An acromioclavicular (AC) joint sprain is a common injury to the shoulder girdle, often called a “separated shoulder.” The AC joint connects the clavicle (collarbone) and the acromion (the highest part of the shoulder blade). A sprain involves damage to the supporting ligaments, ranging from a mild stretch to a complete tear. These injuries frequently occur in athletes involved in contact sports or activities with a high risk of falling.
Understanding the AC Joint and Injury Mechanism
The AC joint is a functionally important joint at the top of the shoulder, formed by the clavicle and the acromion process of the scapula. Stability relies on two primary groups of ligaments: the acromioclavicular (AC) ligaments, which surround the joint, and the coracoclavicular (CC) ligaments. The CC ligaments connect the clavicle to the coracoid process and provide the majority of the joint’s vertical stability.
The most frequent cause of an AC joint sprain is a sudden, forceful impact, usually a direct blow to the point of the shoulder. This often happens when falling onto the shoulder while the arm is tucked close to the body. The force drives the acromion downward, causing the ligaments to stretch or tear.
Symptoms include immediate, localized pain and tenderness directly over the joint. Swelling and bruising may develop shortly after the injury. In severe cases, a noticeable bump may appear above the shoulder, indicating the clavicle has been pushed upward. Movement of the arm, especially raising it above shoulder height, increases the pain.
Grading the Sprain Severity
The severity of an AC joint sprain is classified using the Rockwood system, which divides injuries into six grades based on ligament damage and clavicle displacement. This classification determines the appropriate treatment plan by assessing the disruption of both the AC and CC ligaments.
A Grade I sprain is the mildest form, involving only a stretch of the AC ligaments while the CC ligaments remain intact. Radiographs show no joint separation. Grade II involves a complete tear of the AC ligaments and a partial tear of the CC ligaments. This results in slight widening of the AC joint space and minimal vertical displacement of the clavicle.
A Grade III injury signifies a complete tear of both the AC and CC ligaments. This disruption allows for noticeable superior displacement of the clavicle, resulting in a visible bump at the shoulder’s tip. Grades IV, V, and VI are severe, high-grade injuries involving complex displacements.
Grade IV involves the clavicle being displaced posteriorly into the trapezius muscle. Grade V is a severe superior displacement where the clavicle is significantly elevated. Grade VI, which is rare, involves the clavicle being displaced inferiorly, lodging beneath the coracoid process. These higher-grade injuries (IV-VI) also involve severe detachment of the surrounding muscle fascia, including the deltoid and trapezius muscles.
Management Based on Injury Grade
Treatment for AC joint sprains is tailored directly to the Rockwood grade. Lower-grade injuries (Grade I and Grade II) are almost always managed non-operatively. Initial care focuses on reducing pain and swelling, utilizing the RICE principle:
- Rest
- Ice application
- Compression
- Elevation
Patients with Grade I and II sprains are placed in a sling for comfort and support, typically for up to three weeks. Over-the-counter non-steroidal anti-inflammatory drugs (NSAIDs) are recommended to manage pain. The primary goal of non-operative management is a quick, gradual return to gentle range-of-motion exercises to prevent shoulder stiffness.
The management of Grade III sprains is often debated among orthopedic specialists. Many studies support an initial non-operative approach, involving a similar regimen of pain control and physical therapy. Non-operative treatment for Grade III injuries often leads to a quicker return to daily activities compared to surgery, though a noticeable bump may remain. Surgery may be considered based on the patient’s occupation (e.g., overhead athlete) or if conservative treatment fails.
Operative treatment is reserved for severe high-grade injuries (Grades IV, V, and VI) due to significant displacement and functional loss. Surgical goals involve stabilizing the joint to hold the clavicle in the correct position. Surgery may also be considered for Grade III injuries if non-operative management fails or if the cosmetic deformity is unacceptable. While surgery offers better anatomical reduction, non-operative management often results in comparable long-term functional outcomes for high-grade injuries.
Rehabilitation and Long-Term Recovery
The recovery process relies on a structured rehabilitation program, regardless of the initial treatment. Physical therapy begins once acute pain subsides, focusing first on restoring a full, pain-free range of motion. This is followed by exercises designed to rebuild strength and stability in the shoulder girdle muscles, including the rotator cuff and scapular muscles.
Recovery timelines vary based on the injury grade and treatment method. Mild Grade I sprains allow a return to normal activities within four to six weeks. Grade II injuries require three to six weeks for initial healing, though soft tissue pain may persist for up to three months. Non-operative Grade III injuries often take six to twelve weeks for functional recovery.
For injuries requiring surgery, the recovery period is substantially longer, often requiring four to six months of dedicated rehabilitation before a full return to demanding activities. Long-term outcomes may include residual deformity, especially with non-operatively treated Grade III injuries. Post-traumatic arthritis in the AC joint is also possible, sometimes requiring further intervention years later.