The Schudy Chronicles

The Translation of the Mandible

The translation of the mandible is a concept which is very closely related to the forward positioning of the mandible, and therefore has an important effect on the treatment of Class II malocclusion.

The term "translate" refers to the en masse movement of a body without rotation or angular displacement. It is most remarkable that Nature provides such a manner of movement of the mandible. This type of movement is produced by the growth of the condyles-but by a particular kind of condylar growth.

The growth of the condyles has two components, vertical and horizontal. It is the posterior horizontal component which translates the mandible forward. This horizontal condylar growth can be either posterior or anterior. Obviously anterior condylar growth could not possibly translate the mandible forward. It is usually associated with the rotation of the mandible, and by definition rotation and translation do not go together.

It has been stated many times in the literature that the mandible moves forward the exact amount of the posterior horizontal condylar growth, provided there is no rotation of the mandible. So far as I know and believe, the first reference in the literature to the translation of the mandible was by Creekmore.2

In the October 1998 issue of the AJODO, the authors Pancherz, Ruf and Kohlhas1 referred to the work of Creekmore. This discussion by Pancherz et al pertained to the translation of the mandible by treatment with the Herbst appliance.

They state, "Although measurements of condylar and fossa changes are difficult to perform with any degree of accuracy, the method of Creekmore makes it possible to overcome this problem. With this approach, the effective condylar growth is assessed. This is a summation of condylar remodeling, glenoid fossa remodeling and positional changes of the condyle in the fossa."

The authors stated further, "Chin position changes during and after Herbst treatment are a mirror image of effective condylar growth, provided no autorotation occurs. An anterior mandibular autorotation results in a relatively more forward directed chin movement and posterior mandibular autorotation in a relatively more downward directed chin movement."

We have been discussing these principles since 1960, and with the exception of the work of Creekmore2, Isaacson3, and George Schudy4, this is the first time that we have observed, that other investigators have used these principles. Thank you, Doctors Pancherz, Ruf, and Kohlhas!!! The principles have been discussed in several articles in the Schudy Chronicles and in several publications over the last 35 years.

When the effective vertical condylar growth is equal to vertical molar growth, then there is no rotation; and the mandibular plane remains parallel to its original inclination as it moves downward. This moves the chin forward the exact amount of the posterior condylar growth, provided that the fossa does not move posteriorly. .

The authors make the following statement: "Several growth processes are involved in mandibular autorotation, such as the effective condylar growth, vertical maxillary jaw growth, vertical maxillary and mandibular dentoalveolar growth and steepness of the incisal guidance during mandibular movement. With the use of mandibular superimposition technique of Björk and Skieller5, the present RL line (defined on the T1 head film) could be compared with an implant line. Its changes during the different examination periods show what Björk and Skeiller called 'total mandibular rotation,' which is the true jaw bone rotation not masked by remodeling processes at the lower border of the mandibular corpus." A distinction should be made between effective vertical condylar growth and horizontal condylar growth. Horizontal condylar growth can be either posterior or anterior, and they have opposite effects on chin position.

The article revealed some remarkable statistics. When comparing the herbst group with the Bolton group, the Pg (pogonion) change in the herbst group was three times larger in the vertical and twice as large in the horizontal direction. These changes had a P value of .001.

The authors commented widely concerning the effects of autorotation, but they failed to state whether this rotation was due to excessive or deficient growth-whether, for example, the rotation was due to excessive or deficient vertical molar growth. If I may say so, the term "autorotation" is a misnomer. The mandible does not "self rotate" but is forced to rotate by opposing growth increments.

The authors state, "Chin position changes during and after Herbst treatment are a mirror image of effective condylar growth, provided no autorotation occurs." Yes, this is a correct observation!! But first, there must be a mirror image of the opposing causative increments, the building blocks. Pancherz also states that, "As a result of the effective condylar growth, the chin moves forward and downward." However effective condylar growth cannot move the chin downward. The more the condyles grow, the less the chin moves downward. Please see Figs. 1 and 2 and Art. 12 in the Schudy Chronicles.

