Subacromial Pain Syndrome

Introduction

There has been huge debate in relation to the diagnostic labelling of non-traumatic shoulder pain related to the structures of the subacromial space. The diagnostic label Subacromial Impingment Syndrome (SIS), presenting as anterograde-lateral shoulder pain when the arm is elevated, was first introduced in 1972 by Dr Charles Neer and was based on the mechanism of structural impingement of the structures of the subacromial space.[1][2][3] SIS has been viewed as symptomatic irritation of the subacromial structures between the coraco-acromial arch and the humeral head during elevation of the arm above the shoulder / head [1] and as reflected by the literature is considered by many to be one of the most common causes of shoulder pain. Neers Classification of Impingement identified four types of Shoulder Impingement identified below;

Neer Classification of Subacromial Impingement

According to Neer, impingement syndrome is divided into three stages:

Type I: <25 years old, Reversible, Swelling, Tendonitis, No Tears, Conservative Treatment.

  • Moderate pain during exercise, no loss of strength and no limitation in movement. Edema and/or hemorrhage may be present. This stage generally occurs in patients less than 25 years of age and is frequently associated with an overuse injury. At this stage the syndrome could be possibly reversible. [4]

Type II: 25-40 years old, Permanent Scarring, Tendonitis, No Tears, Subacromial Decompression

  • Pain is usually reported during ADL and especially during the night. loss of mobility is associated with this stage. Type II is more advanced and tends to occur in patients between 25 to 40 years of age. The pathological changes show fibrosis as well as irreversible tendon changes.[4]

Type III: >40 years old, Small Rotator Cuff Tear, Subacromial Decompression with Debridement/Repair

Type IV: >40 years old, Large Rotator Cuff Tear, Sub Acromial Decompression with Repair

While Neers Classification of SIS was key to understanding shoulder pathology at the time, SIS was further broken down into four subtypes associated with either External Impingement (Primary or Secondary) and Internal Impingement.

External Impingement

Figure 1. Acromion Shapes
  • Primary External Impingement related to structural changes, either congenital or acquired, that mechanically narrow the subacromial space such as; bony narrowing or osteophyte formation, bony malposition after a fracture, or an increase in the volume of the subacromial soft tissues.[5][6][7][8][9][10][11][12][13][14][15] The shape of the acromion process of the scapula or shoulder blade may play an important role in recovery and treatment from primary impingement.
  • Secondary External Impingement related to abnormal scapulothoracic kinematics, strength balance alteration resulting in functional disturbance in the centering of the humeral head, leading to an abnormal displacement of the center of rotation when the arm is elevated. Generally caused by weakness of the Rotator Cuff Muscles (functional instability) combined with a glenohumeral joint capsule and ligaments that are to loose (micro-instability). Impingement generally occurs at the coracoacromial space secondary to anterior translation of the humeral head as opposed to the Subacromial space that is seen in primary impingement. Typically occurs in younger individuals with pain is located in the anterior or anterolateral aspect of the shoulder. The symptoms are usually activity specific and involve overhand activities.[8][9][10][11][16][17][18][8][19][20]

Internal Impingement

  • Internal Glenoid Impingement is probably the most common cause of posterior shoulder painin the throwing or overhead athlete. Caused by impingement of the articular surface (intra-articular) of the Rotator Cuff (posterior edge of the supraspinatus and the anterior edge of the infraspinatus) against the posterior-superior-glenoid and glenoid labrum. Mainly seen with repetitive overhead activities, this positioning becomes pathologic during excessive external rotation, anterior capsular instability, scapular muscle imbalances[19], and/or upon repetitive overload of the rotator cuff musculature [20][16][17][18][8][19][20] .These deficiencies result in poor scapulohumral control.

