[1] Univ Ferrara, Res Ctr Study Periodontal Dis, I-44100 Ferrara, Italy
[2] Univ Bern, Dept Periodontol & Fixed Prosthodont, CH-3012 Bern, Switzerland
[3] UCL, Eastman Dent Inst, Dept Periodontol, London WC1E 6BT, England
[4] UCL, Eastman Dent Inst, Dept Oral Pathol, London WC1E 6BT, England