A comparative study on the stress distribution around dental implants in three arch form models for replacing six implants using finite element analysis.

BACKGROUND: Dental implant is a method to replacement of missing teeth. It is important for replacing the missed anterior teeth. In vitro method is a safe method for evaluation of stress distribution. Finite element analysis as an in vitro method evaluated stress distribution around replacement of six maxillary anterior teeth implants in three models of maxillary arch.
MATERIALS AND METHODS: In this in vitro study, using ABAQUS software (Simulia Corporation, Vélizy-Villacoublay, France), implant simulation was performed for reconstruction of six maxillary anterior teeth in three models. Two implants were placed on both sides of the canine tooth region (A model); two implants on both sides of the canine tooth region and another on one side of the central incisor region (B model); and two implants on both sides of the canine tooth region and two implants in the central incisor area (C model). All implants evaluated in three arch forms (tapered, ovoid, and square). Data were analyzed by finite analysis software.
RESULTS: Von Mises stress by increasing of implant number was reduced. In a comparison of A model in each maxillary arch, the stress created in the cortical and cancellous bones in the square arch was less than ovoid and tapered arches. The stress created in implants and cortical and cancellous bones in C model was less than A and B models.
CONCLUSIONS: The C model (four-implant) reduced the stress distribution in cortical and cancellous bones, but this pattern must be evaluated according to arch form and cost benefit of patients.

INTRODUCTION

The teeth are missed due to decay, periodontal disease, diabetes, endodontic treatments, and fracture.[1] Replacement of these teeth to maintain the shape of the arch and the proper occlusion and mandibular join is important.[2] The selection of prosthetic restoration type could be made on to return to natural condition as soon as possible, comfort, and beauty.[3] One of acceptability treatments is soft tissue-supported removable partial denture that this option of treatment is one of the weakest dental treatments. The low survival percentage, health risk of the adjacent teeth, and adjacent tissues of partial denture are some of its undesirable characteristics.[4]

Another option is fixed prosthodontic treatment that fundamental complication with this type of treatment returned to mechanical and biological aspects. Dental decay and endodontic failure of the abutments from one side and periodontal diseases and bone loss on the other side cause to abutment failing and need to prosthesis replacement.[56]

Dental implants are known as the most predictable way to replace missing teeth. The reasons why the implant becomes the first choice are improvements in implant treatment, excellent long-term results, and facility to use of implants for reconstruction of partial edentulous areas.[7]

Implant placement is important affected by many factors such as implant length, implant diameter, jaw location, and implant position.[8]

The anterior maxillary teeth have been a very important and challenge area in esthetic and reconstruction aspects.[9] Anterior missing teeth cause shortening of the lower part of face and gradual protrusion of the chin. The crestal bone resorption and alveolar bone reduction in the height after tooth extraction create esthetic problems, especially in anterior maxilla (includes incisors and canine teeth) site.[10] Fortunately, the shape of the premaxilla arch maintained even after the tooth extraction [11] and classified as ovoid, square, and tapered form.[12]

According general guidelines for implant insertion, the canine is key position and an implant must be insert in this site [13] and short-span prosthetic had a better survival rate than a long-span prosthetic during load time.[14] Following implant restoration, stress around the implant created and this stress affected the long-term implant success rate.[15]

The assessment of stress around the implant is possible by clinical and laboratory methods that have advantages and disadvantages.[1617] The clinically measurement method for understanding effect of arch form and number of implants often can be done, but it was not ethically and its risk, not clear. Nowadays, treatments are simulated in the form of laboratories and its risks are investigated.[18] The main problem in the laboratory simulation of dental implant mechanical behavior is modeling of bone tissue and responses to the applied mechanical forces. Detailed decisions should be adopted for the realization of modeling and analysis; however, this method is an appropriate and useful tool for predicting the effects of stress on the implant and the bone around it. In the past two decades, finite element analysis as an in vitro and laboratory method has been used.[19]

In anterior maxillary area, the jaw shape may effective in implant placement and stress distribution. Since the effects of dental arch forms and number of implants on the stress around dental implant following reconstruction of denudate permaxillary region were not evaluated by finite element analysis method, therefore, in this study, stress distribution evaluated, and according result, the best option of treatment proposed. The aim of this study was to compare the amount of stress distribution around dental implants to replace the six anterior maxillary teeth using the finite element analysis.