These definitions and descriptions of SIS are based on a hypothesis that acromial irritation leads to external abrasion of the bursa, rotator cuff or other structures within the subacromial space.[3] Throughout the years, the description progressed from SIS to ‘impingement related shoulder pain’, with the growing opinion that ‘impingement’ represents a cluster of symptoms and a possible mechanism for the pain, rather than a pathoanatomic diagnose itself. This remained the dominant theory for injury to structures within the subacromial space for the past 40 years, and has been the rationale to guide clinical tests, conservative treatment, surgical procedures and rehabilitation protocols,[2] however the validity of this model of acromial impingement has been challenged from both a theoretical and practical perspective throughout the last decade, with suggestions that the use of SIS terminology can potentially contribute to negative expectations of physiotherapy and conservative treatment for patients, which may compromise outcome, often resulting in an increased incidence for surgery.[21][2][3][21]

While controversy surrounding the diagnostic labelling continues to exist, with many different terms in use to describe subacromial pain and it’s pathology in the literature it has been suggested that the label of SIS does not fully explain the mechanism. Many other diagnostic labels have been proposed but it has been suggested that the term Subacromial Pain Syndrome (SAPS) best describes the pain thought to originate from structures lying between the acromion and the humeral head, most often associated with some degree of shoulder dysfunction,[22] but does not reflect many other causes of shoulder pain located outside the subacromial space.

Definition / Description

Subacromial Pain Syndrome is by defined Diercks et al [22] as all non-traumatic, usually unilateral, shoulder problems that cause pain, localized around the acromion, often worsening during or subsequent to lifting of the arm. It is basically a generic term that encompasses pain associated with any lesion within a structure or structures within the Subacromial Space. As such the SAPS incorporates all conditions related to subacromial structures such as subacromial bursitis, tendinosis calcarea, rotator cuff tendinopathy, rotator cuff tears, biceps tendinopathy, or tendon cuff degeneration as a result of the controversy regarding it’s pathogenesis.[22] [23]

Clinically Relevant Anatomy

The Subacromial Space is the space beneath the acromion (between the acromion and the top surface of the humeral head). This space is outlined by the acromion and the coracoid process (which are parts of the scapula), and the coraco-acromial ligament which connects the two. [24]
Figure 2. Subacromial Structures

The Subacromial Space measures approximately 1cm and contains the; [9][3][25]

  • Coracoacromial Arch, composed of the Acromion, Coracoid Process and Coracoacromial Ligaments[7]
  • Humeral Head
  • Subacromial Bursa[26]
  • Tendons of the Rotator Cuff; Supraspinatus, Infraspinatus, Teres Minor and Subscapularis
  • Tendon of the Long Head of Biceps Brachii
  • G-H Joint Capsule

Prevalence / Incidence

SAPS is the most common disorder of the shoulder, accounting for anywhere between 44% to 65% of all complaints of shoulder pain, [27] with the incidence increasing with age.[28] Peak incidence occurs during the sixth decade of life.[29]

Etiology

There remains debate in regards to the ethiology of subacromial pain, but it is proposed that the mechanisms include intrinsic, extrinsic and combined factors including muscle imbalance and anatomical factors that may affect the subacromial space; variations in the acromial shape; the anterior slope; the angle of the acromion; and the lateral extension of the acromion over the humeral head. Osseous changes of the inferior acromio-clavicular joint or the coracoacromial ligament may also affect the subacromial space. [23]

While it is accepted that multiple factors are involved in the pathetiology, several unresolved issues remain such as: which subacromial structure is first engaged by pathology, and what are the pain-generating mechanisms? [23]

Clinical Presentation

Figure 3. Painful Arc

The affected patients are generally over age 40 and suffer from persistent pain without any known preceding trauma.[30] Patients report pain on elevating the arm between 70 ° and 120 °, the “Painful Arc” (Figure 3), on forced movement above the head, and when lying on the affected side.[16] The symptoms may be acute or chronic. Most often it is a gradual, degenerative condition that causes "impingement", rather than due to a strong external force. Therefore, patients often have difficulties with determining the exact onset of symptoms.

