This is a seminar on the Steps if the Laboratory Procedures carried out in Fabrication of RPD

1. LAB PROCEDURES IN RPD
Presented by
Dr. KELLY NORTON
Post graduate student

2. Contents
• Preparation of master cast
• Duplication of master cast
• Wax pattern & Spruing
• Investing & burn-out
• Casting of framework
• Finishing & Polishing
• Fitting the framework to the master cast
• Sectioning and resoldering the framework
• Review of literature
• Conclusion
• References and cross references
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3. Introduction
• Metal partial denture framework fabrication, as an important constituent part of a
future partial denture, holds a very important place in the fabrication process.
• It is necessary to point out that metals being considerably more resistant to
fractures than acrylic resins enable fabrication of partial denture bases, which
contributes to a better and faster adjustment of the patient to new conditions in the
oral cavity.
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4. Laboratory steps
• Preparation of master cast
• Duplication
• Wax pattern & Spruing
• Investing & burn-out
• Casting
• Finishing & Polishing
• Fitting the framework to the master cast
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5. Preparation of master cast
• Design transfer from diagnostic cast.
• Beading of maxillary major connector.
• Spraying.
• Block out.
• Relief.
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6. Impressions with work authorization forms
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Bomberg TJ, Hatch RA, Hoffman W. impression material thickness in stock and custom tray. J
Prosthet Dent. 1985;54(August).
(1) The signature and license number of
the dentist
(2) The date the authorization was signed
(3) The name and address of the patient
(4) A description of the service
or material ordered.

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Retripoding the master casts
Dental laboratory procedures vol 3;Rudd KD 2nd edition.

8. 75 8
Dental laboratory procedures vol 3;Rudd KD 2nd edition.
MARKING THE HEIGHT
OF CONTOUR

9. Design transfer from diagnostic cast
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Design should mimic the design on diagnostic cast

10. Beading of the maxillary major connector
• Scraping the outline of the major
connector into the master cast.
• A depth of half the diameter of a no.2
round bur.
• Positive contact against palatal tissues.
• Reduce the packing of food particles
underneath major connector.
• 3 - 4mm away from marginal gingiva.
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RAISED EDGE
Beading on thin mandibular
gingival tissues.

11. Spraying
• Treating the master cast with a surface sealer
using a brush or an aerosol spray to seal the
cast and protect the design during the other
steps of cast preparation and duplication.
• Sealer application should be done in front of a
suction vent because inhaling aerosol is
potentially dangerous.
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12. Block out
• Elimination of undesirable undercuts on the master cast
with wax is commonly referred to as blocking out.
• Both hard and soft tissue areas will require block out.
• Types of block out:
• Parallel block out
• Shaped block out
• Arbitrary block out
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13. Parallel block out
• Proximal tooth surfaces used as guiding planes.
• Beneath all minor connectors.
• Tissue undercuts to be crossed by rigid connectors.
• Tissue undercuts to be crossed by origin of bar clasps.
• Deep interproximal spaces to be covered by minor
connectors or linguoplates.
• Beneath bar clasp arms to gingival crevice.
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14. Shaped block out
• On buccal and lingual surfaces to
locate plastic or wax patterns for clasp
arms.
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15. Arbitrary block out
• All gingival crevices.
• Gross tissue undercuts situated below areas
involved in design of denture framework.
• Tissue undercuts distal to cast framework.
• Labial and buccal tooth and tissue undercuts
not involved in denture design.
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16. Relief
• Beneath lingual bar connectors or the bar portion of the
linguoplates when indicated
• Areas in which major connectors will contact thin tissue,
such as hard areas so frequently found on lingual or
mandibular ridges and elevated palatal Raphes
• Beneath framework extensions onto ridge areas for
attachment of resin bases
• The retentive meshwork must be raised above the edentulous
area to allow resin to completely surround it.
• At least 1mm of resin should surround the tissue surface of
mesh.
• Margin of the relief wax forms the internal finish line of the
framework.
• The finish line should be sharply defined.
• Space of at least 1mm and no closer than 1.5-2mm from
abutment. 1675

