Laboratory procedures in RPD

Presented by: Dr. Kamleshwar Singh BDS, MDS, ICMR-IF Assistant Professor Department of Prosthodontics King George’s Medical University, Lucknow • . Laboratory procedures in RPD

There is no area of Dentistry other than framework construction where a major part of patient treatment is delegated outside the dental office. The removable Partial Denture framework constructed in this manner requires a high level of communication between clinician and technician if a quality denture is to be achieved. Introduction

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 this construction.

For laboratory construction of partial denture :- A properly prepared and articulated master cast. A diagnostic cast with a neat and specific design carefully drawn on it. A work authorization order covering all aspects of the desired denture are required. Anything less will compromise the quality.

After recording the master impression, the framework is fabricated for a cast partial denture. The framework is essential for other procedures like preparing occlusal rims, jaw relation, etc. Framework fabrication involves the following steps:- Wax-up Duplication and preparation of refractorycasts Waxing Investing Burn out Casting Finishing and polishing. FRAMEWORK FABRICATION

It deals with all the procedures done to the master cast prior to duplication. We know that the master cast is duplicated to produce the refractory cast, which is used to cast the framework. The refractory cast will not resemble the master cast because the contours of the master cast are altered by various wax up procedures before duplication. Wax up includes design transfer, block out, relief and beading of the master cast. Wax Up Procedure

Design transfer is defined as, "Conveying the outline of the proposed prosthesis from the diagnostic cast to the master cast"—GPT. The outline of the proposed partial denture is drawn on the primary cast during design. This out line is transferred to the master cast with the help of a surveyor. The colour codes used in the primary cast should be followed in the master cast also. Design transfer includes the following steps: Marking the height of contour Measuring the undercut Drawing the clasps Drawing the connectors. Design Transfer

The tripod marks of the primary cast are transferred to the master cast. The master cast can be repositioned on the surveyor using these tripod marks. After positioning the master cast, the survey lines are drawn on the abutment teeth with reference to the survey lines marked in the diagnostic cast Marking the height of the contour As a first step in design transfer, the height of contour is marked on the master cast using the primary cast as the guide

Measuring the undercut The undercut gauge is attached to the survey arm and the undercut areas are marked and measured. Drawing the clasps The clasp should be drawn so that the retentive terminal descends gracefully, into the infrabulge (undercut) area of the tooth. The clasps are drawn in reference to the primary cast. The shape of the clasp should be similar in both primary and master cast

Drawing the connectors The major connector including the lattice work is drawn using brown colour pencils. The minor connectors are drawn such that they unite the clasp with the major connector. The area to be occupied by the denture base may be drawn finally

Blockout or Wax Out Defined as, "Elimination of undesirable under cut areas on the cast to be used in the fabrication of the removable partial denture". It is the process by which the undesirable undercuts on the master cast are eliminated using wax. Since the undercuts are filled with wax, the refractory cast duplicated from the master cast will not have these undercuts.

Parallel block out This is the procedure by which, undercuts below the height of contour of the existing teeth are eliminated in relation to that path of insertion.

Arbitrary blockout This procedure involves filling the soft tissues and other unwanted undercuts in the cast with blockout wax. The purpose of this procedure is to eliminate the unwanted undercuts (ridge, soft tissue), which may interfere with the path of insertion. It is termed arbitrary blockout because the surface of the block out wax need not be parallel to the path of insertion

Formed or shaped blockout It is done in the undercut of the primary abutment along the lower border of the proposed retentive arm. The blockout wax is not trimmed to flush with the tooth surface The excess wax will form a ledge on the occlusal surface. This ledge will follow the lower border of the proposed retentive arm drawn on the master cast. This blockout will be reproduced as a ledge in the refractory cast duplicated from the master cast. This ledge guides the fabrication of the wax pattern for the retentive arm

Formed blockout (Cross-sectional view) Formed blockout

Relief It is defined as, "The procedure of placing a sheet of - wax in strategic areas on the master cast to be ' : duplicated so that a refractory cast can be made". The purpose of relieving the master cast with wax is to provide space between certain components of the framework and the adjacent oral structures such as the minor connector to which the denture base will be attached The area where the relief wax was adapted will be a elevation. Adapting spaces for relief

Beading It is defined as, "Scoring a cast with a sharp instrument or Bur in any desired area to provide a sealbetween the finished prosthesis and the soft tissue". Beading is nothing but a depression created along the borders of the framework drawn on the cast so that the resultant framework will have an elevation at its borders. This is done only for the maxillary cast to get a good seal

Duplication and Preparation of Refractory Casts Either alginate or agar can be used to duplicate the master cast. Agar is the duplicating medium of choice. The blocked, relieved and beaded master cast should be duplicated so that the resultant refractory cast, is ideal to fabricate the framework

Duplicators A duplicator is a machine designed to prepare and load the duplicating material into the duplicating flask. There is an upper reservoir compartment or storage unit where the duplicating material is heated and constantly mixed to improve homogeneity and prevent graininess. The duplicating material is maintained in a particular constant temperature with the help of a thermostat inbuilt within the system.

