The battle in endodontics is against the bacterial infection. To achieve a sterile root canal system the tooth should be isolated to prevent contamination with oral bacteria. The only predictable and effective method is the use of rubber dam, which is also referred to as dental dam (see Figures 13.1B, 13.2 and 13.4 below). The benefits of using rubber dam in relation to endodontics include:

As well as being used in endodontics, rubber dam also has many applications in general operative dentistry, particularly when the dentist is using hydrophobic materials, hence the many references to it in this text.

The rubber dam system consists of a sheet of an elastic material held taut over a frame. Some products have a frame incorporated in them, e.g. OptraDam (Ivoclar Vivadent). A hole or holes are punched into the sheet corresponding to the teeth that need to be isolated. The dam is retained by the use of a clamp or clamps or stabilizing cord such as Wedjets (Hygienic) (Figure 13.1).

Fig. 13.1 (A) Rubber dam stabilizing cord, Wedjets (Hygienic). The orange cord is thicker than the yellow. (B) Cord retaining the dental dam during isolation of the upper anterior teeth.

Rubber dam is available in latex and non-latex (elastic plastomer or polyolefin) presentations (Figure 13.2 and Table 13.1). It should be noted that the words ‘rubber’ and ‘dental’ in relation to dam are used synonymously although the elastic plastomers and polyolefins are polymers.

Fig. 13.2 Three types of dental dam in common usage: latex (Hygenic), elastic plastomer (Roeko) and polyolefin (Hygenic).

Rubber dam is either presented in precut squares of material (Figure 13.2) or in rolls where the dentist cuts off the desired amount. Different sheet thicknesses are available, the thicker sheets may be more difficult to place but they have a greater tear resistance and will retract the tissues more. Products are also available in different colours. Darker colours contrast with the operative site better so decreasing eye strain. Lighter shades, however, can naturally illuminate the area due to their greater translucency. Some products are supplied flavoured or scented in an attempt to improve the patient’s experience.

When rubber dam is placed for endodontics, it is advisable to create a seal around the dam using a caulk material. This prevents irrigants reaching the mouth and equally prevents saliva from leaking back into the operating field. The caulk material used is hectorite clay, which belongs to a group of materials called smectitec clays. In these materials platelets of a metal oxide are sandwiched between plates of silicon dioxide. This layered structure allows the clays to swell in contact with water. The metallic oxide in hectorite clay is magnesium (Figure 13.3). On taking up water the clay will expand to up to 35 times its original size effectively sealing any gaps adjacent to the dam.

Fig. 13.3 Photomicrograph from atomic force microscopy showing the platelet-type layered structure of hectorite.

(Photo courtesy of Elementist.)

An example of such a product on the market is OraSeal (Ultradent). The rubber dam is applied and the sealer is syringed around the now isolated tooth (Figure 13.4).

Fig. 13.4 Latex rubber dam placed around tooth 17 and sealed using OraSeal (Ultradent).

Hand and rotary files both have a cutting edge along the length of the file and at the other end is either a handle or an attachment that fits into the handpiece, respectively (Figure 13.6). The instrument cuts when its radial lands are in contact with the canal wall, the lead angle determining its cutting efficiency. Radial land areas are required for conventional helically fluted files because they prevent the file from over-engagement in the canal (Figure 13.7). Lack of radial land areas reduces friction. If a file becomes suddenly engaged or self-threaded, it may fracture. Radial lands are especially important for files that have positive rake angles. That is, the angle of action of the cutting blade is similar to a snow plough which is forced downward towards the surface of the road. Many files are designed so that the radial lands cannot screw themselves into the canal wall. Thus, the dentinal debris is directed towards the coronal part of the canal so that it is not compacted apically.

Fig. 13.6 The parts of (A) hand and (B) rotary files. The silicone stop is used as a measuring device: the required canal length is measured and the silicone stop is placed at this same distance measured from the tip of the file.

Fig. 13.7 Diagrammatic representation of the microscopic structure of an endodontic file in cross-section showing the radial lands.

