Acetabulum Fracture Imaging 

Most acetabular fractures occur in the setting of significant trauma secondary to either a motor vehicle accident or a high-velocity fall. Blunt force is exerted on the femur, passes through the femoral head, and is transferred to the acetabulum. The direction and magnitude of the force, as well as the position of the femoral head, determine the pattern of acetabular injury. The determination of the pattern of injury is key to the classification of an acetabular fracture.

Once the acetabular fracture is classified, appropriate therapy may be planned and implemented.^{[1, 2, 3, 4, 5]} The anatomy of the acetabulum and computed tomography (CT) images of fractures are depicted below.

Radiographic anatomy

The anteroposterior (AP) view of the pelvis is the primary tool for radiographic evaluation of the acetabulum (see the images below). The iliopectineal, or iliopubic, line is the radiographic landmark for the anterior column. It begins at the sciatic notch and travels along the superior pubic ramus to the symphysis pubis. The ilioischial line demarcates the posterior column. It also begins at the sciatic notch, coursing inferiorly to the medial border of the ischium. The ilioischial line should pass through the acetabular teardrop. If it does not overlap the teardrop, the ilioischial line and, thus, the posterior column are disrupted.^[6]

The iliac wing is considered to be part of the anterior column. An iliac wing fracture in the setting of an acetabular injury indicates anterior column involvement. An iliac oblique radiograph provides a better view of the iliac wing. The posterior wall of the acetabulum is more visible than the anterior wall on the AP view because of its more lateral position. The anterior wall may be difficult to appreciate on the AP view.

The obturator oblique view better depicts the posterior wall, and the iliac oblique view better depicts the anterior wall. The integrity of the obturator ring is an important feature to recognize. Certain fracture patterns (such as those of column and T-shaped fractures) characteristically include fractures through the obturator ring.

The oblique, or Judet, views of the pelvis are named relative to the side of interest (see the images below). For example, if the acetabular fracture is on the left side, the views are named with reference to the left side. The left posterior oblique radiograph displays the iliac wing en face; therefore, this view is termed the left iliac oblique view (see the image below). The right posterior oblique radiograph shows the obturator ring en face; therefore, this view is the left obturator oblique view. The iliac oblique view clearly demonstrates the iliac wing, sciatic notch, and ischial spine. In addition, the posterior column and anterior wall of the acetabulum are seen in profile.

The obturator oblique radiograph provides the best depiction of the obturator ring and shows the anterior column and posterior wall in profile.

Clinical details

Fractures of the acetabulum are most commonly classified according to the system described by Judet and colleagues.^[7] The system is based on the orientation of the fractures and the structures involved. In this system, the orientation of the fracture is based on its depiction on a lateral view of the acetabulum. In order to arrive at the correct classification, AP and oblique (Judet) radiographs of the pelvis are obtained and analyzed. (See the image below.)

Some authors have questioned the necessity of oblique views of the pelvis in the age of multidetector CT scanning. Harris and colleagues have proposed a new classification system based on the multidetector CT scan appearance.^[8] Other authors have defended the utility of the standard radiographic series in the evaluation of acetabular fractures. The Judet system will be presented in the remainder of this article.^{[7, 9]}

In the system described by Judet and colleagues, 10 patterns of acetabular fracture are defined. The 10 patterns are divided into 5 elementary and 5 associated patterns.

Elementary patterns include fractures with a single fracture orientation, whereas associated patterns usually involve combinations of the elementary fractures. Elementary patterns include anterior wall, posterior wall, anterior column, posterior column, and transverse fractures. Associated patterns include both-column fractures, posterior column fractures with posterior wall fractures, transverse fractures with posterior wall fractures, T-shaped fractures, and anterior column fractures with posterior hemitransverse fractures.

For simplicity, the 10 patterns can be grouped into 3 categories: wall, column, and transverse fractures. Some fractures fit into 2 categories. The following fractures are indicated by pattern type.

Wall fractures

These include the following:

• Anterior wall
• Posterior wall
• Posterior column with posterior wall (also a column fracture)
• Transverse with posterior wall (also a transverse fracture)

Column fractures

Column fractures include the following:

• Anterior column
• Posterior column
• Both-column
• Posterior column with posterior wall (also a wall fracture)
• Anterior column with posterior hemitransverse (also a transverse fracture)

Transverse fractures

Transverse fractures include the following:

• Transverse
• T-shaped
• Transverse with posterior wall (also a wall fracture)
• Anterior column with posterior hemitransverse (also a column fracture)

Fracture patterns

Isolated acetabular wall fractures typically do not involve the weight-bearing articular portion of the acetabulum. Fractures of the posterior wall are more common than are those of the anterior wall because of the preponderance of posteriorly directed forces responsible for acetabular fractures. Posterior wall fractures may occur in isolation (see the first three images below) or in combination with posterior column or transverse fractures. Anterior wall fractures are rare (see the last image below).

