laboratory procedures in fabricating a cast partial denture

1. LABORATORY PROCEDURES IN
FABRICATION OF CPD
Dr Prathibha Prasad
Postgraduate
Dept of Prosthodontics
KCDS

2. INTRODUCTION
• Dental Laboratory techniques are varied and require a great
amount of skill and concentration to accomplish
• Each step in the making of a cast partial denture if carefully
controlled and basic principles are followed final results will
be superior

3. Synopsis of Laboratory procedures
1.Care of Impression and Making casts
2.Pouring corrected cast impressions
3.Stock and Custom Impression trays
4.Record Bases and Mounting casts
5.Survey and design
6.Design transfer,Blockout ,Relief and Beading
7.Duplication and refractory casts

4. 8.Waxing and Spruing
9.Investing,Burnout and Casting
10.Finishing and polishing the framework
11.Selecting and arranging teeth
12.Flasking,processing,deflasking and finishing

5. In prosthodontia ,few restoration are made to fit patient directly,
most restoration are made to fit a cast. If the cast is not accurate
the restoration cannot possibly fit the patient satisfactorily
REQUISITES OF CAST
• All surfaces to be contacted by the prosthesis should be
accurate and free of voids or nodules
• The surface of the cast should be hard, dense
• The base of the cast should be not less than 15 mm thick at the
thinnest place

6. IMPRESSION MATERIALS THAT ARE USED COMMONLY:
• Alginate irreversible hydrocolloid
• Elastomeric material
CARE OF THE IMPRESSION
• Impression is rinsed thoroughly in 2% glutaraldehyde solution
to remove impurities.
• Boxing of the impression with wax or pumice should not be
done

7. • Pouring the cast in two stages results in improved cast surface
over the one stage method (Anatomical portion – first
pour/Base – Second pour )
• After 45 minutes to 1 hour from the first pour the cast is
removed from the impression

8. POURING CORRECTED CAST IMPRESSIONS
• CORRECTED CAST : a cast made from a master cast on
which the residual ridges have been recorded by means of a
functional impression
• FUNCTIONAL IMPRESSION : a secondary impression that
is made for the purpose of recording the residual ridges in their
functional form and recording the optimum length and width
of the flanges of the denture bases

9. • A functional impression is generally made by adding an
impression base to the removable partial denture framework.
• made by a variety of impression materials, including
impression wax, zinc oxide eugenol paste, rubber base, and
silicone impression materials

10. TECHNIQUES OF POURING
• Plaster Pumice Mix And Wax
• Beading And Boxing With Wax
• Beading With Wax And The Two Stage Pour
• North Carolina Technique

11. BOXING CORRECTED CAST IMPRESSION WITH
PLASTER/PUMICE MIX AND WAX

15. Completed Corrected cast

16. BEADING AND BOXING CORRECTED CAST
IMPRESSION WITH WAX

19. BEADING CORRECTED CAST IMPRESSION WITH
WAX AND TWO STAGE POUR

22. CORRECTED CAST

23. NORTH CAROLINA TECHNIQUE

26. CORRECTED CAST

27. IMPRESSION TRAYS
STOCK
TRAYS
Metal rim
lock
Perforated
Water cooled
Plastic stock
CUSTOM
TRAYS
Acrylic resin
Thermoplastic
vaccum
adapted
Visible light
cured plastic
trays

28. RECORD BASES AND MOUNTING CASTS
• Record bases with attached occlusion rims are used to record
important information for the laboratory technician such as
maxillo-mandibular jaw relationship and verification
occlusal plane
mid-line
degree of vertical and horizontal overlap

29. RECORD BASE REQUIREMENTS
• same adaptation to the basal seat area as the finished
denture base
• same border form
• sufficient rigidity to withstand occlusal forces
• dimensional stability,
• construction that permits use as a base for tooth
arrangement
• easy,quick, and inexpensive

30. RECORD
BASE
MATERIALS
auto
polymerizing
resins
shellac
baseplate
heat-cured
resins
baseplate wax
Metal
Methods of
fabrication
Sprinkle-on
Method
Finger adapted
Dough Method
Wax Confined
Dough Method

31. SURVEY AND DESIGN
• Survey is the procedure of locating and delineating the contour
and position of the abutment teeth and associated structures
before designing a removable partial denture (GPT9)

32. • Surveyor is a paralleling instrument used in making a dental
prosthesis to locate and delineate the contours and relative
position of abutment teeth and associated structures. (GPT9)