There are many different types of file available. For the clinician’s convenience and from a quality-control perspective, files are standardized according to their physical properties and dimensions (i.e. diameter and taper). To indicate the increase from one instrument size to the next, these are often numbered and colour coded. Files are available in a range of different shapes, tapers, lengths, cutting or non-cutting tips, safe edged (only cutting on one side). The material of which they are constructed is either stainless steel or nickel-titanium.

The traditional material used in the construction of endodontic files is stainless steel. This is an alloy of iron, carbon and chromium. Nickel may also be present. Originally steel was made by forming an alloy of carbon and iron. This was known as carbon steel. This alloy was prone to rusting in the presence of water, but the addition of a percentage of chromium was seen to prevent this rusting. Normally between 13% and 26% of a stainless steel alloy is chromium. The chromium forms a passivation layer of chromium oxide on the surface when exposed to air. This thin layer is not visible but preserves the steel from rust. If scratched, the oxide layer will rapidly re-form, preventing degradation.

Endodontic instruments are manufactured by machining stainless steel wire into a blank of the desired shape (for example either square or triangular in cross-section). This is then twisted into a spiral. During the twisting the material becomes work hardened. The greater the number of twists, the greater the work hardening. The other means of producing the instruments is by directly machining of the shape from a stainless steel rod into the final shape. This machining process also work hardens the material.

Particularly with larger sizes, stainless steel files do not bend easily and in order to maintain the canal shape which is invariably a curve, the clinician should bend or precurve the file prior to its introduction to the canal. It should be remembered that bending the instrument will further work harden the metal and make it more brittle. This can lead to fracture.

Stressing

The mechanical properties of the file will depend on the composition of the material, the geometry of the file and the way in which it is loaded. Anticlockwise twisting of a file is inadvisable as this increasing the twisting of the file, which may result in brittle fracture. Files should therefore not be stressed in this way and definitely not when they are bound in the canal. Some irrigants such as sodium hypochlorite and ethylene diamine tetra-acetic acid (EDTA) can reduce the cutting ability of stainless steel files and so they should be rinsed immediately after use. The risk of this occurring has been substantially reduced as the current recommendation is that files should only be used once and discarded. However, if canal preparation is extended over a long period of time at one appointment, prolonged immersion of these instruments in these solutions in the canal will start the process of degradation.

All files should be regularly inspected for damage after their removal from a root canal. If any damage is seen then the file should be discarded and a new file used. Continued use of a stressed instrument (Figure 13.8) has a high risk of instrument separation with the fractured fragment being retained in the root canal system. If the file cannot be removed, or the canal negotiated beyond it, the success of the treatment will be compromised. Unfortunately, fracture of files can also occur without any visible signs of previous permanent deformation. In order to minimize file breakage, files should only be used in a wet canal and in accordance with the manufacturer’s instructions.

Fig. 13.8A,B A stressed file. Note the difference in light reflection from the instrument, with a matt appearance. This file should now be discarded and not reused.

Debris retained on an endodontic file after use is called swarf. It may contain neural tissue with prions, which are theoretically associated with new-variant Creutzfeldt–Jakob disease (nvCJD). Prions are proteins that cannot be destroyed by the usual methods of surgical disinfection. Endodontic files should therefore be considered as single use (to be used only on the same patient) and then discarded. The other advantage of the single use policy is that instrument separation fracture is decreased as the file is used less. Furthermore, autoclaving can decrease the cutting ability of some files and may result in metal fatigue.

Types of stainless steel file

As previously mentioned, there are many different types of file made of stainless steel, for example, Hedströem and K-Flex (Figure 13.9).

Fig. 13.9 Two examples of stainless steel endodontic files: the upper file is a Hedströem file and the lower file a K-Flex file, both size 35 02. Note the difference in the shape of their cutting parts.

An alloy which has been more recently used to produce endodontic files is nickel-titanium (NiTi). This material is extremely flexible (Figure 13.10), some 500% more than stainless steel and has been termed super-elastic. This means that it is more likely to maintain the canal shape during preparation as it can memorize its original shape. It is also less likely to straighten curved canals as occurs with stainless steel. With nickel-titanium instruments, after a finite number of rotations cyclic fatigue leads to failure. However, they are three times stronger and have a superior resistance to torsional fracture compared with the equivalent stainless steel file. They are also corrosion resistant. It is the alloy of choice for rotary systems although hand files are also available (Figure 13.6).

