Anatomy

• Which bones form the acetabulum?
  The ilium, ischium, and pubis each contribute to the acetabular cup.
• Which landmarks help you judge rotation on an AP pelvis?
  The obturator foramina and iliac wings should appear symmetric, and the sacrum/coccyx should align with the pubic symphysis.
• How do the male and female pelves typically differ?
  The female pelvis is wider and shallower with a larger, more oval inlet; the male pelvis is narrower and deeper with a heart-shaped inlet.
• What type of joint is the hip joint?
  A synovial ball-and-socket joint that permits free movement in all directions.

Positioning 

• Why internally rotate the legs 15–20° for an AP pelvis?
  Internal rotation places the femoral necks parallel to the IR, reducing foreshortening and bringing the greater trochanters into profile.
• Why check that both ASIS are equidistant from the tabletop?
  Equal ASIS-to-table distances ensure the pelvis is not rotated, producing symmetric foramina and iliac wings.
• Why support the knees/ankles after rotating the legs?
  Supports help maintain the required internal rotation and reduce motion during exposure.
• Why center the CR midway between the ASIS and pubic symphysis?
  This levels the beam through the acetabula to include the entire pelvis and proximal femora on most body habitus.
• Why abduct the thighs 45° for the Modified Cleaves method?
• This places the femoral necks parallel to the IR and opens the hip joints for comparison.
• Why is the frog-leg method contraindicated in trauma patients?
  Abduction and external rotation could displace a fracture or exacerbate pain.

Technique & Image Evaluation

• How do you know the pelvis was not rotated on the AP projection?
  Obturator foramina are equal and the sacrum/coccyx align with the pubic symphysis.
• How can you tell the femoral necks were properly positioned on the AP projection?
  The greater trochanters are in profile laterally and the femoral necks are elongated without overlap by the greater trochanters.
• How do you recognize pelvic rotation on the image?
  One obturator foramen appears larger and the contralateral iliac wing appears broader; the sacrum/coccyx deviates off the midline.
• How do you verify motion control?
  The pelvic ring, acetabular rims, and trochanteric cortices are sharply defined without edge doubling.
• How do you ensure complete inclusion of an orthopedic device?
  All components and fixation hardware must be entirely within the field; if not, repeat with wider coverage to include the entire device.
• How can you evaluate the degree of abduction on the frog-leg view?
  If the femoral necks are superimposed by greater trochanters, the legs were abducted too much; if the necks are foreshortened, not enough.
• How do you ensure proper exposure on pelvic images?
  Trabecular patterns of the acetabula and proximal femora should be visible with distinct cortical outlines.

Clinical Applications

• Why is AP pelvis performed first in trauma?
  It provides a rapid overview of the pelvic ring and acetabula to detect unstable injuries before additional or specialized views.
• Why is the frog-leg useful as a supplemental view?
  It shows the femoral heads and necks in profile for joint space comparison and evaluation of mobility or developmental dysplasia.
• Why must orthopedic hardware be completely included on AP pelvis images?
  To evaluate prosthesis alignment and detect loosening or periprosthetic fracture.
• Why might additional views or cross-sectional imaging be requested after an AP pelvis?
  Suspected acetabular column fractures, occult pelvic ring injuries, or preoperative planning often require inlet/outlet views, Judet obliques, or CT.
• Why is precise leg rotation important even outside trauma?
  Accurate rotation standardizes femoral neck visualization for comparison over time and helps avoid false positives for neck shortening or hardware malposition.