DISCRIMINATION OF HARDNESS BY HUMAN TEETH APPARENTLY NOT INVOLVING PERIODONTAL RECEPTORS

Periodontal receptors are generally thought to provide the input used to detect the hardness of food. Whether hardness can be sensed by teet h without periodontal assistance was tested here. A bite-force transdu cer was sandwiched between a hard acrylic strip on one side and rubber on the other side, both sides being covered with masking tape to prev ent participants from sensing a difference in touch. Participants were asked to increase the bite force on the sandwich until, and if, they could detect which material, hard acrylic or rubber, was on the upper side. The positions of acrylic and rubber were randomly chosen. Fiftee n participants each undertook three separate experiments each involvin g 10 tests using (1) bare incisors, (2) incisors capped with acrylic a nd (3) bare molars. The accuracy of responses and the direction and ma gnitude of bite forces were recorded. Participants most correctly dete cted the surfaces with bare incisors (91% correct, SD = 11%). Performa nce was significantly worse with capped incisors (79%, SD = 19%) (p < 0.01) and worse still with molars (57%, SD = 24%) (p < 0.001). The det ection threshold using bare incisors was increased in 14 out of the 15 participants when the incisors were capped, but the increase was stat istically significant (0.002 < p < 0.01) in only six of them. The mola r threshold was significantly increased (0.002 < p < 0.01) in comparis on with the bare incisor threshold in 12 of 15 participants. There was no correlation between bite direction and the position of the rubber. Because in each trial the tipper and lower periodontal input was the same whether the rubber was on the top or on the bottom, it was conclu ded that the periodontal ligament was not involved. The observed discr imination was probably based on a difference in the pressure on the up per and lower teeth. Incisal edges sank more deeply into the rubber an d reduced the pressure (force/unit area) on a tooth drown. (C) 1998 El sevier Science Ltd. All rights reserved.