Orthodontic Diagnosis

Assessment of Developmental Age: In a step particularly important for children around the age of puberty when most orthodontic treatment is carried out, the patient’s developmental age should be assessed. Everyone becomes a more or less accurate judge of other people’s ages—we expect to come within a year or two simply by observing the other person’s facial appearance. Occasionally, we are fooled, as when we say that a 12-year-old girl looks 15 or that a 15-year-old boy looks 12. With adolescents, the judgment is of physical maturity.

The attainment of recognizable secondary sexual characteristics for girls and boys and the correlation between stages of sexual maturation and facial growth are discussed in Chapter 4 and are summarized in Table 6-2. The degree of physical development is much more important than chronologic age in determining how much growth remains.

Facial Esthetics Versus Facial Proportions: Because a major reason for orthodontic treatment is to overcome psychosocial difficulties related to facial and dental appearance and enhance social well-being and QOL in doing so, evaluating dental and facial esthetics is an important part of the clinical examination. Whether a face is considered beautiful is greatly affected by cultural and ethnic factors, but whatever the culture, a severely disproportionate face becomes a psychosocial problem. For that reason, it helps to recast the purpose of this part of the clinical evaluation as an evaluation of facial proportions not esthetics per se. Distorted and asymmetric facial features are a major contributor to facial esthetic problems, whereas proportionate features are generally acceptable even if not beautiful. An appropriate goal for the facial examination therefore is to detect disproportions.

Frontal Examination: The first step in analyzing facial proportions is to examine the face in frontal view. Low-set ears or eyes that are unusually far apart (hypertelorism) may indicate either the presence of a syndrome or a microform of a craniofacial anomaly. If a syndrome is suspected, the patient’s hands should be examined for syndactyly, since there are a number of dental-digital syndromes.

In the frontal view, one looks for bilateral symmetry in the fifths of the face and for proportionality of the widths of the eyes/nose/mouth (Figure 6-8). A small degree of bilateral facial asymmetry exists in essentially all normal individuals. This can be appreciated most readily by comparing the real full-face photograph with composites consisting of two right or two left sides (Figure 6-9). This “normal asymmetry,” which usually results from a small size difference between the two sides, should be distinguished from a chin or nose that deviates to one side, which can produce severe disproportion and esthetic problems (see Figure 6-3).

Prior to the advent of cephalometric radiography, dentists and orthodontists often used anthropometric measurements (i.e., measurements made directly during the clinical examination) to help establish facial proportions (Figure 6-10). Although this was largely replaced by cephalometric analysis for many years, the recent emphasis on soft tissue proportions has brought soft tissue evaluation back into prominence. Farkas’ modern studies of Canadians of northern European origin provided the data for Tables 6-3 and 6-4.⁵

Note that some of the measurements in Table 6-3 could be made on a cephalometric radiograph, but many could not. When there are questions about facial proportions, it is much better to make the measurements clinically because soft-tissue proportions as seen clinically determine facial appearance. During the clinical examination, one can record measurements and literally digitize the face rather than later digitizing a cephalometric radiograph.

The proportional relationship of facial height to width (the facial index) establishes the overall facial type and the basic proportions of the face. It is important to remember that face height cannot be evaluated unless face width is known, and face width is not taken into account when a lateral cephalometric radiograph is analyzed.

The normal values for the facial index and other proportions that may be clinically useful are shown in Table 6-4. Differences in facial types and body types obviously must be taken into account when facial proportions are assessed, and variations from the average ratios can be compatible with good facial esthetics. An important point, however, is to avoid treatment that would change the ratios in the wrong direction, for example, treatment with interarch elastics that could rotate the mandible downward in a patient whose face already is too long for its width.

Finally, the face in frontal view should be examined from the perspective of the vertical facial thirds. The artists of the Renaissance period, primarily da Vinci and Durer, established the proportions for drawing anatomically correct human faces (Figure 6-11). They concluded that the distance from the hairline to the base of the nose, base of nose to bottom of nose, and bottom of nose to chin should be the same. Farkas’ studies show that in modern Caucasians of European descent, the lower third is very slightly longer. The artists also saw that the lower third has a proportion of one-third above the mouth to two-thirds below, and the Farkas data show that this is still true.

It is important to note the cause of vertical problems such as excessive display of the maxillary gingiva, which is done best by examining the position of lips and teeth relative to the vertical thirds of the face (Figure 6-12). It also is important to keep in mind that different ethnic and national groups view facial esthetics somewhat differently (there are differences even in countries as closely matched as the United States and Canada) and that both gender and overall facial attractiveness influence how people are perceived. As the examining doctor, you need to notice and evaluate disproportions, even though you know that as treatment is planned, aspects of facial appearance that would be a problem for some individuals are not a problem for others with a different ethnic background.

Dentofacial characteristics that should be noted as part of the facial examination are shown in Box 6-2. This checklist is just that: a list of things that should be noted systematically during the clinical examination. As in many other things, if you do not look for it, you will not see it. Precise measurements are not necessary, but deviations from the normal should be taken into account when the problem list is developed. Current computer programs already make it possible for an assistant to quickly enter positive findings as the doctor reviews them and have them “flow through” to the preliminary problem list.

