An acromioclavicular (AC) joint separation is an injury to the ligaments that hold the clavicle (collarbone) to the acromion of the shoulder blade. Unlike a shoulder dislocation — which involves the main ball-and-socket joint — an AC separation is a disruption of the separate joint at the very top of the shoulder, ranging from a mild sprain to a complete ligamentous disruption.
An AC joint separation is distinct from a shoulder dislocation, which involves the glenohumeral joint rather than the AC joint, though both injuries can result from similar traumatic mechanisms. Even after a successful recovery, prior AC separations significantly increase the risk of developing AC joint arthritis later in life — a consequence of residual joint incongruity and cartilage disruption at the time of injury. Higher-grade separations can also damage the surrounding rotator cuff and deltotrapezial muscle attachments, contributing to ongoing shoulder weakness if not properly addressed.
Causes
Direct impact to the shoulder.
The most common mechanism is a direct fall onto the tip of the shoulder with the arm at the side, driving the acromion downward while the clavicle remains suspended by its sternoclavicular attachment — effectively separating the two structures. This occurs frequently in contact sports such as football, rugby, and hockey, as well as in cycling and mountain biking falls where the rider goes over the handlebars. The severity of the resulting separation correlates closely with the force of impact and the specific ligaments disrupted.
Fall on an outstretched hand.
Less commonly, a fall onto an extended arm transmits axial force upward through the humerus into the shoulder, stressing the AC ligament complex through a different vector. This mechanism typically produces less severe Type I or Type II injuries than a direct blow, but the outstretched hand mechanism can also result in concurrent injuries such as a shoulder dislocation or distal radius fracture depending on the position of the wrist at impact.
Symptoms
Acute AC separations cause immediate, sharp pain localized to the very top of the shoulder, point tenderness directly over the AC joint, and localized swelling. In higher-grade injuries (Type III and above), a visible step-off deformity is apparent — the distal clavicle is displaced superiorly relative to the depressed acromion, producing the characteristic "piano key" sign where the elevated clavicle can be temporarily reduced with downward pressure and springs back when released. Pain worsens with arm movement, overhead activity, and cross-body reaching. Patients typically hold the arm close to the body and supported at the elbow to minimize discomfort and reduce traction on the injured ligaments.
Diagnosis
A physical exam by a doctor assesses point tenderness, visible deformity, clavicle displacement, and the piano key sign. Weight-bearing X-rays in the Zanca view — taken with a reduced tube tilt to center the beam on the AC joint — of both shoulders allow side-by-side comparison and accurate quantification of coracoclavicular distance and clavicle elevation. An MRI provides definitive soft tissue evaluation that X-rays cannot offer: the integrity of the AC ligaments, coracoclavicular (CC) ligaments — both the conoid and trapezoid components — and the deltotrapezial muscle fascial attachments. This soft tissue characterization is particularly important for Type III injuries, where the extent of CC ligament disruption and muscle detachment is the key factor distinguishing injuries that may be managed conservatively from those more likely to benefit from surgical stabilization. MRI also evaluates the rotator cuff and biceps for co-injuries that may not be apparent clinically in the setting of acute AC joint pain.
Injury grades (Rockwood)
Doctors use the Rockwood classification to grade AC joint separations based on the pattern of ligamentous disruption and the direction and magnitude of clavicle displacement.
- Type I: Sprain of the AC ligament without disruption of the CC ligaments or joint displacement. X-rays appear normal. Treated conservatively with full expected recovery.
- Type II: Complete AC ligament tear with partial CC ligament sprain. Mild superior clavicle displacement — typically less than 25% of the CC distance. Managed conservatively in most cases.
- Type III: Complete tear of both AC and CC ligaments. Clavicle elevated 25–100% relative to the normal side. The most clinically debated grade — initial conservative management is standard, with surgery reserved for failure of non-operative treatment or high-demand patients.
- Type IV: Complete ligamentous disruption with posterior displacement of the clavicle through or into the trapezius muscle. Surgical reconstruction is indicated.
- Type V: Severe clavicle elevation greater than 100% of normal CC distance, with complete detachment of the deltotrapezial muscle fascia from the clavicle. Requires surgical stabilization.
- Type VI: Inferior clavicle dislocation beneath the coracoid process. Extremely rare, associated with high-energy trauma, and almost always requires surgical reduction and reconstruction.
Treatments
Treatment plans depend on the grade of injury, patient age, occupational demands, and activity level.
After the initial injury, ice, a sling for comfort, and anti-inflammatory medications are recommended to manage acute pain and swelling.
Type I and Type II injuries: Non-surgical management with a sling worn for comfort for 1–2 weeks, followed by progressive physical therapy focused on restoring range of motion and rotator cuff strength. Most patients recover full shoulder function within 2–6 weeks and do not experience long-term functional limitations.
Type III injury in young and athletic patients
Management of Type III injuries remains the most debated topic in AC joint surgery. The current consensus favors an initial trial of conservative management for most patients — with surgery reserved for those who fail to regain satisfactory strength and function after 3–6 months of rehabilitation, and for high-demand overhead athletes and manual laborers who require full ligamentous stability to return to their sport or occupation. After surgery, a thorough physical therapy program is critical to restoring full shoulder function.
Type III injury in older, nonathletic patients
Older patients who do not require full shoulder strength for demanding sports or heavy manual work are managed conservatively with physical therapy and activity modification. The cosmetic bump from the elevated clavicle typically persists, but shoulder function is usually acceptable for activities of daily living. These patients are counseled that the risk of later AC joint arthritis is elevated and that symptom monitoring over time is appropriate.
Type IV, V, and VI injuries
Surgical reconstruction is indicated regardless of patient age or activity level. Various techniques are available to reduce the displaced clavicle and reconstruct the CC and AC ligaments, including coracoclavicular fixation with a hook plate, suture button (tightrope) devices, or anatomic ligament reconstruction using autograft or allograft tendon. The choice of technique is individualized based on injury chronicity, bone quality, and surgeon experience. Early surgical intervention — ideally within 2–3 weeks of injury — produces more reliable anatomic reduction and better functional outcomes than delayed reconstruction.
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