Fractures involving the structures of the pelvic girdle are usually sustained in motor vehicle accidents or falls from heights. Although these fractures represent only a small percentage of all skeletal injuries, their importance lies in the significant morbidity and mortality associated with them. Because the clinical signs of pelvic trauma may not always be obvious, radiographic assessment is essential to establish the correct diagnosis.
Radiologic modalities used in evaluation of pelvic trauma include plain film radiography and conventional computed tomography (CT). Other ancillary techniques used for evaluation of concomitant soft tissue and pelvic organ injuries include angiography, cystourethrography, technetium polyphosphate bone scanning, and magnetic resonance imaging.
A single anteroposterior (AP) radiograph used in the initial evaluation of any trauma patient is often sufficient for determining an overview of the pelvic injury. In most instances resuscitation should be instituted immediately on the basis of this single AP radiograph. Reliance on this single view alone, however, may also be misleading because major posterior pelvic displacement can easily be missed in this single view. It is important to remember that in the supine position the pelvis lies 45 ° to 60 ° oblique to the long axis of the skeleton. Therefore, the usual AP radiograph of the pelvis is oblique to the pelvic brim. To obtain more information regarding multidirectional displacements of the pelvic ring Pennal and Sutherland9 recommended the addition of 2 radiographs taken at right angles, the inlet and outlet views. This paper reviews the pertinent radiologic landmarks of the pelvis and their anatomic correlates.
BONY AND LIGAMENTOUS ANATOMY
The pelvis is a ring structure made up of 3 bones, the sacrum and the 2 innominates. The innominate is formed by the fusion of 3 separate centers of ossification, the ilium, the ischium, and the pubis. They meet at the triradiate cartilage, which fuses by age 16 years.
Major ligamentous structures and muscular origins create the stability of the pelvic ring. These ligaments support the posterior pelvis which must transmit the major weightbearing forces across the sacroiliac joint and into the neck of the femur.
Anteriorly, the pubic symphysis acts like a strut, preventing collapse of the pelvis. The opposed bony surfaces of the pubis are covered by hyaline cartilage and are united by layers of fibrocartilage and fibrous tissue. Superiorly and anteriorly, dense ligamentous fibers add stability to the anterior pelvis. Inferiorly, the symphysis is reinforced by the inferior pubic or arcuate ligament.
Posteriorly, the strong ligamentous attachments around the sacroiliac joint include the interosseous sacroiliac ligament, the posterior sacroiliac ligament, and the anterior sacroiliac ligament. The interosseous ligament unites the tuberosities of the ilium and sacrum. This ligament is considered by Grant6 to be the strongest ligament in the body. The posterior sacroiliac ligament is composed of 2 distinct bands: a short posterior sacroiliac ligament that passes obliquely from the tubercle to the posterior superior and posterior inferior spine of the ilium, and a long posterior sacroiliac ligament that runs from the posterior superior iliac spine to the lateral portion of the sacrum. Anterior to the sacroiliac joint, the anterior sacroiliac ligament is a strong, broad ligament composed of transverse and oblique fibers that pass from the anterior surface of the sacrum to the anterior adjacent surface of the ilium.
Several sets of ligaments span the pelvis. The sacrotuberous ligaments are extremely strong, broad ligaments extending from the lateral portion of the entire dorsum of the sacrum and from the posterior surfaces of the posterior superior and inferior iliac spines to the ischial tuberosities. The sacrotuberous ligaments form a portion of the pelvic outlet. These ligaments help to restrain sagittal plain rotation.
The sacrospinous ligaments are strong triangular bands that arise from the lateral margin of the sacrum and the coccyx deep to the sacrotuberous ligament and pass to the ischial spines. They divide the posterior column of the pelvis into the greater and lesser sciatic notch. These ligaments restrain external rotation of the hemipelvis and avulsions representing their discontinuity signify rotational instability.13
The iliolumbar ligaments secure the pelvis to the axial skeleton at the lumbosacral articulation. Bilaterally, this very strong ligament attaches the tip of the fifth lumbar transverse process to the iliac crest. Its avulsion may be a sign of vertical instability. The lateral lumbosacral ligaments run from the L5 transverse process to the ala of the sacrum. Its medial edge may abut the anterior ramus of the fifth lumbar root.
