2. Failure of dental materials 1 Flashcards
What are the 3 classes of materials used?
REDO LAST 10 MIN OF LEC + IMPROVE FLASHCARDS)
- metals
- polymer
- ceramic
(composites are also used = mix of 2 EG. metal + polymer)
Restorative materials are subjected to a
harsh oral environment/ conditions
What are some of these conditions
max occlusal forces range from?
200-3500N
first and second molars and incisors force range?
- First & second molars~ 400-800N
- Incisal forces~ 150-300N
TOOTH STRUCTURE RECAP
Why is it important to understand native tissues?
- important to understand native tissue to try and match properties during restoraions and replacement
TOOTH STRUCTURE RECAP
- Enamel and dentine key features
- rods have hexagonal shape and provide rigidity to enamel
DENTINE
- crystals are much samller, hecne softer than enamel
- acid etch remove smear layer and reveals high energy surface - much more challenging in dentine then enamel
ENAMEL
- Overall structure?
(Rod structure)
• Highly mineralized crystalline structure
• Enamel is made up of rods or prisms, and rod sheaths and a cementing inter-rod substance in some areas
- rods have hexagonal shape and provide rigidity to enamel
• Rods run from DEJ (dentinoenamel junction – interface of dentin and enamel) to the external surface of the tooth. Rods: 4-8μm diameter
• Rods are intertwined, densely packed and run a wavy course approximately perpendicular to the DEJ.
ENAMEL
LARGEST MINERAL CONSTITUENT AND %?
- hydroxyapatite (HA)
- present 90-92% by volume
ENAMEL STRUCTURE
Inorganic and ORGANIC MATTER % WEIGHT
organic = 1-2% by weight
Inorganic = 95-98% by weight
DENTINE STRUCTURE
% Composition and of what?
- 70% mineral and acellular, has hydroxyapatite crystals
- 30% organic content as water, collagen and mucopolysaccharides
DENTINE STRUCTURE
(Dentinal tubules number)
- main structure is = dentinal tubule, which extends from the external surface to the pulp
- approximately 30,000 - 40,000 tubules per square millimetre, which can transmit pain to the pulp if the dentine is exposed
DENTINE STRUCTURE
1
why is denting softer than enamel?
- What happens to smear layer when acid etch placed?
1
- dentine crystals are much smaller, hecne softer than enamel
2
- acid etch remove smear layer and reveals high energy surface - much more challenging in dentine then enamel
features for ideal restorative materials
-Biocompatible
-Exhibit properties similar to enamel and dentine
-Ability to perform in the oral environment
(has it’s own challenges due to changes - 3 main changes/ stresses = temp, pH, stress experienced)
-Assist in tissue regeneration or repair of missing/damaged tissues
Desired properties of restorative materials?
- Restoration remains integral and in place
- Restore occlusion and withstand masticatory forces
- Aesthetics are maintained over time
- Prevent formation of caries and recurrence
- Restore aesthetics
- Provides patient comfort and restore function
why do materials fail?
- combo of forces
displacement
pH
biofilm formation
bacteria
fluids
temp fluctuation
Chaging pH
all contribute to failure
- dental restorations have a limited life span
- 2ndry caries = main cause BUT factors such as patient-clinician variables, material selection + manipulation play a role
-* Replacement of failed restorations
constitutes a large part of operative
work in dental practice
Replacing restorations disadvan compared to repairing
- Replacing a restoration may lead
to an increase in cavity size - Weaken the tooth
- studies show best treatment for defective restoration = conservative management + manage in MI way hence repair instead of replace
diff types of forces that a material experiences
- blue rectangle = body/ tooth
- biomaterials usu experience multiple forces
EG of mechanical properties of biomaterials
SLIDE 13
why may physical failure of a material occur?
- Critical stress is exceeded
- The magnitude of the critical stress
depends on the loading conditions - e.g. in general a material loaded in
shear has a lower critical stress than
one loaded in tension
forces on restorations are variable - they depend on many factors such as?
- age
- gender (occlusal forces usu lower in women then men)
- facial form
- muscle definition
- mandibular angles (high angles can result in lower occlusal forces compare to those with low mandib angles)
EGs (15 minutes) ( slide 15 +16) (write up)
what is stress?
