Coronectomy

Purpose: Damage to the inferior alveolar nerve when extracting lower third molars is often caused by
the intimate relationship between the nerve and the roots of the teeth. The technique of coronectomy,
or intentional root retention, may minimize this problem.
Patients and Methods: Forty-one patients underwent coronectomy on 50 lower third molars with
follow-up of at least 6 months. The technique of coronectomy deliberately protected the lingual nerve
as part of the surgical procedure. All roots were left at least 3 mm below the buccal and lingual plates
of bone. All patients were radiographed preoperatively, immediately postoperatively, and after 6 months.
Results: There were no cases of inferior alveolar nerve–involved damage in this study of 41 patients
who underwent 50 coronectomies. There was 1 case of transient lingual nerve involvement, probably
from the use of the lingual retractor. One patient required subsequent removal of the roots of both lower
third molars because of failure to heal, and 1 patient required subsequent removal of a root because of
subsequent migration to the surface. Root migration was noted in approximately 30% of patients over a
6 month period.
Conclusion: Coronectomy appears to be a viable technique in those cases where removal of the whole
tooth might put the inferior alveolar nerve at considerable risk of damage. The technique appears to be
associated with a low incidence of complications, but subsequent migration of the roots may be an issue
in the long term.

The issue of inferior alveolar nerve involvement
during the removal of lower third molars is a clinical
and medicolegal problem. Any technique that can
reduce the possibility of this involvement is worthy of
exploration. The technique of coronectomy, or deliberate
root retention, has been studied intermittently
in the past, but has no strong body of support.
It does seem appropriate that if this technique is to
be performed, the following rules appear sensible:
1. Teeth with active infection around them, particularly
infection involving the root portion,
should be excluded from this technique.
2. Teeth that are mobile should be excluded from
this technique because it might be felt that the
roots may act as a mobile foreign body and
become a nidus for infection or migration.
3. Teeth that are horizontally impacted along the
course of the inferior alveolar nerve may be
unsuitable for this technique because sectioning
of the tooth itself could endanger the nerve
. The technique is therefore better
utilized for vertical, mesioangular, or distoangular
impactions where the sectioning itself does
not endanger the nerve.. Completed coronectomy on lower right third molar. Note
retained roots are 3 mm below the crest of bone and exposed pulp is
untreated..
4. There does not appear to be any need to treat
the exposed pulp of the tooth and root treatment
appears to be contraindicated.13-16 Animal
studies have shown that vital roots remain vital
with minimal degenerative changes. Osteocementum
usually extends to cover the roots.
5. The technique of leaving the retained root fragment
at least 3 mm inferior to the crest of bone
seems appropriate and does appear to encourage
bone formation over the retained root fragment.
This distance of 2 to 3 mm has been
validated in animal studies.13-15
6. Late migration of the root fragment does appear
to occur in some cases, but is unpredictable.
However, in all cases the root fragments move
into a safer position with regard to the nerve,
and it can be envisaged that should removal
ever become necessary the nerve would not
then be at high risk. The authors are aware of
anecdotal reports from colleagues of retained
root fragments migrating right up to the surface
of the mucosa and appearing through the mucosa
and requiring removal. This happened
only once in the present study, but at least the
root fragments are mobile and easy to remove
without complication and without risk to the
nerve. It is possible that roots will migrate more
if they are mobilized in any way during the
initial surgical procedure.
One difficulty with regard to a study such as this is the
decision as to which patients are at risk and whom to
perform this technique on. In the end it is a personal
decision between the surgeon and the patient. Previous
studies evaluating the risk of inferior alveolar nerve damage
with third molar extraction have relied on either
periapical or panorex radiographs.1-3 Rood and Shehab3
suggested that diversion of the inferior alveolar canal,
darkening of the root interruption of the white line of
the canal, narrowing of the canal, and deflection of the
root were indicators of possible nerve injury. In their
prospective study of 125 teeth with signs suggesting an
increased risk of nerve involvement, 14% developed
nerve injury. Similarly, Blaeser et al,2 in their study,
showed that when increased risk factors are shown on
a panorex radiograph, the incidence of nerve involvement
may rise from a background risk of 1% to between
1.7 and 12%. The advent of low dose cone beam computed
tomography technology, which is now becoming
readily available in a dental outpatient setting,
should provide a much more accurate prediction of the
likelihood of nerve injury, and in cases where the pan-CORONECTOMY
radiograph suggests an increased risk of nerve
involvement, the use of cone beam computed tomography
technology may be indicated to assess the exact
relationship in 3 dimensions. When it is seen that there
is truly an intimate anatomic relationship between the
nerve and the tooth in 3 dimensions, coronectomy may
be a useful option.
There are currently no standards regarding the timing
and frequency of follow-up of patients having coronectomy.
At the present time, we are taking radiographs
immediately postoperatively and 6 months postoperatively.
Later radiographs are taken if the patient becomes
symptomatic. We would not advocate seeing the patient
after 6 months unless he or she becomes symptomatic,
though for research purposes patients may need to be
followed and radiographed for longer periods.