Standard radiographic assessment of the pelvis includes the AP, inlet, outlet, Judet views, and axial CT images. Multiple classification schemes have been developed for the description of pelvic fractures. These can be reviewed in several of the other papers in this symposium and will not be reiterated here.
The AP radiograph is obtained with the patient supine on the xray table, and the beam directed perpendicular to the midpelvis and radiographic plate. A careful inspection of this radiograph gives a wealth of information on the type of pelvic injury sustained (Fig 1).
Evaluation of the pelvis includes the anterior and posterior ring. The anatomic landmarks visible on the AP radiograph include: the symphysis pubis; superior and inferior rami; anterior, superior, and anterior inferior iliac spines; iliac crests; sacral spines; sacroiliac joints; sacral ala; sacral foramen; and L5 transverse processes (Table 1). The anterior ring is evaluated for pubic rami fractures, symphysis disruption, or both. Posterior pathology can be in the form of a sacral fracture, iliac fracture, sacroiliac dislocation, or sacroiliac fracture dislocation. The amount of radiographic displacement of the posterior lesions can yield valuable information on the stability of the pelvis.4,11
Other subtle signs of pelvic instability should also be sought on the standard AP projection. Displaced avulsion fractures of the tip of the transverse process of the L5 vertebra may indicate vertical pelvic instability. The presence of a displaced avulsion fracture of either end of the sacrospinous ligament from the ischial spine or the lateral margin of the sacrum is a sign of rotational pelvic instability. Lastly, the AP radiograph is the proper view on which to evaluate leg length discrepancy. This is done by measuring the difference in the heights of the femoral heads from a line perpendicular to the axis of the sacrum.15 In addition to the pelvic landmarks, the standard anatomic landmarks of the acetabulum are visible. These include the iliopectineal line, the ilioischial line, the tear drop, the acetabular roof, and the anterior and posterior acetabular rims.
The outlet view is obtained with the patient supine on the xray table, and the beam directed from the foot to the symphysis at an angle of approximately 40 ° from vertical. This projection is helpful in disclosing superior displacements in the plane of the pelvis. It is also the best view in which to see sagittal plane rotation (Table 2). This is inferred when the anterior hemipelvis is superiorly displaced, but the posterior ring is not. This view is a true AP view of the sacrum. On the perfect outlet view, the anterior and posterior sacral foramina are clearly seen as perfect circles and fractures through or near the foramen also can be seen (Fig 2). Fractures through the sacral body, especially transversely oriented ones, are also best seen on this projection. Avulsion fractures of the L5 transverse process or from the lateral margin of the sacrum can easily be seen in this projection.
The inlet view is obtained with the patient supine on the xray table, and the beam directed from the head to the midpelvis at an angle of approximately 40 ° from vertical. This projection, which is perpendicular to the pelvic brim, shows the true pelvic inlet (Fig 3).
To fully appreciate the inlet view, it is important to understand the relationship of the promontory of the sacrum as it projects out in front of the S1 body. This is clearly shown in the lateral view of the sacrum (Fig 4). In a true inlet projection, the xray beam is in line with the plane of the anterior S2 and S3 bodies. In this view the anterior cortex of S2 and S3 bodies overlap and appear as a single line just in front of the projection of the sacral bodies. This line appears a few millimeters posterior to the sacral promontory and represents the most anterior limit of sacroiliac screw positioning.
The inlet view shows anterior and posterior displacements in the plane of the pelvis better than any other view.14 In a recent study, the maximal displacement of the posterior pelvic ring was always seen on the inlet view.15 Inward rotation of the iliac wing associated with lateral compression injuries or outward rotation often seen in AP compression and shearing injuries are also manifest. This view is also useful for visualizing sacral impaction or alar fractures. Careful examination of the sacral ala along the arcuate line will reveal sacral crush injuries by a buckling through the arcuate line or a shortening of the sacrum as compared with the other side. Ischial spine avulsion injuries can also be seen on the inlet projection.
