Effects of mechanical and chemicalprocedures on root canal surfacesKISHOR GULABIVALA, BINA PATEL, GLYNIS EVANS & YUAN-LING NG

Root canal treatment may be performed on teeth with

irreversibly inflamed dental pulps to prevent apical

periodontitis or on teeth with apical periodontitis to

treat it. The presenting condition of the root canal

surface may therefore vary from that of an intact pulp–

dentine complex, through partially degraded pulp

tissue with infection, to a dentine surface coated with

a mature bacterial biofilm (1). Subsequent treatment

procedures will alter the surface in ways that depend

upon the root canal anatomy, the instruments used, the

strategy and mode of their use, and the chemicals used

to facilitate debridement. The effects range from

displacement and/or deformation of soft and/or hard

tissue components, to changes in the biological,

mechanical, and chemical properties of the root canal

dentine surface. These changes may have a profound

effect on the survival of the tooth, both in terms of

progression of apical periodontitis and the long-term

integrity of the tooth. An evidence-based synthesis of

the literature on the chain of events associated with the

effects of root canal treatment, on the internal dentine

surfaces, has required subjective assimilation. The mass

of published, largely laboratory data, relevant to the

topic is heterogenous and contradictory, leaving room

for conjecture, differences of opinion, and further

questions. The original questions posed in laboratory

studies were not guided by clinical outcome data and

therefore lacked relevant focus. The synthesized view

presented below is based on the authors’ interpretation

of the literature findings, sought systematically by hand

and electronic search methods.

Presenting condition of root canalsurfaces before treatment

Before the effects of treatment procedures on root

canal surfaces can be evaluated, the condition of the

presenting surfaces must be appreciated. Since root

canal treatment may be carried out on teeth with or

without apical periodontitis and with vital or necrotic

pulp tissue, a diverse range of conditions may present,

especially considering the age of the patient at

presentation.

Influence of canal anatomy

The complexity of the root canal system and the

patterns of prevalence of types of systems in different

teeth and roots are well documented in different racial

groups (2–5) and are reviewed elsewhere in this volume.

These have a dominant effect on the outcomes of

mechanical (6) and therefore chemical preparation (7).

Surface characteristics of the uninfected rootcanal surface

During elective pulpectomy on a tooth with healthy

pulp tissue, a normal pulp–dentine complex would be

encountered. Extirpation of the pulp tissue may leave

odontoblasts either remaining in the dentinal tubules

(8) or torn out. Depending upon the condition of the

pulp tissue, it may fragment or be removed largely in

one piece (Fig. 1). It is likely that the apical parts of the

pulp, which are more fibrous, and those in accessory

anatomies may remain (7, 9, 10–12), particularly in

curved canals (9, 13, 14). A dying pulp, deprived of a

blood supply, may shrink and pull away from the

dentine surface (Fig. 2). Otherwise, an uninfected,

necrotic pulp may remain behind as a dried vestige of

the vital organ.

In contrast, an inflamed pulp would lose its

organization and break down, leaving variable frag-

ments of necrotic tissue over the dentine surface. If the

pulp had been invaded by bacteria, the fragmentation

103

Endodontic Topics 2005, 10, 103–122All rights reserved

Copyright r Blackwell Munksgaard

ENDODONTIC TOPICS 20051601-1538

may be more complete, to the point where little tissue

residue is evident, clinically or at light microscopic level.

At an ultrastructural level, debris will be evident as

irregular, disorganized sludge-like material covering

and masking the openings of dentinal tubules and any

depressions in the canal wall (15) (Fig. 2). A regular

Fig. 1. Extirpated pulp and SEM view of pulp separating from dentine surface with odontoblastic processes drawing outof their tubules.

Fig. 2. SEM views showing dried pulp tissue and remnants of necrotic pulp tissue on the canal wall, in one case forming asludge-like layer.

Gulabivala et al.

104

dentine surface with patent dentinal tubules is there-

fore often taken to mean a clean dentine surface. Teeth

subjected to various microbial insults (through caries,

tooth surface loss, periodontal disease) or restorative

stimuli may demonstrate various degrees of dystrophic

calcification. Depending upon the size, shape, and

position of these relative to the canal walls, they may

obstruct access to the apical anatomy to varying

degrees. Unless formed close to the pulp–dentine

complex, such calcifications in a necrotic pulp are likely

to be loose and potentially capable of apical or coronal

translocation during treatment. From this account, it is

obvious that studies using debris scores as the outcome

measure for evaluating the effects of treatment

procedures should standardize the pre-clinical condi-

tion of the teeth used, a requirement not always

observed.

The root canal dentine surface presents with an

unmineralized front with a hardness value that is lowest

for dentine (30 kg/mm2). Elsewhere and in some areas

of the root canal dentine, particularly in older teeth, the

higher mineralization may raise hardness to 60–70 kg/

mm2 (16). Furthermore, the dentine surface is porous

owing to the patency of dentinal tubules, although they

may sometimes be sclerosed. The presence, density, and

diameter of the dentinal tubules vary with the corono-

apical site in the tooth as well as with age and insult

(17–19).

Where present, the dentinal tubules are irregular in

density and direction in the apical region of roots; while

another complication found is the embedded pulp

stone (19), other descriptions include the so-called

‘denticle’, posing yet another surface complexity on the

root canal wall (20).

