02 - Cephalometric Tracing Techniques

Overview of Cephalometric Technique and Analysis

Learning Objectives

Cephalometric radiographs (often called “cephs” for short) have been an important part of orthodontic diagnostic records since the mid-20th century. They offer two important advantages:

1. the radiographic view makes it possible to directly view the relationship of the dental arches to the underlying jaw structures, and to evaluate the relationship of the jaws to each other and to the cranial base. Although careful clinical examination of patients can yield most of this information, diagnostic precision increases significantly; and
2. serial cephalometric radiographs make it possible to evaluate both growth changes and the response to treatment in a way that was not possible previously.

In this module, we will focus on the first of these advantages, the use of cephs in the analysis of facial form. This extends what you learned in the previous module on facial form analysis, and will help you to further develop the “x-ray vision” needed in clinical examination of patients.

The next module, on cephalometric superimposition, is devoted to the second advantage, using serial cephs and various superimpositions to evaluate patients’ response to treatment.

Early Cephalometrics: America vs Europe

It has happened repeatedly in science (and in other creative areas as well) that a new idea appeared simultaneously in more than one location. You know already that Broadbent in the US and Hofrath in Germany independently realized that serial radiographs of the head and face could be used to study growth. This would allow evaluation of skeletal growth in a way that could not be done with cross-sectional measurements on skulls (craniometry) and longitudinal measurements across the soft tissues that cover the skeleton (anthropometry). Both Broadbent and Hofrath used a head holder so that the individual could be positioned in the same way repeatedly. And you’re already at least somewhat familiar with superimposed cephalometric tracings to view growth changes.

Now let’s look at cephalometric radiology in more detail, focusing on it as a diagnostic tool, not a tool for studying growth. The first thing to remember is that although Broadbent and Hofrath had the same idea, they didn’t standardize the technique in the same way. To this day, cephalometric radiographs (henceforth called cephs) made in Europe differ from those made in the US in one obvious way and one that’s not so obvious. The obvious way is that the patient looks to the right in an American ceph, and to the left in a European one. The not so obvious way is that the American radiograph usually is somewhat more magnified because the x-ray source is closer. Small details in positioning can differ between one machine and another on both sides of the Atlantic, so serial radiographs on the same machine are the most reliable way to detect small differences.

Why Trace Cephalometric Radiographs?

The second thing to consider is why comparisons of the same patient at different times, or comparisons of one patient to another, are based on comparisons of tracings of the cephs instead of direct comparison of the images themselves. The tracings show only a few of the anatomic characteristics that can be seen in the ceph.

The problem is that the x-ray image contains so much information that it’s overwhelming. For both growth studies and cephalometric diagnosis, what you want to know is how the jaws relate to the cranium and cranial base, and how the maxillary and mandibular teeth relate to their own jaw (image 1). To see that clearly, it’s necessary to eliminate most of the detailed image so the areas of interest can be seen clearly (image 2). This was done originally by carefully tracing the outline of the cranial base, maxilla, mandible, teeth of both arches, and the soft tissue profile on a sheet of clear film laid over the illuminated radiograph. Now something that looks a lot like a tracing is generated by a computer program.

The goal was—and is—to evaluate those important relationships. This is the same goal, of course, as the goal of clinical evaluation of facial form.

Image 1: This drawing shows the five major components of the face. An important goal of orthodontic diagnosis is to carefully establish their proportional relationships; cephalometric radiographs are the major tool for doing that.	Image 2: The five major components as seen in a typical ceph.

Cephalometric Analysis: Comparison with Numerical Norms

It did not take long for clinicians who had access to cephs to consider them as possible diagnostic records. The thought was that an individual’s dental and facial proportions could be compared to a population average, or to proportions that were selected as ideal for that person’s racial and ethnic group. This would focus attention on what was different about an individual patient. A comparison of this type is referred to as “cephalometric analysis”.

The first cephalometric analysis (the Downs analysis, named for the faculty member who produced it) was developed at the Univ. of Illinois, where one of the original three Broadbent-Bolton cephalometers was in use. (The Bolton family supported Broadbent’s work with cephalometrics, thus the dual name on the cephalometer). The Downs analysis, and its many successors, were based on defining skeletal landmarks on cephalometric tracings. The key landmarks that can be seen in a lateral ceph are shown on a dissected skull in image 1. The selected measurements in contemporary analyses are different now from those used originally (images 2 and 3), but the procedure still is to compare the patient’s measurements to “normal” values from cephs of a small group of individuals with ideal dental occlusion.

By now, numerical “norms” have been published for most racial and ethnic groups, so you can measure distances and angles for your patient and compare them to norms for the appropriate reference group.

Image 1: Anatomic location of major cephalometric landmarks.	Image 2: Linear measurements frequently made on a ceph tracing.
Image 3: Angular measurements frequently made on a ceph tracing.

Cephalometric Analysis: Comparison with Templates

With the measurement approach, the norms for the various measurements were the averages from a selected reference group. Another way to represent the norms would be to locate landmarks, then create a composite tracing based on the average angles and distances within the reference group. The composite tracing, which contained the same information as a table of numbers but displayed it in a very different way, could be superimposed on the tracing of an individual patient. Then the way the patient differed from the ideal, and by how much, could simply be observed.

