An Update on Sterilization and Disinfection of Endodontic Instruments

Abstract

Infection control is a term used to describe the precautions, policies, and procedures undertaken in a healthcare setting in order to minimize the risk of spreading an infection. It helps prevent the transmission of disease-producing agents such as bacteria, viruses, and fungi from one patient to another, from the dental practitioner and dental staff to patients, and from patients to dental practitioner or other dental staff. Among the many aspects of infection control, sterilization and disinfection of instruments and associated appliances play a crucial role for any infection control regimen to be successful in a health-care setting. Sterilization and disinfection of instruments can be described in general as decontamination procedures for treating instruments, enabling their safe usage. Failure to adhere to recommended sterilization guidelines and protocols can lead to serious cross-infection causing a disease which could be far more incapacitating than what the patient was originally suffering from. It also increases the chances of persistence of the infection. In either case, treatment outcomes are drastically reduced, rendering the entire treatment unfruitful. Sterilization and disinfection of instruments are of particular significance in the field of endodontics due to the intrinsic nature of the various procedures involved as part of the field and the type of instruments that are used. Routine decontamination procedures are usually not effective due to the complex, intricate, and minute design of various endodontic instruments. In addition, it is very important to take into account the effect of various sterilization protocols on the material of the instrument. This review article aims to describe the various sterilization and disinfection procedures for endodontic instruments in a concise manner, taking into consideration the potential effects that the sterilization/disinfection procedure might have on the instrument to be sterilized. Hopefully, it will make it easier for clinicians to choose the most suited/appropriate sterilization and disinfection procedure for any given endodontic instrument.

INTRODUCTION

Disinfection and sterilization are both decontamination processes crucial for any health-care facility and form one of the most important aspects of infection control.

Failure to adequately sterilize and disinfect dental operative instruments can lead to extensive cross-infection and at times can prove to be extremely fatal to the mankind.[¹²]

Sterilization is of great significance especially in the field of endodontics due to the complexity of design that various instruments possess and the intrinsic nature of the endodontic procedures.[³⁴]

Hence, it is very important for every endodontist to have a thorough knowledge of the concepts of sterilization and disinfection. This would not only minimize the chances of cross-infection but will also drastically improve the outcome of any endodontic treatment.

STERILIZATION AND DISINFECTION ARE DECONTAMINATION PROCESSES WHICH CAN BE EXPLAINED AS FOLLOWING:

Sterilization

A process of complete elimination or destruction of all forms of microbial life including both vegetative and spore forms. Technically, there is a reduction of ≥106 log colony-forming units (CFU) of the most resistant spores achieved at the half-time of a regular cycle.[⁵]

• Chemical sterilization: Chemicals are used for a longer duration (3–12 h) to destroy all forms of microbes, e.g., peracetic acid (0.2%), glutaraldehyde (≥2.4%), and hydrogen peroxide (7.5%)
• Physical sterilization: Physical procedures autoclaving, chemiclaving, and dry heat sterilization.

Disinfection

Involves complete elimination of vegetative forms of microorganisms except the bacterial spores from inanimate objects. Technically, there is a reduction of ≥103 log CFU of microorganisms by this method without spores. It can be further classified as:

High-level disinfectant

Used for shorter duration and able to kill 106 log microorganisms except spores, e.g., glutaraldehyde (≥2.0%), hydrogen peroxide (7.5%), and hypochlorite (650–675 ppm).

Intermediate level disinfectant

Mainly used for noncritical items contaminated with blood/body fluids. e.g., alcohols, chlorine based agents etc.

Low-level disinfectant

Used to remove the vegetative form of bacteria, few fungi, and some enveloped viruses from the noncritical items, e.g., 3% hydrogen peroxide, quaternary ammonium compound, diluted glutaraldehyde, and phenols.

Decontamination and cleaning

Decontamination is the process of removal of pathogenic microorganisms from objects, so that they are safe to handle. Cleaning is defined as removal of visible soil (e.g., organic and inorganic materials) from the surfaces and objects. Technically, it achieves minimum reduction of ≥1 log CFU of microorganisms.

