2005 gulavibala - effects of mechanical and chemical procedures on root canal surfaces
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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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