Pelvic Ring Fracture Imaging 

Pelvic ring fractures occur as the result of high-energy blunt trauma, as may result from motor vehicle collisions and falls. These injuries are associated with significant morbidity and mortality, both from the complications of pelvic ring fractures and from commonly associated injuries. Recognition of the pattern of injury to the bony pelvis directs the search for associated soft tissue injuries and allows implementation of the appropriate therapy. (See the images below.)^{[1, 2, 3, 4, 5]}

The Young-Burgess classification system of pelvic ring fractures includes 3 degrees of anteroposterior (AP) pelvic ring compression injuries. (Lateral compression and vertical shear fractures are also described in this system.)^[18]

The hallmark of the AP compression injury is pubic diastasis with or without disruption of the sacroiliac (SI) joints. The location and degree of diastasis is correlated with the magnitude of force imparted to the pelvis and with the amount of resulting instability. The AP compression causes the pelvis to open: 1 or both hemipelves undergo external rotation.

Young-Burgess classification of AP pelvic ring compression injuries is as follows.

Type I AP injuries

Less than 2.5 cm of the pubic diastasis is noted, either at the symphysis or through vertically oriented rami fractures. The SI joints and posterior ligaments remain intact, and stability is maintained.

Type II AP injuries

The amount of anterior diastasis exceeds 2.5 cm. In addition, diastasis occurs in 1 or both of the SI joints. This incomplete posterior arch disruption results in rotational instability. The posterior ligaments are not injured; therefore, vertical stability is preserved.

Type III AP injuries

These injuries extend to the posterior SI ligaments, which are disrupted. Consequently, the pelvis is vertically and rotationally unstable.

AP radiography of the pelvis

AP radiographs of the pelvis and chest and lateral radiographs of the cervical spine are included in the initial radiographic assessment of a patient with major traumatic injuries. (See the image below.)

Radiographs are obtained with the patient in the supine position, with the x-ray beam passing in an AP direction.

Abnormalities depicted on the AP pelvis radiograph direct the need for the next set of radiographs, which include oblique (Judet) views of the pelvis in acetabular fractures,^{[6, 7]} and inlet and outlet radiographs of the pelvis in patients with pelvic ring fractures.

Inlet and outlet radiography of the pelvis

Inlet radiographs of the pelvis are obtained with the patient in the supine position, with the x-ray tube positioned at the patient's head and angled 45° toward the feet. The x-ray beam is perpendicular to the pelvic brim (or inlet). This view allows the evaluation of pelvic brim integrity, AP displacement of the hemipelvis, internal/external rotation of the hemipelvis, and sacral impaction. (See the image below.)

Outlet radiographs of the pelvis are obtained with the patient in the supine position, with the x-ray tube positioned at the patient's feet and angled 45° toward the head. The x-ray beam is perpendicular to the sacrum. This view allows confirmation of vertical (cranial) displacement of the hemipelvis and evaluation of the sacral neural foramina. (See the image below.)

Pelvic computed tomography (CT) scanning

During the initial trauma evaluation, CT images may be obtained of the pelvis in isolation, or CT scans of the pelvis may be obtained at the same time that scans of the abdomen are obtained.^[8]

Axial CT scans may be obtained, but helical CT scans (especially with multidetector CT) yield better 2-dimensional (2D) and 3-dimensional (3D) images.

CT allows the detection of subtle fractures and displacements not appreciated on radiographs.

Limitations of Techniques

Radiographs of the pelvis may not demonstrate subtle fractures that do not affect classification of the injury. Spatial orientation of fracture fragments and joints is visualized better on pelvic CT scans.

Pelvic CT scans require transport of the patient to the CT scanner; most patients need to undergo abdominal and pelvic CT for an assessment of visceral injury.

This section discusses the radiographic views required for the assessment of pelvic ring fractures.

AP radiographs of the pelvis

Usually, the pubic symphysis is approximately 5 mm wide; it should not be more than 1 cm wide. Pubic symphysis diastasis occurs when the fibrocartilage connecting the 2 pubic bones is disrupted. Diastasis of the pubic symphysis indicates an AP compression injury. If overlap of the pubic bones at the symphysis is noted, a lateral compression injury is suggested. The superior pubic rami should be at the same level as they join at the symphysis. In a vertical shear injury, 1 side is displaced in a cranial direction. The lower margins of the rami are a better guide because nonalignment of the upper margins may be a normal variation. (See the images below.)

The orientation of pubic rami fractures provides a clue to the mechanism of injury. Horizontal overlapping fractures of the superior and inferior pubic rami are associated with lateral compression. Vertical fractures of the rami without cranial displacement of the hemipelvis may be seen in AP compression injuries instead of pubic symphyseal diastasis. Vertical rami fractures with cranial displacement are a hallmark of vertical shear injuries. Minimally displaced fractures of the pubic rami may be seen in isolation, usually in an individual with osteoporosis after a low-velocity fall. The integrity of the pelvic ring is maintained.

