Orthodontic tooth movement is associated with orofacial mechanical and thermal hypersensitivities and face sensorimotor cortex neuroplasticity

This study addressed the General Hypothesis that orthodontic tooth movement (OTM) is associated with neuroplasticity of the rat’s primary face motor cortex (face-M1) and somatosensory cortex (face-S1) motor representations and with mechanical and thermal hypersensitivities in the orofacial region. Objective 1 was to develop an OTM rat model, and design and manufacture a cephalostat for standardized radiographic measurements of the amount and the rate of OTM. A constant force of 10 ± 4 cN applied for 28 days produced a uniform rate of OTM of the right three maxillary molars and maxillary incisors of 1.75 ± 0.23 mm. The well-defined force parameters correlated with clinical observations of OTM in humans. Objective 2 was to use intracortical microstimulation (ICMS) and electromyographic (EMG) recordings to test if neuroplastic changes occur in the ICMS-defined motor representations of anterior digastric (LAD, RAD), masseter (LMa, RMa), buccinator (LBu, RBu), and genioglossus (GG) muscles within the rat’s face-M1 and face-S1 during OTM; the analyses to include any alterations in the number of ICMS sites representing these muscles and in the onset latencies of ICMS-evoked responses in the muscles. OTM resulted in significant changes in LAD, RAD, and GG motor representations in face-M1 and face-S1 at days 1, 7, and 28; there were no significant difference in the onset latency for these muscles across the groups, and between face-M1 and face S1 within each group. Objective 3 was to test if orofacial mechanical and thermal hypersensitivities occur in rats during OTM. The mechanical and thermal sensitivities at ipsilateral and contralateral orofacial sites were significantly increased in the early postoperative period (1 – 5 days), with peaks reached on day 1, and then returned to preoperative levels until postoperative day 28. These findings suggest that the cortical neuroplastic changes are unlikely related to maintained OTM-induced pain, although pain may have contributed to the changes in the early postoperative period. Thus, this new OTM rat model was successfully used for neurophysiological and behavioural studies that revealed the relevance of cortical neuroplasticity to orthodontic therapy and concepts of orofacial growth and development.Ph