The acromioclavicular (AC) joint is a common site of injury, particularly among active individuals and athletes. This joint sits at the highest point of the shoulder, connecting the collarbone and the shoulder blade. An AC joint sprain, often called a shoulder separation, involves damage to the ligaments holding the joint together. The severity of the injury dictates the symptoms and the path to healing.
Defining the AC Joint Sprain
The AC joint is the meeting point where the outer end of the collarbone (clavicle) articulates with the acromion, a bony projection extending from the shoulder blade (scapula). This connection is stabilized by a complex of ligaments that prevent the bones from separating. The primary stabilizers are the acromioclavicular (AC) ligaments, which surround the joint capsule.
Further stability comes from the coracoclavicular (CC) ligaments, composed of two distinct bands: the conoid and trapezoid ligaments. A sprain involves stretching or tearing these ligaments, and the extent of this damage determines the injury’s severity. Medical professionals classify AC joint sprains using the Rockwood system, which ranges from Grade I to Grade VI.
A Grade I injury involves a minor stretch or partial tear of the AC ligaments, with the CC ligaments remaining intact and no visible joint displacement. A Grade II sprain involves a complete tear of the AC ligaments, resulting in a slight upward shift of the collarbone. Grade III is the most common severe injury, involving a complete tear of both the AC and CC ligaments, allowing the collarbone to significantly move out of alignment.
The higher classifications, Grades IV through VI, are rare and involve more severe displacement of the clavicle. These grades often involve the bone moving backward into the trapezius muscle or downward beneath the coracoid process. This grading system determines the level of joint instability and guides the treatment approach.
How AC Joint Sprains Occur
AC joint sprains typically result from a direct, forceful impact to the top of the shoulder while the arm is held close to the body. This mechanism causes the acromion part of the shoulder blade to be driven downward, while the collarbone is held in place by muscle attachments. The resulting shear force overloads the stabilizing ligaments, causing them to fail sequentially.
The most common scenarios for this trauma are contact sports, such as football, hockey, or rugby, where a player is tackled or checked directly onto the shoulder. Another frequent cause is a fall, such as from a bicycle or during activities like skiing, where the person lands hard on the point of the shoulder. In both cases, the injury force is concentrated directly on the AC joint structure.
While less common, an indirect force, such as a fall onto an outstretched hand or elbow, can also transmit an upward force through the arm bone to the AC joint. Athletes involved in weightlifting or strenuous occupations may develop AC joint issues due to chronic overuse and strain. However, acute sprains are primarily traumatic, and young, active males are generally the population at highest risk.
Recognizing the Signs of Injury
The most recognizable symptom of an AC joint sprain is sharp, localized pain directly on the top of the shoulder, especially over the bony protrusion of the joint. This pain is often immediate and intense following the traumatic event. Patients may experience significant swelling and tenderness in the area. Any movement that stresses the joint will exacerbate the discomfort, making simple tasks difficult.
Patients often feel increased pain when attempting to lift the arm overhead or move the arm across the body toward the opposite side. In more severe sprains, there may be a noticeable clicking or grinding sensation within the joint during movement, indicating instability. Pain can sometimes radiate outward into the neck or down the arm, potentially confusing the precise source of the injury.
An objective sign of a moderate to severe sprain is a visible prominence or bump on the shoulder. This deformity, known as a “step-off” sign, occurs when the clavicle is displaced upward because the torn CC ligaments can no longer hold it in position. The size of this bump is often proportional to the injury grade, with Grade III and higher injuries showing a more pronounced separation.
Treatment and Recovery Pathways
The management of an AC joint sprain is determined largely by the injury grade, with initial steps focusing on immediate symptom control. Initial care involves the Rest, Ice, Compression, and Elevation (RICE) protocol to minimize swelling and pain. Immobilization in a sling is usually recommended for a brief period to allow the ligaments to begin healing.
For lower-grade sprains (Grade I and Grade II), non-surgical treatment is the standard approach. This involves using nonsteroidal anti-inflammatory drugs (NSAIDs) to manage pain and inflammation. Immobilization is typically maintained for 7 to 10 days for a Grade I sprain, extending up to four to six weeks for a Grade II sprain to ensure adequate rest.
Physical therapy (PT) is a major component of non-surgical recovery, initiated once the pain has subsided sufficiently. The rehabilitation program begins with restoring the shoulder’s full range of motion, progressing from passive to active-assisted movements. This is followed by strengthening exercises to rebuild the stability of the surrounding muscles, preventing chronic instability.
For Grade III sprains, non-surgical treatment is often recommended first, though the decision can be individualized, especially for high-demand athletes. Non-operative management generally results in good functional outcomes, but it may leave a permanent cosmetic bump on the shoulder. Surgery is typically reserved for Grades IV through VI, or for lower grades where non-surgical treatment fails to control persistent pain or instability.
Surgical intervention aims to reconstruct or repair the torn ligaments and stabilize the joint using techniques like ligament grafts or specialized hardware. Following surgery, a prolonged recovery period is necessary, with rehabilitation focusing on regaining strength and function. A full return to contact sports or heavy weightlifting is generally eight to twelve weeks after the initial immobilization period is complete.