SECTIONEDITOR

ct of disinfection procedures base acrylic resins Tehmina Asad, BDS, MSC,~ Adrian C. Watkinson, MDS, FDSRCS, Robin Huggett, MSc, PhD, CGIAC

BChD,b

on flexural

properties

of

and

University of Bristol Dental School, Bristol, England This study evaluates the effects of long-term disinfection immersion on the flexural properties of denture base resins; it was conducted against a background of increasing awareness of the importance of cross-infection control in dental clinics and laboratories. The transverse bend testing procedure used followed that specified in the International Standard for denture base polymers. The results demonstrate that alcohol-based disinfectants are not suitable for use with denture base materials of non-cross-linked acrylic resin. (J PROSTHET DENT 1992;68:191-5.)

T he recent

increase in the incidence of communicable diseases such as hepatitis B and acquired immunodeficiency syndrome (AIDS) has led to a significant change in attitude toward the importance of cross-infection control in dentistry. Because the oral operating environment includes saliva and frequently a mixture of blood and saliva, the risks from blood-borne viruses such as those causing hepatitis B and AIDS are of particular concern.lm5 Potential sources of transmission of infection from patient to dental technician are impressions, impression trays, and gypsum casts3, 6-8; in addition, the dental prostheses at various stages of trial and insertion can transmit infection from dental staff to the patient. If proper measures are not taken, a cycle of cross contamination may occur,g exposing the dentist, dental surgery staff, and the patient to transmission and acquisition of disease (Fig. 1). Since it is not possible to screen every patient for all infectious diseases and since patients with known infections must be treated, transm.ission should be blocked by a method that is practical, easy, and satisfactory; therefore sterilization and disinfection have become the most popular and widely used methods for control of infection.lO-l2 Since many materials used in dentistry cannot be subjected to high heat, chemical agents must be used to sterilize or typically refers disinfect them. I3 The term “disinfectant” to those chemicals that are applied on inanimate surfaces, although some of these agents can be used on skin.14 A denture during its in-service life may need to be exposed to disinfectants several times, and this may adversely affect its strength and structure.133 15-17 Transverse or flexural strength is an important mechanaPostgraduate student, Department of Prosthetic Dentistry. bLecturer, Department of Prosthetic Dentistry. CLecturer, Department of Prosthetic Dentistry. 1011/36609

THE JOURNAL

OF PROSTHETIC

DENTISTRY

TECHNICIAN

TECHNICIAN

v \/

PATIENT

1

PATIENT

2

DSA

$ TECHNICIAN

PATIENT

3

DSA

1 TECHNICIAN

Fig. 1. The cross-infection

chain.

ical property. The International Standards Organization specification for denture base polymers (IS0 1567)18 stipulates that transverse deflection must be between 1 and 2.5 mm for forces of 15 and 35 N, and between 2 and 5 mm for forces of 15 and 50 N. The transverse breaking force for heat-cured resin should not be less than 55 N. This study evaluates the effects of long-term disinfection immersion on flexural properties of denture base resin. The study was conducted against a background of increasing awareness of the importance of cross-infection control in dental clinics and laboratories and the need to limit the effects of such procedures on materials in clinical dentistry.

METHODS AND MATERIAL Disinfectants and denture base resins Three different types of disinfectants were used: glutaraldehyde, chlorhexidine, and an alcohol-based disinfec-

191

ASAD,

Table

I[. Disinfectant Trade

name

Chemistry

Pleat-cured

II.

Trade

name

Transverse

Cross-linked (6% EGDM) polymethylmethacrylate Non-cross-linked polymethylmethacrylate

deformation Deformation 15-35 Mean (x1=6)

Control Chlorhexidine Gluteraldehyde Alcohol-based disinfectant IS0 limits for heat-processed acrylic resins (tested in water)

test results for

N

Table IV. Transverse homopolymer*

deformation

test results for

Deformation

15-50N Mean (n=6)

Average load at fracture N Mean (n = 6)

1.49

2.72 2.53

73.29 79.39

1.25

2.54

73.83

1.46

2.65

72.08

Min 1.0 Max 2.5

Min 2.0 Max 5.0

Not less than 55 N

in air at 20” + 2’ C for control specimens in disinfectant solutions for 7 days (test crosshead

15-35 Mean (n=6)

Group

Control Chlorhexidine Glutaraldehyde Alcohol-based disinfectant IS0 limits for heatprocessed acrylic resins (tested in water)

N

(mm) 15-50 Mean (n=6)

N

Average load at fracture N Mean (n = 6)

