What is an AC Joint Injury?

The acromioclavicular (AC) joint in the shoulder is very important for shoulder strength, motion, and maintaining shoulder position. The joint is stabilized by various ligaments and a capsule, which can cause pain and affect normal joint function if damaged.

Anatomy of the AC Joint

The AC joint is located at the highest point of the shoulder, where the acromion, a bony projection on the shoulder blade, connects to the clavicle or collarbone. This joint enables you to lift your arm above your head and is important for overall shoulder control, motion, and strength.

The AC joint is stabilized by the following structures:

Capsular ligaments: These ligaments are called the acromioclavicular ligaments. They have upper and lower components and resist separation of the joint in the horizontal direction.
Extracapsular stabilizers: These are ligaments extending from the bony process of the scapula called the coracoid process to the clavicle (coracoclavicular ligaments) and the acromion (coracoacromial ligaments). These ligaments resist vertical forces from separating the joint.
Muscular attachments: The deltoid muscle on the outside of the shoulder and the trapezius muscle in the upper back and neck also help stabilize the acromioclavicular joint.

An injury to the AC joint, particularly the ligaments, can result in instability or separation of the AC joint (shoulder separation), causing pain and discomfort and limiting shoulder function.

Causes of AC Joint Injuries

Injuries to the AC joint commonly occur due to:

Falling onto the shoulder
Lifting heavy objects or weights

Risk factors

Various situations which can result in an AC joint injury include:

Participating in contact sports
Participating in activities that increase your risk of falls, such as skiing, biking, or skating.
Weightlifting
Physically strenuous jobs

Symptoms of AC Joint Injuries

Signs and symptoms of AC joint injuries include:

Pain over the AC joint at the top of the shoulder
Tenderness at the top of the shoulder
A bump over the shoulder
Swelling
Bruising
Shoulder tightness and loss of motion
Pain with lifting the arm

Diagnosis of AC Joint Injuries

Your doctor will review your symptoms and medical history. A thorough physical examination is performed to check for a range of motion, blood flow, stability, and strength of the joint. Other diagnostic tests include:

X-ray
MRI
CT scan or ultrasound for a detailed evaluation of the joint.

Treatment for AC Joint Injuries

In most cases, the following conservative treatment methods are quite effective in treating the injury:

Resting the joint for prolonged periods
Application of an ice pack at regular intervals to reduce pain and swelling
Use of anti-inflammatory medications
Physical therapy to help strengthen the shoulder muscles and improve range of motion.