Surface characteristics of the infected rootcanal surface

When teeth have infected root canals, the pattern of

bacterial invasion and associated pulp necrosis has been

revealed by microscopic surveys (light, dark-field,

transmission electron microscopy (TEM) and scanning

electron microscope (SEM)) of such sample teeth (1,

21–26). Bacteria appear to be concentrated in the

coronal part of root canals and appear in smaller

numbers as the apical foramen is reached, particularly in

teeth with closed pulp chambers and residual vital pulp

tissue apically (Fig. 3) (21). In contrast, cariously

exposed canals are evenly coated with a bacterial plaque

(Fig. 4) (24). There may be a difference in the

proportions of morphotypes present in coronal and

apical parts of root canals (27) but this has yet to be

confirmed by cultural and molecular studies (28, 29).

TEM observation of carious teeth (Fig. 5) suggests

that most of the flora in the apical 5 mm of the root

canal is suspended in an apparently moist canal lumen

(1). Less frequently, dense aggregates of morphologi-

cally uniform bacterial cells embedded in extra-cellular

matrix are observed sticking to the dentinal wall.

Sometimes, there are clusters of multi-layered bacterial

condensations containing various morphotypes. The

filamentous forms were often adherent perpendicular

to the canal wall, with coccoid forms either arranged in

strings in the same direction or adherent to the

filaments giving a corn-cob appearance (1, 24). This

was the first true depiction of biofilms in root canals; yet

its full significance for root canal treatment was not

Fig. 3. Lower and higher magnification light microcopicviews (methylene blue stain) showing dying pulp with abacterial front colonizing the dentine surface andbeginning to penetrate the dentinal tubules.

Effects of mechanical and chemical procedures

105

realized until later (30). The physiology of biofilm

development and its relevance for human disease and its

treatment have been reviewed in detail before (31, 32).

Bacteria in biofilms are regarded as more difficult to kill

than those that grow in fluid suspension as planktonic

phenotypes.

Bacterial penetration into dentine is only evident in

the presence of pulp necrosis. The predentine is easily

and commonly infected but the calcified dentine less so

(1, 21). Bacterial penetration into dentine around the

root canal is confined to the close proximity of the

canal, where the tubules end in a vital periodontal

ligament (22, 33–35). Bacteria are observed along the

entire length of the dentinal tubules only when the

tubules end in necrotic periodontal tissue (22). Bacteria

penetrating dentine appear to be dominated by Gram-

positive rods (68%) and cocci (27%). The predominant

types are Lactobacillus (30%), Streptococcus (13%), and

Propionibacterium (9%) species (35, 36). The presence

of Gram-negative bacteria in root canal dentine has

been indirectly confirmed by the detection of high

concentrations of lipopolysaccharide in the inner layers,

up to 300 mm in depth (37).

The overall picture, therefore, is one of a variable

distribution of bacteria within the root canal system and

dentine. The state at any given time may represent a

‘stage’ of a changing microflora, with bacteria extend-

ing up to and sometimes beyond the apical foramina.

The depth of penetration into dentine is variable but

generally appears to be confined within the area close to

the root canal and is probably dominated by Gram-

positive bacteria. The distribution of morphotypes also

appears to be variable.

Mechanical properties of dentine in teethwith vital and non-vital pulps

It is possible that the mechanical properties of the

pulpless tooth are different from those of a matching

vital tooth but definitive proof has been elusive.

Nevertheless, there is convincing circumstantial evi-

dence for the putative causes of fracture of non-vital

and root-treated teeth (38). The main causes may be

Fig. 4. SEM views showing a bacterial biofilm overlyingthe root canal surface from which bacterial cells appear tobe penetrating the dentinal tubules.

Fig. 5. TEM views showing the relationship between thebacterial biofilm and dentine surface, as well as thatbetween the bacterial cells in the root canal. Note thefimbrae extending from cell to cell and the characteristicorientation of the bacterial cells to the canal wall.

Gulabivala et al.

106

loss of tooth tissue, altered physical properties of

dentine, and altered response to occlusal loading. It is

likely that these factors interact cumulatively to

influence tooth loading and distribution of stresses,

ultimately increasing the possibility of catastrophic

failure.

Loss of tooth tissue reduces the force required to

strain and ultimately fracture teeth, with the pattern of

loss influencing the magnitudes of the induced strains

as observed in vitro (39–42). Evidence from clinical

studies confirms these observations (43, 44). The

relative importance of disruption of the marginal ridge

and the width and depth of occluso-proximal cavities

continues to be debated, but tooth anatomy is also

likely to play an important part (45). The presence of an

endodontic access cavity may weaken teeth further,

although the extent of effect is unresolved (42, 46, 47).

Wide coronal flaring of canals has been implicated as an

additional factor in fracture of root-treated teeth (48).

It has been proposed that loss of pulp vitality alters

the properties of dentine; the properties assessed

include: changes in moisture content (49–53), nature

of collagen (54, 55), and other standard laboratory

physical properties (50, 56–58). The findings have

been contradictory or equivocal and as yet no definitive

proof of mechanical weakening of dentine exists. Two

fundamental problems are that: firstly, since all tests are

carried out in the laboratory, the dentine tested is by

definition non-vital; secondly, the science of measure-

ment is still improving and there is evidence that the

methods used have significantly influenced findings on

the properties of dentine (59).