17. Tissue stop
• A small square of 2x2 mm of relief wax is removed at the distal end of the saddle.
• It provides stability of the framework during acrylic resin processing.
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18. Duplication of the master cast
Duplication with reversible hydrocolloid Ticonium flask
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19. 1975
Dental laboratory procedures vol 3;Rudd KD 2nd edition.
Duplicating procedure

20. 75 20
Stewart’s clinical removable partial prosthodontics. Rodney D. phoenix.4th edition.
Reversible Hydrocolloid/Agar :
complex polysacharide extracted from sea weed
Primarily consists of
13-17% colloidal suspension of agar in water
Borates- 0.2-0.5%
Potassium sulphate- 1-2%
Wax- 0.5 - 1%
Thixotrophic material – 0.3- 0.5%
Water- around 84%

21. 2175

22. 2275

23. Investment Materials
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Gypsum bonded
Phosphate bonded
Silica bonded
Robert G. Craig & John M. Powers : Restorative dental materials (Tenth edition)

24. Gypsum bonded
• mixed with water
• composed of a mixture of silica (SiO2) and calcium sulphate hemihydrates
(gypsum product)
• Three types of gypsum bonded investments can be identified as follows:
-Type 1 thermal expansion type; for casting inlays and crowns.
-Type 2 hygroscopic expansion type; for casting inlays and crowns.
-Type 3 casting complete and partial dentures.
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Robert G. Craig & John M. Powers : Restorative dental materials (Tenth edition)

25. Phosphate bonded
• powder contains silica, magnesium oxide and ammonium phosphate
• Can be mixed with water or colloidal silica
• Two types of phosphate-bonded investment can be identified as follows:
-Type 1- for inlays, crowns and other fixed restorations.
-Type 2- for partial dentures and other cast removable restorations
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Robert G. Craig & John M. Powers : Restorative dental materials (Tenth edition)

26. Silica bonded
• consist of powdered quartz or cristobalite which is bonded together with silica gel
or ethyl silicate.
• On heating, the silica gel turns into silica resulting in a completed mould is a
tightly packed mass of silica particles.
• Use for high fusing alloys.
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Robert G. Craig & John M. Powers : Restorative dental materials (Tenth edition)

27. • Refractory materials (also termed investments) must be measured and mixed according to
the manufacturer's instructions if mold expansion is to match the shrinkage of the
associated alloy.
• Gypsum-bonded refractory materials, commonly called low-heat investments, are used for
casting Type IV partial denture gold and Ticonium (CMP Industries, Albany, NY). These
refractory materials can be heated to 704°C (1,300°F) without causing breakdown of the
investment.
• Refractory materials used for chrome cobalt alloys, titanium, and titanium alloys are
termed high-heat investments and may be heated to temperatures approximating 1,037°C
(1,900°F). These high heat investments are phosphate bonded and usually require a special,
silica-containing liquid to mix with the refractory powder.
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28. Investment cast
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29. 75 29

30. 75 30

31. Draw the outline approx. 6mm for trimming
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placed in a drying oven at 93°C (200°F) for 30
to 60 minutes

32. 3275
the cast is positioned on its end to allow excess
wax to run off
Refractory cast is dipped into
beeswax at 138°C to 149°C
(280°F to 300°F) for 15
seconds

33. 75 33

34. • Before the waxing begins, design must once again transferred.
• Master cast is evaluated and the outline of framework design is transferred precisely to refractory
cast.
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Design transfer

35. Waxing technique
• Pre-made wax patterns are used for waxing
the framework.
• Plastic patterns are glued to the cast with a
mixture of acetone and plastic pattern scraps
mixed to a watery consistency.
• Tacky liquid is painted on the design outline
with a fine brush and allowed to dry for a few
seconds.
• Patterns are carefully adapted to the cast with
the help of a soft rubber pencil eraser shaped
into a wedge.
• Exercise extreme care not to stretch the
pattern. 3575

