Prosthetics Flashcards
What percentage of the UK population are edentulous ?
13
What are the stages of denture construction ?
Primary impression, primary cast, special tray, secondary impression, master cast, record block, occlusal registration, shade colour and size, set up teeth, try in, process trim and polish, fit, review
What preparations may be necessary for edentulous patients for dentures ?
Treat infection
Improve contour
What features of the denture should the dentist check at the try in stage?
Vertical height, midline, labial contour, aesthetics, occlusion
What factors does facial surface anatomy depend on ?
Skeletal form, muscles of mastication and facial expression, age, gender, race, disease, dentition
What is occlusal vertical dimension ?
The superior inferior relationship between the maxilla and mandible when in maximum intercuspation
What is resting vertical dimension ?
The superior inferior relationship between the maxilla and mandible when the mandible is at rest. This depends on posture and muscle tone
What is free way space ?
RVD-OVD, 2-3 mm in dentate adults
What is aesthetics ?
Recreating what is considered the norm
What is cosmetics ?
Creating aesthetics above the norm
What is the golden ratio ?
1:1.618 (phi)
Which factors affect the appearance of dentures ?
Tooth selection , modification, position, vertical height, flange colour and design, position and material of clasps
Which factors should be considered when positioning the teeth ?
Occlusal plane, interpupillary and alar tragus lines
Lip support
Arrangement
What alternative materials are there to stainless steel and cobalt chrome for clasps?
Gold
Nylon
Acetal
Sandblasting to give Matt finish
Why replace missing teeth ?
Improve aesthetics, function, speech, confidence, prevent drifting, over eruption, open contacts
What is the Frankfort plane ?
External auditory meatus to the lower border of the orbit
What are the advantages of gypsum ?
Good colour contrast
Dimensional stability
Cheap
What are the disadvantages of gypsum ?
Poor surface detail
Poor tensile strength
Poor abrasion resistance
Poor wetting of rubber based impressions
How is beta type I I gypsum made ?
Heat in open vessel 150-160 degrees
What is type II beta gypsum used for ?
Mount models on articulators, flask dentures, basing models
What is the structure of type II gypsum ?
Large irregular orthorhombic crystals
How is type III gypsum produced ?
Heat in autoclave under steam pressure 120-150 degrees
What are the uses of type III gypsum ?
Can be used 50/50 with white plaster for edentulous models
Used on its own for dentate models and denture flasking to minimise acrylic shrinkage
What structure is type III gypsum ?
Small irregularly shaped prismatic particles
What are the properties of alginate ?
Mucostatic
Viscoelastic
Inexpensive
Susceptible to syneresis and imbition
Dimensional stability depends on humidity
Satisfactory reproduction of hard and soft details
What are the uses of alginate ?
Full and partial dentures - use 3 mm spaced special tray
Duplicate denture moulds
What are the constituents of alginate ?
Sodium alginate Calcium sulphate dihydrate Sodium phosphate Potassium sulphate Filler Sodium silicoflouride
What are the relative amounts of the constituents of alginate (%) ?
Sodium alginate 18 Calcium sulphate dihydrate 14 Sodium phosphate 2 Potassium sulphate 10 Filler 52 Sodium silicoflouride 4
What is The purpose of each of the constituents of alginate ?
Sodium alginate - hydrogel former Calcium sulphate dihydrate - calcium ions Sodium phosphate - control working time Potassium sulphate - setting Filler - consistency Sodium silicoflouride - control pH
What is the setting equation for alginate ?
Sodium alginate + calcium sulphate ——> sodium sulphate and calcium alginate (insoluble )
What is dental wax ?
Thermoplastic material that melts without decomposition and is usually composed of two or more natural or synthetic compounds
What is modelling wax ?
Tough, used for occlusal rim and try ins
What is beading wax ?
Soft, easily distorted at room temperature, used for adjusting impression trays
What is casting wax ?
Used for patterns on metal castings
What is sticky wax ?
