Changes in neuromuscular reflexes in the masseter muscles during functional jaw orthopedic treatment in children

Masseteric reflexes following standardized taps on an upper incisor were recorded electromyographically (1) in children with Class II, Division 1 malocclusions at different times before and during the treatment with the Bionator functional appliance and (2) during an observation period in control subjects who were not being treated. In each experimental session, 72 taps were delivered during clenching at 10% of the maximal electromyographic output. Statistical criteria provided by a previously developed computer program allowed the objective determination of inhibitory and excitatory periods in the electromyographic output following the stimulus, which together constitute the poststimulus electromyographic complex (PSEC). The influence of the change in occlusal relations on the PSEC was determined in ten subjects by comparing pretreatment results with those obtained after 1, 3, and 6 months of treatment. Concurrent with an improvement of the sagittal jaw relation in the patients with a pronounced disharmony, a short latency excitatory peak appeared in their PSECs. It was demonstrated by averaging nonrectified electromyographic activity that this peak was brought about through synchronization of motor units. In the control group, however, the morphology of the PSEC was reproducible over time. Successful functional therapy is thus accompanied by specific transient changes in the reflex response of the masseter muscles to standardized tooth taps. The neural pathways contributing to these observations originate in periodontal and muscular receptors, and might provide a clue for the mechanism underlying successful functional treatment.