Normal Acromioclavicular Joint Space Size on Radiography
The acromioclavicular (AC) joint is a diarthrodial joint connecting the lateral clavicle to the acromion process of the scapula, stabilized by the AC and coracoclavicular ligaments. Accurate measurement of the AC joint space on plain radiography is essential for grading AC joint separations, which are among the most common shoulder injuries seen in emergency and sports medicine settings.
Normal Reference Values
| Orientation | Location | Measurement |
|---|---|---|
| Anteroposterior | Right And Left Shoulder Difference | <2-4 mm |
Clinical Significance
A side-to-side difference of less than 2–4 mm between the right and left AC joint spaces is considered normal on anteroposterior radiography. Asymmetry exceeding this threshold raises suspicion for ligamentous disruption and AC joint separation. Stress views — obtained with the patient holding weights — can accentuate widening and improve diagnostic sensitivity in equivocal cases.
AC joint injuries are classified by the Rockwood system (Types I–VI), with progressive widening reflecting increasing ligamentous and capsular damage. Coracoclavicular (CC) distance widening (normally <13 mm, or >25–50% asymmetry compared with the contralateral side) often accompanies higher-grade injuries. Key pitfalls include patient rotation causing spurious asymmetry, and degenerative osteophytes that may narrow the joint space and obscure the true articular interval.
- AC joint sprain or separation (Rockwood Types I–VI)
- Distal clavicle osteolysis (post-traumatic or overuse)
- Septic arthritis of the AC joint
- Osteoarthritis with joint space narrowing
- Os acromiale mimicking joint widening
Reference: Marincek B, Dondelinger RF. Emergency Radiology, Imaging and Intervention. Springer. (2007).
Imaging Notes
Standard assessment is performed with an anteroposterior (AP) radiograph of both shoulders, ideally on a single cassette to allow direct bilateral comparison of joint spaces. The X-ray beam should be centered on the AC joints with the patient erect and arms at the sides. Zanca view — angled 10–15° cephalad with reduced exposure — provides improved visualization of the AC joint by eliminating superimposition of the scapular spine. Bilateral comparison is critical, as absolute joint space width varies between individuals; side-to-side asymmetry is the most reliable indicator of pathology.
Measure the narrowest vertical distance between the inferior cortex of the distal clavicle and the superior cortex of the acromion. Avoid measurements at osteophyte margins. If clinical suspicion remains high despite normal resting views, weighted stress radiographs can unmask dynamic instability.