Sterilization is a critical part of any surgical tool usage. Every tool needs to be sterilized based on its usage, purpose, and construction. Failure to understand these particular requirements often results in damage to the tools. That is why there exist several methods to sterilize surgical tools. They vary based on their accessibility, usage, and reliability. The end client must choose the sterilization process keeping in mind the special criteria and usable life of the tool.
The most commonly used sterilization methods are moist heat sterilization, dry heat sterilization, ionizing sterilization, vapor sterilization, and chemical sterilization. Economics and reliability play a big role in deciding the best method. You have to decide between several methods justifying their cost, impact on the usable life of instruments, the material of tools to be sterilized, and reliability of the process.
Let us have a look at basic sterilization steps and then we will discuss different methods of sterilization in detail.
Basic Steps of Sterilization
Every sterilization follows the same standard procedure no matter the place. It is standard procedural steps in large manufacturing factories and medical centers alike.
First Step: Cleaning
Every sterilization procedure starts with basic cleaning. It includes the removal of visible debris and dirt from the tools. The process can differ depending on if the tool can be washed with water or not but the purpose stays the same. Most medical facilities also perform an initial inspection at this stage to judge if the tools should be discarded or not.
A trained professional with proper personal protective equipment (P.P.E) cleans dirty tools in an enclosed area. The atmosphere in the area is maintained to reduce any risk of biohazard and contamination. This area is often isolated from the other sterilization areas. This is a highly dangerous area as it contains a lot of contaminants. According to CDC, the American Institute of Architects states negative air pressure and no fewer than six air exchanges per hour in the decontamination area.
Most medical facilities use high-strength enzymatic soap and pressure hoses to clean off any organic matter or debris on surgical tools. Tools with a lot of hidden surfaces are cleaned in an open position to access the areas. Additionally, tools are properly dried after the cleaning procedure.
After enzymatic cleaning and washing, the tool is inspected for any damages. Sharp tools are tested for their cutting action and inserts. If the tools are found to have any damage, it is either discarded or labeled separately to sterilize and sent off for repair.
Second Step: Disinfection
After inspection, the tools are moved to the disinfection area. The primary purpose of disinfection is to reduce the load on the sterilization section. Most of the contaminants are removed in this section. High-level disinfectants are employed for this purpose based on the tools to be disinfected.
Dental Tools waiting for disinfection
This area is relatively sterile and proper sterilization practice to sterilize air is maintained to ensure that tools don’t get contaminated.
This might be the final stage for some less critical tools like stethoscopes and ENT observation instruments. Surgical tools are sent to the Packaging area from here on.
Third Step: Packaging Area
In the packaging area, tools are packaged in sterile packing. There are several different packings from woven or unwoven wraps to blister vacuum peel packs. Packing is chosen based on the type of sterilization to be done. Each packing is rated for different temperatures and heat cycles.
The type of packing material also dictates the storage life of the tools. Depending on the storage facilities, perforated storage material has a reduced shelf life than an air-tight package. Heat-sealed, plastic peel-down pouches, and wrapped packs sealed in 3-mil (3/1000 inch)【0,1 mm】 polyethylene overwrap have been reported to be sterile for as long as 9 months after sterilization.
A properly sterilized and packed tool can remain sterile for as long as 2 years.
Sterilization testing strips and validation media are also included in the packaging to make sure that the tool is properly sterilized. They include many chemical and biological indicators. In addition to several other factors are also monitored like heat penetration in steam and dry heat sterilizations.
Fourth Step: Sterilization
Here, the tools are properly sterilized with the help of an appropriate sterilization method. This area also has a sterile environment and isolation. This is to prevent any mishap or accidental contamination of tools.
A lot of sterilization processes have been created for different kinds of tools. Most commonly used are:
- Steam Sterilization
- Dry Heat Sterilization
- Radiation Sterilization
- Chemical Sterilization
- Gas Sterilization
- Plasma Gas Sterilization
- Vaporized Hydrogen Peroxide (VHP) Sterilization
We will discuss them in detail later.
Fifth Step: Storage Facilities
After proper sterilization packages are forwarded to storage facilities in sterile containers. Storage facilities are specially designated and designed areas to store sterilized tools.
Here tools are properly placed in groups marked with purpose and shelf life. They are organized in storage trays and racks. These storage racks are made according to federal government specifications. Notably, they provide a sterile and dry environment for the tools.
CDC recommends appropriate clearances for the storage of surgical tools in addition to compliance with local fire and building codes. Dryness is an important issue in such facilities as any item that gets wet due to any reason is considered contaminated according to CDC.
Next, Let’s have a look at different types of Sterilization Processes.
Sterilization Methods
Autoclave Sterilization
Steam or autoclave sterilization is the most viable and cheapest option available in medical facilities. The main reason is the versatility of the process. No living thing can survive direct exposure to saturated steam at 250 °F (120 °C) longer than 15 minutes. Since most of the surgical tools are made of surgical grade metals like corrosion-resistant chrome steel, steam is the cheapest and safest option to sterilize them.
