CHAPTER 26 Cephalometrics and Facial Esthetics: The Key to Complete Treatment Planning
In studying a case of malocclusion, give no thought to the methods of treatment or appliances until the case shall have been classified and all peculiarities and variations from the normal in type, occlusion, and facial lines have been thoroughly comprehended. Then the requirements and proper plan of treatment become apparent.
Beauty and harmony are the traditional guiding principles used to assess facial proportions, although the definition of beauty may change as civilizations change. Greek sculpture during the golden age of art (fourth century bc) shows facial proportions very similar to those found desirable today. Basic facial features of Greek male and female figures appear to be depicted identically, with most sculpture angles within 5 degrees of contemporary standards; the exceptions are a more acute mentolabial sulcus and nasofacial angle for the ancient Greek ideal.
In the early twentieth century, dentistry began to include the concepts of facial harmony and balance in the theory and practice of cephalometrics. In 1922, Simon introduced this modern era with the development of gnathostatics, a photographic technique that related the teeth and their respective bony bases to each other, as well as to specific craniofacial structures. Although Racini and Carrera obtained the first x-ray films of the skull by the standard lateral view in 1926, it was not until the introduction of the cephalometer by Broadbent in 1931 that the science of cephalometrics became standardized. This sophisticated form of radiography enabled the practitioner to identify specific problem areas of craniofacial disproportion and devise detailed therapeutic interventions. Through the contributions of investigators such as Brodie, Downs, Reidel, Steiner, Tweed, and Ricketts, the clinical application of cephalometrics has developed the techniques that permit the observation of discrepancies observed in the mandible, maxilla, dental units, and soft tissue profile.
The primary aim of cephalometric analysis is to localize malocclusion within a tracing of facial bone and soft tissue structures. The analysis is performed by using standardized cephalometric landmarks to construct lines, angles, and imaginary planes, which permits linear and angular assessments of dental and facial relationships as seen on radiographic films of the head and face. These findings are compared with established normal values, and an individualized treatment protocol is developed for orthopedic, orthodontic, and orthognathic therapies.
The science of cephalometrics has often been referred to as a “numbers game” and has the reputation of being difficult to master. There appears to be a universal search for a reliable group of numbers that will ultimately lead one to an accurate diagnosis. Such a search is futile, because all cephalometric measurements may at times lead one to an erroneous conclusion. However, an accurate, in-depth analysis provides one with an assessment of dentofacial and craniofacial morphology. A cephalometric radiograph furnishes one with a static analysis, whereas subsequent films allow the clinician to follow the growth patterns of the adolescent patient on a longitudinal basis. In addition, comparison of serial cephalograms of the same patient may allow some developmental predictions to be made.
The use of cephalometrics serves to confirm the diagnosis and makes it possible to include the morphology of the cranium when alternative treatment modalities are considered. In patient care, cephalometrics can provide valuable data when treatment is first initiated and can serve a monitoring function during the course of orthodontic care. On completion of treatment, cephalometric radiology allows assessment of the relative degree of posttreatment stability and evaluation of treatment results produced by various mechanical and appliance selections.
Cephalometric numbers or central tendencies have been developed to serve as guidelines in evaluation of the patient. Dentists must keep in mind that they are treating individuals, not averages, and that the numbers merely help or guide in the formulation of an accurate diagnosis and treatment plan. Because of individual anatomic, biologic, and environmental variations, it is imperative that the clinician consider several factors to achieve a comprehensive case analysis. Any attempt to simplify the analysis is likely to lead to an erroneous conclusion.
The norm is commonly referred to as the mean or average. On the contrary, however, the norm, as it is applied in cephalometrics, is not a set of averages. The average patient in any given population will generally deviate from the norm, because the norm is derived from samples demonstrating ideal dental occlusions of the class I variety.
Most biologic variables are randomly distributed in the population and can be graphically illustrated by a bell-shaped curve (Fig. 26-1). Within this curve, approximately 70% of any given population lies within 1 standard deviation of the mean, whereas 95% of the group falls within 2 standard deviations. Throughout this chapter, the statistical concept of standard deviation is referred to as clinical deviation (CD).
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.
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.
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).
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.
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.
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.
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.
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):
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.
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).