Broadbent took this approach with the serial cephs he collected with financial support from the Bolton family, using his original cephalometer. Acknowledging the family support in producing them by using their name, the resulting Bolton templates show the average proportions at each year of age of 16 males and 16 females who were selected by Broadbent as having ideal facial proportions. He ended up averaging the combined gender groups, so the Bolton templates can be considered unisex data.

Looking at the difference between a Bolton standard template (red) and the patient’s tracing (black), it’s easy to see that the patient differs from the norm in two ways: the maxilla is rotated down posteriorly, and the mandible is rotated down and back. The result is a long face with mandibular deficiency largely due to the rotation.

Cephalometric Analysis in the Era of Digital Radiographs

By the end of the 20th century, film was being replaced by digital images in radiography just as it was in every day use. It’s almost impossible to accurately trace a ceph that exists only on a computer screen. Now what?

The answer is to digitize the location of landmarks on the ceph (which also could be done on a tracing from a film image, so that the tracing could be included in a computer data set). This creates a digital model that, like a tracing, removes most of the extraneous information. Then a computer program can be used to quickly generate as many linear and angular measurements as you want.

More landmarks would be needed in the digital model than usually were identified on a tracing, but otherwise the output would be similar to the tabulated numbers from measurements on a tracing—and the time spent in digitizing landmarks would be regained in not having to make the measurements manually.

Different computer programs require different landmarks. A typical set of landmarks for digitization is shown in this image. You have to digitize a set of at least 40-50 points to get an accurate representation of the patient into computer memory.

With the advent of digital radiographs, it has become the standard to use imaging software (which specifies the landmarks and sequence in which they are to be located) to digitize landmarks and create virtual ceph tracings. This practice has essentially replaced the use of acetate tracings.

Computer Templates

Could you use the template approach to comparisons if your radiographs existed only in computer memory? Yes, if the templates for the normal comparison group also were in computer memory, so they could be called up and superimposed.

In fact, with that approach it wouldn’t be necessary to digitize the patient’s radiograph. You could just pull up the digital ceph on your computer screen (images 1 and 2), then superimpose the reference tracing on the ceph (images 3 and 4), and you’d be able to see the differences between the patient and the template.

A new computer program, developed in 2008 (SmartCeph, OrthoII), offers the ability to bring up the correct Bolton template and superimpose it on the digital ceph. The correct template matches the length of the patient’s anterior cranial base rather than chronologic age, which matches them on developmental status instead of age. Only a couple of clicks of the computer mouse are required. After looking at the template comparison, you can go ahead with digitizing the patient’s landmarks—but that’s not really necessary in most cases.

Just from examing image 4, could you describe how this patient differs from the ideal facial proportions shown in the tracing? You’re going to get some experience doing just that later in this module, but already you can come pretty close—just by looking at it.

Image 1: Digital ceph displayed on laptop computer (probably downloaded from a distant server).	Image 2: Close-up of digital ceph on computer screen.
Image 3: Template (blue-green lines) superimposed (cranial base superimposition) on the patient’s ceph.	Image 4: Close-up of the superimposition, which allows observation of the differences between the patient’s facial proportions and the average for his population group as shown by the template.

3-D Imaging and Synthetic Cephs

At present, there is a strong trend toward replacing radiographs of individual dental and facial areas with a cone-beam computed tomographic (CBCT) image of the head. One CBCT scan could replace the panoramic, lateral ceph and a-p ceph images for a given patient, although there is a modest increase in cost and radiation exposure.

The good news is that the CBCT image can be viewed from any direction, and provides significantly more information than 2-D radiographs. The bad news for orthodontic diagnosis is that the information on a lateral ceph was overwhelming and needed to be reduced for analysis. With a 3-D image, the amount of information is so much greater that a stronger adjective is needed—overwhelming doesn’t describe it.

The interesting result is that a view like a digital ceph, as seen in this image (it’s called a synthetic ceph) often is generated from CBCT data, and then is analyzed by comparing it to reference groups with measurements or templates. For this purpose, templates seem particularly advantageous.

Cephalometric Analysis Using Imaging Software

With the advent of digital radiographs, it has become the standard to use imaging software to trace cepalometric radiographs. This practice has essentially replaced the use of acetate tracings. The accuracy of the digital model that is created using this software relies on identifying the landmarks correctly.

Cephalometric Tracing and Landmarks

Head Positioning in Cephalometrics

A standard cephalometric technique requires two things: (1) putting the patient into a headholder with ear rods. This controls the distances from the x-ray source to the head and from the head to the imaging device, and also controls the a-p, vertical and transverse rotational position of the head; and (2) getting the head level. At that point, the head still can be rotated up-down around the ear rods.

The original cephalometric technique used the standard method for orienting dry skulls for craniometrics (image 1). This called for having the Frankfort plane, an imaginary line extending from the superior portion of the external auditory meatus to the inferior margin of the orbit, parallel to the floor—a position called Frankfort horizontal. Frankfort horizontal is the best estimate of how a skull would have been oriented when the living individual had his head level.

