Shoulder Impingement: Understanding Subacromial vs. General Impingement, Causes, and Treatments

What is the difference between shoulder impingement and subacromial impingement?

Shoulder impingement is a broad, umbrella term describing the pinching of soft tissues within the shoulder joint, while subacromial impingement refers specifically to this pinching occurring in the subacromial space, making it the most common form of shoulder impingement.

Understanding Shoulder Impingement: A Broad Overview

Shoulder impingement, in its most general sense, describes a condition where soft tissues within the shoulder joint are compressed or "pinched" during movement. This compression typically occurs when the arm is raised, particularly overhead, leading to pain, weakness, and restricted range of motion. The shoulder is a highly mobile ball-and-socket joint, and its complex anatomy involves numerous tendons, ligaments, and bursae that can become irritated or inflamed when subjected to repetitive compression or friction.

The term "impingement" itself simply means to strike or collide, and in the context of the shoulder, it signifies that structures are being squeezed between the bones of the shoulder joint. This can lead to a spectrum of issues, from mild inflammation (tendinitis or bursitis) to more severe conditions like rotator cuff tears if left unaddressed.

Delving Deeper: Subacromial Impingement as a Specific Type

Subacromial impingement is the most prevalent form of shoulder impingement syndrome, and it specifically refers to the compression of structures within the subacromial space. To understand this, let's briefly review the key anatomical components:

• Acromion: This is a bony projection from the scapula (shoulder blade) that forms the roof of the shoulder joint.
• Humeral Head: The top part of the upper arm bone (humerus), which forms the "ball" of the ball-and-socket joint.
• Subacromial Space: The narrow space located directly beneath the acromion and above the humeral head.
• Structures within the Subacromial Space:
  • Rotator Cuff Tendons: Primarily the supraspinatus tendon, which passes through this space. The rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis) are crucial for shoulder movement and stability.
  • Subacromial Bursa: A fluid-filled sac that acts as a cushion, reducing friction between the rotator cuff tendons and the acromion during movement.

In subacromial impingement, these soft tissues (primarily the supraspinatus tendon and/or the subacromial bursa) become compressed between the acromion and the humeral head, particularly during overhead activities or internal rotation of the arm. This repetitive compression can lead to inflammation (tendinitis of the rotator cuff, or subacromial bursitis), pain, and functional limitations.

The Key Distinction: General vs. Specific

The fundamental difference lies in their scope:

• Shoulder Impingement is the overarching category. It's a diagnostic umbrella that encompasses any condition where tissues in the shoulder are being pinched.
• Subacromial Impingement is the most common and well-known type of shoulder impingement, specifically identifying the anatomical location (the subacromial space) where the pinching occurs.

While subacromial impingement accounts for the vast majority of shoulder impingement cases, it's important to recognize that other, less common forms of shoulder impingement exist. These include:

• Internal (Posterior) Impingement: Often seen in overhead athletes (e.g., baseball pitchers), where the posterior rotator cuff (infraspinatus, teres minor) and labrum are compressed between the humeral head and the posterior-superior glenoid rim, typically during extreme external rotation and abduction.
• Coracoid Impingement: Less common, involving compression of the subscapularis tendon between the humeral head and the coracoid process (another bony projection of the scapula).

Therefore, all cases of subacromial impingement are forms of shoulder impingement, but not all shoulder impingement cases are subacromial.

Common Causes and Contributing Factors

Regardless of the specific type, several factors can contribute to the development of shoulder impingement:

• Anatomical Factors: The shape of the acromion can predispose individuals to impingement. Some acromial shapes (e.g., hooked or curved) can narrow the subacromial space. Bone spurs (osteophytes) can also develop.
• Biomechanical Factors:
  • Poor Posture: Forward head posture and rounded shoulders can alter scapular mechanics, reducing the subacromial space.
  • Scapular Dyskinesis: Impaired movement or control of the scapula, leading to altered shoulder rhythm and increased impingement.
  • Muscle Imbalances: Weakness in the rotator cuff or scapular stabilizing muscles, coupled with tightness in other shoulder muscles (e.g., pectoralis minor, latissimus dorsi), can disrupt optimal shoulder mechanics.
• Overuse and Repetitive Activities: Activities involving repeated overhead movements (e.g., swimming, throwing, painting, weightlifting) can lead to chronic irritation and inflammation of the tendons and bursa.
• Acute Trauma: A direct blow to the shoulder or a fall can sometimes initiate impingement symptoms.

