JOURNAL OF ENDODONTICS I VOL 2, NO 11, NOVEMBER 1976

E n d o d o n t i c d r y - h e a t sterilizer e f f e c t i v e n e s s Lt. CoL Michael B. Dayoub, DDS, MS, and M. Joanne Devine, AS, Washington

Endodontic instruments usually are decontaminated at chairside by d r y - h e a t s t e r i l i z e r s usin,g s a l t a s a h e a t t r a n s f e r m e d i u m . This s t u d y s h o w s t h a t sterilizers m a y r e a c h i n a d e q u a t e t e m p e r a t u r e l e v e l s , a n d t h a t t h o s e sterilizers w h i c h a r e e f f e c t i v e m u s t b e a l l o w e d a n a d e q u a t e t i m e p e r i o d to r e a c h o p e r a t i n g t e m p e r a t u r e . A l t h o u g h v e g e t a t i v e b a c t e r i a l f o r m s a n d l e s s r e s i s t a n t s p o r e s c a n b e k i l l e d in t e n s e c o n d s in salt, 20 s e c o n d s of i n s t r u m e n t i m m e r s i o n is n e c e s s a r y to e n s u r e k i l l i n g of t h e h e a t - r e s i s t a n t s p o r e s of B a c i l l u s s u b t i l i s v a r . niger.

There is general agreement that aseptic techniques are essential to successful endodontic treatment. Dry heat is less efficient than moist heat, but it is most often used to sterilize root canal instruments because of the deleterious effects of moisture on metal. Decontamination of root canal instruments in the past had been performed with molten metal, and later with glass beads. Oliet, Sorin, and Brown 1 have shown the advantages of salt as a heat transfer medium. At present, salt is used most often. The effectiveness of chairside decontamination procedures has recently been studied. Sterilization has been achieved with glass beads at 218 C (424.4 F ) for 15 seconds and salt at 218 C for ten seconds. 2 H u b b a r d and associates 3 were unable to achieve sterilization with glass beads. This study was undertaken to test the efficiency of dry-heat sterilizers by measuring the sporicidal effects of heat, using a modification of methods outlined by the Association of Official Analytical Chemists. 4

Methods and M a t e r i a l s Three heat sterilizers were initially temperature tested. These were tested for heat-up time with glass beads and with table salt by using a thermocoupie-equipped, temperature-indicating instrument.* The thermocouple was placed 18 m m into the heat transfer medium. Ten other instruments also were tested for heat-up times, using salt as the heat transfer medium. The lyophilized test organism, Bacillus subtilis var. niger, t was reconstituted in soil extract nutrient broth and incubated for 24 hours at 37 C. Tubes of soil extract nutrient broth then were inoculated with 0.1 ml of the original reconstituted culture, incubated at 37 C for 72 hours, and homogenized. Previously sterilized suture loops, porcelain penicylinders (as described in the A O A C spore test), and no. 60 endodontic files were introduced into the homogenized culture and remained there for 20 minutes. They then were removed; placed on sterile,

matted filter paper in glass petri dishes; and dried and kept under vacuum. Acid resistance of the spores was determined by the A O A C acid test. Decontamination procedures were performed in a commercially obtained salt sterilizer that maintained a temperature level of 220 C (428 F ) . Six carriers of each type were buried in the salt to a depth of 18 m m for 5, 10, 15, and 20 seconds. Loops and cylinders were transferred to fluid thioglycollate media.:~ The files were aseptically separated from their handles and then cultured in the aforementioned media. All cultures were incubated at 37 C for 21 days; then, they were heat shocked at 80 C and reincubated for seven days.

Results Initial testing of 13 dry-heat sterilizers showed that seven achieved and maintained an acceptable operating temperature of 218 C; however, six did not. Operating temperatures were reached by some units as rapidly as

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i

15 minutes; of those that eventually reached operating temperature, the maximum time required was 3 8 9 hours. The results of the sporicidal activity test are shown in the Table. In 27 of 30 test tubes growth was evident in media that contained those carriers which had been exposed to the heat for five seconds. When carriers were immersed in salt for ten seconds, media in 24 of 30 test tubes showed growth. The immersion of carriers for 15 seconds showed 13 of 30 test tubes with growth in media. When a 20-second immersion was given, media in all test tubes were negative for growth. Discussion