For a living patient, of course, it would make sense to capture the actual orientation of the head in life. Every individual has a reproducible head position in which he or she “knows” that the head is level, and that’s the head orientation that is presented to the world. It’s the head position you adopt when you look at something on the horizon, or into your own eyes in a mirror if you’re in a small room like an x-ray room. This natural head position (NHP) now is the preferred orientation for taking a ceph (image 2). NHP cephs typically show a hanging chain as a true vertical reference across the front of the image, and the true horizontal line is perpendicular to it. This ceph was taken in NHP with filters in the x-ray beam, so that an outline of the soft tissue profile as well as the hard tissue structures could be obtained.

Image 1: Frankfort plane on a dried skull	Image 2: Ceph taken in natural head position (NHP).

Landmark Positions

Until the advent of digital radiography in the 1990s, a cephalometric radiograph was exposed and developed on plain film. A tracing was created by placing a thin semi-transparent sheet of acetate tracing paper on top of the film. The tracing paper was taped to the film to prevent movement, and the film was placed on a light box to improve the visualization of the anatomic structures. Using a pencil, the functional units of the facial skeleton were outlined and traced onto the acetate paper, and the landmarks were marked with dots.

To create a digital model of a ceph, you still have to find the landmarks accurately, and tracing the anatomic structures that indicate the key landmarks is still a good way to learn where the landmarks are. The most important landmarks for cephalometric analysis are shown on this tracing. We’ll identify them more clearly on the next screen.

Regardless of whether you trace a plain film by hand or digitize landmarks using computer software, the goal is to get the same information—a description of the 5 functional units and their relationships to each other.

Landmark Positions: Cranial Base

Since we are trying to understand the relationships between the 5 functional units of the craniofacial complex, it is important to identify consistent landmarks on each of these functional units. A cephalometric landmark is defined as an anatomic structure that meets two criteria:

1. It can be identified accurately on a cephalometric film.
2. It represents a known part of one of the five major functional units.

Let’s begin with the landmarks for the cranial base. Two critically important landmarks identify its orientation:

Sella (S) – indicates the posterior end of the anterior cranial base, and is located in the center of the cavity of sella turcica (image 1).

Nasion (Na) – indicates the front end of the anterior cranial base, and is located at the anterior end of the junction between the nasal and frontal bones (image 2).

Image 1: Sella, the center of sella turcica, the midline depression in the sphenoid bone that houses the pituitary gland.	Image 2: Nasion, the anterior-superior point at the junction of the nasal and frontal bones.

Landmark Positions: Maxilla

Three critically important landmarks for position and orientation of the maxilla (image 1) are:

Anterior nasal spine (ANS) – indicates the front end of the maxilla, and is located at the tip of the anterior nasal spine (image 2).

Point A (A) –another landmark associated with the anterior maxilla, located at the innermost point of the contour of the premaxilla between the anterior nasal spine and the incisor tooth (image 2).

Posterior nasal spine (PNS) – indicates the posterior end of the maxilla, and is located at the tip of the posterior spine of the palatine bone, at the junction of the hard and soft palates (image 3).

Image 1: Close-up view of the maxilla as seen in a ceph.	Image 2: Anterior maxilla landmarks.
Image 3: Posterior maxilla landmark.

Landmark Positions: Mandible

There are 5 important landmarks for position and orientation of the mandible (image 1):

Point B (B) – indicates the anterior part of the bony base of the mandibular dentition, and is located at the innermost point on the contour of the mandible between the incisor tooth and bony chin (image 2).

Pogonion (Pg) – the most anterior point on the contour of the chin (image 3).

Menton (Me) – the most inferior point on the mandibular symphysis (i.e., the bottom of the chin) (image 3).

Gnathion (Gn) – the center of the inferior contour of the chin, halfway between pogonion and menton (image 3).

Gonion (Go) – indicates the angle of the mandible, and is located at the center of the inferior contour of the mandibular angle (image 4).

Image 1: Close-up of mandible as seen on a ceph.	Image 2: B point, the junction between the skeletal mandible and the alveolar process.
Image 3: Landmarks on the chin.	Image 4: The landmark for the back of the body of the mandible.

Landmark Positions: Max / Mand Dentition

Four landmarks typically represent the a-p and vertical position of the incisor and molar teeth in each arch:

Upper incisor (U1) – maxillary central incisor tip)

Lower incisor (L1) – mandibular central incisor tip)

Upper molar (U6) – maxillary 1st molar, mesial cusp tip

Lower molar (L6) – mandibular 1st molar, mesial cusp tip

These points are indicated on the attached ceph.

Image 1: Close-up view of the maxillary and mandibular teeth as seen on a ceph, with the tooth landmarks identified.	Image 2

Tracing the Cranial Base Landmarks: S and Na

Even if you are in a digital world, if you can trace the areas around the important landmarks, it makes it easier to accurately locate them. Let’s begin some tracing by working with the cranial base landmarks.

Sella (S) appears as a depression in the sphenoid bone that contains the pituitary gland. To trace this area and identify the point (image 1):

• First draw the contour of the depression in the bone, extending anteriorly and posteriorly over the edges of the concavity; then mark point S in the center of the concavity. Extend your line forward along the floor of the anterior cranial base as shown (because this helps when and if you need to do superimpositions).

