Throwing injuries of the upper extremity: Clinical presentation and diagnostic approach" and "Throwing injuries of the upper extremity: Treatment, follow-up care, and prevention".)

CLINICAL PRESENTATION AND EXAMINATION

Clinical presentation — Symptoms of SIS are similar to those of rotator cuff tendinopathy. Patients complain of pain with overhead activity. The pain may localize to the deltoid area or lateral arm and often occurs at night or when lying on the affected shoulder. (See "Rotator cuff tendinopathy".)

Throwing athletes complain of shoulder stiffness and a difficult or prolonged warmup period. Pain occurs during the late cocking phase or the early acceleration phase of throwing. Initially, the athlete may not be able to localize the pain but with time may develop discomfort at the posterior shoulder [10]. Serving athletes (eg, tennis and volleyball players) may complain of pain at follow-through or terminal wrist snap before impingement becomes severe. (See 'Throwing athletes' above.)

Physical examination — Several structures may be involved in SIS, including the subacromial bursa, rotator cuff, biceps tendon, and labrum. Therefore, a number of shoulder examination techniques are used to ensure adequate sensitivity for detecting injury to susceptible structures. The overall approach to examination closely resembles that used to detect rotator cuff tendinopathy and is discussed separately. (See "Rotator cuff tendinopathy".)

Of note, no single examination maneuver is diagnostic for SIS. Performing a combination of tests improves predictive value of disease [29-31]. The Neer straight arm raise and Hawkins-Kennedy internal rotation maneuvers are sensitive for SIS (picture 2 and picture 3).

Performance of the shoulder examination, including special tests for impingement, is reviewed separately (see "Physical examination of the shoulder", section on 'Special tests for shoulder impingement'). Examination for SIS should include the following:

●Complete neck examination.

●Inspection for atrophy or disfigurement.

●Evaluation of glenohumeral range of motion (table 3) (including painful arc testing and a comparison of passive versus active motion).

●Rotator cuff strength testing (including drop arm test and external rotation strength testing).

●Specialty testing (painful arc (picture 4 and movie 1), empty can (picture 5), external rotation resistance (movie 2), and lift-off (picture 6) tests). Single tests, although suggestive if positive, are insufficient to rule in or rule out SIS. Three or more positive tests improve the diagnostic likelihood [32,33].

●Bedside musculoskeletal ultrasound if the technology and a proficient examiner are available.

Patients with SIS can manifest the following findings:

●Neck exam within normal limits.

●Tenderness present in the subacromial space or posterior shoulder.

●Glenohumeral range of motion limited by pain (eg, positive painful arc test).

●Reproduction of pain with specialty testing (eg, Neer (picture 2), Hawkins-Kennedy (picture 3), Yocum).

●Atrophy of posterior shoulder musculature if impingement is chronic.

●Shoulder strength normal, except in some cases of long-standing impingement.

In the throwing athlete, findings of SIS may include the following [10]:

●Asymmetric muscle development unrelated to sport (asymmetry is expected in throwers and racquet sport athletes).

●Tenderness over the region of the posterior rotator cuff and capsule.

●Increase in external rotation and symmetrical decrease in internal rotation compared with the unaffected (nondominant) shoulder (ie, glenohumeral internal rotation deficiency).

●Possible increased laxity of the glenohumeral joint (anterior translation).

●Positive posterior impingement sign. (See "Throwing injuries of the upper extremity: Clinical presentation and diagnostic approach".)

To perform the posterior impingement test, place the patient's shoulder in 90 degrees of abduction, 110 degrees of extension, and maximal external rotation. The test is performed with the patient supine and is positive if it recreates the athlete's shoulder pain (picture 7). Limited external rotation may be associated with SIS, particularly in patients who experience shoulder pain at night [34].

DIAGNOSTIC IMAGING

Plain radiographs — In general, radiographs are unnecessary for the diagnosis of SIS but can be obtained to rule out other associated pathology.