When an external force is applied on a
test specimen an internal force, equal in
magnitude but opposite in direction is set up in the body
calculating stress formula and units
what is strain?
When an external force is applied on a
test specimen it results a change in the
dimension
calculating strain formula and units
- as both have same units, cancel out and strain has NO UNITS
- mechanical properties = physical properties that a material exhibits on application of forces
- stress-strain have close relationship - relationship usu used characterise mechanical prop of materials
- so on application on external force, stress is generated on the body + dimensions change
how do we test dental materials mechanical prop using the stess-strain relationship
- we apply load at a uniform rate and deformation should also occur at a uniform rate
- usu use a universal testing machine to determine these mechanical properties
Stress-Strain curves
1
what do they show?
2
what does the straight line region display?
3
what does the gradient of the straight line region display and the region under the straight line display?
4
what is the yield point?
5
Failure point?
6
what is the elastic limit?
1
- shows relationship between stress and strain
2
- straight line region = linear relationship, both increase proportionally (stress proportional to strain)
3
- the elastic modulus of the material
- elastic region of material and materials will recover from any strain but have no change in dimension
4
- elastic limit of the material
- the point beyond which the material will experience a permanent deformation and change in dimension
- this is the plastic region
5
- depends on the material, more brittle have earlier failure point
6
- maximum stress a material will withstand without permanent deformation
Stress-strain curves
hookes law and young’s modulus and link?
elastic deformation
-
another EG of elastic deformation
Plastic deformation
what is Young’s modulus?
- mechanical property that measures the stiffness of a solid material
- defines the relationship between stress and strain
what determines stiffness of a material?
- the dimensions of a material
what does stiffness depend on?
dimensions
what do we need to know about a material to be able to calculate it;s stiffness?
If the Young’s modulus of a material is known by using standard specimens
then it is possible to determine stiffness of any structure from that material
EGs of stress strain graphs for different materials
- which is the strongest?
- which is the most brittle?
- which is the most ductile?
- help us distinguish diff types of behaviour
eg of brittle materials in dent?
- ceramics
- some polymers
why is it important to know the yield point in dent?
elastic modulus comparisons EG. (these are not absolute values
ADD SLIDE
what is tensile strength?
- ability of a material to resist a force that tends to pull it apart
- the tensile strength of a material = max amount of tensile strength that it can withstand before failure
- failure = material suffering plastic deformation OR breaking due to being brittle
- test specimen placed in universal testing machine
- uniform load applied to pull apart and tensile strength can be determined
is calculating tensile strength easy?
- easy to analyse
- difficult specimen preparation
- alignment is crucial
what is compressive strength of a material?
how is it determenined?
- max compressive load it can withstand without undergoing failure
- using cylindrical specimen with dimensions in accordance to ASTM or ISO standards
- brittle materials tend to show fragmentation of specimen when max compressive strength is exceeded
- ductile materials show ???
amalgam restorations and bulk fracture
Diametral COMPRESSIVE STRENGTH (Indirect tensile strength)
- property for characterising dental composite restorations
- cylindrical specimen loaded onto universal testing machine BUT the axis is changed
REWATCH 35 min ONWARDS
what is flexural strength of a material?
- ability to resists deformation under load
- materials that deform signif BUT don’t break, the load is yield is typically measured at 5 % deformation of the outer surface and reported as flexural strength of a material
- flexural forces = as a result of forces generated in clinical situations + the dental materials need to withstand repeated, twisting, bending and flexing
- higher flexural strength = desirable, as once action of chewing stresses, may induce a permanent deformation
diff between flexure and bend test
- type of material used and information we obtain
- flexure test - designed to measure bend stregth of a brittle material
- bend test - designed to measure crack resistance of ductile material
how is testing done?
- beam = green specimen and supported
- centre load and bending is measured
right image
- 3 loading points (4 point bending test)
- surface defects become more apparent in this test
formula for calc bending strength (DON’t need to memorise formula, just understand)
what is hardness?
- (important property of dental materials)
- defined as the resistance to perm surface indentation.
how is hardness measured?
EG of vickers hardness measured on diff surfaces,
values vary between same materials but rough values
