Acromioclavicular Ligament

1. Overview

The acromioclavicular (AC) ligament is a small but critical ligament that supports the acromioclavicular joint—where the clavicle (collarbone) meets the acromion of the scapula. This ligament functions to stabilize the joint and limit horizontal movement of the clavicle. Though small in size, the AC ligament plays an important role in maintaining upper limb alignment and transmitting forces from the arm to the axial skeleton. Injury to this ligament is common in contact sports and is often referred to as a shoulder separation.

2. Location

The acromioclavicular ligament is located at the superior aspect of the shoulder:

• Proximally: Attaches to the lateral end of the clavicle.

• Distally: Connects to the medial margin of the acromion process of the scapula.

• Forms the roof of the acromioclavicular joint and lies just deep to the skin and subcutaneous tissue in this region.

It is part of a complex joint structure that includes the acromion, clavicle, and supporting capsule and ligaments.

3. Structure

The acromioclavicular ligament is composed of dense regular connective tissue and consists of two main parts:

• Superior acromioclavicular ligament:

  • Stronger and more functionally important; provides horizontal stability.

  • Reinforced by the fibers of the deltoid and trapezius muscles.

• Inferior acromioclavicular ligament:

  • Thinner and less developed; may assist in maintaining joint integrity.

The ligament blends with the fibrous capsule of the acromioclavicular joint and provides static support against translational forces.

4. Function

The acromioclavicular ligament serves several vital mechanical functions:

• Stabilizes the acromioclavicular joint: Maintains articulation between the clavicle and scapula.

• Prevents excessive horizontal movement: Especially anterior and posterior displacement of the clavicle relative to the acromion.

• Supports shoulder girdle integrity: Helps transmit loads from the upper limb to the axial skeleton.

5. Physiological role(s)

Beyond its static structural role, the acromioclavicular ligament contributes to:

• Dynamic shoulder mechanics: Works in concert with the coracoclavicular ligaments and joint capsule to allow controlled movement of the scapula during arm elevation and rotation.

• Force transmission: Transfers stresses from the limb during lifting, throwing, or pushing activities.

• Proprioception: Contains mechanoreceptors that aid in joint position sense and muscular coordination during complex upper limb movements.

6. Clinical Significance

The acromioclavicular ligament is clinically significant due to its vulnerability in shoulder trauma:

• AC joint sprain (shoulder separation):

  • Commonly caused by a fall onto the shoulder or direct impact; graded from Type I to Type VI based on ligament involvement and displacement.

  • Type I: Partial tear of AC ligament; no visible displacement.

  • Type II: Complete tear of AC ligament; partial injury to coracoclavicular ligaments.

  • Type III+: Complete disruption of both AC and coracoclavicular ligaments with visible deformity and instability.

• Clinical signs:

  • Tenderness, swelling, a prominent “step-off” at the AC joint, and pain with shoulder movement.

• Diagnosis:

  • Clinical exam with imaging (X-rays, sometimes MRI) to assess ligament integrity and joint displacement.

• Treatment:

  • Type I–II injuries: Conservative (rest, ice, sling, physiotherapy).

  • Type III+: May require surgical reconstruction, especially in athletes or individuals with persistent dysfunction.

• Degenerative changes:

  • Chronic overuse or minor injuries can lead to osteoarthritis of the AC joint, often seen in weightlifters and older adults.