Plain radiographs may be useful in the following clinical situations:

●No improvement with conservative therapy

●Evaluation of acromion morphology (see 'Pathophysiology' above)

●Evaluation of the acromioclavicular joint (see 'Pathophysiology' above)

●Evaluation of distance between acromion and humeral head (see "Presentation and diagnosis of rotator cuff tears", section on 'Plain radiographs')

●Evaluation for tendon calcification (image 2)

●Anatomical evaluation prior to subacromial or glenohumeral joint injection (not essential prior to injection)

Musculoskeletal ultrasound — Musculoskeletal ultrasound is an accurate tool for the evaluation of superficial tendon and muscle lesions of the shoulder, as well as bursitis, and enables dynamic examination at the bedside. In the hands of experienced users, dynamic ultrasound often reveals the site of impingement and tendons involved (picture 8 and movie 3). Its role in the evaluation of the rotator cuff is discussed in detail separately. (See "Rotator cuff tendinopathy" and "Musculoskeletal ultrasound of the shoulder".)

Additional ultrasound resources — Instructional videos demonstrating proper performance of the ultrasound examination of the shoulder and related pathology can be found at the website of the American Medical Society for Sports Medicine. Registration must be completed to access these videos, but no fee is required.

Magnetic resonance imaging — Magnetic resonance imaging (MRI) is most useful for assessing rotator cuff pathologies other than SIS, including labral tears, biceps tendon injuries, and intra-articular injuries that may benefit from surgery. (See "Superior labrum anterior to posterior (SLAP) tears" and "Biceps tendinopathy and tendon rupture".)

MRI is generally performed in the following circumstances:

●Symptoms and function fail to improve despite appropriate conservative therapy.

●Diagnosis remains unclear after initial evaluation.

●A rotator cuff or labrum tear is suspected based upon clinical presentation. (See "Presentation and diagnosis of rotator cuff tears" and "Superior labrum anterior to posterior (SLAP) tears".)

Subacromial impingent is a dynamic condition not reliably diagnosed by MRI. However, MRI can identify shoulder morphology and pathologic findings associated with SIS [35]. These findings may include a low-lying acromion, inflammation of subacromial structures, bone spurs (which may compress the supraspinatus tendon and subacromial bursa), and abnormalities of the acromioclavicular joint [9,25,36].

According to an observational study of 69 patients with SIS, shoulder pain and dysfunction were associated with increased subacromial bursa width and a "halo sign" around the biceps tendon, suggesting glenohumeral joint effusion [37]. The presence of a subacromial bone spur (osteophyte) in the absence of bursitis did not correlate with shoulder dysfunction. In an observational MRI study of 137 females with symptoms of shoulder impingement, subacromial anatomy characteristics associated with SIS were not found to correlate with supraspinatus or infraspinatus tears [38].

MRI arthrography is not indicated solely for the evaluation of suspected subacromial impingement.

DIAGNOSIS — SIS is a clinical diagnosis made on the basis of a suggestive history and confirmatory examination findings. Patients usually complain of pain with overhead activity that often localizes to the deltoid area or lateral arm and often occurs at night or when lying on the affected shoulder. The Neer straight arm raise and Hawkins-Kennedy internal rotation maneuvers are sensitive tests for SIS (picture 2 and picture 3). Imaging findings, such as diminished subacromial space on plain radiograph or signs of supraspinatus impingement on ultrasound, can help to confirm the diagnosis.

DIFFERENTIAL DIAGNOSIS — Shoulder pain is very common in the general population, and the differential diagnosis can be extensive (table 4). This differential diagnosis and a discussion of how to approach the patient with undifferentiated shoulder pain are provided separately. (See "Evaluation of the adult with shoulder complaints".)

Pain from SIS may result from injury to one or more structures, possibly including the rotator cuff, subacromial bursa, biceps tendon, and labrum. The history and physical examination are used to identify the structures involved and direct treatment. Also, patients with shoulder impingement can concurrently develop cervical radiculopathy, emphasizing the importance of a careful neurologic examination in patients with shoulder complaints [39-41]. Both conditions should be treated [42]. (See "Clinical features and diagnosis of cervical radiculopathy".)

Conditions commonly confused with subacromial impingement include those listed below. It is important to distinguish SIS from rotator cuff tear and adhesive capsulitis, for which management differs.