The reservoir has a dispensing nozzle to dispense the material. The duplicating flask should be placed below the dispensing nozzle of the duplicator during duplication procedure. The duplicator has a horizontal table to hold the duplicating flask. The horizontal table is perforated and a fan is present below the table to cool the duplicating flask. Sophisticated duplicators, which help to pour the duplicating medium in a vacuum chamber, are also available. Vacuum based duplicators prevent porosity in the duplicating material

Duplicating flasks The duplicating flask has two parts namely, the body and a reservoir ring or counter. The body of the duplicating flask forms the base of the flask where the cast to be duplicated is positioned. The reservoir ring is a dome shaped structure with vent holes through which the duplicating material can be poured in. The reservoir ring is designed such that the flange extension of the dome seats on the body of the flask : Duplicating flask. The flange of the dome sleeving over the base

The vent hole present on the upper surface of the counter should be aligned with the dispensing nozzle of the duplicator during duplication procedures.

. Relieved and blocked out master cast is positioned on base of the duplicating flask.

Duplicating flask is filled slowly with hydrocolloid material.

Base of flask rests in a container of cool running water.

Cast is teased from hydrocolloid duplicating material.

The master cast should be soaked in slurrywater for atleast 5 minutes before duplication. The soaked cast is positioned on the base ofthe duplicating flask such that there is at leastI/4th inch clearance all around the cast. The cast is secured in place on the base of theduplicating flask with the help of modellingclay Procedure

The reservoir ring is positioned to fit in the body of flask. The duplicating flask with the positioned cast is placed in the duplicating unit. The vent holes of the flask should be aligned below the dispensing nozzle of the duplicating unit : Duplicating material can be filled by aligning the nozzle with the vent holes of the flask

The nozzle is opened till the agar from thestorage unit fills the entire reservoir ring ofthe flask. When the agar completely fills the reservoirring it will flow out through the other ventspresent in it (Vacuum is created atthis stage when a vacuum based unit is used). The agar is allowed to cool in the duplicatingunit with the help of the fan placed below theperforated table holding the flask in the unit Material is filled till it escapes out through the other vent holes

After the agar cools, the base of the duplicatingcast is gently removed. The clay used forstabilization is removed. A jet of cold air can be blown at the junctionof the cast for the set duplicating material toloosen and remove the cast embedded in theduplicating medium After the agar cools, the master cast can be separated by applying a jet of air spray

Refractory investment (used to make the refractory cast) is poured into the impression of the master cast present in the duplicating material The cast is dried in an oven and treated. Refractory cast poured using refractory material

After blockout and relief, the master cast is duplicated to form the refractory cast

All the blocked out undercuts will be invisible in the refractory cast. All infrabulge undercuts will be blocked out on the refractory cast

The spacer relief will appear as an elevation on the edentulous ridge The spacer area will be an elevation on the duplicated cast

The stopper holes on the spacer will appear as depression on the elevated saddle area Stopper holes in the spacer will be replicated as depressions within the elevated zone on the refractory cast

The gingival relief will appear as an elevated band on the refractory cast The gingival relief will appear as an elevated band on the refractory cast

Refractory Cast Refractory (or investment) materials must be measured and mixed exactly according to the manufacturer's instructions to ensure that the expansion of the mold during burnout will match the shrinkage of the alloy. Gypsum-bonded investments, commonly called low-heat investments, are used for casting Type IV partial denture gold and Ticonium. This refractory material can be burned out at 704 °C (1300°F) without causing breakdown of the investment. Investments used for Vitallium, Nobillium, Jelenko's LG, and other chrome-cobalt alloys are termed high-heat and are burned out at temperatures in the area of 1037° C (1900° F).

The manufacturer's instructions give the time required for complete set of the refractory material. When this stage is reached, the cast is carefully removed from the mold and placed in a drying oven at 93° C (200° F)

Beeswax Dip To ensure a smooth, dense surface on the refractory cast and to eliminate the need for soaking the cast before investing, the dry refractory cast is dipped in hot beeswax. The cast is dried in a hot air oven at 82° to 93 °C (180° to 200 °F) for 1/2 to 1 hour, dipped into beeswax at 138° to 149° C (280° to 300 °F) for 15 seconds, and set immediately on end to allow all excess wax to run off. Once the cast has cooled to room temperature, it is ready to be waxed. Refractory cast is dipped in hot beeswax

Stewart : Clinical Removable Partial Prosthodontics,second edition ,Ishiyaku European American publishers. Carr, McGivney, Brown: McCracken’s Removable Partial Prosthodontics, eleventh edition ,Mosby publishers. Applegate : Essentials of Removable Partial Denture Prosthesis., saunders Company. Bibliography

MCQS

Brown colour used in the dental laboratory is depicting: 1.Height of contour. 2.Resin outline 3.Metal framework 4.Relief area.

The elimination of undesirable undercuts on the master cast with wax is known as 1.Relief 2.Blockout 3.Interference 4.Waxing

The blockout wax is placed at 1.At the height of contour 2. Above the height of contour 3. Below the height of contour 4. On the occlusal surface

Red colour used in the dental laboratory is depicting: 1.Height of contour. 2.Resin outline 3.Metal framework 4.Relief area

Refractory cast for Type IV gold casting is made up of 1.Ethyl slicate investment 2.Phosphate bonded investment 3.Gypsum bonded investment 4. All of the above

To ensure smooth , dense surface on the refractory cast and to eliminate the need of soaking which wax is used 1.Beeswax 2. IOWA wax 3.Korrecta I wax 4.Korrecta II wax

Work authorizations should have 1.The name and address of the patient 2.A description of the kind and type of act or service or material 3.The date of authorization. 4.All of the above

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Laboratory procedures in RPD