Table 1 Thornhill grading: grading of strength of association between mucosal lesions and amalgam/metal
Type I | No association | No lesions in direct contact with amalgam restorations. Lesions have typical bilaterally symmetrical distribution of oral lichen planus or are restricted to areas of the palate, gingivae, or outer surface of the lip that do not come into contact with the teeth |
Type II | Weak association | Some amalgam restorations in contact with affected areas of mucosa. However, < 25% of affected mucosa in direct contact with amalgam restorations. Some amalgam restorations may also be in direct contact with unaffected areas of oral mucosa |
Type III | Strong association |  > 75% of affected mucosa in direct contact with amalgam restorations. No amalgam restorations in direct contact with unaffected areas of oral mucosa |
Type IV | Very strong association | Lesions restricted to but affecting all areas of mucosa in direct contact with amalgam restorations |