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 can be planned and implemented.
One function of the acetabulum is to provide a means of transferring weight-bearing forces from the appendicular skeleton to the axial skeleton via the acetabulum's articulation with the femoral head. The femoral head also transfers high-energy forces to the acetabulum in the setting of trauma. The pattern of acetabular injury is determined by the position of the femoral head at the time of the traumatic event. When the femoral head is rotated internally, the force is transferred to the posterior column. When the femoral head is rotated externally, the force is directed toward the anterior column. If the femoral head is adducted, the force is transmitted to the acetabular roof; if it is abducted, the force is directed inferiorly.
The direction of the force also determines which part of the acetabulum is injured. An anterior force applied to the femoral head is transmitted to the posterior wall and column. Conversely, a posterior force affects the anterior wall and column. A force to the lateral aspect of the femoral head is directed toward the medial wall of the acetabulum, often resulting in transverse acetabular fractures.
Table 1. Relative Frequency of Acetabular Fracture Types
|Fracture Type||Letournel1* (%)||Matta2** (%)||Dakin et al3† (%)|
|Transverse with posterior wall||20.6||23.5||35.3|
|Anterior column with posterior hemitransverse||8.8||5.9||3.5|
|Posterior column with posterior wall||3.5||3.9||18.8|
*n = 567.
**n = 255.
† n = 85.
- Associated injuries
- Significant trauma is required to cause a fracture of the acetabulum. Therefore, acetabular fractures are most often observed in the setting of major trauma, in which injuries elsewhere in the body are common.
- Intracranial, spinal, intrathoracic, and intra-abdominal injuries often are observed in conjunction with acetabular fractures.
- Pelvic ring and extremity fractures also are common in patients with acetabular fractures.
- Bladder injury and clinically significant pelvic hemorrhage are not routinely observed in the setting of acetabular fracture unless a concomitant pelvic ring injury also is present.
- Complications of acetabular fracture include the following:
- Immediate posttraumatic complications include injuries to the sciatic, femoral, or superior gluteal nerves.
- Immediate postsurgical complications include nerve injuries, such as sciatic, femoral, and superior gluteal nerve injuries; wound infection; and thromboembolic disease.
- Late complications include heterotopic ossification, osteonecrosis (avascular necrosis) of the femoral head or acetabular fracture fragment, chondrolysis, posttraumatic osteoarthritis, and acetabular implant failure.
Elderly patients and persons with osteoporosis may occasionally have an acetabular fracture as a result of low-energy trauma, such as a fall from a standing height.
The acetabulum is formed from 3 ossification centers; the ilium, ischium, and pubis each contribute to its development at the triradiate cartilage. The important anatomic components of the acetabulum are the columns, walls, dome, and quadrilateral plate. The acetabulum is divided into 2 columns: anterior and posterior. The 2 columns are described as having the shape of an inverted Y, or of the Greek letter lambda (l).
The anterior column is the larger of the 2 columns. It begins at the iliac wing and extends down the anterior portion of the acetabulum to incorporate the superior pubic ramus. The posterior column begins at the sciatic notch and extends down the posterior acetabulum into the ischium. Both columns are attached to the axial skeleton by the sciatic buttress, which connects the acetabulum to the sacroiliac joint. The column concept is appreciated more easily on the lateral view (see Image 1).
The posterior wall is larger than the anterior wall. The lateral portion of either wall is termed the acetabular rim. The walls help to stabilize the hip joint. The quadrilateral plate is the medial wall of the acetabulum. The dome of the acetabulum is the superior aspect that carries most of the weight-bearing forces. The obturator ring is an important landmark because some acetabular fractures spare the ring, while others disrupt it. The iliac wing is considered part of the anterior column.
The anteroposterior (AP) view of the pelvis is the primary tool for radiographic evaluation of the acetabulum (see Images 2-3). 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.
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 can 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 Images 4-5). 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. 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.
Fractures of the acetabulum are most commonly classified according to the system described by Judet and colleagues.4 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. Some authors have questioned the necessity of oblique views of the pelvis in the age of multidetector CT scanning. 5 Harris and colleagues have proposed a new classification system based on the multidetector CT scan appearance.6, 7 Other authors have defended the utility of the standard radiographic series in the evaluation of acetabular fractures.8 The Judet system will be presented in the remainder of this article.
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 (see Image 6). 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
- Anterior wall
- Posterior wall
- Posterior column with posterior wall (also a column fracture)
- Transverse with posterior wall (also a transverse fracture)
- Column fractures
- Anterior column
- Posterior column
- Posterior column with posterior wall (also a wall fracture)
- Anterior column with posterior hemitransverse (also a transverse fracture)
- Transverse fractures
- Transverse with posterior wall (also a wall fracture)
- Anterior column with posterior hemitransverse (also a column fracture)
Both-column fractures are the most common acetabular injury. As the name implies, the anterior and posterior columns are involved (see Image 6). On AP radiographs, a disruption of the iliopectineal and ilioischial lines, as well as the obturator ring, can be seen (see Image 24). An iliac wing fracture may be seen on the AP view, but often, it is appreciated only on the iliac oblique radiograph (see Image 25). The pathognomonic spur sign (see Radiograph Findings) is present on the obturator oblique view (see Image 26) and confirmed on a computed tomography (CT) scan (see Images 27-29).
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 Images 18-23). Column fractures divide the acetabulum into front and back halves (see Image 7). The posterior column fracture with a posterior wall fracture has the features of each of its components (see Image 6). 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 can be difficult to appreciate radiographically (see Image 30). 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 can be appreciated (see Image 31).
Transverse fractures are transverse because of their appearance when the acetabulum is examined from the lateral view (see Image 6). 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 (see Image 7). The transverse fracture with a posterior wall fracture is a common fracture that incorporates the features of transverse and posterior wall elementary fractures (see Images 13-15). 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 Images 16-17). 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.2 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 as follows:
- Commonly occurring acetabular fractures (90%)
- Transverse with posterior wall
- Posterior wall
- Uncommonly occurring acetabular fractures (10%)
- Anterior column
- Anterior column with posterior hemitransverse
- Posterior column with posterior wall
- Posterior column
- Anterior wall
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 Images 2-3). 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 (see Pelvic Ring Fractures).
Oblique, or Judet, radiographs of the pelvis are obtained with the patient in the left posterior oblique and right posterior oblique positions (see Images 4-5). 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.
Limitations of Techniques
Virtually all acetabular fractures can be correctly classified after careful interpretation of AP and oblique radiographs of the pelvis. Intra-articular fracture fragments can 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.9 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 1 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 can 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 Image 26). In addition, the spur sign can be appreciated on CT scans (see Image 27).
Table 2 shows the combined set of radiographic and CT scan observations that are useful in acetabular fracture classification.
Table 2. Radiographic Features of Acetabular Fracture Types9
*N/A indicates not applicable.
Degree of Confidence
By using Brandser and Marsh's system, the accurate classification of acetabular fractures is possible in almost every patient.
An accessory ossification center, the os acetabulum, can mimic an acetabular wall fracture. Its differentiating features include its characteristic superolateral location and well-corticated margins. Fractures of the anterior puboacetabular junction can 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.