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Chapter n.
Prtparation of Gavitits.
Instruments. Cavities on Labial and Buccal Surfaces. Approxiraal Cavities in
Incisors and Cuspids; Approximo-Iabial Cavities; Approximo-lingual
Cavities; Labio-lingual Cavities. Approximal Cavities
with Step Formation.
It is essential that the beginner in inlay work should understand the
principles of cavity preparation. Disregarding this foundation work,
many dentists have become discouraged, because they began to con-
struct inlays for practical cases before they had studied these principles.
The change in the method of cavity formation is not a difficult one, even
though the operator has been preparing cavities for gold and amalgam
fillings for a long time, provided he understands thoroughly the prin-
ciples of retention for the different fillings.
The preparation of many cavities for inlays requires the sacrifice
of sound tooth structure in order to secure the necessary retentive
resistance, to prevent frail margins of porcelain, and to obtain proper
color effects. In many instances it has been found that more sound
tooth structure is involved in the preparation of the cavity for an inlay
than for gold filling. All frail enamel should be removed. Most por-
celain workers agree that the cavity should be prepared so that the inlay"
will have all the retentive resistance possible ; that parallel walls in
cavities are to be avoided, and that the walls should diverge slightly
toward the margin instead of converging. It should be borne in mind
that porcelain is a friable material, and that its strength is approximately
equal to that of enamel. These facts are frequently disregarded, and
the cavity is not prepared with the view to making the inlay as strong
as possible. Too often carelessness in cavity preparation is the cause
of weak margins of porcelain, and these, in turn, render the filling im-
perfect. In such cases there is a tendency to attribute the imperfection
of the filling to the material used rather than to the method employed.
In illustrating cavity preparation with drawings and photographs
of plaster models of teeth, it must be borne in mind that cavities pre-

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