One disadvantage of this sectional technique was that it was very difficult to retrieve the apical fragment if the endodontic procedure failed and there had to be an attempt at repeating the procedure.

Dental amalgam

Conventional dental amalgam has also been used as an orthograde root filling material with specialized amalgam carriers and pluggers designed to allow the mixed material to be carried to the apex of the root and then condensed into place. Obviously it would not be possible to remove the mercury rich layer from the surface of such a filling (see Chapter 21) but this would be within the root canal system. Again this approach was not amenable to re-treatment and was technically difficult to execute.

Medicated pastes

Medicated pastes have also been proposed as root filling materials. There have been two broad groups, those based on paraformaldehyde (for example N2, SPAD and endomethasone) and those based on iodoform. All of these materials are placed in the canal in a liquid or paste form; the iodoform pastes remain as a paste whilst the paraformaldehyde products set into a hard mass. The principle of using a medicated paste was that the antimicrobial activity of the paste would help to destroy any residual bacteria in the canal system. Whilst this may have been a benefit of both groups there were also some significant drawbacks.

The paraformaldehyde pastes set hard to produce a material that is very difficult to remove making the use of the root canal for a post difficult and retreatment of the endodontic lesion if required very complex. Furthermore if any of the material was expressed beyond the apex of the tooth it is highly toxic to tissues causing lingering pain in bone and permanent dysaesthesia (changed sensation) or anaesthesia (numbness) of nerve tissue. This could be a particular problem if the material was inadvertently expressed beyond the apex of a molar tooth into the inferior dental nerve canal.

Iodoform paste is resorbable and whilst this is a benefit if it is used during endodontic treatment of deciduous teeth it can result in an empty canal in the permanent dentition.

31.6 Contemporary materials

The contemporary approach to obturating the root canal space is to use a malleable bulk fill material in association with a thin sealant that is used to fill the spaces around the bulk fill material and to refine adaptation of the materials particularly to the walls of the prepared root canal.

Sealants

The purpose of a sealant is to fill the spaces between increments of the bulk fill material and to improve the quality of adaptation of the composite of sealant and bulk fill to the walls of the root canal to help to maintain the seal around the root filling.

The ideal properties of a sealant as suggested by Grossman et al. would be:

•tissue tolerance

•insoluble in tissue fluids

•dimensionally stable during setting

•hermetic sealing ability

•radiopaque

•bacteriostatic or bacteriocidal

•exhibit good adhesion to the canal wall when set

•easy to mix

•non-staining

•slow setting time

•easily removed if necessary (may need to be soluble in a solvent to facilitate removal)

They are essentially thin pastes that can be both introduced into the canal system and used to coat the bulk fill material. The setting time should extend over hours rather than minutes to allow for exacting clinical technique.

There have been a number of dental cements adapted for this purpose including glass ionomer, zinc oxide and eugenol, and calcium hydroxide based products. The currently available glass ionomer sealants set rather too rapidly for clinical use. They do however provide a good seal to tooth structure. The two most commonly used materials are either zinc oxide and eugenol or calcium hydroxide based. The calcium hydroxide products provide a good short-term seal but there are some concerns about their longer term solubility in sustaining this seal. Modified zinc oxide and eugenol cements are widely used and are exemplified by ‘Grossman’s formulation’ of

Powder: 42 parts zinc oxide, 27 parts stabellite resin, 15 parts bismuth subcarbonate, 15 parts barium sulphate and 1 part sodium borate Liquid: eugenol

This does have an exceptionally long setting time (up to 2 months), but has been shown to give consistent results with a good peripheral seal and produces a filling that is retrievable.

There is one resin-based product that is commercially available for use with gutta percha, which is based on the epoxy resins with a formulation of

Powder: bismuth oxide 60%, hexamethylenetramine 25%, silver 10%1 and titanium dioxide 5%

Liquid: epoxybisphenol-resin

This material provides a good seal and has a marked antimicrobial action, but there are disadvantages also in that the silver-containing formulation is associated with staining of the dentine and there is some release of formaldehyde from the material once set which is tissue toxic. The most recent versions of the product have lower levels of formaldehyde release than seen previously.