The direction of hemipelvic displacement indicates the mechanism of injury. External rotation of the hemipelvis (open-book pelvis) occurs with AP compression. Internal rotation is seen in lateral compression. Vertical shear injuries result in vertical (cranial) displacement of the hemipelvis. Iliac wing fractures with extension to the vicinity of the SI joint are found in the more severe lateral compression injuries. Avulsion of the ischial spine occurs in external rotation or vertical displacement of the hemipelvis. Isolated iliac wing fractures may occur as a result of a direct blow without disruption of the pelvic ring. With iliac crest, anterior iliac spine, and ischial tuberosity avulsion fractures, the integrity of the pelvic ring is also maintained.

The normal SI joint space is approximately 2-4 mm in width. When the SI joint is analyzed for diastasis, the anterior and posterior aspects should be examined. Disruption of the SI joint with external rotation of the ipsilateral hemipelvis is characteristic of AP compression. If only the anterior SI joint is widened, the posterior ligaments are intact and preserving vertical stability. If the SI joint is anteriorly and posteriorly diastatic, the pelvis is completely unstable. Usually, the SI joint is completely disrupted in vertical shear injuries. Displaced vertical fractures through the sacrum or the iliac wing adjacent to the SI joint have the same implication as SI joint diastasis.

Buckle (anterior crush) fractures of the sacrum are the hallmark of lateral compression injuries (see the images below). The fractures are usually oriented vertically. They may be isolated to the sacral ala, pass through the neural foramina, or extend centrally into the sacral spinal canal. Radiographic findings of the fractures may be subtle. The sacral promontory and arcuate foramina should be carefully examined for cortical disruption. Displaced vertical fractures through the sacrum may be seen in lieu of SI joint disruption in AP compression and vertical shear injuries. Horizontal fractures of the sacrum below the level of the S2 do not affect the integrity of the pelvic ring.

The iliolumbar ligament is inserted at the tip of the L5 transverse process. An avulsion fracture at this site is associated with disruption of the posterior SI ligament complex, as seen in severe AP compression and vertical shear injuries. Hence, an L5 transverse-process avulsion fracture may indicate complete pelvic instability.

Inlet radiographs of the pelvis

The inlet view of the pelvis permits more accurate determination of the following: the degree of posterior displacement at the SI joint, the degree of internal or external rotation of the hemipelvis, the degree of pubic diastasis or overlap, and the presence of subtle sacral fractures. (See the images below.)

Outlet radiographs of the pelvis

The primary purpose of the outlet view of the pelvis is to demonstrate the magnitude of vertical (cranial) displacement of the hemipelvis. Additionally, some sacral and pubic rami fractures are better visualized with the outlet view than with other views. The sacral neural foramina are especially well depicted by using the outlet view. (See the image below.)

Degree of confidence

In most patients, an analysis of the AP radiographs of the pelvis results in the correct determination of the mechanism of pelvic ring injury. Appropriate therapeutic maneuvers may be initiated immediately. Additional radiographic views (eg, inlet and outlet views) and pelvic CT scans allow more precise classification when definitive treatment is considered.

Multidetector CT is now routinely used to evaluate trauma patients; some authors have questioned whether pelvic radiographs should be routinely used for patients who are destined to undergo a CT scan.^{[1, 9]}

False positives/negatives

True pelvic ring fractures must be distinguished from pelvic fractures that do not affect pelvic stability (eg, Tile type A injury).

Pelvic ring fractures should be distinguished from acetabular fractures, which may also occur with pubic rami and iliac wing fractures. The sites that are important for pelvic stability (eg, pubic symphysis, SI joints, sacrum) should be examined to exclude a pelvic ring fracture.

An acetabular fracture may be present in addition to a pelvic ring fracture. With both types, fractures should be analyzed individually.

All radiographic findings should be further assessed on pelvic CT scans, because subtle fractures and disruptions may be more apparent on CT scans. In particular, sacral fractures may be difficult to detect on radiographs. (See the images below.)^{[10, 8, 11]}

The spatial relationship of fracture fragments is often easier to assess with CT scans than with radiographs. Axial CT images may be reformatted into the coronal and sagittal planes. Three-dimensional images of the pelvis may also be reconstructed. Reformatted images are more useful in assessing acetabular fractures than in evaluating pelvic ring fractures.

In addition to the osseous structures, the soft tissues of the pelvis should be examined. The size of a pelvic hematoma secondary to a pelvic ring fracture may be determined. If contrast material is intravenously administered for pelvic CT, active arterial bleeding may be demonstrated, and the information may be used to guide the clinical decision to incorporate angiography into the patient's treatment plan.^[8]

Degree of confidence

The combination of the pelvic CT scans and the AP radiographs with inlet and/or outlet views permits accurate classification of pelvic ring fractures in virtually every patient.

Magnetic resonance imaging (MRI) is not used to evaluate pelvic ring fractures. Research is currently underway to evaluate the use of MRI in the evaluation of deep venous thrombosis in orthopedic patients. MR venography may prove useful in depicting lower extremity and pelvic venous thrombosis.^[12]

Ultrasonography is not used to evaluate pelvic ring fractures. Lower-extremity Doppler ultrasonography is used to assess for the presence of lower-extremity DVT.

Angiography is used to diagnose and treat potentially life-threatening hemorrhage secondary to pelvic ring injury. Pelvic arteriography demonstrates the injured vessels responsible for the hemorrhage. The vessels may then be embolized to control or stop the bleeding.