Shoulder pain affects up to 46 percent of the general population every year, and can be incredibly debilitating. The shoulder is one of the most hypermobile joints in the body, capable of full rotation unlike the hip or knee, and it is one of the largest joint complexes. However, that’s also what makes it so prone to injury. Recurring overuse injuries, unsafe pressing form, improper load management, and a sedentary lifestyle are some of the most common risk factors for shoulder pain, both acute and chronic.
Yet the majority of shoulder pain has nothing to do with the main shoulder joint itself, which is a ball-and-socket joint merging the bone of the upper arm (humerus) with your shoulder blade (scapula). Instead, most shoulder pain results from an inflammation or injury in the acromioclavicular joint, also known as the AC joint.
What is the AC Joint?
The acromioclavicular joint joins the acromion (the bone on the very edge of your shoulders) with the clavicle (your collarbone). Anatomically, the AC joint is located directly above the shoulder capsule, which is the ball of connective tissue (cartilage) surrounding the joint of the upper arm and shoulder blade. The purpose of the AC joint is to enable passive articulation of the pectoral girdle, or the bones of the upper chest – mostly the collarbone.
To understand what that means, think about bringing your elbows together, and then bringing your arms apart, and behind your back. Connecting the top portion of the shoulder joint (the acromion) with the clavicle allows your upper chest bones to follow and move in response to the movement of your shoulders. Your AC joint also facilitates the movement of the pectoral girdle in response to shoulder elevation – such as shrugging.
The AC joint also helps the transfer of force from the upper arm throughout the upper chest – think of overhead pressing, chest flies, incline bench pressing, and elevated push-ups.
What Makes this Pain Different?
Unlike the shoulder joint itself, the AC joint is relatively fragile and small. It is composed of four different ligaments of cartilage, each of which is prone to spraining under undue stress or sudden trauma. Most AC joint sprains occur as a result of trauma, especially from contact sports such as football or rugby. Falling and landing on an outstretched arm can also sprain or tear the AC joint, in addition to other structures in the wrist and arm.
Chronic AC joint pain may be the result of osteoarthritis. Osteoarthritis is also known as wear-and-tear arthritis and is a condition that occurs when the cartilage between two bones is worn down with time.
Certain lifestyle factors, such as excessive manual labor, prior injuries, poor nutrition, and habits such as smoking or alcohol consumption can speed along the degeneration of cartilage, and manifestation of joint pain.
In cases of osteoarthritis, the pain being felt is the result of two bones continuously rubbing together when they shouldn’t be. The results include tissue swelling and bony growths, called bone spurs, which can cause limited mobility (as the joint locks) and inflammation-related pain.
Understanding and Treating Pain and Injuries
Arthritis in the AC joint does not usually cause pain by itself, because the AC joint is rarely load-bearing. However, it can become painful when used excessively, such as while working out or lifting objects overhead. AC joint damage, whether through injury or arthritis, can also result in lowered mobility – such as being unable to raise one shoulder laterally as high as the other.
Pain in the AC joint as a result of overuse injuries (such as construction work or weightlifting) or direct trauma (such as being shoulder-checked in a game) can result in osteolysis, which also causes the erosion of the AC joint.
If you are experiencing AC joint pain during certain activities without a history of prior injuries, your pain may be the result of a form of arthritis or tissue degeneration. However, depending on the origin point and nature of the pain, it may not be related to the joint itself – shoulder pain can also be the result of a different strained ligament, such as a SLAP tear or a rotator cuff tear. Sometimes, shoulder pain can be nerve related.
A physical examination can help a doctor rule out certain injuries and potential signs of separated or damaged ligaments. For an AC joint injury in particular, a doctor may try to see if they can move your collarbone independently from your right or left shoulder, indicating that the AC joint may have been damaged.
What Should I Do About AC Joint Pain?
First and foremost, avoid the activity that is causing you pain. If you are experiencing pain during your workouts, for example, try to swap exercises to a range of motion that you can work through with minor or no discomfort. Explore different alternative exercises until you find something that feels safe.
Go see a professional. Ligament injuries can heal with time, but certain conditions – such as osteoarthritis – are irreversible. If imaging tests (like an x-ray) reveal that the cartilage in your AC joint has worn down considerably, then your doctor may recommend over-the-counter anti-inflammatory medication to reduce immediate pain symptoms and refer you to a kinesiologist or physical therapist to explore different exercises to strengthen the supporting musculature and find out what type of movements you might need to avoid.
How is AC Joint Pain Medically Treated?
Chronic AC joint pain does not typically occur as a result of arthritis or injuries. If your shoulder hurts even when you aren’t moving it through a specific range of motion, the pain may not be caused by your AC joint specifically. Depending on the type of damage – muscle strains, sprained ligaments, or neuropathic pain – a doctor will recommend a different treatment plan.
Pain management is key. Most aggravated joint pain goes away on its own and can be prevented through an individualized physical therapy plan. Medication and short-term pain interventions can help resolve episodes of acute pain, through nerve blocks, over-the-counter painkillers, and local nerve stimulation.
The last thing you should do is ignore the pain. If you are experiencing severe discomfort in your shoulder, regardless of where, go see a doctor. Imaging tests are often needed to pinpoint the exact nature of the pain and determine your timeline for a full recovery.
