When bitewings, full-mouth and panoramic X-rays are not enough, clinicians may turn to cephalometric radiography — it's technology that really lets them see what's going on inside a patient's head
By Rebecca Stone
It's the ultimate headshot, though it may not get you a role in the next Hollywood blockbuster. Cephalometric radiography, commonly known as "ceph," is a method of craniofacial imaging introduced into the United States in 1931 by orthodontist B. Holly Broadbent, DMD. Ceph imaging refers to the two-dimensional X-ray capture of the head and neck. In oral health care, it is primarily used by orthodontists and oral surgeons, and has long been regarded as a must-have tool for prognosis of dental/craniofacial development. It's also helpful for diagnosing and treatment planning for dental malocclusions and skeletal anomalies.
According to Gerald Nelson, DDS, a health sciences clinical professor and interim chair of the University of California, San Francisco School of Dentistry's orthodontic division, the cephalogram is useful for most orthodontic treatment plans, allowing clinicians to set goals for tooth movement, while helping them to avoid unwanted shifts. "It is essential in orthognathic treatment, which is a combination of jaw surgery and orthodontics," adds Nelson. "The head film helps us decide which jaw should be moved, and by how much."
Cephalograms are captured in much the same way as are panoramic X-rays. In fact, many extraoral radiography units either are or can be configured to offer both types of imaging. But where a "pan" provides a flattened frontal view of a patient's dental arches, cephs capture structures from the neck up in lateral and frontal views. To ensure accuracy and diagnostic quality, the head is held in a natural, reproducible position with the aid of a cephalometer. This helps clinicians acquire standardized and comparable craniofacial images, which facilitate the analysis of dentofacial morphology.
DIGITAL TRACING OPTIONS
When employing the graphic approach to cephalometric analysis, the current methods for creating digital tracings include the use of digitizer pads for tracing conventional film; scanners or digital cameras for exporting images to measurement programs; measurement-computing software; and the direct transmission of digital cephalometric images to a computer database.
With digital software systems, tracings can be performed on screen, atop the radiographic image. Landmarks can be easily placed and measurements taken for cephalometric analysis. Such programs even allow illustrations of teeth and hard- and soft-tissue profiles to be placed and adjusted. Finally, tracings and radiographs can be superimposed and compared to diagnose problems, predict and anticipate growth patterns, and plan and track treatments.
Most ceph software is Windows-based, though at least one manufacturer offers ceph software that also works on Macintosh operating systems. Another offers cloud-based tracing software that can be utilized with any operating system. There are also online services that will perform ceph tracings and analyses on uploaded ceph images, as well as store them and facilitate viewing and sharing on a pay-as-you-go basis.
J. Martin Palomo, DDS, MSD, an associate professor and program director of orthodontics at Case School of Dental Medicine in Cleveland, explains that there are five relationships that can be determined with a ceph. "These," he says, "include the position of the upper jaw (maxilla) compared to the cranial base; the position of the lower jaw (mandible) compared to the cranial base; the position of the maxillary teeth related to the maxilla; the mandibular teeth related to the mandible; and the vertical dimension, which evaluates how long the face is, top to bottom."
Palomo explains that by answering these questions of relationship, clinicians can determine if a problem is jaw- or dentition-related. "If we need to move jaws, we may need surgery as part of the treatment plan," he says. "If there were only dental discrepancies, then braces should solve it. In order to arrive at these answers, clinicians usually trace the ceph and extract distances and angles. This is cephalometrics."
POINT of SALE
Cephalometric analysis is the study of the relationship between the teeth and bones in the head in the context of what is considered "normal." According to Palomo, there are dozens of techniques for collecting measurements, all serving similar purposes. "With the cephalometric analysis," he notes, "the clinician is able to extract objective data to answer clinical questions, and arrive at an efficient treatment approach."
Analysis requires the identification of anatomical landmarks to establish and measure these relationships on cephalograms. "We have standards for about 50 measurements on these films," Nelson reports. "So we can have an idea if the upper or lower incisor tip is way off, if one or both of the jaws are too short or too long compared to other parts of the face, if the length of one side of the jaw is longer than the other, and so on."
There are two approaches to this type of analysis. The metric approach measures the distances between landmarks on linear and angular planes. The graphic approach allows visual comparisons by manually pinpointing landmarks on a paper tracing of the cephalogram. Then, by superimposing one tracing over another to compare markings, the clinician can track growth or determine the effect of orthodontic treatment on the jaw and surrounding tissue.
"When we take a cephalogram before and after treatment, we can superimpose them and see rather accurately how far teeth have moved in the jaw, and how that has affected the lip posture," Nelson explains.
"The tracing extracts the main landmarks we use to measure distance and angles, so we can determine what is outside normal limits," notes Palomo. "There are norms and standard deviations for each measurement."
But because traditional tracing methods are time consuming — not to mention, somewhat difficult, as the two-dimensional renderings are meant to reflect a three-dimensional structure — digitized ceph imaging has been gaining traction. Through this new technology, landmarks are converted to numerical values, allowing automatic measurement of landmark distances. With special software, numerical data can be digitized and incorporated onto digital or scanned tracings. Another advantage of digital cephalometric radiography is that it emits less radiation than film-based cephs.
