Polymerization Shrinkage/Stress

Question 1

Polymerization

Answer 1

The connecting of small molecules (mers) into macromolecules (polymers)

Question 2

Macromolecule formation results in

Answer 2

• less space being occupied

Question 3

As polymers form, 3 things happen

Answer 3

i) change in volume (mers become polymers) -> hence a strain
ii) change in shape
iii) Stress
- w/every strain there’s an assoc. amt of stress

Question 4

What is the trend in rlnship btwn amt of filler in resin systems and amt of shrinkage

Answer 4

• The more matrix in the resin system, the less filler in it, hence the more shrinkage

Question 5

The stress following shrinkage, is a bigger issue than the shrinkage itself. T or F

Answer 5

True

Question 6

Strain

Answer 6

• the change in length, or deformation per unit length when a material is subjected to a force

Question 7

A rubber band 2.54cm long is stretched 1.27cm. Calc the strain.

Answer 7

Strain = deformation/length
= 1.27/2.54
=0.5

Question 8

Elastic modulus (2 definitions)

Answer 8

• the measure of the stiffness of a material

- it is the slope of the stress-strain curve in the initial straight-line proportion i.e ration of stress/strain

Question 9

What do yield strength and proportional limits indicate? Explain

Answer 9

• the stress at which the material no longer f(x)’s as an elastic solid
• the strain recovers below these values if the stress is removed, and permanent deformation occurs above these values

Question 10

Proportional limit

Answer 10

• the stress on the stress-strain curve when it ceases to be linear or when the ratio of the stress to the strain is no longer proportional

Question 11

Yield strength

Answer 11

• the stress at some arbitrarily selected value of permanent strain thus is always slightly higher than the proportional limit

Question 12

Why are the two properties - yield strength and prop limit - particularly important?

Answer 12

• coz a restoration can be classified as a clinical failure when a significant amt of perm deformation occurs even tho the material does not fracture

Question 13

How are the materials described in their f(x) below and above the prop. limit/yield strength?

Answer 13

Elastic in f(x) below the prop. limit/yield strength and plastic in f(x) above these stresses.

Question 14

Ultimate strength

Answer 14

• the stress at which fracture occures

Question 15

Ultimate strength is described in 3 properties depending on the type of stress that caused it…

Answer 15

i) tensile strength - occurd from tensile stress
ii) compressive strength - comp stress
iii) shear strength - shear stress

Question 16

Free (pre-gel) shrinkage & stress

Answer 16

• form prior to the gelation of the polymer

disipated thru the bulk of material

Question 17

Rigid (post-gel) shrinkage and stress

Answer 17

• occur after the gelation of the polymer

built up within material and not easily diffused thru’out

Question 18

Factors that contribute to stress relief (2)

Answer 18

i) Flow (in pre-gel condition)

ii) Hygroscopic expansion ( as the materials ex resin, absorb water, stress is relieved)

Question 19

In dealing with the stress if we can maintain an adhesive interface that is strong enough to resist the stress that is generated the material wont de bond from the tooth structure. T or F

Answer 19

True

Question 20

What is the approximate bond strength req’d to predictably resist the polymerization stress that is generated in our materials

Answer 20

• approx. 20 MPa range

Question 21

C (configuration) factor

Answer 21

• ratio of bonded to unbonded surface

= (bonded/unbonded) surf. area

Question 22

Clinical impact of c-factor

Answer 22

• evidence points to improvement of expected outcomes if C-factor is reduced

Question 23

How can c-factor be reduced clinically?

Answer 23

i) Incremental bonding
ii) rounding internal line angles
iii) lab fabrication of resin composite restorations so that the volume of resin that is actually polymerizing during the procedure is the thickness of the luting agent
iv) using flowable resin or GI
v) using vitrebond down and then composite

Question 24

What is the relationship between volumetric shrinkage and stress

Answer 24

• Volumetric shrinkage may not exist in a linear relationship with stress

Question 25

C factor summary of xstics (5)

Answer 25

i) Free surface allows dissipation of stress
ii) Interfacial stress increases directly with the C-factor ratio
iii) C-factor should be as low as possible
iv) Class I composites have a high C-factor
v) Class V composites have lower C-factors

Question 26

How is the amount of bonded surface area related to the experienced stress at the tooth-restoration interface?

Answer 26

• as the amount of bonded surface area increases the amount of stress increases

Question 27

Why are liners, vitrebond or flowable resin, placed on the floor b4 placing composite?

Answer 27

• So that as that overlying resin polymerizes and shrinks the underlying flowable has the ability to deform (MOE) instead of get pulled away from the interface.
  MOE - mod. of elasticity

Question 28

Advantage of Vitrebond as a liner

Answer 28

• compared to flowable and photac fil, vitrebond dissipates shrinkage stress the most (by ~60%)

Question 29

A newer material that is now being used around the world (limited in the U.S) to dissipate stress is

Answer 29

Siloranes

- polymerization by cationic ring-opening process