Profile Analysis: A careful examination of the facial profile yields the same information, though in less detail for the underlying skeletal relationships, as that obtained from analysis of lateral cephalometric radiographs. For diagnostic purposes, particularly to identify patients with severe disproportions, careful clinical evaluation is adequate. For this reason, the technique of facial profile analysis has sometimes been called the “poor man’s cephalometric analysis.” This is a vital diagnostic technique for all dentists. It must be mastered by all those who will see patients for primary care in dentistry, not just by orthodontists.

The three goals of facial profile analysis are approached in three clear and distinct steps. These goals are as follows:

1. Establishing whether the jaws are proportionately positioned in the anteroposterior plane of space. This step requires placing the patient in the physiologic natural head position, which is the head position the individual adopts in the absence of other cues. This can be done with the patient either sitting upright or standing but not reclining in a dental chair and looking at the horizon or a distant object. With the head in this position, note the relationship between two lines, one dropped from the bridge of the nose to the base of the upper lip, and a second one extending from that point downward to the chin (Figure 6-13). These line segments ideally should form a nearly straight line, with only a slight inclination in either direction. A large angle between them (>10 degrees or so) indicates either profile convexity (upper jaw prominent relative to chin) or profile concavity (upper jaw behind chin). A convex profile therefore indicates a skeletal Class II jaw relationship, whereas a concave profile indicates a skeletal Class III jaw relationship.

2. Evaluation of lip posture and incisor prominence. Detecting excessive incisor protrusion (which is relatively common) or retrusion (which is rare) is important because of the effect on space within the dental arches. If the incisors protrude, they align themselves on the arc of a larger circle as they lean forward, whereas if the incisors are upright or retrusive, less space is available (Figure 6-14). In the extreme case, incisor protrusion can produce ideal alignment of the teeth instead of severely crowded incisors, at the expense of lips that protrude and are difficult to bring into function over the protruding teeth. This is bimaxillary dentoalveolar protrusion, meaning simply that in both jaws the teeth protrude (Figure 6-15). Dentists often refer to the condition as just bimaxillary protrusion, a simpler term but a misnomer since it is not the jaws but the teeth that protrude. Physical anthropologists use bimaxillary protrusion to describe faces in which both jaws are prominent relative to the cranium, and the different terminology must be kept in mind when faces are described in the anthropology literature.

Determining how much incisor prominence is too much can be difficult, especially when changes over time in public preference for both lip and chin prominence are taken into account⁶ and ethnic differences are considered. This is simplified by understanding the relationship between lip posture and the position of the incisors. The teeth protrude excessively if (and only if) two conditions are met: (1) the lips are prominent and everted and (2) the lips are separated at rest by more than 3 to 4 mm (which is sometimes termed lip incompetence). In other words, excessive protrusion of the incisors is revealed by prominent lips that are separated when they are relaxed, so that the patient must strain to bring the lips together over the protruding teeth (see Figure 6-15). For such a patient, retracting the teeth tends to improve both lip function and facial esthetics. On the other hand, if the lips are prominent but close over the teeth without strain, the lip posture is largely independent of tooth position. For that individual, retracting the incisor teeth would have little effect on lip function or prominence.

Lip prominence is strongly influenced by racial and ethnic characteristics and to a considerable extent also is age-dependent (see Chapter 2). Whites of northern European backgrounds often have relatively thin lips, with minimal lip and incisor prominence. Whites of southern European and middle eastern origin normally have more lip and incisor prominence than their northern cousins. Greater degrees of lip and incisor prominence normally occur in individuals of Asian and African descent, so a lip and tooth position normal for Asians or blacks would be excessively protrusive for most whites.

Lip posture and incisor prominence should be evaluated by viewing the profile with the patient’s lips relaxed. This is done by relating the upper lip to a true vertical line passing through the concavity at the base of the upper lip (soft tissue point A) and by relating the lower lip to a similar true vertical line through the concavity between the lower lip and chin (soft tissue point B; Figure 6-16). If the lip is significantly forward from this line, it can be judged to be prominent; if the lip falls behind the line, it is retrusive. If the lips are both prominent and incompetent (separated by more than 3 to 4 mm), the guideline is that the anterior teeth are excessively protrusive. Is that a problem? It depends on both the patient’s perception and the cultural setting, not just on the objective evaluation.

In evaluating lip protrusion, it is important to keep in mind that everything is relative, and in this case the lip relationships with the nose and chin affect the perception of lip fullness. The larger the nose, the more prominent the chin must be to balance it, and the greater the amount of lip prominence that will be esthetically acceptable. It can be helpful to look at lip prominence relative to a line from the tip of the nose to the chin (the E-line of cephalometric analysis, which can be visualized easily on clinical examination; Figure 6-17). Vertical facial and dental relationships also play a role here. Some patients with short lower face height have everted and protrusive lips because they are overclosed and the upper lip presses against the lower lip, not because the teeth protrude.

Not only the prominence of the chin but also the submental soft tissue contours should be evaluated. Throat form is an important factor in establishing optimal facial esthetics, and poor throat form is a major contributor to esthetic impairment in patients with mandibular deficiency (Figure 6-18).

