Over the last few years there has been an important change in the cephalometric analysis of growth and treatment changes required by several orthodontic boards including the American Board of Orthodontics, the Angle Society of Europe and the European Board of Orthodontics. The new requirements include “structural superimpositions” of the treated cases presented to the Boards. This is a major change from the previous requirement of analyzing growth and treatment changes using a so-called “best fit” superimposition. The best-fit superimposition technique was in most cases misleading and yielded incorrect information about the changes that had taken place during treatment. Current requirements of a structurally based superimposition are biologically more meaningful and include three superimpositions that demonstrate the changes during treatment as well as post treatment, and example of this technique is shown in Figure 1
Example of “Structural Superimpositions” in a treated patient. A. General facial growth. B. Maxillary growth and treatment with occlusograms. C. Mandibular superimposition with occlusograms. The patient was treated for a Class II, Div. 2 malocclusion. View Hi-Res Image
The superimpositions are now required to be made on biologically stable structures in the cranial base, the maxilla and in the mandible, as advocated by Björk et al., 1
and Dopple, 3
and scientifically supported by their studies using metallic implants, also called radiographic markers; not to be confused with TADs or modern implants to replace missing teeth. The technique is referred to as “the structural superimposition,
” because it uses stable anatomical structures and landmarks.
In a study comparing Anatomical and Implant Superimposition Gu and McNamara4
found that the previous ABO method for superimposing serial headfilms, using a “best fit” technique, provided erroneous information concerning bone growth and remodeling. They also found that tooth movements could be “distorted significantly depending on the method of superimposition.”
Isaacson et al.5
demonstrated this problem by comparing the best-fit with Björk’s implant technique.6, 7, 8
They showed that for instance when the superimposition, to study mandibular changes, is made on the lower border of the mandible and registered at the symphysis, using the so-called “best-fit technique,” the teeth are often seen to move in the opposite direction to the movement seen with an implant superimposition. A further problem is the direction of condylar growth that is completely different between the two techniques (Figure 2
Comparison of superimpositions made on cranial base and implants in maxilla and mandible (A) with best-fit superimpositions (B) in a subject from “Facial Growth and Tooth Eruption” by Björk, A. and Skieller V. AM. J. Orthod. 1972: vol. 62; 4; 339-383. A. A forward-rotating case is superimposed on the anterior cranial fossa registered at sella, left. In the middle figure, the maxilla is superimposed on implants as the mandible is to the right. The mandible dashed lines represent the age of maximum growth rate. The dotted and solid lines represent 3 years before and after the maximum growth rate age. B, Left, tracings of the dotted and solid figures but now are superimposed on the anterior cranial fossa registered at sella. Middle, the maxillae are superimposed on the palatal plane (ANS-PNS) registered at ANS. Right, the mandibles are superimposed on the mandibular plane (Gn-Go) resistered at Gn. (Isaacson. R. J., Worms, R. W. Speidel, M. AJO; vol. 70; no. 3, 1976, Permission Elsevier) View Hi-Res Image
Where the condylar growth direction, when studied with implants as in this case, is upwards and forwards, with the best-fit superimposition it is seen to be upwards and backwards. This has led to the misunderstanding that an upward-backward growth direction, as seen in Figure 2 B, is the most efficient way for the mandible and chin to come forward, when in fact it is the upward forward growth direction of the condyle that results in forward mandibular growth. The explanation is that the latter is associated with a greater vertical component, which is important for posterior face height increase that determines the direction of mandibular displacement.