Nasion (N), the front end of the anterior cranial base, is the anterior end of the junction of the nasal and frontal bones. To trace this area and identify the point (image 2):

• Draw the outer surface of the nasal and frontal bones past the junction; then draw a suture between the bones. Identify point Na as the place where these lines intersect.

Image 1: Outline of sella turcica.	Image 2: Tracing the fronto-nasal junction to locate nasion.

Tracing the Maxillary Landmarks: ANS, Pt A, PNS

ANS is the tip of the anterior nasal spine. Point A, the junction between the skeletal maxilla and the maxillary dentoalveolar process, is the innermost point on the contour of the premaxilla between the anterior nasal spine and the incisor.

To trace this area and identify the points (image 1):

• Trace the superior and inferior surfaces of the anterior nasal spine and mark its tip as ANS; then follow the external contour of the bone downward toward the upper incisor, and mark point A at the depth of this concave line.

PNS is the back end of the bony palate, the point where the hard palate ends and the soft palate begins.

To trace this area and identify the point (image 2):

• Trace the inner contour of the anterior maxillary alveolar process, and continue posteriorly along the roof of the mouth to the end of the bony outline. Mark the end of the palatal bone contour as PNS.

Image 1: Tracing the contours of the anterior maxilla to locate ANS and point A.	Image 2: Tracing the contour of the posterior maxilla to locate PNS.

Tracing the Anterior Mandibular Landmarks (Pt B, Pg, Me, Gn)

Point B, the junction between the skeletal mandible and the mandibular dentoalveolar process, is the innermost point on the contour of the anterior mandible between the incisor and the bony chin. Pogonion (Pg), Gnathion (Gn) and Menton (Me) are points on the contour of the chin. To trace this area and identify the point (image):

Trace the bony chin. Include the inner aspect of the symphysis as shown (useful for superimposition). Follow the external contour of the bone from the chin upward toward the incisor, and mark point B as the depth of this concave line (image 1).

Pogonion is the most prominent point on the anterior aspect of the bony chin. Menton is the most inferior point on the bony chin, and Gnathion is the point on the anterior inferior contour of the chin halfway between pogonion and menton (image 2).

Image 1: Tracing the contour of the mandibular symphysis to locate point B.	Image 2: Locating pogonion, gnathion and menton (top to bottom order) on the contour of the bony chin.

Tracing the Mandibular Gonial Angle Area (Go)

Gonion (Go) is the center of the inferior contour of the mandibular angle. To trace this area and identify the point:

• Follow the lower border of the mandible posteriorly around the mandibular angle and up the posterior surface of the ramus. Mark the center of the curvature at the gonial angle point Go.

Often because of the mild asymmetry that almost everyone has, two shadows of the lower border of the mandible can be seen in the mandibular angle area. In that case, trace both sides, using a solid line for one and a dotted line for the other, and mark point Go as the midpoint between the two sides. (In cases of severe asymmetry, Go for the left and right sides would be the base for separate measurements.)

Orbitale, Mandibular Canal, 3rd Molar

To complete the tracing of the jaws: trace the orbital rims and note point orbitale (Or), the most inferior point on the lower border of the orbit (image 1).

The orbits are bilateral structures, and if you can see both sides separately, trace both and locate point Or halfway between the two sides.

Also trace the shadow of the inferior alveolar canal and the crown of the unerupted third molar if present (image 2). These structures are especially useful as stable areas for superimposition.

Image 1: Tracing the orbital rim to locate point Or. Don’t confuse the dotted line behind the orbit with the orbital rim—a common tracing mistake.	Image 2: Tracing the outline of the neurovascular bundle below where it enters the lower part of the ramus, and the outline of a third molar at the stage of crown formation.

Adding the Incisor Teeth

At this point, you’re ready to trace the teeth. It’s important to locate accurately:

• the facial surface of the crowns of the upper and lower incisors
• the inclination of the roots of the incisors
• the position of the upper and lower first molars.

Templates that represent the typical dimensions of these teeth can make tracing them easier. If you are tracing by hand, you can use a tooth template which helps you draw teeth that are a uniform but consisitent size. If tracing with a computer program, the program has a digital template you can use to trace the shapes of the incisors and molars (it requires the additional points shown in this figure). Remember that the templates are only guides, and if your patient’s teeth differ significantly from the templates, try to trace the real thing.

To trace the upper and lower central incisors:

• Orient your tooth template to accurately represent the axial inclination of the incisor and the position of the tooth’s facial surface. Then use the template to draw in the outline of the incisors.

Adding the Molars and Occlusal Plane

Because the molars are bilateral structures, often two partially superimposed images can be seen. By convention we trace the tooth on the left side, the side closest to the film when the radiograph was taken.

This means that the tooth you want to trace is the one of a nearly-superimposed pair that is slightly above and behind the image of the other side. To place the first molar template accurately, it can help to first trace the sides of the second molar.

To complete the teeth, it is not necessary to trace all the other teeth, but it is important to outline the occlusal plane of the cheek teeth (2nd molar to 1st premolar). To do this digitally, a point on the crown of the first premolar can be added to orient a line across the crown of the first molar.

