Shoulder Dislocation: Causes, Risk Factors, and Prevention

What are the causes and risk factors of shoulder dislocation?

Shoulder dislocation occurs when the head of the humerus (upper arm bone) is forced out of its socket (glenoid fossa), typically due to significant traumatic force, though underlying anatomical predispositions and ligamentous laxity can also contribute.

Understanding Shoulder Anatomy and Stability

To grasp the causes and risk factors of shoulder dislocation, it's essential to understand the unique anatomy of the shoulder joint. The glenohumeral joint, commonly referred to as the shoulder, is a ball-and-socket joint formed by the head of the humerus and the shallow glenoid fossa of the scapula (shoulder blade). This design grants the shoulder an exceptional range of motion, making it the most mobile joint in the human body.

However, this mobility comes at the cost of inherent stability. Unlike the hip joint, which has a deep socket, the glenoid fossa is relatively shallow, resembling a golf ball on a tee. Stability is primarily provided by:

• The Glenoid Labrum: A ring of fibrocartilage that deepens the socket.
• The Joint Capsule: A fibrous sac enclosing the joint.
• Glenohumeral Ligaments: Thickened bands within the capsule that reinforce it.
• Rotator Cuff Muscles: A group of four muscles (supraspinatus, infraspinatus, teres minor, subscapularis) and their tendons that surround the joint, providing dynamic stability by compressing the humeral head into the glenoid.
• Scapular Stabilizers: Muscles that control the movement and position of the scapula, providing a stable base for the humerus.

When these stabilizing structures are overwhelmed or compromised, dislocation can occur.

Immediate Causes of Shoulder Dislocation

Shoulder dislocations are broadly categorized as either traumatic or atraumatic, though traumatic events account for the vast majority.

Traumatic Dislocations

These occur when a significant external force displaces the humeral head from the glenoid fossa. The most common direction is anterior-inferior dislocation (forward and downward), accounting for over 95% of cases. Posterior dislocations are less common but can occur.

• Direct Impact: A direct blow to the shoulder, often from the front or side, can force the humeral head backward or forward.
• Indirect Force: This is the most frequent mechanism. It typically involves:
  • Falling onto an Outstretched Arm (FOOSH): Landing on an extended arm, especially if the arm is abducted (away from the body) and externally rotated, can lever the humeral head out of the socket.
  • Contact Sports Injuries: High-impact collisions common in sports like football, rugby, hockey, or wrestling.
  • Overhead Sports Injuries: Repetitive or sudden forceful movements in sports like baseball (pitching), volleyball (spiking), basketball (shooting), or swimming can, over time, weaken stabilizers or acutely cause dislocation with extreme range of motion.
• Motor Vehicle Accidents: High-impact collisions can cause significant forces leading to shoulder dislocation, often accompanied by other injuries.
• Seizures or Electrocution: Violent, uncontrolled muscle contractions during a seizure or from an electric shock can generate enough force to dislocate the shoulder, often posteriorly.

Atraumatic Dislocations

Less common, these dislocations occur with minimal or no significant trauma. They are often associated with underlying instability.

• Repetitive Microtrauma: Chronic, repetitive movements, particularly in overhead athletes, can gradually stretch the joint capsule and ligaments, leading to instability and eventual dislocation with minimal force.
• Generalized Ligamentous Laxity: Individuals with naturally "loose joints" (hypermobility syndrome) may have inherently lax shoulder capsules and ligaments, making them prone to dislocation even with normal movements.
• Muscular Imbalance/Weakness: Imbalances or weakness in the rotator cuff or scapular stabilizing muscles can compromise dynamic stability, allowing the humeral head to shift out of place more easily.
• Voluntary Dislocation: In rare cases, individuals can voluntarily dislocate their shoulder, often due to significant underlying laxity and learned muscle control. This is distinct from true instability.

Key Risk Factors for Shoulder Dislocation

While a significant force is often the immediate cause, several factors can increase an individual's susceptibility to shoulder dislocation.

• Previous Shoulder Dislocation: This is by far the most significant risk factor. Once dislocated, the shoulder structures (labrum, joint capsule, ligaments, and sometimes bone) are often damaged. Common injuries include:
  • Bankart Lesion: A tear of the anterior-inferior labrum, often with associated capsular detachment.
  • Hill-Sachs Lesion: A compression fracture on the posterior-superior aspect of the humeral head, caused by impact against the anterior glenoid rim during dislocation.
  • These lesions compromise the static stability of the joint, making re-dislocation much more likely, especially in younger, active individuals.
• Age:
  • Younger Individuals (under 30): Have a significantly higher risk of recurrent dislocations after a first episode due to higher activity levels and more forceful injuries. The incidence of recurrence can be as high as 70-90% in adolescents and young adults.
  • Older Individuals (over 40): While less prone to recurrence, first-time dislocations in this age group are more likely to be associated with concomitant injuries, such as rotator cuff tears or nerve damage.
• Gender: Males generally have a higher incidence of shoulder dislocations, largely attributable to higher participation rates in contact and high-impact sports.
• Anatomical Factors:
  • Glenoid Dysplasia: An abnormally shallow glenoid fossa can inherently reduce joint stability.
  • Capsular or Ligamentous Laxity: As mentioned, individuals with naturally loose connective tissue are more prone to dislocation.
  • Bony Defects: Pre-existing Bankart or Hill-Sachs lesions from prior subluxations or dislocations significantly increase the risk of full dislocation.
• Activity Level and Sport Participation: Individuals participating in sports that involve:
  • High-impact collisions: Rugby, football, hockey, wrestling.
  • Repetitive overhead movements: Baseball (pitchers), volleyball, basketball, swimming, tennis.
  • Activities with a high risk of falls: Skiing, snowboarding, gymnastics, rock climbing. are at increased risk.
• Neuromuscular Conditions: Conditions that cause sudden, uncontrolled muscle contractions or significant muscle weakness can predispose individuals to dislocation. Examples include:
  • Seizure disorders (epilepsy).
  • Stroke (leading to muscle imbalance or spasticity).
  • Certain neurological diseases.
• Connective Tissue Disorders: Genetic disorders affecting connective tissue strength and elasticity, such as Ehlers-Danlos syndrome or Marfan syndrome, can lead to generalized joint hypermobility and recurrent dislocations.

Preventing Shoulder Dislocation and Recurrence

While not all dislocations can be prevented, especially those from high-impact trauma, understanding the causes and risk factors allows for targeted prevention strategies. These include:

• Strength and Conditioning: Strengthening the rotator cuff muscles and scapular stabilizers to enhance dynamic stability.
• Proprioceptive Training: Exercises to improve the body's sense of joint position and movement, enhancing reactive stability.
• Proper Technique: Learning and applying correct biomechanical techniques in sports and activities to minimize stress on the shoulder joint.
• Protective Gear: Using appropriate protective equipment in high-risk sports.
• Rehabilitation: Following a comprehensive rehabilitation program after an initial dislocation is crucial to restore strength, stability, and proprioception, significantly reducing the risk of recurrence. In some cases, surgical intervention may be recommended to repair damaged structures and prevent future dislocations.

By understanding the interplay of anatomy, immediate forces, and predisposing risk factors, individuals can take proactive steps to protect their shoulder health and minimize the likelihood of this debilitating injury.