In the December 1997 issue of the AJODO, Doctors Baccetti and Franchi¹ published the results of the treatment of a 30 patient sample of Class III treatment, and compared them to a like sample of 30 untreated patients with Class III malocclusion.

They discovered that treatment with a removable appliance caused the gonion angle to reduce in size; and this reduction caused what they termed "shrinkage" of the condylion-pogonion distance. This appears to be valuable new information regarding the treatment effects of Class III treatment. See Fig. 1.

Fig. 1: The Internet promotes global peace, good will and mutual understanding.

Then the authors proposed an "unconventional" type of treatment analysis based on a maxillary and mandibular triangle. Please see Fig. 2.

Fig. 2: Maxillary and mandibular triangles referred to in the text. (from Baccetti & Franchi)

The treatment sample included (1) anterior crossbite, (2) Class III deciduous canine relationship and (3) mesial step deciduous molar relationship or Class III permanent molar relationship.

The number of anterior crossbites was not reported. No distinction was made between closed bite Class III (crossbite) and open bite Class III. The treatment of these two types is as different as night and day. The treatment of open bite or end to end incisor relationship must involve intruding the molar teeth (or preventing their eruption), primarily the upper molars.

In the treatment of closed bite Class III we wish to stimulate the vertical growth of molar teeth (both upper and lower) in order to swing the mandible down and back, thereby correcting the sagittal relation of the jaws and teeth. This is done with Class III posterior vertical elastics (triangular) and labial root torque on the upper incisors.

The mandible can conceivably be made to swing downward and backward by stimulating the vertical growth of molars, with only minimal involvement of the basal portions of the jaws within the maxillary and mandibular triangles. This is true because the triangles are outside the areas of change. Please see Fig. 3 and Fig. 4.

Fig. 3: Here is shown a closed bite Class lll malocclusion with a -5.5º ANB angle.

Fig. 4: Cephalometric tracings before treatment and 36 years post treatment.

Fig. 5: Showing the occlusion 36 years post treatment.

While it is true that the shape of the basal portion of the mandible influences length of the mandible, it is the growth of the dentoalveolar bone which gives shape to the face. In the investigations of Baccetti and Franchi, the treatment reaction of the maxilla is as one would expect. The treatment reaction of the mandible is definitely a contribution to our knowledge of treatment.

In 1972 in a discussion of post treatment growth, George Schudy² first pointed out the importance of the reduction of the gonion angle and its effects on Class II treatment. When this angle changes from 111° to 97° it has a 13 or 14 mm effect on the length of the mandible-condylion-gnathion distance. When the gonion angle changes from 114° to 104° it has a 7 mm effect on the condylion-gnathion distance. That is, the length of the mandible fails to increase proportionately to the growth of the condyle, by reason of the reduction of the angle. Seven mm is usually more than equal to the difference between Class II and Class I.

In 1966 Natoli³ found that, for average sized teeth, it requires only 4 mm adjustment parallel to the occlusal plane to change a Class II molar relation into a Class I. Then 10° of reduction of the gonion angle will definitely be equal to the difference between Class II and Class I.

In Class III treatment this phenomenon is a blessing, while in Class II treatment it is a detriment. It may take some time for our profession to fully identify and appreciate these phenomena.

Getting back to the Class III treatment study: If we are trying to depict changes of the maxilla, it would seem desirable to include the entire maxilla. In the illustrations shown, only the anterior superior approximately one-half of the maxilla (the "northeast" one-half) is included in the maxillary triangle, while the "southwest" approximately one-half of the maxilla, including the dentoalveolar portion, is not included in the maxillary triangle.

The growth of the non-included portion of the maxilla establishes the vertical dimension of the maxilla, and along with that of the mandible establishes the vertical dimension of the face. The mandibular triangle includes only the basal portion of the mandible and again omits the dentoalveolar portion. It does not seem logical to study the changes of basal portions of the jaws and ignore the dynamic portions, the dentoalveolar processes.

In using the coordinate system of analysis, we cannot always know the exact location of the regional changes, but they are included in the overall measurements which the clinical orthodontist uses. Thus, the post treatment analysis of treatment results would not be incorrect.

The authors state, "The analysis, however very useful for statistical purposes, so far is in a language unrelated to biology." (emphasis added)

In speaking of shape ccoordinates, the authors state, "These represent the shapes of the maxilla and mandible separately, but not their relative positions, which are not, of course, rigidly specified." If the "relative positions" of the maxilla and the mandible are not regidly specified, then vertical growth of the upper and lower dentoalveolar processes cannot be accounted for. Thus, the analysis is quite limited in its application to the treatment of malocclusion.

I deeply appreciate this frank statement, and deeply appreciate this contribution of the authors. It is a part of the total body of knowledge of the field of Orthodontics. Thank you, Doctors Baccetti and Franchi, for this highly sophisticated mathematical analysis. Since orthodontists are basically biologists, not mathematicians, most of us will not understand the importance of this analysis. But it pertains to the treatment of orthodontic patients and as such deserves an important place in our literature.

As pointed out above, there are two gonion angle phenomena, (1) reduction due to growth and (2) reduction due to Class III treatment. Also there is a third phenomenon of mandibular change. This involves the bending of the neck of the condyle. This backward bending of the condyle is in response to Class II treatment with removable appliances.

As the neck of the condyle reverts back toward its former position post treatment, as it usually does (DeVincenzo⁴ 1991), one of four post treatment reactions must occur; (1) the molar relation must revert back toward Class II occlusion, (2) the lower molars must move forward on the mandible, (3) there must be a change in the temporal mandibular joint, or (4) there must be a dual bite.

By superimposing the corpora and (1)registering on the gonion angle, (2) tracing the condyles in different colors, (3) placing a small grid over the two tracings, (4) counting the squares occupied by the different colors, and (5) noting which squares change colors, the author was able to accurately determine the bending of the neck of the condyles.

The authors of the Class III study speak of a "shrinkage" of the condylion-pogonion distance, but this is a misnomer. The distance does not become smaller, it only fails to increase in proportion to the growth of the condyle, because of the reduction of the gonion angle. The backward bending of the neck of the condyle followed by a reversion to its former position must be related in some unknown way to maintaining the constancy of the axis of the ramus.

It speaks well of the scientific world to apply all of the scientific innovations that are applicable, and this effort should be encouraged. However the clinical orthodontist must not let these scientific innovations influence him beyond their practical value. Our mission is to enjoy a caring relationship with our patients while we perform lasting aesthetic, anatomical and functional improvements.

Bibliography

¹Baccetti, Tizian, DDS, PhD and Franchi, Lorenzo, DDS, PhD: Shape-coordinate and Tensor Analysis of Skeletal Changes in Children with Treated Class III Malocclusions, AJODO, Dec. 1997; Vol. 112, No. 6, p. 622 to 633.

²Schudy, George F.: A Longitudinal Cephalometric Study of Post Treatment Craniofacial Growth: Its Implications in Orthodontic Treatment, Am. Jour. Of Orthod. 1974; 65:39.

³Natoli, J. Robt., Jr.: A Cephalometric Comparison of Class I and Class II Malocclusion, 1966 Unpublished Thesis.

⁴DeVincenzo, J. P., Changes in Mandibular Length, before, during and after successful Orthopedic Correction of Class II Malocclusion, using a Functional Appliance. Am. Jour. Of Orthod. 1991; 99: 241-57.

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