1

By Gefei Wang

Surface hardness of denture base materials processed by the traditional and the two-cycle methodBy Gefei WangAdvisors: Scott Schricker and Fengyuan Zheng

2

Background

3

According to recent statistics, the aging population keeps increasing. As people getting old, they start losing their natural teeth. The ability to replace the missing teeth makes dentures indispensable part of old people’s life.

Denture bases contacts the gum directly, so it is necessary to study their properties.

The quality of denture base depends its ability to support and protect oral tissues. The material of denture base has to be structurally stable, anti-microbial and fitting to the gum. Based on these requirements, polymethyl methacrylate is the ideal material for denture bases.

4

The commonly used processing technique is one cycle heat-curing: the denture base is heated gradually to 75C and remains at that temperature for 9 hours.

Recently, a new processing method may improve the denture base properties. This is a two cycle heat-curing: after heated at that temperature for 9 hours, the denture base is cooled down to room temperature; then it is processed under high pressure (15 psi) and temperature for 15 min.

The aim of second cycle is to increase the degree of completion of polymerization. However, the effect of the new method on the material has not been quantified.

(cont.)

5

Hypothesis

The second cycle will improve the degree of polymerization, so the two-cycle processed material will have stronger molecular structure.

Material processed by new method will be harder and more resist to aging.

6

object Compare the differences in the hardness between denture base

materials processed by the old method and the new method. Explore the change in hardness of the material caused by aging.

7

Making the samples…

8

9

Sample: denture base disc, diameter 10mm and thickness 3mm.

Two groups of samples are made for each test. Control group: 5 sample use one cycle processing. Experiment group: 5 samples use two cycle processing.

10

Hardness test Samples are polished manually before test. To determine Vickers values, a load of 100g was applied for

30 seconds on the specimens using a microhardness tester. Each specimen was subjected to five indentations (one at

the center and two at the borders), and a one-way ANOVA was run to test the result.

11

Color stability(may delete) The control group samples are stored in distilled water. Coffee, tea

and Coca Cola are stored in separate jars. Coffee and tea are heated until boiling and sty at 40C, Coca Cola is stored in room temperature (20C), samples are immersed in the solution for a period of one hour/ day. Then the samples are removed, rinse and stored in distilled water. The sample are stored in dark at room temperature to simulated the oral condition. This procedure is done for 30 days.

12

Aging effect The samples are placed in a sealed paper cup at 23oC. The

hardness are then measured. Next, the samples are put in thermocycling machine (hot

water well 37oC, cold water well 5oC) for 600 cycles. The sample are air dried for 24hr.

Then, the hardness are measured again.

one cycle processed two-cycle processedreading diameter (µm) Vicker # reading diameter (µm) Vicker #

#1 #11 217.8 108.9 62.4 1 213.8 106.9 64.92 213.2 106.6 62.9 2 215 107.5 64.23 217.2 108.6 62.9 3 211.8 105.9 66.34 214.2 107.1 64.5 4 207 103.5 69.45 216 108 63.6 5 208.2 104.1 68.6

#2 #21 219.6 109.8 61.5 1 212.5 106.25 65.62 217.8 108.9 62.7 2 209.4 104.7 67.83 217.6 108.8 62.7 3 214 107 64.84 219.2 109.6 61.7 4 211.8 105.9 665 222 111 60.2 5 216.8 108.4 63.1

#3 #31 222.4 111.2 60 1 208 104 68.62 218.8 109.4 62 2 213.6 106.8 653 223 111.5 59.7 3 210.6 105.3 66.94 220 110 61.3 4 211.6 105.8 66.35 218.6 109.3 62.1 5 220 110 61.3

#4 #41 220.6 110.3 61 1 210 105 67.32 216.2 108.1 63.5 2 208.8 104.4 68.13 222.4 111.2 60 3 215.6 107.8 63.84 214.2 107.1 64.6 4 217.2 108.6 62.95 215.2 107.6 64.1 5 212.2 106.1 66

#5 #51 215.6 107.8 63.8 1 213.2 106.6 65.32 220 110 61.3 2 216 108 63.63 223 111.5 59.4 3 206.8 103.4 69.44 216 108 63.5 4 213 106.5 65.35 220.2 110.1 61.2 5 220 110 61.3

62.104 65.67264.6 69.459.4 61.3

maxmin

maxmin

Average Average

Result

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 2554

56

58

60

62

64

66

68

70

72

Hardness

one cycle two cycle

Hard

ness

(Kg/

mm

2 )

SUMMARYGroups Count Sum Average Variance

one cycle 25 1552.6 62.104 2.3529two cycle 25 1641.8 65.672 5.21293333

ANOVASource of Variation SS df MS F P-value F crit

Between Groups 159.1328 1 159.1328 42.0661659 4.52E-08 4.04265213Within Groups 181.58 48 3.78291667

Total 340.7128 49

16

Data Analysis

The mean value of the hardness for the control group is 62.1(Kg/mm2), while for the experimental group, the mean value is 65.7 (Kg/mm2).

There was a statistically significant difference between groups as determined by one-way ANOVA (F(1,48) = 42.06, p = 4.52).

The variance for the two-cycle group is 5.21, which is much higher than that of one-cycle processed group.

17

Aging

18

1 2 3 4 5 6 7 856

58

60

62

64

66

68

70

72

Aging

two-cycle one-cycle

Hard

ness

(Kg/

mm

2 )

19

Data Analysis

The hardness after aging is higher than the original hardness.

One possible explanation is that the samples still contain water, the strong inter molecular force between water molecule and polymer increases the hardness

Another explanation is that the sample size is not large enough.

20

Conclusion Because p<0.05, so the difference is statistically significant.

We can conclude that two-cycle processed denture base is harder than the one-cycle processed denture base.

Because the hardness is positively related to the degree of polymerization, this result support that the second cycle increases the degree of polymerization.

The sample size is not large enough (higher variance in two-cycle group and aging result), so more samples are needed.

For the aging test, more time are needed to air-dry the sample. Temperature differences should be increased.

This suggests that denture base materials with improved hardness can be made via the two-cycle method.

21

Future work

Keep testing the color stability and adjust the thickness of the sample.

Test more samples and check whether the result is replicable.

Do the aging test again but run more cycles(at least 1000) and increase the temperature differences. Give more time to dry the sample.

22

Thank you.

Acknowledgements

I would like to thank Dr. Mostafa Ibrahim forHis help and guidance.