1.50

2.74

72.20

1.46 1.64 1.57

2.68

78.54

Min 1.0 Max 2.5

3.04

71.19

2.75

58.37

Min 2.0 Max 5.0

Not less than 55 N

and for specimens stored speed, 5 mm/min).

tant. Their effects on two different types of heat-cured acrylic resins, a cross-linked 6% ethylene glycol dimethacrylate (EGDM) and a non-cross-linked resin, were observed. The materials used, their trade names, and the manufacturers are shown in Tables I and II.

preparation

Rectangular perspex blocks measuring 65 X 45 X 2.80 mm were used to create uniform mold spaces in gypsum. Blanks of Trevalon (cross-linked polymer) and D80 FC (homopolymer) were produced. Mold separation, packing, and clamping followed standard dental laboratory procedures. The powder/liquid ratio used was 3.2:1. Polymerization was carried out for 7 hours at 70’ C (158’ F) followed by 3 hours at 100’ C (212’ F) in a thermostatically controlled water bath. The flasks were allowed to cool to room temperature before opening. Flash and excess were removed by band polishing on both sides on a Kent automatic lapping and polishing unit (Engis Ltd. Maidstone, Kent, U.K.) by using 600.grade silicon carbide paper, with 192

DeTrey Division, Dentsply Ltd., Weybridge, U.K. Bonar Co., Polymers Ltd., Durham, U.K.

(mm)

1.24

*Tested

Specimen

Manufacturer

TYPO

Trevalon”

Group

Surgikos Ltd., Livingston, Scotland, U.K. Unident SA, Geneva, Switzerland Hales Pharmaceutical Ltd., Wetherby, U.K.

acrylic resin materials

D80 FC homopolymer

III.

HUGGETT

Manufacturer

2% alkaline glutaraldehyde Alcohol based 0.5 % W/V chlorhexidine

Trevalon

Table

AND

solutions

Cidex long life Dermacol Aqueous chlorhexidine

Table

WATKINSON,

flowing water as a coolant, to produce a flat surface. The blanks were then cut into strips by a band saw and returned to the polishing machine to obtain the final measurements of 65 x 10 X 2.5 + 0.05 mm in length, width, and depth, respectively. The specimens were then stored in distilled water in a water bath at 37’ C (98.6” F) for 1 week.

Testing

conditions

Each specimen was immersed for 7 days in one of the three disinfectant solutions. Specimens tested as controls were not subjected to immersion in disinfectants but were kept in water for the same period. A pilot study undertaken on Trevalon specimens after a l-day immersion in the disinfectants demonstrated no significant change. For this reason and to simulate an extreme number of repeated disinfections, the main study was extended to a 7-day immersion period.

Mechanical

testing

The transverse strength of the specimens was measured on a Lloyd Instruments material testing machine, model 2000R (Lloyd Instruments PLC, Southampton, U.K.), JULY

1992

VOLUlME

68

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1

EFFECT

OF DISINFECTION

Table

Modulus

V.

PROCEDURES

of rupture-Transverse

bend strength

for Trevalon”

Mean (n = 6) Nhld

Group Control Chlorhexidine Glutaraldehyde Alcohol based

measurements

Modulus

VI.

Coefficient of variation

11.80 5.95 9.64 4.29

13.60 6.48 11.48 5.14

86.78 91.75 84.00 83.39

disinfectant

A one-way analysis of variance showed that no significant difference was indicated between F = 1.22; df, 3-20; p = 0.3. *Tested in air at 20’ ? 2’ C for control specimens and for specimens stored in disinfectant

Table

Standard deviation

of rupture-Transverse

bend strength

the groups. solutions

measurements

for 7 days (test crosshead

for homopolymer* Comparative

Mean (n = 6) N/mm2

Group

Standard deviation

Coefficient of variation

Control

92.36

6.43

6.95

Chlorhexidine

92.85

3.82

4.12

-

88.85 70.65

3.84 20.20

4.32 28.60

-

Gluteraldehyde Alcohol-based A one-way F = 5.52; *Tested t&dent’s HS is PS is NS is

disinfectant

analysis of variance showed that an overall significant df, 3-20; p = 0.006.

in air at 19” i 2’ C for control t test where: highly significant (p < 0.001). probably significant (p < 0.05). not significant (p > 0.05).

specimens

and

difference

for specimens

stored

which is a continuous rate testing machine attached to a three-point bend testing rig. The rig consists of a loading wedge and a pair of adjustable supporting wedges. The test specimens were centered on the test rig so that the loading wedge, set to travel at a crosshead speed of 5 mm/min, engaged the center of the upper surface of the specimens. Specimens were deflected until fracture occurred. Reference loadings of 15, 35, and 50 N were used, and the load at fracture was recorded. The values for modulus of rupture, often referred to as transverse or flexural strength, and modulus of elasticity, reflecting the rigidity of the specimens, were recorded and computed by the following equations: S=---

