12.48 Fractures of the acetabulum: radiographic assessment and classification

Acetabular fractures represent a serious injury with disruption of the articular surface of the hip joint. Fortunately, these injuries are relatively uncommon with an incidence of about three per 100 000 patients annually in the United Kingdom. In 1998, there were 12 000 fractures of the acetabulum reported in the United States of America. This compares with 323 000 fractures of the femoral neck that occurred in the same time period. The majority of patients sustaining fractures of the acetabulum are injured in a motor vehicle or motorcycle collision, as a pedestrian struck by a motor vehicle, or in a fall from a height. For more than a decade, the reported annual incidence in the United Kingdom has remained stable. However, over time, there has been a change in demographics, with an increase in acetabular fractures caused by simple falls and an increase in the proportion of women with acetabular fractures.

The classification and treatment of acetabular fractures is based on a thorough understanding of the anatomy of the innominate bone. The innominate bone is formed as a condensation of pubis, ischium, and ilium at the triradiate cartilage which fuse at the time of skeletal maturity. The articular surface of the acetabulum can be visualized as being supported between limbs of an inverted Y of bone. Letournel was the first to describe the surgical anatomy of the innominate bone, identifying these two limbs as the anterior column or iliopectineal segment and posterior column or ilioischial segment (Figure 12.48.1). The anterior column refers to the anterior half of the iliac wing that is contiguous with the pelvic brim to the superior pubic ramus, as well as the anterior half of the acetabular articular surface. The posterior column begins at the superior aspect of the greater sciatic notch, and is contiguous with the greater and lesser sciatic notches inferiorly and includes the ischial tuberosity. The posterior column also contains the cortical surface of bone posterior to the acetabulum, known as the ‘retroacetabular surface’. The coalescence of the pelvic brim and greater sciatic notch is referred to as the sciatic buttress and links the anterior and posterior columns (inverted Y of bone) to the axial skeleton (Figure 12.48.2).

Three radiographic projections of the pelvis are used to evaluate fractures of the acetabulum (Figure 12.48.3): the anteroposterior (AP) view of the pelvis, the obturator (or 45-degree internal, Judet) oblique view, and the iliac (or 45-degree external, Judet) oblique view. Interpretation of plain films is based on the understanding of normal radiographic landmarks of the acetabulum and disruption of these landmarks represents a fracture involving that portion of the bone. Computed tomography (CT) and three-dimensional reconstructions are helpful to further define fracture patterns and assess for associated injuries (Box 12.48.1). However, it does not replace the standard radiographic evaluation.

On the AP view, there are six basic landmarks (Figure 12.48.3A and Box 12.48.2). The iliopectineal line is the major landmark of the anterior column. The inferior three-quarters of the iliopectineal line represents the pelvic brim. The superior quarter of this line is formed by the tangency of the x-ray beam to the superior quadrilateral surface and the greater sciatic notch. The ilioischial line is formed by the tangency of the x-ray beam to the posterior portion of the quadrilateral surface, and is considered a radiographic landmark of the posterior column. The radiographic U or teardrop consists of a medial and lateral limb and represents a radiographic finding and not a true anatomical structure. The lateral limb represents the middle third of the cotyloid fossa in the acetabulum, and the medial limb is formed by the obturator canal and the anteroinferior portion of the quadrilateral surface. Because the teardrop and the ilioischial line both result, in part, from the tangency of the x-ray beam to a portion of the quadrilateral surface, they are always superimposed on the AP pelvis view of the normal acetabulum. Dissociation of the teardrop and the ilioischial line indicates either rotation of the hemipelvis, or a fracture of the quadrilateral surface. The roof of the acetabulum is a radiographic landmark resulting from the tangency of the x-ray beam to a narrow portion of the subchondral bone of the superior acetabulum. Interruption of the radiographic line of the roof indicates a fracture involving the superior acetabulum. The anterior rim represents the lateral margin in the anterior wall of the acetabulum and is contiguous with the inferior margin of the superior pubic ramus. The anterior rim is typically medial to the posterior rim, and has a characteristic undulation in its mid-contour in the AP pelvis view. The posterior rim represents the lateral margin of the posterior wall of the acetabulum. Inferiorly, the posterior rim is contiguous with the thickened condensation of the posterior horn of the acetabulum.

