Date post: | 10-Apr-2015 |
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Parts
A.A level platformB.A vertical columnC.A horizontal arm–A surveying arm:
capable moving in a vertical direction and contains a mandrel at its lower end.
A.mandrel–Surveying tools:
placed in the mandrel
Surveying tools
– A analyzing rod:determine relative parallelism of surfaces
– A carbon marker:mark the height of contour on surfaces
– Undercut gauges: .01 .02 .03identify the positions of desired undercuts on dental casts
– A wax knife:used in blockout procedure and in the construction of surveyed restorations
4 critical factors when determine the most favorable tilt
– presents of suitable undercuts– elimination of hard and soft tissue interference– creation of desirable esthetics– the establishment of appropriate guiding planes
1. Retentive undercuts
• The first, unchangeable rule:retentive undercuts must be present on the abutment teeth when horizontal tilt
• surveying procedure always started with the occlusal surface s parallel to the platform of the surveyor
• if retentive undercuts are not present, they must be created in the mouth
• 1. recontouring enamel surfaces• 2. fixed restorations(completed before RPD
fabrication)
1. Retentive undercuts
• each of the proposed abutments should display a .010-inch undercut at the most desirable location ( .015 undercut wrought wire)
• MB line angle, DB line angle, midfacial surface; apical third of the clinical crown
after existence is verified • tilt is changed so that a retentive clasp will be positioned no
farther occlusally than the junction of the gingival and middle third of the tooth (esthetic,torque force)
1. Retentive undercuts
2. Interferences - maxillary archgoal to minimize surgical procedures
palatal torus - design of the major connector | surgical removal exostosis on the buccal surfaces - surgical correction is relatively simple
posterior tooth facial tipping=>height of contour move up=>lever arm longer=>more damaging
facial tipping=>gingiva tissue in an undercut=>infra bulge clasp food trap
just one side=> changing the tipping
if not severe=> enameloplasty
severe=>surveyed crown anterior ridge undercut - cast tipping | modify or eliminate denture flange
2. Interferences - maxillary arch
tori - surgery must be considered if prominent lingual tori posterior teeth lingual tipping -most common solution:lingual surface recountouringplacing restorationorthodontic movement undercuts near pear-shape pads - cast tilting | adjust denture base
2. Interferences - mandibular arch
3.esthetics
– The ideal position for a retentive clasp is in the gingival third of the clinical crown
– appropriate selection, contouring, and placement of the prosthetic teeth; appropriate tooth selection requires consideration of shade, size, and contour
• stock teeth should be modified to reflect the
esthetic and functional characteristics of the individual
• one or more missing anterior teeth =>single path of insertion
surveyor must be used to determine whether recontouring of the remaining natural teeth is indicated, if not possible, crown or other suitable restoration should be planned.
• large undercut may be caused by the
shape of the clinical crown or by tipping of the tooth toward an edentulous space.
• The undercuts produce triangular spaces - esthetics problem, food trap
3.esthetics
4. guiding plane
• guiding plane are always parallel to the path of insertion and are rarely greater than 2~4mm in height
• guiding planes can be prepared on most enamel surfaces• if abutments are to receive cast restorations: teeth should
be prepared, and wax pattern should be shaped so their guiding surfaces are parallel to the established tilt
Path of insertion• most RPD have 2 or more paths of insertion• the most influencing factor: whether or not
edentulous spaces are tooth bounded • unmodified Kennedy Class I,II:
bounded at one end=>variety of angles
• Class II with modification space:
if guiding plane prepared=>single path of insertion
• Kennedy Class III:
entirely tooth bounded;if guiding plane prepared=>single path of insertion
• Kennedy Class IV:single path of insertion
Path of insertion
• minor connectors are the only components that contact the guiding planes of the teeth
• minor connectors remain in contact with the guiding planes throughout the insertion and removal processes
Tripoding
– after ensuing the proper tilt, survey table is locked
– lingual surface of the cast, mark with undercut gauge(.03); widely spaces; on areas that not likely to change from cast to cast
– resulting shallow grooves; 4mm long and narrow; completed as a crosshair configuration
Survey lines,Undercut
•placing survey lines with carbon marker on tooth and soft tissue•
• depth and position of the desired undercut will vary the material and clasping system to be used (.01)
• selected abutment tooth contacts the undercut gauge
• "scrap" contact be marked with red pencil; 2mm in length
Design
Philosophy of design– entirely tooth supported=>
very straightforwarda single impression may be used
edentulous ridge does not offer support, can be recorded in anatomic form
no noticeable rotation forces=> no indirect retainer, flexible direct retention
– main concern is for tooth-tissue supported RPD(I,II,and long-span IV)1. stress equalization2. physiological basing3. broad stress distribution
• forces applied to a RPD are transmitted to abutments• Think:
rigid connection between base and direct retainer are damaging
