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Changing oral surgery

The use of randomised trials in surgery is becoming more common to demonstrate the effectiveness (or otherwise) of new surgical techniques. What is uncommon is a randomised trial to elucidate problems with operative methods that may give rise to uncommon but serious adverse events. We are grateful, therefore, to Derek Richards of Berkshire Health for making us aware of a recent study [1] from Sheffield.

Lower third molar removal

This is one of the commonest UK oral surgery procedures. It is not without morbidity. Apart from pain, swelling and the like, possible nerve damage is a particular concern.

Several nerves may be affected temporarily, and sometimes permanently. The rate of nerve damage is low, but for one nerve there is a great deal of variability in reported problems. One of the nerves to the tongue is reported to be subject to temporary damage in 0% to 25% of operations in 18 reports [1], and permanent damage (undefined) in 0% to 2% of operations.

One particular operative technique using a tool called a Howarth's elevator is intended to protect the lingual tissues and nerves in lower third molar removal, but there was concern that it might be responsible for higher rates of lingual nerve injury.

Randomised Trial

Robinson & Smith from the School of Clinical Dentistry at Sheffield carried out a randomised trial of operations with and without the Howarth's elevator to test its association with lingual nerve damage. Patients were allocated randomly to either operation.

They were seen at one week, and questioned about any subjective alteration in sensation from the lower lip or tongue. If any alteration was reported, patients were seen at a special clinic where sensory testing was carried out. If no recovery of sensation occurred by 4 months, the lingual nerve was explored surgically, and repaired if necessary.

Results

There were 771 operations, of which 378 used a Howarth's elevator. In the 393 where Howarth's was avoided, there were four operations in which the operator felt that visibility and access was inadequate and the Howarth's elevator was needed. The two groups were substantially similar.

The incidence of nerve damage in each operation type is shown in the table.

N erve damage
No Howarth's elevator used
Howarth's elevator used
Odds ratio
(95% CI)
NNT
(95% CI)
Inferior or alveolar sensory disturbance

17

12

1.5 (0.7 - 3.1)

100 (24 - [infinity])
Temporary lingual nerve damage

3

26

5.4 (2.6 - 11)

17 (11 - 29)
Permanent lingual nerve damage

1

3

2.8 (0.4 - 20)

100 (64 - [infinity])
Sensory disturbance from inferior or alveolar nerves was not different between the two operation types. The incidence of lingual nerve disturbance was much higher in the operations using the Howarth's elevator.

There were 26 affected persons compared with three in the control group; a significantly greater incidence. The NNT was 17, meaning that for every 17 patients who have a third molar removal with a Howarth's elevator, one will have temporary lingual nerve damage who would not have if the Howarth's elevator not been used.

Of the 26, 23 had progressive recovery over seven months. Three had permanent damage with complete anaesthesia and no evidence of recovery by four months. Exploratory surgery showed partial (one case) or complete (two cases) division of the nerve. The one case of complete anaesthesia in the non-Howarth's group had complete nerve division. There was no significant difference between permanent damage rates between the operative groups. The point estimate for the NNT was 100 and the low estimate of the 95% confidence interval was 64.

Comment

Adverse events - and especially those that are uncommon but serious - are difficult to handle. Randomised trials which explore the problem are uncommon. This study showed an association between the use of an operative tool - the Howarth's elevator - and temporary damage to the lingual nerve.

How do we interpret these data? With caution: not because the study was flawed, but because different rules apply when we look at information about adverse events. We have a definite association with temporary damage to the lingual nerve - and temporary damage is likely to prove permanent in some fixed but indeterminate proportion of patients. Our best estimate at the moment is that in 3 of 26 (or 1 in 9) patients temporary nerve damage will turn out to be permanent. The rate of temporary nerve damage is eight times higher using a Howarth's elevator.

The other issue is one of significance. Do we care about formal statistical testing here for permanent nerve damage? Is the formal 95% confidence interval too conservative when looking at damage? Is the key figure, the one which puts us on our guard, the low estimate of 64 for the NNT for permanent nerve damage? That could be the correct figure.

The safe conclusion, as Robinson & Smith suggest, is that the use of a Howarth's elevator is contraindicated, and for the majority of cases lingual retraction should be avoided.

Reference:

  1. PP Robinson, KG Smith. Lingual nerve damage during lower third molar removal: a comparison of two surgical methods. British Dental Journal 1996 180: 456-61.



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