CAST RPD DESIGNING
&
RPI SYSTEM
DR . ABHINAV GUPTA
CONTENTS
INTRODUCTION.
DEFINITION.
COMPONENTS OF RPD.
STEPS OF FABRICATION.
BIOMECHANICAL CONSIDERATIONS.
STRESS CONSIDERATIONS.
L-BEAM EFFECT.
RPI SYSTEM.
CONCLUSION.
REFRENCES.
INTRODUCTION
To satisfy the principles of support , retention , stability the removable partial denture must be designed properly and the framework must fit
& adapt accurately to the structures within mouth.
For this, proper fabrication of RPD framework and their components is necessary.
DIFINITION
RPD- “ Replacement of missing tissue and supporting tissues with a
prosthesis designed to be removed by the wearer”
GPT
COMPONENTS OF FRAMEWORK
The various components of RPD Framework are as follows:
1.Major Connector 2.Minor Connector 3.Rest
4.Direct Retainer 5.Indirect Retainer 6.Denture Base
7.Artificial tooth replacements.
MAJOR CONNECTOR
Part of RPD which connects the components one one side of the arch with components of on the opposite side of the arch.
It is the largest and most important component.
Provide indirect retention
MINOR CONNECTOR
The connecting link between the major
connector and the base of RPD and other units of the prosthesis .
Transmits stress evenly to all components.
Transmits force to the ridges and remaining natural teeth.
RESTS
A ridge extension of RPD which connects tooth/teeth to dissipate vertical or horizontal forces.
The are metallic extension in the framework.
Transmit occlusal forces along the long axis of abutment tooth/teeth.
DIRECT RETAINER
A clasp or attachment placed on the abutment tooth for the purpose of holding a removable denture I position.
Prevent the displacement of denture.
INDIRECT RETAINER
A of RPD which assists the direct retainer in preventing displacement of distal extension denture bases by functioning through lever actionon the opposite side of fulcrum line.
It retain the denture in place.
DENTURE BASE
The part of the denture which rests on the oral mucosa and to which teeth are attached.
Made of acrylic/ metal.
Form the foundation of the RPD.
ARTIFICIAL TOOTH REPLACEMENT
PONTIC- An artificial tooth on RPD that
replaces missing tooth, restores functions and occupies the missing space.
It can be made of acrylic, porcelain,
ceramic or metal.
After recording master impression, the framework is fabricated for a cast partial denture.
The framework is essential for other
procedure like occlusal rim preparation, jaw
relation.
STEPS OF FABRICATION
Framework fabrication involves the following steps
• Waxup
• Duplication & preparation of refractory cast.
• Waxing
• Investing
• Burnout
• Casting
• Finishing and polishing.
WAXUP
• Master is duplicated to produce refractory cast,which is use to cast the framework.
• The refractory cast will not resemble the
master cast because the contours of the mater cast are altered by various wax up procedures.
MATERIAL FOR WAXUP
o PERFORMED WAX
o WAX SHEETS
o LIGHT CURE WAX PATTERN
Waxup includes :- Design transfer Block out
Relief
Beading of master cast.
Design Transfer
Conveying the outline of the prosthesis from the diagnostic cast to master cast . GPT
It includes the following steps:-
Marking the height of contour- The tripod masks of primary cast is are transferred to he master cast.
The master cast is repositioned.
Survey lines are drawn on the abutment teeth.
Measuring the undercuts:- the undercut gauge is attached to the survey arm and the undercut areas are marked.
Drawing the clasp:-the clasp is drawn in reference to the primary cast.
Drawing the connector:- drawn with brown pencil
Block Out
Elimination of undesirable undercut areas on the cast to be used in the fabrication of RPD.
It is done by using wax
The undercut is filled with wax and the refractory cast is duplicated.
Relief
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.
Provide space between certain components of the framework and the adjacent oral structures
Areas to be relieved in maxilla are edentulous ridge and tuberosity
In mandible retromolar pad and the crest of alveolar ridge
The free gingival margins of all the teeth shouldbe relieved.
Beading
Scoring a cast with a sharp instrument or bur in any desired area to provide a seal between the finished prosthesis and soft tissue.
DUPLICATION AND PREPERATION OF REFRACTORY CAST
Either alginate or agar can be used to duplicate the master cast.
All the blocked out and undercuts will be invisible.
Relief will appear as an elevation on the edentulous ridge.
