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A simple method to detect undercuts during tooth preparation for fixed prosthodontics

From:ads dental lab    time:2016-9-21 20:29:05
During abutment tooth preparation for fixed partial dentures, the presence of undesirable undercuts, due to clinical complicating factors such as poor visibility and access, frequently is observed.

1The presence of undercuts may hinder the complete seating of the cast restoration, resulting in the need to recontour the pre- pared tooth and make a new impression. This article
presents an easy and quick method for evaluating sin- gle or multiple tooth preparations to detect undercuts
before making the final impression.

1. Make an irreversible hydrocolloid impression after gross tooth reduction using a partial perforated tray, and form the mold with a fast-setting stone.Temporization procedures can be initiated con-comitantly to prevent loss of time.

2. Using a pencil such as the analyzing rod of a sur-veyor, hold the lead parallel to the axial wall of the prepared teeth (Fig. 1, A) and draw 2 lines that encompass the entire circumference of the prepared tooth. The first one must be coincident with the junction between the axial wall and the cervical margin (Fig. 1, B), and the other must correspond
to the junction between the axial reduction and the external cusp reduction (Fig. 1, C). Note that the

occlusal line is not always coincident with the
occluso-axial angle (Fig. 1, D).
3. An occlusal view of the cast allows a clear visual-
ization of the convergence of single or multiple
prepared teeth (Figs. 1, E, and 2). Undercuts or
excessive convergence angles can be detected
when the relative position of the 2 drawn lines are
observed (Fig. 3).
The described method creates conditions for deter-
mining proper abutment convergence and a common
path of insertion in multiabutment preparations. It is a
time-saving procedure because the undercuts can be
detected and corrected in the same clinical visit, before
the final impression is made.

1. Gold HO. Instrumentation for solving abutment parallelism problems in
fixed prosthodontics. J Prosthet Dent 1985;53:172-9.
2. Tylman SD, Malone WF. Tylman’s theory and practice of fixed prostho-

Reprint requests to:

Fig. 2. In cases of multiple prepared teeth, occlusal view
must be performed following same horizontal plan

Fig. 3. Relative position of both lines serves as reference for
analysis of teeth preparation. A, Correct preparation; B,
excessive convergence angle; C, presence of undercut

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