Resins and dentine bonding agents are also used as sealants with the polyester bulk fill materials. These resins are essentially chemically setting composite resin luting agents and they are used in association with a self-etching primer. The concept behind this system is to produce a bonded structure in which the resin sealant bonds to both the root dentine and to the polyester bulk fill material.

Bulk filling materials

The purpose of the bulk filling material is to provide an inert mass that can be used to fill the large defect which comprises the prepared root canal. These materials have to be malleable during the insertion phase and must be dimensionally stable. The most commonly used products are based on a modified natural rubber gutta percha. More recently a polyester resin-based material has

1 A silver-free formulation is now commercially available also to avoid the problems of staining of dentine associated with the silver-containing product.

been developed with similar handling characteristics to gutta percha.

Gutta percha: The most widely used bulk fill material is gutta percha. In its pure form gutta percha is derived from latex as an isomer of rubber known as trans-polyisoprene. It can be produced in two crystalline forms α and β. It is less elastic and harder than natural rubber. The α form is the natural state and is mainly used in thermoplastic manipulation techniques (see below) whilst the β form is produced by cooling α material slowly (at a rate of about 0.5ºC per hour). The β form is more commonly used with cold packing techniques (see below).

Gutta percha as used in dentistry comprises between 19 and 22% trans-polyisoprene, zinc oxide (between 60 and 75%) and a variety of other components including colouring agents, resins, waxes, antioxidants and metallic salts; the latter are incorporated to improve the visibility of the material on radiographs. It is presented to the dentist in either tapered cones which may or may not be ‘matched’ to the sizes of the instruments used to prepare the canal mechanically or as pellets of material to be loaded into a gun-type delivery system.

Polyester resin: Resilon® is a commercially available material which is based on a thermoplastic synthetic polyester, barium sulphate, bismuth chlorate and a bioactive glass. It is claimed that the bioactive glass releases calcium and phosphate ions from its surface on exposure to bodily fluids stimulating bone growth. This material is also available in both tapered and pelleted forms for use with either cold or thermoplastic filling techniques.

Materials for root canal repair and peri-radicular surgery

A variety of materials have been used for this purpose including dental amalgam, zinc oxide and modified zinc oxide pastes and glass ionomer cements. However until recently no material approached the ideal.

Mineral trioxide aggregate (MTA): This comprises tricalcium silicate, dicalcium silicate, tricalcium aluminate, tetracalcium aluminoferrite, calcium sulphate and bismuth oxide. It is

chemically identical to Portland cement except for the incorporation of some bismuth oxide which increases radio-opacity and modifies the setting reaction. The material is made by fusing together the constituents and grinding them to form particles of clinker. It is a strongly alkaline material which sets on exposure to water. The setting reaction involves an initial hydration phase with wetting of the particle surfaces with partial dissolution of the calcium sulphate. Crystals of hydrated calcium aluminium sulphate hydroxide (ettringite) form on the surface of the clinker particles through interaction with the tricalcium aluminate. There is then a delay in the setting reaction during which the material is plastic and can be inserted into defects for repair (it is also used as a root-end filling material during peri-radicular surgery). The final phase of setting is characterised by growth of calcium silicate hydrate crystals along with the ettringite crystals between the clinker particles such that the material forms a rigid mass. This reaction continues to require moisture to occur and progresses over a number of hours until the material reaches maturity. The setting produces a significant expansion by a mechanism similar to that in gypsum products (Section 3.4). This may improve the seal of the canals, but if excessive, could potentially result in root fracture.

In its set condition it is biocompatible (there is some evidence that it can induce cementogenesisis and hence is used during apexification procedures on immature teeth that have been subject to trauma) and can provide a good seal at the root– material interface. Owing to its alkalinity it is also antimicrobial.