36. • Once the patterns are in place on the refractory cast, they must be joined together
with wax.
• The areas where the retentive mesh work joins the major connector must be
reinforced.
• For final contouring ,carvers with miniature rounded blades are most apt to be
used.
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37. Spruing
• Spruing is attaching wax or plastic form to the wax pattern, to provide an entrance to the mold
space and to serve as a reservoir of metal during casting procedure.
• Rules:
• The sprues should be large enough.
• The sprues should lead into the molten cavity as directly as possible.
• Sprues should be attached to the wax pattern at its bulkier sections.
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38. Types of spruing
• There are two basic types of sprues, multiple &
single.
• A single sprue is preferred for complete denture
metal bases.
• The majority of partial denture castings require
multiple spruing.
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39. Types of multiple sprues
• Top spruing: A cone-shaped plastic sprue is
attached to the central sprue from which
auxiliary sprues run to each corner of the wax
pattern.
• 10 guage half round sprues are used.
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40. Inverted spruing
• In which the base of the refractory cast should have a 7 mm
diameter hole in its center.
• A cone-shaped plastic sprue is placed into the hole which
extends about 10mm on the pattern side.
• 6 guage half round leads are used for spruing
• Main and auxiliary sprues should be attached 7 mm below
the central sprue.
• All the sprues have to be attached at the same level to the
central sprue to have equal distribution.
• All sprues should be short and direct.
• All junctions should be reinforced and rounded.
• Auxiliary sprues are attached between the main sprue and
the thick sections of the wax pattern.
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41. ERRORS IN SPRUING
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42. 42
The effect of four sprue shapes on the quality of cobalt-chromium
cast removable partial denture frame-works
Kennedy Class III Mod I
Round with reservoir-sprue design Square-sprue designRound-sprue design Ribbon-sprue design
Evaluation of the fit on master die
Viswambaran M, Agarwal S.. Contemporary Clinical Dentistry. 2013;4(2):132.
21-03-2016 120
Occlusal Radiograph showing internal defect

43. • PURPOSE:
• Provides the strength necessary to resist the forces created by the stream of molten
metal.
• To make a smooth surface of the mold cavity so that the casting obtained will require as
little as finishing required.
• To establish an avenue of release for the gases created by burnout and casting
procedures.
• To compensate for the dimensional changes that occur when the metal goes from the
molten to the solid state.
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INVESTING THE REFRACTORY CAST

44. • Two part mold:
• 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
• Refractory cast is soaked in a slurry water.
• Investment material is measured and mixed.
• A paint on layer about 3-4mm is painted on waxed
refractory cast and ensure that no voids are present.
• As the layer reaches its initial set, the second part of
investing procedure begins.
• The cast is placed in an investing ring , having a
moistened ring liner 3 mm short of the crucible end of the
ring.
• This ring liner permits the escape of gases and mold
expansion.
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45. Burn out
• The time and temperature required to eliminate
wax from the mold cavity is specific to the
refractory-alloy system that is being used.
• Each system is developed to provide mold
expansion that closely matches the anticipated
shrinkage of the alloy as it solidifies..
• Purposes:
• It drives off moisture in the mold.
• It vaporizes & eliminates the wax pattern &
sprues by heating them to evaporation leaving a
cavity in the mold.
• It expands the mold to compensate for
contraction of the metal on cooling.
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46. Burn out furnaces
• Can be either electric or gas and must be vented to
allow the escape of noxious fumes.
• Industrial gas type furnaces can hold 25 casting
molds and small electric furnaces with a capacity of
only 1 or two molds.
• Modern furnaces are controlled electronically to
permit time/temperature relationship to be set exactly
to the alloy manufacturer’s specifications.
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manual
semiautomatic
fully programmable
controls
manual

