intro to dental materials/ biomaterials/ biomechanics

dental material science (DMS)

science of all materials used in, around, by or for dentistry and associated/ adjunct procedures

DMS vs biomaterials

diff definitions:

1. material formed by living organisms

2. materials to be used in contact with internal environment of living systems as implant/ prosthesis (weak)

3. material to be used in contact with living tissue as prosthesis/ treatment device (weaker)

(synthetic biomaterials)

history of dental materials

metals

• metallic bonding in s/l

• metal alloy: combination of diff metallic elements melted together

  • or of metals + non-metals when result is metallic

importance of metals in dentistry

2 types of dental ceramics

ceramic = non-metal inorganic material

glass ceramics

• aka silica-based (SiO2, Li2O5Si2)

• 2 subgroups: porcelain based/ non-porcelain based

oxide cermaics

• </=15% silica with little/ no glass phase

• Al2O3, ZrO2, TiO2, MgO, ZnO

• higher strength

polymer

• macromolecule consisting of large no. of primary structural units, not necessarily identical

metal as dental material

• metal typically crystalline → composed of crystals

• many crystals would form grain structures → mechanical properties

oral environment

• temp

  • affected by exposure to high temp e.g. drink

• pH

• water

• masticatory force

requirements as ideal dental materials

• biocompatible, biochemically non-corrosive

• mechanically strong

• physically stable/ durable

• easy to handle, radiopaque

• aesthetic (shade), tasteless/ odorless

• cost effective

e.g. biomechanics consideration in occlusal forces

max occlusal force and response of underlying tissue change with

1. anatomic location

2. age and gender

3. occlusal scheme

4. placement of dental prosthesis

• material/ design sufficient to withstand occlusal forces in anterior segment may be insufficient for posterior segment

situations (modes of loading) for dental materials

• tension: 2 forces directed away from each other

  • or one end constrained and other with force directed away from constraint

• compression: 2 forces directed towards each other in same direction

• shear: 2 forces directed parallel to each other

• torsion: twisting

• bending

stress (σ)

(assume cross-sectional A of specimen remains constant during test)

strain

stress-strain

characterisation of mechanical properties

• strength

• stiffness (modulus)

• max flexibility

• ductility

• brittleness

• resilience

• toughness

hooke’s law

stress-strain curve

use graph to calc elastic modulus

true breaking strength

• uses cross-sectional area at exact point of rupture

  • usually cross-section min

  • so difficult to obtain

• apparent uses original A

proportional limit

• greatestσ that material will sustain without deviation from linear proportionality of stress to strain

• (straight line section)

• below prop limit

  • no permanent deformation occurs in structure

• within this range ofσ application

  • material = elastic in nature

  • elastic/ reversible strain will occur

• > prop limit

  • permanent/ irreversible strain

• elastic limit (B)

  • as max stress that material will withstand without permanent deformation

  • = yield strength (σy)

comparing stress-strain curves between diff materials

key properties on stress-strain curve

Property	Where to look	High value means	Low value means
Stiffness (elastic modulus, E)	Slope of the initial straight section	Stiff (steep)	Soft / flexible (shallow)
Strength	Height of the curve (peak = tensile strength)	Strong	Weak
Maximum flexibility	Strain at the elastic limit (elastic range)	Bends a lot and still springs back	Little elastic stretch
Ductility	Length of the plastic region after the elastic limit	Ductile (bends before breaking)	Brittle
Brittleness	Absence of a plastic region	Snaps at end of straight line	-
Resilience	Area under the elastic section only	Absorbs much energy without permanent deformation	Absorbs little elastic energy
Toughness	Total area under the whole curve (includes resilience)	Absorbs much energy before fracture	Fractures with little energy

toughness

• resistance of material to fracture

• energy necessary to cause fracture

• whole shaded area under curve (tough vs brittle)

  • plastic + elastic

resilience

• green triangle area beneath elastic portion

• resistance of material to permanent deformation

• amount of energy necessary to deform material to proportional limit

brittleness

• absence of plastic deformation (brittle vs plastic)

• (dashed purple line)

ductility

• length of plastic range (ductile vs brittle)

• how far curve continues after plastic limit before fracture

max flexibility

• elastic range, max elastic strain (flexible vs stiff)

• how far along strain axis straight section reaches before elastic limit

stiffness (modulus)

• slope of elastic portion (soft vs stiff)

• steepness of 1st straight section

• steeper = stifferslope of elastic portion (soft vs stiff)

• steepness of 1st straight section

• steeper = stiffer

strength

• e.g. stress at failure (weak vs strong)

• how high curve goes

• higher = stronger

fracture toughness

• fracture mechanics characterises behaviour of materials with cracks and flaws

• (any defect weakens material

  • sudden fractures can arise at stresses below yield stress)

• sudden, catastrophic fractures typically occur in brittle materials that don’t have ability to plastically deform and redistribute stresses

• in polymer

  • (under stress) intrinsic flaws (void formation)

  • → voids join together → fibrils

  • → craze development → crack

(griffith’s law)

bigger crack = lower stress needed to break brittle material

some fracture toughness values for some dental materials

instrument - universal testing machine

(pulls and measures how hard object pulls back)

example - calc compressive strength

extra terms

• indentation hardness - surface hardness

• nanohardness - from nanoindentation

• 
  

biaxial flexural strength test

pushes down to measure fracture point (force at moment of fracture)

fatigue strength

• stress at which material fails under repeated loading until fracture

• fatigue test

  • mechanical test for implants

  • performed by subjecting alternating stress applications below strength fracture occurs

viscoelasticity and fluids

• elastic = independent of loading rate

• viscoelastic = properties dependent on loading rate

  • faster loading rate

    • > elastic modulus (=stiffer)

    • > proportional limit

    • > strength

teeth

enamel

• human enamel

  • 96% minerals (CA-HAp)

  • 1% org molecules (glycoproteins)

  • 3% H2O

• enamel rods (prisms) have specific orientation w each other

  • maximal strength

• very hard, but soluble and > brittle than dentin (acid etching)

  • mirchomechanical retention

• low permeability

  • enamel seals tooth to foreign molecules

  • BUT bleaching agents e.g. peroxides can penetrate

dentin

• 20% org

• 70% inorg

• 10% H2O

loaded tooth

• decrease crown height to decrease chance of fracture

• or increase size of implant/ increase thickness

enamel microstructure - crack propagation

• multi-level microcracking

• dissipates energy and delays catastrophic fractures

dentin microstructure

dentin tubules widen towards pulp

mechanical properties of enamel vs dentin

>org material in dentine

acid etching and degradation

• remove inorg material

• increase roughness for bonding to resin

• resin penetrates into microscopic structure