Age-dependent saturation of the biological response to different orthodontic forces

In animal studies we have shown that the inflammatory response and rate of tooth movement triggered by orthodontic force reach a saturation point beyond which increased force produces no additional response. In addition, our previous clinical study found that adolescents and adults have different inflammatory response, tooth movement rates, and pain with increasing orthodontic forces. Based on these findings, our goal in this study was two-fold. First, to determine if there is a saturation point for the inflammatory response and tooth movement rates in orthodontics patients. Second, to determine if age impacts the saturation point.

We conducted a non-randomized, single-center, single-blinded clinical study on two groups of healthy subjects who required first premolar extraction and canine retraction to correct their malocclusion. Adolescents (11-14 years old) and adults (21-45 years old) of both sexes were recruited and randomized within their age group (n = 4 per force) to receive one of the following four magnitudes of force during canine retraction: 50, 100, 150, and 200 cN Gingival crevicular fluid (GCF) from the distolabial crevice of the canine was collected to assay inflammatory marker levels before treatment and one day after beginning retraction. IL-1β, CCL2, and RANKL proteins were quantified using a slide-based protein array assay. Study models were used to assess canine retraction after 28 days of force application. Differences within and between treatment groups were assessed statistically.

We found that inflammatory cytokine levels and the rate of tooth movement reached its maximum at 100 cN or 150 cN for adults and adolescents respectively. Adolescents had lower cytokine levels at lower magnitude forces than adults, but the levels increased at higher magnitude forces, with adults reaching saturation sooner than adolescents. The rate of canine movement in 28 days was not significantly different between adolescents and adults at any force level. However, the rate was higher in adolescents for all force groups.

As in our animal studies, humans demonstrate a saturation point for inflammatory responses and the rate of tooth movement to orthodontic force. Adults reached saturation of the biological response earlier, while younger patients had a higher saturation point. This study has important clinical significance as it may guide orthodontists toward optimizing orthodontic forces and treatment duration, while reducing the risk of pathology arising from excessive forces.