13: Materials used in endodontics

Chapter 13 Materials used in endodontics

Introduction

Endodontology is the branch of dental science concerned with the form, function, health, trauma and disease of the dental pulp. Endodontics is the practice of endodontology. Many patients will report that they have had root treatment, which is a misleading term. The correct technical term is root canal therapy (RCT), which more accurately describes the process, i.e. of cleaning and filling all of the root canals in the tooth. The term root filling is often used synonymously with root canal therapy but this only refers to the material that is used to fill the root canal system.

The objective of successful root canal therapy is to access the entire root canal system, and thoroughly remove any necrotic (dead) tissue and microorganisms by canal preparation. This preparation involves cleaning and shaping the canals to facilitate the subsequent obturation (filling) of the dead space. It is critical that a coronal seal is then gained and maintained to prevent subsequent reinfection of the root canal system by oral microorganisms. Should this seal become compromised, the treatment is likely to fail, so underlining the importance of this final stage.

This chapter discusses the materials and the instrumentation used during endodontic procedures.

Rubber Dam

Types and presentation of rubber dam

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.

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.

Endodontic Files

The aim of root canal preparation is to remove as many of the microorganisms as possible and to shape the root canal to facilitate its subsequent obturation (filling) with an inert material. Effective cleaning of the complex root canal system requires using both mechanical and chemical means. Metal files are used to remove necrotic material from the lumen of the canals and also to remove hard tissue from the canal walls. It also removes the microorganisms that have penetrated the dentinal tubules in the canal walls. This process also facilitates the penetration of disinfecting agents so they can reach the bacteria in the root canal system.

Additionally files shape the root canals to provide a uniform-sized dead space, which may be filled using standard-sized cones of plastic material. This is not easy as the root canal system is complex (unlike a chimney!) and the internal anatomy is more akin to a three-dimensional spider’s web of major and minor canals within a network. It is therefore impossible to completely prepare the whole root canal system and the obturating material must be able to be adapted to the root canal walls to achieve a seal.

File preparation of root canals is done either by hand or using rotary files in a speed-reducing, torque-controlled handpiece (Figure 13.5). Rotary systems are much more efficient than those used by hand; however, hand files may be preferred to negotiate very curved root canals as rotary instruments may not be able to get round the curve due to lack of sufficient flexibility. The file may also break (separate) during use. The sharper the curve of the canal, the greater the incidence of fracture of the file due to cyclic fatigue.

Fracture may also occur due to flexural fatigue (i.e. overuse) or torsional fatigue, that is, forces placed on the instrument in rotation while the instrument is prevented from moving. The use of a torque controller is important so that the rotation of the file is stopped prior to receiving excessive torque, which may cause fracture. For further information on speed-reducing handpieces and torque, see Chapter 19. From this brief description, it can be seen that the selection of the material of which the file is made and its method of construction is critical in determining the performance of the instrument.

Anatomy of endodontic files

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.

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.

Stainless steel files

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.

image

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.

Nickel-titanium files

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).

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.

Root Canal Disinfection

Successful outcome of the treatment is dependent on the removal of microorganisms from the root canal system. Studies have shown that root canals free of infection at the time of obturation have a higher success rate, whereas residual bacteria retained in the canal at the time of obturation leads to a higher risk of failure. The dentist has a number of medicaments available to achieve this objective. Irrigants are used during root canal preparation. Other novel antibacterial systems that are now available include ozone and bacterial photo-dynamic therapy, which may also be used at this stage. Root canals may be dressed with antibacterial inter-visit intracanal medications if the procedure is to be carried out over more than one appointment.

Endodontic irrigants

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:

Irrigants for cleaning

Irrigants for disinfection

Many disinfecting agents have been used for endodontic irrigation over the years. These include halogenated compounds such as sodium hypochlorite, iodine potassium iodide, chlorhexidine and a number of potent phenolic disinfecting agents. There is a significant problem with many disinfectants where the concentrations producing effective bactericidal activity are close to those concentrations where tissue toxicity has been reported.

Sodium hypochlorite solution

Of the disinfectants now commonly used, sodium hypochlorite (bleach) solution is preferred by the majority of endodontists. It is a potent organic tissue solvent that is proteolytic and dissolves necrotic organic material. It releases free chlorine, a powerful disinfecting agent which has a wide spectrum of bactericidal effects, so disinfecting the area of application. It is the chlorine that breaks down tissue and proteins into amino acids. These amino acids are then degraded by hydrolysis through the production of chloramine molecules. This is an oxidation reaction with the bleach. The bleach pH can be in excess of 11 and it has no effect on the calcium deposits in the smear layer.

The therapeutic and toxic concentrations of sodium hypochlorite are undesirably close together. There is no difference in the antibacterial effect between 0.5% and 5% solutions but the efficiency of weak solutions decreases rapidly. A solution with concentration greater than 1% is required for pulpal tissue dissolution to occur. However, at higher concentrations, although the disinfecting process is faster, it is more likely that untoward tissue damage will occur as the chemical is more toxic at these concentrations. This is especially the case if the chemical is inadvertently extruded outwith the root canal system.

A further problem with sodium hypochlorite is that it has a higher surface tension than water. It does not wet the root canal walls as well as some other disinfecting solutions. This results in the canal walls being incompletely covered, and consequently the biofilm layer may not be disrupted effectively. This is more likely at higher concentrations as the solution is thicker.

Sodium hypochlorite is not totally effective at killing all the microorganisms found in the root canal system. Pathogens such as Enterococcus faecalis have been isolated from root canals which have been previously treated with this solution.

Care should be taken with all materials used intraorally as the material safety data sheet (MSDS) has instructions on hazards, including those related to swallowing or inhalation. This means that the use in the oral cavity would be regarded as a hazard. In the UK, failure to carry out a risk assessment under COSHH regulations would be a potential problem if anything untoward occurred. This is a particular problem as proprietary solutions specifically for dental use are available.

Many endodontists use sodium hypochlorite solution in combination with an ultrasonic instrument as they believe that the acoustic streaming that occurs enhances the cleaning effect.

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:

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).

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%.

Other irrigants

Bacterial photo-dynamic therapy (bacterial PDT)

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.

Jan 31, 2015 | Posted by in Dental Materials | Comments Off on 13: Materials used in endodontics
Premium Wordpress Themes by UFO Themes