Dental nerve injury: will the numbness go, and when do you see a specialist?

The inferior alveolar nerve runs through a narrow canal inside the bone and carries sensation to the lower teeth, the lower lip and the point of the chin. The lingual nerve, by contrast, does not run inside bone but under the soft tissue on the inner side of the jaw, and it carries both touch and taste from one half of the tongue. Neither is a nerve that works muscles: when they are injured, sensation is lost but you can still move your lip, smile and speak. That is the answer to the fear of paralysis that patients think of first.

Four terms that appear in reports have separate meanings, and which one is written down changes the follow-up. Hypoaesthesia is reduced sensation; anaesthesia is a complete loss of sensation. Paraesthesia is an abnormal sensation such as tingling that does not hurt, while dysaesthesia is an abnormal sensation that is unpleasant, burning or painful. Dysaesthesia is managed differently from the others, so when you describe what you feel, do not stop at "numb"; say whether it is painful, whether it tingles, or whether there is no feeling at all.

The risk varies with the procedure, and the reported ranges are wide. In the surgical removal of a lower wisdom tooth, involvement of the inferior alveolar nerve is reported in studies at between 0.4% and 13.4%, with the part that becomes permanent between 0% and 1.6%. For the lingual nerve the figures run from 0% to 11%, with the permanent part around 0.5% to 0.6%. One review that pooled the published averages puts nerve involvement after lower wisdom tooth removal at about 1%.

In implant placement, temporary involvement is reported in the range of 0% to 24% and permanent involvement in the range of 0% to 11%. There are two mechanisms: the implant or the drill entering the canal directly, and damage caused by the heat of drilling. In root canal treatment the causes are over-instrumentation and the chemicals used reaching the canal. Involvement caused by the anaesthetic injection is rare, and in that group 85% to 94% of patients recover on their own. The ranges are this wide because the studies differ in how they measure, in the position of the tooth and in the experience of the surgeon; so rather than quoting a single percentage, it is more useful to look at your own X-ray.

There are seven signs on a panoramic X-ray from which the risk is read. According to Rood and Shehab, who brought them together in a review, four are seen at the root of the tooth: darkening of the root, deflection of the root, narrowing of the root and a forked root tip. Three are seen in the canal: diversion of the canal, narrowing of the canal and interruption of the white line that surrounds it. If one or more of these signs is present, a close relationship between the tooth and the canal is assumed and the surgery is planned accordingly.

This is why not everyone has a CBCT scan. The usual approach in the field is to move to a three-dimensional image when one of these signs is seen on the panoramic X-ray. On a CBCT scan, the absence of the hard bony border around the canal shows that the root and the canal are in contact. Contact on its own does not amount to an injury, but it does mean that the surgical technique and the options have to be discussed from the outset. A "there is no risk" given without seeing your X-ray is not an assessment.

In implant planning, what a CBCT scan gives you is a measurement in millimetres. A systematic review of the subject shows that once the distance between the implant and the canal falls below one millimetre, the rate of altered sensation rises markedly, while above one millimetre no such disturbance was found in the cases examined. That is the review's own conclusion as well: instead of the traditional two-millimetre safety margin, one millimetre may also be safe. Those few millimetres are why the planning is done beforehand and not on the operating table.

There is a three-level distinction used for the severity of the injury. At the mildest level the structure of the nerve is intact and only its conduction has stopped temporarily, and full recovery is expected. At the middle level the nerve fibres are damaged but the sheath is whole; recovery takes months and may not be complete. At the most severe level the nerve has been cut right through and spontaneous recovery is unlikely. Which level you are at cannot be known for certain in the first few days; the distinction is made by looking at the recovery curve over time.

That is why the timeline matters. Patients who show no improvement at all by the end of three months are recommended for referral to a specialist; if surgical repair is to be considered, the best window is taken to be between three and six months after the injury, and in injuries older than nine months the results are markedly worse. For implant-related pictures a separate window is reported: in one case series, sensation recovered in patients whose implant, thought to be pressing on the nerve, was removed within the first three months, while the same recovery was not seen where removal was carried out after four months. Because it rests on a single series, it should not be read as a firm threshold.

The distinction below is not a diagnosis; it is a framework that helps you read which way the process is going. A definitive assessment becomes clear after an examination and imaging.

The sequence below describes the steps followed when a loss of sensation is reported. The first place to go is the dentist who carried out the procedure; this page does not take the place of an examination.

1. 1

   The history: which procedure, when, which area

   The type and date of the procedure, when the numbness was noticed, which area it covers and whether it is one-sided are all recorded. If there was an electric-shock-like sensation during the anaesthetic injection, that should be written down too.

2. 2

   Mapping the affected area

   The border of the area where sensation is reduced is marked on the face, measured and photographed. The same map is drawn at later check-ups; the area getting smaller is the most concrete indicator of recovery. For your own records, ask for the same method at every check-up.

3. 3

   Sensory tests

   Light touch is tested with the tip of a cotton bud or a fine filament, sharp sensation with the point of a needle, and the difference between hot and cold separately. In the two-point discrimination test, the smallest distance between two points that can still be told apart is measured; in the face, about five millimetres is taken as normal. All the tests are done by comparison with the other side.

4. 4

   Rereading the imaging

   The panoramic X-ray and, where there is one, the CBCT scan are reassessed: the relationship between the root and the canal, the distance between the implant and the canal, and, if root canal treatment was carried out, whether material has been pushed into the canal. This step makes it clear which procedure the cause is related to.

5. 5

   The timeline and the referral threshold are given in writing

   How often you will be reassessed, what you should be watching for and at what point you will be referred to a specialist should all be handed to you in writing. If there is no improvement by the end of three months, a referral comes onto the agenda; timelines given by word of mouth get forgotten.

These do not take each other's place; which one is right depends on the cause, on how long it has been and on the findings.

Knowing the following from the outset sets both your expectations and the speed at which you decide.

The course below is a general framework. It does not move at the same speed in everyone, and it varies with the severity of the injury.

1. The first days and weeks

   In the mildest picture, conduction has stopped temporarily and recovery begins within days to a few weeks. What needs doing in this period is to report it, map it and protect the tissue.

2. Two to six months

   In moderate pictures, recovery usually moves along within this range. The affected area getting smaller and the tingling increasing are good signs.

3. Three months: the decision point

   If there is no improvement at all by the end of this time, a specialist assessment is recommended. Carrying on waiting narrows the best window for surgical repair.

4. Six to nine months

   The best window for surgical repair is taken to be between three and six months; in injuries older than nine months the results are markedly worse.

5. A year and beyond

   At this point the picture is generally taken to have settled. Approaches that make living with the remaining loss of sensation easier, and separate treatment options for the painful type, are discussed.

Under this heading, what determines the cost is less the treatment itself than how far the assessment and the follow-up go. These are the items that determine it:

How far the imaging goes

Making do with a panoramic X-ray and taking a CBCT scan are different items. In surgery close to the nerve, a three-dimensional image is part of the planning.

How often the follow-up is, and how long it lasts

Repeating the sensory examination at set intervals can call for appointments more often than the normal check-up schedule.

Referral to a specialist

In a picture that has gone past three months without improving, a specialist assessment and, where needed, surgical repair are a separate process.

The surgical technique chosen

Between full removal of a wisdom tooth and the technique in which only the crown is taken, there is a difference in both the procedure and the follow-up; with the second, the possibility of a further procedure is also taken into account.