Fig. 1: (Fig. 8 in the Pancherz et al article)
This is an example of a "mirror image" of growth, mentioned in the article by Pancherz, Ruf and Kohlhas. We added the posterior horizontal growth (8 mm) and the forward movement of the chin (8 mm).
Effective condylar growth (Co) and chin position changes (Pg) in 14-year-old boy treated with the Herbst appliance for 0.5 years. (T1, before treatment; T2, after treatment when the appliance was removed; T3, 0.6 years after treatment; T4, 4.0 years after treatment.) No mandibular autorotation occurred during the examination period (T1 to T4).
(From Pancherz, Ruf and Kohlhas)


Fig. 2: It has been said that the remoldeling of the lower border of the mandible offsets the shortening of the condylion/gnathion distance by the reduction of the gonion angle. However, the condylion/gnathion distance failed to increase by several millimeters.
(Fig. 3 in Pancherz article) Measurements of effective condylar growth (Co-point changes) in relation to the RL/RLp reference grid (defined on the first head film). Superimposition of the second head film (2) on the first head film (1) with stable mandibular bone structures used for orientation. The second head film (2) refers to the film from either T2, T3, or T4. We measured the gonion angle change and found it to be approximately a 7° reduction. This would have approximately a 5 mm effect on the condylion/gnathion distance. This distance would fail to increase 5 mm because of the reduction of the gonion angle. Please see Art. 1, Figs. 5 and 6, of the Schudy Chronicles. (From Pancherz, Ruf and Kohlhas)

The vertical effective condylar growth in millimeters must equal the combined vertical growth of the upper and lower first molars in millimeters. This causes the mandible to "translate" forward the exact number of mm that the condyles grow posteriorly. To make this measuring process completely accurate we would need to subtract the number of mm that the fossa moves posteriorly.

In devising a system of measurements to analyze the growth of the craniofacial complex, it would seem important to keep the measurements square with the world, that is, parallel and perpendicular to the earth's surface. Buildings are built on this principle. The World Conference of Scientists held in Frankfort, Germany in 1929, to establish the Frankfort plane, was based on this principle. This plane was selected because it is more nearly parallel to the earth's surface than any plane they could find. Longitude and Latitude are based on this basic principle.

To construct an analysis based on varying degrees of a line, such as the occlusal plane, has many disadvantages. The more a dimension grows, the greater the distortion. But when we base an analysis on lines which are perpendicular, such as sella horizontal and the great divide, regardless of which way an anatomical point moves it can be accurately measured.

Vibeke Skieller5 et al stated, "The mandibular growth rotation is composed of a complex system of movements," but failed to identify these movements. She failed to identify the increments which cause the mandible to rotate. Dr. Skieller et al also stated that, "The lower facial height is, to some extent, dependent on incisal occlusion." Incisal occlusion could not produce lower facial height. Only molar height can produce lower facial height. Again, this indicates that there is a lack of understanding of the effect of molar height. Apparently the function of these growth increments is not well understood. (These concepts have been discussed in lectures for the last 39 years, published in 1964, 1965, 1966, 1968, 1973, 1992 and on the Internet in 1997, 1998 and 1999.)

The relationship between the effective vertical condylar growth (horizontal growth) and vertical growth of the molar teeth, determines whether the mandible rotates backward or forward, or whether it does not rotate. When effective condylar growth exceeds vertical molar growth the mandible rotates forward. When vertical molar growth exceeds effective condylar growth the mandible rotates backward. This simple relationship controls the function of the chewing process. These concepts have been confirmed by Isaacson.4

Measuring from the occlusal plane is probably a valid analysis but seems to apply to only a narrow segment of total growth. The occlusal plane of the permanent teeth does not arrive until about age seven. Please see Figs. 3 and 4. It seems much more logical to start with a landmark which is present at birth, such as sella turcica.

Fig. 3: (Fig. 4 in Pancherz article)
Showing that Pancherz bases his analysis on the occlusal plane. Measurements of chin position changes (Pg-point changes) in relation to the RL/RLp Reference grid (defined on the first head film). Superimposition of the second head film (2) on the first head film (1) with the stable anterior cranial base bone structures used for orientation. The second head film (2) refers to the film from either T2, T3, or T4.
(From Pancherz, Ruf and Kohlhas)

By drawing a line through sella parallel to the earliest Frankfort line, we can establish a stable landmark for vertically measuring all anatomical points in the craniofacial complex. The great divide, a vertical line drawn downward from sella perpendicular to Frankfort, can be used to relate all horizontal growth. Thus, the entire craniofacial complex can be accurately measured both vertically and horizontally.

Also using sella turcica as a starting point seems easier to accurately measure the causative growth increments, the building blocks. Other investigators have failed to recognize the effect of vertical molar growth. It seems easier to visualize the reciprocal effect of vertical and horizontal growth increments when we relate vertical growth to sella horizontal and horizontal growth to the great divide. The use of autorotation is not necessary when we use reciprocal growth increments.

Measuring from the occlusal plane, which is not present until about age 7, as opposed to measuring from sella horizontal are methods as fundamentally opposite as could be found. These two methods could not possibly be of equal value. Since sella horizontal is present at birth and is completely stable it would seem to be preferable. Please see Fig. 4.

