AC Joint Dislocation: Understanding Shoulder Separation, Causes, Symptoms, and Treatment

What is a Dislocated AC Joint?

A dislocated AC joint, often referred to as a shoulder separation, occurs when the acromion (part of the shoulder blade) separates from the clavicle (collarbone) at the acromioclavicular (AC) joint, typically due to a tearing of the supporting ligaments.

Understanding the AC Joint Anatomy

The acromioclavicular (AC) joint is a small but crucial articulation located at the top of the shoulder. It connects two key bones:

• Clavicle (Collarbone): A long, slender bone that runs horizontally across the top of the chest.
• Acromion: The highest point of the shoulder blade (scapula), which forms the roof of the shoulder.

This joint's stability is primarily maintained by a complex network of ligaments:

• Acromioclavicular (AC) Ligaments: These are a series of small ligaments that directly connect the clavicle to the acromion, providing horizontal stability.
• Coracoclavicular (CC) Ligaments: These are much stronger ligaments located slightly below the AC joint, connecting the clavicle to the coracoid process (a hook-like projection of the scapula). They consist of two distinct bands:
  • Conoid Ligament
  • Trapezoid Ligament The CC ligaments provide critical vertical stability to the AC joint.

What is an AC Joint Dislocation?

An AC joint dislocation, or "shoulder separation," is an injury where the clavicle separates from the acromion. It's important to distinguish this from a "shoulder dislocation," which refers to the separation of the humerus (upper arm bone) from the glenoid fossa (shoulder socket).

In an AC joint dislocation, the severity depends on which ligaments are stretched, partially torn, or completely ruptured, leading to varying degrees of displacement of the clavicle relative to the acromion.

Causes of AC Joint Dislocation

AC joint dislocations are almost always the result of direct or indirect trauma to the shoulder. Common mechanisms of injury include:

• Direct Blow to the Shoulder: This is the most frequent cause, often occurring from a fall directly onto the point of the shoulder, with the arm tucked in or close to the body. This impact drives the acromion downwards, while the clavicle remains relatively fixed, leading to separation.
• Fall Onto an Outstretched Arm (FOOSH): Less common, but a forceful fall onto an outstretched hand or elbow can transmit force up the arm to the shoulder, causing the separation.
• Sports Injuries: Highly prevalent in contact sports such as football, rugby, hockey, and wrestling, where direct collisions or falls are common. Cycling accidents, skiing, and snowboarding falls also frequently lead to AC joint injuries.
• Motor Vehicle Accidents: High-impact collisions can result in significant shoulder trauma.

Symptoms of an AC Joint Dislocation

The symptoms of an AC joint dislocation can vary based on the severity of the injury, but commonly include:

• Pain: Localized pain at the top of the shoulder, often sharp and immediate following the injury. The pain usually worsens with movement of the arm, especially overhead or across the body.
• Swelling and Bruising: Visible swelling and tenderness around the AC joint. Bruising may develop over time.
• Deformity: A noticeable bump or "step-off" deformity at the top of the shoulder, where the end of the clavicle appears prominent or elevated relative to the acromion. This is more pronounced in higher-grade injuries.
• Limited Range of Motion: Difficulty and pain when attempting to lift the arm overhead, reach across the body, or perform everyday tasks.
• Instability: A feeling of instability or "clunking" at the joint with movement.
• Audible Pop or Snap: Some individuals may hear or feel a pop at the time of injury.

Grading AC Joint Dislocation Severity

AC joint dislocations are typically classified using the Rockwood Classification system, which categorizes injuries into six types based on the degree of ligamentous damage and clavicular displacement:

• Type I: A mild sprain of the AC ligaments. The ligaments are stretched but not torn, and there is no significant displacement of the clavicle. Minimal deformity.
• Type II: A complete tear of the AC ligaments, but the coracoclavicular (CC) ligaments are only sprained or partially torn. There is some upward displacement of the clavicle, but it remains partially aligned with the acromion. A slight "step-off" may be visible.
• Type III: A complete tear of both the AC and CC ligaments. This results in significant upward displacement of the clavicle, creating a noticeable "step-off" deformity. This is the most common type of complete dislocation.
• Type IV: A complete tear of both AC and CC ligaments, with the clavicle displaced posteriorly (backward) into or through the trapezius muscle. This is a rare and severe injury.
• Type V: A complete tear of both AC and CC ligaments, with severe upward displacement of the clavicle (often 100-300% of normal). The soft tissues surrounding the joint are also significantly disrupted.
• Type VI: A complete tear of both AC and CC ligaments, with the clavicle displaced inferiorly (downward) and lodged under the coracoid process or within the subacromial or subcoracoid space. This is an extremely rare and severe injury.

