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222 PREPARATION OF CAVITIES.
condition perfectly. A (Ictacheil section of enamel j)risnis is represented
at (/, and at h is slu)\vn a portion about being separated by a chisel.
This will explain why enamel unsupported by dentin should not be
allowed to form the margin of a cavity, for it will probably either be
fractured while the filling is being introduced or afterward in mastication.
On all convex surfaces of a tooth the enamel rods radiate outwardly,
and by forming the margins of a cavity on these lines it will have a
slightly flaring or trumpet-shaped orifice, which will not only afford the
greatest strength but will admit of a Ix'tter finish being given to the edges
of the filling. In many cases it will be necessary to give the margins of
a cavity more of an outward bevel than would be obtained by simply
followino- the cleavao-e lines of the enamel rods. This can be secured
by cutting away the outer ends of the enamel rods in an oblicpie direc-
tion as shown at c in Fig. 168. No weakening of the border will result
in such cases, inasmuch as the shorter rods will still rest upon the
dentin. If, however, the rods were cut so as to leave only their outer
ends in place, as shown at d, they would have no substantial support,
and would be liable to be crushed during filling or afterward. All
cavity margins should have the outward bevel to a greater or less
extent in order to secure the best and most permanent results.
In cavities upon depressed or concave surfaces of teeth it would not
^^ ^^ have the enamel margins formed on the lines
Fig. 169
B of enamel cleavage, for this would make the margin
P
of the orifice the most contracted portion and result
in frail marginal edges. Fig. 169, representing a
cross section of a bicuspid tooth with a cavity in the
,. ^ , sulcus, will illustrate this point : A shows the cavitv
Cross section of a bicus- ' i
pid showing treat- orifice prepared on the lines of enamel cleavage,
ment of enamel mar- i xi i • xi, i. i x' i
gins of cavity in the ^^^^^ ^ ^'^^ dressing across the outer edges of enamel
sulcus. required to give the necessary strength.
It may therefore be laid down as a rule that to secure the best results
the line of a cavity wallfrom ivithin outward should form with tlie surface
of the tooth at this point an obtuse angle.
Beside the proper shaping of a cavity margin it should also be made
as smooth as possible. In accessible cavities upon exposed surfaces of
teeth the final marginal smoothing or finish can best be effected by the
use of a bur shaped somewhat like a fissure bur, but having a rounded
end and being simply file-cut upon its surface instead of being bladed.
Such a one is shown in Fig. 170. Its sides being parallel, no rounding
of the cavity margins can occur when it is used with the end inside of
the cavity. Any other form of bur with a short head would unavoidably
give to the cavity margin either a concave or a convex surface, both of
which would be incorrect.
222 PREPARATION OF CAVITIES.
condition perfectly. A (Ictacheil section of enamel j)risnis is represented
at (/, and at h is slu)\vn a portion about being separated by a chisel.
This will explain why enamel unsupported by dentin should not be
allowed to form the margin of a cavity, for it will probably either be
fractured while the filling is being introduced or afterward in mastication.
On all convex surfaces of a tooth the enamel rods radiate outwardly,
and by forming the margins of a cavity on these lines it will have a
slightly flaring or trumpet-shaped orifice, which will not only afford the
greatest strength but will admit of a Ix'tter finish being given to the edges
of the filling. In many cases it will be necessary to give the margins of
a cavity more of an outward bevel than would be obtained by simply
followino- the cleavao-e lines of the enamel rods. This can be secured
by cutting away the outer ends of the enamel rods in an oblicpie direc-
tion as shown at c in Fig. 168. No weakening of the border will result
in such cases, inasmuch as the shorter rods will still rest upon the
dentin. If, however, the rods were cut so as to leave only their outer
ends in place, as shown at d, they would have no substantial support,
and would be liable to be crushed during filling or afterward. All
cavity margins should have the outward bevel to a greater or less
extent in order to secure the best and most permanent results.
In cavities upon depressed or concave surfaces of teeth it would not
^^ ^^ have the enamel margins formed on the lines
Fig. 169
B of enamel cleavage, for this would make the margin
P
of the orifice the most contracted portion and result
in frail marginal edges. Fig. 169, representing a
cross section of a bicuspid tooth with a cavity in the
,. ^ , sulcus, will illustrate this point : A shows the cavitv
Cross section of a bicus- ' i
pid showing treat- orifice prepared on the lines of enamel cleavage,
ment of enamel mar- i xi i • xi, i. i x' i
gins of cavity in the ^^^^^ ^ ^'^^ dressing across the outer edges of enamel
sulcus. required to give the necessary strength.
It may therefore be laid down as a rule that to secure the best results
the line of a cavity wallfrom ivithin outward should form with tlie surface
of the tooth at this point an obtuse angle.
Beside the proper shaping of a cavity margin it should also be made
as smooth as possible. In accessible cavities upon exposed surfaces of
teeth the final marginal smoothing or finish can best be effected by the
use of a bur shaped somewhat like a fissure bur, but having a rounded
end and being simply file-cut upon its surface instead of being bladed.
Such a one is shown in Fig. 170. Its sides being parallel, no rounding
of the cavity margins can occur when it is used with the end inside of
the cavity. Any other form of bur with a short head would unavoidably
give to the cavity margin either a concave or a convex surface, both of
which would be incorrect.
