Procedures · Head, spine and pelvis2 min read5-question check
Pelvis, sacrum and SI joints
Legs in for the pelvis, sacrum angled up, coccyx angled down, and SI obliques show the side up.
Patient and part
For the AP pelvis the patient is supine with the ASIS equal distances from the table and the legs internally rotated 15 to 20 degrees. Internal rotation turns the femoral necks parallel to the IR so they are seen in full length. Do not rotate the legs if a hip fracture is suspected. For the sacrum and coccyx, the patient is supine with no rotation, and the bladder and lower colon should be emptied first. For SI joint obliques, rotate the patient 25 to 30 degrees into posterior oblique positions.
Central ray
The AP pelvis uses a perpendicular CR centered midway between the ASIS and the symphysis pubis, at 40 inches. The AP axial outlet projection angles cephalad and shows the pubic and ischial rami. The AP axial inlet projection angles caudad and shows the pelvic ring from above. The AP axial sacrum uses 15 degrees cephalad and the AP axial coccyx 10 degrees caudad, both centered about 2 inches above the symphysis pubis. The AP axial SI joints use 30 degrees cephalad for men and 35 degrees for women, entering about 2 inches below the level of the ASIS. For SI obliques, center 1 inch medial to the upside ASIS.
What a good image shows
A good AP pelvis shows symmetric iliac wings and obturator foramina, greater trochanters in profile and lesser trochanters barely visible or not visible. A good sacrum is not foreshortened and its foramina are open. A good coccyx is not superimposed by the symphysis pubis. On SI posterior obliques, the joint farthest from the IR is opened, so an LPO shows the right SI joint. Anterior obliques show the joint closest to the IR.
Fixing it
If the lesser trochanters are prominent, the legs were not internally rotated enough. If one obturator foramen looks wider than the other, the pelvis is rotated; level the ASIS and repeat. If the sacrum looks short and its foramina overlap, the cephalad angle was too small for that patient's curve.
Clinical Example
An AP pelvis comes back with both lesser trochanters plainly seen and short femoral necks. The patient's feet had turned out on the table. You rotate both legs in 15 to 20 degrees, tape the toes together and repeat. The necks are now seen in full length.
Common Mistakes
- Rotating the legs internally on a patient with a suspected hip fracture.
- Using a caudad angle for the sacrum; the sacrum is angled cephalad.
- Thinking an LPO opens the left SI joint; it opens the right.
- Centering a pelvis too high, at the iliac crest instead of below the ASIS.
Why It Matters
Correct leg rotation shows the femoral necks where fractures happen. The registry tests the sacrum and coccyx angles, leg rotation and which SI joint each oblique shows.
Key Relationships
- AP pelvis: legs 15 to 20 degrees internal, CR midway ASIS to symphysis
- Sacrum: 15 degrees cephalad; coccyx: 10 degrees caudad
- AP axial SI joints: 30 to 35 degrees cephalad
- SI posterior oblique 25 to 30 degrees: side up shown