Completing the Tracing: Soft Tissue Profile

To complete the tracing, add the outline of the soft tissue profile, from the bridge of the nose to the chin. Your completed tracing should look like the image here.

At this point, it’s ready to use, either by adding reference lines (which already has been done with the tracing shown here), making measurements, or superimposing it on another tracing.

Digitization Rather Than Tracing

Let’s think about what would be different if you were sitting at the computer, as you almost surely will be in your professional future, rather than tracing at a light box.

What would be different if you digitized a digital ceph, instead of tracing a film image? Basically, you’d have to have more points so that the lines you would have traced can added by the computer software.

A minimal digital model (image 1) would ask you to add to the landmarks we’ve identified so far:

• points along the cranial base
• points to mark the posterior maxilla instead of just the orbital rim
• a point at the mandibular condyle and another where the shadow of the zygomatic arch crosses the upper mandibular ramus
• a series of points along the curvatures of the soft tissue profile
• additional points to define the incisor positions instead of just the tips
• points to mark the mesial and distal contact points of the molars.

It would be easier to do that if you had some experience doing ceph tracings—which is one of the reasons we’re doing a little tracing within these modules. More precise digital models would require more points, so to get something that was close to a traced image, you’d have to add additional points as shown in image 2—but if you add the extra points and the the computer program connects them, you do get a reasonable representation of a rather complete tracing. This is what you’ll see most of the time in the future.

Image 1: A minimum set of additional points for digitization—which would be accurate enough for diagnosis but not for research.	Image 2: A more complete set of landmarks for digitization, which would provide greater accuracy.

Cephalometric Analysis

Measurement Analysis

The objective of cephalometric analysis is to establish the relationship of the facial units in the antero-posterior and vertical planes of space. Cephalometric analysis is done by drawing reference lines on the tracing or digital model, to make relationships more visible. Then data are obtained for comparing an individual patient to a reference group, by measuring angles between reference lines, and by measuring distances between landmarks.

Remember, your objective is to define the relationships of the major elements of the head and face. Specifically, you want to know if the relationships, in the a-p and vertical planes of space, are normal or not, and if they’re abnormal, exactly what’s wrong.

Vertical Proportions

The first step in analysis is to examine vertical proportions. Start by drawing five horizontal reference lines:

1. S-N, the inclination of the anterior cranial base.
2. True horizontal, the visual axis, which is drawn perpendicular to true vertical (the chain) through the lower border of the orbit (Or).
3. ANS-PNS, the palatal plane.
4. Functional occlusal plane, drawn along the occluding surfaces of the posterior teeth.
5. Go-Gn, the mandibular plane.

In a well-proportioned face, the horizontal planes project posteriorly toward an approximate common meeting point, as shown in this image.

Vertical Proportions (cont.)

For the patient we have been tracing (image 1), the palatal plane is tilted down posteriorly and meets the occlusal and mandibular planes at a common point relatively close to the back of the jaws, and the SN plane is tilted up anteriorly. This amount of variation would have to be kept in mind as the relationships of the functional components were considered, but it is within normal limits.

Sometimes one or both of the jaws are rotated, or the teeth have not erupted properly. This shows up clearly because then one or more of the planes projects differently from normal. The patient seen in image 2 obviously has a mandible that is rotated down and back, creating a long face and an open bite. Just looking at the orientation of these horizontal planes gives excellent insight into vertical proportions.

If you needed to be more precise, you could measure angles (for instance, the angle between S-N and Go-Gn) or linear distances (Na-Me, the anterior face height) and check them against the norms for your patient’s group.

Image 1: For this patient, angular measurements relative to SN would have to be adjusted because of its inclination to the true horizontal line. Often it is more important to look carefully at the ceph than to depend on measurements to determine relationships of the functional components.	Image 2: Note that the mandibular plane is very steep and projects well forward of the other planes. The downward-backward rotation of the mandible is obvious, and is the major cause of the anterior open bite.

Antero-Posterior Jaw Relationships

The second step in analysis is to evaluate the a-p relationships of the jaws. An excellent way to do this is to draw a true vertical line that extends downward from nasion, and examine the position of points A and B relative to this.

In a normally proportioned face, point A is on or slightly in front of this line; point B is slightly but not very far behind it. More precisely, point A should be about 2 mm in front of the Nasion vertical line, with a range of 0-4 mm. Point B should be about 2 mm behind it, with a range of -4 to zero. But the difference between the vertical lines through points A and B should not be more than 4 mm, otherwise the teeth can’t occlude properly.

For this patient, both jaws are a bit more forward from the cranium than in most individuals, so points A and B are forward from the usual position—but the jaws are positioned closely enough together than normal occlusion is quite possible. The relationships of the jaws to the cranium in this patient would be an example of normal variation.

Antero-Posterior Jaw Relationships (cont.)

In a patient with an antero-posterior displacement of one or both jaws, it will be apparent that the maxilla and/or mandible are not well related to the cranial structures.

Both of the patients seen in images 1 and 2 have the maxilla in a reasonable position relative to the true vertical line dropped from nasion, but there is a large discrepancy in the size and position of the mandible. It’s obvious that the patient in image 1 has a skeletal Class III problem that is due almost completely to a large mandible. The one in image 2 has a skeletal Class II problem that is due to mandibular deficiency.

