Dental Polymers

Lecture #18

Teeth

Humans go through 52 teeth in the course of a lifetime

20 “baby” teeth32 “permanent” teeth

Molars can have contact pressure as great as 25 k.s.i.

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Tooth

EnamelHardest material in human bodyHydroxyapatite (97%)Enamelin (3%)

DentinSupports enamel72% hydroxyapatiteVulnerable to infection and decaySensitive to hot and cold

Pulp/nerveMost active livin part of toothVeins, arteries, nerves, lymph passages

RootFully below gum lineAttach tooth to jaw by means of peridontal membrane

Dental Materials

Dental materialsAll items used in tooth repair or replacementCover broadest range of materials in any one chemical industry

MultidisciplinedOrganic, inorganic, physical, bio-, and polymer chemistryMetallurgy, adhesion, materials science, and coatings technology

Dental Polymers

Most successful, but not oldest group of polymers are the acrylic resins – methyl methacrylate and its derivativesEarly dentures

Carved ivory“Vulcanite” rubber dentures

Acrylic Polymers

First used in 1937Originally injection molded into a pre-cast gypsum mold.Later replaced by a heat curable mixture

CH3 CH3

COOCH3 COOH

- C – CH2 – C – CH2 -

n

poly(methyl methacrylate)

Modifications to Acrylic Resins

Cross-linking monomers to prevent crazing (formation of microcracks) on repair

Ethylene dimethacrylateCo-polymerization with other acrylics for better durabilityInternal pigmentation for shade controlInclusion of chopped fibers for characterization by vein simulation – more “natural” appearanceGrafting with rubbery polymers for better impact resistance

vein simulation

Typical Physical Properties of Denture Base Polymers

<0.04<0.04<0.040.04Solubility (mg/cm2)<0.80.26<0.80.7Water sorption (mg/cm2)

--71--81Coefficient of thermal expansion (°C-1 × 10-6)

4.53.9<4.5 @ 4000g4.05000g (mm)

1.81.9<1.5 @ 2500g2.03500g (mm)

Transverse deflection0.11-0.220.440.110.19Impact strength (kg•m/cm)

1-27-101-11-2Tensile elongation (%)--70-76--76Compressive strength (MPa)

--52--48-62Tensile strength (MPa)Pour ResinVMMAPMMA/SCPMMA/HC

Crown/Bridge

Introduced shortly after full denture use

First use full crowns and facings cemented to natural teethPoor abrasion caused the use of metals such as Au and Co-alloys

Au prosthesis with acrylic crown & bridge resin

Acrylic Resin Teeth

Injection moldedFailed

CrazingComplete disintegration

Now powder/liquid dough processCross-linked with ethylene dimethacrylate or allylmethacrylate for shade control

For better wear, inclusion of up to 50% microfine silica (40 nm)

Restoratives

AmalgamsAlloy powder

Mostly silver and tinMay also include copper, zinc, or gold

Mercury75% of all restorations, 108 amalgams/yr.Mercury poisoning?

Debated pro & conUgly, don’t match tooth color

Composite Restoratives

Methacrylate with fillerE-glass fibersSoda lime glass beadsCalcium phosphateFused silicaLi alumniosilicate glass ceramicAl silicatesBa boroalumniosilicatesCrystalline quartzCalcium silicatePyrogenic silics

Suffer from finishing problemsPolishing produces rough surfacePlaque adhesion

Current uv- or visible light curing restoratives

Other Uses

FacingsPre-formed facings bonded to fractured, deformed, or discolored enamel surface

OrthodonticsBonding of orthodontic device to tooth enamel

Other Polymers

Gutta perchaPoly-trans isopreneEndodontic points for root canal

CelluloseMoisture absorbents

Silicones, polysulfides, and polyethers

Crown and bridge impressions