●Rotator cuff tear – Rotator cuff tears (most often involving the supraspinatus) frequently cause weakness in addition to pain and are associated with a positive drop arm test (movie 4) and weakness with resisted external rotation. Most develop chronically in adults over 40 years of age and may be a complication of long-standing subacromial impingement. Tears in older adolescents and young adults are more often sustained acutely during sport. Musculoskeletal ultrasound and MRI can aid diagnosis. (See "Presentation and diagnosis of rotator cuff tears".)

●Adhesive capsulitis (frozen shoulder) – Patients with adhesive capsulitis complain of increasing motion restrictions associated with pain and dysfunction while performing activities of daily living. They often give a history of recent shoulder injury or surgery and may be diabetic. The condition manifests as restricted active and passive glenohumeral motion. MRI often reveals characteristic capsular thickening and adhesions. (See "Frozen shoulder (adhesive capsulitis)".)

●Biceps tendinopathy – Patients with biceps tendon-related pain typically complain of isolated anterior shoulder pain in the region of the bicipital groove. Examination generally reveals focal tenderness of the tendon within the bicipital groove (picture 9) and a positive Speeds test (picture 10 and movie 5). Musculoskeletal ultrasound is useful for identifying pathology of the biceps tendon. When necessary, MRI may aid diagnosis. (See "Biceps tendinopathy and tendon rupture".)

●Subcoracoid impingement – Subcoracoid impingement is another form of shoulder impingement that occurs when anterior structures (usually the long head of the biceps and subscapularis tendons) are compressed between the coracoid process and the lesser tuberosity of the humerus [43-45]. Subcoracoid impingement usually occurs with forward flexion, adduction, and internal rotation of the shoulder, and symptoms can be reproduced with some combination of these movements. Consequently, subcoracoid impingement is seen with overhead and throwing activities. It may also stem from anatomic changes in the proximal humerus or glenoid following surgery or trauma. Patients generally complain of anterior shoulder pain. Typically, pain can be reproduced by passively moving the patient's shoulder into forward flexion and internal rotation. Tenderness at the coracoid process may be present but is not a consistent finding.

●Shoulder labrum injury – Labral tears, including superior labrum anterior-posterior tears (SLAP lesions), usually manifest as chronic pain with overhead activity, often in combination with weakness, instability, and sport-specific dysfunction. Patients may complain of "clicking" in the shoulder or comparable mechanical symptoms. A combination of specific examination maneuvers (eg, anterior glide test, compression rotation test) help to determine the likelihood of labral injury. Magnetic resonance arthrogram is the most accurate test for identifying labral injuries but should be obtained in consultation with a shoulder surgeon as many such injuries are not amenable to surgical repair. (See "Superior labrum anterior to posterior (SLAP) tears".)

INDICATIONS FOR ORTHOPEDIC REFERRAL — We refer patients with SIS for orthopedic evaluation if three months of conservative treatment (including appropriate physical therapy) fails to improve symptoms and function, if a diagnosis of rotator cuff tear and labrum tear is suspected, or if refractory adhesive capsulitis (frozen shoulder) develops. (See "Presentation and diagnosis of rotator cuff tears" and "Frozen shoulder (adhesive capsulitis)".)

If deemed necessary, surgical intervention for SIS should be individualized by age, comorbidities, and level of physical demand [18]. Surgical intervention may include debridement of the rotator cuff or labrum, acromioplasty with debridement, or rotator cuff repair. If laxity is present in the throwing athlete, a capsular repair may improve outcomes [10,18].

Surgical outcomes — Traditionally, subacromial decompression to increase space and prevent impingement of the rotator cuff was commonly performed for recalcitrant cases of SIS. However, the results of several controlled trials have raised doubts about the effectiveness of surgery for isolated shoulder impingement [46-48]. According to a meta-analysis of 13 randomized trials involving 1062 patients, surgical intervention (most often arthroscopic decompression) failed to produce clinically significant reductions in pain at one- and two-year follow-up or improvements in shoulder function at one-year follow-up compared with conservative treatment (primarily exercise therapy) [46]. Subsequent, well-conducted randomized trials have reported similar results [49].