The autoclave applies saturated steam with high temperature and pressure to destroy all micro-organisms on the surface of the tools. One thing you have to keep in mind is that this is the most commonly used method if the medical device is not made of heat-sensitive material.
The biggest con of Autoclave sterilization is steam penetrations. Complicated instruments or ones with lapping surfaces can block the steam from entering. This may cause some micro-organisms to survive in the crevices of the device. That’s why scissors or holders are always sterilized in an open position.
You also need a compatible packing that does not block steam. Cotton and paper wraps are popular in autoclaves. It must be ensured that every surface of the tool is exposed to steam.
There are some steam penetration markers that can be used with packings. They are mostly chemical reagent inks that change color indicating the penetration of steam.
The temperature at which sterilization occurs depends greatly based on the saturation or amount of water present in the steam. Therefore, autoclaves have very precise control of temperature, pressure, and time.
The sterilization cycle time is measured from the moment tools in the autoclave reach the desired temperature. These parameters are controlled within ±2 °C and ±10 kPa (±0.1 atm) temperature and pressure respectively. There are also chart-based recorders attached that keep a time-temperature record.
Coincidently, the same process can be replicated in a pressure cooker at home minus the strict sterilization environment.
So, if a tool can tolerate moisture and heat, then this is the best method to sterilize it. However, if there is a risk of damage from water then we employ Dry heat Sterilization.
Dry Heat Sterilization
Dry heat sterilization is heating without the presence of any steam. There may be two causes to use dry heat sterilization over steam sterilization. One, heat can destroy your tools and two, steam cannot penetrate your equipment fully. If either of these situations exists then you risk jeopardizing your autoclave sterilization process.
The next best feasible step in such a scenario is using dry heat sterilization. This process is similar to the autoclave in that heat is used to kill pathogens and micro-organisms. The difference is that steam is highly penetrative and to compensate, a higher temperature and exposure time are needed.
Instead of steam, heated air is used to penetrate instruments.
The air is usually heated to around 340 °F【171°C】 and airflow is arranged in such a way that it can penetrate the deepest parts of the tool. Most modern dry-heat sterilization ovens are fitted with forced air systems to ensure the proper distribution of heat.
Dry heat sterilization is used to sterilize glassware, powders, oils, and other oil-based injectables. The process, however, has some detrimental effects on materials that are not stable. The material should also not oxidize or vaporize at this temperature. If a surgical tool manufacturer uses a lubricant or protective coating that is not up to standards, then there is a risk of it vaporizing. Vaporized oils and lubricants leave behind a burnt film that soils and destroy the instrument.
A special type of dry-heat oven worth mentioning is glass bead sterilizers. Glass has a very high melting (1400 °C to 1600 °C). Solids are quicker to transfer and distribute heat than gases. Using both in conjunction can help get quicker sterilization. Glass bead sterilizers have a vessel filled with glass beads which are then heated to around (500 °F) [260°C].
Veterinary surgeons often use it to quickly sterilize tips during operative procedures. This type of sterilizer only needs about ten to fifteen seconds to re-sterilize instruments after initial decontamination. Thus providing a quicker solution for sterilization.
There are chances of mistakes due to the quickness of the procedure in glass bead sterilizers. For that reason, it is mostly used to re-sterilize only.
Sterilization through Radiation
Both ionizing and non-ionizing radiations kill micro-organisms. But non-ionizing radiation does not have much penetrative power. Thus it can only be used to sterilize surfaces. Ionizing radiation is an excellent medium to kill micro-organisms because it has a greater penetration power. It can not only eliminate germs on the surface but also eliminate micro-organisms in highly porous surfaces.
Gamma Radiation is most common in sterilization. It is short-length high-energy radiation that directly reacts with the chromosomal matter. The damaged genetic media ultimately results in the death of pathogenic organisms. Radiation also ensures high sterility assurance level (SAL) as compared to other procedures.
The biggest benefit of the process is that it can sterilize the items in their final packing without any issues. Gamma radiations easily penetrate such packing making the process a lot more streamlined.
There are no temperature cycles and radiation does not leave behind any residue.
In a properly controlled system, there is very little risk of any contamination and personal injury. Additionally, there are no toxic and dangerous chemicals involved.
It also has the ability to sterilize media in any kind of phase (gas, liquid and solid) making it an ideal procedure for several types of equipment.
The only con is its usage. As stated, radiation is also damaging to human beings and other larger organisms. Therefore, special cells and areas are needed to isolate the radiation chamber. This increases the initial cost to buy the required equipment. Being radioactive, medical centers also need governmental approval that further adds to the cost of the procedure.
Furthermore, radiation has the potential to ionize the surrounding material which requires controlling radiation to precise value. Dosimeters are used to ensure that only required dosages of radiation are used.
There is also a risk of personnel exposure during the operation of such sterilizers. Any malfunction can result in dangerous complications. So, humans using such sterilizers are at a greater risk than other procedures.
Nuclear waste, although minimal, is produced during the life of the sterilizer. It requires careful handling and most small medical centers lack the resources to deal with such items.
Although there is minimal degradation to surgical equipment, radiation reacts with common plastics reducing their overall life.
