Shoulder Dislocations: Types, Mechanisms, and Associated Risks

What are the different types of shoulder dislocations?

Shoulder dislocations are categorized primarily by the direction in which the humerus displaces from the glenoid fossa, with additional classifications based on severity, recurrence, and associated injuries.

Understanding Shoulder Dislocations: An Overview

The shoulder joint, anatomically known as the glenohumeral joint, is a ball-and-socket joint renowned for its exceptional mobility. This wide range of motion, however, comes at the expense of inherent stability, making it the most commonly dislocated major joint in the body. A shoulder dislocation occurs when the head of the humerus (the "ball") completely separates from the glenoid fossa (the "socket") of the scapula. When the humerus only partially comes out of the socket and then returns, it is termed a subluxation. Understanding the different types of dislocations is crucial for accurate diagnosis, appropriate treatment, and effective rehabilitation.

Primary Classification by Direction of Displacement

The most fundamental way to classify shoulder dislocations is by the direction in which the humeral head displaces relative to the glenoid fossa.

• Anterior Dislocation:

  • Description: This is by far the most common type, accounting for over 95% of all shoulder dislocations. The humeral head dislocates to the front of the body, typically resting below the coracoid process (subcoracoid).
  • Mechanism: Often results from a combination of abduction (arm lifted away from the body), external rotation (arm rotated outwards), and extension, often due to a fall on an outstretched arm or a direct blow to the back of the shoulder. Sports like rugby, football, and basketball frequently see this injury.
  • Associated Risks: Due to the anterior displacement, there's a higher risk of damage to the anterior labrum (Bankart lesion), compression fractures on the posterior-superior humeral head (Hill-Sachs lesion), and injury to the axillary nerve.

• Posterior Dislocation:

  • Description: A much rarer type, accounting for 2-4% of dislocations. The humeral head displaces towards the back of the body. These can often be missed on initial examination due to subtle clinical signs.
  • Mechanism: Typically caused by a forceful internal rotation and adduction of the arm, often combined with an axial load. Common causes include epileptic seizures, electrocution, or a direct blow to the front of the shoulder (e.g., dashboard injury in a car accident).
  • Associated Risks: Can be associated with a reverse Bankart lesion (posterior labral tear) or a reverse Hill-Sachs lesion (anterior humeral head compression fracture).

• Inferior Dislocation (Luxatio Erecta):

  • Description: This is the rarest form of shoulder dislocation, representing less than 1% of cases. In this type, the humeral head is displaced directly inferiorly, often coming to rest below the glenoid.
  • Mechanism: Caused by extreme hyperabduction of the arm, where the arm is forced directly overhead, causing the humeral neck to impinge against the acromion, levering the humeral head out of the inferior aspect of the glenoid.
  • Clinical Presentation: Patients present with the arm fixed in a position of marked abduction, often with the hand resting on or behind the head, giving it the characteristic "luxatio erecta" (erect dislocation) appearance.
  • Associated Risks: Due to the extreme force involved, there's a high incidence of neurovascular injury (brachial plexus, axillary artery) and rotator cuff tears.

Classification by Severity and Recurrence

Beyond the directional displacement, dislocations are also classified by their completeness and history.

• Complete Dislocation vs. Subluxation:

  • Complete Dislocation: The humeral head is entirely out of the glenoid fossa.
  • Subluxation: The humeral head partially displaces from the glenoid fossa but then spontaneously reduces (returns to its normal position). While less severe, subluxations can still cause pain and instability and often precede complete dislocations or indicate underlying joint laxity.

• Acute vs. Chronic:

  • Acute: Refers to a recent, first-time dislocation.
  • Chronic: Implies that the shoulder has been dislocated for an extended period, often weeks or months, without reduction. These are less common and more challenging to treat.

• First-Time vs. Recurrent:

  • First-Time Dislocation: The initial instance of the injury.
  • Recurrent Dislocation: Occurs when the shoulder dislocates multiple times. This is common after an initial dislocation, especially in younger, active individuals. Recurrence is often due to damage to the static stabilizers of the joint (e.g., labrum, capsule) that occurred during the initial injury, leading to persistent instability.

Associated Injuries and Complications

Regardless of the type, shoulder dislocations can lead to various associated injuries and complications that influence recovery and long-term stability.

• Labral Tears: Tears of the glenoid labrum, particularly the Bankart lesion (anterior-inferior labral avulsion), are very common with anterior dislocations.
• Rotator Cuff Tears: Especially in older individuals, the forceful displacement can tear one or more of the rotator cuff tendons.
• Bony Lesions:
  • Hill-Sachs Lesion: A compression fracture on the posterior-superior aspect of the humeral head, caused by impact against the anterior glenoid rim during an anterior dislocation.
  • Bony Bankart Lesion: A fracture of the anterior-inferior rim of the glenoid, often accompanying a labral tear.
• Nerve Damage: The axillary nerve is the most commonly injured nerve, leading to deltoid weakness and numbness over the lateral shoulder. Brachial plexus injuries are less common but more severe.
• Vascular Damage: While rare, damage to the axillary artery can occur, particularly in high-energy trauma or inferior dislocations.

Why Understanding Types Matters

A precise understanding of the type of shoulder dislocation is paramount for healthcare professionals. It guides the choice of reduction technique, informs the decision regarding surgical versus non-surgical management, and helps anticipate potential associated injuries and long-term complications, such as chronic instability or early onset osteoarthritis. For individuals, recognizing the specific type of dislocation helps in comprehending the injury and adhering to the prescribed rehabilitation protocol for optimal recovery and prevention of recurrence. Always seek immediate medical attention for a suspected shoulder dislocation.