First and second year — the floor first, then every step
Classify by Judet–Letournel from the columns and walls on CT and 3D, then measure what the surgeon acts on: the articular gap and step-off in the weight-bearing dome, marginal impaction, intra-articular fragments and femoral head position.
Orient first
The acetabulum is held by two columns (anterior: iliopectineal line down to the pubis; posterior: ilioischial line down to the ischium) and two walls (anterior and posterior rims).
On axial CT, the orientation of the main fracture line tells you the column: a CORONAL line (running side to side) separates anterior from posterior — a transverse-type fracture; a SAGITTAL line splits the columns.
The roof (dome) is the weight-bearing part; displacement there matters most.
Acquire the study
Thin-section CT through the pelvis with axial, coronal and sagittal reformats, and 3D surface rendering with the femoral head subtracted to see the joint surface.
The manoeuvre
On axial slices from the roof down, follow the main fracture line and decide its orientation (coronal or sagittal) at the level of the dome.
Anterior column: iliopectineal line to the superior pubic ramus. Posterior column: ilioischial line to the ischial tuberosity.
Walls: the posterior wall fragment (size, number of fragments, displacement) and the anterior wall.
Obturator ring: an obturator ring fracture accompanies column fractures, not a pure transverse fracture.
Measure the articular GAP and STEP-OFF in the dome on coronal and sagittal reformats.
Marginal impaction (a rotated, impacted osteochondral fragment at the rim), intra-articular fragments, and femoral head fracture or dislocation.
What confirms it
The pattern is named from the combination of column, wall and obturator ring involvement on axial and 3D images.
What licenses you to exclude it
A normal AP radiograph does not exclude an undisplaced acetabular fracture; CT does.
The classic misread
Missing marginal impaction under a posterior wall fragment — it must be reduced at surgery.
Missing a small intra-articular fragment in the fovea.
Calling an associated both-column fracture a T-type: in both-column fractures no part of the articular surface remains attached to the axial skeleton (the "spur sign").
Articular step-off and gap in the dome, in mm, on coronal and sagittal reformats (≥ 2 mm displacement in the weight-bearing dome is a commonly used operative threshold — verify with the surgical team).
Posterior wall fragment size as a proportion of the posterior wall on axial (larger fragments destabilise the hip).
Femoral head subluxation in mm.
What to report
The Judet–Letournel pattern; posterior wall fragment size and comminution; displacement of each column.
Articular gap and step-off in the dome; marginal impaction; intra-articular fragments.
Femoral head position (subluxed or dislocated), femoral head fracture; extension into the sacroiliac joint or the pelvic ring.
How to report it
CT: "Transverse plus posterior wall acetabular fracture on the right. The transverse component crosses the dome with 4 mm step-off. The posterior wall fragment involves about 40% of the posterior articular surface, with marginal impaction and a 6 mm intra-articular fragment. Posterior subluxation of the femoral head."
What not to report
Do not report "comminuted acetabular fracture" without naming the pattern.
Associated injuries to look for
Posterior hip dislocation, femoral head fracture (Pipkin), sciatic nerve injury, pelvic ring and sacroiliac injury, knee injury (dashboard mechanism).
What changes management
Dome displacement, hip instability or subluxation, marginal impaction and intra-articular fragments — operative fixation.
An irreducible or unstable hip dislocation — urgent reduction.
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.