Spine and Pelvis
36 Acetabulum and Pelvic Ring Exams
Anatomy & Pathology
Anatomy
- Acetabulum — cup-shaped cavity formed by the ilium, ischium, and pubis; receives femoral head
- Anterior and posterior rims — visualized on oblique (Judet) projections
- Obturator foramen — formed by ischium and pubis; changes appearance with rotation
- Pelvic brim — boundary separating true and false pelvis
- Sacrum and coccyx — posterior components of the pelvic ring
- Superior and inferior pubic rami — anterior ring components seen on inlet/outlet projections
Common Pathologies
- Acetabular fractures — disruption of anterior or posterior rim or wall
- Pelvic ring fractures — multiple breaks causing instability
- Pubic symphysis diastasis — widening due to trauma or childbirth
- SI joint subluxation or dislocation — visible on inlet view
- Compression or shearing injuries — deform pelvic brim; shown on inlet projection
These specialized pelvic projections are performed to evaluate traumatic injury to the pelvic ring and acetabulum. Together, the Inlet, Outlet, and Judet Oblique views provide comprehensive assessment of pelvic alignment, sacral and pubic integrity, and acetabular column involvement.
The Outlet view demonstrates vertical displacement of the pubic and ischial rami; the Inlet view reveals anterior-posterior compression injuries; and the Judet obliques isolate the anterior and posterior rims of the acetabulum to evaluate for fractures or dislocation.
Projections
AP Axial Outlet Projection (Taylor Method)
CR Location & Positioning
- SID: 40 inches
- Patient position: Supine, midsagittal plane aligned to midline of table
- Adjustments: ASIS equidistant to table; legs extended; no rotation
- CR: Angled 20–35° cephalad for males and 30–45° cephalad for females, entering at a point 2 inches inferior to the superior border of the pubic symphysis
- Pt. Instructions: Suspend respiration; remain still
- Exposure: Moderate kVp for bone detail through symphysis and rami
Evaluation Criteria
- Coverage: Pubic and ischial rami centered; entire outlet region included
- Rotation checks: Symmetric obturator foramina and equal distance from sacrum to pelvic brim
- Motion checks: Sharp cortical margins; no blur of pelvic rami
- Technique checks: Ischial and pubic rami free of superimposition; contrast sufficient to delineate symphysis
- Clinical aim: Demonstrates pubic and ischial rami to assess displacement or fractures of the pelvic outlet
AP Axial Inlet Projection (Bridgeman Method)
CR Location & Positioning
- SID: 40 inches
- Patient position: Supine, midsagittal plane centered to table
- Adjustments: Ensure pelvis is not rotated; legs extended
- CR: Angled 40° caudad, entering at the level of the ASIS
- Pt. Instructions: Suspend respiration
- Exposure: Similar technique to outlet projection; may increase mAs slightly for tissue thickness
Evaluation Criteria
- Coverage: Entire pelvic inlet and brim visualized
- Rotation checks: Symmetric appearance of pelvic ring and obturator foramina
- Motion checks: Clear trabecular pattern through pubic and ischial bones
- Technique checks: Anterior pelvic bones and brim well demonstrated without burnout
- Clinical aim: Demonstrates the pelvic inlet for assessment of pelvic ring compression or widening
AP Oblique Projection — Judet Method (Acetabulum)
CR Location & Positioning
- SID: 40 inches
- Patient position: Semisupine with body rotated 45°; both RPO and LPO positions obtained for comparison
- Adjustments:
- RPO shows left (up) acetabulum — posterior rim and iliac wing
- LPO shows right (up) acetabulum — posterior rim and iliac wing
- CR: Perpendicular to a point 2 inches distal to the elevated ASIS for the acetabulum of interest
- Pt. Instructions: Suspend respiration
- Exposure: Centered for acetabulum; moderate kVp; ensure grid alignment
Evaluation Criteria
- Coverage: Entire acetabulum and adjacent ilium and pubis
- Rotation checks: Obturator foramina open on the elevated side; closed on dependent side
- Motion checks: Fine trabecular detail of acetabular rims and hip joint margins
- Technique checks: Adequate penetration to visualize anterior and posterior columns
- Clinical aim: Demonstrates anterior and posterior acetabular rims and walls to evaluate fractures or dislocations
Comparison of Acetabulum & Pelvic Ring Projections
| Projection | CR Direction / Angulation | Primary Structures Shown | Clinical Purpose |
|---|---|---|---|
| AP Axial Outlet (Taylor Method) | 20–35° cephalad (males) 30–45° cephalad (females) |
Pubic and ischial rami, symphysis, inferior pubic region | Evaluates pelvic outlet fractures, pubic/ischial displacement, and diastasis |
| AP Axial Inlet (Bridgeman Method) | 40° caudad, centered at the level of the ASIS | Pelvic brim, sacral promontory, superior pubic rami | Assesses pelvic ring compression, inlet widening, and posterior displacement |
| AP Oblique (Judet Method) | Perpendicular, entering 2 in. distal to elevated ASIS (RPO/LPO) | Acetabular anterior and posterior rims, ilioischial and iliopubic columns | Detects acetabular rim or wall fractures, posterior dislocation, and column integrity |
Memory Aids
- Outlet = Out and Up — the CR is angled upward (cephalad) to project the outlet structures below the symphysis.
- Inlet = In and In — the CR is angled inward (caudad) toward the inlet of the pelvis.
- Judet = J for Joint — the Judet obliques show the acetabular joint rims and help identify anterior and posterior column fractures.
Hows & Whys of Acetabulum and Pelvic Ring Radiography
Anatomy
- What three bones form the acetabulum?
The ilium, ischium, and pubis unite to form the acetabular socket that articulates with the femoral head. - What is the difference between the anterior and posterior acetabular rims?
The anterior rim forms the margin of the obturator foramen; the posterior rim lies along the iliac wing and is more posterior on oblique images.
Positioning
- Why is a cephalic angle used for the outlet projection?
It projects the pubic and ischial rami below the symphysis to assess displacement clearly. - Why is a caudal angle used for the inlet projection?
It projects the pelvic brim and inlet plane perpendicularly to the IR for evaluation of ring compression. - Why are both obliques obtained for the Judet method?
To visualize both acetabula and compare anterior and posterior columns for fractures. - Why is the up-side acetabulum demonstrated on RPO or LPO?
The oblique rotation elevates one side to project the acetabulum free of superimposed pelvic structures.
Technique & Image Evaluation
- How do you confirm no rotation on inlet/outlet projections?
Obturator foramina and iliac wings appear symmetric; sacrum centered between pelvic brim margins. - How can you tell which acetabular rim is visualized on a Judet oblique?
In the internal oblique (affected side down), the posterior rim is shown; in the external oblique (affected side up), the anterior rim is demonstrated. - How can you identify correct CR angulation on the outlet view?
If rami are elongated and unobscured by the symphysis, the cephalic angle is correct; excessive angulation elongates symphysis excessively. - How can you check exposure adequacy?
Trabecular detail of acetabular walls and pubic rami should be distinct without overpenetration of the sacrum.
Clinical Applications
- Why are inlet and outlet views often ordered for pelvic trauma?
They evaluate vertical shear and compression fractures by showing displacement of the anterior and posterior pelvic rings. - Why are Judet obliques essential for acetabular fracture evaluation?
They isolate each column of the acetabulum, helping orthopedic surgeons classify fracture type and plan fixation. - Why might CT or MRI follow myelography or Judet exams?
Cross-sectional imaging provides 3D assessment for surgical planning and marrow evaluation beyond what projection images show.
