Techniques of Instrument Sterilization
Any means of instrument sterilization. to be used in office-based dental and surgical care must be reliable, practical, and safe for the instruments. The, three methods generally available for instrument sterilization are dry heat, moist heat, and ethylene oxide gas.
Sterilization with heat. HeatIs one of the ‘oldest means of destroying microorganisms. Pasteur used heat ‘to reduce the number of pathogens in liquids for preservation. Kcch was the first to use heatfor sterilization. He
found that 1.5 hours of dry heat at 1000 C would destroy all vegetative bacteria, but that 3 hours of dry heat at 1400 C was necessary to eliminate the ‘spores of anthrax bacilli. Koch then tested moist heat and found it a more efficient means of heat sterilizatio~ because it reduces the
temperature and ‘time necessary to kill spores. Moist heat is probably more effective because dry heat oxidizes cell proteins, a process requiring extremely high temperatures, whereas moist heat causes destructive protein calculation quickly at relatively low, temperatures.
Because spores are the most resistant forms of microbial life, they are used to monitor sterilization techniques. The spore of the bacteria Bacillus stearothermophilus is extremely resistant to heat and is therefore used
to test the reliability of heat sterilization. These bacilli can . be purchased by hospitals and private offices and run through the sterilizer .with the instruments being sterilized. A laboratory then places the heat-treated spores into culture. If no growth occurs, the sterilization procedure
was successful (Fig. 5-2).
It has been’ shown that 6 months after sterilization the possibility of organisms entering sterilization bags increases, although some individuals feel an even longer period is acceptable as long as the bags are properly handled. Therefore all sterilized items should be labeled with an expiration date that is no longer than 6 to 12 months in the future.
A useful alternative technique for sterilely storing surgical instruments is to place them into cassettes that are double wrapped in specifically designed paper and sterilized as a set foruse on a single patient.
Dry heat. Dry heat is a method of sterilization that can be provided in most dental offices because the necessary equipment is no more complicated than a therrriostatically controlled oven and a timer. Dry heat is most commonly used to sterilize glassware and bulky items that can withstand heat but are susceptible to rust, The success of sterilization depends not only on attaining a certain temperature but also on maintaining the temperature for a sufficient amount of time. Therefore the following three factors must be considered when using dry heat: (1) warm-up time for the oven and the materials to be sterilized, (2) heat conductivity of the materials, and (3) airflow throughout the oven and through the
objects being sterilized. In addition, time for the sterilized equipment to cool after heating must be taken into consideration. The time necessary for dry-heat stenllzation limits its practicality in the ambulatory settingbecause it lengthens the turnover time and forces the dentist to
have many duplicate instruments.
The advantages of dry heat are the relative. ease of use and the unlikelihood of damaging heat-resistant instruments. The disadvantages are the time necessary and the potential damage to heat-sensitive equipment. Guidelines for the use of dry-heat sterilization are provid u in Table 5-2.
Moist heat. Moist heat is more efficient than dry heat for sterilization because it is effective at much lower temperatures and requires less time, The reason for this IS based on several physical principles. First, water boilin 100° C takes less time to kill organisms than does not at the same temperature because water is boiling fransferring heat. Second, it takes approxm a as much heat to convert ·boiling water temperature cause the same amount of room temperature
Guidelines for Dry-Heat and Steam Sterilization
boil. When steam comes into contact with an object, the “:\:’am condenses and almost instantly releases that stored ; .cat energy, which quickly denatures vital cell proteins. Saturated steam placed under pressure (autoclavtng) is even more efficient than non pressurized steam. This is
because increasing pressure in a container of steam increases the boiling point of water so that the new steam entering a Closed container gradually becomes hotter. Temperatures attainable by steam under pressure include.
1090 C at 5 psi (pounds per square inch), 1150 C at 10 psi, 121″ C at 15 psi, and 1260 C at 20 psi (see Table 5-2).
The container usually used for providing steam under pressure is known as an autoclave (Fig. 5-3). It ‘.•o•.rks by creating steam and then, through a series of valves, increasing the pressure so that, the steam becomes superheated. Instruments placed into an autoclave should be packaged to allow the free flow of steam to the equipment, such as by placing instruments in paper bags or wrapping them in cotton ‘cloth.
Simply placing instruments in boiling water or freedflowing steam results in disinfection rather than sterilization, because at the temperature of 100” C, many spores and certain viruses survive.
The advantages 01 sterilization with moist heat are its cttecuvcncss.vpeed. and the relative availability of office proportioned auto claving equipment. Disadvantages include the tendency of moist heat to dull and rust instruments and the cost of autoclaves Crable 5-3).
Gaseous Steriliztion. Certain gases exert a lethal action on bacteria by destroying enzymes and other vital medical structures Of the several gases available for sterilizationl, ethylen oxide is the most commonly used.
It is a highly framablc gas and is mixed with CO, or nitrogen to make it safer to use. Because ethylene oxide gas is at room temperature, it can readily diffuse through
porous materials, such as plastic and rubber.At 50° C it is effective for killing all or&anfsms, including spores, within 3 hours. However, because it is highly tcxic to animal tissue, equipment exposed to ethy.lene oxide must be aerated for 8 to 12 hours..at 50° to 60° C or at ambient temperatures
for 4 to 7 days.
The advantages of ethylerie oxide’ for sterilization are its effectiveness for sterilizing porous materials, large equipment’, and materials sensitive. to heat or moisture. The disadvantages are the need for special equipment and
the length of sterilization and aeration time necessary to reduce tissue toxicity. This technique is rarely practical for dental use, unless the dentist has easy access to a large facility willing to gas sterilize dental equipment (e.g., hospital, ambulatory surgery center).