Imaging: Plain Radiographs

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CHAPTER 3 Imaging

Plain Radiographs

Moritz Tannast, Klaus A. Siebenrock

Introduction

Despite modern radiographic technologies such as computed tomography scanning and magnetic resonance imaging, conventional radiographic evaluation of the hip and the bony pelvis remains the fastest and most inexpensive imaging modality in daily clinical practice. Every orthopedic surgeon should be familiar with basic imaging principles, including patient positioning and radiographic projections. This chapter does not provide a complete overview of all previously described projections around the hip and pelvis. Rather, it focuses on recently described observations and the technical pitfalls of conventional radiographs in the context of the pathomorphologies associated with early osteoarthritis of the hip, particularly femoroacetabular impingement (FAI) and developmental dysplasia of the hip (DDH).

During the initial stage of early hip osteoarthritis, conventional pelvic radiographs typically do not display features of reactive degenerative changes, such as subchondral sclerosis, cysts, joint space narrowing, or osteophytes. Therefore, it is important to recognize typical prearthritic hip pathomorphologies on conventional radiographs before radiographically evident joint degeneration develops.

Basic science

To understand the projected anatomy of the hip and the potential technical pitfalls related to it, it is essential to know the basic geometric radiographic principles. Conventional radiography is based on a point-shaped radiation source. In contrast with computed tomography scanning and magnetic resonance imaging, the image shows a distorted projection of the real anatomy. This conical projection implies that objects lying closer to the x-ray source necessarily will be projected more laterally (Figure 3-1).

Figure 3–1 The effect of the conical projection in conventional radiographs as a result of the point-shaped radiation source is shown. Left, X-ray beams lead to a distortion of the image; points that lie closer to the x-ray source (the blue point on the anterior acetabular rim) are projected more laterally than points that are closer to the x-ray plate (the red point on the posterior acetabular rim). Right, With computed tomography (CT) scanning and magnetic resonance imaging (MRI), no distortion is present.

To obtain reproducible x-ray projections, it is mandatory that the film-focus distance is standardized. A radiograph with a very small film-focus distance can lead to a magnification of anatomic structures that are lying closer to the x-ray beam and vice versa (Figure 3-2).

Figure 3–2 Demonstration of how the projected acetabular morphology depends on the film-focus distance. Left, A decreased distance (here, 80 cm) leads to magnification of the anterior rim (blue), with the appearance of a more pronounced retroversion sign. Right, An increased distance (here, 140 cm) leads to a less-pronounced retroversion sign. AW, anterior wall; PW, posterior wall.

In addition, the proper centering of the x-ray beam is crucial. Two radiographs of the same patient taken with different beam centers can reveal a substantially different radiographic joint anatomy of the hip; this can typically be seen when radiographs of the entire pelvis are compared with radiographs of an individual hip (Figure 3-3).

Figure 3–3 The influence of the position of the x-ray beam on the projected acetabular morphology is shown. Left, A normal acetabular morphology is seen, with the x-ray beam centered on the entire pelvis. Right, A coxa profunda with an increased acetabular anteversion can be created if the x-ray beam is centered over the hip. Yellow, Ilioischial line; green, acetabular fossa; blue, anterior acetabular rim; red, posterior acetabular rim.

Indications

Imaging is fundamental for the screening, diagnosis, treatment, and follow up of patients with hip disorders. Basically, standard, routine, conventional radiographs should be obtained initially for evaluation before additional imaging modalities are considered.

Young patients with painful hip disorders often present with groin pain of slow onset, which may optionally first be noticed after a minor trauma incident. The pain is typically intermittent, and it may be exacerbated by excessive demand on the hip (e.g., athletic activities) or by normal activities of daily living (e.g., walking). The pain may also present after sitting for a prolonged period. Depending on the underlying pathology, physical examination can reveal an increased (DDH) or decreased (FAI) range of motion. A positive anterior impingement sign can be present with either DDH or FAI. This condition is characterized by painful internal rotation in 90 degrees of flexion, and it represents a clinically reproducible sign for the detection of labral pathologies.

Radiographic technique for specific projections

Standard conventional radiographic imaging for the detection of hip pathologies includes at least two radiographs: an anteroposterior (AP) pelvic radiograph and an axial cross-table view of the proximal femur. Optionally, depending on the indication and the findings of these two x-rays, a false profile and an abduction view can also be obtained. In general, gonadal shielding is not recommended, at least for the initial diagnostic analysis, because it can potentially hide important anatomic landmarks for quantifying pelvic tilt and rotation; these landmarks will be described later in this chapter.

Anteroposterior Pelvic Radiograph

To correctly analyze hip joint morphology, an entire pelvic view—not only imaging of the affected hip—is mandatory. The AP pelvic radiograph is taken as a standard in our department, with the patient lying supine (Figure 3-4, A). This can be directly compared with intraoperative AP pelvic radiographs while the patient is under general anesthesia or with follow up radiographs during the early rehabilitation period. The legs have to be internally rotated to adjust for femoral antetorsion, and the film-focus distance should be 1.2 m. The central beam should be directed to the midpoint between the superior border of the symphysis and a line that connects both of the anterior superior iliac spines; these landmarks can easily and reproducibly be palpated by the radiology technician. It is extremely important for the correct interpretation of the radiographs that all radiographs are obtained with the same protocol.

Figure 3–4 The technique of the basic radiographic projections of the hip is shown. A, The anteroposterior pelvic radiograph is taken with the patient in a supine position. The central beam is directed to the midpoint between the pubic symphysis and a line that connects the anterior superior iliac spines. B, The axial cross-table view of the proximal femur is taken with a 45-degree angle and the contralateral hip flexed. C, The false profile of the hip is taken with the patient standing. The index hip is in contact with the radiographic table, the patient is rotated 25 degrees backward, and the long axis of the foot remains parallel with the x-ray table.

Axial Cross-Table View of the Proximal Femur

The axial cross-table view of the proximal femur visualizes the anterior and posterior contour of the femoral head–neck junction, which cannot be seen on the AP pelvic radiograph. This image is taken with the patient in the supine position with the contralateral hip flexed and the ipsilateral leg internally rotated (Figure 3-4, B). Again, the film-focus distance is 1.2 m, and the central beam is directed to the inguinal fold with an angle of 45 degrees. As an alternative to this view, a Dunn-Rippstein view or a frog-leg lateral radiograph may be obtained.

False Profile

The false profile of the hip is taken with the patient standing (Figure 3-4, C). The index hip is in contact with the radiographic table, and the patient is rotated 25 degrees backward while the long axis of the foot remains parallel with the x-ray table. This view is used to quantify anterior acetabular coverage, the narrowing of the joint space posteroinferiorly or anterosuperiorly, and the decentration of the femoral head into the defect.