FEA on the biomechanical behavior of immediately loaded implants with different sizes

Objectives. The purpose of this research was to study, in the case of immediate loading, the biomechanical effect produced by the length of the implants. Material and method. The study material was a CBCT analysis performed on a patient from one dental office in Bucharest. An segment of edentulous mandibular bone was selected from the CBCT, which was processed with Mimics Innovation Suite, respectively Mimics and 3-matic. After processing the bone segment, two implants of the same manufacturer, with identical design, but different length – 10 and 13 mm respectively, were selected from the BIOMAT database. To simulate immediate loading, the bone-implant interface was not blocked and the mandible was defined with properties that characterize a bone with moderate density. A perpendicular masticatory force of 200N was applied to each of the two implants. The software ANSYS calculated the minimum, maximum values and their geometric means for the possible stresses produced on both the shorter implant (10 mm) and the longer implant (13 mm). Results. In the case of short implants, higher average stresses develop along the entire length of the implant, towards the vestibular bone plate, while in the case of long implants the higher stress seems to be cantoned towards the apical side. Conclusions. The present study shows that, in the case of immediate loading, the use of longer implants (13 mm) reduces by more than 50% the geometric mean of the stresses to which the bone-implant interface is subjected than in the case of the use of shorter implants (10 mm). In both types of implants, higher stresses occur at the level of the screw fixing the abutment in the implant.


INTRODUCTION
In recent years, implantology [1][2][3][4][5][6] has become an extremely important branch of dentistry that has developed a lot.The short time between the surgical and prosthetic stages makes the major difference between immediate [7][8][9] and conventional loading [10,11].Immediate loading is an excellent answer to the aesthetic and functional needs of patients, so the long-time frame provided by conventional protocols can be shortened to one hour after surgery.At the interface of some components of a mechanical system, micromovements can occur, resulting in the displacement of one of the component parts in relation to the other [12].In the case of dental implantology, the phenomenon of micro-movement of the components that composed the mechanical system can be detected at the level of two interfaces, either at the connection of the abutment with the implant [13,14], or at the limit between the implant and the patient's bone [15].Over time, multiple studies on the effect of micromovements at the level of the two interfaces have carried out, but for now there is no consensus regarding the phenomena produced exactly at these levels [12].The micromovements produced at the border between the abutment and the implant lead to the formation of a microspace that will be colonized by bacteria and that will cause inflammation of the peri-implant tissues [16].These micromovements will lead to the unscrewing of the abutment [17], affecting the performance of the prosthetic component [18,19].Micromotion produced at this level is an engineering design issue that is the prerogative of implant manufacturers [12].In contrast, micromovements at the bone-implant interface occur when loading a non-osseointegrated implant, as is the case with immediate loading [20].Occlusal pressures lead to displacement of the implant in relation to the implant's alveolus [12].Studies [20][21][22][23][24][25][26] have shown that a displacement of 50-150 μm produced during the period of bone healing can lead to fibrous encapsulation of the implant, endangering the long-term prognosis.Reducing the risk of fibrous encapsulation of the implant is solved by applying the protocol of late prosthetic loading of the implants [27].The introduction of early and immediate loading protocols [20] must take into account this risk factor, represented by potential micromotion at the bone-implant interface [27,28].Experimental approaches in this regard are limited by the difficulty of having available implants at various stages of osseointegration [23,29,30] but finite element analysis (FEA) are particularly useful for simulating micromotions and studying the mode of stress, tensions and deformations transmission when applying a force [21,22,31].The purpose of this research was to study, in the case of immediate loading, the biomechanical effect produced by the length of the implants.