33. WHY???
To determine :
• Acceptable path of placement
• Guiding planes
• Location and measurement of retentive undercuts
• Location of tooth and bony interferences favourable design
• Location of occlusal contacts relative to design
• Placement of tripodal marks

34. PHASES OF SURVEYING
1)Identifying the favorable tilt of the cast
2)Path of insertion
3)Tripoding the cast

35. RECENT ADVANCES
• 3D SCANNERS
• ELECTRONIC SURVEYING
• CAD/CAM
• CNC MILLING
• RAPID PROTOTYPING

36. DESIGN TRANSFER
• Designed diagnostic casts should accompany the master casts
to the dental laboratory.
• Using the designed casts and work order, the laboratory
technician will examine the design and casts, consult with the
dentist if needed, and then transfer the design of the metal
framework to the master casts

37. Designed maxillary and mandibular diagnostic
cast

38. PROCEDURE
• Examine the master casts to determine
acceptability.
• Remount the diagnostic casts on the
surveying table by the aligning the tripod
marks
• New tripod marks touched by the stylus
when it is locked in one position are
chosen. placed on tooth landmarks that are
easily and accurately located on both the
diagnostic and master casts

39. • Place three widely separated marks on the diagnostic casts.
• Mark three spots on the master cast in exactly the same place
as on the diagnostic casts
• Master cast is tripoded using these three marks. Since tripod
marks are the same on both casts, the master cast will be
tripoded in the same plane as the diagnostic cast.

41. • Survey lines should be marked on all teeth that will be
contacted by metal & Soft tissue undercuts near teeth are also
marked
• Retentive undercuts are marked in the master cast in red to
locate the tips of the retentive clasps

42. • Rests are marked on the master cast in red to unmistakably
identify them for the person waxing the framework
• Using the diagnostic cast as a guide, draw the metallic portion
of the design on the master cast with a brown pencil and resin
finish lines with blue pencil.

43. • The design on the master cast should be the same as the design
on the diagnostic cast.

44. BLOCKOUT AND DUPLICATION
Preparing the casts for duplication
• Casts are blocked out to eliminate undercut areas on the
master cast that will be crossed by rigid parts of the
partial denture
• All parts of the framework, except the retentive clasp tips,
are nonflexible and will not spring into an undercut.

45. Additional areas to be blocked out:
• Ledges on which clasp patterns will be placed
• beneath connectors to avoid soft tissues impingement
• To provide for later attachment of the denture base to
the framework

46. Types of blockout
• Parallel
• shaped (ledging )
• Arbitrary
• relief.

47. PARALLEL BLOCKOUTS
• made by using different styli in the surveyor to establish 0-
degree, or 6degree taper in the blockout wax

48. • Parallel blockout extends from the survey line to the gingiva.
• Parallel blockout is used
1)Beneath minor connectors
2)soft tissue undercuts that may be crossed by rigid connectors

49. • In Undercut areas On teeth that will be crossed by clasp arm

50. SHAPED BLOCKOUT OR LEDGING
• used on buccal or lingual surfaces to facilitate accurate
placement of plastic or waxpatterns for clasp arms
• The blockout wax may be placed in the undercut and trimmed
flush with the survey line using the proper stylus or wax
trimmer.

51. • Ledging may be done only in the area of the tip of the clasp;
this is the most desirable method.
• It permits more flexibility in the waxing procedure, and, if the
refractory cast is well made, there is no difficulty in placing
the bracing portion of the clasp.

52. • The ledge can be used as guide for positioning the entire
length of the clasp on the tooth. In this method, the wax must
be placed very accurately, since the ledge cannot be changed
on the refractory cast if improperly placed

53. ARBITRARY BLOCKOUT
• used in
 less critical areas to prevent the encroachment of metal on the
soft tissues
 critical withdrawal of the blocked out cast from the
duplicating material.
 gingival crevices, spaces between the teeth when they are not
in the design

54.  for soft tissue undercuts below areas involved in the design of
the framework
 in the tooth or tissue undercuts not involved in the design but
that would complicate the duplication procedure

55. RELIEF
• Relief is used to create a space between the framework and the
cast or soft tissue
USED IN:
• beneath framework extensions, in edentulous area to provide
for attachment of denture bases, and to form internal finish
lines

56. • used beneath lingual bars and in other area where major
connectors will contact thin soft tissue such as maxillary and
mandibular tori.
• used to form internal finish lines.