It has also been hypothesized that pulpless teeth may

have a reduced capacity to detect occlusal loading and

therefore be more susceptible to fractures (60, 61).

Effect of mechanical instrumentationon root canal surfaces

The role of canal preparation (shaping) has undergone

a paradigm shift from one fulfilling a prime debriding

function, to one regarded more as a radicular access to

the complex root canal systems, for the irrigant and

root-filling material (62) (Fig. 6). Although evidence

had been gathering for some time that mechanical root

canal preparation techniques failed to instrument

a large proportion of the internal dentine surface

(63–65), the conceptual importance of this was not

fully realized.

The proportion of root canal dentine surface planed

by instruments has been quantified recently using high-

resolution computed tomography; it was found that

35–53% of the root canal surface remained uninstru-

mented (66–69). Using a cruder approach, it has also

been demonstrated that anterior maxillary teeth have

significant proportions of their root canal surfaces left

uninstrumented, regardless of access cavity design (70).

In addition to its flushing action, the chief role of the

irrigant is debridement of the uninstrumented canal

walls. This would seem to require two conditions:

firstly that an irrigant capable of dissolving organic

tissue is used, and secondly that a method suitable for

its delivery to the uninstrumented surfaces is used.

Following on from the descriptions above, it is possible

to envisage that as mechanical preparation is com-

menced, in the absence of a chemically active irrigant

(one capable of dissolving organic tissue), several

outcomes may be apparent in a tooth with a vital pulp.

The instruments (depending upon their design) may

remove some of the residual tissue by engaging it, some

will be pushed and compacted apically, and some will be

compacted and burnished against the root canal wall.

Such organic tissue will also be forced into depressions

or accessory anatomies (71, 72).

The irrigant will serve to flush out debris from the

root canal system, but to a certain extent, tags of tissue

may remain bound and merely be displaced apico-

coronally. The final shape of the prepared canal will be

determined by the shape and mode of use of the root

canal instruments. In the absence of an active irrigant,

compaction and burnishing of tissue into the non-

instrumented parts of the root canal system will leave a

space, the boundaries of which are determined by

instrumentation alone. The root-filling material will

therefore trace out a radiographic shape projected by

the instrumentation. In contrast, the use of active

irrigants, such as sodium hypochlorite (NaOCl) and

ethylene-diamine-tetra-acetic acid (EDTA), will help

remove such compacted debris from the non-instru-

mented anatomy and facilitate its display by virtue of

extension of the root-filling material into it. The

classically complex root-filling shapes seen in radio-

graphs used by endodontists to display their technical

prowess is because of the extension of root-filling

material into non-instrumented anatomy such as fins

and lateral canals. In the case of an infected root canal,

Effects of mechanical and chemical procedures

107

any bacterial biofilm on the instrumented canal surfaces

is likely to be disturbed or removed, although some of

the bacterial cells may become embedded within the

smear of tissue and deformed dentine (73). The

bacterial biofilm on the uninstrumented surface should

in theory remain mechanically undisturbed, except by

the displacement of any pulpal tissue or dentinal debris

from the prepared part of the canal. It is probably

fortuitous that changes in the ecology of the root canal

system may influence the demise rather than survival of

bacteria on the uninstrumented surface. Yet, the

uninstrumented surface should be regarded as essen-

tially still contaminated.

Effect on instrumented surface and smearlayer

Hydroxyappatite has the unique property of ‘smearing’

when abraded by another hard surface. Presumably,

evolutionary processes have selected this material for its

resistance to occlusal loading as well as the ability, in

dentine, to deform and cover patent dentinal tubules

during functional abrasion. Equally, such a smear layer

may be formed as a result of instrumentation of the root

canal system (71, 74). The latter group gave a more

detailed description of the layer as a 1–2 mm thick,

amorphous, irregular, and granular layer with a deeper

part that penetrated up to 40 mm into the dentinal

tubules. The penetration into tubules is hypothesized

to be a result of capillary action and adhesive forces

between the dentinal tubules and the smear layer (75,

76). Others have estimated the layer to be up to 5 mm

thick, with inorganic particles of 0.05–0.15 mm dia-

meter (77–79). Essentially, the structure is a complex

mixture of inorganic and organic particles, coagulated

proteins, pulp tissue, saliva, blood cells and in infected

canals, bacteria and fungi (24, 80).

The influence of various pre-operative and intra-

operative variables on the extent of the smear layer is

difficult to gauge because studies show considerable

Fig. 6. A cleared extracted tooth showing the complexity of the root canal system, accompanied by a diagram of the sametooth with the superimposed canal preparation, depicting the discrepancy between the uninstrumented andinstrumented anatomy. It also shows the ‘‘radicular access’’ role of the canal preparation.

Gulabivala et al.

