Acromioclavicular (AC) Joint Injury
What Is an Acromioclavicular (AC) Joint Injury?
The acromioclavicular joint is where the outer end of the clavicle meets the acromion at the top of the shoulder. The AC capsule and ligaments control horizontal movement at the joint. The coracoclavicular ligaments—the conoid and trapezoid—connect the clavicle to the coracoid and provide important vertical and rotational stability.
An AC joint injury, often called a shoulder separation, occurs when force sprains or tears some of these restraints. It is not the same as a shoulder dislocation, in which the upper-arm bone leaves the main shoulder socket. Injury can range from tenderness without displacement to marked separation in more than one direction.
What Causes an AC Joint Injury? Risk Factors
The typical mechanism is a direct fall or blow onto the top or outer side of the shoulder while the arm is close to the body. The acromion is driven down and inward beneath the clavicle, loading the AC and coracoclavicular ligaments. Less often, force transmitted through an outstretched arm can injure the joint.
- Contact and collision sports, including rugby, football, hockey and martial arts.
- Falls from a bicycle, skis, a horse or at work, particularly directly onto the shoulder.
- Traffic accidents or other high-energy trauma that may also injure bone, chest or nerves.
- A previous separation that leaves pain, altered shoulder-blade movement or reduced confidence during contact.
Training level does not prevent every direct-impact injury. The direction and energy of the force, ligament disruption, associated injury and the person's anatomy all influence the resulting pattern.
Rockwood Grades and What They Mean
The Rockwood system describes the injured structures and direction of displacement. It helps communication but does not make the treatment decision by itself. Symptoms, horizontal as well as vertical stability, work and sport demands, skin condition and response to rehabilitation remain important.
- Type I: the AC ligaments are sprained, the coracoclavicular ligaments remain intact and alignment is maintained.
- Type II: the AC ligaments are disrupted and the coracoclavicular ligaments are sprained, producing limited displacement.
- Type III: both AC and coracoclavicular restraints are disrupted with superior displacement; non-surgical care works for many people, while selected persistent or high-demand cases need further discussion.
- Types IV, V and VI: the clavicle has posterior, marked superior or inferior displacement. These uncommon patterns require early specialist assessment because soft tissues, skin and associated injuries affect management.
Symptoms of an AC Joint Injury
Pain is focused over the top of the shoulder and is usually worse when the area is pressed, the arm is raised or moved across the chest, or a bag strap rests on the joint. Swelling and bruising can appear soon after injury. Sleep may be difficult on the affected side.
- A visible or palpable step between the clavicle and acromion in a displaced injury.
- Restricted shoulder movement because of pain rather than true joint locking.
- A feeling of movement, fatigue or loss of power during lifting and overhead activity.
- Altered shoulder-blade rhythm or prominence that becomes more noticeable with repeated use.
The size of the bump does not always match pain or disability. A prominence can remain after the injury has settled, while some less obvious injuries cause ongoing cross-body pain or fatigue.
AC Joint Separation or Another Shoulder Injury?
A clavicle fracture can produce similar pain, bruising and deformity after a fall. Glenohumeral shoulder dislocation usually changes the overall contour of the shoulder and severely restricts movement, while an AC injury is localised at the top. High-energy trauma can cause more than one of these injuries at the same time.
Later pain over the AC joint can also come from post-traumatic or degenerative shoulder arthritis rather than persistent ligament instability. Rotator cuff injury, neck-related pain and nerve injury are considered when weakness, altered sensation or symptoms below the elbow accompany the shoulder complaint.
How Is an AC Joint Injury Diagnosed?
Diagnosis begins with the direction of impact, tenderness, skin condition, deformity and the ability to move the arm. Both shoulders are observed from the front, side and back. The examiner assesses cross-body pain, vertical and horizontal stability, shoulder-blade movement, rotator cuff function and the circulation and sensation in the arm.
X-rays show AC and coracoclavicular alignment and help rule out fracture. Special or comparison views are selected when they will clarify displacement; weighted views are not automatically required. CT is useful for complex fracture patterns, while MRI is reserved for uncertain soft-tissue or associated shoulder problems rather than ordered for every straightforward separation.
Non-Surgical Treatment
Most type I and II injuries and many type III injuries are initially managed without surgery. Early treatment protects the shoulder while pain settles, then restores movement and load tolerance. The aim is useful, comfortable function; non-surgical care may not remove the visible bump.
- Use a sling briefly for comfort, avoiding prolonged immobilisation once safe movement is tolerated.
- Apply cold through a protective layer and use suitable pain medicine only when medically appropriate.
- Avoid heavy lifting, pushing, pulling and painful cross-body loading during the irritable phase.
- Begin a progressive rehabilitation programme as pain and clinical stability allow.
Follow-up considers pain, movement, work demands and whether the shoulder blade and arm function together without fatigue or instability. Persistent limitations after a consistent programme justify reassessment rather than an automatic assumption that the grade was wrong.
Exercise and Rehabilitation
Rehabilitation starts with hand and elbow motion, posture and comfortable shoulder movement. Cross-body adduction and heavy elevation may be limited early because they compress or stress the AC joint. Exercise progresses according to symptoms on the day and the following day.
- Restore comfortable active and assisted range without forcing the painful end range.
- Rebuild rotator cuff, deltoid and shoulder-blade strength with controlled resistance.
- Develop endurance for carrying, reaching and gradual overhead work.
- Add contact, falling and sport-specific drills only after strength and confidence return.
Are Injections Useful for AC Joint Injuries?
Injection is not routine treatment for a recent traumatic separation and cannot reconnect torn AC or coracoclavicular ligaments. Masking pain too early may encourage loading before the shoulder is ready. Initial care therefore focuses on protection, movement and progressive rehabilitation.
A carefully placed local anaesthetic or corticosteroid injection may occasionally help determine whether chronic, localised AC joint pain comes from the joint itself, or provide temporary symptom relief in selected non-acute cases. It does not correct deformity or mechanical instability and should follow a clear diagnosis and discussion of limitations.
When Is Surgery Needed?
Early surgery is considered more often for Rockwood IV, V and VI patterns, open injury, threatened skin or displacement that remains unstable in several directions. Type III remains individualised: persistent pain, scapular dysfunction or instability during high-demand work or sport after adequate rehabilitation may support a surgical discussion. Appearance alone is not enough.
Surgery reduces the clavicle and restores AC and coracoclavicular restraint using fixation, ligament repair or graft reconstruction according to injury age and tissue quality. The AC joint repair guide explains preparation, techniques and postoperative protection in more detail. No procedure guarantees a flat contour or return to the previous sporting level.
Recovery Time and Prevention
Low-grade injuries may recover daily function over several weeks, while displaced injuries, heavy work and contact sport take longer. Surgical recovery extends over several months because ligament or graft healing must be protected before strengthening and contact. Return depends on pain, useful motion, strength, control and task demands rather than grade or time alone.
Not every direct-impact injury can be prevented. Sport-specific protective equipment, safe falling technique where trainable, gradual return after a previous injury, and shoulder-girdle strength may reduce exposure or improve control. Cyclists and workers should also use appropriate protective equipment and address avoidable fall hazards.
When Should You See a Doctor?
Seek prompt assessment after trauma if the shoulder is visibly deformed, the skin is stretched over the clavicle, movement is very limited or pain prevents safe use of the arm. Urgent care is needed for an open wound, chest symptoms, a cold or pale hand, new numbness or weakness, or high-energy trauma that may involve other injuries.
For related shoulder conditions and treatments, visit the Shoulder page. If pain, deformity or instability persists and limits work or sport, use the Contact page to request an individual assessment.