3. Reevaluation of vertical facial proportions and evaluation of mandibular plane angle. Vertical proportions can be observed during the full-face examination (see previous section) but sometimes can be seen more clearly in profile. In the clinical examination, the inclination of the mandibular plane to the true horizontal should be noted. The mandibular plane is visualized readily by placing a finger or mirror handle along the lower border (Figure 6-19). A steep mandibular plane angle usually accompanies long anterior facial vertical dimensions and a skeletal open bite tendency, while a flat mandibular plane angle often correlates with short anterior facial height and deep bite malocclusion.

Facial form analysis carried out this way takes only a couple of minutes but provides information that simply is not present from dental radiographs and casts. Such an evaluation by the primary care practitioner is an essential part of the evaluation of every prospective orthodontic patient.

Tooth–Lip Relationships: Evaluation of tooth–lip relationships begins with an examination of symmetry, in which it is particularly important to note the relationship of the dental midline of each arch to the skeletal midline of that jaw (i.e., the lower incisor midline relative to the midline of the mandible, and the upper incisor midline relative to the midline of the maxilla). Dental casts, even if mounted on an articulator, will show the relationship of the midlines to each other but provide no information about the dental-skeletal midlines. This must be recorded during the clinical examination.

A second aspect of dental to soft tissue relationships is the vertical relationship of the teeth to the lips at rest and on smile. During the clinical examination, it is important to note the amount of incisor display. For patients with excessive incisor display, the usual cause is a long lower third of the face, but that is not the only possibility—a short upper lip could produce the same thing (see Figure 6-12). Recording lip height at the philtrum and the commissures can clarify the source of the problem.

A third important relationship to note is whether an up-down transverse rotation of the dentition is revealed when the patient smiles or the lips are separated at rest (Figure 6-20). This often is called a transverse cant of the occlusal plane but is better described as a transverse roll of the esthetic line of the dentition (see the section in this chapter on classification by dentofacial traits). Neither dental casts nor a photograph with lip retractors will reveal this. Dentists detect a transverse roll at 1 mm from side to side, whereas laypersons are more forgiving and see it at 2 to 3 mm—but at that point, it is a problem.⁷

Smile Analysis: Facial attractiveness is defined more by the smile than by soft tissue relationships at rest. For this reason, it is important to analyze the characteristics of the smile and to think about how the dentition relates to the facial soft tissues dynamically, as well as statically. There are two types of smiles: the posed or social smile and the enjoyment smile (also called the Duchenne smile in the research literature). The social smile is reasonably reproducible and is the one that is presented to the world routinely. The enjoyment smile varies with the emotion being displayed (for instance, the smile when you are introduced to a new colleague differs from the smile when your team just won the year’s most important game). The social smile is the focus of orthodontic diagnosis.

In smile analysis, the oblique view and the frontal and profile views are important. The following variables need to be considered along with the viewing perspective (Box 6-3).

Amount of Incisor and Gingival Display: Using computer-altered photographs, recent research has established a range of acceptability for incisor and gingival display (Figure 6-21).⁸ Although some display of gingiva is acceptable and can be both esthetic and youthful appearing, the ideal elevation of the lip on smile for adolescents is slightly below the gingival margin, so that most of the upper incisor can be seen. More importantly, up to 4 mm display of gingiva in addition to the crown of the tooth, or up to 4 mm lip coverage of the incisor crown, is acceptable. Beyond that, the smile appearance is less attractive.

It also is important to remember that the vertical relationship of the lip to the incisors will change over time, with the amount of incisor exposure decreasing (see Chapter 4).⁹ This makes it even more important to note the vertical tooth–lip relationships during the diagnostic evaluation and to keep it in mind during treatment.

Transverse Dimensions of the Smile Relative to the Upper Arch: Depending on the facial index (i.e., the width of the face relative to its height), a broad smile may be more attractive than a narrow one—but what does that mean exactly? A dimension of interest to prosthodontists, and more recently to orthodontists, is the amount of buccal corridor that is displayed on smile, that is, the distance between the maxillary posterior teeth (especially the premolars) and the inside of the cheek (Figure 6-22). Prosthodontists consider excessively wide buccal corridors (sometimes called “negative space”) to be unesthetic, and orthodontists have noted that widening the maxillary arch can improve the appearance of the smile if cheek drape is significantly wider than the dental arch. Although minimal buccal corridors are favored by most observers, especially in females,¹⁰ the transverse width of the dental arches can and should be related to the width of the face (Figure 6-23). Too broad an upper arch, so that there is no buccal corridor, is unesthetic. The relationship of the cheeks to the posterior teeth on smile is just another way of evaluating the width of the dental arches.

The Smile Arc: The smile arc is defined as the contour of the incisal edges of the maxillary anterior teeth relative to the curvature of the lower lip during a social smile (Figure 6-24). For best appearance, the contour of these teeth should match that of the lower lip. If the lip and dental contours match, they are said to be consonant.