Diagnosis

Diagnosis of an AC joint dislocation typically involves:

• Physical Examination: A healthcare professional will examine the shoulder for tenderness, swelling, and deformity. They will also assess the range of motion and stability of the joint.
• X-rays: Standard X-rays are crucial to confirm the diagnosis, assess the degree of clavicular displacement, and rule out other fractures. Stress X-rays (taken while the patient holds weights) may be used to highlight instability in less severe cases.
• Magnetic Resonance Imaging (MRI) or Computed Tomography (CT) Scans: These advanced imaging techniques may be used in complex cases to evaluate the extent of soft tissue damage (ligaments, muscles) or to visualize the displacement more clearly, especially in higher-grade injuries.

Treatment Options

Treatment for an AC joint dislocation depends heavily on the severity (grade) of the injury, the patient's activity level, and individual factors.

Non-Surgical (Conservative) Treatment

Most Type I, II, and many Type III AC joint dislocations are managed non-surgically.

• Rest and Immobilization: The arm is typically rested in a sling for a period (usually 1-3 weeks) to support the joint and allow healing.
• Ice Application: Applying ice packs to the affected area helps reduce pain and swelling.
• Pain Management: Over-the-counter non-steroidal anti-inflammatory drugs (NSAIDs) like ibuprofen or naproxen are often prescribed to manage pain and inflammation.
• Physical Therapy: Once initial pain and swelling subside, a structured rehabilitation program is essential. This typically involves:
  • Range of Motion Exercises: To restore shoulder mobility.
  • Strengthening Exercises: To rebuild strength in the shoulder and surrounding muscles (rotator cuff, scapular stabilizers) to improve joint stability and function.
  • Proprioception and Neuromuscular Control: Exercises to re-educate the joint's sense of position and movement.

Surgical Treatment

Surgical intervention is generally reserved for:

• High-Grade Dislocations (Type IV, V, VI): These severe injuries often require surgery to anatomically reduce (realign) the clavicle and stabilize the joint.
• Failed Conservative Treatment for Type III: If a Type III injury continues to cause pain, instability, or functional limitations despite adequate non-surgical management, surgery may be considered.
• Specific Patient Populations: Athletes or individuals whose occupations demand full overhead function may opt for surgery for Type III injuries to ensure optimal recovery and return to activity.

Surgical procedures aim to reduce the dislocated joint and reconstruct or repair the torn ligaments. Common techniques include:

• Direct Repair/Reconstruction: Repairing the torn AC and CC ligaments or reconstructing them using grafts (from the patient's own tissue or a donor).
• Internal Fixation: Using screws, plates, or strong sutures to temporarily hold the clavicle in place while the ligaments heal. These implants may be removed later.

Rehabilitation and Recovery

Regardless of whether treatment is surgical or non-surgical, a comprehensive rehabilitation program is critical for optimal recovery. This process typically progresses through several phases:

1. Acute Phase (Pain and Swelling Management): Focus on rest, ice, pain control, and protecting the injured joint.
2. Mobility Phase (Early Range of Motion): Gradual introduction of passive and then active range of motion exercises to prevent stiffness.
3. Strengthening Phase: Progressive strengthening of the shoulder girdle muscles (deltoids, rotator cuff, scapular stabilizers) to improve stability and function.
4. Functional and Return-to-Activity Phase: Sport-specific or activity-specific drills to prepare for a safe return to pre-injury activities.

The recovery timeline varies significantly based on the injury's severity and the chosen treatment path, ranging from a few weeks for mild sprains to several months for surgical repairs. Adherence to physical therapy protocols is paramount for restoring full function and preventing re-injury.

Prevention

While not all AC joint dislocations can be prevented, certain measures can reduce the risk:

• Proper Technique in Sports: Learning and practicing correct falling techniques in sports like cycling or skiing can minimize the impact on the shoulder.
• Strength and Conditioning: Maintaining strong shoulder and core muscles can improve overall stability and resilience.
• Protective Gear: Using appropriate padding or protective gear in contact sports can help absorb impact forces.
• Awareness of Surroundings: Being mindful of potential hazards in environments where falls are common.