Both-column fractures are the most common acetabular injury. As the name implies, the anterior and posterior columns are involved. On AP radiographs, a disruption of the iliopectineal and ilioischial lines, as well as the obturator ring, may be seen (see the first image below). An iliac wing fracture may be seen on the AP view, but often, it is appreciated only on the iliac oblique radiograph (see the second image below). The pathognomonic spur sign is present on the obturator oblique view (see the third image below) and confirmed on a computed tomography (CT) scan (see the last 3 images below).

Isolated anterior and posterior column fractures are uncommon. Anterior column fractures disrupt the iliopectineal line while preserving the ilioischial line. Conversely, posterior column fractures disrupt the ilioischial line but not the iliopectineal line (see the images below).

Column fractures divide the acetabulum into front and back halves (see the first image below). The posterior column fracture with a posterior wall fracture has the features of each of its components (see the second image below). The slightly more common anterior column fracture with a posterior hemitransverse fracture is the most complex acetabular fracture to classify.

The combination of column fractures and transverse fractures may be difficult to appreciate radiographically (see the first image below). The iliopectineal and ilioischial lines are broken, and an iliac wing fracture should be evident. Unlike the both-column fracture, which shares these features, the obturator ring is intact and the spur sign is not present. On CT scans, the anterior column and the posterior transverse fracture planes may be appreciated (see the second image below).

Transverse fractures are transverse because of their appearance when the acetabulum is examined from the lateral view. The iliopectineal and ilioischial lines are interrupted, but the obturator ring is spared. On CT scans, the fracture is oriented vertically (front to back).

Transverse fractures divide the acetabulum into top and bottom halves, as seen on the lateral view of the acetabulum. The transverse fracture with a posterior wall fracture is a common fracture that incorporates the features of transverse and posterior wall elementary fractures (see the images below).

The T-shaped fracture is a fairly common acetabular injury. This fracture has the characteristics of an elementary transverse fracture with the addition of a medial acetabular wall fracture extending through the obturator ring (see the images below). The anterior column with posterior hemitransverse fracture is discussed earlier.

In a study by Brandser and colleagues, the following 3 most common types of acetabular fracture accounted for roughly two thirds of all fractures: both-column fractures, transverse fractures with posterior wall fractures, and posterior wall fractures.^{[10, 11]} This number increased to 90% when the next 2 most common fracture types were considered: T-shaped and transverse fractures. The frequency of the fractures types is described below.

Commonly occurring acetabular fractures (90%) include the following:

• Both-column
• Transverse with posterior wall
• Posterior wall
• T-shaped
• Transverse

Uncommonly occurring acetabular fractures (10%) include the following:

• Anterior column
• Anterior column with posterior hemitransverse
• Posterior column with posterior wall
• Posterior column
• Anterior wall

Preferred examination

AP radiography of the pelvis is used in the initial radiographic assessment of patients with major trauma that is suggestive of pelvic and/or acetabular injury (see the images below). Images are obtained with the patient in the supine position and with the radiographic beam passing in an AP direction. Abnormalities depicted on the AP pelvis radiograph direct the need for the next set of radiographs. Acetabular fractures are imaged by using oblique (ie, Judet) views of the pelvis. Pelvic ring fractures are imaged by using inlet and outlet views of the pelvis.

Oblique, or Judet, radiographs of the pelvis are obtained with the patient in the left posterior oblique and right posterior oblique positions (see the images below). The patient should be at a 45º angle relative to the radiographic beam, which remains perpendicular to the cassette. This technique results in 2 orthogonal radiographs of the pelvis. The patient must be moved to the oblique position; the radiographic tube is not moved so as to be at a 45º angle relative to the patient and film cassette. Angling the tube results in unacceptable radiographic distortion.

A common mistake in this radiographic technique is the positioning of the patient in an oblique position that is not steep enough, with a resultant angle of less than 45º. On an oblique view obtained with good positioning, the coccyx should project over the femoral head.

Pelvic CT scans may be obtained alone or in combination with abdominal CT scans during the initial trauma evaluation. Axial CT scans may be obtained, but helical CT scanning yields better 2-dimensional and 3-dimensional reformatted images. Pelvic CT scans allow the detection of subtle fractures and displacements that are not appreciated on radiographs.^[12]

Virtually all acetabular fractures may be correctly classified after careful interpretation of AP and oblique radiographs of the pelvis. Intra-articular fracture fragments may be difficult to recognize on radiographs.

