Shoulder Joint Dislocation: Classifications, Types, and Clinical Significance

What are the classification of shoulder joint dislocation?

Shoulder joint dislocations are classified based on several key characteristics, primarily the direction of the humeral head's displacement, but also by the completeness of the dislocation, its recurrence, and whether the skin is intact, all of which critically inform diagnosis, treatment, and prognosis.

Understanding Shoulder Dislocation

The shoulder, or glenohumeral joint, is a highly mobile ball-and-socket joint, formed by the head of the humerus (upper arm bone) and the glenoid fossa of the scapula (shoulder blade). Its extensive range of motion comes at the cost of stability, making it the most commonly dislocated major joint in the body. A shoulder dislocation occurs when the head of the humerus completely separates from the glenoid fossa, while a subluxation refers to a partial or incomplete separation. Understanding the specific classification of a dislocation is paramount for effective clinical management and rehabilitation.

Classification by Direction of Dislocation

This is the most crucial and widely used classification, describing where the humeral head ends up relative to the glenoid fossa.

• Anterior Dislocation:

  • Prevalence: Accounts for approximately 95-97% of all shoulder dislocations.
  • Mechanism: Typically results from indirect forces, such as a fall on an outstretched arm with the shoulder abducted, externally rotated, and extended. Common in contact sports or falls.
  • Subtypes: Can be further classified based on the final position of the humeral head relative to the glenoid:
    • Subcoracoid: Most common anterior type, where the humeral head is inferior and medial to the coracoid process.
    • Subglenoid: Humeral head is inferior to the glenoid.
    • Subclavicular: Humeral head is medial to the coracoid process, beneath the clavicle (rare).
    • Intrathoracic: Extremely rare, where the humeral head enters the thoracic cavity.
  • Associated Injuries: Often associated with Bankart lesions (injury to the anterior-inferior labrum), Hill-Sachs lesions (compression fracture of the posterior-superior humeral head), and rotator cuff tears (especially in older individuals).

• Posterior Dislocation:

  • Prevalence: Far less common, accounting for 2-4% of shoulder dislocations. Often missed on initial examination due to subtle clinical signs.
  • Mechanism: Typically results from direct posterior force applied to the anterior shoulder, or indirect forces involving adduction and internal rotation. Common causes include seizures, electrocution, direct trauma (e.g., dashboard injury in a car accident), or falls on an outstretched arm with internal rotation.
  • Subtypes: Can be subacromial, subglenoid, or subspinous.
  • Associated Injuries: May involve reverse Bankart lesions (posterior labral tear), reverse Hill-Sachs lesions (anterior-medial humeral head defect), or fractures of the lesser tuberosity.

• Inferior Dislocation (Luxatio Erecta):

  • Prevalence: The rarest type, comprising less than 1% of dislocations.
  • Mechanism: Occurs due to extreme hyperabduction of the arm, forcing the humeral head inferiorly and often tearing the inferior capsule.
  • Presentation: The arm is typically fixed in a fully abducted position, pointing upwards, resembling a "salute" or "stick-up" posture, hence the name "luxatio erecta."
  • Associated Injuries: High incidence of neurovascular injury (axillary nerve and artery), rotator cuff tears, and fractures.

• Superior Dislocation:

  • Prevalence: Extremely rare.
  • Mechanism: Usually results from severe, direct superior force to the adducted shoulder, often associated with significant trauma such as a fall on the shoulder with the arm adducted, driving the humeral head upwards, potentially through the acromion or clavicle.
  • Associated Injuries: Almost always accompanied by fractures of the acromion, clavicle, or coracoid, and severe soft tissue damage, including rotator cuff tears.

Classification by Completeness of Dislocation

This classification distinguishes between a full separation and a partial one.

• Complete Dislocation (Luxation):

  • The articular surfaces of the humeral head and glenoid fossa are entirely separated from one another. This is what is typically referred to as a "dislocation."

• Partial Dislocation (Subluxation):

  • The articular surfaces are partially separated but maintain some contact. The humeral head may transiently move out of alignment and then spontaneously reduce back into place. Subluxations can still cause significant pain and instability.

Classification by Recurrence

This classification is crucial for understanding the long-term prognosis and guiding treatment decisions, especially regarding surgical intervention.

• First-Time Dislocation:

  • The initial occurrence of a shoulder dislocation. The management often involves conservative measures (reduction, immobilization, rehabilitation).

• Recurrent Dislocation:

  • Subsequent dislocations of the same shoulder. The risk of recurrence is higher in younger patients (especially those under 20-25 years old) and those with significant associated soft tissue or bony injuries (e.g., Bankart or Hill-Sachs lesions). Recurrent instability often warrants surgical stabilization.

Classification by Onset

This relates to the time frame since the dislocation occurred.

• Acute Dislocation:

  • A recent dislocation, typically within hours or days of the injury. These are usually reduced as soon as possible.

• Chronic (or Old) Dislocation:

  • A dislocation that has remained unreduced for an extended period, typically weeks or months. These are more challenging to reduce due to soft tissue contractures and potential bony changes, and may require open surgical reduction.

Classification by Open vs. Closed

This distinguishes whether the skin integrity is maintained.

• Closed Dislocation:

  • The skin overlying the dislocated joint remains intact. This is the vast majority of cases.

• Open Dislocation:

  • The skin and soft tissues are disrupted, creating a direct communication between the joint space and the external environment. These are rare but represent a surgical emergency due to the high risk of infection.

Clinical Significance and Implications

The precise classification of a shoulder dislocation guides every aspect of patient care:

• Diagnosis: Knowing the typical mechanisms and presentations of each type helps clinicians identify subtle dislocations (e.g., posterior) and anticipate associated injuries.
• Imaging: Different types of dislocations may require specific radiographic views (e.g., axillary view for posterior dislocations) or advanced imaging (MRI for soft tissue injuries).
• Reduction Technique: The direction of dislocation dictates the appropriate reduction maneuver (e.g., traction-countertraction for anterior dislocations).
• Prognosis: Factors like age, direction, and recurrence risk influence the likelihood of future instability and the need for surgical intervention.
• Rehabilitation: Tailored rehabilitation protocols are developed based on the specific structures injured and the stability of the joint post-reduction.

Conclusion

The classification of shoulder joint dislocations is not merely an academic exercise; it is a fundamental framework for healthcare professionals. By accurately categorizing a dislocation based on its direction, completeness, recurrence, and other factors, clinicians can ensure precise diagnosis, appropriate immediate management, and effective long-term strategies for rehabilitation and prevention of future instability, ultimately optimizing patient outcomes and facilitating a return to activity.