MATERIAL AND METHOD
The study was carried out according to a protocol approved by the Ethics Committee of the Faculty of Dental Medicine of the Titu Maiorescu University in Bucharest (no.5/12.01.2017), respecting the Helsinki declaration and human rights, without harming the patients or the environment.The study material was a CBCT analysis performed on a patient from one dental office in Bucharest.The simulations were carried out with the support of the BIOMAT Research Center -Politehnica University of Bucharest.An edentulous mandibular bone segment was selected from the CBCT, which was processed with Mimics Innovation Suite, respectively Mimics and 3-matic.After processing the bone segment, two implants of the same manufacturer, with identical design, but different length -10 and 13 mm respectively, were selected from the BIOMAT database.First the short implant and then the long implant were positioned and exported to NASTRAN format.NAS-TRAN is a finite element analysis (FEA) program that was originally developed for NASA in the late 1960s under United States government funding for the aerospace industry.The MacNeal-Schwendler Corporation (MSC) was one of the principal and original developers of the publicly available NASTRAN code [32,33].NASTRAN source code is integrated in a number of different software packages, which are distributed by a range of companies [34].This format is necessary to be able to transmit the geometrical bone-implant assembly to ANSYS for finite element analysis at masticatory forces.To simulate immediate loading, the bone-implant interface was not blocked and the mandible was defined with properties that characterize a bone with moderate density.A perpendicular masticatory force of 200N was applied to each of the two implants.The ANSYS software calculated the minimum values, maximum values and their geometric means for the possible stresses produced on both the shorter implant (10 mm) and the longer implant (13 mm).

RESULTS
The analyzed parameters varied for the two types of embankments, and some of them also varied in terms of distribution.The calculated results for the two types of implants are recorded in tables 1-2, and those parameters for which the distributions varied are illustrated comparatively in figures 1-6.In the case of short implants, higher average stresses develop along the entire length of the implant, towards the vestibular bone plate, while in the case of long implants the higher stress seems to be cantoned towards the apical side.In both types of implants, higher stresses also develop at the level of the screw fixing the abutment in the implant.

DISCUSSIONS
Since the experimental capabilities in implantology are greatly limited by the ethical aspects of research on human subjects, finite element analyses are useful procedures for understanding the micro-displacements that occur in oral implantology.For a relevant assessment in the field, we measured both the displacements of the implants and the bone, in contrast to other authors [23,35] who performed measurements at the level of photographs capturing the displacement of the implants to compare the effect of the occlusal stress of some provisional restorations on the implants' micromotion.The present study demonstrates that implant length has an important influence on the absolute values and mode of transmission of stresses, strains, and deformations at the bone level, with direct implica-    [20][21][22][23][24][25][26], the values calculated in the present study are low and do not exceed a threshold at which osseointegration could be endangered, but it is still remarkable that in the case of short implants, stresses and strains develop with double and even triple average values compared to the values calculated in the case of long implants.

CONCLUSIONS
1. Finite element analysis is a reliable tool that, although it involves additional time and the creation of a multidisciplinary team to work together to determine the optimal therapeutic solution for a given clinical situation, allows for the accurate determination of vulnerable areas, in which may produce biomechanical over stresses, with an impact on the osseointegration of immediately loaded im-plants.2. At axial stresses of up to 200 N of the immediately loaded implants, excessive stresses and strains do not occur, which could jeopardize the achievement of osseointegration and the maintenance of long-term therapeutic success.3. The present study shows that, in the case of immediate loading, the use of longer implants (13 mm) reduces by more than 50% the geometric mean of the stresses to which the bone-implant interface is subjected than in the case of the use of shorter implants (10 mm). 4. In both types of implants, higher stresses occur at the level of the screw fixing the abutment in the implant.5. Regarding the stress transmitted to the bone, the use of shorter immediately loaded implants is associated with stressing the bone-implant interface along the entire length of the implant, towards the vestibule, and the use of longer immediately loaded implants is associated with higher stress on the mandibular bone especially at the apex of the implant.

FIGURE 4 .
FIGURE 4. Normal Elastic Strain Y for short (left) and long implant (right)

FIGURE 3 .FIGURE 5 .
FIGURE 3. Normal Elastic Strain X for short (left) and long implant (right)