Compared with radiography, pelvic CT scanning allows a more precise determination of the degree of articular involvement, as well as of fragment displacement and orientation. Pelvic CT scanning also permits the identification of intra-articular fracture fragments. In complex acetabular fractures, 3-dimensional reformatted images may help conceptualize the fracture pattern and, thereby, aid in the planning of orthopedic surgery.

Brandser and Marsh devised a system of observations leading to the correct classification of most acetabular fractures.^[11] The answers to the following questions about the radiographic observations are used to determine the acetabular fracture pattern.

Is a fracture of the obturator ring present?

A fracture of the obturator ring indicates either a T-shaped or a column fracture (with the exception of the hemitransverse type of fracture). An intact obturator ring eliminates these fractures from consideration.

Is the ilioischial line disrupted?

Disruption of the ilioischial line occurs in fractures involving the posterior column or fractures in the transverse group.

Is the iliopectineal line disrupted?

Disruption of the iliopectineal line indicates anterior column involvement or one of the transverse-type fractures.

Is the iliac wing above the acetabulum fractured?

Iliac wing fractures are observed in fractures involving the anterior column.

Is the posterior wall fractured?

Posterior wall fractures may occur in isolation or in combination with posterior column or transverse fractures.

Is the spur sign present?

The spur sign is observed exclusively in the both-column fracture. The spur is a strut of bone extending from the sacroiliac joint. Usually, this strut of bone connects to the articular surface of the acetabulum. In the both-column fracture, this connection is disrupted; a fractured piece of bone that resembles a spur remains.

The spur sign is best depicted on the obturator oblique view (see the first image below). In addition, the spur sign can be appreciated on CT scans (see the second image below).

Table 1 shows the combined set of radiographic and CT scan observations that are useful in acetabular fracture classification.

Table 1. Radiographic Features of Acetabular Fracture Types^[11] (Open Table in a new window)

Degree of confidence

By using Brandser and Marsh's system, the accurate classification of acetabular fractures is possible in almost every patient.

False positives/negatives

An accessory ossification center, the os acetabulum, may mimic an acetabular wall fracture. Its differentiating features include its characteristic superolateral location and well-corticated margins. Fractures of the anterior puboacetabular junction may be observed in pelvic ring fractures. These fractures may extend into the anterior column of the acetabulum, but they are not anterior column fractures per se. Such fractures are more correctly considered to be superior pubic ramus fractures.

Brandser and Marsh described several CT scan–based observations in the classification of acetabular fractures.^[11] The answers to the following questions may help in the classification of these fractures.

Does the fracture divide the acetabulum into top and bottom halves or into front and back halves?

Transverse-type fractures divide the acetabulum into top and bottom halves, as seen from the lateral perspective of the acetabulum. Column fractures divide the acetabulum into front and back halves. Isolated wall fractures do not divide the acetabulum.

Can an intact strut of bone be followed from the sacroiliac joint to the acetabular articular surface?

In other words, is the CT scan spur sign present? Only the both-column fracture causes the spur sign.

What is the orientation of the major fracture line on CT scans?

Transverse-type fractures have a vertical (sagittal) CT scan orientation. Column fractures have a horizontal (coronal) orientation. Wall fractures are oriented obliquely.

CT scan features of acetabulum fractures are demonstrated below.

Table 1 shows the combined set of radiographic and CT scan observations that are useful in acetabular fracture classification. (See Radiograph.)

Durkee and colleagues presented an algorithm for classification of the 5 most common types of acetabular fracture based on several observations.^[3] The observations are similar to those presented in Table 1.^[3]

Degree of confidence

Interpreted in conjunction with the pelvic radiographs, CT scans allow accurate fracture classification and appropriate surgical planning.

False positives/negatives

False findings can occur.

Magnetic resonance imaging (MRI) is not routinely used in the primary evaluation of acetabular fractures. Potter and colleagues described the use of MRI in the evaluation of occult femoral head injuries and in the detection of subclinical sciatic nerve injury.^[13] Intra-articular fracture fragments may be missed on magnetic resonance images.

Currently, the use of MRI in imaging deep venous thrombosis in patients with pelvic or lower extremity injuries is being evaluated. Magnetic resonance venography may be useful in depicting lower extremity and pelvic venous thrombosis.^{[14, 15, 16]}

Ultrasonography is not used to evaluate acetabular fractures. Lower extremity Doppler ultrasonography is used to assess lower extremity deep venous thrombosis.