Subscapularis Dysfunction Contributing to Subacromial Impingement Syndrome

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Subscapularis Dysfunction Contributing to Subacromial Impingement Syndrome

Shoulder pain is one of the most prevalent musculoskeletal complaints in clinical practice. Research from Western populations estimates that its prevalence in the general population ranges from 11% to 27%. Notably, approximately 50% of individuals experiencing shoulder pain report persistent or recurring symptoms, even after three years. This highlights the chronic nature of many shoulder conditions, underscoring the need for effective management strategies to prevent long-term disability and improve patient outcomes.

Pain originating from structures within the subacromial space is one of the most common causes of shoulder discomfort. Key structures within this space include the tendons of the supraspinatus and the long head of the biceps, along with the subacromial bursa. While these structures are frequently implicated in subacromial impingement syndrome, the role of the subscapularis muscle is often overlooked, leading to underdiagnosis and inadequate treatment.

Subscapularis dysfunction refers to impaired performance of the subscapularis muscle, which is crucial in stabilizing the glenohumeral joint and ensuring smooth shoulder movement. Dysfunction of this muscle can result in abnormal glenohumeral motion, which in turn may contribute to the development or exacerbation of subacromial impingement syndrome. Recognizing and addressing subscapularis dysfunction in patients presenting with shoulder pain can offer critical insights for accurate diagnosis and targeted treatment, potentially improving outcomes by addressing a frequently missed biomechanical factor.

Direct causes of shoulder pain related to subscapularis dysfunction include tendonitis, tendinopathy, and tears. Subscapularis tears can occur in isolation or in combination with injuries to other rotator cuff muscles. Rotator cuff tears involving the subscapularis have been reported in 19% to 40% of cases, with isolated subscapularis tears accounting for approximately 4% of all rotator cuff tears. Traumatic tears are more common in younger individuals, whereas degenerative tears typically occur in older patients without a history of trauma. A study examining patients undergoing glenohumeral arthroscopy found that 50% of individuals aged 50 years or older had evidence of a subscapularis tear. Risk factors for chronic subscapularis tears include metabolic syndrome, diabetes, overuse activities, posterior-superior rotator cuff tears, increased tendon retraction, and associated biceps tendon injuries.

Clinical manifestations of subscapularis dysfunction vary based on the extent and severity of the injury and often overlap with other shoulder conditions, making diagnosis challenging. Recognizing the involvement of the subscapularis is essential to ensure appropriate treatment, particularly when addressing symptoms that may not respond to conventional therapies targeting other rotator cuff muscles.

Functional Anatomy

A thorough understanding of the anatomy and function of the subscapularis muscle is essential for healthcare professionals involved in the diagnosis and treatment of subacromial impingement syndrome and other shoulder pathologies. The subscapularis is one of the four rotator cuff muscles crucial in shoulder stability and movement. This large, triangular, multipennate muscle is located on the anterior surface of the scapula (subscapular fossa). It originates from the subscapular fossa on the costal surface of the scapula and runs anterolaterally, crossing the anterior aspect of the glenohumeral joint. The subscapularis muscle inserts on the lesser tubercle of the humerus, enabling internal rotation of the shoulder and contributing to glenohumeral joint stability.

The subscapularis muscle is innervated by the upper and lower subscapular nerves, branches of the brachial plexus, originating from spinal levels C5 and C6. As the largest of the four rotator cuff muscles, the subscapularis plays a critical role in shoulder movement and stability. It provides powerful internal rotation of the humerus and acts as a dynamic stabilizer for the glenohumeral joint, helping to maintain the head of the humerus within the glenoid fossa during movement.

The function of the subscapularis muscle is particularly important in preventing anterior displacement of the humeral head and for ensuring smooth, coordinated shoulder movements. Impairment or dysfunction of the subscapularis can disrupt the shoulder’s biomechanical integrity, potentially contributing to conditions such as subacromial impingement syndrome. Understanding its role in movement and joint stability allows clinicians to design targeted treatment strategies that address underlying biomechanical deficits, enhancing outcomes for patients with shoulder pain.

The subscapularis synergizes with the infraspinatus, teres minor, and supraspinatus muscles to stabilize the shoulder joint during various activities. It prevents anterior displacement of the humeral head, especially during the lower range of shoulder abduction. Additionally, it counteracts upward displacement of the humeral head caused by the deltoid during arm elevation, working alongside the infraspinatus to ensure smooth, stable shoulder motion. Without this balancing mechanism, the humeral head can shift upward, leading to subacromial impingement and causing friction on the supraspinatus tendon and long head of the biceps tendon, increasing the risk of tears.

