The acromioclavicular (AC) joint is located at the top of the shoulder, connecting the collarbone (clavicle) and the highest part of the shoulder blade (acromion). This small joint provides stability and is fundamental to shoulder movement. Understanding the specific sensations associated with AC joint pain is a crucial first step toward identifying the issue. This article describes the distinct feelings and mechanical symptoms that characterize AC joint discomfort.
Pinpointing the Location and Quality of AC Joint Discomfort
AC joint pain is highly localized, felt directly on top of the shoulder where the collarbone meets the shoulder blade. The area is often tender to the touch, and pain can be easily reproduced by pressing directly onto the joint space. Unlike deeper shoulder problems, AC joint discomfort feels superficial, close to the skin surface.
The quality of the pain depends on whether the issue is acute or chronic. Acute pain from a sprain or separation is frequently described as sharp, stinging, or stabbing, especially when the shoulder is moved unexpectedly. This sharp sensation results from sudden stress placed on the joint’s ligaments.
Chronic discomfort, often associated with arthritis, typically manifests as a persistent, dull ache or underlying soreness. This constant pain may feel like a low-grade throb that lingers even when the arm is at rest. However, sudden movement or pressure can transform this dull ache into a sharp spike of pain.
How Specific Movements Exacerbate AC Joint Pain
AC joint pain is triggered by movements that either compress the joint surfaces or apply a shearing force across them. A common movement that provokes sharp pain is horizontal adduction, which involves bringing the arm across the chest. This action forces the acromion and clavicle together, compressing the joint space and irritating damaged tissue.
Lifting objects overhead or raising the arm above 90 to 120 degrees can also cause pain. When the arm moves into this range, the joint reaches its closed-pack position, increasing tension and shear forces. This makes activities like reaching for a high shelf or overhead pressing difficult.
Sleeping on the affected side commonly causes nighttime pain because body weight directly loads and compresses the injured joint. Additionally, motion that pulls the arm downward, such as carrying a heavy object, puts traction on the surrounding ligaments, resulting in discomfort.
Associated Symptoms Beyond Primary Pain
AC joint injuries produce mechanical symptoms beyond localized pain. A frequent complaint is a clicking, popping, or grinding sensation, known as crepitus, which occurs during shoulder movement. This feeling often arises from the irregular movement of joint surfaces or the presence of debris from degenerative cartilage.
In cases of severe injury, such as a complete joint separation, a visible bump or deformity may appear on the top of the shoulder. This bump signifies the collarbone end being displaced upward due to torn supporting ligaments. Localized swelling, bruising, and increased tenderness may also be present over the joint.
The pain and mechanical symptoms often reduce the shoulder’s overall range of motion and cause a perceived loss of strength. The body restricts movement to avoid discomfort, leading to stiffness and weakness in the surrounding muscles.
Differentiating AC Joint Pain from Other Shoulder Conditions
Distinguishing AC joint pain from other common shoulder issues, like rotator cuff problems or biceps tendonitis, relies on pinpointing the exact location and specific movements that cause pain. Rotator cuff tendonitis and bursitis typically result in pain felt deeper within the shoulder or radiating down the side of the arm. AC joint pain, in contrast, is sharply focused on the superior surface of the shoulder.
Rotator cuff pain is often most intense during active lifting, especially in the mid-range of motion or when lifting the arm straight out to the side. While AC joint pain can hurt with overhead lifting, its strongest trigger is the cross-body motion (horizontal adduction). This movement compresses the AC joint directly and is often used as a diagnostic test, but it does not typically bother someone with an isolated rotator cuff problem.
Biceps tendonitis causes pain localized toward the front of the shoulder, where the biceps tendon runs through a groove in the arm bone. This condition is worsened by movements involving resisted supination, such as turning the palm upward against resistance, or flexing the elbow. These actions do not specifically stress the AC joint and do not provoke the sharp, superior pain characteristic of an AC joint issue.