Shoulder Impingement: Understanding Primary, Secondary, Internal, and Coracoid Types

What are the 4 types of shoulder impingement?

Shoulder impingement, a common cause of shoulder pain, occurs when soft tissues within the subacromial space are compressed. While often generalized, it can be broadly categorized into four primary types based on their underlying mechanisms and anatomical locations: primary, secondary, internal (posterior-superior), and coracoid impingement.

Understanding Shoulder Impingement Syndrome

Shoulder impingement syndrome describes the compression and irritation of soft tissues—primarily the rotator cuff tendons (supraspinatus, infraspinatus, subscapularis, teres minor) and the subacromial bursa—as they pass through the narrow subacromial space. This space is bounded superiorly by the acromion, coracoacromial ligament, and acromioclavicular (AC) joint, and inferiorly by the humeral head. Repetitive arm movements, especially overhead activities, can exacerbate this compression, leading to pain, inflammation, and potential damage to the affected structures.

The Four Primary Classifications of Shoulder Impingement

Understanding the specific type of impingement is crucial for effective diagnosis and targeted rehabilitation. The four main classifications are:

1. Primary Impingement (Outlet Impingement):

   • Mechanism: This type arises from a structural or anatomical narrowing of the subacromial space. The "outlet" refers to the space beneath the coracoacromial arch.
   • Causes: Common causes include variations in acromial shape (e.g., hooked or curved acromion, classified as Type II or Type III Bigliani classification), degenerative changes like osteophytes (bone spurs) on the acromion or AC joint, and thickening or calcification of the coracoacromial ligament. These structural anomalies directly reduce the available space for the rotator cuff tendons and bursa.
   • Affected Structures: Primarily the supraspinatus tendon and the subacromial bursa.

2. Secondary Impingement (Functional/Non-Outlet Impingement):

   • Mechanism: Unlike primary impingement, secondary impingement is not due to structural narrowing but rather functional instability or poor dynamic control of the humeral head within the glenoid fossa. This leads to superior migration of the humeral head, effectively reducing the subacromial space during arm elevation.
   • Causes: Often linked to muscle imbalances, such as weakness or fatigue of the rotator cuff muscles (which normally depress and stabilize the humeral head) or scapular dyskinesis (improper movement of the shoulder blade). Other factors include glenohumeral joint laxity or instability, and poor posture.
   • Affected Structures: Rotator cuff tendons and subacromial bursa, but the root cause is dynamic dysfunction rather than static anatomical constraint.

3. Internal Impingement (Posterior-Superior Impingement / Articular-Sided Impingement):

   • Mechanism: This type involves the compression of the articular (joint-side) surface of the posterior-superior rotator cuff tendons (supraspinatus and infraspinatus) and/or the posterior labrum. This compression occurs between the humeral head and the posterior-superior glenoid rim, typically when the arm is in a position of maximum abduction (arm raised out to the side) and external rotation (e.g., during the late cocking phase of a throwing motion).
   • Causes: Most common in overhead athletes (e.g., baseball pitchers, tennis players, volleyball players) due to repetitive extreme ranges of motion. It can be associated with adaptive changes in the shoulder capsule (e.g., posterior capsular tightness and anterior capsular laxity) and scapular dyskinesis.
   • Affected Structures: Articular side of the supraspinatus and infraspinatus tendons, and the posterior glenoid labrum.

4. Coracoid Impingement (Subcoracoid Impingement):

   • Mechanism: This relatively less common type involves the compression of the subscapularis tendon and/or the biceps tendon between the humeral head and the coracoid process, particularly during shoulder flexion, adduction, and internal rotation.
   • Causes: Can be due to an abnormally shaped or prominent coracoid process, hypertrophy of the subscapularis muscle, or a congenitally small interval between the coracoid and the humerus.
   • Affected Structures: Subscapularis tendon, long head of the biceps tendon, and the anterior capsule.

Common Symptoms of Shoulder Impingement

Regardless of the type, shoulder impingement often presents with a consistent set of symptoms, including:

• Pain with overhead activities, reaching behind the back, or lifting objects.
• Pain at night, especially when lying on the affected side.
• Weakness in the affected arm.
• Limited range of motion, particularly in elevation and rotation.
• A catching or clicking sensation with certain movements.
• Gradual onset of pain, though acute exacerbations can occur.

Diagnosis and Management

Accurate diagnosis of shoulder impingement requires a thorough clinical evaluation by a healthcare professional, including a detailed history, physical examination, and often imaging studies like X-rays, MRI, or ultrasound. Identifying the specific type of impingement is critical as it guides the treatment strategy.

Management typically begins with conservative approaches, including:

• Rest and activity modification to avoid aggravating movements.
• Anti-inflammatory medications (NSAIDs) to reduce pain and swelling.
• Physical therapy focusing on improving posture, strengthening rotator cuff and scapular stabilizing muscles, and restoring proper shoulder mechanics and range of motion.
• Corticosteroid injections into the subacromial space to reduce inflammation.

In cases where conservative treatments fail, surgical intervention may be considered to address underlying structural issues (e.g., acromioplasty to reshape the acromion, debridement of inflamed tissue).

Conclusion

Shoulder impingement is a complex condition with varied etiologies. While often grouped under a single umbrella, understanding the distinctions between primary, secondary, internal, and coracoid impingement is fundamental for fitness professionals, kinesiologists, and individuals experiencing shoulder pain. A precise diagnosis, followed by a targeted and comprehensive rehabilitation program, offers the best pathway to alleviating symptoms, restoring function, and preventing recurrence. Always consult with a qualified healthcare provider for an accurate diagnosis and personalized treatment plan.