Rigid, brittle, used to fix things together
What are the thermal properties of dental wax ?
Coefficient of thermal expansion that minimises dimensional change
Well defined melting point
Softening point above the mouth
Poor thermal conductivity meaning that the outer surface hardens before the inner mass
What are the mechanical properties of dental wax ?
Flows well at moulding point
Little or no flow at mouth or room temperature
Brittleness which allows it to be removed from undercuts
What are the constituents of dental wax ?
Paraffin wax
Bees wax
Synthetic materials eg carnauba to improve toughness and melting range
What is the melting range of dental wax ?
49-58 degrees
What are the special tray design principles ?
2 mm from sulcus
Relief for frenae
What angle should the handle be at for an upper special tray ?
45 degrees
What angle should the handle be at for the lower special tray ?
10 degrees
When might a finger rest be included in a special tray ?
For close fitting lower trays a finger rest may be placed on the crest of the ridge in the premolars region
What are the properties of light cured special tray wafer ?
Transverse strength 80 mpa
Shelf life 3 years
What are the uses of the special tray wafer ?
Special trays
Occlusal rim and try in baseplate
Neutral zone base plates
What are the constituents of the special tray resin wafer ?
Glass filler Additives Catalyst Pigment Pyrogenic resin Vinylic ester resin UDMA TEDMA
Which factors influence dimensional stability?
Continuation of setting reaction after setting time
Slow elastic memory
Internal stress from mouth to room temperature
Loss of volatile products leading to shrinkage
When is plaster used as an impression material ?
Edentulous, mobile soft tissue
Describe plaster as an impression material
Similar to Dental plaster with higher water to powder ratio to record fine detail
Additives increase working time and reduce setting expansion
May give patient dry mouth
How is plaster disinfected ?
Places in sodium hypochlorite for 10 mins
What is impression compound used for ?
Primary impressions for full dentures
Base for wash impression
What types of impression compound are there ?
Type 1 low fusing impression material
Type 2 high fusing tray material
For how long is impression compound heated and under what temperature ?
Several minutes 55-60 degrees
Flow rate for type 1 is substantial over 45 degrees but poor below 37
What are the properties of impression compound ?
Muco displasive
Non elastic
High viscosity
Can be re adapted
What causes internal stress in impression compound ?
High coefficient of thermal expansion
Change in temperature from softening to room
Poor thermal conductivity
What are the constituents of impression compound ?
47 % thermoplastic material eg wax or raisin characterises softening temperature
50% filler eg limestone increases viscosity and reduces Thermal contraction
3% lubricant eg stearin acid or gutter percher improves flow properties
Which other material are impression waxes most commonly used with?
Zinc oxide eugenol
What so zinc oxide eugenol impression material used for ?
Edentulous master casts
What are the properties of zinc oxide eugenol impression material ?
Non toxic Irritant Persistent taste Good dimensional stability Good detail reproduction Moco displasive
What are the constituents of paste 1 of zinc oxide eugenol impression material ?
Zinc oxide
Water
Olive oil
Zinc acetate
What are the constituents of paste 2 of zinc oxide eugenol impression material?
Eugenol
Kaolin
What is agar ?
15% colloidal suspension of agar in water
Includes borax to give body and potassium sulphate to increase setting of gypsum as this is retarded by borax
What is agar susceptible that could compromise its dimensional stability?
Imbition
Syneresis
What should the casting of addition cured silicone impression materials be delayed ?
They release hydrogen which could cavitate the gypsum
What chemical on some synthetic gloves can retard the setting of some silicone impression materials?
Dithiocarbonate
Do addition cured silicones have a high or low setting shrinkage ?
Low so no space for cement
What is the by product of condensation cured silicone ?
Alcohol
Do condensation cured silicones have a high or low setting shrinkage ?
High so allows space for cement
What sis a disadvantage of condensation cured silicone impression materials ?
TES is hydrolysed easily and can become inactive if contacts water
To which component of a poly ether can patients be allergic ?
