THE EDITOR'S CORNER

It has been contended by more than one author that the normal position of the first molar is in a marked mesial inclination, with the distobuccal cusp farther to the occlusal than the mesiobuccal cusp. This places the upper first molar in a kind of reverse toe-hold position on the distobuccal and centrobuccal cusps of the mandibular first molar. It is contended that this position permits a better interdigitation of the bicuspids and cuspids, while a maxillary molar position with the buccal cusps on line or with the mesiobuccal cusp lower than the distobuccal, causes the bicuspids and cuspids to fall into a too-mesial position.

My education in dental anatomy and occlusion was at variance with this idea. My instructor in dental anatomy was Dr. Moses Diamond and in his book, Dental Anatomy, he wrote: "The occlusal plane of the posteriors usually describes a slight curve in the direction of the lower teeth. The lowest point of the curve is the summit of the mesiobuccal cusp of the upper first molar . . . The curve is known as the curve of Spee". He went on to say that the absence of a curve of Spee, with posteriors on a flat plane is within the bounds of normal variation. He also stated: "The bicuspids are slightly inclined towards the mesial about five degrees from the central axis. This inclination, how ever, diminishes with the first molar and disappears with the second and third molars. The mesial inclination of the upper first molar, however, seems to increase considerably in degree with advancing years and may be due to the fact that a much greater stress is directed against the mesial half of the upper first molar, which tends to incline the upper first molar further mesially". Also, "The mesiobuccal cusp is slightly longer than the distobuccal cusp as well as slightly wider mesiodistally".

If the description in Dr. Diamond's book is correct, it is possible that the other authors are taking their substantiating material from adults and adult skulls. Applying that to children may not be correct.

I have a feeling from looking at numerous models before and after treatment, that the upper first molars in treated and untreated cases, in malocclusion and good occlusion, are in a variety of axial inclinations. With all the possibilities for variables including variations in tooth size naturally and restoratively, it would be difficult to assume that the mesial axial inclination is the correct one. From what I have retained from my early education and the brainwashing on forward resultant forces from mesially inclined occlusion, I would favor upright upper first molars or a slight distal axial inclination if I were aiming for one of the three.

To gain some further insight into this, I went to my files and pulled fifty consecutive successfully treated cases. I subjectively decided whether the upper first molars were in mesial axial inclination in the before-treatment models and in the after-treatment models.

Of the fifty cases, twenty were extraction cases and thirty were nonextraction cases. Of the twenty extraction cases, nine had mesial inclination of upper first molars before and after treatment.

Eleven did not have mesial inclination before treatment, but ten of these had it after treatment. Therefore, of the twenty extraction cases, nineteen had mesial axial inclination of upper first molars after treatment.

Among the thirty non-extraction cases, sixteen had mesial axial inclination of upper first molar before treatment and ten did after treatment. Of the fourteen who did not have the inclination before treatment, four did after treatment.

What I deduce from these figures is that space closure in extraction cases may be responsible for the mesial axial inclination of upper first molars. They also make me think I am not doing too well with the uprighting of upper first molars in non extraction cases. Those who favor mesial axial inclination for upper first molars would say that I am not doing well at all in achieving that mesial axial inclination in my non-extraction cases. How about pulling fifty of your treated cases and charting their inclinations, extraction and non-extraction, before and after treatment. I would be interested in your results.