Anatomy and biomechanics of the acetabulum
The acetabulum (acetabulum) is a key element of the pelvic ring, forming an inverted bony cup that articulates with the femoral head. It is formed at the junction of three innominate bones: the ilium (ilium), ischium (ischium), and pubis (pubis).
Spatially, the acetabular structure relies on two massive bone columns and two walls:
- Anterior column: includes the anterior part of the iliac wing, the superior pubic ramus, and the anterior articular surface.
- Posterior column: a robust structure extending from the greater sciatic notch to the ischial tuberosity.
- Walls: the anterior and posterior walls of the acetabulum are distinguished.
The biomechanics of this region are strictly distributed: the superior part of the acetabulum (the roof) serves as the primary weight-bearing surface and absorbs body weight, while the posterior part is a critical stabilizer of the hip joint. Fractures in these specific zones carry the highest risk of delayed post-traumatic arthritis.
Injury mechanism and complications
Destruction of the acetabulum occurs when the femoral head is forcefully driven into the articular surface. The specific fracture pattern (which exact walls or columns are affected) directly depends on the position of the femoral head at the moment of impact.
- High-energy trauma: predominates in young patients. The main causes are motor vehicle collisions (especially motorcycle crashes) and falls from a height. A classic example is a vehicular injury where the flexed knee strikes the dashboard, and energy is transmitted along the axis of the femur into the pelvis. In such cases, management follows a strict ATLS protocol ("life before limb").
- Low-energy trauma: typical for geriatric patients. Against the background of senile asthenia and osteoporosis, the bone may fail from a routine fall.
Acetabular injuries are frequently combined with hip dislocations. Depending on the direction of the force, the dislocation can be anterior, posterior, or central (when the femoral head causes a protrusion through the acetabular floor directly into the pelvic cavity). A high-energy fracture in this area is always a "red flag" requiring the exclusion of associated internal organ injuries and fractures of other tubular bones.
Diagnostic radiology
Primary diagnostics for any pelvic trauma begin with an anteroposterior (AP) pelvic radiograph. Specifically for the acetabulum, the physician must trace the integrity and smoothness of six critical anatomical contours:
- Anterior wall line.
- Posterior wall line.
- Acetabular roof (dome) line.
- Iliopectineal line (located more medially).
- Ilioischial line.
- Radiographic "teardrop figure" — a specific shadow reflecting the bone thickness between the inner and outer surfaces of the walls.
It is also mandatory to evaluate Shenton's line, which should normally transition smoothly and without steps from the inferior border of the superior pubic ramus to the inferior contour of the femoral neck.
If an injury is identified on radiographs, computed tomography (CT) becomes strictly mandatory. 2D CT slices with subsequent sagittal, coronal, and 3D reconstructions allow for detailed visualization of all fracture lines and fragment displacement. This is critically necessary for quality preoperative planning and surgical approach selection.
Briefly: answers to topical questions
What is the classification system used for acetabular fractures?
Acetabular fractures are classified according to the internationally recognized Judet and Letournel anatomical classification, as well as by the localization of the injury.
According to the Judet and Letournel classification, they are divided into:
- Elementary types — anterior or posterior wall, anterior or posterior column fractures, and transverse fractures.
- Associated types — combinations of elementary fractures (T-in-beer, both-column, and column fractures associated with posterior wall fractures).
Additionally, isolated fractures (of the rim and floor of the acetabulum) and fractures combined with pelvic ring injuries are distinguished.
What structures form the posterior column of the acetabulum?
The posterior column of the acetabulum is shorter and wider than the anterior column, extending from the greater sciatic notch (incisura ischiadica major) to the ischial tuberosity.
It is composed of the following structures:
- Roof and posterior hemisphere — a portion of the acetabular articular surface.
- Body of the ischium — forms the inferior part of the column.
Radiographically, the posterior column is identified by the ilioischial line.
What late complications occur following an acetabular fracture?
Chronic pain and functional limitations may persist following acetabular fractures. A specific late complication is post-traumatic coxarthrosis; a high risk of its development persists even with perfect anatomical reduction. Nonunion, malunion, implant failure, and wound or deep infections are also possible.
To easily memorize the 6 radiographic lines of the acetabulum, imagine a "House with walls and a roof where tears are shed": Anterior/posterior walls (1,2), Roof (3), "Teardrop figure" (4) + two forming lines (Iliopectineal and Ilioischial).
Class and exam questions
Why is an acetabular fracture dangerous in the long term?
Due to the high risk of developing post-traumatic coxarthrosis. It occurs in approximately 30% of patients, especially if the congruence of the weight-bearing acetabular dome is disrupted.
What is a central hip dislocation?
This is a severe complication where the impact of the femoral head fractures the floor of the acetabulum, and the head shifts (protrudes) into the pelvic cavity.
Why perform a CT scan if the fracture is already clearly visible on radiographs?
Pelvic CT with 3D reconstruction is vital for the precise identification of all hidden fracture lines, assessment of column displacement, and preoperative planning.
What else to study on this topic
- Organization of care in high-energy pelvic trauma according to the ATLS protocol.
- Causes and mechanisms of coagulopathy and hemorrhagic shock in pelvic fractures.
- Classification of pelvic ring fractures (lateral compression, anteroposterior compression, vertical shear).
- Avulsion fractures of the pelvis in adolescent athletes: typical localizations.
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