Shoulder Impingement: Understanding Its Root Causes and Contributing Factors

What is the root cause of shoulder impingement?

Shoulder impingement syndrome, also known as subacromial impingement, is fundamentally caused by the compression and irritation of the soft tissues within the subacromial space of the shoulder, primarily due to a reduction in this critical anatomical clearance.

Understanding Shoulder Impingement

Shoulder impingement occurs when the tendons of the rotator cuff (especially the supraspinatus tendon) and/or the subacromial bursa are "pinched" or compressed between the head of the humerus (upper arm bone) and the acromion (a bony projection of the shoulder blade or scapula) during arm elevation. This repetitive compression leads to inflammation, pain, and, if left unaddressed, can progress to tendinopathy, bursitis, and even rotator cuff tears.

Anatomy of the Subacromial Space

To grasp the root causes, it's essential to understand the intricate anatomy of the shoulder joint:

• Humerus: The head of the humerus forms the ball of the ball-and-socket glenohumeral joint.
• Scapula (Shoulder Blade): This bone features several key structures:
  • Glenoid Fossa: The shallow socket that articulates with the humeral head.
  • Acromion: A bony arch that extends over the top of the humeral head, forming the "roof" of the subacromial space.
  • Coracoid Process: Another bony projection that, along with the acromion, forms part of the coracoacromial arch.
• Rotator Cuff: A group of four muscles (supraspinatus, infraspinatus, teres minor, subscapularis) and their tendons that surround the glenohumeral joint, providing stability and enabling a wide range of motion. The supraspinatus tendon passes directly through the subacromial space.
• Subacromial Bursa: A fluid-filled sac located between the rotator cuff tendons and the acromion, designed to reduce friction during movement.

The subacromial space is naturally narrow. Any factor that further reduces this space or causes the soft tissues within it to swell can lead to impingement.

The Primary Mechanism: Subacromial Space Narrowing

The immediate mechanism of impingement is the mechanical compression of the soft tissues (rotator cuff tendons, bursa) within the subacromial space. This compression is exacerbated during overhead arm movements (abduction and flexion), as the humeral head moves superiorly towards the acromion. When the space is compromised, the tissues get squeezed, leading to inflammation and pain.

Key Contributing Factors: The "Root Causes"

While the immediate cause is space narrowing, several underlying factors contribute to or directly cause this reduction, making them the true "root causes" of shoulder impingement. These can be categorized as structural/anatomical, biomechanical, and activity-related.

1. Anatomical Variations

Some individuals are predisposed to impingement due to the natural shape of their acromion.

• Acromial Morphology: Dr. Charles Neer classified acromion types:
  • Type I (Flat): Least likely to cause impingement.
  • Type II (Curved): Moderate risk.
  • Type III (Hooked): Highest risk, as the hooked shape significantly reduces the subacromial space.
• Acromial Spurs (Osteophytes): Bone spurs can develop on the underside of the acromion, often due to age-related degenerative changes, further encroaching on the subacromial space.
• Thickened Coracoacromial Ligament: This ligament, part of the coracoacromial arch, can also thicken and contribute to impingement.

2. Poor Posture and Scapular Dysfunction

The position and movement of the scapula are crucial for maintaining adequate subacromial space.

• Forward Head and Rounded Shoulder Posture: This common postural deviation can protract and internally rotate the scapula, bringing the acromion closer to the humerus and reducing the subacromial space, even at rest.
• Scapular Dyskinesis: Impaired or uncoordinated movement of the scapula during arm elevation. Normally, the scapula should upwardly rotate and posteriorly tilt to clear the acromion from the humeral head. If this motion is deficient (e.g., due to weakness of the serratus anterior or lower trapezius, or tightness of the pectoralis minor), the acromion fails to move out of the way, leading to impingement.

3. Muscle Imbalances and Weakness

Dysfunction in the muscles surrounding the shoulder joint can significantly alter joint mechanics.

• Weak Rotator Cuff Muscles: Particularly the supraspinatus, which helps depress the humeral head during arm elevation. Weakness can lead to superior migration of the humeral head, causing it to ride up and pinch against the acromion. External rotators (infraspinatus, teres minor) are also critical for centring the humeral head.
• Tightness of Internal Rotators: Overactivity or tightness in muscles like the pectoralis major, latissimus dorsi, and subscapularis can pull the humerus into excessive internal rotation, further narrowing the subacromial space.
• Weak Scapular Stabilizers: As mentioned under scapular dysfunction, weakness in the serratus anterior and lower trapezius impairs proper scapular upward rotation and posterior tilt, leading to an impingement-prone position.
• Overactive Upper Trapezius: Can contribute to scapular elevation without proper upward rotation, further reducing subacromial space.

4. Overuse and Repetitive Overhead Activities

Repetitive movements, especially those involving the arm above shoulder height, are a common trigger.

• Microtrauma: Repeated compression and friction can cause microscopic tears and irritation to the rotator cuff tendons and bursa, leading to inflammation (tendinitis, bursitis) and swelling.
• Inflammation: Swelling of the tendons or bursa further reduces the already limited subacromial space, creating a vicious cycle of impingement and inflammation.
• Occupational/Sporting Demands: Athletes (swimmers, baseball pitchers, tennis players, volleyball players) and workers (painters, carpenters, electricians) who regularly perform overhead tasks are at higher risk.

5. Acute Trauma or Injury

While often chronic, impingement can also result from a single traumatic event.

• Direct Blow: A direct impact to the shoulder can cause inflammation and swelling within the subacromial space.
• Fall on an Outstretched Arm (FOOSH): Can damage the rotator cuff or cause acute inflammation, leading to impingement symptoms.

6. Degenerative Changes (Age-Related)

As we age, the tissues in the shoulder can undergo wear and tear.

• Tendinosis: Degeneration of the rotator cuff tendons, making them less resilient and more prone to irritation and tearing.
• Bursitis: Chronic inflammation of the subacromial bursa.
• Osteoarthritis: Degenerative changes in the acromioclavicular (AC) joint can lead to bone spur formation that impinges the underlying structures.

The Vicious Cycle of Impingement

It's important to recognize that these root causes often interact, creating a self-perpetuating cycle. For instance, poor posture can lead to muscle imbalances, which causes scapular dysfunction, increasing impingement. This impingement then causes pain and inflammation, which can further alter movement patterns (e.g., guarding, avoiding full range of motion), exacerbating the issue. Over time, chronic inflammation can lead to structural changes like tendon thickening or even rotator cuff tears.

Conclusion

The "root cause" of shoulder impingement is rarely a single factor but rather a complex interplay of anatomical predispositions, biomechanical dysfunctions, and repetitive stress. Understanding these contributing factors is crucial for effective prevention, diagnosis, and rehabilitation. Addressing issues such as poor posture, muscle imbalances, and improper movement mechanics is paramount to alleviating symptoms and preventing recurrence, allowing for optimal shoulder health and function.