Incisor Tipping Moment Effects on the EMG-Bite Force Relationship

Predetermined masticatory muscle forces have been used to simplify calculations for computer modeling of human jaw mechanics. Force magnitudes have been based on an assumed relationship between EMG and bite force. Recent work suggests that the EMG-bite force relationship varies depending on the tooth tipping moment produced by angled bite forces on mandibular first molars. Objective: To investigate the EMG-bite force relationship for a range of incisor tipping moments. Methods: Seven healthy male volunteers each produced unilateral static bites on a 5 mm steel ball positioned between custom acrylic occlusal copings affixed to mandibular central and lateral incisisors and to opposing maxillary teeth. Bite force magnitudes were recroded using electrically resistive film (TekScan ForceFlex®). Bite positions were systematically changed to produce a range of tipping moments on the mandibular incisors from buccal (+) to lingual (-). EMG activities were recorded bilaterally using surface (temporalis and masseter muscles) and indwelling (lateral pterygoid muscles) electrodes. Positions of the indwelling electrodes were verified using CT scans. EMG activity (RMS) versus bite force (N) regression slopes were calculated for each muscle and tipping moment. ANOVA was used to determine effects of biting side, biting position on the EMG-Bite Force slope for each muscle. Levels of significance were set conservatively at p < 0.01. Results: For each subject, muscle, and tipping moment, the EMG-bite force relation was linear (R²  0.75). For the same individual and muscle, slopes for different tipping moments varied up to 10:1. ANOVA results showed that slopes for + versus – tipping moments were significantly different (p < 0.001) for all muscles. Conclusions: EMG-bite force relationships were linear and slopes varied depending on the direction (+,-) of the tipping moment on the incisors. [This project was funded in part by a UNMC College of Dentistry Summer Studentship, and D.H. Reinhardt Scholar Fund].