RADIOGRAPHIC TECHNIQUE

The technique employed in cephalometric radiology has been standardized to permit the comparison of initial and subsequent films for the same patient so that growth can be assessed and treatment progress monitored.

This standardization requires that the equipment include a headholder (cephalostat) and an x-ray tube positioned at a distance of 60 inches from the mid sagittal plane of the subject and that the distance from the midsagittal plane of the patient to the film be approximately 7.5 inches (Fig. 26-2). The cephalostat maintains a reproducible spatial relationship with respect to the position of the patient’s head, the film, and the x-ray source. The most common device uses a counterbalanced beam with the radiographic tube on one end and the cephalostat on the other. This entire unit can be adjusted vertically to compensate for variations in patient height.

Figure 26-2 Wall-mounted, counterbalanced cephalometer.

(Courtesy Dr. William W. Merow.)

The patient is positioned in the cephalostat by means of laterally adjusted ear rods and a vertically adjusted nasal piece (Fig. 26-3). The nasal piece allows the clinician to orient the patient’s head so that the Frankfort horizontal plane (a plane extending from the tragus of the ear to the inferior border of the orbital rim) is parallel to the floor. The ear posts should be centrally aligned to the source of radiation so that a transporionic axis is established.

Figure 26-3 Patient positioned in the cephalostat.

LATERAL HEAD FILM

For a lateral head radiograph, the patient is first positioned so that the left side of the face is tangent to an 8- by 10-inch film cassette, which permits less magnification and less distortion of the left-sided structures (Fig. 26-4).

Figure 26-4 Lateral cephalometric film.

The film cassette should be positioned as close as possible to the patient to minimize the effects of magnification, maximize resolution, and standardize the technique. The distance from the film cassette to the patient’s midsagittal plane should be recorded to allow for comparison of serial films. Generally, the film is obtained with the mandible in its most retruded position and the lips in repose. Use of additional positions may be indicated. Once the patient has been positioned, the x-ray beam should enter through the ear rods perpendicular to the film.

Grids and intensifying screens are accessories used to improve the quality of the radiographic image. Rare-earth intensifying screens allow for a reduction of radiographic exposure while increasing the clarity of the radiographic image. Because the film range does not provide for sharp skeletal and soft tissue contrast, a movable aluminum screen attached to the cassette must be used over the soft tissue profile area to reduce the radiation and provide a better differential contrast between the two tissue types.

FRONTAL (POSTEROANTERIOR) FILM

Most diagnostic features related to vertical and anteroposterior (AP) problems are evident from the lateral film, though severe maxillary transverse deficiencies or facial asymmetries may be better diagnosed by the use of a posteroanterior (PA) film (Fig. 26-5). The patient is oriented facing the film cassette, with the ear rods and nasion piece positioning the patient so that the midsagittal and Frankfort planes are at right angles to the film cassette. After the patient’s head is positioned so that the central x-ray beam passes through the head at the level of the transporionic axis and at its midpoint, the film cassette is moved into contact with the patient’s nose. Because more radiation is required for this view, the milliamperage must be increased over that used in the lateral film technique.

Figure 26-5 Frontal (posteroanterior) cephalometric film.

(Courtesy Dr. William W. Merow.)

CEPHALOMETRIC TRACING TECHNIQUE

Precise localization of the anatomic landmarks used in cephalometric analysis requires adequate knowledge of the radiographic and anatomic appearance of the facial bones and their relationships to adjacent structures. Various features are discernible: lines, shadows, the projections of bony structures, and contours of varying density. All of these make it difficult for the clinician to interpret and identify the anatomic relationships. A clear understanding of craniofacial structures and their relative spatial relationships is imperative before a lateral head film is traced.

Fig. 26-6 depicts a lateral cephalometric tracing. The lateral tracing should include the soft tissue outline, bony profile, outline of the mandible, posterior and anterior cranial base, odontoid process of the axis, anterior lip of the foramen magnum, clivus, planum orbitale, sella turcica, orbit, pterygomaxillary fissure, floor of the nose, roof of the palate, and body of the hyoid bone. In addition to the bony tissues, at least the first permanent molars as well as the most anterior maxillary and mandibular incisors are commonly included. In certain situations it may be desirable to trace other teeth or the complete dentition as shown in Fig. 26-6.