47. Steps of burn out
• For the investment to heat uniformly, it should be moist at the
start of the burnout cycle. Steam will then carry the heat into
the investment during the early stages of the burnout.
• Also because of the water trapped in the pores of the
investment it reduces the absorption of wax .
• The mold should be placed in the oven with the sprue hole
down.
• Burnout should be started with a cold oven, or nearly so.
• The temperature of the oven should be increased slowly and
should be maintained for the period to a temperature
recommended by the manufacturer.
• When the cycle is complete all the wax of the pattern is
eliminated.
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48. Casting
• Molten metal is forced into the pre-heated mold.
• Centrifugal force or air pressure is used.
• If little force is used, the mold is not completely
filled before the metal begins to freeze.
• If excess force is used,turbulence may result in
the entrapment of gases in the casting.
• Metal is melted with a gas-oxygen blow torch
or by an electric muffle surrounding the metal.
• Induction casting is the method of choice for
base metal alloys.
• Titanium is melted using an argon arc melting
and casted in a vacuum pressure chamber
• Modern induction casting machines are
normally programmed to cast when the alloy
has reached the desired temperature.
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49. Recovery of the casting
• The mold is removed from the machine and
allowed to cool according to manufacturer’s
directions.
• At the appropriate time the mold is broken by
tapping it with a wooden mallet to break off
the outer layer of investment.
• The remaining investment is then removed
by sand blasting using aluminium oxide
particles.
• If gold casting alloys are used then pickling
is done to remove the base metal deposits,
oxidation and other contaminants
• If not removed can cause tarnish and
corrosion
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50. Errors during burnout and casting
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51. Rubber wheels,rubber points,knife edge wheels
Magnification and bright light for defects
Point is formed on truing stone or heatless stone
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Finishing the framework
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52. Sprue stumps removed with heatless stone or separating disk
Inaccessible areas-carbide bur or small mounted stone Major connector finished with abrasive stones
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Separating disk to cut sprues

53. Inverted cone stone used with sharp edge in undercut
Flat surface against finish line
Remove nodules from latticework
5375Felt cylinder used with Tripoli on surfaces that contact teeth
Felt wheel to polish tissue side and
tongue side of major connector

54. Knife edge wheels for finish lines
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Rag wheel with Tripoli to polish rest of framework
Scrub casting to remove tripoli.
Chloroform on cotton can be used to
remove last traces of tripoli

55. ELECTROPOLISHING
• Electropolishing, a form of electrolytic stripping, is
usually the first step in polishing the framework.
• In this process, atoms of metal from rough
projections on the framework go into solution
before those in smooth areas do.
• This results in a very consistent, satin-like surface.
• The polishing occurs in a bath of 85%
orthophosphoric acid, which is heated to 49°C
(120°F) to 60°C (140°F).
• The anode is attached to the cast metal frameworl
and is immersed in the solution.
• Each square inch of metal surface area requires 2
amperes of current for 6 minutes.
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56. Fitting the framework
• The technician begins to fit the framework by carefully
seating the casting on the master cast and attempts to identify
the first spots that bind.
• Special powdered sprays and liquid disclosing media are
commercially available to aid in this process
• The seating and spot grinding continues until the rests
completely contact the cast.
• As the retentive clasp tip engages an undercut area, they will
most likely be the first area that binds.
• Technician must carefully relieve the cast in the area of the
clasp tip undercuts to allow the clasps to pass the height of
contour. 5675

57. • Some technicians bend the retentive clasp arms away from the cast and then attempt to
recontour the clasp back into contact.
• This attempt requires good deal of skill and seldom results in a truly passive clasp.
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58. Rubber wheeling and final polish
• Once the casting seats completely on the master cast
without rocking, proceed with rubber wheeling and
polishing.
• Each alloy manufacturer sells a specific variety of
polishing compound that best suits alloy.
• Ultrasonic cleaning is commonly used to remove all traces
of polishing materials.
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59. 5975
FINISHED PROSTHESIS

60. Sectioning and resoldering the framework
• Framework must be fitted to oral cavity before denture bases
and prosthetic teeth are added.
• If the framework cannot be seated, the clinician must:
• 1)decide to have new framework made or
• 2)attempt to correct fit of the existing framework by
sectioning and soldering.
• Sectioned segments must be satisfactorily related in the mouth
and then transferred with either a plaster index or a resin
matrix to the laboratory.
• Technician carefully pours a soldering cast against the
properly related segments.
• Foil is placed between the cast and the components to be
soldered.
• Electrosoldering tips are placed on each side of the joint to
heat the solder
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61. Adjusting occlusion
• Preliminary adjustments to the occlusion of a removable partial denture framework
should be accomplished in the laboratory.
• The clinician should provide an opposing cast and suitable interocclusal records.
• Occlusal and incisal rests are sometimes made excessively thick to ensure complete
casting.
• Once cast, these areas are adjusted to return to the proper occlusal vertical dimension.
• Once the framework has been adjusted to the minimum required dimensions, the
interfering part of the opposing cast should be relieved and clearly marked with a red
pencil.
• In this way, a potential problem area is called to the attention of the clinician.
• This approach allows the clinician the option of performing the final framework
adjustment personally, reducing the opposing tooth, or both.
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62. 62
Review of literature
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63. The effects of different types of investments on the alpha-
case layer of titanium castings
• The aim of this study was to evaluate the effect
of 3 types of investments on the
microstructure, composition, and
microhardness of the a-case layer on titanium
castings.
• Patterns were invested using 3 types of
investment materials: SiO2-, Al2O3-, and
MgO-based investments and were cast in pure
titanium.
• Results: The use of an MgO-based investment
material reduces the thickness of the reaction
layer and decreases the surface microhardness
of titanium castings compared to SiO2- and
Al2O3-based investment materials
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Guilin Y, Nan L, Yousheng L, Yining W. The Journal of Prosthetic Dentistry. 2007;97(3):157-164.