The obturator oblique view (also known as the internal oblique view) is taken with the patient rotated so that the injured hemipelvis is rotated 45 degrees toward the x-ray beam (Figure 12.48.3B and Box 12.48.3). This view shows the obturator foramen in its largest dimension and profiles the anterior column. The iliopectineal line has the same relationship with the pelvic brim as on the AP view. The posterior rim of the acetabulum is best seen in the obturator oblique view. Comparison of the relationship of the femoral head with the posterior wall on the normal hip and the injured hip on the obturator oblique view will allow the surgeon to detect subtle amounts of posterior subluxation.

The iliac oblique view (also known as the external oblique view) is taken with the patient rotated so that the injured hemipelvis is tilted at 45 degrees away from the x-ray beam (Figure 12.48.3C and Box 12.48.4). This view shows the iliac wing in its largest dimension, and profiles the greater and lesser sciatic notches, as well as the anterior rim of the acetabulum. Involvement of the posterior column is often best seen on this view. Fractures of the anterior column traversing the iliac wing can also be detected.

Two-dimensional (axial) and three-dimensional CT scans are used as an adjunct to the analysis of the AP and oblique plain radiographic projections. After studying the plain films, the surgeon should use the two-dimensional CT to answer specific questions about the fracture that remain unanswered (Box 12.48.5). In add ition, two-dimensional CT has been shown to be superior to plain radiographs in the detection of fracture step and fracture gap deformities. In order to obtain reliable and useful information, the CT scan should consist of contiguous sections of no more than 3mm in thickness. Although the two-dimensional CT has also been advocated as a means to determine hip joint stability, this has proven unreliable. Furthermore, CT analysis may overestimate the extent of fracture comminution.

Three-dimensional CT scan technology has improved to the point that it is helpful in further defining the fracture pattern and thereby assisting in preoperative planning. However, it does not provide the diagnostic detail of the two-dimensional CT scan.

The basic acetabular fracture classification developed by Judet and Letournel has been incorporated into the alphanumeric comprehensive fracture classification systems of the Orthopaedic Trauma Association and AO. Therefore, the ‘Letournel’ acetabular fracture classification continues to remain the international language of the majority of surgeons treating these complex injuries. The classification is based on the anatomy of the fracture pattern and has ten distinct categories, which are divided into five elementary types and five associated types (Figure 12.48.5 and Box 12.48.6). Variants of these ten basic types are not uncommon. However, they can usually be easily integrated into the system. This system is important not only for its ability to describe the fracture, but it also serves as a guide for subsequent operative treatment. High rates of interobserver and intraobserver reliability have been reported using this classification system.

Fractures of the posterior wall are the most common type of acetabular fractures. The posterior wall fracture involves disruption of the posterior rim of the acetabulum, along with a portion of the articular surface and retroacetabular surface. Fractures can vary in size from a very small portion of articular surface to nearly the entire posterior wall, and can vary in location from the posteroinferior to the posterosuperior aspect of the acetabular articular surface. On the AP and obturator oblique x-rays, posterior wall fractures disrupt the posterior rim, but no other radiographic landmarks (Figure 12.48.6). Impaction of the articular surface (marginal impaction) (Figure 12.48.4) is a common associated finding. The typical posterior wall fracture does not disrupt the greater and lesser sciatic notches or extend into the quadrilateral surface. Extended posterior wall fractures represent a more severe injury that involve a disruption of the greater or lesser sciatic notches, and a portion of the quadrilateral surface. These fractures do not extend into the obturator foramen.