Although the heat-up time of instruments in endodontic dry-heat sterilizers has been investigated thoroughly,5 scant attention has been given to the time period necessary for sterilizers to reach operating temperatures. As these sterilizers are supplied without accurate means o f monitoring the temperatures that are reached, the periodic measurement of temperatures is important. Because only 7 of 13 sterilizers tested reached and maintained operating temperatures and two of these reached only a minimally acceptable temperature (218 C ) , the likelihood of inadequate decontamination with untested equipment is obvious. The results of this study vary with those of Windeler and Walter,2 who achieved sterilization in salt at 218 C for ten seconds. Their files were dipped into a saline suspension of spores for inoculation. H u b b a r d and his co-workers 3 demonstrated that mechanical action can decrease spore numbers in decontamination procedures. In our test the immersion of instruments in salt probably removed a minimal number of spores because the organisms had been suspended in an adhesive medium. Under these test 344

VOL 2, NO 11, NOVEMBER 1976

Table. Growth o~ Bctetllus m~tii|8 vat. n~ger on callers after endodontic

~--~-htmt sterilization. Test tube culture resul~s Heating time

/ :~

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(in sec at 220 C) :

Suture loops No. +

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10

I0 15

12 4 0

20

Penieylinder~

No,-

No. +

~,Z, 0 8 I2 II

conditions sterilization of no. 60 endodontic files was achieved in 15 seconds. Koehler and Hefferren s have shown significant gradients of heat transfer occurring within larger instruments. The suture loops and penicylinders used in this experiment added to spore heat resistance by their greater bulk and also through the heat insulation caused by their porosity. Spores and vegetative microorganisms can be killed in dry-heat sterilizers; however, the time required to sterilize varies greatly, depending on the instrument's size, the heat transfer medium, and the heat actually generated by the sterilizer. Summary and Conclusions

Thirteen heat sterilizers were temperature tested, and it was found that six of them did not achieve an acceptable operating temperature of 218 C. Heat-up time varied from 15 minutes to 3 8 9 hours. Suture loops, porcelain penicylinders, and no. 60 endodontic files were decontaminated in an endodontic hotsalt sterilizer maintained at 220 C. The following conclusions were drawn: - - E n d o d o n t i c dry-heat sterilizers may be unable to reach and maintain an effective temperature and should be temperature tested to ensure their effectiveness. - - B e c a u s e of heat-up times, sterilizers must be adequately preheated to operating temperature before use.

.

Files

No. -

No. +

No, -

12

0

5

I

I1 9

I 3

1 0

$ 6

0

12

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6

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,

- - E n d o d o n t i c dry-heat sterilizers effectively sterilized endodontic files in 15 seconds. - - L a r g e r instruments and more porous materials should be immersed in the salt sterilizer for a minimum of 20 seconds. *Model 32-JP-7, JBT Instruments, Inc., New Haven, Conn. tAmerican type culture collection no. 9372, Rockville, Md. *Difco Laboratories, Detroit. Dr. Dayoub is a member of ~he USA Institute of Dental Research, and Ms. Pevine is a research assistant, Walter Reed Army Medical Center, Washington. Requests for reprints should be directed to: Lt. Col. Michael B. Dayoub, USA Institute of Dental Research, Walter Reed Army Medical Center, Washington, 20012. References

I. Oliet, S.; Sorin, S.; and Brown, H. A temperature analysis of thermostatical-

ly controlled root canal sterilizers using molten metal, glass beads, or salt. Oral Surg 11:37 Jan 1958. 2. Windeler, A.S., Jr., and Walter, R.G. The sporicidal activity of glass bead sterilizers. J Endod 1:273 Aug 1975. 3. Hubbard, T.M., Jr.; Smyth, R.N.; Pelleu, G.B., Jr.; and Tenca, J.I. Chairside decontamination of endodontic files. Oral Surg 40:148 Iuly 1975. 4. Horwitz, W., ed. Official methods of analysis of the Association of Official Analytical Chemists, ed 1 I. Washington, 1970, p 64. 5. Koehler, H.M., and Hefferren, J.J. Time-temperature relations of dental instruments heated in root-canal instrument sterilizers. J Dent Res 41:182 Jan-Feb 1962.

Endodontic dry-heat sterilizer effectiveness.

JOURNAL OF ENDODONTICS I VOL 2, NO 11, NOVEMBER 1976 E n d o d o n t i c d r y - h e a t sterilizer e f f e c t i v e n e s s Lt. CoL Michael B. Dayo...
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