31.7 Clinical handling

The complexity of canal anatomy illustrated in Fig. 31.2 gives the clinician a significant challenge when trying to clean, shape and then fill the space. Contemporary thinking involves the use of a combination of hand or mechanical instrumentation to prepare the canals along with an irrigant to remove debris and to disinfect the canal system, prior to filling the space in three dimensions with an inert material. There have been legions of techniques described for canal preparation which are beyond the scope of this text. Three-dimensional obturation is usually undertaken with either a cold packing technique or by warming the bulk fill material so using a thermoplastic approach. It

is no longer acceptable to have a single cone of gutta percha sitting in a sea of sealant.

Cold packing (or lateral condensation)

The technique used for lateral condensation is illustrated in Fig. 31.3. The root canal comes to a natural constriction close to the apex of the root. The canal is prepared using hand or rotary instru-

(A)

(B)(C)

Fig. 31.3 Cold lateral condensation is one of the classic techniques for obturation of root canals. A gutta percha ‘master cone’ is selected which matches the instrument size used to prepare the apical portion of the canal. The cone is inserted coated in sealant to length (A) and then a finger spreader is used to squash the gutta percha apically and laterally within the canal (B). An accessory cone is then inserted into the space created by spreader (C) and the spreader re-inserted. This process is repeated until the canal is completely filled in three dimensions with the combination of gutta percha and sealant. This technique can also be used with the more recently introduced Resilon material.

ments until this constriction is exaggerated to create an apical stop. A gutta percha cone (the master cone) of matching taper and size to the instrument used to create the apical stop is coated thinly in the sealant of choice and inserted to the working length in the canal. There have been techniques described to customize this master cone by softening the tip of the gutta percha point in chloroform and then inserting it to length in the canal. This allows the softened gutta percha to distort to more closely match internally the shape of the root apex. This technique is only of value if a careful note is made of the orientation of the gutta percha point in the canal during the customization process as it has to be replaced in the canal with similar orientation during final seating.

Once the master cone is fully seated in place a smooth tapered instrument (a spreader) is used to squash the gutta percha against the sides of the root canal. This creates a space in the canal into which an accessory gutta percha cone is inserted, once again coated in a thin film of sealant. This lateral condensation process continues until the canal is full of a homogenous mixture of gutta percha and sealant (Fig. 31.3).

Both Resilon and β phase gutta percha can be used with this approach.

Thermal packing methods

Gutta percha is a thermoplastic material; consequently adaptation of the material to the walls of the prepared root canal can also be achieved with thermally softened material. There are two basic technologies used to achieve this, softening of the material prior to its insertion into the canal and application of heat to the cold mass of gutta percha in the canal. Whichever technique is used sealant is still required in the canal to help to achieve the best quality seal between the obturation material and the canal walls. Obviously it is also possible to apply heat after insertion to a material that was placed in the root canal in a softened state.

Heat can be applied to the material using some form of external heating source; these can be in the form of a gun which is then used to inject softened gutta percha into the tooth (Fig. 31.4) or an oven to soften the material prior to insertion. Oven softened material would be difficult to handle on its own; this approach relies on a thermally stable carrier down the centre of the gutta

Fig. 31.4 A thermoplastic gutta percha unit designed with accurate temperature control and to give a standard rate of delivery (flow) of material from the tip of the unit into the root canal system. Illustration courtesy of J.M. Whitworth.

percha. This gutta percha on a stick is inserted into the canal once the gutta percha has been warmed to beyond its softening point. The carrier has a similar function to the spreader in the cold lateral condensation method, pressing the material against the walls of the root canal; α phase gutta percha should be used for these thermal preheating techniques and again there are commercial versions of Resilon which can be injected into the canal using a gun system.

Heat can be applied using a rotary instrument and friction. Thermomechanical compaction uses a tapered instrument which is shaped to have an action like an Archimedean screw. The instrument is inserted adjacent to gutta percha points in the canal and then is driven at relatively high speed in a clockwise direction. The action of the steel instrument causes frictional heating of the gutta percha which is then driven down into the canal and forced sideways by the action of the compactor. Obviously it is critical that this instrument is driven in a clockwise direction; anti-clockwise movement of the compactor would simply lift the gutta percha out of the canal.