108

variation in experimental design, making comparison of

results futile. The experimental teeth vary from those

with single, straight roots to molars with various canal

curvatures. In some studies, the root canals were

instrumented and irrigated prior to extraction (9, 10)

but the pre-operative pulpal status was not always known,

especially in laboratory studies. Furthermore, extracted

teeth may be stored in a variety of media or be frozen,

introducing another factor that may confound findings

(81–83). The delivery and type of irrigants vary

considerably, with crucial details often omitted from

the published papers. The quality of standardization and

reporting is only occasionally better (84). It should be

noted that the use of EDTA as an irrigant is likely to

influence the residual smear layer (85, 86). The position

of the working length relative to the root canal terminus

is indicated in some studies, when it is frequently 1 mm

short of the apical foramen (87–89). Less often, it was

0.5 mm short (90) or at the apical foramen (63). Patency

filing was used by several groups (89, 91, 92) but was not

always reported. The number of uses of the files before

discarding, a factor that may influence the amount of

smear layer, is quite variable. Depending upon the study,

file re-usage is not always reported; where reported, files

have been re-used in 10 canals (91, 92), three canals (93),

two canals (94), or not re-used at all (11, 95).

The vast majority of studies comparing various

mechanical methods of debridement attempt to

quantify the retained debris and smear layer. Prepared

sections may be examined under a microscope with a

calibrated eyepiece micrometer (11) or the image may

be captured by a grid system (85), photomicrograph

(86), or digitized (94). The image is then quantified by

a scoring system that is invariably subjective. Such

systems vary from simple criteria, such as ‘debris

present or absent’ (12, 96) to arbitrary three-, four-,

five-, or seven-point scoring systems (11, 72, 88, 97).

Scores may be expressed in terms of amount of debris

or smear layer per root level or canal, or alternatively, as

percentage area of root surface occupied (90, 94, 98).

Given the subjective nature of the scoring, some form

of reproducibility tests should be performed (11) but

are rarely reported. The latter studies also took the

additional step of blinding the examiners to the

treatment groups. Standardization of the experimental

protocol may aid comparison of studies.

More crucially, the important question centers on the

clinical relevance of the quantity of residual canal debris

and smear layer. There has been considerable debate

about its impact on treatment outcome and the merits of

removing it (24, 79, 80, 99). One view is that it is

undesirable because it may: (1) harbor microorganisms

(24, 71); (2) prevent or delay diffusion of irrigants and

medicaments into dentinal tubules (100, 101); and (3)

reduce the sealing ability of obturation materials (102,

103). In truth, although, the clinical significance of the

residual debris and smear layer is unknown. A recent

study reported that root canal isolates grew only when

exposed to tissue fluids, such as blood, serum, and saliva;

they failed to thrive in pulp tissue or tooth components

(104). The inference is that although ‘residual debris’

has become a marker for canal cleanliness in laboratory

studies, it is a poor outcome measure because a standard

amount cannot be guaranteed pre-operatively and it has

no obvious clinical relevance.

Residual bacterial infection in the root canalsystem after mechanical debridement

Numerous studies have evaluated the effect of different

stages of root canal treatment on the bacterial flora, in

qualitative and sometimes also quantitative terms. They

represent a multitude of methodologies as well as

treatment protocols. Some studies have merely re-

ported positive culture tests, whereas others have

speciated and quantified the bacterial flora before and

after various stages of treatment. Accepting the

differences in methodologies as limitations for direct

comparison, it was still possible to discern trends that

may be potentially helpful in framing new hypotheses.

A number of studies have evaluated the effect of

‘mechanical preparation’ on the bacterial flora, using

water or saline as the irrigant (105–110). They all noted

a reduction in the bacterial flora with the achievement

of negative cultures in a proportion (mean 25%, range

4.6–53%). Data on individual bacterial species and their

respective reduction rates were not available but one

study made the broad observation that none of the pre-

treatment species was especially persistent after treat-

ment (108).

Effect on mechanical properties of dentine

Irrigation of the root canal system with water or saline is

unlikely to induce changes in the mechanical properties

of root canal dentine (111, 112).

The mechanical properties of root dentine may be

affected by the extent of dentine removal; it is therefore

Effects of mechanical and chemical procedures

109

prudent to be cautious about overinstrumentation.

Interfacial forces are generated during push–pull filing

and can vary considerably by operator and instrument

size (113, 114). The actual forces acting along the

length of the instrument are likely to be dictated by its

relative flexibility and displaceability on the one hand

and the cushioning effect of the periodontal ligament

and alveolar bone, on the other. That is, dentine will be

cut in those places where the interfacial and transla-

tional forces exceed the fracture strength of the dentine

engaged by the sharp edges of the instrument. In

contrast, rotational instrumentation techniques, such

as ‘balanced force’, rely on actively engaging dentine

across opposing parts of the canal, in order to effect the

fracture of microchips of dentine. This allows stress to

develop both within the dentine and the instruments

(115, 116).

The use of rotary nickel–titanium instruments has

introduced numerous other variables as potential

contributors to induction of stress within the dentine,

including type of instrument, motor, tooth, canal

anatomy, and experience of operator (115, 117–119).

So far, most of the research has focused on the effect of

stress on the instrument, little effort has been put into

the effect of the same stress on the root dentine (116).

It is possible that such stress could also induce cracks or

fractures in the roots, although the sole study on this

concluded that this was not a danger.

Effect on chemical properties of dentine

Irrigation of the root canal system with water or saline is

unlikely to induce significant chemical changes in the

root canal dentine (120).