Differential Diagnosis

There are a variety of shoulder conditions that can initially be confused with subacromial subacromial pain syndrome [31]. A thorough physical examination should exclude other conditions such as [32]:

Physical Examination

Detailed History and clinical examination are necessary for diagnosis of SAPS. No single test alone is accurate to diagnose SAPS or sufficiently differentiate between various shoulder disorders [33][34], but using a combination of specific tests increases the post-test probability of the diagnosis of SAPS. [35][36][37][38]

The Dutch Orthopaedic Association Guidelines [22] for diagnosis and treatment of subacromial pain syndrome recommend a combination of the following tests to aid in the diagnosis;

The combination of the Hawkins-Kennedy Test, the Painful Arc and the Infraspinatus Resistance Test have a considerably higher predictive value;[22][39]

  • 3 tests are positive: the probability that the patient has SAPS is (10,56) [39]
  • 2 tests are positive: the probability that the patient has SAPS is (5,03) [39]
  • 1 test is psitive: the probability that the patient has SAPS is (0,90) [39]
  • 0 tests are positive: the probability that the patient has SAPS is (0,17) [39]

The following additional test should also be used to rule out a Rotator Cuff Tear;

Diagnostic Procedures

Radiographs may be used to detect anatomical variants, calcific deposits or acromioclavicular joint arthritis. The three recommended views are[42]:

  • Antero-posterior View with the arm at 30 degrees external rotation which is useful for assessing the glenohumeral joint, subacromial osteophytes and sclerosis of the greater tuberosity.
  • Outlet Y View is useful because it shows the subacromial space and can differentiate the acromial processes.
  • Axillary View is helpful in visualizing the acromion and the processus coracoideus, as well as coracoacromial ligament calcifications.

The size of the subacromial space can also be measured. MRI can show full or partial tears in the tendons of the rotator cuff, and inflammation to weak structures.[43] Ultrasound and arthrography are being used when rotator cuff tears are suspected or in complex cases.[4]

Ultrasound of the shoulder is a sensitive and specific method. The diagnostic accuracy is good and comparable to that of conventional MRI for identification and quantification of complete (full-thickness) rotator cuff injuries. The sensitivity and specificity of ultrasound and conventional MRI are not significantly different in the detection of partial- or full-thickness rotator cuff tears with MR Arthrography an accurate method to rule out partial rotator cuff injuries.[22]

The Dutch Orthopaedic Association Guidelines arise the use of Ultrasoundas the most valuable and cost-effective diagnostic imaging if a first period of conservative treatment fails, which they recommend can be combined with conventional radiography of the shoulder to determine osteoarthritis, osseous abnormalities, and presence/absence of calcium deposits. They suggest that where reliable ultrasound is not available or inconclusive, then MRI of the shoulder is indicated and should be used in patients who are eligible for surgical repair of a rotator cuff tear to assess the degree of retraction and atrophied fatty infiltration. An MRI study with intra-articular contrast can be considered if any intra-articular abnormality or a partial rotator cuff injury have to be ruled out. It is preferable for a study in abduction and external rotation (ABER) to be part of an MR arthrography protocol. [22]

Treatment/Management

The treatment depends on age, activity level and general health of the patient. The goal is to reduce pain and regain function. Conservative treatment is the first line of treatment, and should be considered for up to about a year until improvement and return to function are noticed.[44] Surgery should only be considered if the patient does not respond to exhaustive non-operative treatment. [22]

Conservative treatment consists of; relative rest, reduction of aggravating activities, in particular overhead activities, NSAID’s to reduce pain and swelling, physical therapy management, and subacromial injection. Cortisone is often used because of its anti-inflammatory and pain reducing effect, but this is controversial depending on the structure involved and should not be utilised in tendon related pain.[45]

There is no convincing evidence that surgical treatment is more effective than conservative treatment and surgery should only be considered when the conservative treatments fail to reduce the pain or restore function. Several surgical techniques are available, depending on the character and severity of the injury but no clear preference for surgical technique can be indicated currently.[22][46]

  • Surgical Repair of torn tissues, mostly of supraspinatus muscle, long head of biceps tendon or joint capsule. Note: a rotator cuff tear is not an indication for surgery. [31]
  • Bursectomy or removal of the subacromial bursa.
  • Subacromial Decompression to increase the subacromial space by removing bony spurs or prominences on the underside of the os acromiale 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.

