The standard radiological diagnostics of acetabulum fractures include conventional X-rays including an anteroposterior (AP) view of the hip; the whole pelvis (pelvic AP view); and oblique views according to Judet (iliac oblique view [IOV], obturator oblique view [OOV]).1
In addition, thin-section computed tomography (CT) is always recommended for detailed diagnosis with multiplanar reconstructed images and, if possible, surface-rendered three-dimensional (3D) reconstruction with or without disarticulating of the proximal femur. CT allows for better assessment of intraarticular fragments and better visualization of the articular surface of the acetabulum and 3D-CT offers better understanding of the fracture geometry. Additionally, removing the contralateral hemipelvis allows for better understanding of the medial major fracture lines.2,3
The interpretation of the acetabulum fracture on conventional X-rays is based on Letournel’s and Judet’s fundamental work.1,4 Understanding the radiographical lines provides the basis for understanding fracture pathology as well as classification of the fracture.
3.2 Conventional Diagnostics
3.2.1 Anteroposterior View of the Pelvis
The pelvic AP view is still the gold standard in the evaluation of acetabular fractures. Prerequisite for optimal evaluation of the fracture is a well-centered symmetrical view in standard technique.
In the fracture situation, use of a Gonad shield should be avoided not to cover the relevant osseous structures. On this view, six lines (▶ Fig. 3.1) are relevant for fracture classification.4
Fig. 3.1 Characteristic lines on the anteroposterior view of the pelvis (see text): 1, iliopectineal line; 2, ilioischial line; 3, tear drop figure; 4, acetabular roof; 5, anterior wall line 6, posterior wall line.
Iliopectineal Line
The iliopectineal line (1) corresponds in its lower parts to the upper pubic rami and therefore to the pelvic plane entering the true pelvis. Dorsocranially, this line ends near the sacroiliac (SI) joint and forms part of the dense bone superior to the greater sciatic notch (sciatic buttress). Here, this line no longer corresponds to the terminal line or pelvic brim, as it runs slightly below the plane of the terminal line. Radiologically, this line usually ends at the level of the second sacral arcuate line (▶ Fig. 3.2).
Fig. 3.2 Iliopectineal line marked on the uninjured (right) side and disrupted on the injured (left) side.
Clinical Relevance
The iliopectineal line represents to ¾ the medial part of the anterior column.
Ilioischial Line
The ilioischial line (2) starts at the lateral border of the obturator foramen and corresponds in its further course to an intraosseous portion of the quadrilateral surface until it reaches proximally the sciatic buttress near the second sacral arcuate line. Often, an overlapping with the iliopectineal line is present, but sometimes both lines are separated proximally. The ilioischial line is closely related to the teardrop figure and runs lateral to the ischial spine (▶ Fig. 3.3).
Fig. 3.3 Ilioischial line marked on the uninjured (right) side and disrupted on the injured (left) side.
Clinical Relevance
The ilioischial line represents the posterior column in its lower and middle parts.
Posterior Wall Line
In the majority of cases, the line of the posterior wall (6) is the most lateral part of the hip joint on the pelvic AP view, due to the physiological anteversion of the acetabulum.
Its proximal to distal course is slightly S-shaped, showing a small medial orientated curve at the posterior horn of the acetabulum at the groove proximal to the ischial tuberosity (▶ Fig. 3.4). Variations exists in acetabular retroversion where a crossing with the anterior wall line is visible (crossing sign).
Fig. 3.4 Posterior wall line marked on the uninjured (right) side and disrupted on the injured (left) side.
Clinical Relevance
The line of the posterior wall is usually the most lateral border of the acetabulum and represents the posterior wall or part of the posterior column.
Anterior Wall Line
The line of the anterior wall (5) is often only visible in X-rays with near-perfect X-ray quality and corresponds to the anterolateral wall of the acetabulum. It is normally a double S-shaped line and runs medial to the posterior wall line (▶ Fig. 3.5). In its distal part, this line is passing to the anterior and superior margin of the obturator foramen. In acetabular retroversion, the proximal posterior wall line is medial to the anterior wall line with a crossing at various localizations.5,6,7
Fig. 3.5 Anterior wall line marked on the uninjured (right) side and disrupted on the injured (left) side.
Clinical Relevance
The typical line of the anterior wall is medial to the posterior wall line and runs to the anterior parts of the obturator foramen.
Line of the Acetabular Roof
The acetabular roof (sourcil, acetabular dome) (4) corresponds to a narrow bone area of subchondral osseous condensation of about 2–3 mm width at the superior acetabulum. It represents the major weight-bearing surface (▶ Fig. 3.6).