Fig. 13.10 A nickel-titanium hand file being gently curved to demonstrate its flexibility.

When the alloy is stressed during use, its structure changes from an austenitic crystalline structure to a martensitic crystalline structure. This happens progressively and as a result the stress on the file reduces even though the strain may be increased. The modulus of elasticity is much higher for an austenitic structure than for the martensitic form, meaning that the latter state is more brittle than the former. When the stress on the alloy decreases, spring back occurs without permanent deformation and the alloy returns to the austenitic phase. This allows an 8% strain deformation with complete recovery compared with less than 1% with stainless steel.

The nature of this alloy dictates that to make a file, the desired shape must be machined out of the blank rather than just twisting a blank before grinding the cutting pattern into it. The production costs are thus increased as this process is more complicated than merely twisting the blank as with stainless steel instruments.

There are so many files and file systems available to the endodontist these days that to list even a small selection would be prohibitive from a space perspective! Many systems have various nuances which may appeal to various operators. Practitioners are therefore advised to select the file system which they find most effective and are comfortable using. They should use the instruments within the principles of their recommended usage and should consult the manufacturer’s instructions prior to clinical use. Comprehensive lists of files and file systems are available from companies manufacturing or distributing them.

It is imperative that root canals are not prepared dry. Fluids introduced into the root canal system have a number of purposes. They should:

The irrigants available can be divided into two groups based on their mode of action:

Biofilm and smear layer

Bacteria colonize not only on the necrotic material in the lumen of the root canal but also infect its walls. Microorganisms can penetrate into the dentinal tubules as well as forming complex, highly developed ecologies on the surface of the canal wall. This has been termed a biofilm. In order to kill these organisms the biofilm must be disrupted and penetrated. Furthermore, during instrumentation organic material is left behind on the dentinal surface, termed the smear layer, which must also be removed to allow the disinfectant to gain access to lateral canals (Figure 13.11). If this is achieved, then the disinfection process will be enhanced.

Fig. 13.11 Dentine surface with the smear layer removed.

(From Pathways to the Pulp, 8th edition, Cohen and Burns, p. 306, Mosby.)

The removal of the smear layer (and so opening of the dentinal tubules) is achieved primarily by using chelating agents such as citric acid or EDTA. These also have some benefit in disrupting the biofilm but they are ineffective at killing bacteria. Disruption of the biofilm may be achieved, albeit slowly, by using sodium hypochlorite, which will break up proteinaceous material. The action of a chelating agent also provides a cleaner surface against which the final filling materials will adapt.

Irrigants for cleaning

Citric acid

Citric acid is a chelating agent that removes calcified tissue. It was first used in dentistry as a surface cleaner in conjunction with glass ionomer cements to prepare the tooth surface for bonding to these materials. It has been reported to successfully remove the proteinaceous material on the surface of dentine, allowing an interaction between the cement and the clean dentine surface. It is also effective as a decalcifying agent. It is used in solution at a concentration of 10–12.5%.

Bespoke citric acid-containing endodontic irrigation solution products are available such as d2d Endo citric acid or it can be obtained from a pharmacy on prescription by a registered dentist.

EDTA

EDTA may also be used as a chelating agent as an alterative to citric acid, its action being more aggressive. It is used as either a synthetic amino acid or sodium salt of EDTA and is highly effective at removing the smear layer and emulsifying soft tissue. EDTA demineralizes and softens root canal wall dentine by 20–50 μm. Although it is neither bactericidal nor bacteriostatic, EDTA-containing compounds (Figure 13.12) will eventually kill bacteria by starving them of the metallic ions needed for growth as the chemical chelates the ions. It is non-toxic and non-corrosive to instruments.

Fig. 13.12 Example of an EDTA-containing endodontic irrigation product (OdontoEDTA, Smart Dental).

Bacteriostatic: the ability of a chemical to inhibit the growth or reproduction of bacteria.

Bactericidal: the ability of a chemical to kill bacteria outright.

