Comparative Assessment of Three Microimplant Assisted Biomechanical Strategies for the Simultaneous Intrusion and Retraction of Anterior Teeth in Relation to Alveolar Bone Stress and Change in Incisor Inclinations – A 3D Finite Element Analysis Study

Abstract

Microimplants are widely used to provide absolute anchorage in cases with bimaxillary dentoalveolar protrusion especially in those that require simultaneous retraction and intrusion of anterior teeth. The position of the microimplant significantly affects the build up of stress in the alveolar bone as well as the incisor inclinations, and is therefore a critical factor in treatment plannning. Keeping this in mind, this finite element method(FEM) study was taken up to identify the most suitable combination of implant placement sites for cases in which en-masse anterior retraction is done along with intrusion. The geometric model was constructed from a CBCT scan of the maxilla of an adult patient with full permanent dentition.The images were saved as DICOM files and were later exported to the 3D image processing software (Mimics,version 17). The center of resistance for the 6 anterior teeth was 9 mm superiorly and 13.5 mm posteriorly from the midpoint of crown tip of central incisors. The working archwires were assumed to be 0.019 / 0.025-in stainless steel. The three mini-implant placement sites compared were – S1- Midline micro implant between the maxillary central incisors with two placed posteriorly between maxillary 2nd premolar 1st molar roots. S2 – Micro implant placed between the lateral incisor and canine along with posterior mini implants as above. S3- Micro implant placed high up between the maxillary second premolar and first molar roots. The amount of tooth displacement after finite element analysis was compared with central and lateral incisor and canine axis graphs. For the system S1, intrusive components were seen on the archwire mainly in the anterior region with maximum displacement between central incisors and gradually decreasing away from point of force application. In S2, the intrusive component of force was more evenly distributed. In S3, pattern of intrusive component was similar to S2 but the maximum displacement was slightly lower. Greatest value of minimal principal stress was seen on cervical and apical third of central incisors as well as apical third of lateral incisors in S1; and cervical third of lateral incisors and apical and cervical third of canines in S2. Maximum retraction of anterior teeth was seen in S3. In all of the three systems of force application, tooth inclinations were maintained. Maxillary anterior teeth showed more tendency towards retraction in the case where two micro implants were placed posteriorly high up above the roots of maxillary premolars and molars such that the force is directed diagonally having both horizontal and vertical components, and hence eliminating the need for anterior implants. Greater intrusion tendency was seen when implants were placed between the roots of maxillary central incisors.