A flattened (nonconsonant) smile arc can pose either or both of two problems: it is less attractive and tends to make you look older (because older individuals often have wear of the incisors that tends to flatten the arc of the teeth). The characteristics of the smile arc must be monitored during orthodontic treatment because it is surprisingly easy to flatten it in the pursuit of other treatment objectives. The data indicate that the most important factor in smile esthetics, the only one that by itself can change the rating of a smile from acceptable to unesthetic, is the smile arc.⁸

It is important to keep in mind that these features of the smile are viewed differently by patients when the full face is the context (i.e., they are looking into a large mirror mounted so that they can see their whole face) instead of just seeing their lips and teeth (in a hand-held mirror that shows only part of the face). With the full-face view, the smile arc is judged most attractive when the upper incisal edges and canines parallel the curvature of the lower lip. The preferred buccal corridors are small, significantly smaller than when judged using the smaller mirror. A transverse cant of the occlusal plane is less tolerated in the full-face view, but more upper to lower midline discrepancy is acceptable.^11,12 When patients have complaints about these specific smile components, it is best to have them point out what concerns them while they are looking into a large mirror that lets them see their entire face—just as others will view them in real life encounters.

Because facial attractiveness and gender do make a difference for some of these features, Box 6-4 shows a range of acceptability for characteristics in which this is important. Although there are modest differences between ethnic groups¹³ and nationalities (even Canadians and Americans)¹⁴ in their judgment of smile esthetics, the safe ranges had some commonality for groups that were predominantly of European descent. Similar data for Asian and African groups do not exist at present.

Although smile arc, gingival display, buccal corridor, and upper midline to the face all are viewed significantly differently against a full-face background, the features described below that constitute micro-esthetics are unaffected by the size of the view. Patients can view these characteristics close-up in a hand mirror or in a full-face view and make similar judgments. The facial context and attractiveness make little difference, and there is no gender difference.

Tooth Proportions: The smile, of course, reveals the maxillary anterior teeth, and two aspects of proportional relationships are important components of their appearance: the tooth widths in relation to each other and the height–width proportions of the individual teeth.

Width Relationships and the “Golden Proportion.”: The apparent widths of the maxillary anterior teeth on smile, and their actual mesiodistal width, differ because of the curvature of the dental arch. In particular, only a portion of the canine crown can be seen in a frontal view. For best appearance, the apparent width of the lateral incisor (as one would perceive it from a direct frontal examination) should be 62% of the width of the central incisor, the apparent width of the canine should be 62% of that of the lateral incisor, and the apparent width of the first premolar should be 62% of that of the canine (Figure 6-25). This ratio of recurring 62% proportions appears in a number of other relationships in human anatomy and sometimes is referred to as the “golden proportion.” Whether it has any mystical significance or not, it is an excellent guideline when lateral incisors are disproportionately small or (less frequently) large, and the width ratios of the central and lateral are the best way to determine what the posttreatment size of the lateral incisor should be. The same judgment is used when canines are narrowed to replace missing lateral incisors.

Height–Width Relationships: The range in height–width relationships for maxillary central incisors is shown in Figure 6-26. Note that the width of the tooth should be about 80% of its height. In examining an orthodontic patient, it is important to note both height and width because if disproportions are noted, this allows a determination of which is at fault. The central incisor seen in Figure 6-26, B, looks almost square. Its width measures 8.7 mm and its height 8.5 mm. From the table, the 8 mm width is in the middle of the normal range, and the height is short. There are several possible causes: incomplete eruption in a child, which may correct itself with further development; loss of crown height from attrition in an older patient, which may indicate restoration of the missing part of the crown; excessive gingival height, which is best treated with crown lengthening; or perhaps an inherent distortion in crown form, which suggests a more extensive restoration with facial laminates or a complete crown (see Chapter 18). The disproportion and its probable cause should be included in the patient’s problem list to focus attention on doing something about it before orthodontic treatment is completed.

Gingival Heights, Shape, and Contour: Proportional gingival heights are needed to produce a normal and attractive dental appearance (see Figure 6-24). Generally, the central incisor has the highest gingival level, the lateral incisor is approximately 1.5 mm lower, and the canine gingival margin again is at the level of the central incisor. Maintaining these gingival relationships becomes particularly important when canines are used to replace missing lateral incisors or when other tooth substitutions are planned. Both laypersons and dentists readily recognize differences of more than 2 mm.

Gingival shape refers to the curvature of the gingiva at the margin of the tooth. For best appearance, the gingival shape of the maxillary lateral incisors should be a symmetric half-oval or half-circle. The maxillary centrals and canines should exhibit a gingival shape that is more elliptical and oriented distally to the long axis of the tooth (Figure 6-27). The gingival zenith (the most apical point of the gingival tissue) should be located distal to the longitudinal axis of the maxillary centrals and canines, while the gingival zenith of the maxillary laterals should coincide with their longitudinal axis.

