The antibacterial effect of mechanical preparation
with saline as an irrigant has been shown to be
inefficient in the elimination of bacteria from the root
canal.
This implies that mechanical instrumentation
of the canal must be supplemented by antibacterial
irrigants and dressings for efficient elimination of
micro-organisms from the root canal. There are many
other benefits to be gained by the use of chemical
agents during the preparation of the root canal. The
agents used for chemical disinfection can be separated
into two types — those used for irrigation during canal
preparation and those used as an intracanal dressing
between appointments.
Irrigation
Irrigation of the root canal is an essential component
of root canal preparation. The main benefits of using
irrigants during the cleaning of the canal include
wetting of the canal walls and removal of debris by
flushing, destruction of micro-organisms, dissolution of
organic matter, removal of smear layer and softening of
dentine and cleaning in areas that are inaccessible to
mechanical cleansing methods.
When applied to infected tissue, an irrigant should
ideally destroy micro-organisms and their toxins with-out damaging normal tissues. Sodium hypochlorite
solution (NaOCl) was identified early last century as a
promising microbicide that did not cause tissue damage
or interfere with wound healing.
A clearly superior antibacterial effect has been
demonstrated when NaOCl solution is used as an
irrigant. Although the concentration of NaOCl solution
seems to have little apparent effect on antimicrobial
activity in the root canal system,
95,120
the efficacy of
weak solutions decreases rapidly and consequently
irrigation should be frequent and copious. In addition
to its powerful antimicrobial activity, NaOCl solution
has a strong capacity for dissolution of organic
matter.
101,102,121-123
The tissue dissolving ability of NaOCl
solution is influenced by the amount of organic matter,
the fluid flow around this matter and the surface area
available for interaction.
124
The chelating agent ethylenediamine tetra-acetic acid
(EDTA) is commonly used as an irrigant in conjunction
with NaOCl solution, because EDTA is highly effective
in removal of the smear layer and opening dentinal
tubules,
101
which potentiates the reach of antimicrobial
irrigation and dressing.
There are many regions of the root canal system that
are simply inaccessible to mechanical instrumentation
and all of these areas have the potential to harbour
micro-organisms and necrotic pulp tissue. These areas
include accessory canals, fins and webs that branch
from the main canal or canals. Areas that are
inaccessible to mechanical instrumentation can only be
cleaned by antimicrobial irrigants that are able to
permeate into these recesses. Any further antibacterial
effect will only occur with the support of an intracanal
dressing.
Dressings
Mechanical cleaning, irrigation and dressing with
antibacterial medicaments achieve a reduction of the
number of living bacteria in infected root canals.
Evaluation of the relative efficacy of these measures in
eliminating the bacteria has shown that mechanical
cleansing supported by irrigation significantly reduces
the number of bacteria in the root canal, but that
approximately 25 to 50 per cent of canals treated in
this way still contain bacteria at the end of the appoint-ment.
The number of persisting bacterial cells is
usually low, but these remaining bacteria can recover
and rapidly increase in number between treatment
visits if no antibacterial dressing is present in the root
canal. The growth of bacteria between appointments
can ultimately lead to the re-establishment of the
number of bacteria that were initially present in the
root canal before treatment.
The principal goal of dressing the root canal between
appointments is to ensure a safe, antibacterial action
with a long-lasting effect. If the active agent in the
medicament is rapidly lost then the duration of its
antibacterial activity is likely to be short, and thus
ineffective. The antibacterial effect of dressing root
canals with camphorated paramonochlorophenol and
camphorated phenol has been assessed in vivo and been
shown to be of limited efficacy.
The clinical effectiveness of calcium hydroxide in
infected canals has been tested in a number of in vivo
studies and been shown to be an efficient antibacterial
treatment eliminating micro-organisms in previously
untreated cases from 75 per cent
to more than
90 per cent of dressed canals.
Application of an
interappointment calcium hydroxide dressing prior to
obturation has been shown to yield improved healing
responses over non-calcium hydroxide treated teeth in
human and animal teeth.
Treatment with
calcium hydroxide has also been shown to dissolve
necrotic tissue and enhance the tissue dissolving effect
of NaOCl solution.
It is critical that the calcium hydroxide dressing be
placed carefully in the instrumented canal as a thick,
moist paste fully filling the entire canal
and that it be
left for sufficient time to achieve the desired anti-microbial effect.
The paste consistency helps prevent
influx of the periapical fluid, which is an important
nutrient source for any remaining bacterial cells. The
hydroxyl ions that are responsible for the strong
antibacterial effect are rapidly potent when in intimate
contact with target micro-organisms in vitro,
but
need time under in vivo conditions to diffuse into the
adjacent dentine. The reason for the slow diffusion of
hydroxyl ions into dentine is the powerful buffering
capacity of dentine, which creates a concentration
gradient across the root wall.
Dressing the canal
for one week has been shown to be an efficient method
in the clinical setting.
After canal preparation and final irrigation, residual
fluid should be aspirated leaving the canal walls moist
since the antimicrobial effectiveness of calcium
hydroxide depends on an aqueous environment.
Calcium hydroxide is easily applied as a paste into the
canal with a spiral paste filler. Any residual calcium
hydroxide on the walls of the access cavity should be
carefully removed before the temporary filling is
placed. A well placed temporary filling of >4mm depth
is essential, for without it the many antibacterial steps
preceding its placement are rapidly undone.