The value of cephalometric superimposition resides mostly in its importance in demonstrating the gross effects of a given orthodontic treatment. For example, superimposing both pre-treatment and end-of-treatment cephalograms provide a fair estimation of the change in the position of the incisors, or the amount and the nature of the displacement of the mandible following class II or III therapy (Figure 1).
The emphasis that was once towards numerical measurements using well-known and accepted analysis may not suit today’s clinicians in their desire to achieve their personal treatment objectives.
In an era of evidence-based care to patients, all clinicians need a precise method for the assessment of natural growth and changes brought by the correction of a malocclusion.
Even though many methods of cephalometric superimposition that seem handy and simple to use exist, they don’t all carry the same diagnostic value and authority.
Fundamental flaws in measuring distances and angles with landmarks include working with periosteally located structures that won’t remain stable in time, and providing little or no information on change of size and shape of the studied structures.
The most powerful scientific method that estimates overall treatment changes is the structural superimposition method described by Arne Bjork (1911-1996). It allows a global point of view of craniofacial changes over time, since it is based on a unique longitudinal implant study. The technique did not get popular acceptance in the past for different reasons, two of them being that: 1) it was difficult to obtain high quality cephalograms compared with today’s modern equipment and 2) good knowledge of growth and development when using that method is required.
Structural superimposition is a true scientific approach and the only evidence-based technique. The observation that superimpositions on landmarks and lines have been so popularly used since the 18th century cannot replace the fact that the former are based on an ides and circumstance reasoning instead of scientific proofs.
The first Illustration published that compared different stages of growth of the skull and the face came from John Hunter in 1771. His work came from experiments using pigs and observations on dry mandibles of young children.
Petrus Camper (1722-1789) evaluated growth changes of the external contours of the head using an instrument called dioptra. He gave his name to some well known measures, namely the Camper plane and Camper angle.
Herman Welcker (1822-1897) superposed newborn and adult skulls on Na-Ba line at tuberculum sellae (internal contours).
In 1922, Keith and Campion brought the theory that the skull was made of separate parts, each of which had its own growth pattern (Welcker considered the skull as a whole).
Keith and Campion were the first to introduce the structural superimposition concept. They used the pituitary fossa and the cribiform plate of ethmoid bone for this purpose since they strongly believed, without true scientific evidence available at the time, that these regions were stable areas.
Of course, radiographic cephalometers improved and more standardized methods allowed great studies to be conducted that led to extensive literature being published, like the Bolton Study of the Development of the Face of the Normal Child that started in 1928.
Lack of defined maturational markers in craniofacial region brought problems and controversies in the search for stable points, lines, contours, or structures.
In 1955, Dr. Arne Bjork published on a developed method of superimposition that used stable markers (metallic implants placed at different positions in the jaws). Superimpositions on very stable structure in the maxilla and the mandible were therefore created. As for the cranial base, he used data from human autopsy material that came from Professor Birte Melsen, an orthodontist from Danemark who published in the fields of growth and development, that allowed him to identify areas that were stable in that area after a certain age.
The structural method was evidence-based and used natural structures present in the skull that were clearly visible on cephalograms. To this day, it is still the only evidence-based superimposition technique.
Quality of Cephs and Technique
X-rays of excellent quality are a prerequisite for a possible superimposition of two or more consecutive cephalometric tracings. It is also necessary that these x-rays be taken with the same machine and under the same conditions.
Double contours are normal in cephalometry, since the left side is closer to the film than the right side. Many structures are therefore seen as double images (the gonial angle regions for example). They may also be of skeletal origin if an asymmetry is present, which is frequent. Patient positioning errors also will have an impact on the double images. These have been precisely studied in 1990.
One rule is that of the two images, the smaller one (left) is closer to the film, because the enlargement factor is smaller.
Ear rods have to be properly inserted in the acoustic meatus and the cephalometer has to be well-adjusted to prevent double images.
When double images are present, if tracing for superimposition purposes, clinicians should draw the equidistant contour of the two visible lines.
By convention the colors that are used for cephalometric tracings are as follows
Black for pre-treatment tracing
Blue for in-treatment tracing
Red for end of treatment tracing
Green for after the end of treatment tracing
Three methods are known for superimposing purposes:
The use of landmarks and lines (for example, Na-S line with sella registered as the stable landmark)
Best-fit, or anatomical fit, that uses contours where there seems as no change has occurred with time, based on personal visual perception (ex.: De Coster Line)
Structural method that uses references inside the bony or periosteal contours that have been scientifically proven to be stable within a given frame
The use of ‘best fit’ started as very popular, but it is based on the assumption that neither osseous deposition nor resorption is taking place on the periosteal surface of the ‘stable’ chosen contour. Brodie discussed on a method using best-fit of the lower border and symphysis for mandibular superimposition.
