15.4 Pelvis & SI Joints

Anatomy of the Pelvis

The bony pelvis is formed by two hip (innominate) bones, the sacrum, and the coccyx. Each hip bone, or os coxae, is three fused bones meeting at the acetabulum: the ilium (superior, with the fan-shaped ala and the iliac crest), the ischium (posteroinferior, with the ischial tuberosity and ischial spine), and the pubis (anterior, meeting its partner at the pubic symphysis). Landmarks the exam expects you to locate include the anterior superior iliac spine (ASIS), the obturator foramen, and the greater and lesser sciatic notches. The pelvic brim (inlet) divides the false (greater) pelvis above from the true (lesser) pelvis below. The sacroiliac (SI) joints connect the sacrum to the ilia posteriorly and angle obliquely, which is why straight AP projections do not open them.

AP Pelvis

The AP pelvis centers the CR perpendicular midway between the level of the ASISs and the pubic symphysis, roughly 2 inches inferior to the ASISs and 2 inches superior to the symphysis, with the midsagittal plane centered to the grid. For a routine, non-trauma study, the feet and lower limbs are internally rotated 15-20 degrees so that the great toes touch; this overcomes femoral anteversion and places the femoral necks in full profile while the greater trochanters project laterally. Evaluation criteria include symmetric iliac wings, symmetric obturator foramina, the femoral necks in profile (not foreshortened), and the lesser trochanters barely or not visible when rotation is correct.

Trauma Exception

The trauma rule overrides the rotation rule. If a hip or pelvic fracture is suspected, do not internally rotate the limbs; radiograph the pelvis in the position the patient presents. Forcing internal rotation on a fractured pelvis or proximal femur risks displacing fragments. On such an image the femoral necks may appear foreshortened and the lesser trochanters prominent, which is acceptable given the clinical priority. Additional trauma projections of the pelvic ring include the AP axial inlet (CR about 40 degrees caudad, showing anterior/posterior displacement of the ring) and the AP axial outlet (Taylor) (CR 20-35 degrees cephalad, showing the pubic and ischial rami).

Sacroiliac Joints

Because the SI joints angle posteriorly and medially, they are imaged with an AP axial and posterior obliques:

The key tested rule for the posterior obliques is direction: the projection demonstrates the SI joint farthest from the image receptor, that is, the elevated (up) side. Therefore an LPO (left side down) demonstrates the RIGHT SI joint, and an RPO (right side down) demonstrates the LEFT SI joint. (This is the opposite convention from anterior obliques, where the down-side joint is shown.) The AP axial Ferguson method with a 30-35 degree cephalad angle opens both joints simultaneously and projects the pubis away from the sacrum.

Male Versus Female Pelvis

The exam sometimes asks candidates to distinguish the sexes on a radiograph. The female pelvis is generally wider and shallower, with a rounder, larger inlet, more flared ilia, and a subpubic arch greater than 90 degrees. The male pelvis is narrower and deeper, with a heart-shaped inlet and a subpubic arch less than 90 degrees. These differences reflect the female pelvis's obstetric role; the true pelvis below the brim houses the birth canal, and the SI joints transmit weight from the axial skeleton to the lower limbs.

Evaluation Criteria and Technical Factors

The pelvis is a grid examination at about 75 to 85 kVp with respiration suspended. A correctly positioned AP pelvis shows symmetric iliac alae, symmetric obturator foramina, the ischial spines equally demonstrated, the femoral necks in profile, and the lesser trochanters minimally visible. Asymmetry of the obturator foramina or iliac wings indicates the patient was rotated; correct by making the ASISs equidistant from the table. A repeat is warranted if rotation obscures the acetabulum or SI joints.

Worked Scenario

A young adult is struck by a car and has pelvic pain with an unstable ring on exam. The technologist images an AP pelvis as-presented (no limb rotation) and, per the ordering physician, adds the AP axial inlet (about 40 degrees caudad) to assess anterior-posterior ring displacement and the AP axial outlet, or Taylor method (20-35 degrees cephalad) to show the pubic and ischial rami. Never rotate a suspected unstable pelvis; stabilize and image in place. For focal SI complaints without trauma, the Ferguson AP axial (30-35 degrees cephalad) plus posterior obliques is the workup, remembering the up-side rule: an RPO shows the left SI joint.

Common Traps

The first trap is applying internal rotation to a trauma pelvis; when the stem mentions a fracture, the correct answer is to image as-is. The second is reversing the SI oblique rule; memorize that a posterior oblique shows the up-side (farthest from the IR), so LPO = right SI joint. A third is confusing the SI axial angle (30-35 degrees cephalad, Ferguson) with the sacrum/coccyx angles from the spine chapter. Keeping the AP-pelvis centering point (midway between ASISs and symphysis) and the internal-rotation rationale (femoral anteversion) firmly in mind resolves most pelvis questions quickly.