LAB PROCEDURES INVOLVED IN THE FABRICATION OF FPD

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LAB PROCEDURES INVOLVED IN THE FABRICATION OF FPD
Prepared By
dr shabeel pn
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INTRODUCTION

• Fabrication of a fixed partial denture involves
  the fabrication of the wax pattern, casting,
  soldering and finishing. Here we shall study in
  detail about these procedures.

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MODELS,CAST, DIES

• MODEL It a positive replica of the teeth and
  adjacent structures used primarily for
  observation and diagnosis.
• CAST A working model, master cast.
• DIE Die is a positive replica of the
  individual pre-pared tooth on which the
  inlays,crowns,bridges are made. These are
  individual tooth replicas prepared for easier
  handling during wax pattern fabrication and
  finishing of inaccessible areas of the cast
• .

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TYPES OF CAST AND DIE MATERIALS

• GYPSUM
• Type IV-Dental stone, Highstrength
• Type V-dental stone ,High Strength, high
  Expansion
• Dental stonelignosulphonates
• METALS
• Electroformed/Eletroplated.
• Sprayed metals.
• Amalgam.

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• POLYMERS
• Metal-filled resins or inorganic filled resins.
• Epoxy.
• CEMENTS
• silicophosphate or polyacrylic acid bonded
  cements.
• CERAMIC OR REFRACTORY DIE MATERIALS
• For direct baking of porcelain crown or
  preparation of wax pattern for casting.
• Type IV Type V Stone dies, Electroformed dies,
  Epoxy resins , Refractory or materials are
  recommended die materials.

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IDEAL PROPERTIES OF DIE MATERIALS

1. It should be dimensionally accurate.
2. It should have high abrasion resistance, should
   possess good strength, have smooth surface .
3. Toughness to allow burnishing of foil resist
   breakage.
4. Ability to reproduce all fine details in the
   impression.
5. Compatibility with all impression materials.
6. Colour contrast with wax, porcelain alloys.
7. Easy quick manipulation rapid fabrication.
8. Non-injurious to health by touch or inhalation.
9. Economic.

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IDEAL REQUIREMENTS OF A DIE SYSTEM

• The die should be easy to remove and replace in
  its original position.
• The die must be stable when placed in the cast.
• It should be easy to mount in the articulator

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TYPES OF DIES

• Based on the design, die systems can be
  classified into
• 1 WORKING CAST WITH SEPARATE DIE SYSTEM
• 2 WORKING CAST WITH REMOVABLE DIE SYSTEM. There
  are three commonly used removable die systems,
  they are
• a)Dowel pin system
• -Straight
• -Curved
• b)Di-lok tray system
• c)Pindex system
• 3 ACCUTRAC SYSTEM

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WORKING CAST SEPARATE DIE SYSTEM

• Here, two casts are poured from a single
  impression and one cast is sectioned and used as
  a die and the other is not sectioned and is used
  as the working cast. The wax pattern is prepared
  on the die and later transferred to the working
  cast. The die is shaped and finished using an
  acrylic trimmer and the portion of the die below
  the cervical line is finished using a scalpel.
  Sharp undercuts in the base should be avoided.
• ADVANTAGE
• Simplest and easiest method
• DISADVANTAGE
• The wax pattern may get distorted while
  transferring it from the die to the cast.
• Proximal margins tend to get overcontoured.

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WORKING CAST WITH A REMOVABLE DIE SYSTEM

• In this system a special type of working cast is
  prepared and the dies are carefully sectioned so
  that the individual dies can be removed and
  replaced in their original position in the cast.
  Dowel pin systems, di-lok tray systems and the
  pindex systems come under this category.

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DOWEL PIN SYSTEMS

• Dowel pins may be straight or curved
• STRAIGHT DOWEL PIN SYSTEMS
• They are prepared using two techniques, namely
• the pre-pour and post-pour techniques.

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• PRE-POUR TECHNIQUE
• Here the die pins are positioned in their
  appropriate places within the impression using
  bobby pins and sticky wax.

The dowel pin is positioned within the impression
using sticky wax

• After positioning the pins, the impression is
  poured up to the level of the alveolar process of
  the remaining teeth.

First pour is poured upto the level of the
alveolar process

• Separating medium is applied over the first pour
  and die pin and the remaining portion of the cast
  is poured.

Separating medium is applied and the base is
poured
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• After the cast is set, the dies are sectioned
  by placing vertical sections on the interproximal
  regions of the cast using a die sectioner or a
  manual saw. Vertical sections at the inter
  proximal regions should be made without damaging
  the contour of the adjacent teeth.

• Since the separating medium was applied before
  pouring the second half of the cast, the dies can
  be easily separated with vertical sections alone.
• The die is carefully separated from the cast
  without chipping. After removing the die, a small
  hole will be visible in the cast. This indexing
  hole will guide the pin into position while
  replacing the die.

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• POST-POUR TECHNIQUE
• Here, the cast is poured upto the level of the
  crowns of the teeth in the impression.
  Subsequently, small holes are drilled within the
  first pour in the required places and the dowel
  pins are cemented into the holes The remaining
  part of the cast is poured and the dies are
  sectioned as described in the pre-pour technique.

ADVANTAGES Easy to prepare No special
equipment is necessary DISADVANTAGES The pins
may get displaced while pouring the cast.
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• 2. CURVED DOWEL PIN SYSTEM
• This is similar to straight dowel pin technique
  except that curved pins are used. The curved pins
  will project from the sides of the base of the
  cast. When the projecting pins are pressed, the
  die unit attached to the respective pin will pop
  out from its place.

Curved dowel pin system (pushing the pin from the
side pops the die out).
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• B) DI-LOK TRAY SYSTEM
• This technique uses a special tray to pour the
  cast. This special tray has orientation grooves
  on the inner aspect. Actually the tray is made of
  multiple components, which can be assembled or
  dismantled as required (Fig. 33.7).
• Impression is poured using a two-pour technique.
  The first pour is poured up to the level of the
  impression and the second or base pour is poured
  after positioning the rim of the di-lok tray over
  the impression. Before the second pour is set,
  the base of the di-lok tray is assembled and the
  cast is allowed to set. Later, the di4ok tray is
  dismantled and the grooves on the base of the
  cast formed by the di-lok tray is used as a guide
  to do die sectioning.
• Advantages
• Simple and easy to prepare
• The cast can be mounted in an articulator
• Disadvantages
• Requires special equipments.

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• C) PINDEX SYSTEM
• This system is similar to the post-pour dowel pin
  technique. Here a special drill press equipment
  is used to do die sectioning. The drill press has
  a platform with a slot like opening through which
  the drill pin will project during the procedure.
  The entire platform is spring mounted and when
  pressed down automatically starts the drill,
  which will project through the slot. The unit has
  a red pilot light lamp on top, which will shoot a
  point exactly on the drill. This red pilot
  pointer will act as a guide while drilling (Fig.
  a).

Fig - guide to drill holes in the base of the
cast. The platform of the pindex machine is
spring loaded. When the cast is passed against
the platform, the motor will get activated and
drill a hole in the casta. The cast is placed on
the pindex machine and the pilot light is used as
a
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• Die sectioning procedure is simple wherein the
  cast of 15 mm base is placed on the drill press
  and drill holes are prepared in the under surface
  of the base of the cast using the pilot light as
  a guide. After making the drill holes to the
  required depth, sleeved die pins are placed and
  cemented using cyanoacrylate adhesive. Remember
  that the holes will not be very deep. Hence a
  portion of the pins will be projecting from the
  base of the cast.

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• After cementing the pins, a base is poured over
  the base of drilled cast. Consecutively the dies
  are sectioned but not upto the base as described
  in the dowel pin system.
• ADVANTAGES .
• Allows accurate placement of the die pins.
• DISADVANTAGES .
• Special equipment is required.

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THE ACCUTRAC SYSTEM

• These dies are
  exclusively used in the fabrication of laminate
  veneers. Basically this system involves the
  fabrication of two casts as described in the
  working castwith separate die system. One cast
  (master cast) is sectioned into dies and used to
  prepare the coping. After preparing the wax
  pattern, the sectioned dies are reassembled in
  the master cast.

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• The master cast is blocked out with wax (filled
  with wax to seal the crevices). The second cast
  (refractory cast) is duplicated from the blocked
  out, sectioned, die master cast using addition
  silicone. The second cast is poured using
  specially formulated stone, whose expansion will
  compensate for the shrinkage of porcelain when
  fired in the furnace.

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Thank you