Sterilizing Instruments With Dry Heat

Editor’s Note: The opinions expressed are those of Chris H. Miller, PhD. 

The safe provision of oral health care services is a top priority for both patients and providers. Ensuring that infection control protocols are precisely followed is the responsibility of all dental team members. While infection ­con­­trol protocols encompass a wide array of policies and procedures, the sterilization of dental instruments is a critical component of ensuring safety in the dental operatory. In dentistry, there are three ways to sterilize instruments: steam under pressure, dry heat, and chemical vapor. In this informative Q&A, infection control expert Chris H. Miller, PhD, shares his insight on the dry heat sterilization method.

How does dry heat sterilization work?

Dry heat sterilization causes proteins to lose their distinctive shapes as microbes dehydrate. Free oxidative radicals are created as microbial proteins coagulate and microbial death ensues. The amount of water in and around microbes influences the temperature at which proteins are destroyed. The lower the water content, the higher the temperature needed for destruction.¹ This is why steam sterilizers, which use moist heat, operate at lower temperatures (about 121° C to 134° C) compared to dry heat sterilizers (about 160° C to 190° C).²

It’s important to ensure that the packaging materials and the instruments and other items being processed can withstand the high temperatures of dry heat sterilization. As with any sterilizer, successful sterilization occurs only after the prescribed exposure time has been achieved at the indicated sterilizing temperature. So “warm-up” time (time required for the chamber to reach sterilizing temperature) is not included in the sterilizing exposure time. Also, interrupting the exposure time—such as opening the door of a dry heat sterilizer mid-cycle (if allowed) to add a forgotten item—circumvents the sterilization process. Only equipment that has been cleared by the United States Food and Drug Administration (FDA) as a sterilizer should be used to process dental instruments, and the manufacturer’s directions need to be followed precisely to assure successful sterilization.³

What are the two types of dry heat sterilizers and how are they different?

Static-air and forced-air are the two types of dry heat sterilization, and they are based on how the hot air circulates in the sterilizing chamber.

The static-air (gravity convection) type depends on natural air convection. As the air is heated by coils on the bottom or sides of the chamber, it rises and the cooler air falls to create circulation. The sterilizing time usually ranges from 60 minutes to 120 minutes (after the warm-up time is complete), depending on the equipment used.

Forced-air (mechanical convection) uses a blower motor to mechanically circulate the chamber air, which decreases the heat-energy transfer time from the hot air to the items being processed. Thus, the sterilizing cycle is shorter than static-air models and may be as brief as 12 minutes for wrapped instruments, depending on the equipment used.

What are the benefits of dry heat sterilization?

A major benefit of dry heat sterilization is minimal or no instrument corrosion. Oxygen, moisture, and high temperature facilitate corrosion of susceptible metal items (eg, carbon steel). In dry heat sterilization, moisture is eliminated, so corrosion is minimized. This is why it’s especially important to dry the cleaned instruments prior to packaging and using dry heat sterilization. Another benefit is that instrument packages are dry at the end of the sterilization cycle and are ready to use after the appropriate cooling time. This feature of forced-air dry heat sterilization helps provide a relatively short instrument turnaround time, depending on the equipment used.

How does dry heat sterilization compare to steam sterilization?

There are a variety of FDA-cleared steam and dry heat sterilizers available. Steam sterilization (ie, autoclaving) is dependable and economical and is the most widely used sterilization method for items that are not sensitive to heat and moisture. Autoclaving can cause corrosion of nonstainless steel (eg, carbon steel) items such as burs and some cutting instruments. Also, the sterilized items are wet at the end of a steam sterilizing cycle.

Dry heat sterilization can be used on items that corrode in steam or are impenetrable to moist heat (eg, powders, petroleum products).^3,4 The sterilized items are dry at the end of a dry heat sterilizing cycle. The static-air dry heat sterilizers have longer sterilizing cycles than steam sterilizers. Some forced-air dry heat sterilizers, however, have been reported to provide shorter instrument turnaround times than steam sterilizers.⁵

FDA-cleared packaging materials and chemical monitoring devices are available for both methods of sterilization. Steam and dry heat sterilizers can be biologically monitored with appropriate bacterial spores, including Bacillus atrophaeus for dry heat and Geobacillus stearothermophilus for steam. This step is important in ensuring the success of the sterilization process.

Dry heat sterilization is an effective method for sterilizing instruments in the dental setting, and may be particularly well suited for practices in which instrument corrosion is an issue.

REFERENCES

1. Joslyn, LJ. Sterilization by dry heat. In Block SS, ed. Disinfection, Sterilization, and Preservation. 5th ed. Philadelphia: Lippincott Williams & Wilkins; 2001:715.
2. Miller CH. Instrument processing. In: Infection Control and Management of Hazardous Materials for the Dental Team. 5th ed. St Louis: Elsevier; 2014:130.
3. Centers for Disease Control and Prevention. Guidelines for infection control in dental healthcare settings—2003. MMWR Morb Mortal Wkly Rep. 2003;52:42.
4. Centers for Disease Control and Prevention (Healthcare Infection ControlPractices Advisory Committee). Guidelines for disinfection and sterilization inh ealthcare facilities. Available at: cdc.gov/hicpac/ Disinfection_ Sterilization/13_0Sterilization.html. Accessed August 24, 2015.
5. Slavik NS. Instrument sterilization: the case for high velocity hot air sterilization in the orthodontic office. Orthodontic Practice. 2013;4(4):34–37.

From Dimensions of heat Hygiene. September 2015;13(9):38,40–41.