Differential Diagnosis of Anterolateral Shoulder Pain

Patients presenting with anterolateral shoulder pain may be suffering from several conditions. The differential diagnosis includes the following:

• Biceps strain, tendinopathy or tear
• Rotator cuff strain, tendinopathy or tears
• Deltoid strain, tendinopathy or tear
• Subacromial impingement syndrome
• Myofascial pain syndrome (e.g., the infraspinatus, supraspinatus, and pectoral muscles)
• Glenoid labral tears
• Acromioclavicular joint dysfunction
• Glenohumeral dysfunction, instability or arthritis
• Adhesive capsulitis

Common History in Subacromial Impingement Syndrome

• Most commonly affects individuals over the age of 40, with or without a history of trauma.
• Pain typically develops gradually over weeks or months but can have a sudden onset following trauma.
• Pain is often localized to the anterolateral acromion, with possible radiation to the lateral shoulder and upper arm.
• Pain provoked by overhead activities and movements involving shoulder elevation.
• Night pain is common, especially when lying on the affected side or sleeping with the involved arm overhead.
• Generalized shoulder weakness may also be present.

Specific History in Subscapularis Dysfunction

• Pain in the front of the shoulder, sometimes radiating to the upper arm.
• Pain is commonly triggered by internal shoulder rotation, such as reaching behind the back or tucking in a shirt.
• History of traumatic shoulder injury may be present, especially involving forced external rotation of the arm, which is common in cases of acute subscapularis tears. These injuries are more frequently observed in younger individuals engaged in sports or high-impact activities.
• Degenerative subscapularis tears are typically seen in individuals aged 50 years or older and often develop gradually over time.

Common Examination Findings in Subacromial Impingement Syndrome

• Atrophy of shoulder muscles, particularly the supraspinatus or infraspinatus, may be present.
• Positive Painful Arc test with pain occurring within 60 to 120 degrees of shoulder abduction.
• Pain during the Apley’s Scratch test, especially when reaching across the body or behind the back.
• Tenderness over the anterolateral acromion, as well as over the supraspinatus muscle, biceps tendon, or acromioclavicular (AC) joint, depending on the specific underlying pathology.
• Weakness in the rotator cuff muscles and scapular stabilizers (e.g., serratus anterior, trapezius), depending on the severity of the condition.
• Positive Neer Impingement, Hawkins-Kennedy, Yocum, and Scapular Assistance tests.
• Positive Empty Can test if supraspinatus tendon pathology is present.
• Disturbed Scapulohumeral Rhythm during arm elevation.

Specific Examination Findings in Subscapularis Involvement

• • Loss of shoulder internal rotation range of motion.
  • Pain and weakness with resisted internal shoulder rotation.
  • Pain with passive external rotation.
  • Tenderness over the anterior glenohumeral joint or at the lesser tubercle of the humerus.
  • Tenderness and hypertonicity at the lateral border of the subscapularis near its insertion on the lesser tubercle.
  • Positive Belly Press, Bear Hug, and Lift Off tests.
  • Positive Internal Rotation Lag Sign.

• Plain Radiography – Shoulder radiographs are typically the first-line imaging modality for evaluating undifferentiated shoulder pain. While radiographs may appear normal in cases of subscapularis injury, they are useful for identifying bony abnormalities such as avulsion fractures or osteophytes that could indicate tendon or ligament detachment.
• MRI Imaging – MRI is the gold standard for diagnosing rotator cuff injuries, including those involving the subscapularis tendon. It provides a comprehensive evaluation of both tendons and muscle bellies. MRI is highly sensitive in detecting a range of subscapularis pathologies, from mild degenerative changes and partial-thickness tears to complete full-thickness ruptures. It can also assess associated findings such as fatty infiltration, atrophy, and muscle retraction, which are important for determining the extent of the injury and guiding treatment options. In cases of subscapularis injury, MRI offers a detailed view of not just the tendon but also the muscle’s structural integrity, allowing for a more accurate diagnosis and assessment of potential involvement of other rotator cuff muscles.

The following are examples of red flags for patients presenting with shoulder pain:

• A history of significant injury
• Severe pain
• Unrelenting pain
• Nocturnal pain
• Unexplained weight loss
• Fever
• Deformity
• Significant swelling
• Presence of upper extremity tingling, numbness, burning, or other neurological impairment
• Significant loss of range of motion
• Severe tenderness on palpation or severe pain with any examination procedure

If any of these ‘red flags’ are identified during history taking or clinical examination, it is crucial to refer the patient for urgent medical evaluation and further investigation.

Myofascial Pain Syndromes Mimicking Rotator Cuff Tendinopathy

Myofascial pain syndromes can mimic the pain patterns of rotator cuff tendinopathy. Trigger points in muscles such as the infraspinatus, supraspinatus, subscapularis, deltoid, and biceps brachii can produce symptoms that resemble rotator cuff pathology, including shoulder pain, limited range of motion, and discomfort with certain arm movements. Identifying and treating these trigger points is essential, as myofascial pain may not respond to traditional treatments for rotator cuff tendinopathy. A thorough examination of the shoulder and surrounding musculature, including palpation for trigger points, can help differentiate between true tendinopathy and myofascial pain syndrome.

The trigger points associated with myofascial pain syndrome may be located anywhere within a muscle and its fascia. Therefore, the entire muscle should be examined thoroughly. However, based on our clinical experience, the common locations for trigger points are presented below: