Laboratory steps of crown and bridge fabrication

1. Dr.Muhammad Rafay Imran
House Officer
Department of Prosthodontics

2. Introduction
 CROWN: A prosthetic appliance or a prosthesis which is permanently attached to
teeth and replaces a single tooth individually.
 Cannot be removed by the patient.
 BRIDGE: A prosthetic appliance or prosthesis which is permanently attached to
teeth and replaces one or several teeth.
 Cannot be removed by the patient.

3. Laboratory steps of Crown and Bridge
Fabrication
 Impression
 Pouring of impression by dental plaster or dental stone
 Removal of Undercut

4. Wax up
 Armamentarium:
 Bunsen Burner
 Inlay wax
 Waxing instruments
 Cotton cleaning cloth
 Soft toothbrush
 The finished wax pattern should be an accurately shaped
anatomic replica of the original teeth.
 Should not be too bulbous or too flat

6.  Spruing: a sprue is the passage through which a molten material is introduced into a
mold.
 Functions of Sprue:
 Forms a mount for the wax pattern
 Creates a channel for elimination of wax
 Forms a channel for entry of molten metal
 Selection of Sprue:
 Diameter:
 It should be of the same size of the thickest
Portion of the wax pattern
 Attachment:
 It should be attached to the thickest portion of the
wax

7.  Burn out by lost wax technique: Oven is preheated to approximately 200 Degrees
Centigrade for 30 minutes
 Then temperature is raised slowly to 600 degree centigrade for 45 minutes

8.  Casting: The action of pouring or injecting a flow able material into a refractory mold
 Source: The Journal of Prosthetic Dentistry
 The Glossary of prosthodontic Terms
 Ninth Edition

9. Investment Materials
 Gypsum Bonded
 Silica Bonded
 Phosphate Bonded

10.  TYPES OF GYPSUM-BONDED MATERIALS
 Primary use: Mold gold casting alloys
 Type I: thermal expansion type; for casting inlays and onlays
 Type II: hygroscopic expansion type; for casting inlays and
 onlays
 Type III: for casting complete and partial dentures

11.  SILICA-BONDED INVESTMENT MATERIALS :
 Silica-bonded investment materials are used for casting high-melting alloys

12.  PHOSPAHTE-BONDED INVESTMENT MATERIALS:
 They are the most commonly used investment for high-melting alloys
 Primary use: Molds for base metal and gold casting alloys, molds for cast ceramics and glasses
,Refractory die for ceramic
 There are two types of phosphate-bonded investments:
 Type I: for inlays, crowns and other fixed prosthesis
 Type II: for partial dentures and other cast removable restoration

13. Armamentarium for Casting Technique
 Armamentarium:
 Centrifugal Machine
 Crucible
 Blow Torch
 Protective Colored goggles
 Casting Alloy
 Flux

14.  Procedure: the casting machine is given three clockwise turns (four if metal-
ceramic alloys are used) and locked in position with the pin.
 Then we light up the torch and adjust it.
 The crucible is preheated (Preheating avoids slag formation)
 Then, the alloy is heated in the reducing part of the flame until it is ready to cast.
little flux can be added to conventional gold alloys.
 The mold is placed in casting machine and kept on alloy with the reducing flame
until the crucible is moved into the position.
 The casting machine arm is then released to make the casting. The machine is
allowed to spin until it has slowed enough that it can be stopped by hand and
ring is removed.

15. Recovery of the Casting
 After the red glow has disappeared from the button, the casting ring is plunged
under the running cold water into a large rubber mixing bowl.

16. Defects in the casting.
 Roughness: can be caused by improper water/power ratio, excess burnout
temperature
 Nodules: Air trapped during investing procedure
 Incomplete casting: Wax pattern too thin, inadequate metal

17.  Finishing of metallic crown or bridge.

18. Porcelain fused to Metal
 Oxidizing: To establish chemical bond between metal and porcelain, a controlled
oxide layer must be created on the metal surface.
 Oxide layer is typically obtained by placing substructure
On a firing tray, inserting it into the porcelain furnace and
raising the temperature to a specified level that sufficiently
Exceeds the firing temperature of porcelain

19.  Apply Opaque Porcelain
This is applied as a first ceramic coat and performs 2 major functions:
• It masks the color of the alloy
• It is responsible for the metal ceramic bond

20.  Body Porcelain:
Body porcelain is fired onto the opaque layer, usually in conjunction with with the
incisal porcelain. It provides some translucency and contains metallic oxides that aid
shade matching.

21.  Incisal Porcelain:
It is usually translucent, as a result the perceived color of the restoration is
influenced by the color of the underlying body porcelain