The refractory cast is porous should be coated with a sealer( liquid sealer or bees wax).
WAXING
It is the fabrication of wax pattern for the framework.
A sheet of the pattern material is cut according to the outline of the major connector and gently
adapted over the refractory cast.
The bead groove created during wax up should be filled .
Wax is finished to a thin edge in areas where it contact the tooth.
Depending on the type of minor connector either cross struts used to make meshwork or nail beads created on the saddle areas.
Finally the pattern is polished.
INVESTING
The process of covering wholly or in part , an object such as denture,tooth, waxform,
crown,etc with a suitable investment material
before processing, soldering or casting.
GPT.
Done by placing the pattern with the cast in an investing ring
BURNOUT
It is done for :-
Dry the moisture out of the mould:- heating at 100C for 1 hour to vapourize the moisture.
Expand the mould to compensate for
solidification shrinkage of the metal:- heating at 538C for 1hr.
Eliminate the pattern plastics and waxes by
melting and vapourization:- at 675-710C for 1- 2 hr.
CASTING
The mould is ready for casting after burnout . Casting ring is paced and locked in position.
Required amount of metal is placed in front of the sprue way of the casting ring.
The tip of the blue flame use to heat the metal pellet.
The casting ring is bench cooled for 12 mins.
FINISHING AND POLISHING
Sprue are cut using carborundum disc
Surface irregularity are removed with tungsten carbide bur.
The casting is trimmed and finished using a mounted stone
Framework are polished using rubber abrasive wheels and points.
BIOMECHANICAL CONSIDERATIONS
The application of mechanical laws to living structures, specifically the locomotor systems of the body.
GPT
PRINCIPLES
Lever Principle Inclined Plane
Snowshoe Principle
LEVER PRINCIPLE
A simple machine consisting of a rigid bar pivoted on a
fixed point and used to transmit force, as in raising or moving a weight at one end by pushing down on the other
INCLINED PALNE
Forces against an inclined plane may result in deflection of that which is
applying the forces or may result in
movement to the inclined plane,
neither of these is desirable.
SNOWSHOE PRINCIPLE
This principle is based on distribution of forces to as large an area as possible.
Like in a snow shoe which is designed to distribute forces on the entire base area of the shoe, a partial denture should cover
maximum area possible within the physiologic limits so as to distribute the forces over a larger are.
STRESS CONSIDERATION
The stresses can be divided as:
Vertical
1.Displacing stresses
2.Dislodging stresses
Horizontal Torsional
Vertical stress
Horizontal Stress Torsional Stress
STRESS CONTROLL
1. Retention as a means of stress control.
A) forces of adhesion and cohesion.
B) atmospheric pressure.
C) Frictional contact.
D) Neuromuscular control.
2. Clasp design.
a) circumferential clasp.
b) bar clasp.
c) combination clasp.
3 Indirect retention.
4 Functional basing.
5 occlusion.
a) harmonious occlusion.
b) size of food table.
c) occlusal pattern of posterior teeth.
6 Denture base.
a) Size & configuration.
b) accuracy of adaptation of base to the tissues.
RPI SYSTEM
Developed by Krol (1973).
Concept advocated reduction in the amount of tooth preparation and coverage involved with I-bar system.
RPI denotes: rest, proximal plate and I-bar clasp.
REST
Preparation is less extensive, extends only to mesiocclusal surface. It covers the
mesiolingual embrasures connecting then to the minor connector.
PROXIMAL PALATE
The prepared guide plane is only 2-3mm and contacts only 1mm of the gingival portion.
Distal guide plane is added to the proximal
plate located at the distal surface of the tooth.
I BAR
It is placed either labially or buccally connecting the proximal plate and the rest.
Its terminus should be not more than 2mm of its tip contacting the abutment.
The I bar connects the rest and the proximal plate.
I-bar retainer is located at greatest prominence of tooth in gingival third
CONCLUSION
FABRICATION OF RPD COMES UNDER
PHASE 5 OF THE TREATMENT PLANNING.
EVERY STEP OF FABRICATION NEED TO BE DONE PRECISELY.
THE COLLABORATIVE RESEARCH HAS DEMONSTRATED THAT THE DIGITAL
DESIGN AND DIRECT MANUFRACTURE OF RPD FRAMEWORK IS ENTIRELY FEASIBLE.
REFRENCES
McCrackens- RPD
V Rangarajan, TV Padmanabhan- RPD