Fig. 4: Here the "sella horizontal analysis" and the "occlusal plane analysis" are both displayed on the same head film tracing, to show their effects on facial measurements.
Please note that the great divide and a perpendicular to the occlusal plane, both passing through sella turcica, result in a remarkable difference in the horizontal dimension of the face. Their divergence causes approximately a 5 mm difference in the horizontal dimension at about the midface, approximately a 10 mm difference at the gonion angle and approximately a 15 mm difference at the vertical level of the chin. This same divergence caused an 11° difference in the vertical dimension of the face.

When the occlusal plane changes its inclination, as it usually does, then a vertical plane perpendicular to this plane passing through sella would unfavorably affect horizontal measurements which are measured perpendicular to this vertical plane.

The occlusal plane is the effect, not the cause, of anatomical relationships. The position of the occlusal plane is determined largely by the vertical growth of the maxillary teeth. The inclination of the occlusal plane is determined largely by the growth of the dentoalveolar bone, but the changes in the inclination are caused by the relative amounts of vertical and horizontal bone growth.

The cause of the changes in the inclination of the occlusal plane is the relative amounts of (1) the vertical effective condylar growth and (2) the vertical growth of the molar teeth. This is the basis for (1) the vertical growth of the craniofacial complex and (2) the anterior growth of the chin. In other words, these growth increments control the functional activity of the complex. They do not control the anteroposterior growth of the maxilla.



In Fig. 5 and Fig 6 are the growth studies of two individuals. In Fig. 5 the occlusal plane was 4° steeper with relation to SN in the beginning. At the end of the study the occlusal plane of the individual in Fig. 5 was 8° flatter. In Fig. 6 the occlusal plane became 5° steeper-a total of 13° in the inclinations of the occlusal planes. .


Fig. 5: This illustrates that condylar growth (as related to vertical growth) is the key to changes of the occlusal plane. The posterior growth analysis shows that the condyles grew 23 mm and the vertical growth in the molar area was 18 mm (9+6+3). The result was an 8° change of the occlusal plane. (From the growth study of the U. of Mich.)


Fig. 6: Again showing that condylar growth when related to vertical growth is the key to the behavior of the occlusal plane. Poor condylar growth (4 mm) could not keep pace with 10 mm of vertical growth (4+2.5+3.5). The result was a 5° change of the occlusal plane. (From the U. of Mich. Growth study.)



Yet, through all of these complex growth changes, the relationship between vertical and saggital increments, accurately depicted these changes. Please note that as the effective condylar growth exceeded the vertical molar growth, the occlusal plane tipped down on the posterior end. When the vertical molar growth exceeded the effective condylar growth, the occlusal plane tipped downward anteriorly. This is as one would expect, since the relationship of the increments caused the rotation of the occlusal plane. Again this is objective proof that the "sella horizontal" analysis is scientifically valid.

The divergence of the great divide and a perpendicular to the occlusal plane, both passing through sella, causes approximately a 5 mm anteroposterior difference in the two methods at midface, approximately a 10 mm difference at the gonion angle and approximately a 15 mm difference at the vertical level of the chin. Also this divergence causes the same differences vertically.

These shocking differences make it obvious that the analysis based on sella horizontal is "square with the world," and scientifically valid.

Dr. Pancherz et al are to be highly complimented on this marvelous discussion of treatment with the Herbst appliance. The size of the sample and the period of time studied gives us an accurate report on the overall effects of treatment with the Herbst appliance.

It is hoped that this discussion will help colleagues to understand mandibular translation and its importance in Class II correction.

Bibliography

1. Pancherz, Hans, D.D.S., Ruf, Sabine, D.D.S. and Kohlhas, Peter, D.D.S., "Effective Condylar Growth" and Chin Position Changes in Herbst Treatment: A Cephalometric Roentgenographic Long-Term Study. AJODO: Oct. 1998; 114: 437-46.

2. Creekmore, T.D., Inhibition and Stimulation of Vertical Growth of the Facial Complex: Its Significance to Treatment, Angle Orthodontist, 1967; 37: 285-97.

3. Isaacson, Robert J., D.D.S., Ph.D., Zappel, Richard J., D.D.S., M.S., Worms, Frank, D.D.S., M.S.D., Bevis, Richard R., D.D.S., and Speidel, T. Michael, D.D.S., M.S.D., The Effects of Mandibular Growth on the Dental Occlusion and Profile, Angle Orthodontist, April 1977.

4. Schudy, G. F., A Longitudinal Cephalometric Study of Post-Treatment Craniofacial Growth: Its Implications in Orthodontic Treatment. AJO 1974; 63: 39.

5. Skeiller, Vibeke, Björk, Arne, and Linde-Hausen, Torkild, Prediction of Mandibular Growth Rotation Evaluated From a Longitudinal Implant Sample, AJODO 1984, Vol. 86, No. 5.

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