Sir, - Your correspondent Eoin Licken (June 1st) quotes Dublin dentist Nick Armstrong as questioning the association of lichen planus with amalgam. Lichen planus is a benign disease of skin and mouth which has been well known since before the widespread use of amalgam fillings in dentistry. The oral form affects between ten and 20 people per 1,000 in European populations. It tends to occur in the sixth decade of life whereas amalgam restorations are usually first placed in the first or second decade.
A significant proportion of cases are caused by prescribed medication, mostly for high blood pressure, arthritis and diabetes. In a small proportion of cases, removal of amalgam fillings leads to healing of the oral disease. Mercury is the prime suspect; although the precise mechanism is not understood there is some evidence for an allergic basis. Allergy to mercury and its compounds is not particularly uncommon; lichen planus patients with positive patch tests to mercury may or may not benefit from removal of amalgams.
Lichen planus of the skin was once thought to be premalignant; this is now known to be false. There is still some debate on the malignant potential of oral lichen planus; it may become cancerous at a rate of about one case in 1,000 per year but there are those, including myself, who find the evidence less than convincing.
If the one per cent of the population with oral lichen planus are getting oral cancers at the rate of 0.1 per cent per year, then we would have to assume that at least half of all mouth cancers in Ireland arise in oral lichen planus. This is not our experience; in 25 years of dealing with both mouth cancer and oral lichen planus in Dublin, I have seen only two cases where both conditions were present in the one patient.
We can assume, therefore, that mercury causes oral lichen planus in only a very small proportion of those with amalgam fillings and that the risk of subsequent cancer in those patients is extremely small, if present at all. This is consistent with the lack of evidence that mercury is a carcinogen.
Any patients with amalgam fillings who have been diagnosed with oral lichen planus should not be concerned nor should they rush to have their fillings replaced. There is a simple test which can be applied to establish a possible relationship with the fillings: the lesions should be present anywhere the lining of the mouth tends to lie against the amalgam fillings and should be absent from all other parts of the mouth. A dentist will be able to ascertain this and will be able to advise on removal. - Yours etc., Bernard McCartan
Consultant in Oral Medicine, Dublin Dental Hospital and Hume Street Hospital, Dublin 2.