Chemical Sterilization
There are certain chemicals that can eliminate nearly all the micro-organisms depending on surfaces they are used.
They are classified into several categories depending on their effectiveness. Physicians refer to them from high-level sterilizing agents to low-level sterilizing agents.
Chemical sterilization agents can be divided into a lot of groups.
The first one is from the aldehyde group. The three most famous chemicals used in this group are formaldehyde, paraformaldehyde, and glutaraldehyde. This group is an excellent sterilization agent but often requires long exposure times and is toxic to living beings.
Formaldehyde is a suspected carcinogen. Glutaraldehyde is toxic to human beings and Paraformaldehyde produces formaldehyde gas during usage.
Aldehyde group is used in a controlled environment with appropriate ventilation with chemical denaturing agents.
The second group is halogen-based sterilizing agents. They mainly include chlorine and iodine chemicals.
Chlorine-based compounds are excellent biocidal activity but they lose their effectiveness quickly when exposed to air. The most commonly available is liquid bleach. It is a good sterilization solution but should be used fresh.
Another problem with them is they corrode metals, rubbers, and minerals. Furthermore, if a tool is treated with bleach, it must be cleaned thoroughly first before being sterilized in an autoclave. Otherwise, it will rust rapidly.
Iodine-based solutions are widely available and cheap but they are less effective than other chemicals. As disinfectants, they have been widely used as Povidone-Iodine, tincture of iodine, etc.
The third is group phenolic compounds. They are also widely available commercially but they suffer from the same problem as iodine-based disinfectants.
They have their own pros and cons. The biggest problem with chemical-based sterilization is exposure to toxic chemicals and their disposal. Both of which can lead to personnel and environmental damage. That is the reason that they are not used as frequently.
Even FDA recommends that liquid chemical sterilizing agents are only used with critical devices that are heat-sensitive and incompatible with other sterilization methods.
Plasma Gas Sterilizers
It is a relatively new sterilization method that uses hydrogen peroxide gas. The plasma is then created by exciting the molecules of gas with the help of radio frequency or microwaves.
The process can be separated into vaporization and plasma phases. Hydrogen peroxide is itself a good disinfectant, and when combined with free radicals produced in the plasma phase it makes a good and reliable sterilization medium.
The by-products of the process are mainly water vapor and oxygen. So, no toxic waste or gas is produced. After the removal of excess hydrogen peroxide gas, there is even no need for aeration.
Another benefit is that operating temperature is low (37 – 44 °C) and cycle duration is short (50-75 minutes).
The process is used for medical devices that cannot tolerate high temperatures and moisture. Since there is no major deterioration of tools during the process, this process is compatible with more than 95% of them.
Vaporized Hydrogen Peroxide (VHP) Sterilizers
Vaporization of Hydrogen Peroxide is the first phase of Plasma Gas Sterilization. And it should be treated as such. VHP sterilizers remove humidity from within an enclosure and a generator rapidly injects hydrogen peroxide to sterilize instruments at a specific concentration.
This eliminates any humidity in the environment, eliminating the risk for any kind of corrosion during the process. The process as a whole is low temperature making it viable for heat-sensitive devices.
Another benefit is when the process is complete, the generator breaks down the VHP into water and oxygen. This makes the whole process a lot more efficient with no substances or gases to treat afterward.
Furthermore, the cycle time of the sterilizer is low which allows it to process a large number of tools in a short time.
The only drawback here is that it may not be as good at sterilization as its plasma variant. Which can be enhanced by changing a few parameters.
Gas Sterilizations
Vaporized Hydrogen Peroxide sterilization can also be called gas sterilization. But there are other gases that we can use to sterilize surgical tools.
Using ethylene oxide gas (ETO) for sterilization is more commonly referred to as Gas Sterilization. ETO in itself is flammable and can cause explosions.
Some Parameters of the whole process:
Each of these parameters is relative and thus others can be manipulated by changing one.
Two types of ETO mixtures are commonly used. ETO-CFC (ETO-HCFC now) mixture and ETO-CO2 mixture. Later one of which is less expensive but requires higher pressure.
ETO cycle consists of six phases namely preconditioning & humidification, gas introduction, exposure time, gas evacuation, air washing, and aeration.
This is a widely used method, especially for heat and moisture-sensitive devices. This procedure has a relatively high cycle time along with greater cost. The biggest drawback is its potential hazard.
Even minor exposure to ETO can lead to irritation in the skin, eyes, gastrointestinal tracts, respiratory tracts, and issues in Central Nervous System. Ethylene Oxide gas is also known as a human carcinogen.
Due to these side effects, the ETO sterilized devices have to be aerated which adds to the exposure risk and cycle time. However, most modern ETO sterilizers combine the sterilization and aeration chamber into one which eliminates this possibility. There are specific ANSI and AAMI guidelines for the usage of ETO in medical facilities.
Conclusion
There are many other sterilization methods but steam sterilization, dry heat sterilization, radiation sterilization and chemical sterilization are the most common methods employed widely in the medical industry. This is nowhere an exhaustive explanation of the procedures but a brief view from the point of a manufacturer of surgical tools. It sheds light on the benefits and issues in the usage of the process.
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