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 teeth without periodontal assistance was tested here. A bite-force transducer was sandwiched between a hard acrylic strip on one side and rubber on the other side, both sides being covered with masking tape to prevent 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. Fifteen participants each undertook three separate experiments each involving 10 tests using (1) bare incisors, (2) incisors capped with acrylic and (3) bare molars. The accuracy of responses and the direction and magnitude of bite forces were recorded. Participants most correctly detected the surfaces with bare incisors (91% correct, SD=11%). Performance was significantly worse with capped incisors (79%, SD=19%) (p<0.01) and worse still with molars (57%, SD=24%) (p<0.001). The detection threshold using bare incisors was increased in 14 out of the 15 participants when the incisors were capped, but the increase was statistically significant (0.002<p<0.01) in only six of them. The molar threshold was significantly increased (0.002<p<0.01) in comparison 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 upper and lower periodontal input was the same whether the rubber was on the top or on the bottom, it was concluded that the periodontal ligament was not involved. The observed discrimination was probably based on a difference in the pressure on the upper and lower teeth. Incisal edges sank more deeply into the rubber and reduced the pressure (force/unit area) on a tooth crown.