Orthodontic Force Facilitates Cortical Responses to Periodontal Stimulation

Somatosensory information derived from the periodontal ligaments plays a critical role in identifying the strength and direction of occlusal force. The orthodontic force needed to move a tooth often causes uncomfortable sensations, including nociception around the tooth, and disturbs somatosensory information processing. However, it has mostly remained unknown whether orthodontic treatment modulates higher brain functions, especially cerebrocortical activity. To address this issue, we first elucidated the cortical region involved in sensory processing from the periodontal ligaments and then examined how experimental tooth movement (ETM) changes neural activity in these cortical regions. We performed in vivo optical imaging to identify the cortical responses evoked by electrical stimulation of the maxillary and mandibular incisor and the first molar periodontal ligaments in the rat. In naive rats, electrical stimulation of the mandibular periodontal ligaments initially evoked neural excitation in the rostroventral part of the primary somatosensory cortex (S1), the ventrocaudal part of the secondary somatosensory cortex (S2), and the insular oral region (IOR), whereas maxillary periodontal ligaments elicited excitation only in S2/IOR rostrodorsally adjacent to the mandibular periodontal ligament-responding region. In contrast, maximum responses to mandibular and maxillary periodontal stimulation were observed in S1 and S2/IOR, and the 2 responses nearly overlapped. One day after ETM (maxillary molar movement by Waldo's method), the maximum response to stimulation of the maxillary molar periodontal ligament induced larger and broader excitation in S2/IOR, although the initial responses were not affected. Taken together with the histologic findings of IL-1 beta expression and macrophage infiltration in the periodontal ligament of the ETM models, inflammation induced by ETM may play a role in the facilitation of S2/IOR activity. From the clinical viewpoints, the larger amplitude of cortical excitation may induce higher sensitivity to pain responding to nonnoxious stimuli, and enlargement of the responding area may reflect radiating pain.