Image 1: Skeletal Class III due mostly to a very large mandible.	Image 2: Skeletal Class II due mostly to a small mandible, with protrusion of mandibular incisors.

A-P Jaw Relationships: Measurements

There are other measurements that you could make to estimate a-p jaw relationships.

For instance, you could draw the NA and NB lines (image 1), and measure the angle between them (ANB) to judge how great the difference in jaw position was.

And you could measure the SNA and SNB angles to estimate whether the maxilla or mandible was in the wrong position (image 2).

Standards for these and many other measurements have been published and are readily available.

But remember, these measurements are estimates of the vertical and a-p jaw relationships we’re ultimately interested in. The goal of cephalometric analysis is to accurately describe the facial and dental relationships, not to measure some specific characteristic.

Image 1: The ANB angle is an estimator of the a-p relationship of the jaws. It is quite small in patients with normal jaw relationships, like this one.	Image 2: The SNA and SNB angles indicate the degree of protrusion of the jaws relative to the cranial base, but they must be used carefully because the reference standards assume a normal inclination of SN, and we have seen already that this is not always correct.

A-P Relationship of Teeth to Jaw

The third step in analysis is to examine the a-p relationship of the upper teeth to the maxilla and lower teeth to the mandible.

Let’s consider the maxlla first. The question is, do the upper teeth protrude relative to the maxilla, is their a-p position within normal limits, or are they too far back?

A good way to answer that is to move the true vertical line so that it runs through point A, and examine the position of the upper incisor to the line. The incisor should be slightly but not very much ahead of the line, as it is in the patient shown here—so the a-p position of the upper incisors is normal.

A-P Relationship of Teeth to Jaw (cont.)

In the patient shown here, when the upper incisor position is compared to the position of point A, it becomes obvious that the upper teeth are a little forward from the maxilla. This individual’s overjet is due largely to a deficient mandible (note where point B is), but some protrusion of the maxillary teeth relative to the maxilla contributes to it.

In the same way, we can examine the position of the lower teeth relative to the mandible, by moving the true vertical line through point B and looking at how the incisor teeth relate to it. The guideline is the same: the lower incisors should be slightly but not very far in front of the line. More precisely, the lower incisors should be about 2 mm in front of point B, with a range of 1-5 mm.

The prominence of the chin is a factor in how much lower incisor protrusion is appropriate. The position of Pg can be noted relative to the line through point B.

The lower incisor can be 2 mm more forward than Pg, but not a lot more than that, and should not be a great deal more prominent than the chin. Note that this patient has a lot of protrusion of the lower incisors relative to the mandible—which is why his overjet is smaller than the discrepancy between maxilla and mandible.

Incisor Angulation and Vertical Position

There are a number of other measurements you might make, to establish more precisely how the incisors are angulated and positioned vertically.

Incisor angulation usually is measured relative to the S-N line (cranial base plane) for the maxillary incisors, and relative to the Go-Gn line (mandibular plane) for the lower incisors (image 1). Remember, though, that an angle that is larger or smaller than the ideal could be due to the inclination of the S-N or Go-Gn line, so if one of the reference lines is tipped, you might not want to judge incisor inclination from this angle. You could see inclination of the cranial base or mandibular planes by drawing the lines as illustrated in the previous screens. If you want to measure, the S-N line should be about 6° up relative to the true horizontal line.

You also might note some excess overbite (image 2) (you can see that on a ceph, but of course it’s also obvious clinically), and suspect that the incisors have erupted more or less than they should have. One way to check that would be to measure how far the maxillary incisor root apices are from the palatal plane, or the how far the mandibular incisors root apices are from the mandibular plane (image 3). The normal distances are part of the data available for comparison.

Image 1: Evaluating maxillary incisor inclination from the angle between the upper incisors and SN, and mandibular incisor inclination from the angle between the incisors and the mandibular plane (Go-Gn) will be accurate only if the reference lines (SN, Go-Gn) are normally inclined.	Image 2: Overbite can be measured on a ceph, but that doesn’t tell you the underlying cause of excess overbite.
Image 3: One way to judge whether maxillary or mandibular incisors have erupted too much or too little is to measure the distance from the root apices to the palatal or mandibular planes and compare this to the normative values.

Cephalometric Analysis: Summary

This combination of inspection of relationships seen on a ceph tracing or digital model, and measurements to confirm discrepancies from the normal, is a simplified approach to cephalometric analysis.

Perhaps one way to look at it is that the more your patient differs from the norm, the more measurements you might have to make to determine exactly what the discrepancies really are. Cephalometric analysis shouldn’t become a “numbers game”, with a standard set of measurements for every patient. The objective, as we have emphasized, is to determine the relationships between the components of the face. The analysis succeeds or fails depending on how well it defines those relationships.

Referral to Self-Test

Before you go to the self-test, be sure you read the material in Contemporary Orthodontics: pages 184-199 in the 5th edition, and 202-218 in the 4th edition. Then use the self-test to be sure you have understood the basics of cephalometric analysis.

Self-Test

Question 1

Which of the following are ways that typical American and European cephs differ?