Patient-care items (dental instruments and equipment) can be categorized depending on the potential risk for infection associated with their intended use as proposed in Spaulding's Classification of Surfaces:[⁵⁶]

Critical items

They are used to penetrate soft tissue or bone. They have the greatest risk of transmitting infection. Hence, they should always be sterilized using heat or disposable instruments should be chosen, e.g., pressure heating (autoclaving), dry heat, or heat/chemical vapor. These include endodontic files and probes, matrix bands, forceps, scalpels, bone chisels, and surgical burs.

Semicritical items

These come in contact with mucous membranes or skin which may or may not be intact. Hence have a lower risk of transmission. If a semicritical item is heat-sensitive, it should be replaced with a heat-tolerant or disposable alternative. High-level disinfection should be chosen as the last alternative.

These include mouth mirrors, condensers, ultrasonic tip, impression tray, radiographic sensor etc.

Noncritical items

They come in contact only with intact skin and hence pose the least risk of cross-infection. In the majority of cases, cleaning followed by disinfection with an Food and Drug Administration-registered hospital disinfectant is adequate. Protecting these surfaces with disposable barriers is advised. e.g.-Apex locator, curing lamps, radiograph head etc.

We must remember that it is important to remove the visible debris (saliva/blood/hard or soft tissue) adhering to the object by manual scrubbing before appropriate disinfection, as debris can shield the microbes and render any further disinfection/sterilization procedures ineffective [Figure 1].[⁶⁷]

Figure 1: gives a general outline for the different steps involved in sterilization and disinfection of any object^[7]

Preferred method of sterilization/disinfection

Burs

Although most burs can be reused, it is advisable to use them as single use objects whenever possible as it can get difficult for steam to penetrate into the irregularities present in the bur and at the same time the chances for corrosion increase. As corrosion occurs, it helps in harboring microbes.

The following are some of the commonly used burs in endodontics:[⁸]

Stainless steel burs

They become easily blunt and corrode after multiple uses and hence must be discarded into the sharps waste sooner. This is commonly seen after the third or fourth cycle of use, particularly when a corrosion inhibitor is not used during reprocessing. They are also more difficult to clean as compared to other types of burs. Hence, it is recommended to use lower cost stainless steel burs as single-use items.

Diamond burs

They are designed for reprocessing. However, the resin carrier for the diamond does degrade with multiple sterilization cycles. Some brands employ chrome cobalt alloy as a matrix for the diamond particles, which is long lasting.

Surgical burs

Are commonly used for dentoalveolar surgery and are designed for reprocessing. They are made of materials such as tungsten carbide that minimally degrades under steam sterilization. Likewise, silicon nitride burs are also designed for reprocessing.

The following sterilization/disinfection methods were compared for their efficacy in different articles-manual scrubbing, hot air oven, glass bead sterilization, ultrasonic cleaner, autoclave, chemiclave and various chemical disinfectants.[⁹¹⁰¹¹]

From almost all the articles, it is evident that steam sterilization (autoclaving) continues to be more efficacious than other methods evaluated, followed closely by dry heat sterilization. The reason for this mainly is that moist heat has better penetrative potential as compared to dry heat. However, steam sterilization did not lead to complete sterility of the burs in any of the articles. From most of the articles it was concluded that steam sterilization (autoclaving) was successful in reducing bacterial threshold only by 75% to 85%. This strongly indicates that whenever possible, burs should be disposed after a single use.[⁹¹⁰¹¹]

It is important to note that presoaking of burs in soapy water/disinfectant is crucial to loosen the debris that is adhering on the burs. Ultrasonic systems can also be used for enhanced debridement.[⁴]

A study evaluated the following sterilization methods on the cutting efficacy and durability of burs-dry heat, autoclave, microwave irradiation, and glutaraldehyde. From this study, it was evident that dry heat sterilization had the least deteriorating effect on the burs. Autoclaving and chemical sterilization had corrosive effects on the bur.[¹²]