Symptoms and Diagnosis

Symptoms of shoulder impingement typically include:

• Pain: Often described as an ache in the outer shoulder or upper arm, worsening with overhead activities, reaching behind the back, or sleeping on the affected side.
• Weakness: Difficulty lifting the arm, particularly against resistance.
• Limited Range of Motion: Especially noticeable during abduction (lifting the arm out to the side) and internal rotation.
• Clicking or Catching Sensations: May be felt during certain movements.

Diagnosis typically involves a thorough physical examination by a healthcare professional, including specific orthopedic tests that provoke impingement symptoms. Imaging studies such as X-rays (to assess bone structure), MRI (to visualize soft tissues like tendons and bursa), or ultrasound may be used to confirm the diagnosis and rule out other conditions.

Management and Rehabilitation Principles

The management of shoulder impingement, including subacromial impingement, typically follows a conservative approach:

• Rest and Activity Modification: Avoiding activities that exacerbate pain.
• Pain and Inflammation Management: Ice application and non-steroidal anti-inflammatory drugs (NSAIDs) can help reduce pain and swelling.
• Physical Therapy: This is the cornerstone of conservative management. A physical therapist will design a program focusing on:
  • Improving Scapular Stability: Strengthening muscles that control the shoulder blade (e.g., serratus anterior, rhomboids, lower trapezius).
  • Rotator Cuff Strengthening: Exercises to improve the strength and endurance of the rotator cuff muscles, particularly external rotators and abductors.
  • Flexibility and Mobility: Stretching tight muscles (e.g., pectoralis major/minor, latissimus dorsi, posterior capsule) to restore normal shoulder mechanics.
  • Posture Correction: Addressing postural imbalances that contribute to impingement.
  • Movement Pattern Retraining: Educating on proper mechanics for daily activities and exercises to avoid re-impingement.
• Corticosteroid Injections: In some cases, an injection of corticosteroids into the subacromial space may be used to reduce inflammation and pain, providing a window for more effective physical therapy.
• Surgical Intervention: If conservative measures fail after an extended period (typically 6-12 months), surgical decompression (acromioplasty) may be considered to create more space for the tendons.

Preventing Impingement Issues

Prevention is key, especially for individuals engaging in repetitive overhead activities or strength training:

• Prioritize Proper Form: Always use correct technique during exercises, especially overhead movements like overhead presses, pull-ups, and lat pulldowns. Avoid "ego lifting" with excessive weight that compromises form.
• Balanced Strength Training: Incorporate exercises that strengthen the entire shoulder girdle, including the rotator cuff, scapular stabilizers, and postural muscles. Don't neglect posterior chain muscles.
• Maintain Thoracic Mobility: Stiffness in the upper back can negatively impact shoulder movement. Incorporate thoracic extension and rotation exercises.
• Warm-up and Cool-down: Prepare your shoulders for activity with dynamic warm-ups and cool down with static stretches.
• Listen to Your Body: Do not push through shoulder pain. If an exercise causes discomfort, modify it, reduce the weight, or seek professional advice.
• Address Postural Habits: Be mindful of prolonged slumped postures, especially when sitting at a desk.

Conclusion: A Unified Understanding

In summary, "shoulder impingement" is a general term for the painful pinching of any soft tissue within the shoulder joint. "Subacromial impingement" is the most common and specific type, identifying the compression occurring within the subacromial space, typically involving the rotator cuff tendons and bursa. Understanding this distinction is crucial for accurate diagnosis, effective treatment, and targeted prevention strategies. By addressing the specific anatomical and biomechanical factors at play, individuals can mitigate the risk and manage the symptoms of these common shoulder conditions, promoting long-term shoulder health and function.