Shoulder Dislocation: Humerus, Causes, Symptoms, and Treatment

Which bone is most commonly dislocated?

The humerus, specifically at the glenohumeral (shoulder) joint, is the bone most commonly involved in dislocations. This high incidence is due to the shoulder's exceptional range of motion, which inherently compromises its stability.

The Shoulder Joint: A Vulnerable Ball-and-Socket

While many joints in the human body can dislocate, the shoulder joint stands out as the most frequently affected. This is primarily because of its unique anatomical structure, which prioritizes mobility over inherent stability. The shoulder is a ball-and-socket joint where the head of the humerus (the "ball") articulates with the shallow glenoid fossa of the scapula (the "socket"). This design allows for a remarkable 360-degree range of motion, crucial for activities ranging from throwing to reaching overhead. However, this extensive movement comes at the cost of stability, making the humerus prone to displacement from its socket.

Anatomy of the Glenohumeral Joint

Understanding the shoulder's anatomy illuminates its predisposition to dislocation:

• Humerus Head: The large, spherical head of the humerus is significantly larger than the glenoid fossa it articulates with. This size mismatch means a large portion of the humeral head is not contained within the socket at any given time.
• Glenoid Fossa: This shallow, pear-shaped depression on the scapula provides minimal bony containment for the humeral head.
• Glenoid Labrum: A fibrocartilaginous rim that deepens the glenoid fossa, increasing its surface area and enhancing stability. However, it can be torn during a dislocation (Bankart lesion).
• Joint Capsule: A fibrous sac enclosing the joint, reinforced by ligaments. While it provides some stability, it is relatively loose to allow for wide range of motion.
• Ligaments: The glenohumeral ligaments (superior, middle, inferior) and coracohumeral ligament provide static stability, but their primary role is to limit extreme movements.
• Rotator Cuff Muscles: Comprising the supraspinatus, infraspinatus, teres minor, and subscapularis, these muscles dynamically stabilize the joint by compressing the humeral head into the glenoid fossa during movement. Weakness or injury to these muscles can increase dislocation risk.

Types of Shoulder Dislocations

Shoulder dislocations are typically classified by the direction of the humeral head's displacement:

• Anterior Dislocation: The most common type (over 95% of cases), where the humeral head dislocates forward and downward, often beneath the coracoid process.
• Posterior Dislocation: Less common, occurring when the humeral head moves backward out of the socket. Often associated with seizures or electric shocks.
• Inferior Dislocation (Luxatio Erecta): The rarest type, where the humeral head displaces directly downward, often leaving the arm fixed in an overhead position.
• Subluxation: A partial dislocation where the humeral head comes partially out of the socket but then spontaneously reduces (slips back in).

Common Causes of Shoulder Dislocation

Dislocations are typically caused by significant force or trauma:

• Falls: Falling onto an outstretched arm (FOOSH) is a very common mechanism, especially in older adults or during sports.
• Sports Injuries: High-impact sports (e.g., football, rugby, hockey) and activities involving overhead movements or falls (e.g., gymnastics, wrestling, skiing, volleyball, basketball) frequently lead to dislocations.
• Motor Vehicle Accidents: Direct impact to the shoulder or a forceful twisting motion can cause dislocation.
• Repetitive Strain/Overuse: While less common as a direct cause, chronic overuse or repetitive microtrauma can lead to capsular laxity, making the joint more susceptible to dislocation with less force.
• Previous Dislocation: Once dislocated, the shoulder is significantly more prone to future dislocations due as the soft tissues (labrum, capsule, ligaments) are stretched or torn, leading to chronic instability.

Recognizing a Dislocated Shoulder: Symptoms

A dislocated shoulder is usually immediately apparent due to a combination of severe symptoms:

• Intense Pain: Sharp, immediate pain at the moment of dislocation.
• Visible Deformity: The shoulder may appear "squared off" rather than rounded, with a noticeable bulge under the skin (the humeral head) either in front, back, or below the joint.
• Inability to Move Arm: The arm is often held in an awkward position and any attempt to move it causes excruciating pain.
• Numbness or Tingling: Due to potential nerve compression (e.g., axillary nerve) in the arm or hand.
• Swelling and Bruising: May develop rapidly around the joint.

Management and Treatment

A dislocated shoulder requires immediate medical attention. Do not attempt to reduce it yourself.

• Reduction: A healthcare professional will manually manipulate the humerus back into the glenoid fossa. This is often done under sedation or anesthesia to relax the muscles and minimize pain.
• Immobilization: After reduction, the arm is typically immobilized in a sling for a period (e.g., 1-3 weeks) to allow the injured soft tissues to heal.
• Rehabilitation: A crucial phase involving physical therapy to restore range of motion, strengthen the rotator cuff and scapular stabilizing muscles, and improve proprioception (the body's sense of joint position). This helps prevent recurrent dislocations.
• Surgery: May be recommended for recurrent dislocations, significant labral tears (Bankart lesions), bone loss from the glenoid or humeral head (Hill-Sachs lesion), or in athletes requiring high levels of shoulder stability. Surgical options include arthroscopic repair of the labrum and capsule.

Prevention Strategies

While not all dislocations can be prevented, certain measures can reduce the risk, especially for those with a history of shoulder instability:

• Strengthening Exercises: Focus on strengthening the rotator cuff muscles and scapular stabilizers (e.g., rhomboids, serratus anterior, trapezius).
• Proprioception Training: Exercises that challenge balance and joint position awareness can improve dynamic stability.
• Proper Technique: Learning and using correct biomechanics in sports and activities can minimize undue stress on the shoulder joint.
• Protective Gear: Using appropriate padding or braces in contact sports.
• Avoid High-Risk Positions: For individuals with a history of anterior dislocation, avoiding positions like abduction and external rotation (e.g., arm cocked back for throwing) can be critical.

Conclusion

The humerus, at its articulation with the scapula in the shoulder joint, is indeed the most commonly dislocated bone. Its design, favoring an extensive range of motion, inherently makes it more susceptible to displacement. Understanding the anatomy, mechanisms of injury, and the importance of appropriate management and rehabilitation is vital for anyone involved in fitness, sports, or healthcare. Prioritizing shoulder health through targeted strengthening and smart training practices can significantly reduce the risk of this painful and often recurring injury.