MANAGEMENT — Initial management of SIS is similar to that for rotator cuff tendinopathy. Some aspects of management are supported by randomized trials, but many are not, and the overall quality of available evidence pertaining to treatment is weak [50]. A description of the management approach and basic treatment for SIS follows. Issues related to the specific management of SIS, as opposed to rotator cuff tendinopathy, are discussed here; evidence for treatments used to manage both SIS and rotator cuff tendinopathy is reviewed elsewhere. (See "Rotator cuff tendinopathy", section on 'Treatments'.)

General approach — We approach patients with suspected SIS in the manner described here but recognize that evidence is limited and alternative approaches may be reasonable. We begin with a focused history and physical examination. (See 'Clinical presentation and examination' above.)

If, based on the history and examination, we suspect a clinically significant rotator cuff tear, a labral tear, or other significant pathology not amenable to conservative therapy, we generally refer the patient to an orthopedic surgeon. We maintain a lower threshold for referral in the case of high-functioning athletes. If we suspect a rotator cuff tear but the patient has minimal weakness and reasonable motion, we generally embark on a course of conservative medical management consisting primarily of physical therapy. Conservative, nonoperative management is appropriate for adhesive capsulitis. (See "Presentation and diagnosis of rotator cuff tears" and "Superior labrum anterior to posterior (SLAP) tears" and "Frozen shoulder (adhesive capsulitis)" and "Rehabilitation principles and practice for shoulder impingement and related problems".)

If we suspect acute SIS, we initiate conservative medical management including the acute treatments described below and an appropriate physical therapy program. (See 'Acute treatment' below and 'Physical therapy' below.)

The duration and success of physical therapy depends upon many factors, including underlying pathology, compliance with treatment, and the appropriateness of the program prescribed. We believe that whenever possible, it is important for patients to begin rehabilitation under the guidance of a knowledgeable professional (eg, athletic trainer or physical therapist with experience managing shoulder disorders).

Subacromial injection of a glucocorticoid and analgesic (eg, lidocaine) may be used to augment physical therapy, particularly when pain prevents or hinders a patient's participation. It is best to refrain from such injection if symptoms or signs consistent with a tear of the rotator cuff are present, particularly if surgical repair is a possibility, as such injections have been associated with worse surgical outcomes. (See 'Subacromial injection' below.)

If function and symptoms improve over several weeks of physical therapy, we have the patient continue therapy and begin a gradual, stepwise resumption of activities, including sports. Should function fail to improve despite adequate rehabilitation, we obtain imaging studies. Musculoskeletal ultrasound, if not performed previously, may be obtained first. A plain radiograph should be obtained for persistent symptoms, if not performed at initial presentation, to assess for anatomic variants, such as a downsloping acromion or os acromiale, and acromioclavicular or glenohumeral osteoarthritis. We obtain an MRI if the musculoskeletal ultrasound is nondiagnostic or not available, a rotator cuff or labral tear is suspected, or the diagnosis is unclear. (See 'Diagnostic imaging' above and "Musculoskeletal ultrasound of the shoulder".)

Subsequent management depends upon the results of imaging studies. Alternative diagnoses are managed accordingly. We refer patients with rotator cuff tears, labral tears, and refractory adhesive capsulitis to an orthopedic surgeon. For rotator cuff tendinopathy or subacromial bursitis, we continue conservative management. Some clinicians may choose to incorporate adjunct treatments, such as glucocorticoid injection, at this point or earlier if pain is severe. Physical therapy may require several months before adequate shoulder function is achieved. We refer the patient to an orthopedic surgeon if, after six to nine months of conservative treatment, function and symptoms fail to improve significantly. (See 'Indications for orthopedic referral' above.)

Acute treatment — Research to direct management of SIS is limited, but the following is a generally accepted approach for acute symptoms:

●Cryotherapy – Ice may decrease acute swelling and inflammation and provide some analgesia.

●Rest – This means avoiding activities that aggravate symptoms, including all overhead activities.

