An acromioclavicular (AC) joint sprain is an injury to the ligaments stabilizing the joint between the acromion of the scapula and the lateral end of the clavicle. It often presents as pain over the top of the shoulder and may be associated with a visible bump or deformity. AC joint sprains are common in sports and traumatic falls.
Because the AC joint plays a role in shoulder girdle stability and scapular motion, injury can disrupt normal shoulder mechanics and lead to pain, weakness, or long-term degeneration if not managed appropriately.
Anatomy and Biomechanics
The AC joint is a small planar synovial articulation between the distal clavicle and acromion. It is stabilized by several ligamentous structures:
- The acromioclavicular ligaments (superior, inferior, anterior, posterior) provide horizontal stability.
- The coracoclavicular ligaments (conoid and trapezoid) provide vertical stability, resisting upward displacement of the clavicle.
- The joint often contains a fibrocartilaginous disk or meniscus.
- Dynamic support is provided by the deltoid and trapezius muscles and their attachments, which also help stabilize the region in shoulder motion.
Under normal load, small translations occur in the AC joint to accommodate scapular motion. When force exceeds the tolerance of these ligamentous supports, sprain or separation may occur.
Etiology and Risk Factors
Mechanism of Injury
The most common mechanism is a direct blow or fall onto the lateral aspect of the shoulder (often with the arm adducted), driving the acromion downward relative to the clavicle. An indirect mechanism, such as a fall onto an outstretched hand (transmitting force through the arm to the shoulder), can also contribute. The severity of injury depends on the direction and magnitude of force.
Risk Factors
- Participation in contact or overhead sports (e.g., football, hockey, rugby)
- Previous injury or chronic AC joint stress
- Poor muscle conditioning or shoulder fatigue
- Anatomical variations in AC joint shape or ligament strength
Classification and Injury Grades
AC joint sprains are usually classified by severity, often using the Rockwood classification (Types I to VI). Lower grades indicate ligament stretching or partial tears; higher grades indicate complete rupture and displacement:
- Type I: mild sprain, ligaments stretched but intact, no joint displacement
- Type II: complete tear of AC ligaments, partial injury to coracoclavicular (CC) ligaments, mild elevation of clavicle
- Type III: complete disruption of both AC and CC ligaments, with noticeable clavicle displacement
- Type IV–VI: more severe displacements (posterior, exaggerated superior, or inferior dislocations) and soft tissue disruption
As grade increases, joint instability and deformity become more pronounced, and surgical intervention is more often considered.
Clinical Presentation
Symptoms
Patients typically report acute shoulder pain localized to the AC joint after trauma. The pain may worsen with cross-body adduction of the arm, overhead reach, or pushing down on the shoulder. A “bump” or prominence of the distal clavicle may be visible in moderate to severe cases.
They may also have swelling, bruising, and tenderness to palpation at the AC joint.
Physical Examination
- Local tenderness over the AC joint
- Cross-arm adduction test (bringing the arm across the chest) elicits pain at the joint
- Piano key sign: pressing down on the clavicle causes it to depress and rebound upward
- Examination of full shoulder movement, strength, and scapular motion to detect associated injuries
- Comparison with the contralateral (uninjured) shoulder for asymmetry
Because AC sprains often occur with other shoulder injuries (rotator cuff, glenoid labrum), a full shoulder exam is essential.
Diagnostic Evaluation
Imaging
- Plain radiographs (AP shoulder, Zanca view, and bilateral comparison) to assess alignment, displacement, and joint widening
- Stress views (weighted views) may help detect vertical instability or displacement
- In uncertain or chronic cases, advanced imaging (MRI or CT) can assess soft tissue injury, ligament disruption, joint cartilage, and associated lesions
Imaging helps classify the injury grade and plan treatment accordingly.
Treatment
The goals of treatment are pain relief, stability of the joint, restoration of function, and prevention of chronic symptoms or degeneration.
Nonoperative Management
Nonoperative treatment is often successful for lower grade injuries (Types I and II, many Type III). Key components include:
- Sling immobilization for comfort and soft tissue rest
- Ice, nonsteroidal anti-inflammatories, analgesics
- Gradual progression to passive, then active range of motion exercises
- Strengthening of the deltoid, trapezius, rotator cuff, and scapular stabilizers
- Activity modification and return to sport when tolerable
Early motion (within limits) and rehabilitation are central to avoiding stiffness and promoting recovery.
Surgical Management
Surgery is considered for high-grade injuries (Types IV, V, VI) and for some Type III cases, especially in athletes or those with persistent symptoms or instability after nonoperative care.
Techniques may include:
- Open or arthroscopic reduction of the joint
- Ligament reconstruction (coracoclavicular ligament repair or grafting)
- Fixation with plates, screws, suture-button devices, or other constructs
- Supplementary soft tissue repairs (capsule, deltoid/trapezius attachments)
Postoperative rehabilitation follows stabilization and repair, with gradual return of motion, strength, and function.
Rehabilitation and Follow-Up
- Rehabilitation usually begins with protective motion and strengthening under guidance
- Gradual progression to full shoulder use over weeks to months
- Return to sports or heavy labor often delayed until adequate stability and strength achieved
- Serial monitoring for residual instability, joint degeneration, pain, and function
Close follow-up is necessary to detect complications early and adjust rehabilitation or surgical plans.
Complications and Prognosis
Possible complications include:
- Persistent pain or discomfort over AC joint
- Chronic instability or subluxation
- Posttraumatic osteoarthritis of the AC joint
- Hardware irritation or failure (in surgical cases)
- Loss of reduction or recurrence
- Surrounding soft tissue problems, muscle weakness, or altered scapular mechanics
Prognosis is generally good for low to moderate grades treated nonoperatively. Many patients return to full function, though some may have residual symptoms under heavy load. Surgical repair for higher grades offers better stability in many cases, but carries surgical risks and a longer recovery period.
Prevention and Patient Education
- Use protective gear and safe techniques in contact sports
- Encourage shoulder strengthening, conditioning, and proprioception training
- Early evaluation of shoulder trauma and proper diagnosis to avoid missed injury
- After injury, adherence to rehabilitation and gradual progression
- Avoid premature return to high-demand activity until sufficient recovery
References
- Boufadel, P., Fares, M. Y., Daher, M., Lopez, R., Khan, A. Z., & Abboud, J. A. (2025). Epidemiology of acromioclavicular joint separations presenting to emergency departments in the United States between 2004 and 2023. Shoulder & Elbow, 17585732251320015.
- Moyal, A. J., Burkhart, R. J., Adelstein, J. M., Voos, J. E., Apostolakos, J. M., & Calcei, J. G. (2025). Acromioclavicular and sternoclavicular joint injuries in contact sports: a narrative review of conservative and surgical treatments. Annals of Joint, 10, 31.
- Rubenstein, A. Y., Salamatian, A. D., Denegar, C., Capetta, M., & Sudduth, R. M. (2025). Diagnostic utility of acromioclavicular joint clinical examination tests a systematic review. Physical Therapy Reviews, 30(2), 106-117.
- Gupta, G. K., Amaravathi, R. S., Jeevo, J., Babu, V. M., Selvin, B., Pilar, A., … & Sekaran, P. (2025). Arthroscopy-Assisted Anatomic Global Reconstruction of Acromioclavicular Joint Separation. Arthroscopy Techniques, 103915.
- Todjalievich, S. M. (2025). Complications of treatment for acromioclavicular joint injuries. Web of Medicine: Journal of Medicine, Practice and Nursing, 3(3), 102-111.
