Incidence of Nerve-Damage relating to Wisdom Tooth Removal:
Inferior Alveolar Nerve. IAN function is disturbed in 4 – 5% of procedures (range
1.3 – 7.8%). Most patients will regain normal sensation within a few weeks or
months and < 1% (range 0 – 2.2%) have a persistent sensory disturbance.
A higher incidence of IAN injury has been reported with wisdom teeth that are
horizontally or mesio-angularly impacted and have complete bone cover.
One study has also demonstrated that increasing age is associated with a higher
frequency of IAN injury (14 – 24 year old patients 1.2%; 35 – 81 year-old patients,
9.7%).
Lingual Nerve. There is a wide range in the reported frequency of LN injuries
during lower wisdom tooth, with 0.2 – 22% of patients reporting sensory
disturbances in the early post-operative period and 0 – 2%, a permanent
disturbance.
A higher incidence of IAN injury has been reported with certain types of surgical
technique (using an ‘elevator’ to ‘protect’ the LN) together with deeply impacted
teeth when the surgery is consequently difficult, particularly if distal bone removal is
required.
Most cases of nerve damage during wisdom tooth removal are not identified at the
time of lower wisdom teeth removal but in the post-operative period.
However, careful monitoring of sensory recovery over a three month period should
distinguish between these different types of injury.
Monitoring sensory recovery is undertaken by the application of stimuli to the ‘numb’
area. Responses of the patient will indicate first the arrival of the regenerating
nerve ends and then subsequently the level of recovery.
However, the most sensitive indicator of a sensory abnormality is the patient’s own
subjective report, as minor sensory disturbances may not be detected by testing.
Simple Sensory Testing
A standard protocol for sensory testing does not exist and attempts to standardise
objective evaluation of nerve injuries have been unsuccessful.
Evaluation techniques are subjective or semi-objective at best.
Suggested techniques include:
Mapping out and photographing the area
involved
Light touch is most commonly tested by gently
applying a wisp of cotton wool to the skin or lining
of the cheek or lips.
However, it is difficult to apply this stimulus in a
reproducible manner and the use of a cotton wool
wisp on moist oral mucosa is difficult.
Greater consistency and reproducibility can be
obtained using Von Frey hairs. Stimuli are applied
at random and the area of anaesthesia can be
stimulus is felt.
Pin Prick Sensation
Testing pin prick threshold is often performed using a dental probe or needle but
reproducibility is poor.
Areas of anæsthesia can be mapped. If sensation is present within the affected
area on the injured side, then the pin prick sensation threshold is determined.
The probes are drawn a few millimetres across the surface, at a constant pressure
and the patient asked to indicate the point at which the sensation becomes sharp
rather than dull.
The pin prick sensation threshold is noted for a series of randomly chosen
points on both the ‘injured’ and the ‘uninjured’ side.
Two Point Discrimination
probes with different separations (2 – 20 mm) are
mounted around a disc.
The probes are applied at a series of fixed sites
chosen on the lips or tongue, depending on which
has been damaged.
The probes are drawn a few millimetres across
the surface, at a constant pressure and the
patient is asked whether one or two
points are felt.
The minimum separation, that is consistently reported as two points, is termed the
two point discrimination threshold.
This threshold varies in different regions of the mouth (2 – 4 mm on the tongue and
lip, 8 – 10 mm on the skin over the lower border of the chin).
Taste Stimulation
Cotton wool pledgets soaked in saline solution, sugar solution, vinegar or quinine
solution are drawn 1 – 2 cm across the side of the tongue and the patient asked to
indicate whether they taste salt, sweet, sour, bitter or no taste, before
Stimuli should be applied in random order, to each side of the tongue and rinsing
with tap water between tests.
Treatment
Inferior Alveolar / Dental Nerve:
If a sensory disturbance is first noted at review, recovery should be monitored
using the sensory tests described above.
Patients with paræsthesia in the distribution of the IAN (evoked by touching the lip
or chin) usually require no surgical intervention.
Patients with complete anaesthesia post-operatively should be evaluated
radiographically (such as an OPG or a CT scan) to ensure that the roof of
the nerve canal has not been displaced downwards to create an
obstruction to nerve repair and regeneration. In the extremely rare event that this
has occurred, removal of the bony fragment would seem to be appropriate, without
undue delay.
Referral to an Oral & Maxillofacial surgeon familiar with this type of procedure or
a neurosurgeon or a micro-neurosurgeon is important. The patient should know
that full recovery may not be achieved even with surgery though some recovery
may occur even if surgical ‘decompression’ is not performed.
If, after 3 months after the injury, monitoring reveals little or no sensory recovery,
referral is again indicated. A further X-ray to assess the continuity of the IDN canal
is obtained and surgical exploration and ‘decompression’ of the nerve is considered
if the canal is disrupted, if there is very little recovery of sensation or if there is
significant dysaesthesia.
However, the results of surgery are variable and sometimes disappointing.
Lingual Nerve:
If the LN is knowingly cut during wisdom tooth removal, it should be immediately
repaired.
This may not be possible in dental practice and immediate referral to an
appropriate experienced Oral & Maxillofacial surgeon is indicated. In the majority
of patients, the injury is only discovered post-operatively.
At early review, the presence of some sensation in response to stimulation of the
tongue suggests that the nerve is at least partially intact; no treatment is
indicated but sensory monitoring is required.
Complete anæsthesia could be caused by both a crush or cutting injury and so
surgical intervention is not indicated initially.
However, the absence of progressive sensory recovery by 3 – 4 months post-injury
is an indication for surgical exploration at an appropriate Oral & Maxillofacial
unit.
If, at the time of surgery, the nerve is found to be intact and of fairly uniform
thickness but merely constricted by scar tissue, it should be freed (external
neurolysis) and the wound closed. This is unusual however and more commonly
the nerve is found to have been cut.
If a neuroma has developed, this can be seen as a marked expansion at the site of
the injury and must be removed together with the damaged segment of
the nerve. A nerve graft is then used. The results of surgery are very variable;
some patients regain good sensation whilst others show little if any improvement.
One study showed a success rate of 80% and a recent prospective study has
shown that the majority of patients consider the surgery worthwhile. Surgery
should therefore be offered to all patients with LN injury who show few signs of
spontaneous recovery.