Figure 26-6 Lateral cephalometric tracing.

To make the tracing, the radiograph is placed on a view box with the facial profile to the right side. Acetate tracing paper (0.003 matte) is then placed over the radiograph with the matte side up. With a sharp No. 2 or 3H drawing pencil, all the necessary structures are traced. Because all x-rays become divergent once they emanate from the collimator, magnification of the subject will result, and a double-image effect will occur along the inferior border of the mandible and the area of the posterior teeth. All paired structures will produce double images on the head films. Because left-sided structures are magnified less by the radiographic beam and are considered more accurately rendered, the outline of these structures can be traced, although some prefer to make the tracing lines bisect bilateral images.

A PA cephalometric radiograph, as illustrated in Fig. 26-5, can be of significant diagnostic value in cases demonstrating mandibular displacement, facial asymmetry, severe posterior crossbite, or other types of bony dysplasia. Cephalometric analysis and a thorough and systematic clinical examination of these patients often reveal malocclusions accompanied by mandibular shifts when the patient is in maximum occlusion.

The PA radiograph is traced in the same manner as the lateral film. Fig. 26-7 illustrates the important skeletal and dental structures that must be traced for an accurate and complete analysis.

Figure 26-7 Frontal (posteroanterior) cephalometric tracing (see also Fig. 26-10). ANS, Anterior nasal spine; I, I (incisor) point; LAG, left antegonial notch; LJ, left jugal process of maxillary tuberosity; LZF, left zygomaticofrontal suture; Me, menton; RAG, right antegonial notch; RJ, right jugal process of maxillary tuberosity; RZF, right zygomaticofrontal suture.

REFERENCE POINTS FOR LATERAL TRACING

The ultimate diagnostic value of the cephalometric analysis is dependent on the initial accurate identification and localization of anatomic and anthropologic points (Fig. 26-8). These landmarks are used to construct the lines, angles, and planes used to make a two-dimensional assessment of the patient’s craniofacial and dental relationships. Although each analysis is completed in two dimensions, when the lateral analysis and the PA analysis for the same patient are considered together, a threedimensional simulation emerges to contribute to the overall diagnosis and treatment plan. The following reference points are used in this chapter (see Fig. 26-8):

Figure 26-8 Lateral tracing with cephalometric reference points.

(Adapted from Dr. William W. Merow.)

REFERENCE LINES, ANGLES, AND PLANES

Linear assessment is derived when two reference points are connected. Angular measurements are possible when three points are used. Planes (and some lines) are actually imaginary when the cephalometric tracing is viewed because the planes are at right angles to the tracing and can be seen only as a line on the two-dimensional tracing (Fig. 26-9). In cephalometric analysis, the dentist must become accustomed to thinking in three dimensions while viewing a two-dimensional representation. Therefore a point on the tracing may not only be a point but also may represent a line (or axis). A line on the tracing may actually be a line (or axis) or it may represent a plane.

Figure 26-9 Cephalometric reference lines and planes.

(Adapted from Dr. William W. Merow.)

Several lines or planes are used in the different cephalometric analyses, although one line or plane generally serves as the major reference on which the entire analysis is based. Two common references are the sella-nasion plane (anterior cranial base) and the Frankfort horizontal plane.

The basic units of cephalometric analysis are angles and distances (lines). Measurements may be treated as absolute values, or they may be related to one another and expressed as relative proportions. These measurements and interrelationships provide the basic framework for describing craniofacial abnormalities. The following definitions help explain the planes of reference used in this chapter (see Fig. 26-9).

INTERPRETATION OF MEASUREMENTS

The objectives of cephalometric interpretation are summarized as follows:

LATERAL CEPHALOMETRIC ASSESSMENT