Shoulder Dislocation: Affected Structures, Associated Injuries, and Recovery

What structures are affected by shoulder dislocation?

A shoulder dislocation, specifically of the glenohumeral joint, primarily involves the separation of the humeral head from the glenoid fossa of the scapula, leading to damage or stretching of the joint capsule, glenoid labrum, and associated ligaments, with potential secondary involvement of rotator cuff muscles, nerves, blood vessels, and bone.

Understanding the Shoulder Joint: A Primer

The shoulder is a marvel of human anatomy, renowned for its incredible range of motion. This mobility, however, comes at the cost of inherent stability. The glenohumeral joint, the true shoulder joint, is a classic ball-and-socket articulation where the large, spherical head of the humerus (arm bone) articulates with the small, shallow glenoid fossa (socket) of the scapula (shoulder blade). This anatomical mismatch necessitates a robust network of soft tissues—ligaments, tendons, and the joint capsule—to maintain stability. When these stabilizing structures are overwhelmed, a dislocation occurs.

Primary Structures Involved in a Dislocation

A shoulder dislocation, most commonly anterior (forward), directly impacts the core components of the glenohumeral joint:

• Glenohumeral Joint Capsule: This fibrous sac encloses the joint, providing a sealed environment and contributing significantly to stability. During a dislocation, the capsule is severely stretched or torn, often permanently compromising its integrity.
• Glenohumeral Ligaments (Superior, Middle, Inferior): These are thickenings of the joint capsule that act as primary restraints against excessive movement. In an anterior dislocation, the inferior glenohumeral ligament complex is most commonly injured, often avulsed (torn away) from its attachment on the glenoid rim.
• Glenoid Labrum: This is a fibrocartilaginous ring that surrounds and deepens the glenoid fossa, effectively increasing the surface area of the socket and providing a crucial attachment point for the joint capsule and glenohumeral ligaments. A dislocation frequently causes a tear in the labrum, most often an anterior-inferior tear (Bankart lesion).
• Bones:
  • Humerus: The head of the humerus is forced out of the glenoid fossa.
  • Scapula: Specifically, the glenoid fossa, which serves as the socket.

Secondary Structures Often Compromised

Beyond the immediate joint components, a dislocation can inflict collateral damage on surrounding neurovascular structures and musculotendinous units:

• Rotator Cuff Muscles and Tendons: The four rotator cuff muscles (supraspinatus, infraspinatus, teres minor, subscapularis) and their tendons are vital for dynamic shoulder stability and movement. They can be stretched, bruised, or even torn during a dislocation, particularly in older individuals or with high-energy trauma.
• Nerves:
  • Axillary Nerve: This is the most commonly injured nerve in shoulder dislocations due to its proximity to the inferior aspect of the humeral head. Injury can lead to weakness in deltoid and teres minor muscles (affecting shoulder abduction and external rotation) and sensory loss over the lateral shoulder.
  • Musculocutaneous Nerve: Less common, but can be affected, leading to weakness in elbow flexion and sensory loss on the lateral forearm.
  • Brachial Plexus: In severe dislocations, the entire brachial plexus (a network of nerves supplying the arm and hand) can be stretched or damaged, leading to widespread neurological deficits.
• Blood Vessels:
  • Axillary Artery: Located close to the axillary nerve, this major artery can be compressed, stretched, or torn, leading to compromised blood flow to the arm.
  • Circumflex Humeral Arteries: These smaller arteries, which supply the humeral head, can also be damaged, potentially contributing to avascular necrosis (bone death due to lack of blood supply) in rare, severe cases.
• Bursae: The subacromial bursa, a fluid-filled sac that reduces friction between the rotator cuff tendons and the acromion, can become inflamed (bursitis) due to the trauma.

Common Associated Injuries (Bone)

The forces involved in a dislocation can also lead to fractures:

• Bankart Lesion: While primarily a labral tear, it often involves an avulsion of the anterior-inferior labrum with a piece of the glenoid bone attached. This is a bony Bankart lesion.
• Hill-Sachs Lesion: This is a compression fracture or indentation on the posterior-superior aspect of the humeral head. It occurs when the humeral head impacts the anterior rim of the glenoid during the dislocation event.
• Greater Tuberosity Fracture: An avulsion fracture where one or more rotator cuff tendons pull off a piece of the greater tuberosity of the humerus.
• Glenoid Rim Fracture: A fracture of the bony rim of the glenoid fossa itself, which can significantly reduce the stability of the joint.

The Impact of Recurrent Dislocations

Each subsequent dislocation can progressively worsen the damage to the stabilizing structures. The joint capsule becomes more lax, the labrum may sustain further tears, and any associated bone lesions can enlarge. This leads to chronic shoulder instability, making future dislocations more likely and often requiring surgical intervention to restore stability.

Rehabilitation and Long-Term Considerations

Proper diagnosis, including imaging (X-rays, MRI), is crucial to identify all affected structures. Rehabilitation after a shoulder dislocation is paramount. It focuses on:

• Pain and Swelling Management: Initial acute phase.
• Restoring Range of Motion: Gradual, controlled movements.
• Strengthening: Targeting the rotator cuff muscles, deltoid, and scapular stabilizers to provide dynamic stability.
• Proprioception and Neuromuscular Control: Re-educating the joint's sense of position and movement.

Understanding the complex array of structures affected by a shoulder dislocation underscores the importance of prompt medical attention and a comprehensive rehabilitation program to optimize recovery and minimize the risk of recurrence.