Connectors and Embrasures: These elements, illustrated in Figure 6-28, also can be of real significance in the appearance of the smile and should be noted as problems if they are incorrect. The connector (also referred to as the interdental contact area) is where adjacent teeth appear to touch and may extend apically or occlusally from the actual contact point. In other words, the actual contact point is likely to be a very small area, and the connector includes both the contact point and the areas above and below that are so close together they look as if they are touching. The normal connector height is greatest between the central incisors and diminishes from the centrals to the posterior teeth, moving apically in a progression from the central incisors to the premolars and molars. The embrasures (the triangular spaces incisal and gingival to the contact) ideally are larger in size than the connectors, and the gingival embrasures are filled by the interdental papillae.

Embrasures: Black Triangles: Short interdental papillae leave an open gingival embrasure above the connectors, and these “black triangles” can detract significantly from the appearance of the teeth on smile. Black triangles in adults usually arise from loss of gingival tissue related to periodontal disease, but when crowded and rotated maxillary incisors are corrected orthodontically in adults, the connector moves incisally and black triangles may appear, especially if severe crowding was present (Figure 6-29). For that reason, both actual and potential black triangles should be noted during the orthodontic examination, and the patient should be prepared for reshaping of the teeth to minimize this esthetic problem.

Tooth Shade and Color: The color and shade of the teeth changes with increasing age, and many patients perceive this as a problem. The teeth appear lighter and brighter at a younger age and darker and duller as aging progresses. This is related to the formation of secondary dentin as pulp chambers decrease in size and to thinning of the facial enamel, which results in a decrease in its translucency and a greater contribution of the darker underlying dentin to the shade of the tooth. A normal progression of shade change from the midline posteriorly is an important contributor to an attractive and natural appearing smile. The maxillary central incisors tend to be the brightest in the smile, the lateral incisors less so, and the canines the least bright. The first and second premolars are more closely matched to the lateral incisors. They are lighter and brighter than the canines.

At present, even young patients are quite likely to be aware of the possibility of bleaching their teeth to provide a more youthful appearance and may benefit from having this done at the end of orthodontic treatment. If color and shade of the teeth are a potential problem, this should be on the orthodontic problem list so that it is included in the final treatment plan if the patient desires it.

Physical versus Virtual Casts: Whether physical or virtual orthodontic diagnostic casts are to be produced, an impression of the teeth that also gives maximum displacement of the lips and cheeks is desired. Being able to visualize the inclination of the teeth, not just the location of the crown, is important. If the impression is not well extended, important diagnostic information may be missing. If the impressions are to be poured in dental stone without great delay, alginate impressions are satisfactory; if virtual models will be produced, a more accurate and stable impression material (such as modified alginate or polysiloxane) should be used.

At the minimum, a wax bite or polysiloxane record of the patient’s usual interdigitation (maximum intercuspation) should be made, and a check should be made to be sure that this does not differ significantly from the initial contact position. An anterior shift of 1 to 2 mm from the retruded position is of little consequence unless it creates a pseudo–Class III relationship, but lateral shifts or anterior shifts of greater magnitude should be noted carefully and a bite registration in an approximate centric relation position should be made.

Dental casts for orthodontic purposes are usually trimmed so that the bases are symmetric (Figure 6-31) and then are polished (or, if electronic records are used, the images are prepared to look like trimmed and polished casts). There are two reasons for doing this: (1) if the casts are viewed with a symmetric base that is oriented to the midline of the palate, it is much easier to analyze arch form and detect asymmetry within the dental arches; and (2) neatly trimmed and polished casts are more acceptable for presentation to the patient, as will be necessary during any consultation about orthodontic treatment. By convention, these trimmed and polished casts are then referred to as models. In specialty practice, virtual models are rapidly replacing physical models because they eliminate the need for storage space and can be used for computer-assisted fabrication of appliances.

There are two ways at present to generate digital casts: from laser scans of impressions or from scans of casts poured up from impressions. Despite repeated efforts over the last decade, a way to visualize the undercuts within impressions with laser beams still has not been worked out. Creating temporary casts and then scanning the casts works efficiently enough that several commercial companies now offer satisfactory digital models produced in this way.

The ideal way to generate digital casts would be from an intraoral laser scan, eliminating impressions that are unpleasant for patients, as well as requiring additional processing to obtain a physical or virtual model. As with scanning impressions, undercuts would create a problem if the scan were only from the occlusal so that a model created directly from an intraoral scan of the occlusal surface would only show the teeth down to their height of contour. Although that would not be acceptable as a diagnostic record, it is enough for fabrication of archwires by a computer-controlled wire-bending robot. This technology now is commercially available and is discussed in Chapter 10. An alternative would be the use of a wand for intraoral scanning to generate data of the undercut areas, and this probably will become the standard method as costs decrease.

Articulator Mounting: Whether it is desirable to mount casts on an adjustable articulator as part of an orthodontic diagnostic evaluation is a matter of continuing debate. There are three reasons for mounting casts on articulators. The first is to record and document any discrepancy between the occlusal relations at the initial contact of the teeth and the relations at the patient’s full or habitual occlusion. The second is to record the lateral and excursive paths of the mandible, documenting these and making the tooth relationships during excursions more accessible for study. A third, which increasingly is superseded by posteroanterior (P-A) cephalograms or CBCT images, is to display the orientation of the occlusal plane to the face.