An alternative to keep the burs protected from deteriorating during autoclaving is to submerge them in 2% sodium nitrite solution.[¹³¹⁴]

Endodontic files

Hand operated endodontic files

Most hand files are typically labeled as single-use items. Sterilizing them is both ineffective and unsafe as it can result into sharps injury. Protocols with proven effectiveness for cleaning rotary nickel titanium files have limited effectiveness in removing bioburden from hand files made from either stainless steel or nickel titanium alloy. Besides this the complex structure of endodontic files with their microscopic crevices left behind by the manufacturing process retains more debris to which prions adhere becoming impervious to most of the decontamination and sterilization methods. Prions are highly heat resistant and have been known to cause a variety of neurodegenerative disorders. The WHO guidelines suggest the following procedures for inactivating prions: Immersion in sodium hypochlorite (20,000 ppm of available chlorine) for 1 h and heating with 1 M sodium hydroxide for 1 h, or autoclaving under vacuum at 121°C for 30–90 min in the presence of sodium hydroxide. These procedures are inappropriate to be used on endodontic files as they can cause corrosion and deterioration of mechanical properties of the instruments. Besides this, the timing is not suitable for outpatient activity. Hence, it is recommended that whenever possible, hand files should not be reused.[^15161718]

The Department of Health of the United Kingdom recommended in 2007 that either endodontic files be considered as disposable instruments or that the most effective sterilization method available be employed.[¹⁹]

Although first preference continues to be to use hand files as single use items, various studies compared different sterilization and disinfection protocols to sterilize hand files. Through these, it can be concluded that appropriate disinfection followed by autoclaving continues to be the most efficacious method compared to the rest of the methods (hot air oven, glass bead sterilization, ultrasonic cleaner, autoclave, chemiclave and various chemical disinfectants) tested.[^20212223]

From studies, it is also evident that although glass bead/salt sterilizers have been intended for this purpose and require less time, they do not effectively sterilize files as they are based on dry heat sterilization which has poor penetration potential. Furthermore, the handle portion of files continues to be unsterilized.[^20212223]

Further using an endodontic box instead of plastic packaging for autoclaving gave marginally better results.[²⁴]

A study indicates that enzymatic disinfection (immersing the files in an enzymatic disinfecting solution) in an ultrasonic bath before autoclaving was highly efficacious. Enzymatic solutions contain various proteinases and collagenases which help break down pulpal tissue adherent to files.[¹⁵]

Furthermore, mechanical debridement (scrubbing) with appropriate brush and soap water prior to immersing in a disinfecting solution and subsequent autoclaving is crucial to eliminate the bulk of the pulpal remnants.[^15202122]

Rotary NiTi files

When using rotary files, it is not economical to discard the files after a single use. Besides this, sterilizing Niti files is more efficacious than sterilizing stainless steel files due to the difference in their surface characteristics.[⁴²⁰]

However, physical and mechanical properties of Niti instruments recover only partially following autoclaving. Hence, it is recommended that they should not be used for more than five cycles.[²⁵]

When NiTi endodontic rotary files are reprocessed, the presterilizing cleaning process must be validated as being effective. A verifiable process has been described below:[⁴]

1. Immediately after use remove stoppers and insert the files into a scouring sponge soaked with chlorhexidine gluconate aqueous solution
2. Clean the files by using 10 vigorous in-and-out strokes in the sponge
3. Place the files in a wire mesh basket and immerse in a suitable enzymatic cleaning solution for 30 min
4. Follow this by 15 min ultrasonification in the enzymatic cleaning solution;
5. Drain and rinse in running water for 20s
6. Proceed to steam sterilization.

