NW revision

Question 1

Alginate impression materials
Discuss how alginate impression materials (non-reversible) will perform in terms of their accuracy in deep undercut areas, storage before pouring and cross-infection control processing

Answer 1

Alginates are irreversible aqueous hydrocolloid impression materials

accuracy:

• requirement to be flexible in order to fit past undercut areas
• must have adequate tear strength to enable removal without damaging the impression material
• use sharp snap technique for removal because alginate is viscoelastic = slow causes flow within the material = plastic deformation
• explain viscoelastic spring + dashpot?
• is hydrophilic so able to displace moisture, blood + fluids = accuracy, no bubbles
• important to select appropriate tray size by looking at amount of undercut + determine the amount of space needed from the tray: smaller space = material more squished = less elastic recovery thus should allow more space

storage

• alginate not dimensionally stable
• can undergo shrinkage or expansion depending on conditions
• if left in inert environment, continuation of polymerisation = shrinkage
• synaeresis = contraction of a gel accompanied by the separating out of a liquid
• wrapping in wet paper towel introduces moisture which can be absorbed = expansion
• dimensional changes are irreversible and reduce the accuracy of the impression
• to minimise must pour the impression within 20 minutes after allowing 5 minutes for viscoelastic recovery
• recommended storage in a sealed plastic bag which prevents water absorption and synaeresis

Cross infection

• must disinfect impression before pouring the material
• soaking in Haztab solution for 5 minutes maximum
• during this time material also undergoing viscoelastic recovery

Question 2

Gypsum products
Briefly explain how gypsum products are classified and discuss the type of gypsum that should be used to produce a working case for a denture base resin complete denture; cast all-metal crown and metal removable partial denture

Answer 2

Gypsum is calcium sulphate dihydrate

Classifications:
Class I - impression plaster
Class II - plaster
- low grade cases, imbeding casts in flasks and articulator mountings
Class III - dental stong
- diagnostic casts and working cases for restoration made from denture base resins
Class IV - die stone low expansion
- working casts for fixed restorations
Class V - die stone high expansion
- working casts for metallic removable restorations

Class 1 and 2 - dry calcination = prolonged heating of a substance at some temperature below its melting temperature
- causes conversion of calcium dihydrate to calcium hemihydrate
Class 3 - wet calcination + autoclaving, drying then grinding
Class 4 and 5 - wet calcination, boiling in CaCl2, drying and grinding
- wet calcination = conversion of calcium dihydrate to calcium hemihydrate in the presence of liquid
- liquid causes recrystallisation of the hemihydrate = less porous more regular crystal structure

Denture base resin complete denture = use Class 3
- dental stone has greater expansion (placing in water during setting allows for greater expansion = hydroscopic expansion)
- expansion useful because can compensate for the polymer forming the denture base which undergoes shrinkage
Cast all-metal crown = class 4
- die stone low expansion is useful due to its strength, hardness, and minimal setting expansion
Metal removable partial denture = grade 5
- die stone high expansion useful because high expansion is required in the stone to aid in compensating for the alloy solidification shrinkage

Question 3

Elastomeric impression materials

Briefly explain how surface tension, wetting angle and viscosity influence the accuracy of an impression material and discuss how this would inform your selection of an elastomeric impression material type to take the impression of a fully edentulous maxilla

Answer 3

Wetting angle, surface tension, and viscosity all strongly affect the accuracy of an impression material.

Wettability

• ability of a liquid to wet a solid surface
• measured by the wetting angle
• lower wetting angle = more hydrophilic
• hydrophilic material can flow and adhere to tooth structure and periodontal tissues by displacing blood, saliva, and water
• means the material is able to spread more and capture more details, thus increasing the accuracy of the impression

Surface tension

• wetting angle is dependent on surface tension and external forces (e.g. gravity)
• lower surface tension = smaller wetting angle + fluid can maximise its contact with the material
• liquid able to wet the material and spread out into a thin film

viscosity

• resistance to the flow of movement
• viscosity of an impression material is dependent on the filler content
• high filler = more viscous material
• dont want too low viscosity because material would run out of the impression tray and not hold intimate contact with the impression site
• dont want too high viscosity because may induce elastic strains which result in distorted or inaccurate impressions

hydrophilicity is important in order for the impression material to be compatible in the inherent moisture of the edentulous mucousal tissues, in order to adapt to it and take an accurate impression.
- must use hydrophilic impression material.

there are 4 types of elastomers: polysulfide, condensation silicone, addition silicone, and polyether

• edentulous patient require hydrophilic material
• cannot use condensation and (most) addition because they are hydrophobic
• polysufides are mildly hydrophilic while polyethers are more hydrophilic = low wetting angle for polyether impression material = significantly greater accuracy

Question 4

Dental porcelains and ceramics
Discuss the different ceramic options (PFM and all-ceramics) and their advantages/disadvantages as they relate to an anterior crown

Answer 4

it is important to consider aesthetics and longevity for anterior and posterior crowns. however because anterior teeth experience low bite forces, aesthetics is more important and materials with low fracture toughness can be used.

