Anatomy of the AC Joint and Mechanism of Injury

The collarbone is the only bony connection between the trunk and the arm. Its outer end forms a small joint with the acromion of the shoulder blade, and two groups of ligaments provide the stability of this joint.

The acromioclavicular ligaments surround the joint itself and control front-to-back stability. The coracoclavicular ligaments (conoid and trapezoid) connect the collarbone to the coracoid process and act as the principal suspension in the vertical direction.

Injury typically occurs with a fall onto the point of the shoulder with the arm at the side. The blow drives the acromion downwards while the collarbone stays in place, and the ligaments fail in sequence: first the acromioclavicular ligaments, then, if force continues, the coracoclavicular ligaments.

When the coracoclavicular ligaments rupture, the outer end of the collarbone stands proud under the skin. In reality it is not the collarbone rising but the shoulder dropping — a distinction that explains the logic of treatment.

AC joint arthritis is a separate condition, caused by repetitive loading and age-related wear. It is common in weightlifters and those who bench press or train overhead, and causes pain and tenderness at the outer end of the collarbone.

The Rockwood Classification

• Type 1: sprain of the acromioclavicular ligaments without rupture. No displacement on radiographs
• Type 2: the acromioclavicular ligaments are ruptured, the coracoclavicular ligaments intact, with partial displacement
• Type 3: both ligament groups are ruptured and the collarbone is clearly elevated. This is the type whose treatment is most debated. We prefer surgery in athletes and active young patients; the indication is always set by the patient rather than by the injury type, weighing age, occupation, level of sport, expectations and the functional demand on the shoulder
• Type 4: the outer end of the collarbone is displaced backwards into the trapezius muscle. Requires surgery
• Type 5: a severe form of type 3, with marked prominence and stripping of the muscle attachments. Requires surgery
• Type 6: a very rare type in which the collarbone is displaced beneath the acromion or coracoid. Requires surgery

Symptoms

• Sharp pain and marked tenderness at the point of the shoulder, over the outer end of the collarbone
• A visible prominence and change in shoulder contour, more obvious with greater separation
• Pain on bringing the arm across the body towards the opposite shoulder
• Difficulty raising the arm overhead and lying on that side
• Piano key sign: the elevated end of the collarbone can be pushed down and springs back up on release
• A sense of the arm being dragged down and the shoulder tiring when carrying weight
• In AC joint arthritis: insidious pain aggravated by bench press and overhead work, with point tenderness over the joint

Making the Diagnosis

The diagnosis is usually clinical. Point tenderness over the joint, a visible prominence and the piano key sign are decisive. The cross-body adduction test reproduces pain arising from the AC joint.

Plain radiographs are the standard investigation. Alongside a standard shoulder view, dedicated views demonstrate the AC joint, and comparison views of both shoulders allow the degree of separation to be measured. An axillary view is necessary to identify a type 4 injury.

Weighted stress views are no longer used routinely; they increase pain and rarely alter the treatment decision.

MRI is obtained to identify associated rotator cuff and labral injury, particularly after high-energy trauma and in athletes. In AC joint arthritis, MRI shows joint oedema, but this finding is also common in people without symptoms and must be interpreted with the examination.

Non-operative Treatment

Almost all type 1 and type 2 injuries, and the majority of type 3 injuries, are treated successfully without surgery. The contour change may be permanent, but shoulder strength and function return fully in most patients.

• Sling: usually 1 - 3 weeks until pain settles, giving early comfort
• Ice and anti-inflammatory medication
• Early movement: pendulum and passive exercises within pain limits to prevent stiffness
• Graded strengthening of the deltoid, trapezius and scapular muscles to distribute load
• Temporary avoidance of heavy lifting and contact sport, usually for 6 - 12 weeks
• In AC joint arthritis: activity modification (limiting the range of bench press and dips), anti-inflammatory treatment and intra-articular corticosteroid injection
• In type 3 injuries non-operative treatment is tried first; if symptoms persist, later surgery gives results comparable to early surgery

General Surgical Risks

• Infection
• Bleeding and haematoma
• Risks related to anaesthesia and the nerve block
• Wound healing problems and a visible scar, as the incision lies over the shoulder
• Shoulder stiffness

Risks Specific to This Operation

• Loss of reduction: the commonest problem, appearing as partial re-elevation of the collarbone. Function is usually preserved, but the contour changes
• Fracture of the coracoid or clavicle: can occur through the drill tunnels, particularly with early loading
• Implant-related problems: button migration, suture failure, widening of the bone tunnels
• Persistent AC joint pain: degeneration within the joint can cause ongoing pain despite stable fixation, sometimes requiring distal clavicle excision
• Instability from excessive distal clavicle excision
• Graft-related problems: donor site pain, graft stretching
• Heterotopic ossification in the coracoclavicular region: usually symptomless
• Need for further surgery, in cases of lost reduction or implant problems