Adverse effects of sterilization/disinfection on Niti files can be summarized as follows:

1. Disinfecting agents such as sodium hypochlorite and autoclaving are known to increase surface roughness of the metal and initiate micropitting type of corrosion.[²⁶²⁷]
2. A reduction in the cutting efficacy of NiTi files occurs in alloys which are not treated during the hot making process (twisted file with the M-wire alloy) has been noticed. The researchers have predicted that the corrosive effect after autoclaving produced a reduction in the cutting capacity of 20% of the instruments tested after seven autoclave cycles. Data from another study reported that there is a 1%–12% reduction in the cutting capacity after 5–10 autoclave cycles.[²⁸²⁹]
3. Partial recovery of cyclic fatigue suffered by NiTi instruments (majority but not all studies) in an autoclave – recently, manufacturers of endodontic instruments have made nickel-titanium alloys which through a thermomechanical production process, produce a superelastic NiTi alloy that is capable of maintaining a stable martensitic phase during clinical use. These instruments have the ability to undergo an additional thermal treatment during the sterilization phase which increases their flexibility.[^30313233]
4. Varying results have been obtained while studying the influence of autoclaving on torsional stress (resistance) with some suggesting an improvement with autoclaving while others suggesting that it may have no effect or is detrimental.[^34353637]

Hence, taking into consideration the abovementioned effects, we can clearly see that although rotary endodontic instruments are reusable, their reuse should be limited to only a few cycles of autoclave. This will help in maintaining adequate clinical efficacy and prevent chances of instrument separation.

Ultrasonic tips

Although the sterilization protocol may differ based on the manufacturing company, most of the ultrasonic tips are autoclavable.[¹⁴]

Endodontic post

The post can be wiped with alcohol and then allowed to air dry before use.[¹⁴]

However there seems to be a lack of evidence about the effect of contaminated dental post on success of endodontic treatment. Furthermore, as such there is no verifiable process for the disinfection and sterilization of all types of dental posts.[³⁸]

Gutta-percha cones and resin points (root filling materials)

The synthesis of the cones occurs under aseptic conditions, but from various studies it has been noticed that they get colonized by bacteria subsequently. Although gutta-percha cones contain a certain amount of zinc which can partly inhibit the growth of microorganisms, the proliferative action of bacteria occurs anyways.[³⁹⁴⁰]

Hence, from these studies, it is evident that they require disinfection prior to use even if they are aseptically removed from a new packet.

Sterilization by heat would alter the cones, so autoclaving is not suggested. Following are some of the methods that have been tested and compared:[⁴⁰]

1. Sodium hypochlorite in concentrations of 0.5%–5%[⁴¹]
2. Chlorhexidine, 2%[⁴²]
3. Glutaraldehyde[⁴³]
4. Alcohol[⁴⁴]
5. Hydrogen peroxide, 3%[⁴⁵]
6. MTAD[⁴⁶]
7. Rosmarinus officinalis extract[⁴⁷]
8. Quaternary ammonium[⁴⁸]

Among all the above methods, sodium hypochlorite immersion with concentrations ranging from 2% to 5.25% for a minimum time of 5–10 min, continues to be the most commonly followed method. This can be attributed to the fact that sodium hypochlorite is extremely efficacious, easily available in the dental office, inexpensive and has a quick action. Alternatively, chlorhexidine can be used in a concentration equal to 2% for a shorter period (1–2 min).[^40414243]

Other agents which demonstrated disinfecting capacity similar to sodium hypochlorite and chlorhexidine were hydrogen peroxide, glutaraldehyde and quaternary ammonium compounds. However, it was observed that all these disinfectants required prolonged immersion time of at least 10 min to achieve comparable results.[^40434548]

Rosmarinus officinalis extract derived from the plant rosemary with its bactericidal and a fungicidal properties has also showed great promise. It contains carnosic acid and carnosol, which may disturb the bacterial cell membrane.[⁴⁷]

The physical alterations caused on the cones of gutta-percha and resin by the various disinfectants can be summarized as follows:[⁴⁰⁴⁷⁴⁹]

1. Alterations of the surface due to the action of hypochlorite, MTAD and chlorhexidine
2. The formation of cuboidal crystals on the surface due to precipitation of hypochlorite
3. A reduction of tensile strength following prolonged exposure to sodium hypochlorite.

