Definitions in sterilisation

This page is based on an article by Prof. George Freedman

Cleaning

The technique of removing visible contamination from a dental instrument or surface, either through physical scrubbing (manual decontamination), an energy/chemical process (ultrasonic cleaning), or the innovative, automated washing of instruments (Hydrim C51w, SciCan, Toronto, Canada) that ensures that they are thoroughly cleaned, free of debris and ready for effective sterilisation. The automated method of washing instruments eliminates the need to pre-soak or scrub instruments. This process should be used only as a preamble to sterilisation, and is of course totally unacceptable as a standalone procedure for dental instruments.

Disinfection

The destruction of microorganisms, pathogenic or otherwise, by physical or chemical means. This process destroys most known pathogens but may not affect bacterial spores. In the dental practice, the concept of 'most but not all' pathogens is a very questionable path to follow. The risk of the surviving pathogens causing cross infection is a major one indeed.

Sterilisation

The destruction of all viable microorganisms, including many of the resistant bacterial spores, by a physical (including heat) or chemical process. Since this process covers the broad range of pathogens that may be found in the oral cavity and in the teeth, it is the only clinically acceptable means for dealing with reusable instruments between patients.

Classification of autoclave cycles

The various types of autoclave cycles¹ are differentiated by what they will or will not sterilise and how long they take to complete the process. In terms of sterilisation material targeting, the two most important parameters are whether the instruments are solid or hollow, and whether they can be wrapped, or must be left unwrapped. The concept of 'porous load' also plays a big role in cycle definition. It makes sense that a sterilising agent will have more difficulty penetrating and sterilising a hollow object such as a handpiece than surface-contacting a solid object such as a dental mirror. The chemical or physical agent can be simply passed over a solid surface, while it must be forced or sucked into internal spaces that have constricted access. It is important to remember that many dental instruments are 'hollow', in that they have lumens or difficult-to-access areas. The key issue is that the air trapped inside these hollow areas cannot be easily removed to allow the sterilant to contact the instrument surface. The sterilising agent also takes longer to penetrate to instruments through wrapping than simply contacting unwrapped ones. The wrapping effectively envelops the instruments in a 'hollow' into which the agent must be forced or sucked. On the other hand, the convenience and organisation of wrapping instrument 'kits' together can greatly increase practice efficiency, and hence practice success. Another advantage of wrapping is that the instruments maintain their sterility during storage. The ideal sterilisation cycle for the dental practice is one that can handle both hollow and solid instruments, wrapped or unwrapped. The least noticed phase of sterilisation is the drying time. Instruments or wrapped packs should be allowed to dry inside the sterilising chamber prior to removal and handling. If they are handled while they are moist and hot, the packing can act as a wick, absorbing moisture and bacteria from the outside, and transporting them to the instruments inside.

The N-cycle is suitable for sterilising unwrapped, solid instruments. N-cycle sterilisers are the most popular bench top autoclaves, and are classified as passive systems (also known as gravity, non-vacuum, or downward displacement). Typically, as steam is admitted into the sterilisation chamber, it forces unsaturated and saturated air out through a vent. The major concern with the N-cycle sterilisers is the non-removal of trapped air (especially air pockets in lumens and difficult-to-access areas of the load) during gravity displacement. Errors in packaging or overloading the steriliser chamber can result in cool air pockets where items are not sterilized.²

The B-cycle is used to sterilise solid, hollow, and porous instruments, be they wrapped or unwrapped. It involves the use of active (forced) air removal, usually with a vacuum pump. These prevacuum sterilisers are usually fitted with a pump that creates a vacuum that removes air from the sterilizing chamber before the chamber is pressurised with steam. This technique allows faster and more effective steam penetration throughout the entire instrument load than the gravity displacement technique typically employed in sterilizers with N-cycles. Any air that is not removed from the chamber will interfere with steam-instrument contact, and may compromise sterilisation. B-cycle sterilisers must be tested periodically for adequate air removal.

The S-cycle is indicated for unwrapped solid products; porous, hollow, single-wrapped products; or multi-layer wrapped products. Thus, all dental instruments can be sterilised with this cycle. The S-cycle utilises forced air removal, this action being created by vacuum or steam pulsing. The positive pressure pulse system removes air from the sterilization chamber without a vacuum pump. Pressurised steam is injected into the chamber, gradually forcing the air out through a valve. Once the chamber is pressurised, the chamber is vented to near-atmospheric conditions. This process is repeated multiple times until effective air removal is achieved. S-cycle autoclaves such as the Statim 2000s and 5000s use Positive Pressure Pulsed Air Displacement to offer a sterilisation cycle that is specifically designed for clinical convenience in that they are able to sterilise products effectively (with validation) with reduced cycle times.

The length of the cycle time is the other important issue in selecting a steriliser. The longer the cycle time, the more instrument sets and handpieces a busy practice requires.

The time required for a sterilisation cycle is dependent on a number of factors. The type of cassette (light, thin cassette walls promote rapid heating and cooling) and the size of the sterilisation chamber are both critical; the smaller the chamber, assuming that it is large enough for the typical instrument load, the more quickly air can be removed and steam and pressure introduced. Expelling the excess steam at the end of the cycle will speed up the drying stage and shorten the cycle time.

Since the limitations of N-cycle sterilization make it impractical for dental office utilisation, the clinical choices should be limited to B- and S-cycle autoclaves. The B-cycle steriliser takes a minimum of 47 minutes to complete versus the 6-9 minutes of a pressure pulsed S-cycle (without drying). From a strictly time management point of view, the pressure pulsed autoclave systems make the most sense. Furthermore, since the S-cycled instruments (particularly expensive and delicate handpieces) spend less time in the corrosive environment of hot air and moisture, this technology offers not only a quicker but also a more gentle sterilisation for the dental practice.

Small Sterilizers
Cycle type	N-cycle	B-cycle	S-cycle (Statim)
Materials that can be sterilized	solid only	solid/hollow	solid/hollow
	non-wrapped only	multi-wrapped/non-wrapped	wrapped/non-wrapped
		porous
Air removal		forced	pressure pulsed
	25 minutes	45 minutes	6-9 minutes

Validation

Manufacturers should be able to provide autoclave users with microbiological validation confirming that their autoclaves are effective in sterilising the instruments that they are indicated for, under the conditions listed in the instructions.

Furthermore, each autoclave must be monitored on a regular basis to ensure the ability of the equipment to attain the physical parameters that are required to achieve effective sterilisation. This can be accomplished with chemical, biological, and temperature monitoring test kits. These tests can be cumbersome, however, and are easily forgotten. Statim autoclaves are microprocessor controlled, and all the important parameters are constantly monitored internally. All the necessary information is presented on the user-friendly screen display, including prompts for continued operation as well as any error messages. Since Statim automatically aborts the sterilisation cycle in case of any malfunction, every completed cycle guarantees successful sterilisation of the contents. However, load control and monitoring (chemical and biological indicators) are essential parts of the effective sterilization process.

References

¹ European Committee for Standardisation. Voluntary European Standard EN 13060:2004
² US Centers for Disease Control and Prevention. Guidelines for Infection Control in Dental Health-Care Settings 2003

About the author
George Freedman is past president of the American Academy of Cosmetic Dentistry and associate director of the Esthetic Dentistry Eduction Center at the State University of New York at Buffalo. He is also director of postgraduate programmes in Esthetic Dentistry at the Eastman Dental Center, University of Rochester Medical Center, university programmes in Seoul, South Korea and Schaan, Liechtenstein, and the chairman of the Clinical Innovations Conferences in London.