Effect of chemical agents on root canalsurfaces

As inferred earlier, the use of an ‘active’ irrigant would

seem desirable, given that a large proportion of the root

canal surface remains uninstrumented. The goal is to

deliver the irrigant into the prepared radicular access

and from there to disperse it into the uninstrumented

parts of the root canal system (62). In considering the

effects of the chemical or ‘active’ agents used on the

root canal contents and surfaces, it is necessary to take

account of canal preparation dimensions, canal con-

tents, irrigation dynamics, chemical properties, and

exposure to canal surfaces (instrumented and unin-

strumented). It is self-evident that penetration of the

irrigant or medicament will be dependent upon

adequate apical enlargement (121, 122) and likely

canal taper (12, 123), as well as the delivery system and

fluid properties of the irrigant. It is surprising to note

that the issue of irrigation dynamics has been so poorly

researched (124).

Effect on canal contents

Vital healthy pulps will be extirpated as previously

described. However, the added benefit of a chemically

active agent will be to promote organic tissue dissolu-

tion (10). The tissue-dissolving ability of NaOCl has

been found to be related to the duration of exposure

(125) and its concentration and temperature (126). It is

also dependent on the amount of organic tissue present,

the frequency and intensity of the irrigant fluid flow, and

the available surface area for interaction (127).

Partially or completely necrotic pulps are dissolved

more easily (65, 82, 127–129) but the efficacy of

dissolving solution on the uninstrumented surfaces is

dependent on an effective irrigation regime.

Effect on instrumented surface and smearlayer

The smear layer is amenable to removal by chemical,

ultrasonic, and laser treatments (99). The present

review focuses on the current evidence in relation to the

efficacy of various chemical preparations that have been

used to remove the smear layer, either as a sole agent, in

conjunction with other solutions, or with ultrasonic

energization.

Assuming that removal of the smear layer is a

desirable outcome, an ideal root canal irrigant should

be biologically compatible, chemically able to remove

both organic and inorganic substrates, be antibacterial,

demonstrate good surface wetting, have no adverse

effects on remaining tooth structure, and be easy to use

and effective within clinical parameters. No single agent

appears to meet these criteria; those agents used and

tested are shown in Table 1. Their chemistry of action is

covered elsewhere (127, 130–133).

The vast research efforts on smear layer removal are

naturally predominantly laboratory studies, but un-

fortunately are difficult to compare because of lack of

standardization of methodology. Most researchers have

Gulabivala et al.

110

used decoronated teeth with unlimited access, perhaps

giving false insight into effectiveness. Other experi-

mental variables include the age, type, and sample size

of teeth used, instrumentation techniques, irrigant

delivery systems, depth of penetration, volume, con-

centration and pH of agent, and duration of its use. As

before, the outcome measures vary and include

subjective scoring systems for debris and smear layer,

as well as erosive effects on dentine. Reproducibility of

scores by examiners and blinding of observations are

often overlooked, to add to the bias created by non-

randomized, selective examination of roots at different

levels. Most images have been captured from the SEM

but a diverse range of sample preparation methods and

varying magnification has been used.

The most common solutions used for smear layer

removal include: varying concentrations of NaOCl (15,

74, 65) and EDTA preparations (134–139). These are

used either as sole irrigants or in conjunction with each

other (75, 140–149).

The quality and quantity of the smear layer produced

may vary as chemo-mechanical instrumentation pro-

ceeds, depending on the mechanical approach, irrigant

properties, and mode of delivery. During the early stages

of instrumentation, the smear layer may have a higher

organic content because of the presence of pulp tissue in

the canal. With the progressive dissolution of organic

substrate, the inorganic component may increase and be

more amenable to removal by EDTA (150). The nature

of the smear layer created with current nickel–titanium

rotary techniques may vary considerably from

that formed using stainless-steel instrumentation be-

cause of the different mechanical and chemical forces

in play. Furthermore, the chelating gels routinely

recommended for use with nickel–titanium instruments

to avoid instrument breakage (139, 147, 151)

may significantly alter the nature of the smear layer

formed (147). In the latter study, use of ‘Glyde prep’

in conjunction with 2.5% NaOCl resulted in a residual

smear layer. The differences in flow properties of

the agents (fluid vs. gel) may be a contributory factor.

The plethora of liquid and paste-type chelators curr-

ently available, their mode of action, advantages,

and disadvantages have been well reviewed elsewhere

(133).

EDTA was introduced to endodontics as a tool for

negotiating narrow or sclerosed canals, where demi-

neralization of root dentine on application of 15%

EDTA was proportional to the observation time (152).

However, the demineralizing effect of the chelating

agent is self-limiting, because it is exhausted (134).

Furthermore, organic material inhibits the action of

EDTA when used on its own; but when combined with

NaOCl, the quantity of inorganic material becomes the

limiting factor (141). The combination of NaOCl and

EDTA produces a synergistic effect, resulting in

effective removal of the entire smear layer (142, 149).

On the other hand, the latter study demonstrated a

reduced antibacterial effect of NaOCl when used in the

presence of EDTA.

