Assessment of the Maximum Amount of Orthodontic Force for Dental Pulp and Apical Neuro-Vascular Bundle in Intact and Reduced Periodontium on Bicuspids (Part II)

Abstract

This study examines 0.6 N–4.8 N as the maximum orthodontic force to be applied to dental pulp and apical NVB on intact and 1–8 mm reduced periodontal-ligament (PDL), in connection with movement and ischemic, necrotic and resorptive risk. In addition, it examines whether the Tresca finite-element-analysis (FEA) criterion is more adequate for the examination of dental pulp and its apical NVB. Eighty-one (nine patients, with nine models for each patient) anatomically correct models of the periodontium, with the second lower-premolar reconstructed with its apical NVB and dental pulp were assembled, based on X-ray CBCT (cone-beam-computed-tomography) examinations and subjected to 0.6 N, 1.2 N, 2.4 N and 4.8 N of intrusion, extrusion, translation, rotation, and tipping. The Tresca failure criterion was applied, and the shear stress was assessed. Forces of 0.6 N, 1.2 N, and 2.4 N had negligible effects on apical NVB and dental pulp up to 8 mm of periodontal breakdown. A force of 4.8 N was safely applied to apical NVB on the intact periodontium only. Rotation and tipping seemed to be the most invasive movements for the apical NVB. For the dental pulp, only the translation and rotation movements seemed to display a particular risk of ischemia, necrosis, and internal orthodontic-resorption for both coronal (0–8 mm of loss) and radicular pulp (4–8 mm of loss), despite the amount of stress being lower than the MHP. The Tresca failure criterion seems more suitable than other criteria for apical NVB and dental pulp.

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].…”

“…Dental pulp and neuro-vascular bundle (NVB) are ductile-like materials [2], being highly vascularized tissues, with NVB being more sensitive to circulatory disturbances due to its topographical position in the apical third PDL [2,5].…”

“…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 .…”