Acromioclavicular (AC) Joints

Carla M. Allen; Taylor M. Otto

Upper Extremities

24 Acromioclavicular (AC) Joints

Anatomy & Pathology

Anatomy Overview
The acromioclavicular (AC) joints connect the lateral end of the clavicle (acromial extremity) to the acromion process of the scapula. Along with the sternoclavicular (SC) joints, they anchor the shoulder girdle to the axial skeleton.

Key Structures: acromioclavicular joint, sternoclavicular joint, clavicular notch, acromial extremity, sternal extremity, acromion, coracoid process, scapula
Function: allows limited gliding and rotary movement, stabilizing the shoulder during elevation and rotation of the arm

Common Pathologies

AC Joint Separation: Partial or complete tearing of AC and/or coracoclavicular ligaments, leading to superior displacement of the clavicle.
Subluxation: Mild misalignment due to ligamentous laxity or trauma.
Arthritis or Degenerative Changes: Narrowing and irregularity of joint surfaces with possible osteophyte formation.
Fracture Extension: Clavicle fractures extending into the AC or SC joints.

Projections

AP (Bilateral) – Pearson Method

CR Location & Positioning

SID: 72 inches (reduces magnification and beam divergence)
Patient position: Upright, seated or standing; recumbent position should be avoided because it can reduce dislocations
Adjustments:
- Center midline of body to midline of IR
- Shoulders in the same horizontal plane
- Distribute weight evenly on both feet
- Take two exposures: one without weights, one with 5–10 lb weights attached to wrists (not held in hands)
CR: Perpendicular to midline of body at the level of the AC joints
Pt. Instructions: Suspend respiration
Exposure: Moderate kVp to include soft tissue and joint detail

Evaluation Criteria

Coverage: Both AC joints entirely included on one (or two) images
Rotation checks: Sternoclavicular joints symmetric; no leaning or shoulder elevation
Motion checks: Cortical margins and soft tissue edges sharply defined
Technique checks: Proper density—soft tissues visible, joints not overexposed
Clinical aim: Demonstrates joint alignment and detects separation or dislocation, especially with weights applied

AP Axial – Alexander Method (Supplemental Projection)

CR Location & Positioning

SID: 40 inches
Patient position: Upright, back against grid
Adjustments: Center affected AC joint to IR; align coracoid process with center of field
CR: 15° cephalic angle to the coracoid process (projects AC joint above acromion)
Pt. Instructions: Suspend respiration
Exposure: Slightly higher kVp than standard AP to penetrate shoulder girdle

Evaluation Criteria

Coverage: Entire AC joint and distal clavicle visualized
Rotation checks: AC joint projected slightly above acromion; no body rotation
Motion checks: Sharp cortical outlines of joint and clavicle
Technique checks: Proper brightness and contrast to visualize joint space
Clinical aim: Demonstrates subluxation or subtle dislocation by elevating the AC joint from overlying structures.

Hows & Whys of AC Joint Radiography

Anatomy

What bones form the AC joint?
The acromion process of the scapula and the acromial end of the clavicle.
What type of joint is the AC articulation?
A synovial gliding joint, allowing slight movement to accommodate shoulder motion.

Positioning

Why are AC joints imaged upright rather than supine?
Because lying down can reduce or mask dislocations—gravity helps show separation when upright.
Why is a 72-inch SID used?
To reduce magnification and beam divergence, enabling both joints to appear on one image.
Why are exposures taken with and without weights?
Weights stress the AC and coracoclavicular ligaments, revealing subtle separations not visible without traction.
Why should the patient not hold the weights?
Holding weights activates shoulder muscles, which can mask ligamentous separation by pulling the joint together.

Technique & Image Evaluation

How can you tell the patient was not rotated?
The sternoclavicular joints are equidistant from the vertebral column.
How can you tell alignment was correct before exposure?
Both shoulders are equal distance from the bucky, and the shoulders are level.
How can you confirm correct collimation and exposure?
Both AC joints are included; soft tissue detail is visible without overexposure, and Rt/Lt and weight markers are clearly present.