●Nonsteroidal antiinflammatory drugs (NSAIDs)

For acute injuries, we give a short course (two to four weeks) of scheduled NSAID therapy. If the patient has no major contraindications to high-dose NSAIDs (eg, bleeding gastric ulcer), we often prescribe a two-week regimen (eg, ibuprofen 800 mg three times daily, naproxen 500 mg twice daily). If there are relative contraindications to high-dose NSAIDs (eg, mild renal insufficiency), we may give a two-week course of an over-the-counter NSAID using a lower dose (eg, ibuprofen 200 to 400 mg). Thereafter, patients may use an NSAID as needed. (See "NSAIDs: Therapeutic use and variability of response in adults" and "Nonselective NSAIDs: Overview of adverse effects".)

Physical therapy — Although studies of physical therapy regimens for SIS are limited, available evidence and our clinical experience suggest that properly designed and performed physical therapy programs effectively treat most patients with SIS and should be implemented prior to surgical referral. According to a network meta-analysis, exercise and exercise-based therapies (ie, physical therapy) are ideal treatment for early SIS [51]. In addition, a meta-analysis of nine randomized trials involving 1014 patients concluded that subacromial decompression provided no additional benefit in symptom relief or improved function compared with physical therapy or placebo surgery [52]. (See 'Pathophysiology' above.)

The physical therapy programs used most often for SIS closely resemble those used for rehabilitation of rotator cuff tendinopathy. Physical therapy for these conditions is discussed in detail separately. (See "Rehabilitation principles and practice for shoulder impingement and related problems".)

A rehabilitation program for SIS should follow a progression from an initial focus on restoring mobility, to stability and strength training, and finally to integration of shoulder rehabilitation into overall functional training. A qualified therapist or trainer can help the clinician to design and implement an effective rehabilitation program for SIS, which generally includes:

●Range-of-motion exercises to improve motion in all planes (flexion, extension, abduction, adduction, and internal and external rotation).

●If SIS is associated with motion restriction, glenohumeral joint mobilization, including specific maneuvers for capsular structures.

●Strengthening exercises focusing on the rotator cuff and scapular stabilizers. Eccentric exercises (application of a load during muscle lengthening) are included in the program.

●Biomechanical training to improve the throwing motion or other repetitive activity that led to injury.

●Exercises to improve the strength and stability of the torso (core musculature).

●Integration of shoulder rehabilitation into patient- or sport-specific functional activities.

Overhead athletes should generally refrain from all throwing activities for two to four weeks while performing physical therapy for the rotator cuff and scapular stabilizers. As symptoms resolve, athletes begin a graded return to throwing [10]. Approximately 95 percent of throwing athletes return to their previous level of function if started early on a well-designed rehabilitation program [15]. (See "Throwing injuries of the upper extremity: Clinical presentation and diagnostic approach" and "Throwing injuries: Biomechanics and mechanism of injury" and "Throwing injuries of the upper extremity: Treatment, follow-up care, and prevention".)

In addition to standard rehabilitation exercises, the authors have found that having patients work with a physical therapist to create simple postural modifications to improve ergonomics at home and work often helps to reduce the symptoms of SIS. As a general guideline, modifying specific activities to limit abduction and rotation of the shoulder, while keeping the arms close to the body and below shoulder level, should diminish symptoms associated with impingement (table 1). Proper ergonomics for such activities as sleeping, driving, and working on a computer are discussed separately (see "Joint protection program for the upper limb" and "Overview of joint protection"). Inconsistent diagnostic criteria, disparate outcome measures, small numbers of participants, differing follow-up intervals, and variable treatment protocols make studies of physical therapy for SIS difficult to interpret [4,50,53,54]. Nevertheless, the available evidence suggests that stretching and strengthening exercise programs consistent with the guidelines listed above provide effective treatment for SIS [4,6,17,55,56]. The addition of glenohumeral mobilization exercises to rehabilitation further improves outcomes. One small prospective study found that patients with less acromion-humeral narrowing showed greater functional improvements following physical rehabilitation [57].

