Several studies have reported the possibility that the teeth of those animals investigated may contain low-threshold mechanoreceptors. The central destination of impulses was unknown, although some of the cell bodies may be in the mesencephalic nucleus. Painless electrical stimulation of human incisor teeth elicited inhibitory effects in the masseter muscle. This again suggests that tooth pulps may contain sensory receptors not involved with the sensation of pain.

Individuals are able to increase their maximum bite force if their incisors are covered with an acrylic cap. We explained this result by concluding that the pulps of the teeth contain high-threshold mechanoreceptors. These receptors may subconsciously protect a tooth from potentially damaging bite forces by monitoring the stress (force per unit area) on the dentine. When the tooth is capped, the bite force is spread over a larger area instead of being concentrated at its incisal edge. This reduces the stress on the dentine, which in turn permits a larger bite force.

The hypothesis that tooth pulps contain mechanoreceptors is supported by the following observation. Individuals can detect whether their incisors are biting on hard acrylic or soft rubber when the input to all receptors, except those assumed to exist inside the tooth pulps, remains unchanged. We concluded that a person (consciously) detects the reduction in the stress on a tooth when biting with the same force into soft rubber as opposed to hard acrylic. The incisal edge sinks into the rubber. This spreads the load over the tooth crown and, for the same bite force, reduces the stress on the compressed dentine.

If the above conclusions are correct, individuals should be able to develop a larger maximum bite force when biting on a softer surface because the load is spread over a larger area and the dentine is therefore less stressed. But if the tooth is covered with a crown the load is already spread and there should be little or no difference in the maximum bite force whether biting on a hard or a soft surface. We here test these predictions.

A 2 mm thick piece of tough rubber was glued over the end of one side of a 1.5 cm wide, thin metal spatula. A 2 mm thick piece of acrylic (Dura Lay; Dental Mfg, IL, U.S.A.) was glued over the other side. The sandwich was wrapped in masking tape to give all surfaces the same appearance and texture. Participants were asked to bite as hard as possible on the sandwich. By rotating the sandwich they could be made to bite with their upper incisors on either the hard acrylic or the soft rubber surface without being aware of any change in the experimental procedure. They would only be able to detect a difference by means of receptor mechanisms associated with either the jaws and/or the teeth.

Fig. 1. Diagrams showing experimental set-up. (A) The bite block: the acrylic and the rubber strips are glued to opposite sides of the metal spatula creating a bite-block sandwich; the sandwich is wrapped with masking tape. (B) The U-shaped bite-force transducer with the bite block symmetrically placed on top. The transducer is fitted over the lower incisors with an acrylic overlay. Note the upper and lower teeth during biting are edge-to-edge. Inset shows the transducer with the transverse acrylic wedge on top. The edge of the wedge is shaped like that of the upper incisor. (C) A lateral view of the transducer in situ. The free end of the spatula is lightly held by the participant.

The study was approved by the human ethics committee. A total of 15 university students aged 21–36 years, with no existing tooth and jaw-muscle pain, took part in each of two experiments. Having given informed consent, alginate impressions were taken and plaster models of the upper and lower jaws were made for each participant. A U-shaped transducer was fitted on to the cast of the lower incisors by means of an acrylic overlay. The transducer was positioned over the lower incisors with its centre between the upper incisors and its horizontal platform as close as possible parallel to the lower occlusal plane. The directions of bite forces recorded from the transducer were therefore the same as those with respect to the lower occlusal plane. A wedge made from Dura Lay and shaped like the upper incisal edge was cemented to the centre of the upper surface of the transducer. This ensured that the bite force from the upper incisor, which was transferred to the tip of the wedge, was directed down the centre of the transducer in the frontal plane. Lines drawn transversely across the centre of both acrylic and rubber sides of the sandwich helped position the lower jaw so that the upper incisor maintained a constant anteroposterior relation to the lower jaw.

The transducer was fitted over a participant’s lower incisors. Next, the bite sandwich was placed symmetrically in position with the line marked on either the hard or soft surface aligned with the line marked on the centre of the acrylic wedge on the transducer. Participants were guided to move their lower jaw until the transverse line marked on the upper side of the sandwich made contact with the edges of the upper incisors. They stabilized the block by lightly holding its handle. When they felt comfortable they were directed to bite rapidly as hard as possible. The direction and magnitude of the maximum bite forces were measured with the transducer.

During earlier studies, several participants complained of slight discomfort after they had exerted a maximum bite force on the transducer more than about 12 times. We therefore set a limit of 12 on the number of maxima for a single recording session. Unfortunately, some measurements had to be rejected because participants, for reasons unknown, occasionally used an unusual bite direction. A change in the direction of the bite force can greatly affect the maximum because it changes the moment arms and mechanical advantage of the system. In order to standardize the procedure, we therefore collected measurements of the first six maximum bite forces whose associated directions clustered within a cone of 4°, the resolution of the transducer obtained during calibration. As far as possible the sequence of hard and soft sides was randomly chosen, but towards the end of an experiment we arranged things so that, ultimately, we obtained three maximum bite forces with upper incisors biting on the soft side of the bite block and three biting on its hard side.

On a separate day, in order to limit the number of maxima produced in a single session, the whole experiment was repeated with the difference that the upper incisors were covered with a full acrylic crown (made from Ortho-Resin; Caulk, DL, U.S.A.) about 1 mm thick. Only 12 of the original 15 participants were available for this second experiment. Three new participants volunteered.

The percentage change in maximum bite force when biting on (soft) rubber or (hard) acrylic was calculated in both bare-tooth and capped-tooth experiments. The ?² goodness-of-fit method was used to test if the percentage changes were normally distributed. A paired t-test for statistical differences was applied if the distribution was normal. Possible differences in bite directions between biting on acrylic and rubber were tested by the Watson–Williams test for directional data.

Bite directions ranged from 14° forward to 16° backward and from 12° right to 11° left in the frontal plane. The change of biting surface from rubber to acrylic did not cause a significant change in the bite direction used by any participant (p>0.05; Watson–Williams test).