Anatomy

• What bones form the sacroiliac joint?
  The auricular surface of the sacrum articulates with the auricular surface of the ilium on each side.
• What type of joint is the SI joint?
  An amphiarthrodial (slightly movable) synovial joint with limited gliding movement.

Positioning

• Why should a support be placed under the patient’s knees for the AP axial projection?
  Flexing the knees straightens the lumbar spine, reducing lordosis and improving alignment of the SI joints.
• Why must both ASIS be equidistant from the table top?
  To ensure the pelvis is not rotated, keeping both SI joints symmetrical.
• Why do males require less CR angulation than females on the AP axial projection?
  Males have a narrower, less curved pelvis, requiring less cephalad angulation to open the L5–S1 joint.
• Why is each oblique performed?
  Both sides are obtained for comparison because disease processes often affect one joint more than the other.
• Why does the posterior oblique demonstrate the joint on the side up?
  The side farther from the IR is projected open because of the divergent beam and body rotation.
• Why does the anterior oblique demonstrate the joint on the side down?
  When the patient is prone, the side nearest the IR is best demonstrated due to reversed beam direction.

Technique & Image Evaluation

• How can you tell if the patient was rotated on an AP axial image?
  The sacrum will not be aligned with the symphysis pubis, and one SI joint will appear more open than the other.
• How can you confirm that the L5–S1 joint space is open?
  The joint space should appear as a clear, horizontal gap without superimposition or foreshortening of vertebral endplates.
• How can you verify proper obliquity on posterior oblique SI joints?
  The joint should appear open with minimal overlap of the ilium and sacrum; too little rotation closes the joint, too much causes overlap.
• How can you check for motion?
  Sharp cortical outlines of the sacrum, ilium, and SI margins indicate adequate immobilization and exposure timing.
• How can you tell if the contrast and exposure are appropriate?
  Trabecular bone patterns and joint margins should be visible; the image should not appear “burned out” or overly gray.

Clinical Applications

• Why are SI joint radiographs performed?
  To evaluate pain, inflammation, trauma, or degenerative changes in the sacroiliac articulation.
• Why are oblique views especially important?
  They allow separation of the sacrum and ilium to assess for joint narrowing, widening, or erosive changes seen in sacroiliitis or ankylosing spondylitis.
• Why might a CT or MRI follow a radiographic SI joint study?
  To further assess subtle erosions, bone marrow edema, or early inflammatory changes not visible on radiographs.