3 PI 2 bde

(1) where

E = Stress Strain=4

was indicated

Alcohol-based disinfectant

Glutaraldehyde NS 0.1 NS 0.1

PS 0.02 HS 0.0003 NS 0.8

-

between the groups.

in disinfectant

for 7 days (test

solutions

crosshead

speed,

5 mm/min)

where E = Modulus of elasticity (N/mm2) F = Force at point P (N) I = Distance between supporting wedges b = Width of specimen (mm) d = Depth of specimen (mm) y = Deflection at point P (mm) Point curve.

P was taken

at the initial

part

(mm)

of the load/deformation

The transverse bend testing procedure used in this study followed that specified in the International Standard.ls The surfaces of the specimens were observed after 7 days’ immersion in disinfectant solution under a scanning electron microscope (SEM model 600 Stereoscan, Cambridge Instruments, Cambridge, U.K.).

The data obtained from the investigations described are given in Tables III through VIII. The data comprise the mean values obtained, together with the standard deviation and coefficient of variation. To identify any significant differences between the data obtained for the groups of materials tested, a one-way analysis of variance was

(mm)

F13

(2) JOURNAL

Cblorhexidine

@)

RESULTS

S = Modulns of rupture (N/mm2) P = Peak load exerted on specimen (N) I = Distance between supporting wedges b = Width of specimen (mm) d = Depth of specimen (mm)

THE

signifieancet

NS 0.8

Control

speed. 5 mm/min).

OF PROSTHETIC

DENTISTRY

undertaken;

if

a significant

difference

was

established, 193

ASAD,

Table

VII.

Modulus

of elasticity-Elastic

modulus Mean

GlWLlp

Control Chlorhexidine Glutaraldehyde Alcohol-based

measurements (n = 6) N/Ulmz

Standard deviation

VIII.

Modulus

of elasticity-Elastic

modulus Mean

Group

Control Chlorhexidine Glutaraldehyde Alcohol-based

disinfectant

measurements (n = 6) N/mmz

HUGGETT

CoetEcient of variation

102 97 62 45

A one-way analysis of variance showed that no significant difference was indicated between F = 2.87; df, 3-20; p = 0.06. *Tested in air at 20” I 2’ C for control specimens and for specimens stored in disinfectant

Table

AND

for Trevalon*

2220 2075 2100 2166

disinfectant

WATKINSON,

4.59 4.67 2.96 5.08

the groups. solutions

for 7 days (test crosshead speed, 5 mm/min).

for homopolymer” Standard deviation

Coefficient of variation

2237 2250

157 33

2080

231

7.00 1.49 11.09

2057

75

3.65

one way analysis of variance showed that no significant difference was indicated between the groups. F = 2.92; df, 3-20; p = 0.06. *Tested in air at 19O + 2’ C for control specimens and for specimens stored in disinfectant solution for 7 days (test crosshead speed, 5 mm/min).

A

Student’s t test was used to determine the probability values. The transverse deformation results from Trevalon and the homopolymer investigated are shown in Tables III and IV. Modulus of,rupture values for Trevalon are shown in Table V and for the homopolymer in Table VI. Modulus of elasticity values for the two materials are presented in Tables VII and VIII. Visual and SEM observations showed no significant change in the surface of Trevalon specimens; an obvious crazing effect was found on the surface of the non-crosslinked homopolymer specimens immersed in alcohol-based disinfectant.

DISCUSSION The objective of immersing a denture in a disinfectant is to obtain a clean, decontaminated prosthesis by the destruction of microorganisms. It is desirable that the process should not involve any physical, mechanical, or chemical changes in the denture. Such changes may include alterations of the surface morphology and changes in transverse strength and rigidity. This study was conducted to identify the effect of immersion in various disinfectants for 7 days on the above-mentioned properties of acrylic resin, to test the worst scenario, and to study the possible effect of repeated short immersions. The authors realize that since most agents can disinfect in 10 minutes and many can

194

sterilize in 6 hours, a 7-day immersion time would be unrealistic in normal practice. The transverse deformation and load-to-fracture values obtained for both materials, before and after soaking, were within the limits imposed by the IS0 specifications (Tables III and IV). It was found that (1) the modulus of rupture and modulus of elasticity for Trevalon specimens remained unaffected after a 7-day immersion period in disinfectant solution and (2) the modulus of rupture for homopolymer was significantly affected (p = 0.02) when the specimens stored in alcohol-based disinfectant were compared to the control group. This reduction may be explained by the action of alcohol present in the disinfectant; the alcohol acts as a solvent that causes crazing on the surface of non-cross-linked homopolymers, making them more prone to fracture around the crazing lines, which act as fracture initiation sites. It appears that the modulus of rupture seems to be more sensitive to testing procedures than the elastic modulus. Chitchumnong et al. lg (1989) reported a similar finding in their study on a comparison of three- and four-point flexural strength testing of denture base polymers, and this could be the reason for the absence of any significant change in the elastic modulus of homopolymer specimens after immersion in disinfectant solution, although the mean values for the alcohol-based disinfectant group remained the lowest.