EDTA is normally used in solution at a concentration of 15–17%. Products available include Endo-Solution EDTA (Cerkamed Dental-Medical Company). Alternatively, in the UK, a dentist can request a pharmacist to make up the required concentration by writing a prescription.

EDTA gel

EDTA is used in liquid form for endodontic irrigation but is also available in gel form (Figure 13.13). These products are used to coat endodontic files. They act as lubricants as well as helping to remove smear when the file is used within the root canal, making canal preparation easier and faster. The increased viscosity of this presentation is better for holding debris in suspension.

Fig. 13.13 Example of a product containing EDTA in a gel form (Glyde File Prep, Dentsply Maillefer).

Some products contain carbamide or urea peroxide, which effervesce during use, resulting in an ‘elevator action’ that helps to remove debris from the root canal system, so optimizing cleaning. Effervescence is caused by release of oxygen, which occurs when the product comes into contact with sodium hypochlorite solution. The release of oxygen may also kill anaerobic bacteria.

Commercially available products

Commercially available materials are shown in Table 13.2.

Table 13.2 Some EDTA-containing gels used during root canal preparation currently available on the market

• Chelating materials should be thoroughly washed from the root canal system as their retention for any length of time will continue to soften the dentine. This effect is eventually self-limiting as the chelator is used up. For best results, these irrigants should:

 Always be used in combination with a disinfecting irrigant

 Not be used as the last irrigant.

• Chelating materials soften the dentine within the tooth and so should not be initially used to negotiate the canals as a false canal may be cut iatrogenically.

Iatrogenic event: inadvertent complications as a result of a clinical intervention.

Extracanal extrusion of endodontic irrigants

It is not uncommon for endodontic irrigating solutions to be extruded outwith the root canal system during use. Depending on the chemical involved the effects can range from insignificant to very serious. There are many documented cases in the literature where sodium hypochlorite solution has been inadvertently extruded outwith the root canal system. When sodium hypochlorite comes into contact with vital tissues it can cause severe inflammation and tissue necrosis. Severe complications such as neurological damage, facial atrophy, anaphylaxis and airway problems have also been reported.

Clinically, the problem manifests as immediate severe pain for the patient (even though local anaesthetic has been administered), rapid swelling and ecchymosis (bruising). Secondary infection and persistent pain may result subsequently. Management of this distressing problem for both patient and clinician involves:

1. Stopping the irrigation, and considering giving additional local anaesthesia

2. Irrigating with sterile water or saline

3. Closure of the root canal with no medication and dressing the tooth

4. Analgesic advice

5. Considering prescription of an antibiotic, a steroid (such as dexamethasone) and an antihistamine.

6. Considering referral to an oral and maxillofacial surgical unit for specialist care and monitoring.

To reduce the risk of extracanal extrusion of endodontic irrigants, the dentist is advised to use a side exiting endodontic irrigating needle. This should not bind tightly in the canal so that irrigating fluid may pass out in a coronal direction. The needle and syringe should be kept moving while performing slow, gentle irrigation.

Chlorhexidine

Chlorhexidine digluconate (Figure 13.14) is used by many endodontists as it has a number of beneficial properties. This chemical is a cationic bis-biguanide that is bacteriostatic at low (0.2%) and bactericidal at higher (2%) concentrations. Its mode of action is to cause cell wall decomposition, leading to the loss of cellular components. It does not, however, dissolve any organic tissue. It is active against a wide spectrum of microorganisms, with its antibacterial properties similar or greater to that of sodium hypochlorite. It is known that the bacterial flora and ecology of endodontic cases which have failed is different. In these cases, chlorhexidine is thus preferred because it may have an effect on microorganisms resistant to sodium hypochlorite. Other clinicians use it in preference to sodium hypochlorite as it is a safer alterative (see below).

Fig. 13.14 Examples of products containing chlorhexidine that can be used for endodontic irrigation: (A) Corsodyl (GlaxoSmithKline) and (B) R4 (Septodont). The former product is also very effective against periodontal pathogens and is thus widely used in the treatment of periodontal disease (see Chapter 17).

