1
Q
What is lost wax technique?
A
- fabrication of metal or pressed-ceramic fixed prosthesis
2
Q
What is another name for lost wax technique?
A
pressing technique
3
Q
What does lost wax casting of metals involve?
A
- construction of wax pattern
- coating of a wax model with plaster
- melting the wax out of the resulting mould
- filling the space with molten metal
4
Q
Why is wax used to make patterns?
A
because it can be conveniently manipulated and precisely shaped and it can be completely eliminated from the mould after investing using heat
5
Q
What is wax?
A
thermoplastic material which is solid at room temperature but melt without decomposition to form mobile liquid
6
Q
What are the types of inlay casting wax?
A
- type 1: medium-hardness wax (generally used with direct technique for making patterns in the oral cavity
- type 2: softer wax (generally used for the indirect fabrication of castings)
7
Q
What is the composition of inlay casting wax?
A
- paraffin: usually the main constituent (40-60%), is likely to flake when it is trimmed (disadvantage) and does not produce a smooth glossy surface
- natural resin: dammar resin (to reduce flaking and increases toughness)
- carnauba, ceresin, candelilla wax (to raise melting temperature) or beeswax
- dyes: provide colour contrast (for fine details of margin)
8
Q
What are the requirements for wax patterns?
A
- adequate flow at typical forming temperatures
- flow requirements are necessary to control the stability of the wax once it had reached temperature (37 degrees for type 1 and 25 degrees for type 2) at which it is carved, burnished ad polished
- burn out cleanly to allow escape of gases and complete entry of molten alloy, there can be no residual ash
- wax should be uniform when softened with no granules on the surface and no hard spots within surface when the wax is softened
- wax should not fragment into flakes or similar surface particles when it is moulded after softening, such flakiness is likely to be present in paraffin wax so modifiers must b added to minimise this effect
- dimensionally stable (does not disslve)
- contrast colour than die material
- easily carved without chipping (easy to manipulate)
- can be burnished and polished
- should fracture rather than deform upon removal from cavity in case of undercuts
9
Q
What are the methods of wax pattern construction?
A
- addition method
- dipping method
- CAD/CAM system
10
Q
How is printed wax pattern (3D printing) made?
A
- either digital impression or scanning of cast
- digital designing of wax pattern
- 3D printer expels microdroplets of wax, developing a wax pattern in a layer-by-layer manner
11
Q
What are the advantages of CAD/CAM and 3D printing?
A
- more precise and accurate
- faster
- less stress between wax layer
- less distortion
12
Q
What are molten press and injection methods used for?
A
industrial purposes
13
Q
Will every defect or void in the wax appear in the casting?
A
yes
14
Q
Can defects be corrected easily in metal casting?
A
no
15
Q
Can defects be corrected easily in wax?
A
yes
16
Q
When is compensating for an error in a waxing technique impossible?
A
when metal casting has been formed
17
Q
What are the prerequisites of wax pattern?
A
- correction of defects
- die trimming
- die ditching
- marking the margins
- provision of adequate cement space
18
Q
What happens during correction of defects?
A
- small defects on the die like dimples due to caries or loss of previous restorations
- normally blocked out intraoral by glass ionomer cement (GIC)
- can be blocked on the die as long as the defect doesn’t extend to within 1mm of cavity margin
- blocked out using zinc phosphate cement, autopolymerising resin
19
Q
What happens during die trimming?
A
- remove most of the excess stone then trim the die apical to (under) the finish line of the preparation
- over trimming will result in over contoured restoration
20
Q
What are the advantages of die trimming?
A
- accentuate finish line
- resembling the normal contour of the natural root for proper cervical contouring of wax pattern
- produce smooth area gingival to the finish line
21
Q
What happens during die ditching?
A
- ditching of die below the finish line to 0.3mm
- ditching is a circumferential groove that facilitates the establishment of good margin (i.e. proper accentuation for the finish line)
22
Q
What happens during making the margins?
A
- margins should be marked in order to locate it during wax carving
- should be delineated with a coloured pencil
- colour should contrast with that of wax that will be used
- graphite (lead) pencil shouldn’t be used
23
Q
Why shouldn’t we use graphite pencils during marking of margin?
