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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.

What is a shoulder separation?

The clavicle, or “collarbone” is connected to the shoulder blade at the top of the shoulder blade. This region is called the acromion. The joint between the collarbone and the acromion is called the AC joint (acromioclavicular joint). Three sets of important ligaments hold the AC joint together. When part or all of these ligaments are injured, this is called a shoulder separation. This most commonly occurs by a direct fall onto the shoulder.

What is the role of the clavicle?

The clavicle is the only bone connecting the shoulder blade and the arm to the central skeleton. In essence, the clavicle is like a suspension bridge between the spines central skeleton and the shoulder blade (scapula) and arm. The shoulder blade and arm are actually suspended from the distal end of the clavicle. The ends of the clavicle are held in place by very strong ligaments.

The inside end of the clavicle (medial) creates the sternoclavicular joint (SC joint) at the central skeleton (the sternum). The outside end (lateral or distal) creates the acromioclavicular joint (AC joint). Injuries to the clavicle and or the AC joint will result in a disruption of the connection between the central skeleton and shoulder blade/arm. This results in a loss of position of the shoulder blade/arm.

When this happens, the shoulder blade/arm are dropping down away from the clavicle. Muscles attached to the clavicle will now pull unopposed by the weight of the shoulder blade/arm. The result of the arm dropping down and away and the muscles pulling the clavicle up creates the typical deformities seen with clavicle fractures and AC joint injuries.

Will it heal on its own?

Fortunately, most of the injuries from Grade 1 to 3 will heal on their own. This is because the ligaments can generate scar tissue. This scar tissue can hold the torn ligaments together. This can restore the normal function to the ligaments that were torn. Once the ligaments are back to normal function, the shoulder will return to normal function.

These injuries occur in four degrees of severity.

Grade 1: Injury to only the ligaments at the AC joint itself.

Grade 2: Injury to the ligaments of the AC joint and a partial injury to the CC ligaments. There is a partial displacement of the collarbone upwardly.

Grade 3: Injury to both sets of ligaments. There is a complete displacement of the collarbone upwardly.

Grade 4+: Injury to both sets of ligaments. There is a complete displacement of the collarbone through the trapezius muscle and it is stuck up and behind the AC joint. Or it could even be displaced below and in front of the AC joint.

When is surgery needed?

In certain cases, the function of the shoulder can be affected by a shoulder separation. Usually, surgery is reserved for injuries that involve all three sets of ligaments (Grade 3), in patients who put very high demands on their shoulders. For example, a quarterback with a Grade 3 injury is a patient who puts high demand on his shoulder and may need surgery. A recreational tennis player with a Grade 3 injury usually does not need surgery. All patients with Grade 4 injuries or higher will need surgery. The type of surgery performed varies with the situation and the severity of the injury.

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