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Upper Extremities

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.

Routine Projections (ARRT Required)

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.

 

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Radiographic Procedures Review Guide Copyright © 2025 by Carla M. Allen and Taylor M. Otto is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License, except where otherwise noted.

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