An acromioclavicular (AC) joint sprain is a common injury affecting the shoulder girdle, often called a shoulder separation. This injury occurs when the ligaments connecting the collarbone to the shoulder blade are stretched or torn. It is frequently seen in athletes or individuals who experience a direct fall onto the shoulder. This article explains the anatomy of the AC joint, how the sprain is classified, and the expected path to recovery.
Anatomy of the AC Joint and Injury Mechanism
The acromioclavicular joint is the articulation where the outer end of the clavicle (collarbone) meets the acromion, a bony projection extending from the scapula (shoulder blade). This connection point allows for gliding movement and helps transmit forces from the upper limb to the skeleton. The joint’s stability depends heavily on two main sets of ligaments.
The acromioclavicular ligaments reinforce the joint capsule, controlling horizontal motion and anterior-posterior stability. The coracoclavicular (CC) ligaments, consisting of the trapezoid and conoid ligaments, provide the joint’s vertical stability. An AC joint sprain results when these stabilizing ligaments are damaged, ranging from a mild stretch to a complete tear.
The most frequent cause is a forceful, direct blow to the top of the shoulder, which drives the acromion downward. This mechanism is common during a fall, such as off a bicycle, or from an impact in collision sports like football or rugby.
Recognizing Symptoms and Classification of Severity
A person with an AC joint sprain typically experiences immediate, localized pain and tenderness directly over the joint. Swelling commonly develops around the injury site, and movement of the arm, especially lifting it above shoulder height, greatly intensifies the discomfort.
Healthcare providers use the Rockwood classification to categorize the injury based on the degree of ligament disruption and joint separation.
Grade I and II Sprains
A Grade I sprain is the mildest form, involving only a stretch or partial tear of the acromioclavicular ligaments, with the coracoclavicular ligaments remaining intact. There is no physical displacement of the joint, and pain is generally mild with no visible bump.
A Grade II injury signifies a complete tear of the acromioclavicular ligaments and a partial tear of the coracoclavicular ligaments. This partial loss of stability allows for a slight upward elevation of the clavicle. Symptoms include moderate to severe pain and a slight, palpable step-off or bump at the joint.
Grade III and Higher Sprains
The more severe Grade III injury involves a complete tear of both the acromioclavicular and coracoclavicular ligaments. This ligament failure results in a significant separation, causing the end of the clavicle to elevate noticeably above the acromion, creating a pronounced visible deformity. Grades IV, V, and VI are rare, highly unstable injuries that involve even more extreme displacement.
Treatment Options and Recovery Expectations
The management approach for an AC joint sprain is determined primarily by its grade of severity. Treatment for Grade I and II sprains is almost always non-surgical, focusing on controlling pain and allowing the torn ligaments to heal. Initial care involves resting the shoulder, applying ice to reduce swelling, and using a sling for one to three weeks to immobilize the joint.
Once the initial pain subsides, physical therapy begins to restore full range of motion and strengthen the surrounding shoulder muscles, including the rotator cuff and deltoid. Non-operative recovery for a Grade I sprain takes approximately one to two weeks, while a Grade II sprain generally requires three to six weeks before returning to full activity.
For most Grade III injuries, non-surgical treatment is often successful, although the decision is individualized based on a patient’s activity level and occupation. Conservative recovery for Grade III sprains may take six to twelve weeks to reach a high level of function.
Surgical intervention is generally reserved for the highly unstable injuries (Grades IV, V, and VI) or for Grade III cases where non-operative treatment fails to provide sufficient function. Recovery following a surgical stabilization procedure is significantly longer, typically requiring four to six months of extensive rehabilitation before a full return to sports or heavy labor is possible. Ongoing physical conditioning and a gradual return to activity are necessary to ensure the best long-term outcome.