• stress directors are essential to protect abutments• simple hinges interposed between base and clasping
assemblies• minimal direct retention
1. stress equalization
• used in conjunction with intracoronal attachments
• can be used in Class I,II• minimizing the tipping
force on abutments=>limiting bone resorption
• fragile• costly• constant maintenance• impossible to repair
1. stress equalization
2. physiologic basing
• recording the edentulous ridge in its functional form
• this can be accomplished by depressing the mucosa during impression-making procedure or by relining the denture base after it has been constructed
• prosthetic teeth and occlusal rest will be positioned above the existing occlusal plane when the prosthesis is not in function
• direct retainers provide minimal retention
• physiological stimulation effect on the tissue of residual ridge
• lightweight prosthesis requiring minimal maintenance and repair
• reduction in the forces transmitted t the abutment teeth
• premature contact• difficult to produce
effective retention• not well stabilized against
lateral forces
2. physiologic basing
3. Broad stress distribution
• distributing forces over as many teeth and as much soft tissue area as possible
• additional rests and clasp assemblies and by broad base
• multiple clasp not for more retention,but resistance for horizontal movement (splinting of perio compromised tooth)
• no moving parts• rigid, minimize rotational
movements and provide horizontal stabilization
• ridge do not bear too much occlusal load
increased coverage may not accepted by some ptcomplicated oral hygiene
3. Broad stress distribution
Essentials of design
Essentials of design
1.Direct Retention
2.Clasps
3.Rests
4.Indirect retention
5.Major connectors
6.Minor connectors
7.Occlusion
8.Denture bases
Direct retention
not primary objective
main objective:
− restoration of function and appearance
− maintenance of comfort
− preservation of remaining oral structures
1.clasp assemblies
2.adaption of the framework to guiding planes
extension and adaptation of denture bases
Clasps
simplest clasps
remain passive until activated by functional Stresses
Clasps: Class I 2 clasp assemblies
Most posterior tooth on the R/L side of arch
DB undercut: infrabulge clasp(T,1/2T)
MB undercut:wrought wire claspcast circumferential clasp
Reciprocal : rigidbracing arm / lingual plating
3 retentive clasp assemblies
Distal extension side: Most posterior tooth
Opposite side: 2 retentive clasp arm− One as far posterior
− One as far anterior
If modification space is present:clasp the teeth adjacent to the edentulous space
circumferential clasps
Reciprocal : rigidbracing arm / lingual plating
Clasps: Class II
Rests
Abutment for max support for prosthesis
Rest seats prepared:forces directed within long axis of the abutments
adjacent tooth to the edentulous spaces
Indirect retention Neutralize unseating forces
As far from the fulcrum line; not anterior than the canines
2 indirect retainers in a Class I design
1 indirect retainer in a Class II design;on the opposite side of distal extension base
forces directed within long axis of the abutments
Lingiual plating:must be supported by rest-rest seat combinations
Major connectors Rigid
目的 : comfort
Cleanliness
placement of artificial teeth
Occlusion MIP should coincide
with C.R.
Harmony
Prosthetic teeth
1.Fewer teeth / narrower buccolingually
2.Over the residual ridge
3.Sharp cutting edges / escapeways
Denture bases
Broad coverage
Not interfere with functional movements of the surrounding tissues
Selective pressure impressions are recommended for extension base areas of Class I and Class II
External surface of a denture base should enable the pt to exercise max neuromuscular control
Kennedy Class III Direct retention:
retentive undercuts on individual abutment teeth are not critical
Clasps: quadrilateral position of direct retainer
simple circlet clasp
reciprocal arm must be rigid
Rests: adjacent to the edentulous spaces
support major connectors and lingual plating
Kennedy Class III
Indirect retention: not required
if posterior abutment only vertical support, no retention, entire design must satisfy Class I or II
Basesfunctional impressions are not required
Kennedy Class IV
For esthetics, pronunciation:prosthesis tooth placed anterior to residual ridge
Reduce forces Preservation of soft/hard structure of residual ridge
Central incisor/other tooth preservation, reduce rotation
quadrilateral position of direct retainermost anterior/posterior
Kennedy Class IV
Indirect retainer Not needed if quadrilateral
Far away from fulcrum line
Selective pressure impression if extensive residual area
Design procedure
Design procedures
Armamentarium:.01 inch: clasp of chrome alloy and titanium alloys.015 inch: wrought wire (.02 inch: wrought wire).03 inch: tripod Color:•red: tripod, prepared(rest:solid ,recontoured( 斜線 ),relieved( 寫 relief)•black:survey line, information(type of tooth replacement, clasp,depth of undercut)•blue: resin base•brown:metallic portion
1. examine occluded Dx cast
• Vertical black line for rest area
• Red reshape(for rest)
• Lingual view, black line
2. tooth placements
• Black pencil
• Tooth on base
• Reinforced acrylic pontic(RAP)
• Braided post(BP)
• Tube(T)
• Metal pontic(M)
1.place cast on survey table
2.Tripod
• Survey line with carbon marker
• Undercut on abutment(2mm)
• Solid redrest seats
• Diagonal redrecontouring surface
• Blue, resin base
• Brownmajor connector(cast stop)minor connectorindirect retainerretentive/reciprocal elements