Infection Control: Factors influencing the sterilization process

April 1, 2001
The killing of these spores is the hallmark of sterilization because the spores are more difficult to kill than any disease agent ...

Sterilization is a process designed to kill all microorganisms. The different forms of sterilization include: heat, chemical, radiation, low-temperature plasma, and filtration. Since heat sterilization is the most popular form in dentistry, these remarks will be restricted to heat sterilization (steam, unsaturated chemical vapor, and dry heat).

The rationale for sterilizing dental instruments is straightforward. It makes the instruments safe to use on patients. Without proper instrument-cleaning, packaging, and sterilization, microorganisms from a patient or from contaminants in the environment may be spread to a subsequent patient via the contaminated instruments. Although sterilizers have built-in safety factors to provide "overkill," the sterilization process is influenced by several factors. These factors include:

  • The resistance of the microorganisms present
  • Amount of bioburden (saliva or blood mixed with the microorganisms) present
  • The nature of the items to be processed
  • Access of the sterilizing agent to the items to be processed
  • The function and use of the sterilizer.
  • Resistance of the microbes - Sterilizers are designed to operate in conditions known to kill certain highly resistant bacterial spores. The killing of these spores is the hallmark of sterilization because the spores are more difficult to kill than any disease agent (except certain prions). Prions are unique proteinaceous particles involved in causing rare degenerative brain diseases of little significance in private-practice dentistry.

Thus, demonstrating the kill of the special bacterial spores (called biological indicators or BIs) provides the main guarantee of sterilization. This is accomplished by routinely spore-testing sterilizers as recommended by the CDC and ADA and as specifically required in several states. The microbes present on dental instruments used in patients' mouths are less resistant than the bacterial spores used to monitor the use and functioning of the sterilizers.

  • Amount of bioburden - Saliva, blood, and debris on instruments form a layer that may insulate the microbes from direct contact with sterilizing agents (steam, chemical vapor, and dry heat). That's why it is extremely important to thoroughly clean instruments prior to processing them through a sterilizer. Cleaning is performed to give sterilization the best chance to work. Ultrasonic cleaning or instrument washers are key equipment in instrument-processing procedures.
  • Nature of items to be processed - Some instruments are more difficult to clean and sterilize than others because they have deep grooves, hinges, lumens, or other "nooks and crannies" that collect debris. Because of this, all hinged instruments need to be opened during cleaning and sterilization, and cleaning times need to be set to clean the most difficult-to-clean instrument in the lot. Items with lumens (metal three-way syringe tips, high-volume evacuator tips) need to be ultrasonically cleaned or the lumens need to be brushed and/or flushed. Particular attention also must be given to serrations, flutes in burs, and to the deep grooves where instrument tips fit into the handles.
  • Access to the sterilizing agent - As mentioned above, access to the sterilizing agent can be limited by the presence of bioburden on the instruments. Improper packaging also can interfere with this access. The "wrapped cycles" of sterilizers have been shown to yield sterilization of properly packaged instruments. However, use of the wrong type of packaging material can cause sterilization failure. For example, using aluminum foil or closed metal boxes, trays with tight-fitting lids, or closed glass containers to package items for steam or unsaturated chemical vapor sterilization will prevent the steam or chemical vapor from contacting the instruments inside. Wrapping packages with more than two layers of sterilization paper also may impede the penetration of sterilizing agents.

Improper loading of the sterilizer chamber also can prevent access to the sterilizing agent. Space for circulation of the sterilizing agent needs to exist around each pack, bag, tray, or cassette. This space is best achieved by placing packages on their edges whenever possible and not stacking or layering items one on top of the other. If there is ever a question about packaging materials or techniques or about loading the chamber, place spore strips inside of the packages, bags, trays, or cassettes to measure spores killed during a routine cycle.

  • Function and use of the sterilizer - Sometimes a sterilizer may not function properly. These problems may be detected by use of chemical monitors with every load, along with periodic spore-testing. It's also a good idea to periodically perform physical monitoring of the sterilizer. This involves documenting the physical conditions of time and temperature of a cycle. Some sterilizers have printouts that document these parameters, but those that do not can be checked by simply observing the gauges or read-outs during a cycle. While most sterilizers are fairly automatic and easy to operate, some require special attention to assure the desired result. For example, some dry heat sterilizers do not have automatic timers that start only after the correct temperature has been reached. So, the warm-up time must be accounted for in the overall timing of the cycle. Also, the doors of most dry heat units can be opened during the cycle (to add forgotten items, for example), but this causes a drop in temperature, requiring the cycle timing to begin again.

In summary, sterilization is a very important patient safety aspect of an infection control program. Several factors influence the success of sterilization, such as the proper preparation and packaging of the instruments, as well as the proper use and function of the sterilizers.

Chris Miller, PhD, is professor of oral microbiology and executive associate dean at the Indiana University School of Dentistry.