A
- can abrade the die
- darker colour can interfere with efforts to verify that the wax has been properly adapted at the margin )(should have unique colour)
- traces of the graphite can prevent complete casting of margins (act as antiflux)
24
Q
What happens during provision of adequate cement space?
A
- space should exist between internal surface of casting (crown) and prepared surface of tooth everywhere except immediately adjacent to the margin (1mm)
- space provides room for luting agent (a material that on hardening fills the space and binds the tooth and crown together) and allows complete seating of restoration during cementation
- adequate cement space is 20-40 micrometres for each wall so the internal diameter of the wall of crown would be 40-80 micrometres
- 1mm band around the margin shouldn’t be given any luting space
25
What happens if the luting space is too narrow?
restoration will not seat properly during cementation because of hydraulic pressure that develops when the viscous mass of the luting agent (cement) cannot escape through the narrow gap between crown and preparation as restoration is seated
26
What happens if the luting space is too wide?
- casting is loose on the tooth, resistance form is reduced and the position of the crown is difficult to maintain accurately during evaluation and occlusal adjustment
- the risk that the crown will loosen during function increases considerably, affecting its longevity
- decrease retention
27
What are the uses of die spacer?
- this material is applied to the die to increase cement space between axial walls of the prepared tooth and the restoration
- it is formulated to maintain constant thickness when painted on the die
- a band of about 1mm immediately adjacent to the preparation margin must be left unpainted for adequate marginal adaptation
28
What are the factors affecting the size of cement space?
- choice of impression material
- die material and technique
- investment
- casting alloy
29
What is wax pattern construction?
finished wax patterns should be an accurately shaped anatomic replica of original teeth that meets all functional dynamic requirements
30
What are some information needed to shape restoration correctly?
- contour of unprepared tooth surface
- adjacent tooth surface
- opposing occlusal surfaces
- reproduction of mandibular movement in dental laboratory
31
What are advantages of electric wax instruments?
- enable precise control of wax temperature
- minimise carbon build-up which easily results from overhearing a waxing instrument in a Bunsen flame
32
What are the disadvantages of electric wax instruments?
because instrument remains hot, it is not possible to draw solidifying wax in the required direction
33
What is the sequence recommended for waxing posterior teeth?
- internal surface
- wax pattern removal and evaluation
- proximal surfaces
- axial surfaces
- occlusal surfaces
- margin finishing
34
What is done to the internal surface during waxing of posterior teeth?
the objective of the first waxing step is a perfectly adapted reproduction of the prepared tooth surfaces
35
What is wax pattern removal and evaluation during waxing of posterior teeth?
- careful evaluation of the pattern, preferably under low-power (up to 10 times) magnification, is crucial to obtain a good casting
- make sure it isn't stuck to prepared tooth and can be removed
36
What are the contat areas like in the proximal surfaces of waxing up of posterior teeth?
- most posterior contact areas are located in the occlusal third of the
crown
- the maxillary first and second molars make contact in the middle third
- between maxillary premolars and molars, the contact areas are usually toward the buccal surface (which makes the lingual embrasure larger than the buccal)
37
What happens if the contact area is abnormally large during waxing up of posterior teeth?
contact areas make plaque control more difficult, which can lead to periodontal disease
38
what happens if the contact area is very small during waxing up of posterior teeth?
may be unstable and cause drifting.
Deficient contacts can also lead to food impaction; although this is not
a direct cause of chronic periodontal disease, it can be very
uncomfortable and painful for the patient
39
What are the characteristics of proximal surface of waxing of posterior teeth?
- proximal surfaces of natural teeth from the contact area to the
cementoenamel junction tend to be flat or slightly concave from the contact area to the cementoenamel junction and any restoration must reproduce this feature to provide adequate room for the free gingival tissues of adjacent teeth surfaces.
- the cervical contour of the restoration should be continuous, with the contour of the unprepared tooth structure immediately cervical to the preparation margin
40
What is done to axial surfaces of waxing of posterior teeth?
