Page 266 - My FlipBook

264 CAPILLARITY OF APPROXIMAL SURFACES.

The rationale of (H'i surfaces is readily made apparent by considering certain facts and prin-
ciples of physics.
Wh(Mi a tube is inserted in a li([uid capable of wetting its surface
the liquid will rise to a higher level within the tube than the surface
level of the surrounding liquid. This phenomenon is known as capil-
lary attraction, and is explained upon the principle of " surface tension
of liquids." If, instead of a tube, two rounded or flat plates are im-
mersed in the licpiid, the same rising of the fluid between them will
be noticed. The smalh^r the tube, or the nearer the two plates are
together, the higher will the liquid rise.
Applying the princij)les governing these facts to the teeth and con-
sidering them as bodies immersed in a liquid (saliva), it will readily be
seen that if the appnjximal surfaces of the teeth were parallel and
close together the saliva would rise to a higher level between them and
cover more tooth surface than if they stood farther apart, and being re-
tained in this narrow space with little opportunity for motion the saliva
would soon assume an acid character and destruction of the tooth tissue
begin. This is exactly what takes place upon approximal surfaces
made flat by filing, and will occur whether fillings have been placed
in such surfaces or not.
Normally, however, the crowns of the human teeth are more or less
convex upon their approximal surfaces and touch each other only at the
point of their greatest transverse diameters, which is near to and just
above the occlusal surfice. From this point their diameters gradually
become less until they reach the cervical border, where they are smallest.
This leaves a triangular interdental space with the base of the tri-
angle at the gum, as shown in Fig. 221, in which the saliva will rise but
a short distance owing to the separation near the
Fig "^21 . ^
gum and the consequent lessening of the capil-
lary attraction. For this reason teeth preserving
their normal forms are less liable to approximal
decay than they could possibly be under any
Showing nornicil eonUct of other COuditionS.
teeth.
The earliest treatment of approximal sur-
faces with a view to the prevention of caries consists in gaining access
to them by wedging, and if found to be superficially affected by caries
the removal of the injured structure and the perfect polishing of the
surfaces.
When cavities of moderate size are discovered they should be care-
fully prepared and filled, preserving the original contour as far as
possible. Decay may recur, but it is less likely to do so with advan-
cing age, increased density of tissue, and proper prophylactic treatment.