JULY

1992

VOLUME

68

NUMBER

1

EFFECT

OF DISINFECTION

PROCEDURES

There was no significant change in the surface characteristics of Trevalon (cross-linked acrylic resin), and the surface of homopolymer (non-cross-linked acrylic resin) showed obvious lines of crazing on the specimens treated with alcohol-based disinfectant.

ONCLUSION Alcohol-based disinfectants are not suitable for use with non-cross-linked acrylic resins. In addition, although there was no significant change in cross-linked acrylic resin (Trevalon) specimens after immersion for up to 7 days, further work should be undertaken to determine the effectiveness of cross-linking agents, at different percentages and of different types, in resisting the action of solvents such as alcohol.

1. Federation Dent&e Internationale. A revision of technical report No. 10. Recommendations for hygiene in dental practice including treatment of infectious patients. Int Dent J 1987;37:142-7. 2. Cottone JA, Nolinan JA. Selection for dental practice of chemical disinfectants and sterilants for hepatitis and AIDS. Aust Dent J 1987; 32~368-74. 3. Watkinson AC. Disinfection of impressions in UK dental schools. Br Dent J 1988;164:22-3. 4. Langone J. AIDS. Discover 1985;6:28-53. 5. Porter SR, Scully C, Cawson RA. AIDS update and guidelines for general dental practice. Dent Update 1987;14:9-17. 6. Rowe AHR, Forrest JO. Dental impressions, the probability of contamination and a method of disinfection. Br Dent J 1978,145:184-6. 7. Stern KA, Whitacre RJ. An investigation of methods available for sterilizing impressions. Br Dent J 1981;151:217-9.

JOURNAL

PROSTHET

DENT

1979;42:619-23.

16. Rudd RW, Senia ES, McCleskey FK, Adams ED Jr. Sterilization of complete dentures with sodium hypochlorite. J PROSTHET DENT 1989;51:318-21.

REFERENCES

THE

8. Leving RL, Schonfeld SE. Gypsum casts as a potential source of microbial cross contamination. J PROSTHET DENT 1983;49:210-1. 9. American Dental Association Council on Prosthetic Services and Dental Laboratory Relations. Guidelines for infection control in the dental office and commercial dental laboratory. J Am Dent Assoc 1985, 110:969-72. 10. British Dentai Association. Guide to blood borne viruses and the control of cross infection in dentistry. London: British Dental Association, 1987. 11. Ronnel RR. The world wide need for basic infection control procedures. J Corm State Dent Assoc 1982;62:37-9. 12. Martin MV, Bartzokaz CA. The boiling of instruments in general dental practice-a misnomer for sterilization. Br Dent J 1985;159:18-20. 13. Shen C, Javid NS, Colaizzi FA. The effect of glutaraldehyde base disinfectants on denture base resins. J PROSTHET DENT 1989;61:583-9. 14. Ciancio SG. Drugs in dentistry. Disinfectants. Dent Manage 1986;10:62-3. 15. Budtz-Jorgenson E. Materials and methods of cleaning dentures, J

OF PROSTHETIC

DENTISTRY

17. Robinson JG, McCabe JF, Storer R. Denture bases, the effects of various treatments on clarity, strength and structure. J Dent 1987;15:15965. 18. International Organization for Standards. International Standard IS0 1567,2nd ed. Denture base polymers. Geneva: International Organization for Standards, 1988. 19 Chitchumnong P, Brooks SC, Stafford GD. Comparison of three and four point flexural strength testing of denture base polymers. Dent Mater 1989;5:2-5. Reprint requests to: DR. ROBIN HUGGETT DEPARTMENT OF PROSTHETIC DENTISTRY UNIVERSITY OF BRISTOL DENTAL SCHOOL BRISTOL BS12LY ENGLAND

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The effect of disinfection procedures on flexural properties of denture base acrylic resins.

This study evaluates the effects of long-term disinfection immersion on the flexural properties of denture base resins; it was conducted against a bac...
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