A systematic review of randomized trials of conservative interventions for SIS reported the overall quality of studies to be very low but found specific exercises for the shoulder and scapular muscles to be among the most significant interventions for improving shoulder function (four trials, n = 202, standardized mean difference [SMD] -0.57, 95% CI -0.85 to -0.29) [50]. One randomized trial of 97 patients diagnosed with SIS reported that those treated with such an exercise regimen had significant improvements in shoulder function and were less likely to undergo surgery than patients treated with standard exercises (20 versus 63 percent, odds ratio [OR] 7.7, 95% CI 3.1-19.4) [58].

Scapula-focused therapy (including strengthening exercises for scapular stabilizers and manual mobilization) appears to improve scapular muscle strength, but its effects on shoulder function and pain reduction remain unproven [59]. The results of a broad systematic review of studies of scapular rehabilitation suggest that scapular stabilizing exercises reduce SIS-related pain, even in the absence of appreciable change to scapular position or motion [60].

Taping — Kinesio tape (ie, kinesiology tape) is a type of elastic tape that some clinicians apply to specific injured regions with the intention of stimulating improved blood and lymph flow and kinesthetic sense and providing support to muscles and tendons. Adjunct treatment with kinesio tape has become popular despite a dearth of high-quality studies supporting its efficacy.

A well-designed randomized trial involving 100 patients diagnosed with subacromial impingement reported no added benefit from kinesio tape (or NSAID use) when performed in combination with a short-term exercise program [61]. Studies of taping were included in a more comprehensive meta-analysis of nonsurgical treatments for SIS [50]. In this review, tape treatment was found to be superior to sham taping for pain (five studies, n = 272) and short-term functional improvement (three studies, n = 161), but the overall quality of these studies was limited. A subsequent systematic review of 10 clinical trials found no conclusive evidence that kinesio taping reduced pain, improved overall shoulder function, or increased mobility [62]. Significant concerns for bias and inconsistency were noted in most studies. A subsequent randomized trial of 120 patients with SIS comparing therapeutic exercise plus taping versus therapeutic exercise alone found that taping provided some additional benefit for symptom reduction and improved mobility [63].

Subacromial injection — We offer a subacromial injection of glucocorticoid to patients with persistent pain and dysfunction despite six weeks of acute treatment and physical therapy. In some cases (eg, severe, intractable pain), injection prior to six weeks is appropriate. The symptomatic relief gained from such injections may improve a patient's effort and compliance with physical therapy, thereby accelerating recovery.

In such cases, we administer a combination of glucocorticoid and local anesthetic. Typically, this includes 1 to 2 mL of a glucocorticoid and 1 to 2 mL of 1 or 2% lidocaine. An intermediate (eg, triamcinolone 40 mg) or high-potency (eg, betamethasone) glucocorticoid is used most often (table 5). Ultrasound-guided injection is preferred when possible and supported by the results of a systematic review [50].

Evidence supporting the use of subacromial glucocorticoid injection for the treatment of SIS is limited [6,64-67]. Several randomized trials report a short-term reduction of symptoms following injection [50,68-71]. Other trials report that injection of an NSAID (eg, ketorolac) provides similar, or possibly better, short-term pain relief than glucocorticoid injection without the potential adverse effects from repeated exposure to glucocorticoids [72-75]. (See "Joint aspiration or injection in adults: Complications", section on 'Glucocorticoid-associated toxicity'.)

Dry needling — Limited evidence suggests that dry needling may provide short- and medium-term improvements in pain and disability [76,77]. The effectiveness of dry needling for tendinopathy is discussed separately. (See "Overview of the management of overuse (persistent) tendinopathy", section on 'Dry needling'.)

Alternative and unproven treatments — Only very weak evidence exists to support the use of the modalities listed below for the treatment of SIS, and we do not routinely use them in the care of our patients [50]. Alternative treatment modalities may include:

●Electrical stimulation, phonophoresis, and iontophoresis. (See "Rotator cuff tendinopathy".)

●Therapeutic ultrasound – No evidence supports the use of ultrasound for SIS [4,6,55,78,79].

●Laser – As a single intervention, laser may provide greater relief than placebo, but studies suggest it provides no added benefit when used in combination with other conservative treatments (eg, physical therapy) [4,55,79].

●Acupuncture – Limited evidence from clinical studies suggests that acupuncture may provide some benefit when combined with physical therapy [4,80].

