Fixed Prosthodontics Flashcards
what is fixed prosthodontics?
the replacement/ restoration of teeth by artificial substitutes that are not removed from the mouth
what is retention and resistance of a prepared tooth?
retention:
- prevents removal of restoration along the path of insertion
resistance:
- prevents dislodgement of the restoration by forces directed in apical or oblique direction and prevents any movement of restoration under occlusal forces
what factors affect retention and resistance?
- taper
- freedom of displacement
- length
- substitution of internal features
- path of insertion
why is the margin of the restoration important?
the placement has direct bearing on the ease of fabrication and the success of the restoration
what is an impression?
a negative reproduction of anatomical parts
what is a die?
a positive reproduction of the prepared tooth, that is sectioned, trimmed and ditched to reveal the prepared margins before placed in a solid base
what is the criteria for selecting suitable die materials?
- should reproduce surface detail accurately
- allow dimensionally accurate cast and be strong and resistant to abrasion
- easy to section and trim
- compatible with separating agent
- colour that contrasts with wax so margin can be seen
- easily wettable by wax
what is gypsum?
a widely used die material that has different types
what is type IV die stone?
a high strength stone that has 0.1% or less expansion
what is type V die stone?
a high strength and high expansion stone that can expand as much as 0.3%
what are some advantages of gypsum?
- inexpensive
- easy to use
- can produce consistent results
what is a die spacer?
used to produce relief space for dental cement
what is a die hardener?
used to reduce susceptibility to abrasion during wax carving
what are some advantages to polymeric die materials?
- stronger than gypsum
- abrasive resistance
- good detail production
what are some disadvantages of polymeric die materials?
- expensive
- epoxy die materials may shrink
- not suitable for polysulfide and hydrocolloid impressions
what are electroplated dies?
known as electroforming, when dies are produced from electroplated non-aqueous elastomeric impression material and are coated with fine powder silver or graphite
what are some advantages of electroplated dies?
- excellent abrasion resistance
- moderately high strength
- restore friction on worn telescopic crowns
what are some disadvantages of electroplated dies?
- labour intensive
- time-consuming
- some procedures can be toxic
- costly equipment
- can distort if not done properly
- not suitable for all types of impression materials (only silicone and polyether)
what are the things in a pindex system?
sectioned part, removable die, ditch, base and the anatomical part
what is the solid “check” cast?
to check the fit of the restoration at the final stage to verify the accuracy of the restoration
what is dipping wax?
reinforces the wax copings in crown and bridge work
what is cervical wax?
cervical region of all wax patterns that are susceptible to quick warpage
why is margin finishing important?
the cervical regions is susceptible to quick warpage, that means wax can be used to remodel it to be reliable and extremely thin
why are proximal contacts important?
proximal contact points help stabilise the teeth within the arch
what are embrasures?
when two teeth in the same arch are in contact, their curvatures adjacent to the contact area form a spillway space
what are the functional cusps?
the palatal and buccal cusps of maxillary and mandibular teeth
what are the non-functional cusps?
buccal cusps of the maxillary and lingual cusps in the mandibular molars
what is a provisional restoration?
used to protect prepared tooth during time between the preparation of the tooth and the placement of definitive restoration
what should provisional restorations have?
- pulpul protection
- good marginal fit and seal
- stability
- function
- strength and retention
- good aesthetics
what are the resins used for making custom provisional restorations?
- polymethyl methacrylate
- polyvinylethyl methacrylate
- bis-acryl composite resin
- visible light-cured urethane dimethacrylate
why is structural durability important for restorations?
restorations must contain bulk of material to withstand the forces of occlusion
what is required to achieve structural durability?
- occlusal reduction
- functional cusp bevel
- axial reduction
what is occlusal reduction?
produces adequate bulk of metal and strength to the restoration
what is functional cusp bevel?
a wide bevel is placed on the functional cusp to provide space for an adequate material in area of heavy occlusal contact
what is axial reduction?
secures space for adequate thickness of the restorative material. if inadequate axial reduction, it will have thin walls that can distort
what are some indications for complete cast crowns?
- extensive destruction from caries or trauma
- endodontically treated teeth to compensate for loss of tooth structure
- necessity for maximum retention and strength
- provide contours to receive a removable appliance
- correction of occlusal plane
what are the sections to the anatomy of a post crown?
the post:
- fills and is cemented into the root canal
the core:
- replaces prepared tooth and serves to provide retention for the crown
post crown:
- replaces missing tooth
what is important for direct sprues in single units?
- avoid many curves in sprue system
- extend direct sprue 4mm beyond the crown
- maintain 5-6mm spacing between pattern and mould wall and bottom
- point of attachment should be on thickest part of wax pattern and placed on bulkiest non-functional cusp
- angle of attachment should be at 45 degrees to allow easy flow of molten alloy
how to prevent warpage of wax?
- apply in small increments to prevent rapid or uneven cooling
- should not stand uninvested for more than 30 minutes
- store at room temperature
- avoid unnecessary removal from die
- avoid inclusions in pattern
- avoid exposure to extreme temperatures
What is the di-lock system?
the use of segmented trays for the base of the die, the die stone is inserted into the base while its still wet. After it has set, the locking and curved arm trays are removed and cast is removed by gentle tapping. from there, the die are sectioned.
why does tapering affect retention and resistance?
when it is nearly parallel to opposing wall of preparation, the greater the retention
what is the ideal/optimum angle of tapering?
16 degrees, each with an 8 degree inclincation
how does the freedom of displacement affect retention and resistance?
limits the path of withdrawal/ removal
why does length affect retention and resistance?
the longer preparation will provide more surface area for retention and resistance to tipping forces
how does the substitution of internal features affect retention and resistance?
internal preparations features, such as grooves, box and pinholes are frequently substituted for each other
adds extra surface area, adds roughness to increase friction
how does the path of insertion affect retention and resistance?
it must be parallel to adjacent proximal contacts
what are some types of polymeric die materials?
epoxy resin and polyurethane
what is the ideal height of the anatomical part of the working cast?
10-15mm thick exclusive of the teeth
what is the DVA model system?
uses precision drill and special baseplates that are aligned and drilled to provide die removal
what is the zeiser model system?
uses a metal base and pre-positioned pins to provide additional dimensional stability
what is the multi-pour technique?
1st pour is for preparation area
2nd pour is for individual dies
3rd is for entire arch
what are the requirements of wax patterns of a crown?
- accurately fit the die
- thickness, should not produce thin, weak or flawed cast (no less than 0.5mm before casting)
- occlusion must be correct
- frequent removal from die will increase warpage/distortion
- marginal shape of wax pattern should be exact copy of margin of tooth
what is the emergence profile of a crown?
part of axial contours that extends from base of gingival sulcus past margin of gingiva and extends to height of contour
should be straight to facilitate cleansing, aids food deflection
what are some disadvantages of a complete cast crown?
- reduced aesthetics
- removal of large amount of tooth structure (during tooth preparation)
- possible adverse effects on tissues
what are some advantages of a complete cast crown?
- high strength
- high retentive qualities
- resistance against torquing if prepared correctly
- option to modify form
what are some contraindications of a complete cast crown?
- not necessary when a more conservative restoration would do
- not suitable for high aesthetic requirements
