A STUDY OF THE PATTERN AND COMBINATIONS OF CONGENITALLY MISSING TEETH IN MAN

¹ University of Minnesota, Minneapolis, Minn.

The pattern observed in a group of persons with congenitally missing teeth is repeated in group after group and suggests that the process is going on in an orderly fashion. The observation may lead to the speculation that the human dentition is gradually readjusting from the original formula of 32 to a lesser number sufficient to satisfy the modern demands.

Individual types of teeth vary in their frequency of absence. As shown by Montagu, the upper second incisors have an anatomical association with the points of articulation of the bones which may be suggestive of a causal relationship. A similar association can be shown for the lower first incisors. However, there is no such anatomical structure relationship with the second premolars that can explain the high incidence with which those teeth are missing. Yet they fit into the general pattern in such a way that one can readily believe that the relationship between deficiencies of the teeth and the articular joints of the bones may be merely coexistent rather than causal, or at best may explain only some part of the cases of congenitally missing teeth. Moreover, the deficiency of the anterior teeth is often observed in more than one member of a family, indicating that the anomaly has not occurred purely as a matter of chance. Whether the frequently missing third molars might be explained on the basis of their relationship to the growth of the arch is uncertain.

Combinations of missing teeth in which more than one kind of tooth is absent are found in many persons. In the cases reported, approximately one-half of the individuals had the multiple deficiencies. Based upon the frequencies with which the individual types of teeth are involved, the combinations occurred too often to be due to chance alone. Not over 7 of the cases of multiple conditions can be explained as due to definite diseases, and those 7 cases lacked from 14 to 19 teeth. The other cases tend to involve only the upper second incisors, second premolars and third molars and not the teeth which are more stable in the arches.

In some families, the dental deficiencies suggest that the inherent defect is not always expressed alike in every affected member of the family. Different kinds of teeth are involved in different members of a family. The probable relationship between peg-shaped and missing teeth will be discussed in another report.

In our data the incidences observed for missing teeth are lower than those reported by some other workers. In part, the differences in frequencies may be explained by the fact that our data include, almost without exception, only one affected member of a family.

Arch size and tooth deficiency seem to coexist but seem to have no causal relationship. Some arches are too small to permit an even alignment of the number of teeth in the arch. In some arches, spaces are retained even though the teeth do not develop. Other arches are small enough that the smaller number of teeth fit the available room and no spaces are observable even though the number of teeth is less than normal. In some persons Adho lack posterior teeth, the arches have continued to grow, thus leaving space for the teeth which do not develop. As a consequence, the facial angles in those cases do not differ from the normal expectancy.