Page 30 - My FlipBook

4 PATHOLOGY OP THE HARD TISSUES OF THE TEETH.
In this there is a failure of the formation of the cementing sub-
stance between the enamel rods in the outer third only and in
very irregular patches, the cementing substance between other
rods being perfect. The fact is well established that these
defects occur in the teeth of more than 80 per cent of persons
who live in such geographic areas during the period of enamel
formation. If such individuals reside in one of these known
geographic areas during the period of formation of a number of
the teeth, and elsewhere during the formative period of other
teeth, only those which are formed during residence in the area
will show the characteristic defects. These present the most
difficult problem of all in relating the histologic defect to the
cause, since the formation of perfect cementing substance and
the lack of formation of this substance are contemporaneous
in the individual tooth or several teeth. There is a local failure
of cells to functionate, which is evidently the result of some
general systemic condition.
These observations indicate that there are separate forma-
tive cells for the enamel rods and for the cementing substance
between the rods ; that under certain conditions both processes
are interfered with, while in others there is a failure in the for-
mation of the cementing substance, but not of the rods, and in
others a failure of rod formation. Future studies may deter-
mine how these things come about.
In the following pages the various dystrophies will be
described. To get a proper understanding of these conditions
in relation to the causes, it is necessary that one have a good
knowledge of the histological structure and development of the
teeth. There should be in mind for each tooth the average time
of beginning formation and the period required for growth to
completion. This is necessary to determine the age at which
the cause of the particular defect was operating. If it was a
condition of malnutrition its effect should be recorded by
defects in the portions of all teeth in process of formation at
the time. For example, at the age of three about two-thirds
of the crown from occlusal to gingival of the first permanent
molar has been formed ; at the same age only about the incisal
third of the central incisor has been formed, usually a little less
of the lateral incisor and only the tip of the cuspid. Therefore
an illness at the age of three which resulted in a defect of any
one of these teeth in the position mentioned should involve all
of them. The defect in the cuspid could not be as far away from
the incisal edge as in the central incisor, because the central is
always in advance of the cuspid in its formation. (Figure 9.)