Table 1. A classification of types of chemicals used for root canal irrigation

Type of chemical Generic and brand examples

Chelating agents

(EDTA containing)

EDTA, EDTAC, REDTA, Salvizol, Tublicid, RCPrep; Glyde; EGTA

Halide complexes Sodium hypochlorite, tincture of iodine, povidone–iodine, iodine potassium iodide, oxidative potential

water (electrochemically activated water)

Acids

(organic and inorganic)

Phosphoric acid, citric acid, lactic acid, polyacrylic acid, tannic acid, DMSA (dimercaptosuccinic acid)

Antibiotics Tetracycline hydrochloride, doxycycline hydrochloride

Oxidizing agents Hydrogen peroxide

Others Cetrimide, bardac-22 (quaternary ammonium compound), tergensol (0.2% lauryl sodium sulfate),

chlorhexidine, MTAD (tetracycline isomer, an acid, detergent), ethylenediamine, methylene blue dye,

titanium tetrafluoride, trientine hydrochloride (Syprine), Succimer (Chemet)

Organic solvents Chloroform, halothane, xylene, eucalyptus oil, orange oil

Effects of mechanical and chemical procedures

111

Several agents have been combined with EDTA

in an attempt to improve surface wettability and

penetration into dentine. Earlier studies explored the

use of NaOCl in conjunction with hydrogen peroxide

but the combined cleaning effect was found to be

weakened (71, 142, 153, 154). A comparison of the

cleaning effects of 2% chlorhexidine and NaOCl gave

similar residual debris scores in the cervical third of roots

with both agents, although smear layer removal was

poor (155).

Numerous studies have evaluated the effectiveness of

inorganic and organic acids (Table 1) for smear layer

removal and found them to be highly effective, but too

aggressive; their use has therefore not been universally

adopted (136, 140, 154, 156–161).

The effects of ultrasonic agitation of irrigants have

been evaluated with contradictory results (85, 150,

162–167). The reasons for this may include: lack of

attention to variation in power outputs of the ultrasonic

generators; frequencies of output; dimensions of files

or tips used; and their mode of use.

Despite the experimental variables inherent in the

studies mentioned, it may be concluded that NaOCl is

efficient at debris removal in the coronal and middle

thirds of root canals but fails to disperse the smear layer

and plugs from dentinal tubules (15). In addition, the

challenge of debridement of the apical anatomy has not

been fully resolved (71, 139, 155). Nevertheless, the

combination of agents, and the sequence in which they

are used, clearly can enable better apical cleaning (140).

A final flush of NaOCl has been advocated, as EDTA

may leave the organic part of the smear layer behind

(168) and it also neutralizes the acidic effects of any

residual EDTA (147).

The counter-side of the picture is that these alternat-

ing regimes of NaOCl and EDTA have adverse effects

too (145, 146, 148, 169, 170). Two groups have

independently observed significant intertubular and

peritubular dental erosion in the middle third of roots

treated with both 17% EDTA and 5.0% NaOCl.

Shorter application times and/or reduced volumes of

irrigants were proposed to minimize such damage,

particularly in young patients. An irrigation regime

incorporating 4% titanium tetrafluoride (TTF), follow-

ing irrigation with NaOCl and EDTA, has been

advocated to help re-mineralize the dentine (143).

TTF supposedly forms a tenacious coating over the

enamel and cementum. This proposal remains to be

confirmed by other researchers.

Various agents with surface wetting and antibacterial

properties have been added to EDTA or new agents

have been tested in an attempt to improve the efficacy

of smear layer removal without deleterious effects on

dentine. Experimental evidence suggests that various

EDTA-based solutions are not more successful at

removal of the smear layer than those with EDTA

alone (138, 158, 171, 172). Other agents introduced

include oxidative potential water (OPW) (electroche-

mically activated water) and tetracycline-based agents.

Electrochemically activated water has been used as a

commercial disinfectant, sterilizing agent and for

agricultural and industrial processes, without adverse

effects on biological tissues (173, 174). It has proper-

ties similar to OPW developed by Japanese researchers

(175, 169). Collective experimental findings from

endodontic investigations indicate that these agents

are ineffective in removing the smear layer efficiently,

unless combined with NaOCl or EDTA.

Tetracyline-based solutions may be potentially suc-

cessful irrigants because of their chelating and sustained

antibacterial actions. The efficacy of doxycycline

hydrochloride in removing the smear layer in the

middle and apical thirds of root canals has been noted

(176); it was attributed to its acid pH of 2. There is no

information on its potential interaction with NaOCl

regarding smear layer removal.

A new solution for root canal irrigation, which

combines a tetracycline isomer, an acid, and a detergent

(Biopure, Dentsply Tulsa, Tulsa, OK, USA) has

recently been proposed (177, 178). The agent appears

to be partially effective at removing the smear layer on

its own but exhibits superior cleaning when used in

conjunction with NaOCl. The erosive effects of this

combination are less than those of NaOCl and EDTA,

and it has been proposed for use with NaOCl (2.65%)

as a final rinse. Added benefits apparently include

broad-spectrum antibacterial effects sustained over

time. The irrigant remains to be tested clinically.

Effect on uninstrumented surface andbiofilm layer

Where there is an absence of a bacterial infection on the

uninstrumented surface, 2.5% and 5% NaOCl may

dissolve most of the predentine, exposing the globular

mineralizing front, the calcospherites (Fig. 7) (179).