57. • required when altered cast impression procedures will be used
• used when ridge to be covered by denture base is not well
healed

58. Relief – Where not to use
• Under Metal Complete Denture Bases
• Bead or Nailhead Retention
• Open Retention when the outer support is against the cast

60. BEADING
• to prevent food particles from collecting beneath the
framework and producing discomfort for the patient.
• depth should not exceed 1mm
• should fade out as the gingival margins are approached or as
prominent area in the midline of the palate

61. Cast ready for duplication

62. DUPLICATION AND REFRACTORY CAST
• Master cast is duplicated in refractory material so that it can
withstand burnout temperatures of about 1300 F
• Framework pattern is formed on the refractory cast using wax
and plastic.

63. Refractory cast
Duplication of waxed up master cast
Relief and block out
Surveying and design transfer from diagnostic cast to master cast
Master cast
Surveying and designing
Diagnostic cast

64. REVERSIBLE HYDROCOLLOID (AGAR) MOLDS
• used most often to make the mold for duplication
• prepared by grinding or chopping the colloid into small pieces
and heating it until it liquefies at 212 degree F(100degree C).
• The melted colloid can be stored for a few days in a
hydrocolloid storage unit at 135degree to 140degree F

65. A duplicating flask will be used
to support the cast and confine
the hydrocolloid, since it aids in
controlling shrinkage
Cast is placed on the base of the
duplicating flask to make sure
that space around and above
cast is sufficient for the
duplicating material.

66. Agar mixer is used to melt
and store gel duplicating
material. the material is
cut into small pieces so
that it melts soon
Fill the duplicating flask
with agar hydrocolloid
duplicating material.

67. Continue to fill the flask
until the agar starts to
run out of the holes on
tip of the flask.
Cool the duplicating
material in the flask in
tray of cool running tap
water

68. Gently remove the
cast from the flask
with knife without
tearing agar
Examine the
hydrocolloid
carefully for tears
and voids

69. INVESTMENT CAST
Pour the investment into the
mold immediately. Mechanical
vibration is needed because the
mix will be thick.
Permit the investment to set for
1 hour, then separate it from
the duplicating material very
carefully.

70. It may be necessary to
trim the base to make it fit
the casting ring.
The trimmed and cleaned
cast is ready for treating
with wax or model spray.

71. HARDENING OF REFRACTORY CAST
• DUROL hardening liquid is used to harden the refractory cast.
• Cast is dipped in it for 2 mins qand left to dry out at room
temperature.
• Casts placed in drying oven at temperature between 180 and
200 degree F for 15-20 min
• After removing the casts they are ready for wax up

73. WAXING AND SPRUING
• After the master cast is duplicated to make a refractory cast, it
is necessary to transfer the design to the refractory cast and
complete the wax pattern
• The outline of the framework can be transferred from the
master cast to the refractory cast and positioned quite
accurately without using a surveyor
• The relief, blockout, and beading can be easily seen on the
refractory cast

74. Design transferred to
refractory cast

75. WAXING THE FRAMEWORK
• Full palate, 26 gauge
• Palatal strap, 22 gauge
• Horseshoe and double palatal bar, 24 gauge
• Lingual plating, 6 gauge half pear shape with 28 gauge wax
adapted over it and extending onto the teeth
• Lingual bar, 6 gauge half pear shape reinforced with a layer of
28 gauge wax

78. SPRUING
Spruing for casting of removable partial dentures involves a
large quantity of metal, so special attention must be given :
• points of attachment
• directions of the sprues
• the direction of flow of the molten metal.

79. PROCEDURE
• Check the size of the investment cast in the investment flask
• When the wax pattern is centered in the ring there should be at
least 7mm clearance between the wax pattern and the side of
the ring.

80. 8 or 10 gauge Flat sprue
attached to the junction
major and minor connector
of either side of the cast
14 gauge sprue attached one
from the indirect retainer
and lingual bar junction and
one from I bar clasp and
minor connector junction

81. Preformed casting
funnel attached
Add a casting channel
wire of 4mm thick at
the transition to the
retaining elements on
the framework

82. 1.2 mm wax wire
attached as a ventilation
and pressure
compensation method
Casting funnel is
attached

83. INVESTING SPRUED WAX PATTERN
• Investing is the process of covering or enveloping wholly or in
part an object such as trial denture tooth,wax pattern or crown
with an investment material.

