Evaluation of pain in rats through facial expression following experimental tooth movement

This study was carried out to evaluate pain in rats by monitoring their facial expressions following experimental tooth movement. Male Sprague‐Dawley rats were divided into the following five groups based on the magnitude of orthodontic force applied and administration of analgesics: control; 20 g; 40 g; 80 g; and morphine + 40 g. Closed‐coil springs were used to mimic orthodontic forces. The facial expressions of each rat were videotaped, and the resulting rat grimace scale (RGS) coding was employed for pain quantification. The RGS score increased on day 1 but showed no significant change thereafter in the control and 20‐g groups. In the 40‐ and 80‐g groups, the RGS scores increased on day 1, peaked on day 3, and started to decrease on day 5. At 14 d, the RGS scores were similar in control and 20‐, 40‐, and 80‐g groups and did not return to baseline. The RGS scores in the morphine + 40‐g group were significantly lower than those in the control group. Our results reveal that coding of facial expression is a valid method for evaluation of pain in rats following experimental tooth movement. Inactivated springs (no force) still cause discomfort and result in an increase in the RGS. The threshold force magnitude required to evoke orthodontic pain in rats is between 20 and 40 g.