Knowing the centric occlusal relationship when the condyles are positioned “correctly” obviously is important for orthodontic diagnostic purposes if there is a significant difference between it and the usual intercuspation. Unfortunately, there is no current agreement as to what the “correct” centric position is, though the “muscle-guided” position (the most superior position to which a patient can bring the mandible using his or her own musculature), seems most appropriate for orthodontic purposes. It is now generally accepted that in normal individuals this neuromuscular position is anterior to the most retruded condylar position. Lateral shifts or large anterior shifts are not normal and should be recorded. Articulator-mounted casts are one way, but not the only way, to do that.

The second reason for mounting casts—to record the excursive paths—is important when restorative dentistry is being planned because the contours of the replacement or restored teeth must accommodate the path of movement. This is much less important when tooth positions and jaw relationships will change during treatment.

The current consensus is that for preadolescent and early adolescent orthodontic patients (i.e., those who have not completed their adolescent growth spurt), there is little point in an articulator mounting. In these young patients, the contours of the TM joint are not fully developed, so that condylar guidance is much less prominent than in adults. The shape of the temporal fossa in an adult reflects function during growth. Thus, until mature canine function is reached and the chewing pattern changes from that of the child to the normal adult (see Chapter 3), completion of the articular eminence and the medial contours of the joint should not be expected. In addition, the relationships between the dentition and the joint that are recorded in articulator mountings change rapidly while skeletal growth is continuing and tend to be only of historic interest after orthodontic treatment.

The situation is different when growth is complete or largely complete. In adults with symptoms of TM dysfunction (clicking, limitation of motion, pain), articulator-mounted casts may be useful to document significant discrepancies between habitual and relaxed mandibular positions. These patients often need therapy to reduce muscle spasm and splinting before the articulator mounting is done. An articulator mounting may also be needed to plan orthodontic or surgical treatment of adults with significant cants of the occlusal plane or asymmetries.

Virtual Articulators: A virtual articulator certainly is a possibility. Although software used for surgical treatment planning already has produced something like it, the virtual articulator still is some distance from reality as this is written because it requires not only accurate dental casts but also an accurate way to relate them to each other and to the jaws.

To relate the virtual casts to each other, two approaches have been evaluated. The first is using the palatal rugae as a stable structure to which the casts can be related. This has the advantage of creating a landmark independent of the teeth that is quite close to them, and the disadvantage that the accuracy is questionable because the rugae are far from ideal as a stable reference. The second is to use three scans: the maxillary and mandibular casts separately and then a scan with the casts in occlusion, which shows only the facial surfaces (Figure 6-32). Although this method has been validated and can be considered as potentially better than using the rugae,¹⁶ its accuracy also is somewhat questionable because there is no external point as a reference for the articulated casts.

A more difficult task is to develop a way to relate the virtual casts to the facial skeleton. In theory one could use 3-D radiographs to develop a virtual facebow transfer that would provide a better relationship to the face than can be obtained with a physical facebow. In fact, this simply has not been developed yet but almost surely will be.

Choice of a Horizontal (Cranial) Reference Line: In any technique for cephalometric analysis, it is necessary to establish a reference area or reference line. The same problem was faced in the craniometric studies of the nineteenth century. By the late 1800s, skeletal remains of humans had been found at many locations and were under extensive study. An international congress of anatomists and physical anthropologists was held in Frankfort, Germany, in 1882, with the choice of a horizontal reference line for orientation of skulls an important item for the agenda. At the conference, the Frankfort plane, extending from the upper rim of the external auditory meatus (porion) to the inferior border of the orbital rim (orbitale), was adopted as the best representation of the natural orientation of the skull (Figure 6-43). This Frankfort plane was employed for orientation of the patient from the beginning of cephalometrics and remains commonly used for analysis.

In cephalometric use, however, the Frankfort plane suffers from two difficulties. The first is that both its anterior and posterior landmarks, particularly porion, can be difficult to locate reliably on a cephalometric radiograph. A radiopaque marker is placed on the rod that extends into the external auditory meatus as part of the cephalometric head positioning device, and the location of this marker, referred to as “machine porion” is often used to locate porion. The shadow of the auditory canal can be seen on cephalometric radiographs, usually located slightly above and posterior to machine porion. The upper edge of this canal can also be used to establish “anatomic porion,” which gives a slightly different (occasionally, quite different) Frankfort plane.

An alternative horizontal reference line, easily and reliably detected on cephalometric radiographs, is the line from sella turcica (S) to the junction between the nasal and frontal bones (N). In the average individual, the SN plane is oriented at 6 to 7 degrees upward anteriorly to the Frankfort plane. Another way to obtain a Frankfort line is simply to draw it at a specific inclination to SN, usually 6 degrees. However, although this increases reliability and reproducibility, it decreases accuracy.

The second problem with the Frankfort plane is more fundamental. It was chosen as the best anatomic indicator of the true or physiologic horizontal line. Everyone orients his or her head in a characteristic position, which is established physiologically, not anatomically. As the anatomists of a century ago deduced, for most patients the true horizontal line closely approximates the Frankfort plane. Some individuals, however, show significant differences, up to 10 degrees.