64. Comparative study of circumferential clasp retention force for
titanium and cobalt-chromium removable partial dentures
• The objective of this study was to compare
circumferential RPD clasps made of commercially pure
titanium and identical clasps made of 2 different cobalt
(Co)-chromium (Cr) alloys by testing insertion/removal
and radiographically inspecting the casts for defects,
• 36 frameworks were cast from commercially pure
titanium and 2 Co-Cr alloys with identical prefabricated
patterns and the manufacturer-designated investment and
casting technique. Each group was divided into 2
subgroups, corresponding to .25-mm and .50-mm
undercuts, respectively
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Main testing device comprised
metallic matrix (A), load cell (B),
and test specimen (C).
Rodrigues R, Ribeiro R, de Mattos M, Bezzon O. The Journal of Prosthetic Dentistry. 2002;88(3):290-296.

65. • RESULTS:
• A total of 20% of the titanium specimens demonstrated porosity, showing casting
difficulties, and any defect detected on the clasps determined the sample replacement. For
Co-Cr alloys, casting difficulties were not found.
• The results suggest that commercially pure titanium clasps maintained retention over a
simulated 5-year period, with lower retention force than identical Co-Cr clasps.
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Porosities in titanium clasps.

66. Use of CAD/CAM technology to fabricate a removable partial
denture framework
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Unrestored condition of patient.
Use of tool in FreeForm.
Completed virtual build.
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Williams R, Bibb R, Eggbeer D, Collis J. The Journal of Prosthetic Dentistry.
2006;96(2):96-99.

67. 67
Screen capture of virtual RPD and
supports prepared for building.
Framework emerging from selective laser melting(SLM).
75SLM Co/Cr RPD fitted to cast.
Finished framework fitted to patient

68. Conclusion
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• Although the clinician’s direct involvement with construction phases of
the removable partial denture framework is impractical, each dentist
should be aware of the techniques and processes used in prosthesis
fabrication.
• A good working relationship between the dentist and the dental
laboratory technician permits the construction of quality prostheses and
the delivery of quality care.

69. References
• McCracken’s Removable partial prosthodontics.11th edition
• Stewart’s clinical removable prosthodontics.3rd edition
• Dental laboratory procedures-Removable partial dentures.Morrow,Rhudd.Vol-3.
• Robert G. Craig & John M. Powers : Restorative dental materials (Tenth edition
• Viswambaran M, Agarwal S. The effect of four sprue shapes on the quality of cobalt-chromium
cast removable partial denture frame-works. Contemporary Clinical Dentistry. 2013;4(2):132.
• Guilin Y, Nan L, Yousheng L, Yining W. The effects of different types of investments on the
alpha-case layer of titanium castings. The Journal of Prosthetic Dentistry. 2007;97(3):157-164.
• Rodrigues R, Ribeiro R, de Mattos M, Bezzon O. Comparative study of circumferential clasp
retention force for titanium and cobalt-chromium removable partial dentures. The Journal of
Prosthetic Dentistry. 2002;88(3):290-296.
• Williams R, Bibb R, Eggbeer D, Collis J. Use of CAD/CAM technology to fabricate a removable
partial denture framework. The Journal of Prosthetic Dentistry. 2006;96(2):96-99.
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