Also, the practice of superimposition brought multiple errors (i.e. positioning of subjects, quality of images, quality of tracings, inadequate superimposition) that led clinicians towards a more simple way of identifying easy-to-find landmarks on the cephalograms.
Some clinicians have searched other methods based on 2-3 landmarks, like the S-N line at Sella, but doing so implies that change in N position in straight line with original S-N line. Studies have proved that to be erroneous.
Even though it does have certain limitations, the structural method is the only evidence-based approach, but it does require great knowledge of the anatomy of the skull. Superimpositions usually can be done using one of three areas: the cranial base, the maxilla, or the mandible.
A superimposition on the anterior cranial base will reveal the changes that took place in the maxilla, the mandible, and the soft tissues.
Cranial Base: After the age of 5 to 6 years, the anterior wall of the sella turcica is a stable structure and should be used for horizontal positioning when superimposing different tracing. It is also well-defined because its cortex is greatly
calcified. It has been shown that point sella is not stable and is not a recommended superimposition landmark.
The cribiform plate of the ethmoid bone (which reaches stability at 4 years of age) and squamous part of the frontal bone can be used as a vertical guide to orient the tracings to superimpose (Figure 2.1 and 2.2).
Maxilla: The anterior contour of the zygomatic process is stable after 9 years of age and should be the maxillary element used for superimposition purposes. Care must be taken not to trace the anterior contour of the maxillary sinus.
First, locate the zygomatic crest and the orbital floor and trace them. Then trace the image of the anterior wall.
The use of the triangle for horizontal positioning is very accurate. Note that the lower end contour of the triangle is closely just above the upper first molar apices (Figure 3).
Points ANS or PNS are not recommended for maxillary superimposition since they have been shown to be unstable. The same goes for the nasal and oral borders of the palate.
Mandible: Local mandibular superimposition often used include, for example, Best-fit and Ricketts. Because the landmarks they use are not stable points, interpretation with such superimpositions could be misleading.
As a result of the superimposing x-rays on metallic implants, Bjork and Skeiller were able to describe areas in the mandible that appeared stable during determined periods of time. These
areas were named natural reference markers and should be safely used for superimposition purposes (Figure 4).
Natural reference markers are:
- outside cortical outline of the chin
- any trabecular structure in the lower part of the symphysis
- internal cortical outline of the symphysis
- fudus of the crypt of the 3rd molar (rarely used because of short time frame and developmental variation)
- contours of the mandibular canal
Guide: the contours of the anterior border of the ramus on the second tracing should always be posterior, not anterior, to the first tracing.
Example of Superimposition and Interception (Figure 5.1 to 5.3)
After superimposing on very stable structures both pre-and post-treatment x-rays of this pseudo class III patient, it is possible to appreciate what effects this author’s treatment had on upper teeth position and mandibular position. This patient was treated using an upper hyrax-type appliance and reverse facemask, followed by sectional fixed orthodontics from upper second primary molar to the same tooth on the other side of the same arch. Superimposition shows correction of third order angulation of the upper incisors, as well as downward and backward position of the mandible, which increased lower anterior facial height.
It is, however not possible to separate the amount of change that is due to natural growth of the patient and the changes caused by treatment unless x-rays are superimposed for growth evaluation purposes only without any treatment performed.
For example, the superimposition in the pseudo class III patient shows to what extent the upper incisors were brought forward and how the mandible responded to its new vertical dimension, but also that the maxilla stayed where it was at the beginning.
When practicing orthodontics, we certainly enjoy having the best available methods of evaluating the true changes our treatment has made in a patient’s skeletal and dental characteristics. But science being what it is, we need to
follow what strongest evidence recommends: cephalometric superimposition is no exception to that. Some methods of evaluation have been the best ones available so far.
On the subject of cephalometric superimposition, I found a true work-of-art and a very complete book by authors Herman S. Duterloo and Pierre Geroges Planche`, Handbook of Cephalometric Superimposition (Quintessence Publishing Co, 2001). I would not be shy to recommend it to any dentist interested in digging deeper into this somehow complex topic.
Duterloo HS, Planche` P-G. Handbook of Cephalometric Superimposition. Quintessence Publishing Co, Inc 2011, p.x
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Bjok A, Skieller V. Normal and abnormal growth of the mandible: A synthesis of longitudinal cephalometric implant studies over a period of 25 years. Eur J Orthod 1983;5:1-46