Which muscles are affected by an anterior shoulder dislocation?

Anterior Shoulder Dislocation: Affected Muscles

Anterior shoulder dislocation primarily affects the subscapularis muscle, which is the major active stabilizer of the glenohumeral joint and frequently sustains injury during the dislocation event. Additionally, the rotator cuff muscles (particularly the supraspinatus, infraspinatus, and external rotators) are commonly affected, especially in older patients 1.

Primary Muscular Injuries

Subscapularis Muscle (Most Critical)

The subscapularis is the most important muscle affected by anterior shoulder dislocation:

• Acts as the primary dynamic stabilizer preventing anterior translation, particularly at 0 degrees of abduction 2
• Sustains interstitial fibrosis and muscle scarring following traumatic anterior dislocation 3
• Shows modifications in fiber type ratios consistent with disuse atrophy after injury 3
• These pathologic changes alter strength and stability of the glenohumeral joint 3

Clinical implication: Rehabilitation specifically targeting the subscapularis is essential after anterior dislocation 3.

Rotator Cuff Muscles

The rotator cuff complex is frequently involved:

• Age-dependent pattern: Older patients (>40 years) are significantly more likely to have rotator cuff tears associated with shoulder dislocation 1
• Specific muscles affected: Supraspinatus, infraspinatus, and external rotators demonstrate weakness and hypotrophy 4
• Mechanism: Repetitive eccentric stress on these muscles during the dislocation event 5

Deltoid Muscle

While the deltoid may be examined as a comparison muscle, it is not typically pathologically affected by anterior shoulder dislocation 3.

Stabilization Mechanism by Position

The muscles involved in preventing dislocation vary by arm position 2:

• At 0° abduction: Subscapularis provides the majority of stability
• At 45° abduction: Subscapularis, middle glenohumeral ligament, and anterosuperior fibers of inferior glenohumeral ligament work together
• At 90° abduction: Inferior glenohumeral ligament becomes the primary stabilizer during external rotation

Clinical Assessment Priorities

When evaluating muscle involvement after anterior shoulder dislocation:

1. Test subscapularis function specifically with internal rotation strength testing
2. Assess rotator cuff integrity - particularly in patients over 40 years old 1
3. Evaluate for muscle weakness and hypotrophy of supra- and infraspinatus muscles 4
4. Check for decreased range of motion during abduction with external or internal rotation 5

Rehabilitation Implications

Muscle training is essential for preventing recurrent instability 4:

• Specific strengthening of rotator cuff muscles and deltoid using isokinetic training
• Focus on muscle strength, coordination, and endurance
• Training shoulder flexors, internal and external rotators three times weekly for 8 weeks
• Success rate: Most patients achieve relief from pain and decreased/cessation of dislocations with appropriate muscle rehabilitation 4

Important caveat: Exercises alone may be inadequate for patients with abnormal skeletal anatomy, multidirectional instability, or those in whom the humeral head migrates antero-superiorly under load 4, 6. These patients may require surgical intervention despite optimal rehabilitation efforts.