Department of Orthodontic Abstracts and Reviews

Edited l1.v

DR. EGON NETJSKUYP .CTD 1)~. .JOSF,PH I). Ew, NRW Yoeri CITY

All communications concerning further information about abstracted material and the accept- ance of articles or books for consideration in this department should be addressed to Dr. Egon

Neustadt. 133 East Fifty-Elghth Street, New York City.

The BEgntion of Tooth Germs. Abstracted from “Present Status of Knowl- edge Concerning Movement of t,he Tooth Germ Through the Jaw” by Dr. Allan G. Rrodie, *T. A. D. A., October, 1934.

THE FACE AT IIIRTH

At birth, the face is in a very low st,ate of clevelopment as compared with the cranium. The hard palat,e is at a level only slightly below the orbit.. The floor of the orbit practically constitutes the base of the alveolar process.

The germs of the maxillary teeth (five deciduous and six permanent ones) are, therefore, crowded in a very small area of bone. They lie close to the floor of t,he orbit. The alveolar process is at, this stage hardly indicated, and the maxillary sinus a narrow slit. How does this baby structure develop into the adult face? Mainly by deposition of bone on the outside of the maxilla and by absorption on its inside.

GROWTH OF THE MAXILLA

In the downward growth of the maxilla, new bone is laid down on t,he lat- eral surface of t,he alveolar process and on the under surface of the palate. The thinness of the palate is maintained by an accompanying absorption of the palatal bone on t,he side which forms the floor of t,he nose. The growth of t,he orbit takes place in a similar manner, but is completed at an early age.

The maxilla increases in width by the deposition 03 new bone on its outside, while simultaneously its inside is absorbed and t,he size of the sinus increased. The zygoma also grows by addition of bone on its outer surface and by absorp- tion on its inner surface.

There are mainly two sources for ~&reuse in depth of the face: first, deposition of bone on the labial surface of the alveolar process ; second, addition of bone on the site of the tuberosit,y in an amount equal to the combined width of three permanent molars.

GROWTH OF THE MANDIRLE

Similar changes occur in the mandible. Its height is increased by additions to the alveolar process and to the lower border. Its width is enlarged by deposition of bone on the outer (buccal) surface with a corresponding resorp- tian on the inner (lingual) surface. Its length is increased by deposition on the posterior side of the ramus with corresponding resorptions on its anterior side.

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Orthodontic A&h-acts and Reviews 1231

FIRST STAGE OF ERUPTION

(Migration of the Unerupted Tooth in the Jaw)

During these growth changes, a constant transformation of the bone on the inside of the maxilla and mandible takes place. Compact bone is being changed to cancellous bone, and vice versa.

What do the tooth crypts and their germs do during this period? In the maxilla, they start from under the baby orbit, and they finally lie an inch and a half below it, to say nothing of their lateral and forward movements. Evidently their speed in movement is even greater than the growth of the bone, because they start the migration above the alveolar surface (unerupted) and later on pass it (erupted).

Furthermore, if the alveolar process moves laterally through deposition of bone on its buccal and through absorption on its lingual surface, the tooth crypt, holding the unerupted tooth, would ultimately be exposed on its lingual side. As this is not the case, it must be assumed that the unerupted tooth travels in its crypt together with the bone.

SECOND STAGE OF ERUPTION

(Emergence of the Tooth Into the Mouth)

As the tooth crypts are formed at the same early age at which the tooth crown calcifies, it becomes evident that they have to travel a considerable distance before they reach their final place where the crypt ruptures and the tooth emerges. Their movement may be due to interstitial bone changes, but in all probability it is part of that process which is called eruption.

The life cycle of a tooth may, therefore, be divided in two stages: first, its development in the crypt and its journey through the jaw to the place where the crypt ruptures; second, the emergency of the tooth from the rup- tured crypt into the mouth, and its consequent further eruption.

For the eruption of the tooth into the mouth several factors have been made responsible :

(1) Elongation of the tooth root. (2) Multiplication of the pulp cells. (3) Deposition of new cement layers. (4) Gr0wt.h of the alveolar bone.

(Every one interested in the problem of tooth eruption will find in this article a new and instructive analysis of this complex phenomenon.)

E. N.

Changes in the Mouth Cavity During the Period of Menstruation. Abstracted from “die Verlnderung der Mundhohle wiihrend der Menstruation” by Dr. Anny Klein, Zahnlrztliche Rundschau, September, 1934, Berlin.

At the Women’s Clinic at the University of Erlangen, eighty women were examined for the condition of their teeth during and after the period of menstruation. About one-third of them suffered from some changes that were taking place in their mouths.