Hence to minimize the above mentioned effects, it is recommended that prolonged immersion times should avoided and the disinfectants should be completely rinsed off. Rinsing with distilled water has been strongly recommended after disinfection procedures.[⁴⁷]

Absorbent paper points

Studies indicate that absorbent paper points just like gutta-percha cones can be contaminated with bacteria. This indicates that it is important for us to sterilize them prior to use.[⁵⁰⁵¹]

A study also indicates that absorbent paper points maintain their sterility only for a maximum duration of 1 week and hence require resterilization after that.[⁵¹]

Both dry and wet heat can be used for sterilization of paper points without significantly affecting their absorption capacity.[⁵²]

Hand piece

All dental hand pieces must be cleaned and lubricated in accordance with the manufacturer's instructions and must be sterilized after each patient. Debate continues about the most effective decontamination method for hand piece. Some of the methods compared in past are dry heat, moist heat and ethylene oxide gas. Among these, according to most of the studies, moist heat and ethylene oxide gas have shown the best results. This can be attributed to their excellent penetration capacity into the intricate parts of the dental hand piece. However, ethylene oxide gas is preferred over moist heat as it does not cause corrosion.[⁵³⁵⁴]

The following steps have been recommended for sterilizing hand piece:[⁵⁵]

1. Clean the outside of the handpiece with detergent and water–never clean or immerse the handpiece in disinfectant solutions or the ultrasonic cleaner
2. Lubricate the handpiece with pressurized oil for the recommended period
3. Clean off excess oil
4. Sterilize in a steam sterilizer or by using other suitable sterilization method
5. Run the handpiece briefly before use to clear excess lubricant that might be there.

Curing lamp/light

Curing light tips are regarded as semicritical pieces of equipment. The curing tips should be either heat sterilized or have an appropriate barrier placed over the tip for each patient. However, it is not possible to heat sterilize all curing tips and hence high level disinfectants can be used, provided a barrier was placed.[⁵⁵]

Dental radiology

It is recommended to use heat-tolerant or disposable intraoral radiograph devices (unless digital radiography is utilized) wherever possible and semicritical items (e.g. film-holding and positioning devices) must be either heat sterilized or barrier protected before use on subsequent patients. Use of protective barriers should always be done where possible on the developing equipment and when surfaces become contaminated, they must be cleaned using an appropriate disinfectant.[⁵⁶]

Specialized intraoral equipment and devices

Commonly used ones endodontics include air abrasion unit, intraoral cameras, lasers, apex locators, endomotors, occlusal analyzers, and electrosurgery units. Some of them come in direct contact with the oral mucosa and hence are considered as semicritical items. Most of them are heat sensitive. Hence, high level disinfection in conjunction with disposable barriers should be carried out. It is advisable to first read the manufacturer's instructions pertaining to their disinfection as some may not be tolerant to high level disinfectants, e.g.-lenses of intraoral cameras.[⁵⁵]

Single-use items

These are items which need to be discarded after they have been used once a patient. Whenever possible, they should be used as they reduce the chances of cross-contamination. These include disposable triple-syringe tips, plastic evacuator tips, prophylaxis cups, microbrushes, plastic dappen dishes, disposable impression trays, local anesthetic needles, cartridges, sutures, and scalpel blades. For common simple triple-syringe designs, disposable ones are preferred due to difficulty of cleaning and challenges in removal of air for steam penetration during autoclaving.[⁵⁷]

Very small or sharp instruments with minute complex designs are difficult to clean and should be considered single use. This is very relevant to matrix bands, wedges, reamers, and broaches. There is currently no validated cleaning method for such items.[⁵⁷]

To conclude, the method chosen for sterilization and disinfection largely depends on the object that is undergoing the procedure. At same time, it is also important to take into consideration, the implications that the chosen method is going to have on the object which would be undergoing the sterilization/disinfection procedure.

Financial support and sponsorship

Self-supported (Contributor 1).

Conflicts of interest

There are no conflicts of interest.