27. rpd lab procedures

LABORATORY
PROCEDURES FOR
REMOVABLE PARTIAL
DENTURES

Learning outcomes
1. Describe the laboratory steps in
making a RPD.
2. Differentiate the types of block-
out.
3. Describe what work authorization
is.
4. List the functions work
authorization

Why we need to know lab
procedures????

 Knowledge of the laboratory
phase of partial denture
construction is essential for the
clinician, who must assume total
responsibility for the design and
the quality control of all aspects
of its construction.

 It is important to have an understanding of
the laboratory procedures involved.
 1. This enables the dentist to design a
RPD framework.
 2. Complete a laboratory work
authorization that tells a technician to
design and make the framework and
 3. Evaluate the quality of the framework.

STEPS IN MAKING A
REMOVABLE PARTIAL
DENTURE

STEP 1: DUPLICATING A
STONE CAST

A stone cast may be duplicated
for many reasons:
i. Preservation of original casts
ii. Processing of interim prosthesis
iii. To check the fit of the framework

A stone cast may also be duplicated
with block-out of wax for:
Formation of investment
(refractory) cast for framework
fabrication

Duplicating Materials and
Flasks
 Duplicating materials are:
1. Colloidal (agar)
2. Silicone

 Colloidal materials are made fluid by
heating and return to gel while cooling.
The cast to be duplicated must be
placed in the bottom of a flask, called a
duplicating flask.

Duplicating Procedure:
i. Blockout on the master cast with
Ney blockout wax (can withstand high
temperature)

4 types of blockout are there:
 Paralleled Blockout,
 Shaped Blockout,
 Arbitrary Blockout, and
 Relief

Paralleled Blockout
 Proximal tooth surfaces to be
used as guiding planes and
below height of contour.

Shaped Blockout
 On buccal and lingual surfaces to
locate wax patterns for clasp
arms.

Arbitrary Blockout
 Labial surfaces and labial
undercuts not involved in RPD
design.

Relief
 Relief wax is placed in the edentulous
areas
 Beneath lingual bar connectors
 Areas in contact with thin tissues

ii. Attach cast to the bottom of the
flask so that it does not move

Making refractory cast (investment
cast) - can withstand high temperature

Investment cast Dry the cast
Dip it in hardener Hardened refractory cast

STEP 2: WAXING THE
REMOVABLE PARTIAL DENTURE
FRAMEWORK
Preformed wax patterns

 Preformed plastic wax patterns
are available which are made
from soft plastic material which
tend to stretch on removal from
their backing.

Design Transfer: Before the actual
waxing can begin, the design must
once again be transferred on the
refractory cast (beading the
master cast helps in this
procedure).

Wrought wire: can be made at
the time of wax patterns or
soldered to the framework

..
 Sprues (ingates) have the purpose
of leading the molten metal from the
crucible into the mold cavity.

.. For this purpose, they should be
large enough to accommodate
the entering molten metal and of
the proper shape to lead the
metal into the mold cavity as
quickly as possible.

..
 Sprues have the further purpose of
providing a reservoir of molten metal
from which the casting may draw during
solidification, thus preventing porosity
caused by shrinkage.
Thrad Dam in

STEP 4: INVESTING THE
SPRUED PATTERN

..
 The investment for a removable
partial denture casting consists
of two parts: the investment cast
on which the pattern is formed,

..
 and the outer investment
surrounding the cast and
pattern. The outer investment is
confined within a metal ring or
silicone ring.

..
 The investments used for RPD
casting is ethyl silicate bonded
investments and the alloys
mainly used are chromium-
cobalt or titanium.

..
 If the silicone ring is removed
before the casting is done and is
called ringless casting
technique.

..
 If you use the metallic ring, it is
not removed.

The burnout procedure serves three
purposes:
i. it drives off moisture in the mold;
ii. it vaporizes and thus eliminates the
pattern, leaving a cavity in the mold;
and
iii. it expands the mold to compensate
for contraction of the metal on cooling.

The metal is melted by an electric
muffle and the molten alloy is
forced into the mold cavity by
centrifugal force.

Dental laboratory casting[72

STEP 7: REMOVING THE
CASTING FROM THE
INVESTMENT (DIVESTING)

Once the alloy cools it is divested
and the remaining investment
material is sandblasted using
aluminum oxide.

STEP 8: FINISHING AND
POLISHING

The following rules for finishing the casting
are important:
1. High speeds are preferable to low
speeds.
2. The wheels or points and the speed of
their rotation should do the cutting.
3. A definite sequence for finishing should
be adopted and followed for every
framework.

4. Clean polishing wheels should be
used.
5. Be sure that each finishing
operation completely removes all
scratches left by the preceding one.

Once the polishing with abrasives and
polishing agents are done then
electropolishing is done as a last step.

Electropolishing:The polishing occurs
in a bath of 85% orthophosphoric
acid, which is heated t o 49 °C
(120°F) t o 60 °C (140°F). The anode
is attached to the framework and is
immersed in the solution for 6
minutes.

Electropolishing unit used for
the final surface polish of
frameworks in a heated
polishing liquid.

After electropolishing
 Try-in of framework is done in
patients mouth

http://www.remanium-
kompendium.de

Work Authorizations for
Removable Partial Dentures

Work Authorization
A work authorization contains
the written directions for
laboratory procedures to be
performed for fabrication of
dental restorations.

The responsibility of a dentist to
the public and to the dental
profession to safeguard the quality
of prosthodontic services is
controlled in part through
meaningful work authorizations.

 If work authorizations are
properly completed, they provide
a means for increased
professional quality assurance
and satisfaction in a removable
partial denture service.

Content
Information contained in a work
authorization should include the following:
1. the name and address of the dental
laboratory;
2. the name and address of the dentist
who initiates the work authorization;
3. the identification of the patient;
4. the date of work authorization;

5. the desired completion date of the
request;
6. specific instructions;
7. the signature of the dentist; and
8. the registered license number of
the dentist.

Function
The following four important functions
are performed by a work authorization:
1. It gives definite instructions for
laboratory procedures to be done and
implies an expectation of a level of
acceptable quality for the services
rendered.

2. It provides a means of protecting
the public from the illegal practice of
dentistry.
3. It is a protective legal document for
both the dentist and the dental
laboratory technician.
4. It completely delineates the
responsibilities of the dentist and the
dental laboratory technician.

Characteristics
 A work authorization must be
legible, clear, concise, and
readily understood.

27. rpd lab procedures

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27. rpd lab procedures

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