Restorative Dentistry #1

Notes

Dental Materials

Creep = slow plastic deformation occurs with application of static of dynamic force over time

Elastic modulus = rigidity of material defined by ratio of stress to strain

Fatigue = cyclic forces applied, a crack my nucleate and increase by small increments

Resilience = elastic deformation up to elastic limit

Stiffness = bend without permanent deformation / fracture

Dental Amalgams

Silver (Ag) - corrosion resistance, strength, resistant for tarnish

Tin (Sn) - control setting expansion / contraction (↑ working time)

Copper (Cu) - reduce formation of weak and corrosion-prone gamma 2 phase

Mercury (Hg) - liquid at room temperature prior amalgamation

Types : lathe cut, spherical, dispersion (lathe cut + spherical)

Amalgam takes 3 months to produce bond with tooth

Resin-based Composites

Polymerization shrinkage : 1 - 4%

Polymerization : induction → propagation → termination

Components

Filler (quartz, borosilicate glass, silica) - ↓ curing shrinkage and thermal expansion

Resin (bisGMA, UDMA, TEGMA) - ↑ stiffness and ↓ shrinkage

Resin/filler composition affects light scattering, translucency, aesthetics

Acetone - eliminate water in dentine, then filled by resin tags

Types

Macrofilled (2.5 - 5µm) - good mechanical properties, hard to polish, soon roughens

Microfilled (0.04µm) - good surface polish, unsuitable for load bearing, poor wear resistance, ↑ contraction shrinkage

Nanofilled - ↑ wear resistance, polishability, lustre

Hybrid (1-50µm) - higher modulus of elasticity

Packable (condensable) - ↑ wear resistance and less polymerization shrinkage

Glass Ionomers

Setting reaction : acid-base reaction between glass and polyalkenoic acid

3 overlapping stages : dissolution → gelation → maturation

Releases aluminium, fluoride (replaced by hydroxyl ions), calcium, sodium ions

Types of GIC

I Luting
II Restorative
III Liners and base
IV Pit and fissure sealant
V Luting for orthodontic purpose
VI Core build-up

VII High fluoride-releasing
VIII ART
IX Primary teeth

Resin Ionomer Hybrid Materials

RMGIC (powder-liquid) - methacrylate resin, poly acid, ion leachable glass, water, HEMA, acid-base, light-cured and chemically cured

Compomer (polyacid-modifed composite) - hydrophobic resin filled with acid-leachable glass particles, mainly set by resin polymerization

Giomers - pre-reacted surface / fully reacted GI filler particles, sustained fluoride release in absence of water (unlike GI and compomer)

Ormocers - organically modified ceramics : organic polymers, ceramic glasses, polyvinylsiloxane

Cerments - ion-leachable glass fused with fine silver powder

Ceramic conditioning - readily etchable ceramics by hydrofluoric acid / acidulated monofluorophosphate

Luting Cements, Linings and Bases

Calcium hydroxide (setting / non-setting) : alkaline, weak material, calcific bridge formation

Zinc oxide-eugenol : no adhesion to tooth, set accelerated by moisture (interfere with resins polymerization)

Ethoxybenzoic acid (EBA)-based cements : intermediate restoration, retrograde seal in endodontic surgery

Zinc phosphate : no adhesion to tooth, exothermic

Zinc polycarboxylate : some adhesion to tooth, non-irritant to pulp

Glass ionomer, RMGIC

Impression Materials

Rigid : (edentulous, no undercuts)

• Impression compound (Type I - 1 impression, Type II - border moulding)
• Zinc oxide-eugenol (use with close-fitting individual tray)

Elastic :

Hydrocolloids sol → gel (poor dimensional stability)

Syneresis = continued cross-link formation after initial set → shrink

Imbibition = water imbibed by osmosis due to electrolytes between polymer chains → swell

Evaporation = water evaporates → shrink, hard, brittle

• Reversible (agar) - sulphated polysaccharide, accurate but poor dimensional stability, for duplicating cast
• Irreversible (alginates) - carboxylated polysaccharide, sodium sulphate (retarder)

Elastomers - crown and bridge, cobalt-chromium denture, implant impressions

• Polyethers - longest shelf-life
• Polysulphides
• Silicones (addition / condensation) - most stable

Dental Ceramics

Predominantly glass - feldspathic porcelain → low fracture toughness, brittle, wear resistant

Particle filled ceramic composites - glass network with crystalline filler particulates e.g. albite ~40% (Vita Mark II), leucite ~40-50% (Empress Esthetic, Mirage), lithium disilicate (glass-ceramics), alumina ~70% (e.max Press, e.max CAD, In-Ceram alumina) → reasonable translucency, ↑ fracture toughness, low-medium strength

High-density polycrystalline - little / no glass component e.g. yttrium-stabilized polycrystalline zirconia ~99%, polycrystalline alumina ~99%, CAD/CAM processed → low-medium translucency, high strength, for multiple-unit bridgework

Metal-ceramic crowns - nickel-chrome / high and low gold / silver-palladium alloys → poorer aesthetics, good for 'tight occlusions'

Dental Ceramics Processing

30% shrinkage on firing, vibrate and blot ↓ shrinkage

Vacuum fired

Pressure cooling and slow firing ↓ porosity

Self-glaze using short post-process firing 

Slow cooling rates → prevent residual stress between ceramic layers and consequential crack formation under tension

Acrylic Denture Processing

👉 3 main faults (porosities) : contraction / gaseous / granular ← ratio / heating processes

👉  polymer (powder) : monomer (liquid) = 3.5 : 1

Insufficient monomer → dry, spongy / crumbly appearance of acrylic

Excess monomer → unreacted monomer → irritation of mucosa

Too little volume of acrylic → contraction porosities → incomplete flange areas , short gingival margins / papilla, streaks

Gaseous porosities → resin exceed boiling point (100.3℃) before polymerization completed

Inclusion of moisture → pale, cloudy area within acrylic

Insufficient tightening → excessively thick denture base, ↑ OVD

Aesthetics

Hue = family e.g. red, green, blue (L : yellow, R : red)

Chroma = intensity e.g. the amount of hue ("M" value)

Value = brightness / dullness (1→5)

Problems in choosing shades

Metamerism - objects appear as different colours in different lights

Colour washout - objected stared at for too long a time appears lighter

Observer errors - different people are 'better' than others at shade selection

Technical problems - different technicians and laboratories produce 'different shades'

Dental Gold Alloys

Gold and copper, biocompatible, good corrosion resistance, easy to cast, aesthetically appealing

Type I - class III / V inlays

Type II - most inlays

Type III - crowns / bridges

Type IV - posts and dentures

Cobalt-chromium Alloys

Cobalt 40-60%, chromium 25-35%, small amounts of nickel (↑ ductility), carbon (hardens), iron (solution hardening), molybdenum (refines metal grains), strong and hard, corrosion resistance, little ductility, very work hardenable, less expensive than gold, 2x casting shrinkage compared with gold, less flexible, ✖ bond to porcelain

Do not bend a cobalt-chromium clasp due to brittle grain boundary carbides

Steel Alloys

Martensite - hard and brittle, not corrosion resistant e.g. surgical blades

Austenite (Stainless steels) - 18% chromium / 8% nickel, good corrosion resistant e.g. clasps