Sterilization is the killing of all microbes. Microbes subjected to a heat sterilization process are killed within a short period of time after direct contact with a heat-sterilizing agent that is at the proper temperature. Thus, exposure, time, and temperatures are key factors influencing heat sterilization.
The actual surface of the instruments to be sterilized must be exposed to the sterilizing agent for the appropriate time. Anything that interferes with this exposure or with the temperature of the sterilizing agent will either prevent the killing of all microbes or will extend the time needed to kill all microbes.
You cannot assume that sterilization always will occur in a sterilizer. This is why the sterilization process is to be monitored by spore testing and by the use of chemical indicators. Several things may cause sterilization failure:
- Improper packaging - This can slow or prevent the sterilizing agent from reaching the instruments inside. This may involve using a packaging material that is not compatible with the sterilization method used. Also, it could involve using a closed container or excessive layers of wrap that prevent the sterilizing agent from contacting the instruments.
- Improper loading of the sterilizer - The sterilizing agent must have access to all items in the chamber, which means that packages and cassettes must be slightly separated and not packed too tightly. The best way to achieve this is to place packages and cassettes on their edges rather than in stacks or layers.
- Improper timing of the sterilization cycle - It takes a certain amount of time for the proper temperature to be reached in the sterilizing chamber and for the sterilizing agent to penetrate to and through all of the packages and cassettes. Most sterilizers are automatic and won`t start timing until after the proper temperature has been reached. Timing problems can occur:
* If the sterilizer is not operated correctly.
* If the timing is started before the proper temperature is reached (in units that are not automatic).
* If the door of a dry-heat sterilizer is opened before the end of the normal cycle.
* If the sterilizer malfunctions.
- Improper temperature in the sterilizer - A lower than normal temperature will either prevent or retard sterilization, and may result due to incorrect operation of the sterilizer or sterilizer malfunction.
- Improper instrument cleaning - Debris left on instruments from improper cleaning may insulate microbes from the sterilizing agent.
The first four causes of sterilization failure listed above can be detected by periodic sterilization monitoring (spore testing), if the biological indicator (spore test) is placed in the appropriate part of the load. Placing a biological indicator (spore test) inside of a package or cassette will determine if the sterilizing agent is penetrating the packaging material and reaching the instruments inside. Placing this spore test package or cassette in the center of the load will determine if the sterilizing agent is spreading around all of the packages and reaching the most challenging spot in the chamber.
In some instances, the sterilizer manufacturer`s instructions may state that the spore test should be placed in a site other than the center of the load (on the bottom shelf near the door, for example). Those instructions should be followed when given.
Once spore testing has shown that sterilization can be achieved within a particular load configuration, that same configuration (arrangement of packages and/or cassettes) should be used routinely and spore-tested periodically. If more than one load configuration is used in the office, each should be spore-tested periodically to help assure sterilization success. Placing a chemical indicator inside of each package and cassette can determine if the sterilizing agent has at least reached the instruments inside no matter where the package is placed within the sterilizer chamber.
The fifth cause of sterilization failure (improper instrument cleaning) cannot be detected by sterilization monitoring. Placing a spore strip or chemical indicator inside a package next to the instruments can determine if the sterilizing agent has at least reached the instruments, but it cannot determine if debris on the instruments may have insulated microbes from that agent. It is very important to perform instrument cleaning properly every time to help assure sterilization success.
Common types of heat sterilization
There are three common types of heat sterilization:
- Steam sterilization - Steam sterilization (autoclaving) is the most common sterilizing method in dentistry. Its main advantage is its ability to achieve excellent heat penetration to all of the surfaces of properly cleaned and packaged instruments. Unfortunately, it will cause rusting of carbon steel items. Also, packages are wet at the end of the sterilizing cycle and they should be allowed to dry before being removed from the chamber.
The cycle times range from 2.5 minutes to 30 minutes depending on the temperature (134 to 121 degrees C) and load/packaging conditions. These times are in addition to the initial "come-up" time and subsequent "comedown" and drying times.
- Dry-heat sterilization - There are two types of dry-heat sterilizers. One is a rapid heat transfer type in which the heated air is mechanically circulated around the load. This unit operates at temperatures near 191 degrees C with sterilizing cycles of six to 12 minutes depending on packaging conditions plus the initial heat-up and subsequent cool-down times.
The other type of dry-heat sterilizer, sometimes referred to as an oven type, does not mechanically circulate the air and has sterilizing cycle times of 60 to 120 minutes depending on the unit (plus heat-up and cool-down times).
The advantages of both types of dry-heat sterilizers are that they do not cause rusting of the instruments, and they yield dry packs at the end of the cycle.
- Unsaturated chemical-vapor sterilization - This type of sterilization is achieved in a unit referred to as a Chemiclave or a Harvey sterilizer. The sterilizing agent is a hot chemical vapor generated from a solution of formaldehyde and alcohol. The sterilizing cycle is 20 minutes at 134 degrees C plus heat-up and comedown times.
The advantages are that it does not cause rusting of the instruments and it yields dry packs at the end of the cycle. Some have concerns about the formaldehyde.
Chris Miller is director of Infection Control Research and Services and professor of oral biology at Indiana University.