Even though the use of rotary NiTi instrumentation
allows curved roots to be widened to sizes 45 to 80,
there is a question about the effect of such enlargement
on tooth structure. More than 25 years ago, cleaning to
large apical sizes was advocated
and was fraught with
procedural clinical problems, mainly associated with
iatrogenic damage to the fine structure of the apical
third of the root. The difference between the stainless
steel instruments of that time and NiTi instruments of
today is that the increased flexibility of NiTi
instruments reduces the chance of deviation from the
original canal anatomy during instrumentation.
However, an open question is: compared with
conservative techniques of apical preparation, does
apical enlargement provide better clinical results and
with a suitable margin of safety?
Observation of the results of those practising
techniques of apical enlargement has shown that, apart
from in the hands of the most highly skilled clinicians,
there are many procedural risks associated with apical
enlargement. Some of the potential procedural problems
associated with enlarged apical instrumentation are
summarised in Table 5.
As a direct consequence of apical enlargement, the
tooth structure surrounding the root canal space is
thinned and the risk to the integrity of the root
structure is greatest where there is the smallest amount
of original tooth structure. Consequently, the risk of
apical perforation is progressively higher with
increasing instrument size and may result in root
weakness and splitting of the delicate apical root
structure.
A lower margin of safety applies with apical enlarge-ment
since a minor miscalculation in measurement may
result in a significantly more damaged apex. An over-
instrumentation error with a size 60 or 80 file leaves a
much larger apical wound than a size 25 file — the
wound surface area is 6–10 times greater with a size 60
or 80 instrument than a size 25 file, respectively. Thus,
with apical enlargement the operator skill becomes a
much more critical factor for the outcome of clinical
treatment and there is a noticeably lower margin of
safety.
Apart from the risks to the integrity of tooth
structure, apical enlargement also has the potential to
adversely impact on subsequent endodontic
procedures. Once the canal has been instrumented to a
larger size (>#45), the filling material is more difficult
to control during obturation. If, in addition, the apex
has been opened there is a higher risk of overfilling,
which has been shown to be associated with a reduced
success rate.
Lastly, if an apically enlarged root
filling fails, it may be much more difficult or
impossible to successfully retreat the tooth. This is
because the more dentine removed during initial treat-ment,
the less dentine is available for preparation at re-treatment.
Further instrumentation of an apically
enlarged canal significantly increases both the risk of
procedural errors (perforation, zipping, etc.) and
heightens the risk of excessive apical root weakness or
splitting.
Based on current knowledge, the answer to the
question ‘does increased apical enlargement predictably
eliminate bacteria?’ is no. To the question ‘does apical
enlargement provide better clinical results and with a
suitable margin of safety?’, the answer is no. The results
achievable with a suitable antimicrobial dressing are
more predictable and do so in a conservative way.
Taken together, the thin evidence for apical enlarge
ment as a means of bacterial eradication, and the
significantly increased risk of procedural errors, the
disadvantages and risks of apical enlargement far out
weigh the perceived benefits.