- buccal and lingual surfaces should be shaped to follow the contours of
the adjacent teeth
- the location of the height of contour is generally located in the gingival third of most teeth, although on mandibular molars it is usually in the middle third of the lingual surface
- the tooth surface gingival to its height of contour immediately adjacent to the gingival soft tissues, sometimes called the emergence profile which is usually flat or concave
-t he creation of convexity in this area or a shelf or ledge makes bacterial plaque removal more difficult and cause inflammatory and hyperplastic changes in the marginal gingiva
41
What is emergence profile?
tooth surface gingival to its height of contour immediately adjacent to the gingival soft tissues and is usually flat or slightly concave
42
What is the occlusal surface of waxing of posterior teeth?
- the cusps and ridges of the occlusal surfaces should be shaped to allow even contact with the opposing teeth while the teeth are stabilized and forces are directed along their long axes
- nonfunctional cusps (buccal cusps of the maxillary teeth, lingual cusps of the mandibular teeth) should overlap vertically and horizontally, preventing accidental biting of the cheek or tongue and keeping food on the occlusal surface
43
What is the cusp height and location of waxing o posterior teeth?
- cusps should be positioned so that they follow the anteroposterior curve (curve of Spee) and the mediolateral curve (curve of Wilson)
- adjacent marginal ridges should be of the same height
- occlusal contacts in excursive movements must also be evaluated
- if there is unwanted contact, grooves can be created in the cuspal ridges to allow the
passage of opposing cusps
44
What are the occlusal schemes of waxing of posterior teeth?
- cusp-marginal ridge
- cusp-fossa
-
45
What is cusp-fossa occlusal scheme?
- mandibular functional cusps contact farther distally into the fossa of the maxillary tooth and contact only one tooth (tooth-to-one tooth)
- lingual functional cusps of the maxillary teeth contact the fossae of the mandibular teeth in both schemes
46
What is cusp-marginal ridge occlusal scheme?
- buccal cusps of the mandibular premolars and the mesiobuccal cuspsof the mandibular molars (functional cusps) contact the embrasures between the maxillary teeth (tooth-to-two teeth)
47
What are the advantages of cusp-fossa arrangement over the cusp–marginal ridge?
- food impaction is prevented
- centric relation closure forces are nearer the long axes of the teeth
- improved stability results from the tripod contacts for each functional
cusp
48
What happens during marginal finishing of waxing of posterior teeth?
- margins must be reflowed over a band approximately 1mm wide and
refinished immediately before the wax pattern is invested to optimize the adaptation of the wax pattern (and the cast restoration) to the die
- minimizing dissolution of the luting agent
- facilitating plaque control
49
What are wax connectors?
connectors that join the separate components of a fixed dental prosthesis are created in wax just before the margins are finalized
50
What should the characteristics of connectors be?
- mechanically: the connectors should be as large as possible to ensure optimal strength
- biologically: the connectors should not impinge on the gingival tissues
and should be at least 1mm above the crest of the interproximal soft
tissue
- embrasure form gingival to the connectors must enable optimal plaque control
- the cervical aspect of the connector must be shaped to a smooth, archlike configuration
- esthetically: I=in esthetic areas, connectors should be hidden behind the esthetic ceramic veneer, therefore, connectors are often placed slightly lingually when connectors are waxed for anterior prosthesis
51
What are the types of dental waxes?
- impression wax
- processing wax
- casting (inlay) wax
52
What is the main disadvantage of paraffin wax?
causes flaking and does not produce a smooth surface
53
What are the manual methods of wax paattern construction?
- addition (drop by drop)
- dipping
54
When is die hardener added and what is its use?
- after die spacer
- prevents abrasion
55
56
Wht is the shape of the area under the contact aea?
slightly concave or flat (never convex)
57
What should the contact area contour NEVER be?
concave
58
What happens if there is absence of axial contour (slight bulge)?
1. pressure on gingiva
2. accumulation of food
59
What happens if there is overcontoured axial surface?
1. no massaging and cleaning of food
2. accumulation of food
3. gingivitis
60
What are wax connectorsused for?
bridges
crown
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