Acromioclavicular Joint Injuries

The acromioclavicular joint is the small joint where the lateral end of the clavicle meets the acromion of the scapula. It is one of the few bony links between the arm and the axial skeleton, and it is injured surprisingly often - typically in young, active people who fall directly onto the point of the shoulder during contact sport, cycling, or a simple trip.

For the exam, the whole topic hangs on three ideas: the anatomy (two ligament groups controlling two directions of stability), the Rockwood classification (which grades the injury and decides treatment), and the management debate (low grades are easy - nonoperative; high grades are operative; type III is genuinely contested). Get these three threads clear and you can answer almost any question on this joint.

The acromioclavicular joint is a plane synovial joint with a small fibrocartilaginous disc that degenerates from early adulthood. Its stability comes from two separate ligament groups, and understanding which controls which direction is the single most useful piece of knowledge in this topic.

The practical message is that an injury progresses in a predictable order: first the AC ligaments (horizontal instability), then the CC ligaments (vertical instability and an increased CC distance), and finally the deltotrapezial fascia (gross displacement). This sequence is exactly what the Rockwood classification captures.

When coracoclavicular reconstruction is chosen, restoring the anatomic footprints of the conoid and trapezoid matters, because anatomic reconstruction controls both vertical and rotational/horizontal stability better than a non-anatomic transfer.

• Anatomic reconstruction places the graft and tunnels at these two footprints to recreate both the conoid and trapezoid, restoring vertical stability and the rotational/horizontal control that a single central construct cannot.
• Non-anatomic reconstruction (the modified Weaver-Dunn coracoacromial-ligament transfer) is biomechanically weaker than the native CC ligaments and is now mostly reserved for chronic cases or as an augment.
• Tunnels must be carefully sized and placed - drilling at or near these footprints is exactly where the recognised complication of a clavicle or coracoid fracture around the tunnel arises.

The injury is a mechanical failure of restraints in sequence, driven by a single force. A direct downward blow to the point of the shoulder (with the arm adducted) drives the acromion and scapula downward and medially while the clavicle, strutted against the sternum, stays put. The result is that the energy is concentrated across the acromioclavicular ligaments and then the coracoclavicular ligaments.

The failure cascade:

• Low energy stretches or partially tears the AC ligaments only - the joint stays reduced (type I) or subluxes a little (type II), and because the CC ligaments hold, the clavicle does not ride up.
• Higher energy completes the AC tear and then ruptures the CC ligaments - the clavicle loses its vertical restraint and the coracoclavicular distance increases (type III).
• The greatest energy also strips the deltotrapezial fascia, removing the last soft-tissue restraint and allowing gross, fixed displacement - posterior (IV), severe superior (V), or inferior (VI).

Because the clavicle is anchored medially at the sternoclavicular joint, the apparent "elevation" of the clavicle is really the arm and scapula dropping under the weight of the limb once the restraints fail - the basis for comparing both shoulders rather than relying on weighted stress views.

The late pathophysiology explains the chronic problems: a torn intra-articular disc and an unstable joint drive post-traumatic osteoarthritis and distal clavicle osteolysis, while a malreduced joint produces scapular dyskinesis and persistent pain even when the acute injury looked modest.

View larger

The Rockwood classification divides acromioclavicular injuries into six types based on the direction and degree of displacement and which structures are torn. It expanded the older three-type Tossy system to capture the severe displacement patterns (types IV to VI).

Some surgeons split type III into IIIA (stable, asymptomatic with rehabilitation) and IIIB (symptomatic, dynamically unstable scapula) to help decide who might benefit from surgery - but the headline for the exam is simply that III is the grey zone between clearly nonoperative (I and II) and clearly operative (IV to VI).

A lateral (distal) clavicle fracture is the close cousin of the acromioclavicular dislocation and is examined alongside it, because its behaviour - and the decision to operate - turns on the same coracoclavicular ligaments. The Neer classification grades these fractures by the fracture line's relationship to the CC ligaments.

The typical patient is a young athlete who fell directly onto the point of the shoulder and now has pain, swelling, and often a visible step or bump at the lateral clavicle.

Diagnosis is clinical and radiographic. The aims of imaging are to confirm the injury, exclude an associated fracture, and grade it accurately, because the grade decides treatment.

View larger

View larger

View larger

Treatment is decided by the Rockwood grade, modified by the patient's symptoms, demands, and dynamic instability.

The key exam point is that surgery is not risk-free: the complication profile (infection, hardware problems, loss of reduction, and fractures around tunnels) is exactly why the evidence pushes toward nonoperative care for type III injuries that are settling.

Acromioclavicular injuries are bread-and-butter for the shoulder, trauma, and sports clinics and a favourite viva topic because they test clear, structured thinking. In clinic the everyday task is grading the injury correctly and counselling a young athlete on a realistic recovery. In the viva the examiner will push you on the anatomy (which ligament controls which direction), on not missing a type IV (axillary view), and on the type III debate - where a candidate who quotes the evidence and offers a sensible "trial of nonoperative care, surgery for failures" plan stands out. Knowing which grade goes which way, and why, is the core that examiners probe.

• Consensus across regions is strongest at the extremes: types I and II are managed nonoperatively, and types IV, V and VI are managed operatively. This is reflected in major trauma and sports-medicine teaching worldwide (AAOS-aligned US practice, BOA/UK practice, and AO Foundation principles for the surgical constructs).
• Type III is where guidance differs, but the difference is one of threshold rather than principle. The pooled evidence (multiple meta-analyses) shows no clear functional advantage for early surgery, so most guidance now favours an initial trial of nonoperative care with surgery reserved for persistent symptoms - a position increasingly shared across the FRCS, FRACS, and ABOS curricula.
• The "type IIIA versus IIIB" refinement (stable and asymptomatic versus dynamically unstable and symptomatic) is used in many centres to select the minority of type III patients for surgery, particularly overhead athletes and heavy manual workers.
• Global practice variation in the surgical technique (hook plate versus suture-button versus tendon-graft reconstruction, open versus arthroscopic) largely reflects local expertise, implant availability, and cost rather than clear evidence that one is superior - the systematic-review data show comparable outcomes between open and arthroscopic methods.