Based on work evaluating the instrumented and

uninstrumented root canal surfaces, it may be reason-

Gulabivala et al.

112

able to assume that the combination NaOCl and EDTA

would help to remove the biofilm layer (141). However,

there is no specific research investigating the degrada-

tion of the residual biofilm on the uninstrumented

surface. Indirect evidence, if it may be called that,

suggests that the combined use of NaOCl and EDTA

facilitates better reduction of the bacterial load in root

canals of single-rooted teeth (180). The precise

mechanism is unknown but it may be hypothesized that

it is because of a combination of EDTA: (1) helping to

remove debris obstructing access to the uninstrumented

surfaces; and (2) chelating heavy metal ions that help to

bind bacterial cells together in the biofilm.

Residual bacterial infection in the root canalsystem after chemomechanical debridement

Numerous studies (108, 180–185) have used NaOCl

irrigation (concentration range 0.5–5.25%) to supple-

ment mechanical preparation and the increased fre-

quency of negative cultures immediately after

debridement shows the benefit of the procedure (range

25–98%, mean 73%) (181, 186). The majority of

studies have reported culture reversals during the inter-

appointment period without the aid of further active

antibacterial dressing between appointments. The

reversals were attributed to re-growth of residual

bacteria or re-contamination by bacterial leakage

around the access cavity restoration (105, 186–189).

Other antibacterial irrigation and dressing agents

have also been used experimentally, including Biosept

(a quaternary ammonium compound) giving 32%

(107) and 40% (190) negative cultures, respectively;

Nebacin antibiotic giving 60% negative cultures (107);

and Cresatin/CMCP/polyantibiotic paste giving 76%

negative cultures (189).

The most significant series of studies (100, 108, 180,

182, 183) evaluated the effect of various root canal

treatment procedures on the bacterial flora both

qualitatively and quantitatively using standardized

methodology. The effects of mechanical preparation,

NaOCl irrigation (0.5%, 5.0%, 5.0% with EDTA), the

addition of ultrasonic activation, and calcium hydroxide

dressing were evaluated in series and each showed a

better antibacterial effect than the last. They collectively

also observed that the antibacterial action reduced the

number of bacteria from an initial range of 102–108 cells

to 102–103 fewer cells after initial debridement, further

reducing down to no recoverable cells after inter-

appointment dressing with calcium hydroxide.

The benefit of dressing the root canal system with

calcium hydroxide directly after irrigation with water

(following mechanical preparation) has been confirmed

(109, 184), in addition to its use after irrigation with

NaOCl (100, 191, 192). Only Peters et al. (193) found

no obvious benefit of dressing with calcium hydroxide

between visits.

Most importantly, Sundqvist’s group noted that the

collective antibacterial action during root canal treat-

ment in their material did not give rise to the

persistence of any particular species in the later visits.

They therefore concluded that there was an absence of

evidence that specific bacteria were implicated in

persistent infections (108). This view has been con-

firmed for primary root canal treatment by several

groups (181, 185, 194). Gomes et al. (185) did,

however, reach the overall conclusion (based on both

primary and secondary root canal treatments) that

certain species were more resistant to biomechanical

procedures than others.

Fig. 7. SEM views across the dentine surface showing themineralized dentine, and the irregular surface formed bythe mineralizing front of overlapping calcospherites.

Effects of mechanical and chemical procedures

113

The residual species in previously root-filled teeth

with apical periodontitis appear to have root canal

infections that are dominated by Gram-positive bacter-

ia (195, 196), suggesting that incomplete root canal

debridement may allow these less fastidious bacteria to

dominate the infection. These types of bacteria are

found infecting the dentine (35) and therefore may be a

source for recontamination of the root canal system.

Numerous in vitro studies have evaluated dentinal

tubule infection and its treatment (33, 35, 197, 198).

While such studies are important for understanding the

nature of tubule infection, the clinical relevance of

studies evaluating the efficacy of eliminating single

species from radicular dentine remains questionable.

Effect on mechanical properties of dentine

Medicaments and root-filling materials may influence

the physical and mechanical properties of dentine.

Eugenol-containing root canal sealers, for example, can

harden intra-canal dentine (199), while chloroform,

xylene, and halothane soften dentine (200). NaOCl is

known to reduce the modulus of elasticity of dentine

(111, 112), as well as its flexural strength (111, 112).

Dynamic mechanical analysis has revealed that while

the visco-elastic properties of dentine are not altered by

NaOCl alone, when used in combination with EDTA, a

significant change is elicited (201).

Irrigation with a 5.25% solution of NaOCl signifi-

cantly increased the tooth surface strain of teeth using

cyclic non-destructive loading in a whole-tooth model.

Furthermore, sequential repeated 30 min irrigation

steps with 5.25% NaOCl did not result in a linear

increase in tooth surface strain, but one that plateaued

after the first two steps (111, 202). In contrast,

alternate irrigation with NaOCl and EDTA eliminated

the plateau effect, with a continuously increasing tooth

surface strain (202), suggesting a more severe effect.