1. direction in which the patient faces
2. vertical orientation of the head
3. distance from x-ray source to patient
4. positioning of the ear rods
5. 1 and 2
6. 2 and 3
7. 3 and 4
8. 1 and 3 ✓
9. 2 and 4

Correct

That’s right, the direction in which the patient faces (nose to the right in America, to the left in Europe) and the distance from the x-ray source to the patient (greater in Europe) are different. The vertical orientation of the head and the positioning of the ear rods are the same.

Incorrect

No, that wrong. The direction in which the patient faces (nose to the right in America, to the left in Europe) and the distance from the x-ray source to the patient (greater in Europe) are different. The vertical orientation of the head and the positioning of the ear rods are the same.

Question 2

(A) The major goal of cephalometric analysis is to establish the relationship of the teeth of each jaw to that jaw because (B) correcting these relationships is the primary objective of orthodontic treatment.

1. A true, B true, A and B related
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true
5. A and B false ✓

Correct

You’re correct, both statements are false. Establishing the relationship of the teeth to their jaw is one goal of cephalometric analysis, but establishing the relationship of the jaws to the cranial base and to each other also are important goals of the analysis. Correcting the patient’s malocclusion in a way that provides maximum benefit to the patient is the primary objective of orthodontic treatment—the relationship of the teeth to their jaw is just one thing to be considered relative to that primary objective.

Incorrect

No, that’s wrong. Both statements are false. Establishing the relationship of the teeth to their jaw is one goal of cephalometric analysis, but establishing the relationship of the jaws to the cranial base and to each other also are important goals of the analysis. Correcting the patient’s malocclusion in a way that provides maximum benefit to the patient is the primary objective of orthodontic treatment—the relationship of the teeth to their jaw is just one thing to be considered relative to that primary objective.

Question 3

(A) Creating an image like a lateral ceph from a cone-beam CT image is impossible because (B) the amount of information in a CBCT image is much greater than the information in a standard ceph.

1. A true, B true, A and B related
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true ✓
5. A and B false

Correct

That’s correct. The first statement is false—it’s quite possible to produce a “synthetic ceph” from CBCT data—but the second one is true. A CBCT file indeed has much more information than is found in a standard ceph.

Incorrect

No, that’s wrong. The first statement is false—it’s quite possible to produce a “synthetic ceph” from CBCT date—but the second one is true. A CBCT file indeed has much more information than is found in a standard ceph.

Question 4

Why is orienting the patient so the Frankfort plane is level, now the preferred method for taking a ceph?

1. reproducibility
2. allows comparison with craniometric data
3. allows comparison with anthropometric data
4. correlates with NHP
5. it isn’t, NHP is preferred ✓

Correct

That’s correct, NHP is preferred in modern cephalometric technique. Although the Frankfort plane correlates with the natural head position, and is the best way to orient your patient if he’s dead, NHP gives you the patient’s real head position in life.

Incorrect

That’s incorrect. NHP is preferred in modern cephalometric technique. Although the Frankfort plane correlates with the natural head position, and is the best way to orient your patient if he’s dead, NHP gives you the patient’s real head position in life.

Question 5

Which of the following are characteristics of a good cephalometric landmark?

1. marks the position of specific teeth
2. relates the position of a tooth to the jaw
3. can be identified accurately on a ceph
4. represents a known part of one of the major functional units
5. 1 and 2
6. 2 and 3
7. 3 and 4 ✓
8. 1 and 3
9. 2 and 4

Correct

That’s correct, a good cephalometric landmark can be identified accurately on a ceph, and represents a known part of one of the major functional units. Landmarks aren’t restricted to the teeth, and no single landmark can relate one thing to another.

Incorrect

No, that’s incorrect. A good cephalometric landmark can be identified accurately on a ceph, and represents a known part of one of the major functional units. Landmarks aren’t restricted to the teeth, and no single landmark can relate one thing to another.

Question 6

Which landmark is found at the junction of the fronto-nasal suture?

1. Sella
2. Nasion ✓
3. Pogonion
4. Gonion
5. Menton

Correct

That’s correct, it’s nasion. Nasion is defined as the anterior point of intersection of the nasal bone and frontal bones, which of course is the fronto-nasal suture. It represents the anterior end of the cranial base and is a key point in establishing the length and inclination of the cranial base.

Incorrect

That’s wrong, it’s nasion. Nasion is defined as the anterior point of intersection of the nasal bone and frontal bones, which of course is the fronto-nasal suture. It represents the anterior end of the cranial base and is a key point in establishing the length and inclination of the cranial base.

Question 7

Which landmark is at the base of the contour above the chin?

1. point B ✓
2. pogonion
3. gnathion
4. menton
5. gonion

Correct

That’s right, it’s point B. Point B indicates the anterior part of the bony base of the mandibular dentition, and is located at the innermost point on the contour of the mandible between the incisor tooth and bony chin.

Incorrect

That’s wrong, it’s point B. Point B indicates the anterior part of the bony base of the mandibular dentition, and is located at the innermost point on the contour of the mandible between the incisor tooth and bony chin.

Question 8

How do you locate the landmark S (sella)?