Posterior column fractures (Figure 12.48.7) disrupt the posterior or ischial portion of the acetabulum and are best seen on the iliac oblique view. The majority of the retroacetabular surface is displaced with the posterior column. A vertical or oblique fracture line starting near the apex of the greater sciatic notch and crossing in the acetabular fossa, separates the anterior and posterior columns, and commonly enters the obturator foramen. There is usually an associated fracture of the inferior pubic ramus. The femoral head usually follows the posterior column posteriorly and medially. The ilioischial line is typically displaced relative to the teardrop. However, when a large portion of the quadrilateral surface remains intact with the posterior column, the teardrop will displace with the ilioischial line.

Anterior wall fractures disrupt the central portion of the anterior column (Figure 12.48.5C) and are rare fractures (1–2%). The inferior pubic ramus is typically intact. The iliopectineal line commonly has displacement in its mid-portion on the AP and obturator oblique views and the ilioischial line is intact. An anterior wall fracture variant has been described (Figure 12.48.8), which does not involve the pelvic brim and is a morphological equivalent to the posterior wall fracture.

Anterior column fractures can occur at a variety of levels. The fracture is named based on where it exits the bone anteriorly. High fractures exit the iliac crest, intermediate fractures exit at the anterior superior iliac spine (ASIS), low fractures exit below the anterior inferior iliac spine (AIIS), and very low fractures exit at the iliopectineal eminence. The fracture typically involves disruption of the superior acetabular articular surface in the coronal plane. The iliopectineal line and anterior rim are typically displaced. Displacement of the superior articular surface is often best seen on the obturator oblique view. Inferiorly, the fracture disrupts the obturator ring through the ischiopubic ramus. Many variants have been described (Figure 12.48.9).

Transverse fractures divide the innominate bone into two portions, with a horizontally displaced fracture line, an intact superior acetabular fragment, and an inferior ischiopubic segment. The fracture begins at the pelvic brim, superomedially, and extends obliquely laterally and distally. The fracture line can cross the acetabulum at various levels and have been subdivided into three groups: transtectal fractures—the fracture line crosses the superior acetabular articular surface; juxtatectal fractures—the fracture line crosses at the junction of the superior acetabular articular surface and superior cotyloid fossa; and infratectal fractures—the fracture line crosses through the cotyloid fossa. Disruption of the vertical landmarks on the AP view (i.e. iliopectineal line, ilioischial line, anterior, and posterior rims) are noted. On CT scan, the fracture line is oriented in an AP direction in the axial section (see Figure 12.48.10D).

The association of a posterior column and posterior wall fracture represents a typical posterior column fracture, complicated by injury to the posterior wall. The femoral head usually dislocates with the wall component. Radiographic signs of a posterior column fracture with a dissociation of the ilioischial line and teardrop are commonly noted. Posterior wall fractures will be best seen on the obturator oblique view.

Associated transverse and posterior wall fractures are an extremely common type of acetabular fracture, approximately 20% of all acetabular fractures. The femoral head is often dislocated. This fracture combines a normal transverse configuration with a post erior wall fracture component. When the posterior wall fragment is minimally displaced, it may be missed on the AP pelvis, but is commonly seen on the obturator oblique view, as well as the CT.

T-shaped fractures are similar to transverse fractures with the addition of a vertical fracture in the ischiopubic segment along the quadrilateral surface and acetabular fossa, separating the anterior and posterior columns inferiorly (Figure 12.48.10). This inferior fracture is known as the stem of the T. The stem will typically extend through the obturator foramen, exiting through the ischiopubic ramus; however, it may also extend posteriorly (exiting through the ischium) or anteriorly (exiting near the pubic body). Medial or posterior subluxation of the femoral head can occur.