Computed Tomography of Pelvis
Computed tomography has been able to add valuable information about pelvic fractures and has revolutionalized the assessment of the posterior interosseous ligamental structures of the pelvis. This information is useful in assessing pelvic stability. In a study by Gill and Bucholz5 comparing plain radiography and CT scan in evaluation of 25 pelvic disruptions, ⅓ of the injury classifications were changed on the basis of additional anatomic information provided by the CT scan.
Computed tomography clearly delineates fractures about the sacrum and the sacroiliac joint manifesting the degree of comminution and separation that may otherwise go undetected. Rotational deformities and translational displacement of the hemipelvis are better seen on the CT scan than plain films. Sacral separation, foraminal impingement, sacral alar fractures, and the L5-S1 region are seen clearly on the axial CT images. Completely unstable sacroiliac dislocation can be diagnosed if the sacroiliac joint is widened anteriorly and posteriorly. This is in contradistinction to vertically stable but rotationally unstable injuries that manifest anterior widening but no posterior widening (Fig 5).
The CT scan is also helpful in defining concomitant acetabular injuries. High pubic rami fractures may occur near the acetabulum or enter the acetabulum inferiorly. These should be considered part of the pelvic injury pattern.
Computed tomography is recommended in all cases in which plain radiographs are inadequate to judge pelvic stability, in fractures with extension into the adjacent acetabulum, and in injuries that are to be treated by open reduction and internal fixation. Finally, 1 of the important roles of CT scanning in preoperative surgical planning lies in its ability to discern alar or impacted sacral fractures which would preclude anterior sacroiliac fixation.
Anterior Pelvic Lesions
Anterior pelvic lesions can be classified as a disruption of the pubic symphysis, fracture of the pubic rami, fracture of pubic body, or combined lesions (Table 3).
The most common types of anterior ring injury are rami fractures. These are usually due to lateral compression injuries. A fracture in the coronal plane is pathognomonic of this injury.17 The number of rami fractures has been linked to genitourinary injuries.16 In most cases of symphyseal dislocation, the pubic symphysis becomes avulsed from 1 hemipelvis. Rarely, the pubic symphysis may be seen overlapped from a lateral compression injury. A bucket handle pattern is a special example of this phenomenon. The amount of symphysis pubic disruption seen radiographically yields valuable information on the presence or absence of major ligamentous injuries. Biomechanical studies have shown that if one cuts the symphysis pubis only, it opens less than 2.5 cm. If the sacrospinous ligaments and the anterior sacroiliac ligaments are cut as well, the pelvis opens like a book until the posterior superior spines abut the sacrum.12 Further displacement represents a complete posterior lesion. Combined injury of the rami with symphyseal dislocation occur regularly. These can be from any of the force vectors described by Young et al.17 The most likely cause, however, is lateral compression.
Posterior Pelvic Lesions
The 3 major posterior pelvic lesions are sacral fracture, pure sacroiliac joint dislocation, and fracture dislocation of the sacroiliac joint (Table 4). Isolated iliac wing fractures are also possible, but are less common. A pure sacroiliac dislocation occurs through the sacroiliac joint without any associated fractures. A fracture dislocation of the sacroiliac joint begins in the ilium and exits through the sacroiliac joint anteriorly (Fig 1B). The posterior portion of ilium remains intact to the sacrum via the posterior ligaments and may be of various sizes. The size of the intact ilium has implications on the surgical management of this injury.1 Fractures of the sacrum may be vertical or horizontal below the sacrogluteal line. Horizontal or transverse sacral fractures are considered spinal injuries and will not be addressed here. Vertical sacral fractures represent a pelvic ring disruption. They may occur through the sacral foramina, medial or lateral to it.3 The most common fractures of the sacrum are caused by lateral compression and cause impaction of the cancellous bone of the sacrum. Foraminal impingement can be associated with these injuries and is best shown by CT. Occasionally, fractures of the sacrum may be markedly comminuted in the shape of a T or an H. These are usually associated with falls from a height.
