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BDS3 BMS - Dental Materials > 2. Failure of dental materials 1 > Flashcards

2. Failure of dental materials 1 Flashcards

1
Q

What are the 3 classes of materials used?

REDO LAST 10 MIN OF LEC + IMPROVE FLASHCARDS)

A
  • metals
  • polymer
  • ceramic

(composites are also used = mix of 2 EG. metal + polymer)

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2
Q

Restorative materials are subjected to a
harsh oral environment/ conditions

What are some of these conditions

A
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3
Q

max occlusal forces range from?

A

200-3500N

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4
Q

first and second molars and incisors force range?

A
  • First & second molars~ 400-800N
  • Incisal forces~ 150-300N
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5
Q

TOOTH STRUCTURE RECAP

Why is it important to understand native tissues?

A
  • important to understand native tissue to try and match properties during restoraions and replacement
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6
Q

TOOTH STRUCTURE RECAP

  • Enamel and dentine key features
A
  • 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
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7
Q

ENAMEL

  1. Overall structure?

(Rod structure)

A

• 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.

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8
Q

ENAMEL

LARGEST MINERAL CONSTITUENT AND %?

A
  • hydroxyapatite (HA)
  • present 90-92% by volume
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9
Q

ENAMEL STRUCTURE

Inorganic and ORGANIC MATTER % WEIGHT

A

organic = 1-2% by weight

Inorganic = 95-98% by weight

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10
Q

DENTINE STRUCTURE

% Composition and of what?

A
  • 70% mineral and acellular, has hydroxyapatite crystals
  • 30% organic content as water, collagen and mucopolysaccharides
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11
Q

DENTINE STRUCTURE

(Dentinal tubules number)

A
  • 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
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12
Q

DENTINE STRUCTURE

1
why is denting softer than enamel?

  1. What happens to smear layer when acid etch placed?
A

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

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13
Q
A
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14
Q
A
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15
Q
A
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16
Q

features for ideal restorative materials

A

-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

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17
Q

Desired properties of restorative materials?

A
  • 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
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18
Q

why do materials fail?

A
  • combo of forces
    displacement
    pH
    biofilm formation
    bacteria
    fluids
    temp fluctuation
    Chaging pH

all contribute to failure

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19
Q
A
  • 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

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20
Q

Replacing restorations disadvan compared to repairing

A
  • 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
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21
Q

diff types of forces that a material experiences

A
  • blue rectangle = body/ tooth
  • biomaterials usu experience multiple forces
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22
Q

EG of mechanical properties of biomaterials

A

SLIDE 13

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23
Q

why may physical failure of a material occur?

A
  • 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
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24
Q

forces on restorations are variable - they depend on many factors such as?

A
  • 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)
25
Q

EGs (15 minutes) ( slide 15 +16) (write up)

A
26
Q
A
27
Q

what is stress?

A

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

28
Q

calculating stress formula and units

A
29
Q

what is strain?

A

When an external force is applied on a
test specimen it results a change in the
dimension

30
Q

calculating strain formula and units

A
  • as both have same units, cancel out and strain has NO UNITS
31
Q
A
  • 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
32
Q

how do we test dental materials mechanical prop using the stess-strain relationship

A
  • 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
33
Q

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?

A

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

34
Q

Stress-strain curves

hookes law and young’s modulus and link?

A
35
Q

elastic deformation

A

-

36
Q

another EG of elastic deformation

A
37
Q

Plastic deformation

A
38
Q

what is Young’s modulus?

A
  • mechanical property that measures the stiffness of a solid material
  • defines the relationship between stress and strain
39
Q

what determines stiffness of a material?

A
  • the dimensions of a material
40
Q

what does stiffness depend on?

A

dimensions

41
Q

what do we need to know about a material to be able to calculate it;s stiffness?

A

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

42
Q

EGs of stress strain graphs for different materials

  • which is the strongest?
  • which is the most brittle?
  • which is the most ductile?
A
  • help us distinguish diff types of behaviour
43
Q

eg of brittle materials in dent?

A
  • ceramics
  • some polymers
44
Q

why is it important to know the yield point in dent?

A
45
Q

elastic modulus comparisons EG. (these are not absolute values

A

ADD SLIDE

46
Q

what is tensile strength?

A
  • 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
47
Q
A
  • test specimen placed in universal testing machine
  • uniform load applied to pull apart and tensile strength can be determined
48
Q

is calculating tensile strength easy?

A
  • easy to analyse
  • difficult specimen preparation
  • alignment is crucial
49
Q

what is compressive strength of a material?

how is it determenined?

A
  • 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 ???
50
Q

amalgam restorations and bulk fracture

A
51
Q
A
51
Q

Diametral COMPRESSIVE STRENGTH (Indirect tensile strength)

A
  • property for characterising dental composite restorations
  • cylindrical specimen loaded onto universal testing machine BUT the axis is changed

REWATCH 35 min ONWARDS

52
Q

what is flexural strength of a material?

A
  • 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
53
Q

diff between flexure and bend test

A
  • 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
54
Q

how is testing done?

A
  • 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

55
Q

formula for calc bending strength (DON’t need to memorise formula, just understand)

A
56
Q

what is hardness?

A
  • (important property of dental materials)
  • defined as the resistance to perm surface indentation.
57
Q

how is hardness measured?

A
58
Q

EG of vickers hardness measured on diff surfaces,

values vary between same materials but rough values

A

BDS3 BMS - Dental Materials (12 decks)

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