Orthodontic Tooth Movement Influences Face Mechanical Sensitivity and Motor-Cortex

Objective: To determine if altering the rat's dental occlusion by an orthodontic appliance produces mechanical hypersensitivity and influences the intracortical micro-stimulation (ICMS)-defined face primary motor cortex (face-M1). 

Method: Sprague-Dawley rats had a closed-coil spring placed to mesialize the right maxillary molars. Electromyographic (EMG) wires were implanted in the anterior digastric (AD-jaw-opening), masseter (MAS-jaw-closing), genioglossus (GG-tongue-protrusion), and buccinator (BUC-cheek) muscles. ICMS (20, 40, 60µA) was applied in microelectrode transdural tracks to define the right and left face-M1 in different groups of rats on days 1 (n=5), 7 (n=6), and 28 (n=8) subsequent to spring attachment. Sham groups with springs with no activation for days 1 (n=5), 7 (n=5), and 28 (n=6) served as controls. The evoked EMG activities of the muscles were analyzed by a customized program developed in LabVIEW. A similar set of experimental and control groups were tested for thresholds of mechanical sensitivity to facial mechanical stimuli applied by von Frey monofilaments. Group data were compared using one-way ANOVA and post-hoc Tukey test.

Result: A significant reduction in the number of M1 sites from which AD and/or GG activity could be evoked occurred in the day 1 experimental group (67.6 ± 12.8) compared to sham (127.6 ± 10.5; p<0.001). The number of sites increased in the day 7 experimental group (135.2 ± 12.9) compared to sham (94.2 ± 6.5); p<0.001), and decreased in the day 28 experimental group (86.4 ± 13.6) compared to sham (127 ± 5.4; p<0.001). A significant decrease in mechanical thresholds was observed on day 1 and was maintained until day 5, but only in the experimental groups.

Conclusion: Orthodontic tooth movement is associated with short-lasting changes in facial mechanical sensitivity but longer-lasting neuroplastic changes in face-M1 that may reflect the initial mechanical hypersensitivity and subsequently the changes in the dental occlusion.