Sulphonic acid catalyst system
What is any advantage of poly ether imoression materials ?
Less hydrophobic than silicones
How long does poly sulphide impression material take to set ?
10 mins
What are the advantages of polysulphides?
Can be electroplated
Highest tear strength
What are the disadvantages of polysulphides?
Bad smell and taste
Humid and temperature dependent
Erratic setting
What are the main types of articulators ?
Simple hinge
Average value articulators
Semi adjustable
Fully adjustable
What are the 2 types of average value articulators called ?
Ash freeplane 75 and dentatus ARS
What are the average vales for average value articulators and what plane are they in ?
Incisal plane 10 degrees, horizontal
Condylar plane 30 degrees horizontal
Bennett angle 15 degrees Saggital
What are the 2 types of semi adjustable articulators called ?
Dentatus ARH non arcon
Denar mark 2 arcon
What are the angles and planes for a semi adjustable articulator ?
Incisal, angle custom table, horizontal
Condylar, set to patients protrusive wafer, Frankfort
Bennett, set to patients lateral wafer, Saggital
What is the type of fully adjustable articulator called ?
Denar D5A
What are the angles and planes for the fully adjustable articulator ?
Incisal, custom table, horizontal Condylar, set to pantograph , Frankfort Bennett , set to pantograph , Saggital Inter condylar distance, adjustable width Pantograph, recordings are transferable
What are the uses of heat cured acrylic resin ?
Denture bases
Splints
Artificial teeth
Rebases
What are the physical properties of acrylic resin ?
Thermal conductivity
Radiopaque
Low specific gravity and density
High glass transition temperature to prevent distortion
What are the mechanical properties of acrylic resin ?
Flexural and impact strength
High modulus of elasticity
High elastic limit
What are the chemical properties of acrylic resin?
Does not absorb water
Inert
Insoluble
What are the biological properties of acrylic resin?
Non toxic
Non irritant
Resists bacteria and fungi
What Is the boiling point if the acrylic resin monomer ?
100.3 degrees
What is the monomer in acrylic resin ?
Methylmethacrylate
What is the ideal ratio of powder to liquid for acrylic resin ?
2.5 to 1
What is the ideal polymerisation shrinkage for acrylic resin?
5-6 %
What are the transitional stages of the polymerisation of acrylic resin ?
Sand Sticky Dough Rubber Hard
What should be taken into consideration regarding the polymerisation of acrylic resin?
Above 65 degrees benzoyl peroxide evaporates forming free radicals
Above 100.3 degrees the monomer boils in the hottest parts of the denture leading to gaseous porosity
The polymerisation is very exothermic
High conversion rate of monomer to polymer
Describe a typical polymerisation cycle for acrylic resin ?
Heated to 70 degrees over several hours then 100 degrees for 3 hours
Majority of polymerisation occurs at 70 degrees, 100 degrees at end ensures thinnest parts of denture polymer missed as less exothermic heat is generated in these areas
How many times greater is the coefficient of thermal expats nil of acrylic than gypsum ?
Ten
What is the powder on acrylic resin composed of ?
Polymer - polymethylmethacrylate Initiator benzoyl peroxide Pigment, slats of iron or organic dyes Synthetics eg acrylic or nylon Plasticiser dibutyl phthalate
What is in the monomer of acrylic resin ?
Methyl methacrylate
Inhibitor hydroquinone
Cross linking agent ethylenglycoldimethacrylate
What lowers the flexural modulus in high impact acrylic resin ?
Butadiene styrene
What are the ideal properties of denture base materials ?
High thermal conductivity,
Low density,
Resistant to bacterial growth and absorption of oral fluids,
Radio opaque
What amount of undercut is required for a cobalt chrome clasp ?
0.25mm
What amount of undercut is required for stainless steel or cast gold clasps ?
0.5 MM.
What amount of undercut is required for wrought gold claps ?
0.75mm
