Assessment of the Maximum Amount of Orthodontic Force for PDL in Intact and Reduced Periodontium (Part I)

Abstract

This study examines 0.6 N and 1.2 N as the maximum orthodontic force for periodontal ligament (PDL) at multiple levels of periodontal breakdown, and the relationships with the ischemic, necrotic, and resorptive risks. Additionally, this study evaluates if Tresca failure criteria is more adequate for the PDL study. Eighty-one 3D models (from nine patients; nine models/patients) with the 2nd lower premolar and different degrees of bone loss (0–8 mm) where subjected to intrusion, extrusion, rotation, translation, and tipping movements. Tresca shear stress was assessed individually for each movement and bone loss level. Rotation and translation produced the highest PDL stresses, while intrusion and extrusion determined the lowest. Apical and middle third PDL stresses were lower than the cervical stress. In intact periodontium, the amount of shear stress produced by the two investigated forces was lower than the 16 KPa of the maximum physiological hydrostatic pressure (MHP). In reduced periodontium (1–8 mm tissue loss), the apical amount of PDL shear stress was lower than MHP for both applied forces, while cervically for rotation, translation and tipping movements exceeded 16 KPa. Additionally, 1.2 N could be used in intact periodontium (i.e., without risks) and for the reduced periodontium only in the apical and middle third of PDL up to 8 mm of bone loss. However, for avoiding any resorptive risks, in the cervical third of PDL, the rotation, translation, and tipping movements require less than 0.2–0.4 N of force after 4 mm of loss. Tresca seems to be more adequate for the study of PDL than other criteria.

Investigating the Ability of the Tooth and Surrounding Support Tissues to Absorb and Dissipate Orthodontic Loads during Periodontal Breakdown—Finite Elements Analysis

“…In dental field, the numerical studies' results are regarded with care since they often contradicted clinical knowledge and displayed various results from one report to another . This issue was not addressed, except for our previous research [5,6,9,10,16,48].…”

“…Some of the dental tissular components are more sensitive to the orthodontic pressures due to a better circulatory vessel system (i.e., PDL, NVB, and dental pulp); thus, the physiological maximum hydrostatic pressure MHP of 16 KPa (approx. 80% of systolic pressure) is exceeded and both ischemic and resorptive risks are inevitable [5,6].…”

“…If the tooth's surrounding support system is intact, common orthodontic forces up to 1.2 N [4] are safely applied. However, when there are various levels of bone loss, due to mechanical changes, the stress absorption-dissipation ability changes, with higher ischemic and resorptive risks and altered treatment prognosis [5,6,10]. When analyzing small movements under light forces, the issue related to the loading conditions must be carefully addressed.…”

“…There is only one single available method for the study of stress distribution in dental tissues, the finite elements analysis (FEA), which individually investigates the stress distribution and biomechanical behavior of each dental component [5,6,10]. In dental field, the numerical studies' results are regarded with care since they often contradicted clinical knowledge and displayed various results from one report to another .…”

“…FEA accuracy depends on the selection of proper failure criteria which are suitable for the analyzed material, anatomically correct models, and proper boundary conditions. Our previous studies [5,6,9,10,16,48] reported that since dental tissues are ductile-like materials (with a certain brittle flow mode) only a failure criterion specially designed for ductile materials is suitable (i.e., Von Mises overall stress and Tresca shear stress). Moreover, for validation, the quantitative results must be correlated with MHP, while qualitative results with acknowledged clinical data.…”