First and second year — the floor first, then every step
A posterior sternoclavicular dislocation can compress the great vessels, trachea and oesophagus and is invisible on a standard chest film — CT is the test. At the other end, grade an AC injury by the coracoclavicular distance.
Orient first
The medial clavicular physis fuses late (early to mid-twenties), so in a young adult a "dislocation" is often a physeal fracture.
Posterior SC dislocation is the emergency: the brachiocephalic vessels, trachea and oesophagus lie directly behind the joint.
AC injuries are graded by which ligaments fail — the AC ligaments first, then the coracoclavicular ligaments.
Acquire the study
Bilateral AP view of both AC joints on one film (or side by side), and a Zanca view (10–15° cephalic tilt).
The manoeuvre
Measure the coracoclavicular distance in mm (top of coracoid to under-surface of clavicle) and compare with the contralateral side.
Vertical and horizontal displacement of the clavicle relative to the acromion, in mm on the AP projection.
Serendipity view (40° cephalic tilt) for the SC joints when CT is not available: a posteriorly dislocated clavicle projects lower.
What confirms it
Clavicular head displaced behind the manubrium on axial CT.
What licenses you to exclude it
A normal chest radiograph does not exclude posterior SC dislocation.
The classic misread
Grading an AC injury without the contralateral comparison.
Reporting the injury
Classification to use
AC joint: Rockwood I–VI. SC joint: anterior versus posterior dislocation; physeal fracture in the skeletally immature.
Measurements — and how to take them
Coracoclavicular distance in mm on each side (a 25–100% increase over the normal side is Rockwood III territory — verify); posterior displacement of the clavicular head in mm.
What to report
Direction and degree of displacement; the structures behind a posterior SC dislocation; mediastinal haematoma; physeal status; for AC — coracoclavicular distance on both sides and the Rockwood grade.
How to report it
CT: "Posterior dislocation of the left medial clavicle, which indents the left brachiocephalic vein; the brachiocephalic artery and trachea are displaced but patent. Small mediastinal haematoma."
What not to report
Do not report an SC "dislocation" in a patient under about 25 without considering a physeal fracture.
Associated injuries to look for
Great vessel, tracheal and oesophageal injury; brachial plexus injury; rib and scapular fractures.
What changes management
Posterior SC dislocation with mediastinal compromise → urgent reduction with cardiothoracic cover.
Rockwood IV–VI → operative.
Reference values
Each value carries the caveat that keeps it from being misused. Normal limits and diagnostic criteria are kept apart on purpose: a disease cut-off read as a normal range is the more dangerous mistake.
up to approximately 6 mm (some texts accept 8 mm); narrows with age
Side-to-side comparison outperforms the absolute figure — more than about 2–3 mm of asymmetry, or inferior offset of the acromion, matters more than the millimetre. Weighted views are largely abandoned.
approximately 11–13 mm; an increase of more than about 25% over the contralateral side indicates coracoclavicular ligament disruption
The PERCENTAGE against the normal side, not the absolute distance, separates the higher Rockwood grades — image both sides on the same projection before grading.
X-ray
Diagnostic criteria
Acromioclavicular joint (Rockwood) · Rockwood grade — how to apply the registered AC and CC figures
a named I–VI grade; the AC-width and coracoclavicular-distance entries are the measurements — this entry exists so the grade is not assigned from a single millimetre
Grade III versus IV is a horizontal-stability / posterior-displacement question, not a CC-percentage question. Image both sides on the same projection.
X-ray
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.