What Is AC Joint Arthritis?

Your acromioclavicular (AC) joint is the part of your shoulder where your collarbone and shoulder blade meet. It contributes your shoulder’s mobility, such as raising your arm overhead.

Arthritis in the AC joint is a most common cause of shoulder pain in adults. Arthritis here usually develops when the smooth cartilage between your scapula and collar bone wears down, becoming thin and rough. This can cause pain and inflammation in the joint when the end of your bones rub against each other.

In this article, we examine the role of your AC joint in healthy shoulder movement and how AC joint arthritis is treated.

Your shoulder is a ball-and-socket joint that provides you with a large range of motion. The trade-off for this large range is that your shoulder is prone to injury. As many as 67 percent of people will experience shoulder pain in their life and up to 26 percent experience it at any given time.

Your shoulder joint consists of three bones:

  • humerus (upper arm bone). The head of your humerus makes up the ball of your shoulder joint.
  • scapula (shoulder blade). Your scapula makes up the socket of your shoulder joint, and many muscles and ligaments help support and stabilize your joint capsule.
  • clavicle (collarbone). The end of your clavicle connects to a part of your scapula called the acromion. They’re connected with a thick ligament called the acromioclavicular ligament. This area is your AC joint.

The delicate structures of your AC joint make it particularly prone to getting hurt. Injuries to this area make up more than 40 percent of shoulder injuries.

A healthy AC joint only allows limited motion. During certain motions like raising your arm over your head, your acromion meets the end your clavicle. If the joint becomes inflamed or painful, it can restrict your range of motion when you move your arm.

AC joint pain is usually caused by traumatic injuries or arthritis. Arthritis is a group of conditions that cause inflammation and degeneration of a joint. The most common form of arthritis is called osteoarthritis. It’s caused by a wearing down of a joint due to repetitive use, making age the biggest risk factor.

Sudden trauma to the AC joint can lead to post-traumatic arthritis that tends to develop soon after the injury. Most of the time it’s temporary and goes away over time.

Traumatic injury

AC joint pain is reported in anywhere from 0.5 to 2.9 per 1000 people per year. The most common cause of pain is an injury from catching yourself on an outstretched arm. This type of impact can lead to sprains to the ligaments around the joint or joint separation.

A sprain is a tearing of the ligaments that support the joint. A joint separation is when your collarbone and shoulder separate.

AC injuries are common among young people because they occur frequently in collision sports like football or ice hockey, making up almost 10 percent of injuries in these sports.

Other causes of trauma to the joint can include:

  • falling off a bicycle
  • car accidents
  • repeatedly lifting objects overhead with poor mechanics (improper form)
  • direct impact on the joint (such as a fall)

Arthritis

The primary cause of AC arthritis is repetitive stress on the joint, particularly from repeated overhead lifting.

Arthritis develops in the AC joint primarily due to the wearing down of the joint over time. AC joint arthritis can also develop as a complication of AC joint injuries, affecting 30 to 50 percent of people who have an AC joint separation.

Risk factors associated with the development of AC joint arthritis include:

  • being over the age of 50
  • having a history of shoulder instability or traumatic injury
  • working a job that requires heavy lifting
  • manual work
  • repetitive trauma through sports like weightlifting, basketball, or swimming
  • having inflammatory arthritis, such as psoriatic arthritis or rheumatoid arthritis
  • having septic arthritis, an infection of the joint capsule

Symptoms of AC joint arthritis usually get worse over time and with specific motions such as lifting objects overhead or crossing your arms. They often include:

  • pain near the top of your shoulder and sometimes into the neck and arm
  • limited range of motion through your shoulder
  • clicking or snapping when moving your shoulder

Most injuries are treated without surgery and people usually regain functional movement within 6 weeks and return to normal activity within 12 weeks.

Nonsurgical treatment options include:

  • pain and anti-inflammatory medication
  • physical therapy
  • changing exercise habits
  • corticosteroid injections

Your doctor may recommend surgery if these treatments fail to give your relief.

A 2021 review of studies found that conservative and surgical treatment can both effectively treat AC joint osteoarthritis. However, there isn’t enough evidence to establish one as superior to the other.

Let’s go over these treatment approaches in depth.

Medicines

Painkillers such as Tylenol and anti-inflammatory medications like nonsteroidal anti-inflammatory drugs (NSAIDs) may be able to help reduce your pain. These medications are generally safe, but your doctor can best advise you on how to use them to treat your shoulder. Make sure your doctor knows about any medications you’re taking to prevent adverse drug interactions or side effects.

You may get relief from topical anti-inflammatories, such as creams, gels, or patches.

Ice or heat can also help reduce pain and swelling, or relax muscles around the joint. Learn more about when to use cold and hot therapies.

Physical therapy

Physical therapy may help you control pain, maintain your range of motion, and build strength through your shoulder. Physical therapy may help treat other shoulder issues you have that might contribute to your pain such as damaged rotator cuff muscles.

Changes in physical activity

Avoiding motions that cause pain may help reduce discomfort. Some motions such as push-ups, overhead lifting, and exercises that require your arm to cross your body are particularly likely to cause pain.

Steroid injections

Steroid injections can potentially offer pain relief and reduce swelling in the short term. A positive response to steroid injections can also help confirm the diagnosis. However, these injections aren’t considered to be an effective long-term treatment.

Surgery

If more conservative options fail, you may need a type of surgery called distal clavicular excision (DCE). This procedure involves creating a space between your shoulder blade and your collarbone by removing bone from the end of the collarbone.

DCE has been performed regularly to treat AC joint problems since the 1940s. In recent years, it’s become common to perform the surgery arthroscopically. Surgeons perform arthroscopic surgery with a long flexible tube with a camera called an arthroscope. Using an arthroscope minimizes the need for a large incision, and is associated with faster recovery.

AC joint arthritis is usually caused by wear and tear of the joint between your shoulder blade and your collarbone. Arthritis can also develop after sudden injuries, and less commonly, due to autoimmune conditions or joint infections.

First-line treatment for AC joint arthritis usually consists of conservative options such as painkillers and physical therapy. If you don’t respond to these treatments, you may need surgery.

Your doctor can explain what treatment options are best for you based on the severity of injury, and taking into consideration your other individual health factors.

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