## 4 Discussion
However, due to the internal micro-architecture [1; 11; 13], the dentine component displayed an increase in variable percentage values which is correlated with the progression of periodontal breakdown (8.98-39.67% in 0 mm loss, 15.57-49.57% in 1 mm loss up to 58.35-92.9% in 8 mm loss). The comparison between stresses displayed in the tooth with bracket and tooth without bracket showed a limited absorption-dissipation ability of stainless-steel stress bracket (15.48-18.73% in 0 mm, 16.43-18.16% in 1 mm loss up to 4.02-16.35% in 8 mm loss) except for translation which displayed doubled values (probably due to the movement's biomechanical specificity). The enamel component (comparison between tooth without bracket and coronal dentine) displayed a higher absorption-dissipation ability (50.29-62.26% in 0 mm, 49.09-62.26% in 1 mm loss up to 38.53-65.44% in 8 mm loss) than bracket. There is little difference (i.e., % of stress) at root level between the three structures (tooth with bracket, tooth without bracket, and dentine), the main differences being visible only in coronal part (where the main absorption-dissipation seems to take place). Thus, the largest absorption-dissipation is performed by dentine (i.e., approx. 40-93%) and enamel (i.e., approx. 40-65%) components, while the stainless-steel bracket base has a limited ability (i.e., approx. 16%). Nevertheless, biomechanically, all three above components act similar to a single-stand structure that absorbs-dissipates most of the stresses produced by orthodontic loads, allowing for only a fraction of these stresses to be manifested in the circulatory sensitive tissues (i.e., approx. 0.3-8.4% in PDL, 0.2-0.7% in NVB, 0.02-0.12% in dental pulp, which are quantitatively under a physiological amount of 16 kPa of MHP). These agree with our previous reports about the absorption-dissipation ability found in PDL, dental pulp, NVB, tooth, and bone [5; 6; 9; 10; 16; 48]. Regarding the bone alveolar socket, only approx. 10-20% in intact periodontitis and 35-50% in 8 mm reduced periodontitis reached the bone cervical third (with reduced circulatory vessels) and exceeded MHP, but with smaller percentages for apical and middle third where the circulatory component is better represented. The above biomechanical behavioral data were found for all three orthodontic loads since they displayed similar qualitative results and increased quantitative values (doubling for 1.2 N and quadrupling for 2.4 N). A previous research [2; 3; 9; 10; 48] of our team (0.5-1.2 N of force, five movements, Tresca criteria, intact and reduced periodontitis) reported a tooth structure absorption-dissipation ability of approx. 85% of stresses before reaching circulatory sensitive tissues (i.e., 86.66-97.5% dissipation before reaching PDL, 98% before reaching NVB, and 99.6-99.94% before reaching pulp) similar to the research herein. Thus, the herein numerical simulation confirmed that 0.6-1.2 N are safe for application in intact periodontium, producing only minor limited ischemic and resorptive risks especially in the cervical third of PDL and bone alveolar socket, which is also in agreement with previous reports [5; 6; 9; 10; 16; 48]. Nevertheless, in reduced periodontium, the same amounts of force increased ischemic and resorptive risks for radicular dentine and PDL, which are strictly correlated with the periodontal breakdown process, as previously reported [5; 6; 9; 10; 16; 48]. This FEA simulation confirmed the importance of support tissues in biomechanical orthodontic behavior and the need to reduce orthodontic loads applied after 4 mm bone loss (to 0.2-0.6 N) to eliminate ischemic and resorptive risks, as previously recommended [5; 6; 9; 10; 16; 48]. Therefore, the maximum force safely applied in these tissues should be the one that is safer for the weakest component (i.e., PDL and NVB).