In their comparative study of best-fit versus implant superimposition Isaacson and coworkers retraced all 21 cases from Björk and Skieller’s article on “Facial development and tooth eruption: An implant study at the age of puberty.” 9
The process was as follows; tracings from the original article were copied, retraced and then superimposed to illustrate the differences between best fit and implant superimposition and included general facial growth, maxillary and mandibular growth and tooth movements. One of the most striking differences was in the actual tooth movements in both maxilla and mandible, but there were also distinct differences in the growth direction of the condyles. Figure 2 A and B demonstrate the two different superimpositions in a subject seen side by side (Case 15), and it can be seen that the teeth move quite differently between the two analyses. The individual superimpositions on maxilla and mandible, seen in Figure 2
, demonstrate the tooth movements within the maxilla and mandible that clearly are very different. On the implant superimposition the lower incisors move forward or proclined slightly (Figure 2A
), whereas on the “best fit” superimposition they move posteriorly (Figure 2B
). Differences can also be seen with respect to the lower molars that with best-fit superimposition move distally whereas with implants they move mesially. It is an interesting fact that it took so many years for the “structural superimposition,” despite numerous well-documented implant studies, to finally become the recognized and recommended method for superimposing serial headfilms. However, despite the recent changes in requirements there is still work to be done in order to achieve a more precise analysis of the molar positions on the headfilm and also their movement during treatment. It is notoriously difficult to precisely determine molar positions on the lateral headfilm by simple visualization. In a recent study we presented a new method for achieving a more precise determination of the first molar position by using measurements from occlusograms.10
The difference between a best fit and an implant superimposition is especially pronounced during the most active growth period at puberty, which is when most patients are treated. In cases where the mandible shows pronounced forward or anterior growth rotation these differences are
Variations in mandibular condylar growth direction, tooth movement and modeling of the mandibular lower border. The red arrows indicate the effective vertical component of condylar growth. The period of growth includes six years around puberty. From Björk, A. Variations in the growth pattern of the human mandible: Longitudinal radiographic studied by the implant method. J. Dent. Res. 1963: v42; 1; 400-411.1 View Hi-Res Image
more noticeable. Remodeling changes typically include apposition of bone under the anterior half of the mandible and resorption of the lower posterior border of the mandible. Both changes are adaptations to the masticatory muscles that are attached to the mandible. These remodeling changes vary depending on facial type as illustrated by two examples from Björk’s early implant studies, seen in Figure 3
.1 Note the differences in modeling between the two more extreme types of mandibular growth and also the difference in condylar growth direction and amount. These remodeling changes relate to changes in position of the mandible within the soft tissue matrix during the growth period and are in response to changes in muscle length and attachment .11
The tooth movements seen clearly differ between the two superimpositions. In the case seen in Figure 3A
, the incisors move forward, and the molars migrate mesially, whereas in the case in Figure 3B
the incisors erupt posteriorly and the molars vertically with no forward movement.
The resorption of the lower border of the mandible is a biological response to the rapid lowering of the mandibular ramus resulting from condylar growth. One might then ask what causes this resorptive modeling to take place? The best understanding, we have is that the muscle fibers of the pterygo-masseteric sling attached to the mandibular ramus are not capable of lengthening fast enough to keep up with the rapid growth changes, thus affecting these changes to maintain their insertion in the bone. The opposite muscle-bone reaction takes place anteriorly in cases with forward growth rotation of the mandible. Below the symphysis, along the posterior border of the symphysis and along the anterior part of the lower border of the mandible, bone is often added in order to maintain the insertion of the muscles. The result over time in a continuous thickening of the inferior lower and posterior border of the symphysis, and of the anterior lower border of the mandible (Figure 3A
). Note that there is no apposition on the anterior part of the symphysis or the chin area, so this area can safely be used for superimposition. The tracing of the mandible of the subject on the right in Figure 3B
, on the other hand, shows a different direction of condylar growth. The condylar growth direction in this case is upwards and backwards, and the amount of vertical growth (indicated by an arrow) is much less than in the case seen in Figure 3A
. As a result, there is little or no need for lower border modeling. Björk recognized early on that facial growth was complex and that modeling changes varied between facial types.6, 7, 8
He also found that these anatomical changes could only be studied by in detail by using a technique that eliminated the influence of surface modeling of the bones, and began using small metallic implants or radiographic markers that could be embedded in the jaw bones. As there is no interstitial bone growth, these markers are permanent and remain stable over time. In the following we will describe the three most typically used superimpositions to demonstrate facial growth and treatment changes.