1. junction of anterior and posterior clinoid processes
2. 5 mm distal from anterior wall of depression in sphenoid bone
3. 5 mm mesial from posterior wall of depression in sphenoid bone
4. center of the space created by the depression in the sphenoid bone ✓
5. junction of ethmoid and frontal bones

Correct

That’s right, the point S is the center of the space (called sella turcica) created by the depression in the sphenoid bone in which the pituitary gland is located. The normal variations in the shape of sella turcica make it impossible to be consistent in measurements based on the position of the bony constituents of this region, but the center of the space can be located with acceptable accuracy.

Incorrect

No, that’s wrong. The point S is the center of the space (called sella turcica) created by the depression in the sphenoid bone in which the pituitary gland is located. The normal variations in the shape of sella turcica make it impossible to be consistent in measurements based on the position of the bony constituents of this region, but the center of the space can be located with acceptable accuracy.

Question 9

How do you find the landmark PNS?

1. trace the superior and inferior surfaces of the anterior nasal spine and mark its tip
2. follow the external contour of the maxilla downward toward the upper incisor, and mark the depth of this concave line
3. trace posteriorly along the roof of the mouth to the end of the bony outline, and mark the end of the palatal bone contour ✓
4. trace anteriorly along the base of the nose, and mark the height of contour
5. trace the bony chin, following the external contour of the bone upward toward the incisor, and mark the depth of this concave line

Correct

That’s right, PNS is located at the posterior end of the palate, and is best located by tracing posteriorly along the roof of the mouth to the end of the bony contour.

Incorrect

No, that’s wrong. PNS is located at the posterior end of the palate, and is best located by tracing posteriorly along the roof of the mouth to the end of the bony contour.

Question 10

(A) Creating a digital model of a ceph instead of a tracing requires more landmarks because (B) otherwise there is not enough information for the computer to add the lines needed to simulate a tracing.

1. A true, B true, A and B related ✓
2. A true, B true, A and B not related
3. A true, B false
4. A false, B true
5. A and B false

Correct

That’s right, the statements are true and related. In digitization, additional landmarks are need to outline the cranial base, add the soft tissue profile, and refine the display of the maxilla, mandible and teeth.

Incorrect

No, that’s wrong, the statements are true and related. In digitization, additional landmarks are need to outline the cranial base, add the soft tissue profile, and refine the display of the maxilla, mandible and teeth.

Question 11

Which of the following lines is not used in establishing vertical facial proportions?

1. S-N
2. N-Me ✓
3. ANS-PNS
4. occlusal plane
5. Go-Gn

Correct

That’s right, N-Me is a measure of face height, but it’s not used to establish vertical proportions. The orientation of the other 4 lines to each other does help to establish vertical proportions.

Incorrect

No, that’s wrong. N-Me is a measure of face height, but it’s not used to establish vertical proportions. The orientation of the other 4 lines to each other does help to establish vertical proportions.

Question 12

Which of the following statements usually would not correctly describe a patient with a long face?

1. open bite
2. maxilla rotated down anteriorly ✓
3. mandible rotated down anteriorly
4. increased anterior face height
5. they’re all correct

Correct

That’s right, in a long face patient the maxilla is much more likely to be rotated down posteriorly rather than anteriorly, but the other characteristics are present. Note that the tracing for this severe long face patient shows the palatal plane almost parallel to the true horizontal line, so there’s no maxillary rotation at all for him.

Incorrect

No, that’s wrong. In a long face patient the maxilla is much more likely to be rotated down posteriorly rather than anteriorly, but the other characteristics are present. Note that the tracing for this severe long face patient shows the palatal plane almost parallel to the true horizontal line, so there’s no maxillary rotation at all for him.

Question 13

Which of the following measurements would be most useful in establshing the a-p position of the mandible relative to the cranial base?

1. SNA
2. SNB ✓
3. ANB
4. Gonial angle
5. N-Me distance

Correct

That’s correct, of these chacteristics SNB would be the most useful in establishing the a-p position of the mandible relative to the cranial base.

Incorrect

That’s wrong. Of these chacteristics SNB would be the most useful in establishing the a-p position of the mandible relative to the cranial base. It’s the only one with information relating the cranial base (S-N) to the mandible (B).

Question 14

To compare whether an upper incisor had erupted too much, which of the following characteristics would be most useful?

1. Max incisor-SN angle
2. distance from incisal edge to NA line
3. distance from incisal edge to ANS-PNS line
4. Max incisor-palatal plane angle
5. distance from root apex to ANS-PNS line ✓

Correct

That’s right, the distance from the root apex to the palatal plane (ANS-PNS line) would be the best indicator of excessive eruption of the upper incisor. For the lower incisor it would be the distance of the root apex from the mandibular plane (Go-Gn). Angular measurements wouldn’t work, and the distance from the root apex is a better indicator than distance from the incisal edge.

Incorrect

That’s incorrect. The distance from the root apex to the palatal plane (ANS-PNS line) would be the best indicator of excessive eruption of the upper incisor. For the lower incisor it would be the distance of the root apex from the mandibular plane (Go-Gn). Angular measurements wouldn’t work, and the distance from the root apex is a better indicator than distance from the incisal edge.