No difference in outcome (shoulder function, complications) has currently been shown between an arthroscopic approach and an open approach, with a bursectomy likely to give the same clinical outcome as a bursectomy with acromioplasty.[22]

[47][48]

Physical Therapy Management

There is strong evidence that supervised non-operative rehabilitation decreases pain in the shoulder and increases function.[26] Non-operative treatment should therefore be attempted first, assuming there is no tear that requires surgery.  (Level of evidence 4)

Physical therapy management includes:

  • RICE therapy in the acute phase to reduce pain and swelling
  • Stability and postural correction exercises
  • Mobility Exercises
  • Strengthening Exercises
  • Stretching exercises, including capsular stretching
  • Manual therapy techniques of the shoulder
  • Acupuncture
  • Electrical stimulation
  • Ultrasound and musculoskeletal ultrasound
  • Low-level laser therapy has positive effects on all symptoms except on muscle strength[49] (Level of evidence 1b)
  • Corticosteroid injections, in the first 8 weeks
  • High-energy extracorporeal shockwave therapy (ESWT) is more effective than low-energy ESWT, ESWT is not recommended in the acute phase.[50](Level of Evidence 2a)

When patient is presented with acute pain, it should be relieved first then strengthning exercises are implemented for prevention of future injuries. Although exercise therapy alone has proved efficient, the addition of manual therapy insures further increase in muscle strength.[51] Exercise therapy is a vital part of treatment for subacromial impingement but results showed no significant difference between home-based exercises and clinical exercise. (Level of evidence 1b)

Strengthening exercises should include[52]: (Level of evidence 1a)

  • Rotator Cuff strengthening such as; external rotation with thera-tubing, Horizontal abduction exercises
  • Lower and Middle Trapezius strengthening such as; Eated press up,Unilateral scapular rotation, Bilateral shoulder external rotation, Unilateral shoulder depression.
  • 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.[53] (Level of evidence 3b

A specific exercise strategy, focusing on strengthening eccentric exercises for the rotator cuff and concentric/eccentric exercises for the scapula stabilisers, is effective in reducing pain and improving shoulder function. [54] (Level of Evidence 1b)

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.[53] (Level of evidence 3b)

A combination of physical therapy and surgical treatment, would give better clinical results than physical therapy alone.[55] (Level of Evidence 2b)

The motions of the rotator cuff that are emphasized for strengthening are internal rotation, external rotation and abduction. It is important to remember that the function of the rotator cuff, in addition to generating torque, is to stabilize the glenohumeral joint. Thus, stronger rotator cuff muscles result in better glenohumeral joint stabilization and less impingement. A typical initial exercise program involves the use of 4 to 8 weights, with 10 to 40 repetitions performed three to five times a week.

Patients with Stage II impingement may require a formal physical therapy program. Isometric stretches are useful in restoring range of motion. Isotonic (fixed-weight) exercises are preferable to variable weight exercises. Thus, the shoulder exercises should be done with a fixed weight rather than a variable weight such as a rubber band. Repetitions are emphasized, and a relatively light weight is used. Sometimes, sports-specific techniques are useful, particularly for strengthening the throwing motion, the serving motion or swimming motions. In addition, physical therapy modalities such as electrogalvanic stimulation, ultrasound treatment and transverse friction massages can be helpful.[56](Level of evidence 2b)

[57]
[58]
[59]
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