Fig. 3.6 Acetabular roof line marked on the uninjured (right) side and disrupted on the injured (left) side.
Clinical Relevance
Acetabular roof line involvement represents injury to the superior weight-bearing area without defining its extent.
Teardrop Figure
The teardrop figure or U-figure (3) is a radiographic feature resulting from an end-on projection of a bony ridge running along the floor of the acetabular fossa and, therefore, corresponds to parts of the lateral (acetabular fossa) and medial wall (quadrilateral surface) of the acetabulum. Its distal part corresponds to the proximal obturator canal (▶ Fig. 3.7).
Fig. 3.7 Teardrop figure marked on the uninjured (right) side and disrupted on the injured (left) side.
Clinical Relevance
Analysis of the teardrop figure indicates involvement of the quadrilateral surface in acetabular fractures.
Roof-Arc Measurement
Matta introduced roof–arc measurements in the assessment of acetabular fractures.8,9 On the AP view of the pelvis, the medial roof–arc angle is determined (▶ 5). The technique and consequences of roof–arc measurements regarding indication are described in ▶ 5.
Fig. 3.8 Medial roof–arc angle according to Matta8,9: The angle between a vertical line through the femoral head center and a line from femoral head center to the first medial fracture line.
Further Radiological Criteria
In addition to the six standard lines, analysis of the pelvic AP view includes the iliac bone. Here, additional fracture lines extending to the iliac crest, as typically seen in fractures with a high anterior column component, can be visible. Furthermore, an orientated assessment of the obturator foramen is performed to analyze possible disruptions.
3.2.2 Judet Views
In addition to the pelvic AP view, no true radiological perpendicular plane is available. The oblique views of the hemipelvis are the basis for an additional radiograph in two perpendicular planes. Considering the morphological anatomy of the hemipelvis, the iliac wing/fossa is rotated 90 degrees in relation to the obturator segment.
In the true frontal plane of a pelvic AP view, the obturator segment and the iliac wing have an angulation of approximately 45 degrees (▶ Fig. 3.9). Thus, these oblique views are taken for two-plane analysis with the pelvis rotated either right or left by 45 degrees. By using both views, the obturator segment and the iliac fossa have perpendicular orientations (▶ Fig. 3.10).
Fig. 3.9 Schematic view of the perpendicular orientation of the planes of the iliac fossa and the obturator foramen.
Fig. 3.10 Complete visualization of the whole pelvis in iliac and obturator oblique views by 45-degree rotation allows comparison of the injured and uninjured sides.
To allow optimal comparison, X-rays of the whole pelvis should be performed, as one side shows an OOV and the contralateral side shows an IOV.
Iliac Oblique View
The patient is placed in the supine position and the contralateral hemipelvis (uninjured side) is rotated 45 degrees anteriorly. The X-ray beam is directed vertically toward the affected hip and is perpendicular to the iliac wing, whereas the obturator foramen shows superimposed bony structures (▶ Fig. 3.11).
Fig. 3.11 Characteristics of the iliac oblique view. Illustration of the lines of the posterior column (orange), the anterior wall (yellow), and the iliac fossa (blue).
Correspondingly, the contralateral side presents as an OOV and therefore allows comparison to the OOV of the fractured side.
The most medial part of the hemipelvis on the IOV corresponds to the posterior column (▶ Fig. 3.12) and the most lateral part at the acetabulum corresponds to the anterior wall (▶ Fig. 3.13) (reversed on the OOV, see below). The iliac crest and the iliac fossa are most clearly seen on the IOV for analysis of possible fractures extending to these regions (▶ Fig. 3.14).
Fig. 3.12 Illustration of the lines of the posterior column (orange). By mirroring the uninjured side, comparison of the injured and uninjured side is possible.
Fig. 3.13 Illustration of the anterior wall line (yellow). By mirroring the uninjured side, comparison of the injured and uninjured side is possible.
Fig. 3.14 Illustration of the line of the iliac crest (blue). By mirroring the uninjured side, comparison of the injured and uninjured sides is possible.
Clinical Relevance
The IOV allows analysis of the iliac fossa, the posterior column, and the anterior wall.
Obturator Oblique View
The patient is placed in the supine position and the ipsilateral hemipelvis (injured side) is rotated anteriorly at 45 degrees. The X-ray beam is directed vertically toward the affected hip and is perpendicular to the obturator foramen, whereas the iliac wing shows superimposed bony structures (▶ Fig. 3.15).