For their long-dead skulls, the anatomists had no choice but to use an anatomic indicator of the true horizontal. For living patients, however, it is possible to use a “true horizontal” line, established physiologically rather than anatomically, as the horizontal reference plane (Figure 6-44). This approach requires taking cephalometric radiographs with the patient in natural head position (i.e., with the patient holding his head level as determined by the internal physiologic mechanism). This position is obtained when relaxed individuals look at a distant object or into their own eyes in a mirror and incline their heads up and down in increasingly smaller movements until they feel comfortably positioned. The natural head position can be reproduced within 1 or 2 degrees.²³

In contemporary usage, cephalometric radiographs should be taken in the natural head position (NHP), so that the physiologic true horizontal plane is established. Although NHP is not as precisely reproducible as orienting the head to the Frankfort plane, the potential errors from lower reproducibility are smaller than those from inaccurate head orientation.²⁴ The inclination of SN to the true horizontal plane (or to the Frankfort plane if true horizontal plane is not known) should always be noted; if the inclination of SN differs significantly from 6 degrees, any measurements based on SN should be corrected by this difference.

Steiner Analysis: Developed and promoted by Cecil Steiner in the 1950s, Steiner analysis can be considered the first of the modern cephalometric analyses for two reasons: it displayed measurements in a way that emphasized not just the individual measurements but their interrelationship into a pattern, and it offered specific guides for use of cephalometric measurements in treatment planning. Elements of it remain useful today.

In a sense, Steiner analysis was based on evaluating the compensations necessary to compensate for the difference between SNA and SNB, which indicates the magnitude of the skeletal jaw discrepancy (Figure 6-45). To Steiner, this difference (the ANB angle) was the measurement of real interest. One can argue, as he did, that which jaw is at fault is of mostly theoretical interest: what really matters is the magnitude of the discrepancy between the jaws that must be overcome in treatment, and this is what the ANB angle measures.

Steiner then measured the angular and millimeter relationship of the upper incisor to the NA line and both the lower incisor and the chin to the NB line, thus establishing the relative protrusion of the dentition (Figure 6-46). The millimeter distance establishes how prominent the incisor is relative to its supporting bone, while the inclination indicates whether the tooth has been tipped to its position or has moved there bodily. The prominence of the chin (pogonion) compared with the prominence of the lower incisor establishes the balance between them: the more prominent the chin, the more prominent the incisor can be, and vice versa. This important relationship is often referred to as the Holdaway ratio. The final measurement included in Steiner analysis is the inclination of the mandibular plane to SN, which is its only indicator of the vertical proportions of the face (see Figure 6-45). Tabulated standard values for five racial groups are given in Table 6-8.

There were significant problems with Steiner analysis, however, that led to its replacement. First, its reliance on ANB is problematic. The ANB angle is influenced by two factors other than the anteroposterior difference in jaw position. One is the vertical height of the face. As the vertical distance between nasion and points A and B increases, the ANB angle will decrease. The second is that if the anteroposterior position of nasion is abnormal, the size of the angle will be affected. In addition, as SNA and SNB become larger and the jaws are more protrusive, even if their horizontal relationship is unchanged, it will be registered as a larger ANB angle. The validity of these criticisms has led to use of different indicators of jaw discrepancy in the later analyses presented in the following sections.

Second, it should not be overlooked that relying on tooth movement alone to correct skeletal malocclusion, particularly as the skeletal discrepancies become large, is not necessarily the best approach to orthodontic treatment. It is usually better to correct skeletal discrepancies at their source than to attempt only to achieve a dental compromise or camouflage. It is fair to say that the Steiner compromises reflect the prevailing attitude of Steiner’s era, that the effects of orthodontic treatment are almost entirely limited to the alveolar process.

Sassouni Analysis: The Sassouni analysis was the first to emphasize vertical, as well as horizontal, relationships and the interaction between vertical and horizontal proportions. Sassouni pointed out that the horizontal anatomic planes—the inclination of the anterior cranial base, Frankfort plane, palatal plane, occlusal plane, and mandibular plane—tend to converge toward a single point in well-proportioned faces. The inclination of these planes to each other reflects the vertical proportionality of the face (Figure 6-47).

If the planes intersect relatively close to the face and diverge quickly as they pass anteriorly, the facial proportions are long anteriorly and short posteriorly, which predisposes the individual to an open bite malocclusion. Sassouni coined the term skeletal open bite for this anatomic relationship. If the planes are nearly parallel so that they converge far behind the face and diverge only slowly as they pass anteriorly, there is a skeletal predisposition toward anterior deep bite, and the condition is termed skeletal deep bite.

In addition, an unusual inclination of one of the planes stands out because it misses the general area of intersection. Rotation of the maxilla down in back and up in front may contribute to skeletal open bite, for instance. The tipped palatal plane reveals this clearly (Figure 6-48).

Sassouni evaluated the anteroposterior position of the face and dentition by noting the relationship of various points to arcs drawn from the area of intersection of the planes. Unfortunately, as a face becomes more disproportionate, it is more and more difficult to establish the center for the arc, and the anteroposterior evaluation becomes more and more arbitrary.

