Methods: Experiments were performed on six volunteers. EMG recordings were made from a masseter muscle. The subjects activated the muscle to approximately 12.5% of maximum with the help of visual feedback of the EMG. Reflexes were produced by applying gentle mechanical stimuli to an upper central incisor. The stimuli consisted of "ramp and hold" forces with a 5ms rise time and a 350ms duration; the forces plateaued at 0.25-4N. For each subject, the forces were applied in two positions – close to the incisal edge and close to the cervical margin. The resulting reflexes were recorded along with the actual forces applied and the resulting movements of the tooth crown.
Results: There was a linear relationship between force and movement regardless of whether the force was incisal or cervical (Pearson's r=0.91 and r=0.93 respectively). There was no difference between the slopes for these two relationships (ANCOVA p=0.42). There were also linear relationships between force or movement and the resulting inhibitory reflexes (r=0.84, 0.8, 0.8 and 0.64 for incisal and cervical forces and incisal and cervical movements respectively). The estimated thresholds for the reflexes were 0.33±0.1N (equivalent to 15±4 microns) and 0.46±0.1N (equivalent to 20±5 microns) for incisal and cervical stimuli respectively.
Conclusions: Interestingly neither the amounts of movement nor the resulting jaw reflexes were dependant on the position at which forces were applied to the tooth. These findings are not supportive of the notion that the tooth movement takes place around a fulcrum in response to the application of horizontal forces.