Wisdom tooth extraction under local anaesthetic is far less likely to result in damage, the Welsh study says, adding that general anaesthesia should only be used in the case of "the most serious surgical complications."

But dental experts are still puzzled as to why an unconscious patient should fare worse.

Dentists have already been told not to perform operations under general anaesthetic unless there is a specialist anaesthetist present an there is immediate access to emergency care facilities.

But the new study suggests, even under these circumstances, a general anaesthetic is likely to be inappropriate for wisdom tooth removal.

The research at Cardiff Dental School, published in the British Dental Journal, has found that patients opting for a general anaesthetic are six times more likely to sustain nerve damage than those choosing simply to stay awake during the operation.

The findings reveal that almost all patients suffered pain, swelling or difficulty opening their mouths.

Around one in 10 initially had temporary numbness of lip, tongue or cheek, but for one percent of patients, this was still the case six months later.

Three quarters of the 367 patients studied had chosen general anaesthetic.

Overall, 18 per cent of those who had general anaesthetics were damaged, compared to only three per cent of those on local anaesthesia.

Discouraging general anaesthesia

The dental school is now persuading patients having wisdom teeth extracted to have a local anaesthetic.

The researchers could find no link between the difficulty of the surgery and the subsequent damage.

But they suggested that the more horizontal position of patients under general anaesthetic may be to blame, or possibly that dentists may be able to use more of their strength on an unconscious patient.

The report says: "It is possible that the degree of surgical force is greater under general anaesthetic, and that a conscious patient, whether sedated or not, presents the surgeon with a series of cues which tends to limit soft tissue retraction and surgical force, and therefore the risk of nerve damage.

"It is unlikely that the actual anaesthetic itself had an effect on the incidence of nerve damage."