Anterior column with associated posterior hemitransverse fracture combines an anterior wall or anterior column fracture with a transverse component posteriorly. The posterior component of this fracture pattern is identical to the posterior half of a transverse fracture. The distinction between the associated anterior column and posterior hemitransverse and the T-shape patterns is often subtle. The anterior injury is typically at a higher level and more displaced than the posterior hemitransverse component. Anterior subluxation of the femoral head is typical with an anterior column and posterior hemitransverse injury.

Both-column fractures by definition have the anterior and posterior columns separated from each other, and all articular segments of the acetabulum are detached from the intact portion of the posterior ilium (Figure 12.48.11). The both-column fracture is unique, in that it represents an acetabulum completely disconnected from the axial skeleton. If the femoral head medializes, the articular fragments can rotate around the head because the labrum is usually intact. This is called ‘secondary congruence’ and is unique to both column fractures. Both-column fractures are associated with the ‘spur’ sign. This represents the fracture edge of the intact posterior ilium that is seen prominently on the obturator oblique view. This spur sign is pathognomonic of a both-column fracture (Figure 12.48.11B). The surgeon should recognize that transverse fractures, transverse and posterior wall fractures, T-shaped fractures, and anterior column and posterior hemitransverse fractures all involve the anterior and posterior columns of the acetabulum, but are not ‘both-column’ fractures. In these four fracture types, a portion of the articular surface remains intact with the ilium and the axial skeleton.

The acetabulum is a complex three-dimensional structure and effort is required to develop the required interpretive skills. The majority of fractures can be classified from the information gleaned from high-quality plain radiographs. An organized approach to the examination of the three radiographs (AP and obliques) and the CT scan of a patient with an acetabular fracture must be used. One method is as follows.

First, the lines on the AP radiograph (Figure 12.48.3A) should be carefully analysed in turn. Is the iliopectineal line disrupted? If so, the fracture possibilities include the anterior wall, anterior column, transverse types, T-shaped, anterior column and posterior hemitransverse, and both-column. If the ilioischial line is disrupted, possibilities include the posterior column types, transverse types, T-shaped, anterior column and posterior hemitransverse, and both-column. With both lines disrupted, the possibilities are reduced to transverse types, T-shaped, anterior column and posterior hemitransverse, and both-column. Is the line along the posterior rim disrupted? If so, this will add the possibility of a posterior wall fracture. Is the ilioischial line displaced from its normal relationship to the teardrop? If so, this would indicate, in general, that the two columns are separated from each other.

Next the obturator oblique is examined. This view will refine the diagnosis made from the AP view. A suspected posterior wall component will become obvious, as well as disruption involving the anterior wall or column. Is the obturator ring fractured? If so, this, again, would suggest that the two columns are separated from each other. The presence of a spur sign (Figure 12.48.11B) is pathognomonic for a both-column fracture. The iliac oblique is viewed next. Injury to the posterior column is further defined, as well as the presence and location of fractures involving the iliac wing (for example; anterior column, anterior column and posterior hemitransverse, and both-column fractures).

Finally, the CT scan is studied to reveal the additional information described previously (Box 12.48.5). After this analysis, the plain films should be revisited to refine the diagnosis for fracture subtypes (for example, the level of transverse fracture or the path of the stem of the T-shaped fracture). If the diagnosis continues to be unclear, the three-dimensional CT scan can prove helpful. However, as noted previously, the three-dimensional CT scan has its limitations.

In some clinical situations, high-quality plain radiographs, especially the Judet oblique views, may be difficult to obtain. Investigators have reported on the advantages of equivalents to the plain oblique radiographs, which are reconstructed from CT scan data using three-dimensional CT modelling software. However, the use of this approach and an overall increased role for the three-dimensional CT scan, awaits further study, as well as technological improvements.

Acetabular fracture patterns can be complex; however, classification can be achieved with the use of appropriate imaging and a systematic approach. Classification is not only important for communication but is essential in allowing the surgeon to understand the fracture pattern and select the most appropriate surgical strategy.