●Radial extracorporeal shock wave therapy (rESWT) – No additional benefit for subacromial shoulder pain, except possibly in patients with rotator cuff calcification [81]. (See "Calcific tendinopathy of the shoulder", section on 'Extracorporeal shock wave therapy'.)

FOLLOW-UP CARE — We have patients return to clinic within two weeks of the start of their initial treatment. This allows us to reassess our original clinical impression and to determine the effectiveness of interventions. Sometimes pain limits the initial physical examination, and rehabilitation programs may need modification.

Monthly follow-up thereafter is appropriate. More frequent evaluation may be needed to facilitate efficient return to sport or work activity. If nonoperative therapy does not provide relief within three to six months, orthopedic referral is appropriate. (See 'Indications for orthopedic referral' above.)

COMPLICATIONS — Untreated, chronic SIS can result in a significant loss of glenohumeral motion, possibly leading to adhesive capsulitis (frozen shoulder). This complication can be difficult to treat and may require surgical intervention. For the athlete, long-standing SIS can result in weakness, dysfunction, chronic pain, and the inability to perform effectively.

RETURN TO SPORT OR WORK — Guidelines are the same as those for rotator cuff tendinopathy. (See "Rotator cuff tendinopathy".)

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Shoulder soft tissue injuries (including rotator cuff)".)

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5^th to 6^th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10^th to 12^th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on “patient info” and the keyword(s) of interest.)

●Basics topic (see "Patient education: Shoulder impingement (The Basics)")

●Beyond the Basics topic (see "Patient education: Shoulder impingement syndrome (Beyond the Basics)")

SUMMARY AND RECOMMENDATIONS

●Definition, epidemiology, risk factors – Subacromial (shoulder) impingement syndrome (SIS) refers to a combination of shoulder symptoms, examination findings, and radiologic signs attributable to the compression of structures around the glenohumeral joint that occurs with shoulder elevation. Repetitive activity at or above the shoulder during work or sports represents the main risk factor. (See 'Epidemiology and risk factors' above.)

●Pathophysiology – SIS consists of a spectrum of clinical findings, not injury to a specific structure (ie, rotator cuff). Increased translation of the humeral head, acromion morphology that predisposes to impingement, and osteophytic change of the acromioclavicular joint all may play a role. Throwing athletes may develop posterior SIS. (See 'Pathophysiology' above.)

●Differential diagnosis – It is important to distinguish SIS from rotator cuff tear and adhesive capsulitis (frozen shoulder). Rotator cuff tears generally cause weakness, occur in older patients, and are associated with a positive drop arm sign and weakness with external rotation. Adhesive capsulitis is often associated with a history of shoulder injury and manifests as restricted active and passive glenohumeral motion. (See 'Differential diagnosis' above.)

●Clinical presentation and physical examination – Patients with SIS complain of pain with overhead activity. Examination techniques for the shoulder are sensitive for the presence of SIS but cannot reliably distinguish among specific causes of pain and dysfunction. In addition to general tests of shoulder motion and strength, the Neer (picture 2) and Hawkins-Kennedy (picture 3) impingement tests are useful. The neck and neurologic function must be carefully examined. (See 'Clinical presentation and examination' above.)

●Diagnostic imaging – Plain radiographs are unnecessary for the initial evaluation of suspected SIS. Ultrasound often reveals the site of impingement and tendons involved. (See 'Diagnostic imaging' above.)

●Indications for surgical referral – We refer patients for orthopedic evaluation only if three months of conservative treatment (including appropriate physical therapy) fails to improve symptoms and function or if a rotator cuff tear, labrum tear, or refractory adhesive capsulitis, is suspected. (See 'Indications for orthopedic referral' above.)

●Management – Evidence to guide management of SIS is limited. We suggest that patients diagnosed with SIS be treated with physical therapy rather than surgery or therapeutic injection (Grade 2B). Properly designed and performed physical therapy programs effectively treat most patients. In select patients, subacromial injection of a glucocorticoid and analgesic can reduce symptoms and allow for full participation in physical therapy. (See 'Management' above and "Rehabilitation principles and practice for shoulder impingement and related problems".)