Application to Sacroiliac Screws
During the past several years the technique of percutaneous posterior sacroiliac screw fixation has gained increasing popularity among the orthopaedic trauma surgeons. Among its many advantages include the limited dissection necessary, minimal blood loss, and decreased risk of infection. Alternately, many surgeons prefer sacroiliac screw fixation after open reduction of the posterior pelvic ring.8,15
When performing sacroiliac screw fixation it is mandatory that one understands the 3 radiographic views of the pelvis as described earlier. As little as 5 ° under or over rotation of the inlet view can affect the precise radiographic landmarks (Fig 6). These lines are extremely important because they guide the surgeon in proper positioning of the posterior sacroiliac screws. Incorrect placement of the drill bit may damage the S1 nerve root, cauda equina, L5 nerve root, or the common iliac artery or vein.
Over rotation of the inlet view will show the anterior S2 and S3 bodies as separate radiographic lines within the S1 body (Fig 6). The distance between the lines of the sacral promontory and the anterior S2 body is increased.
Under rotation of the inlet view will show the sacral promontory line overlapping with the sacral bodies (Fig 6). The distance between the lines of the sacral promontory and the anterior S2 and S3 bodies is decreased. If significantly under rotated, the line of the anterior S3 body can also appear anterior with respect to the sacral promontory line. The coccyx is also more prominent. The posterior aspect of the S1 body appears distorted.
Understanding the relationship of the promontory of the sacrum as it projects out in front of the body is particularly important for placement of posterior sacroiliac screw fixation. In the inlet view a screw aimed at the sacral promontory has a high chance of exiting the S1 body anteriorly, especially if the screw is inferior on the outlet view (Fig 7). In this view the ideal position for a posterior sacroiliac screw is in the center of the S1 body and parallel or anteriorly directed to the line tangent to the anterior S2 and S3 bodies.7 By staying parallel to the front of the S1 body, the screw may be inserted well across the midline gaining better purchase.2 By starting more posteriorly there is less danger of exiting the ala anteriorly or superiorly. Both methods have their proponents.
A second important anatomic relationship that must be kept in mind is the anterior inferior slope of the sacral ala (Fig 8). In the outlet view a screw that is placed superiorly in the S1 body, has a high chance of exiting the sacrum and causing L5 nerve root injury, especially if it is anterior in the inlet view. As a result, in the outlet view, the ideal position for a single posterior sacroiliac screw is just superior to the S1 foramen. The screw should be directed parallel or inferiorly with respect to the superior endplate of the S1 body. This, also, will help to avoid injury to the L5 nerve root as superiorly directed screws may go in out in over the sacral ala.10
When inserting posterior sacroiliac screws the reduction must be obtained before the insertion of the screw. The path of the drill bit must be checked many times by the inlet and outlet views as the instrument is advanced slowly. Careful attention must also be given to the number of different zones of cortical resistance encountered as the drill bit is advanced. Three different zones of resistance corresponding to the outer cortex of the ilium, the iliac side of the sacroiliac joint, and the sacral side of the sacroiliac joint should be felt.15 If a fourth cortex is felt, drilling must be stopped immediately. Drilling through a fourth cortex indicates that the drill has violated the sacral body, which can cause significant damage to the cauda equina, L5 nerve root, iliac vessels, or the retroperitoneal space.
Iliosacral screw placement is absolutely contraindicated when the pertinent radiographic landmarks are not clearly seen fluoroscopically. Several factors may cause decreased visualization. These include the presence of bowel gas or contrast, obesity, and fractures involving the sacral foramina.
Radiographic assessment of patients with pelvic trauma begins with the AP radiograph. The AP radiograph in conjunction with the pelvic inlet and outlet views reveals significant information on the probable mechanism of injury, status of pelvic stability and any associated injuries. A thorough knowledge of the radiographic landmarks of the 3 standard pelvic views and their anatomic correlates is mandatory when treating patients with pelvic fractures.
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