Fig. 3.15 Characteristics of the obturator oblique view. Illustration of the lines of the anterior column (orange), the posterior wall (yellow), and the obturator foramen (blue).
In the optimal technique, the coccyx is projected slightly superior to the acetabular roof.
The most medial part of the hemipelvis on the OOV corresponds to the anterior column (▶ Fig. 3.16) and the most lateral part at the acetabulum corresponds to the posterior wall (▶ Fig. 3.17), reversed on the IOV). The obturator foramen is most clearly seen on the OOV for analysis of possible fractures extending to this region (▶ Fig. 3.18).
Fig. 3.16 Illustration of the anterior column (orange). By mirroring the uninjured side, comparison of the injured and uninjured side is possible.
Fig. 3.17 Illustration of the posterior wall line (yellow). By mirroring the uninjured side, comparison of the injured and uninjured side is possible.
Fig. 3.18 Illustration of the obturator foramen (blue). By mirroring the uninjured side, comparison of the injured and uninjured side is possible.
Additionally, parts of the iliac bone can be analyzed. Here, it is necessary to pay attention to the spur sign. The anatomical basis of the spur sign is a fragment of the iliac bone that remained attached to the SI joint, whereas the fractured acetabular segment displaced medially (▶ Fig. 3.19). This sign is pathognomonic for the both-column fractures.4,10
Fig. 3.19 Spur sign, indicative of both-column fractures, superimposed on the pelvic AP view (dotted line) and optimally visualized on the OOV.
Clinical Relevance
The OOV allows analysis of the obturator foramen, the anterior column, and the posterior wall.
Value of Conventional Lines
Petrisor et al analyzed the interobserver reliability with respect to conventional imaging and analyzed the value of the characteristic lines.11
The consistency of different examiners was dependent on their individual experience with an overall good agreement. Using the pelvic AP view, best results were observed when assessing the posterior wall (fragment), the ilioischial, and iliopectineal line, whereas assessment of the acetabular roof line and the anterior wall line showed poor results. Analysis of the Judet views showed good reliability analyzing the column lines.
Considering the potential higher radiation dose using computed tomography (CT), the five pelvic and acetabular standard views had a higher effective dose to the patient, whereas the CT had better organ doses, except for the ovarial organ dose.12,13
Recently, the value of the Judet views was questioned.14 In an intra- and interobserver agreement study, three different image sets were analyzed. Compared to sets with two dimensional (2D)- and 3D-CT scan, the image set without oblique views showed best results with a kappa value of 0.60.14 Thus, additional Judet views do not seem to improve the reliability of fracture classification.
Clinical Relevance
Although Judet views seem to be of minor value for classification of acetabular fractures, their clear value is during intraoperative analysis of the reduction results. Thus, their understanding and interpretation is of major practical value.
3.3 Computed Tomography
CT of acetabular fractures is crucially important in providing detailed analysis of intraarticular lesions.15,16,17,18,19,20,21,22,23
The axial CT allows safe identification of intraarticular fragments, marginal impaction zones, comminution zones, accompanying soft tissue injuries, and femoral head pathologies (▶ Fig. 3.20).
Fig. 3.20 Representation of intraarticular fragments, including osseous avulsion of the ligament of the femoral head, as well as articular comminution in the 2D-CT.
Adam et al detected 30% unrecognized intraarticular fragments compared to conventional X-rays using CT.15 St. Pierre et al and Vas et al found a similar high rate of overlooked fragments.22,23
These potential prognostic relevant modifiers in acetabular fractures are inadequately recognized on conventional X-rays.24 This problem is especially observed in pediatric fractures.25 Resnick et al recommended a mandatory CT examination26 for more precise classification in patients with acetabular fractures.
Clinical Relevance
The axial CT allows for accurate detection of the intraarticular pathology.
A substantial advantage of the axial CT is easier differentiation of certain fracture types. The knowledge of the typical fracture line in transverse fracture or in wall fractures is relevant for decision making.
A clear identification of typical fracture lines on the axial CT images in the assessment of acetabular fractures is essential.
3.3.1 Column Separation
The anterior column is seen anteriorly on the axial CT images (at the top of the image) and the posterior column is located posterior (at the bottom of the image; ▶ Fig. 3.21). The column separation is anatomically approximately in the middle of the acetabular fossa (▶ Fig. 3.22). For practical analysis:
Fig. 3.21 Separation of the anterior and posterior column on axial CT images.
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