Chlorhexidine binds to hydroxyapatite on the root canal walls and has a good substantivity, that is about 12 hours after application (with some products claiming much longer times than this). It should therefore be the last irrigant to be used in the canal.

Substantivity: the length of time that a medicament remains effective after application.

If chlorhexidine is used in direct combination with sodium hypochlorite solution, an acid–base reaction between the two may occur with formation of an insoluble precipitate that can be difficult to remove. This potential problem may be circumvented by irrigation with sterile water or saline between these two chemicals.

Some chlorhexidine products have been specifically designed for endodontic irrigation. They commonly have a higher concentration of chlorhexidine (2%) and also contain a wetting agent to lower surface tension to improve its penetration into dentinal tubules and small canals. Examples include R4 (Septodont), which is 20% chlorhexidine digluconate in denatured alcohol and commonly used as a final soak, and Gluco-Chex 2.0% (Cerkamed Dental-Medical Company). Concentrations lower than this are likely to be ineffective.

Some products are supplied as a gel that can be coated onto endodontic files prior to their insertion in the root canal to lubricate their passage. Two examples are Hibiscrub (Regent Medical), which contains 4% chlorhexidine, and Gluco-Chex gel 2% (Cerkamed Dental-Medical Company) which is 2%.

Iodine potassium iodide

Iodine potassium iodide is an organic compound that releases iodine. Iodine is a potent antibacterial agent with a broad spectrum of action: it is bactericidal, fungicidal, tuberculocidal, virucidal and sporicidal. It is effective for a limited period of time, about 2 days. Its mode of action involves attacking proteins, nucleotides and fatty acids leading to cell death. It has a low toxicity but some patients are allergic to iodine so its use in these individuals is contraindicated. It is supplied as an irrigating solution containing 2% solution of iodine in 4% aqueous potassium iodide. In the UK, Videne (Adams), which is povidone iodine, can be sourced from the pharmacy.

Iodine is know to discolour teeth as it stains dentine but use of sodium hypochlorite solution early in the procedure will help negate this side effect. It can also stain clothing so care should be taken during its use to avoid contact with the patient’s or dental team’s clothing.

Hydrogen peroxide

Hydrogen peroxide degrades to form water and oxygen, produces hydroxyl free radicals, which are effective against bacteria, yeasts and viruses as they attack proteins and DNA. Unfortunately it has not been shown to reduce bacterial load in root canals significantly. Furthermore due to effervescence of the chemical, oxygen may penetrate into the periradicular tissues causing surgical emphysema. In contemporary endodontics, it is considered to be a product of the past and should not be used. It is still available as Acqua ossigenata 12 V (Septodont), which is a 3.6% hydrogen-stabilized peroxide solution.

Ozone is used in the food and environmental industries for large-scale sterilizing of water supplies. The gas is a very effective sterilizing agent but at levels close to which it has toxicological effects can damage normal tissue. Devices are now available whereby the gas can be produced and delivered down a handpiece to the operating site in the mouth (Figure 13.15). In one device the area is sealed from the surroundings. The residual gas is then drawn back to the generator and neutralized using a platinum filter. More recent devices deliver the ozone down a fine tip directly into the canal for very short exposure times. This system does not require a seal around the tooth.

Fig. 13.15 (A) Healozone (CurOzone) device for using ozone and (B) handpiece delivery.

Despite major advances in instrumentation and techniques, it is still not possible to consistently disinfect the root canal system. Accessing parts of the complex internal root canal system anatomy where bacteria are harboured can be challenging with conventional instruments and irrigants. This possibly explains why the success rate of endodontics even under ideal conditions is no better than 87%. Another reason is that currently available disinfectants such as sodium hypochlorite, chlorhexidine and calcium hydroxide are ineffective against some organisms, such as E. faecalis and Streptococcus faecalis.

It has been known for a number of years that the combination of a photo-sensitizer and a specific wavelength of light is effective against all microorganisms found in the mouth. This system is called Bacterial photo-dynamic therapy (bacterial PDT). Some clinicians are using such a system to disinfect the root canal system during root canal therapy in an attempt to eradicate all microorganisms prior to obturation.