EFFECT OF CORTICAL BONE QUALITY AND QUANTITY ON FIVE MAXILLARY POSTERIOR INTRUSION MECHANICS USING MINISCREWS: A FINITE ELEMENT ANALYSIS STUDY

Abstract

Title: effect of cortical bone quality and quantity on five maxillary posterior intrusion mechanics using miniscrews: A FINITE ELEMENT ANALYSIS STUDY

Introduction: Orthodontic mini-implants (MI) have been used as an alternative modality to correct posteriorly extruded maxillary molars notably in anterior open bite cases, which constitute a challenging clinical situation for Orthodontists. The influence of cortical bone on the rate of tooth movement in intrusion has not been studied. Moreover, the definite mechanics involved in predictable outcomes should be explored.

Aims: Compare in a finite element analysis (FEA) the stresses and displacements generated on maxillary teeth through five intrusion modalities, accounting for individual variation. Our hypothesis was that cortical bone quality and quantity influence the rate of tooth movement in all modalities.

Methods: A 3D simulation model of a maxilla containing the different components (teeth, PDL, trabecular and cortical bones) was developed with 5 intrusion modalities:

Bone stiffness/thickness measurements of 11 subjects utilizing the data generated by Peterson et al. (2006), in human cadavers. The specimens were meshed through using the software ScanIP™ 7.0 (Simpleware, Synopsys, Mountain view, CA, USA) according to data from measurements made on the CT scans of 11 patients. FEA was ran using ABAQUS 6.13 (Dassault Systèmes, Tokyo Japan) software by simulation of five intrusion modalities: intrusion was replicated with a force equivalent to 400gms. Stress levels and displacement were measured at the molar and adjacent teeth. Outcome measures included stress distribution and displacement of the following permanent teeth: canine, first and second premolars, first and second molars.

Potential clinical implications that may be illustrated are the determination of the modality causing the least stress. In parallel, evaluating different mechanical assemblies used will help determine the difference in efficacy among these assemblies about extruded teeth. Results: The highest stress was concentrated on the root surface of the second premolar in the first modality, mostly on its buccal and palatal aspects, in all the other modalities, the first premolar withstood the highest stresses. The least on the extremity teeth, the canine and the second molars. Similar displacement patterns were registered in all modalities especially on the premolars with the highest rate of stress/displacement in the fourth and the fifth model. The second molar intruded the most in the second modality where the force is concentrated posteriorly. Secondary effects varied between models. Upon stiffness and thickness variation, stress configurations in the PDL and initial displacement on the mesial, distal, buccal, and lingual sides of each tooth differed significantly (p<0.05) between modalities. Bone stiffness configuration correlated negatively with the stress on the molars and positively with the displacement, the opposite applied to bone thickness. Conclusion: Generalizing on best intrusive modality is not suitable, as anatomical, and biologic individual traits may influence one or the other in individualized treatment. Further investigations must untangle anatomy conditions on which one protocol is better than the other. This pure numerical method is unable to precisely predict long-term orthodontic tooth movement and may not provide definitive formula for the whole mechanotherapy planning. But at the same time, the capacity for FEA in combination with clinical data input and time-dependent protocol in the analysis can help determining the specificity of the movement in a patient for a better planning of the treatment.