CEPH AND THREE-DIMENSIONAL X-RAYS
But regardless of the analytic technique employed, if the future looks bright in radiography, it is positively aglow thanks to cone beam computed tomography (CBCT) technology. After all, few can dispute the ultra-cool look of three-dimensional (3D) CBCT imagery compared to two-dimensional (2D) ceph shots. Says Lorne Lavine, DMD, founder and president of The Digital Dentist, a technology consulting firm based in Studio City, Calif., "Since many CBCT machines can extrapolate out the ceph image, I'm seeing far greater demand for CBCT than for pure pan/ceph machines." Lavine adds that once the pricing for CBCT models drops to the point that it's competitive with pan/ceph, he expects the need for ceph units to plummet.
Palomo agrees, saying that though we have learned a great deal from 2D cephs, CBCT is a much more accurate representation of the patient's head. "We need 3D images because the right and left side will not always be the same as far as jaws and even teeth," he explains. "A 3D image allows a more complete analysis, which may increase the chances of arriving at a more efficient treatment plan. At this point, the ceph is still part of treatment planning, and now that one can be created out of a CBCT image, it is very possible that in the future cephs will play a much smaller role or even disappear."
But there is another factor in this equation, and the key word here may just be "aglow." Enhanced ability to identify anatomical landmarks on a 3D image notwithstanding, CBCT emits considerably more radiation than cephalometric radiography. According to Nelson, cephs emit only about 6 microsieverts (µS), while a CBCT can create anywhere from 40 to 100 µS. He adds that traditionally, orthodontists have taken a pan (around 25 µS), a ceph, and then may also request a full mouth series (at around 120 µS), or even other films, such as temporomandibular joint tomograms. "The exposure can add up with multiple films," he observes.
But radiation dosage can be minimized — and, in fact, some ceph units allow exposure of only targeted portions of the head on a lateral scan. This is said to reduce scanning time, and limit radiation dosage by as much as 62% compared to a nontargeted view. In an effort to reduce patient exposure to radiation, the Board of Trustees of the American Association of Orthodontists and the American Academy of Oral and Maxillofacial Radiology have appointed a council tasked with creating guidelines for CBCT use. One proposal is to combine 2D images and 3D images taken with a smaller field of view.
"Orthodontists don't want to give up the head film, since the research standards are so helpful," notes Nelson. He adds that orthodontists who order a full head CBCT will be able to section a lateral head film from the CBCT — as well as any other view of the face they want. "But," he cautions, "a full-head CBCT is a higher dose. An alternative is to take a CBCT that is a smaller field of view that only exposes from the jaw joints to the chin (sort of a 3D panogram). This reduces exposure, but doesn't include enough for the head film, so the option is to also take the 2D cephalogram with the medium field of view CBCT. Then the exposure is around 50 µS total, and the information available for planning the treatment is extraordinary."
But for all the appeal of 3D radiography, ceph will most likely be around for some time to come. Besides dishing out more miserly doses of radiation than their more souped-up counterparts, ceph units' image quality has vastly improved thanks to better image-acquisition and high-definition technologies — and they still cost significantly less than CBCT machines. And when you add it all up, that's a blockbuster of a picture worth the price of admission.
EYE OF THE BEHOLDER
The beginnings of cephalometric analysis appear to be somewhat ethnocentric, as they are based on the Caucasian perception of beauty. As Gerald Nelson, DDS, a health sciences clinical professor and interim chair of the University of California, San Francisco School of Dentistry’s orthodontic division, tells it, this type of radiography, which became popular around 1947, was developed from two studies. Says Nelson, “Large audiences were shown photos of patients with good occlusion, who also had a lateral head film. The audience was asked to choose the attractive faces. Once the researchers had a group of appealing faces, they looked at the head films and established measurements, both linear and angular, that could then be used as a standard to set goals of treatment.
“Another study,” he reports, “involved taking a lateral head film on children every year from age 6 to age 21. These records have been used for decades to serve as a database for research into facial growth.”
Fortunately, according to Nelson, there are now ceph standards for a wide range of ethnicities, and orthodontists can use the head film to see how their patient, regardless of heritage, compares to standards of an “attractive person.” They can also better anticipate where the facial growth trend will probably go. “This is so important,” he says, “because as we move teeth, we don’t want to be fighting the growth trend.”
BONUS WEB CONTENT : CEPH LANDMARKS
The identification and measurement of landmarks — and their alignment with planes and angles — are the heart and soul of cephalometric radiography. But orthodontists speak in code when identifying these points. Here are some of the key landmarks that are likely to be marked on a cephalogram tracing, and their translations:
S = Sella tursica: Saddle-shaped depression in the sphenoid bone, which is a butterfly-shaped formation located at the base of the skull
N = Nasion: Where the top part of the nose joins the forehead
Or = Orbitale: The lowest point on the lower margin of the orbit or eye socket
Po = Porion: Central point on the upper part of the external auditory meatus or ear openings
ANS = Anterior Nasal Spine: Pointed projection at the front of the location where the maxillary bones join
PNS = Posterior Nasal Spine: Pointed posterior end of the nasal crest
GO = Gonion: The most outer portions of the lower mandible, at which point the jawbone angles upward
Me = Menton: Lowermost point on mandibular symphysis, or where the two halves of the jawbone join
A point = Deepest concavity on anterior profile of maxilla
B point = Deepest concavity on anterior profile of mandibular symphysis
Mandibular Plane: Follows angled plane of mandible and can help assess the direction of growth
Maxillary Plane: Follows angled plane of maxilla
N-Pog = Facial Plane: The line that goes from the nasion through the pogonion or most anterior point on the chin
Sagittal Plane: Vertical plane that divides the body into right and left halves
S-GN = Y-axis: The line through the sella to the gnathion or lower curvature of the chin
SN = Sella-Nasion: The line from the sella to the nasion