The dressing of root canals with calcium hydroxide

may also reduce the flexural strength of dentine but not

the modulus of elasticity (112). If the dressing is left

long term, it could render teeth more susceptible to

fracture (203). A similar in vitro test protocol applied to

MTAD (Biopure) suggested that if used according to

clinical protocol, there was no change in the flexural

strength and modulus of elasticity of dentine. If,

however, a longer duration of contact was used, then

changes in both properties were evident with MTAD

and EDTA (204).

Effect on chemical properties of dentine

The changes in mechanical properties of dentine as a

result of root canal irrigants and dressings are almost

certainly because of the altered chemical composition

of dentine. It has been conclusively shown that the

organic element of dentine (collagenous component) is

depleted by soaking in NaOCl (120, 201), while the

mineral component is left relatively intact. If irrigation

with NaOCl is alternated with EDTA, the hydroxyap-

patite is also degraded and consequently leads to

greater dentine strain and a change in visco-elastic

properties (201). The combined chemical effect of

NaOCl and EDTA explains both the changes in

mechanical properties as well as the surface erosions

noticed in dentine as a result of aggressive irrigation.

Priorities for improvement in successrates of root canal treatment

The average success rate of root canal treatment has

been reported to be 74% with a range of 31–100%

(205), while, using meta-regression in a framework of

multi-level modelling, the mean probability of success

was estimated at 84% (206). The pre-operative pulpal

and periapical status of teeth are the most significant

factors affecting the success rate of root canal treatment

and therefore imply the predisposition of some teeth to

failure, regardless of treatment protocol (207–209).

The single most important treatment factor influen-

cing success is the apical extent of root filling, although

this probably implies both apical extents of canal

preparation as well as filling (207, 208, 210). The

probability of success is reduced if the root filling is

extruded beyond the radiographic apex, regardless of

the presence or absence of pre-existing periapical

disease. The effect of root fillings flush with or short

of the radiographic apex depends upon the pre-

existence of periapical disease. In the presence of

periapical disease, flush root fillings result in a higher

probability of success, while short root fillings would

result in the reverse. In the absence of periapical disease,

short and flush root fillings result in an approximately

equal probability of success. The influence of the

mechanical preparation technique on success rate has

rarely been investigated (209, 211–213). An important

clinical guideline in root canal treatment is the size to

which the canal is prepared apically; yet, its effect on

outcome has never been properly investigated and

Gulabivala et al.

114

where it is assessed, it gives contradictory results (209,

211, 212). Similarly, the effect of canal taper on

outcome has also not been specifically analyzed, but

one study (208) suggested increased success rates with

greater canal flare. Instrumentation with nickel–tita-

nium files may result in higher success rates compared

with stainless-steel files because of better maintenance

of canal shape and access to apical anatomy (214).

Although numerous irrigants and medicaments have

been used during root canal treatment, their effects on

success rate have never been properly compared in

randomized clinical trials. A number of studies (215–

220) have reported a significantly higher chance of

success after obtaining a negative culture prior to

obturation, compared with a positive culture; the

success rates were between 10% and 26% higher with

a mean of 12%. However, others (210, 221–224) have

found no significant difference in success rates between

pre-obturation positive and negative culture tests.

Despite being researched extensively ex vivo, the

influence of ‘canal cleanliness’ (presence of debris and

smear layer) prior to obturation, on success rate, has

never been studied. There is a great need for properly

designed randomized-controlled trials to compare the

effect of different mechanical and chemical root canal

debridement protocols on the outcome of root canal

treatment.

Conclusions

Root canal treatment procedures bring about a multi-

tude of changes to the root canal surface, which can be

described in mechanical, chemical, and biological

terms. The changes may be considered to be beneficial

and/or damaging. Much of the above research has

been driven by contemporary concepts upon which

root canal treatment procedures are based. Unfortu-

nately, these in turn are not always founded upon

clinical outcomes’ research. The latter suggests that the

presence or absence of residual infection in the apical

anatomy and length of root canal treatment are the

prime determinants of success.

The principal aim of root canal preparation is

therefore to obtain and maintain access to the apical

anatomy, for the purpose of delivering antimicrobial

agents to the infection in this site. A combination of

NaOCl and EDTA remains the irrigant of choice for

both smear layer removal and bacterial debridement;

however, their effectiveness in the apical anatomy

depends upon a careful regimen and adequate mechan-

ical preparation. Overenthusiastic mechanical or che-

mical root canal preparation has severe consequences

on the mechanical properties of dentine and may

render teeth more susceptible to fracture. Therefore, a

balance has to be achieved in delivering antibacterial

agents effectively to the apical anatomy while main-

taining tooth strength and integrity.

The quantity of literature on smear layer removal

seems in exaggerated proportion to that on the

biological and clinical factors that are likely to influence

success rates of root canal treatment. It may be that this

obsession, partly driven by the desire for observing the

filling of root canal anatomy, radiographically, has

coincidentally helped bacterial biofilm degradation in

the uninstrumented parts of the root canal system. The

precise dynamics and biological mechanisms leading to

successful root canal treatment still remain to be

determined. Upon achievement of such an under-

standing, modifications to root canal treatment should

lead to an evidence-based improvement in success rates,

including apical healing and tooth survival.

Acknowledgments

The authors would like to thank Nicky Mordan, Naomi

Richardson, and Shailesh Rojekar for producing the micro-

scopic views of the root canal surface.

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