Dental material science Flashcards
Why is dental materials science important?
dentists constantly use materials, need to communicate with other professionals e.g. technicians, need to evaluate new materials
What are the types of dental materials?
restorative, impression, dentures, metals and alloys, …others
What are restorative materials used for?
to fill a cavity after caries removal
What are the restorative materials?
amalgam, composite resin, (resin-modified) glass ionomer cement
How does the cavity design differ for amalgam and composite resin?
amalgam has a more flared out cavity (need to remove more hard tissue) whereas composite resin only requires caries to be removed (more rounded shape)
Example of a failure of an amalgam restoration?
ditched margins
Possible cause of ditched margins in amalgam restoration
amalgam creep
Outline the process of amalgam creep
- low level forces applied continuously
- amalgam protrudes around margins - vulnerable to fracture
- exposed amalgam fractures - forms ditched margins
Danger of ditched margins
possible site for secondary caries
What technique is used to bond composite resin to the tooth?
acid etch technique
What chemical is applied to the tooth surface and for how long for acid etching?
30% phosphoric acid for 20 sec
Function of acid etching
improves the shear strength of composite resin (doesn’t slide of enamel when force applied due to rough surface created)
Which restorative material has a thermal expansion rate almost identical to enamel?
(renin-modified) glass ionomer cement
What is the cavity shape for (resin-modified) glass ionomer cement?
shape of caries (rounded) as glass ionomer cement binds to enamel so no excess hard tissue needs to be removed
What is an advantage specific to the use of (renin-modified) glass ionomer cement?
releases 1ppm fluoride per day which increases enamel’s resistance to caries
What is the pattern of fluoride release by glass ionomer cement?
high F release initially (8ppm/day) which rapidly decreases to 1ppm/day from day 10 onwards (is this sufficient?)
What quality of the restorative material determines how well it resists fracture?
compressive strength
Important features fore restorative materials?
high shear strength, high compressive strength, thermal expansion similar to tooth
What happens if the restorative material does not expand/contract as the tooth does?
Microleakage
How does microleakage occur?
cold stimulus causes thermal contraction of the restoration only. Gaps form between the enamel and restoration which allows saliva and bacteria to ingress
Function of impression materials
to record a patient’s dentition (size, shape, position, orientation of each tooth)
Definition of viscosity
a measure of a fluid’s resistance to flow
Which has the lowest viscosity: water, ketchup, syrup?
water
Which has the highest viscosity: water, ketchup, syrup?
syrup
What is the advantage of using a low viscosity impression material?
Captures the most accurate record of tooth surface details
What is the advantage of using a high viscosity impression material?
most dimensionally stable after setting and removing
Describe the two step technique that can be used for impression taking
high viscosity material is placed in the tray and impression taken. Then, low viscosity material is placed around individual teeth and the impression tray is reinserted
Important property for impression material
elasticity
Why is it important for an impression material to have elasticity?
once impression material is set around tooth, it can endure elastic strain during removal and then elastic recovery to its original shape
What is an elastic material?
material that undergoes full recovery of shape after force removal
What is a non-elastic material?
material that has a permanent deformation after force removed
What type of replica of the dentition is created by the impression material?
negative replica
How is a positive replica of the dentition obtained?
dental stone (gypsum) is poured into the negative replica (impression material)
Example of denture material
acrylic resin denture
Why does an upper acrylic denture require transverse strength?
under pressure of 3 point loading (masticatory force acting down from palate, supportive force acting up from teeth)
What are metals and alloys used for?
partial denture frameworks, orthodontic wires, denture bases
Feature of partial dentures used to retain denture
clasps
Definition of a rigid material
undergoes a little change in shape when large stress is applied
opposite of rigid
flexible
Describe the rigidity of alloys
very rigid (particles different sizes so cannot slide)
What material may be used for orthodontic wires?
stainless steel
What are the categories that properties are divided into?
mechanical, thermal, flow, miscellaneous (e.g. creep)
List mechanical properties
strength (shear, tensile, compressive, flexural, tear), rigidity, hardness (abrasion resistance)
List thermal properties
expansion coefficient, conduction
List flow properties
viscosity, visco-elasticity
Example of several terms illustrating same idea (importance of property definitions)
rigidity = stiffness = elastic modulus = Young’s modulus
Definition of viscosity *
ability to flow (resistance of flow)
Definition of elasticity
ability of material to return to its original shape on removal from mouth using an applied force
Definition of rigidity
measure of how much force is needed to cause a temporary change in shape of material
Definition of compression strength
ability to withstand applied compressive forces without fracturing
Definition of hardness *
ability of surface to withstand indentation and resistance to abrasion (e.g. removal of surface layer)
Definition of creep
change in shape due to repetitive application of small forces