PFM

• consists of a metal substructure with a class 1 predominantly glass porcelain layer on top.
• have much higher strength and cheaper but not as aesthetic as all-ceramic
• metal substructure may tint the overlying porcelain a grayish shade
• underlying metal sometimes has a darkline by the crowns edge and if gingival recession occurs in the patient, this line would become visible.
• crevice corrosion can lead to PFM smile which is a dark line that can stain the gums
• not ideal in this situation as it is an anterior tooth so dont require high strength

ceramics:

• recommended if patient more concerned about aesthetics or they have a periodontal problem
• predominantly glass, particle-filled, and polycrystalline classes
• very aesthetic
• very low strength so should be veneered to substructure such as metal (PFM) or other ceramic (bilayer all-ceramic crown)
• veneered ceramics are high in aesthetics due to similar layering effects as natural teeth with similar translucency
• if wanted higher strength all-ceramic can use particles filled glass which contain filler particles to improve mechanical stress properties = better strength
• polycrystalline e.g. zirconia have no glass = the highest fracture toughness but not as aesthetic + must be veneered with porcelain

Question 5

Dental alloys casting and investment materials

Discuss and explain the role of the refractory elements in a casting investment for typical dental alloys

Answer 5

• refractory metals are a class of metals with high resistance to heat
• used for making die stones on which ceramic restorations are constructed
• it is a requirement that refractory die materials have the ability to retain their structural integrity at the temperatures required to fire ceramics
• required to have sufficient strength to withstand the stresses set up when molten alloy enters the investment mould

role:

• when molten alloy is poured into the mould, due to temperature it is expanded
• as it cools, it contracts
• contraction can result in the restoration e.g. crown etc. being to small to fit back into the mouth
• refractory element has the role of expanding via: setting expansion, thermal expansion, and hygroscopic expansion
• refractory element expansion causes the mould to become larger than the actual tooth thus when the alloy is poured into the mould and it contracts as it cools, it contracts from a large size to the right size and thus will still be able to fit inside the mouth

hygroscopic expansion is when the setting expansion of the gypsum occurs in water = further expansion

Question 6

Corrosion and restoration failure
Explain the process of fatigue failure in metallic dental restorations and discuss how corrosion can affect the rate of fatigue failure in dental alloys

Answer 6

The first stage of the fatigue process is known as crack initiation. A fatigue crack usually initiates in a micro defect on the surface or within the material of the restoration. The stress intensity at the tip of the defect can be much greater than the normal yield strength of the material as a result of the stress being concentrated at the tip resulting in a crack being initiated.
In metals and alloys, when there are no macroscopic or microscopic discontinuities, the process starts with dislocation movements, eventually forming persistent slip bands that nucleate short cracks.

Stage two in the process is where the crack propagates and advances incrementally with each stress cycle.

The final stage is where the propagation phase continues until the crack achieves a critical dimension and catastrophic failure occurs. The structure has reached its fatigue life.

Areas of stress concentration, such as surface defects and notches, are particularly dangerous and can lead to the initiation of the fatigue process. Therefore it is vitally important that the surfaces of the restorations are left with a smooth highly polished finish and that subsequent intraoral adjustment and treatment does not compromise this.

Corrosion fatigue is another problem that the practitioner should be aware of in treatment planning and selecting materials. The mouth is a harsh environment and the drivers for these corrosion processes are influenced by variables such as:
• Oral temperature and their fluctuations
• Diet effecting halogenation or sulfurisation reactions
• Differing oxygenation of the surrounding oral fluids
• Circulation of oral fluids around the corroding surfaces and the solubility of the corrosion products

This situation includes continuous cyclic loading with variable loads and is therefore an ideal environment for corrosion of the various materials we use in prosthodontic restoration. In this environment, it is corrosion that creates the defects that subsequently lead to the initiation of the fatigue process. In addition, corrosion drives the fatigue process at a faster speed. Therefore it is important that we try to limit corrosion by prescribing appropriate materials and ensuring that all surfaces are smooth and highly polished