Although the total arcial analysis described by Sassouni is no longer widely used, his analysis of vertical facial proportions has become an integral part of the overall analysis of a patient. In addition to any other measurements that might be made, it is valuable in any patient to analyze the divergence of the horizontal planes and to examine whether one of the planes is clearly disproportionate to the others.

Harvold and Wits Analyses: Both the Harvold and Wits analyses are aimed solely at describing the severity or degree of jaw disharmony. Harvold, using data derived from the Burlington growth study, developed standards for the “unit length” of the maxilla and mandible. The maxillary unit length is measured from the posterior border of the mandibular condyle to the anterior nasal spine, while the mandibular unit length is measured from the same point to the anterior point of the chin (Figure 6-49). The difference between these numbers provides an indication of the size discrepancy between the jaws. In analyzing the difference between maxillary and mandibular unit lengths, it must be kept in mind that the shorter the vertical distance between the maxilla and mandible, the more anteriorly the chin will be placed for any given unit difference, and vice versa. Harvold did quantify the lower face height to account for this factor. The position of the teeth has no influence on the Harvold figures (Table 6-9).

The Wits analysis was conceived primarily as a way to overcome the limitations of ANB as an indicator of jaw discrepancy. It is based on a projection of points A and B to the occlusal plane, along which the linear difference between these points is measured. If the anteroposterior position of the jaws is normal, the projections from points A and B will intersect the occlusal plane at very nearly the same point. The magnitude of a discrepancy in the Class II direction can be estimated by how many millimeters the point A projection is in front of the point B projection, and vice versa for Class III.

The Wits analysis, in contrast to the Harvold analysis, is influenced by the teeth both horizontally and vertically—horizontally because points A and B are somewhat influenced by the dentition and vertically because the occlusal plane is determined by the vertical position of the teeth. It is important for Wits analysis that the functional occlusal plane, drawn along the maximum intercuspation of the posterior teeth, be used rather than an occlusal plane influenced by the vertical position of the incisors. Even so, this approach fails to distinguish skeletal discrepancies from problems caused by displacement of the dentition, and it does not specify which jaw is at fault if there is a skeletal problem. If the Wits analysis is used, these limitations must be kept in mind.

McNamara Analysis: The McNamara analysis, originally published in 1983, combines elements of previous approaches (Ricketts and Harvold) with original measurements to attempt a more precise definition of jaw and tooth positions. In this method, both the anatomic Frankfort plane and the basion–nasion line are used as reference planes. The anteroposterior position of the maxilla and mandible are evaluated with regard to their position relative to the “nasion perpendicular,” a vertical line extending downward from nasion perpendicular to the Frankfort plane (Figure 6-50). The maxilla should be on or slightly ahead of this line, the mandible slightly behind. The second step in the procedure is a comparison of maxillary and mandibular length, using Harvold’s approach. The mandible is positioned in space utilizing the lower anterior face height (ANS-menton). The upper incisor is related to the maxilla using a line through point A perpendicular to the Frankfort plane, similar to but slightly different from Steiner’s relationship of the incisor to the NA line. The lower incisor is related as in the Ricketts analysis, primarily using the A-pogonion line (Figure 6-51).

The McNamara analysis has the following two major strengths:

Counterpart Analysis: A major problem with any analysis based on individual measurements is that any one dimension is affected by others within the same face. Not only are the measurements not independent, but also it is quite possible for a deviation in one relationship to be compensated wholly or partially by changes in other relationships. This applies to both skeletal and dental relationships. Compensatory changes in the dentition to make the teeth fit in spite of the fact that the jaws do not are well known and often are the goal of orthodontic treatment. Compensatory changes in skeletal components of the face are less well known but occur frequently and can lead to incorrect conclusions from measurements if not recognized.

The basic idea of interrelated vertical and horizontal dimensions leading to an ultimately balanced or unbalanced facial pattern was first expressed well by Enlow et al in “counterpart analysis.” If anterior face height is long, facial balance and proper proportion are preserved if posterior face height and mandibular ramus height also are relatively large (Figure 6-52). On the other hand, short posterior face height can lead to a skeletal open bite tendency even if anterior face height is normal because the proportionality is disturbed.

The same is true for anteroposterior dimensions. If both maxillary and mandibular lengths are normal but the cranial base is long, the maxilla will be carried forward relative to the mandible and maxillary protrusion will result. By the same token, a short maxilla could compensate perfectly for a long cranial base.

One way to bring the insights of counterpart analysis into clinical practice is from examination of the patient’s proportions versus those of a “normal” template (as discussed in the next section). Another, increasingly popular in the last few years, is the use of “floating point” norms for measurements.²⁵ The idea is to use standards derived from the individual’s facial type rather than relating individual cephalometric values to population means, taking advantage of the correlations between the individual values. Rather than judging normality or abnormality based on individual values, the judgment then would be based on how the values were related to each other—some combinations would be acceptable as normal even if the individual measurements were outside the normal range. Other combinations could be judged as reflecting an abnormal pattern even though the individual measurements were within the normal range. Assessing skeletal relationships in this way is particularly valuable for patients who are candidates for growth modification therapy or orthognathic surgery.