84. PURPOSE
• To provide necessary strength
• To make a smooth surface for mould cavity
• To establish an avenue of escape for most of the gases created
by burnout and casting procedure
• To compensate for dimensional changes

85. Soak the cast
for 5 minutes
in room
temperature
water
Attach the
waxed
refractory cast
on the base of
silicone casting
ring
Spray the wax
pattern and
sprues with a
wetting agent

86. Connect the
silicone ring
to the silicone
base and
ensure that
there is no
water
leakage
Mix
investment
and coat
pattern
uniformly
with paint
brush
Fill ring
with
investment
under mild
vibration

87. Set investment ready for burnout

88. Place the ring in a cool furnace
sprue side down
Turn on the furnace and adjust it to
reach 500 deg c in about 1 hour
Increase temperature to 850-1030 C
based on metal used
Mold must heat soak at this
temperatutre for atleast half an hour
Mold is ready to cast

89. CASTING
Prepare and balance the casting machine
before casting ring is placed in burnout oven
Place metal in crucible
Light the torch and adjust it to form a
reducing flame

90. Heat the metal keeping tip of the blue cone on surface of metal
When surface of the metal is clean and a swirling action is seen
position the casting ring in the casting machine
Release the casting machine
When the machine stops remove ring and allow it to bench cool

91. FINISHING AND POLISHING
• Process of smoothing the metal with progressively finer
abrasive agents to remove scratches and to give a high lustre to
the completed framework
• Cobalt chromium casting – high speed lathes
• Avoid use of heavy pressure- warpage of casting

92. Cut the sprues from the casting with
separating disc
Grind off sprue stumps with grinding
wheel
Smooth framework surface with mounted
stones

93. Refine butt joints with inverted cone
stone
Use series of sand paper disc to
smoothen the surface
Finish casting with rubber wheels

94. Disclosing agent applied over cast
Electropolishing framework
Universal polishing paste

95. FINISHED FRAMEWORK

96. SELECTING AND ARRANGING TEETH
• Acrylic is the material of choice for denture teeth
• Acrylic retentive devices do not interfere with the desired
tooth position
• If porcelain teeth given – severely abrade gold and amalgam
restorations as well as the natural dentition.

98. PROCESSING
• FLASKING
• WAX ELIMINATION
• PACKING
• PROCESSING

99. REMOUNTING

100. FINISHING AND POLISHING
• Vulcanite bur
• Pumice
• Dry clean cloth with
resin polishing agents

101. DUPLICATE MASTER CASTS
• Made at the time when refractory casts are made
• After cast has been removed from duplicating medium for
refractory cast remove any block out or relief on soft tissue
areas of denture base
• Place the casts in duplicating flasks and pour hydrocolloid.

102. • This cast can be used for fitting the metal framework when
polishing.
• Partial dentures can be processed on duplicated master cast
remounted and occlusion corrected
• Finish and polish
• Master casts will be intact so finished partial denture can be
fitted and sent to dentist.

104. • This article provides proof of concept for the use of intraoral
scanning technology to record hard and soft tissue morphology
for the fabrication of a cast partial removable dental prosthesis.
• An open source intraoral scanner was used to scan the hard
and soft tissues to create a stereolithographic file that was
subsequently imported into a computer-aided design software
program for the digital/virtual design of a partial denture
framework.

105. • Computer-aided design and computer-aided manufacturing
technology was then used to fabricate a resin framework that
was trial placed to evaluate accuracy and for conventional
investing and casting with a cobalt-chromium alloy.

108. SUMMARY
• In this example, the resulting resin framework was judged to
fit the digitally fabricated polyurethane cast as well as the
mouth accurately. The finished definitive prosthesis was
successfully placed in the mouth and is being used by the
patient.

109. CONCLUSION
Those who have worked in the dental laboratory whether in a
office or a commercial laboratory know that techniques are varied
and one requires a great amount of skill and concentration to
accomplish
As long as one remembers that the most important step in
prosthodontics is the one being done at any given time the end
result will be well fitting restoration in which the dentist and
technician can take pride

110. REFERENCES
• Rhoads JE, Rudd KD, Morrow RM. Dental laboratory
procedures: removable partial dentures. St. Louis, Missouri:
CV Mosby. 1986.
• Kattadiyil MT, Mursic Z, AlRumaih H, Goodacre CJ. Intraoral
scanning of hard and soft tissues for partial removable dental
prosthesis fabrication. Journal of Prosthetic Dentistry. 2014
Sep 1;112(3):444-8.