MATERIALS AND METHODS

In this in vitro study, a toothless patient's cone-beam computed tomography (CBCT) was used to build a three-dimensional model of the anterior maxillary teeth.[20] The CBCT data were transferred to the MATERIALISE MIMICS ver. 10.01 (Materialise, Leuven, Belgium) modeling software, and three-dimensional (3D) maxillary contour was exacted. According to the Lekholm and Zarb classification, the type II of bone model was chosen.[21] The bone that included a cancellous bone in the center with 1 mm circumferential cortical bone was simulated.[2122]

The created models were transferred to the Geomagic Studio software (Raindrop, South Carolina, United States) to create meshes and nonnecessary points and fields were removed, and an editable 3D volume was prepared. Furthermore, in this study, screw-form dental implants (ITI; Institut Straumann AG, Waldenburg, Switzerland) with 10 mm length and 4.1 diameters were used to replace teeth.[23] The canine and incisor teeth were the same size, so both implants were used the same size.[24]

Moreover, cobalt-chromium (Wiron 99; Bego, Bremen, Germany) and feldspathic porcelain were selected for crown metal frame and occlusal surface. The thickness of porcelain and metal was 0.8–2 mm and the thickness of the cementum layer was ignored.

The different parts of the jaw consist of cortical and cancellous bones (obtained from Mimics software); veneer and implant were assembled together in the environment of ABAQUS software (Simulia Corporation, Vélizy-Villacoublay, France).[25] In all cases, to simulate the functional forces, a 100-N force with a 30-degree angle was imposed buccolingually from long axis of each veneer.[2627] In this software, the models for veneer and implant were designed using available references and manufacturer's instructions.[28] According to the shape of the jaw arch,[12] the number of two to four implants for reconstruction of six maxillary anterior teeth replaced, and on this basis, nine models were simulated.

The square arch

SA model: Two implants in the canine areas on the both sides [Figure 1].

Figure 1

The stress distribution of Square A model

SB model: Two implants in the canine areas on the both sides and another implant in the central incisor area [Figure 2].

Figure 2

The stress distribution of Square B model

SC model: Two implants in the canine areas and two implants in the central incisor areas [Figure 3].

Figure 3

The stress distribution of Square C model

The ovoid arch

OA model: Two implants in the canine areas on the both sides.

OB model: Two implants in the canine areas on the both sides and another implant in the central incisor area.

OC model: Two implants in the canine areas on the both sides and two implants in the central incisor areas.

The tapered arch

TA model: Two implants in the area of the canines on the both sides.

TB model: Two implants in the canine areas on the both sides and another implant in the central incisor area.

TC model: Two implants in the canine areas on the both sides and two implants in the central incisor areas.

The data were analyzed by ABAQUS finite element analysis software 6.8 (Simulia Corporation, Vélizy-Villacoublay, France).

RESULTS

The stress distribution of von Mises was examined in every nine models and around all components (bone, implant, and prosthesis). The stress distribution, in the three models of arch shapes, was nearly similar and only minimum differences were seen. The amount of stress created around the implants in the cortical bone was more than cancellous bone and stress reduced with increased number of implants [Charts 1 and 2].

Chart 1

The stress distributions in the cortical bone in the various arches. A model – Two implants in the area of canines on the both sides; B model – Two implants in the canine area on the both sides and another in the central area; C model – Two implants in the canine area and two implants in the central areas

Chart 2

The stress distributions in the cancellous bone in the various arches. A model – Two implants in the area of canines on the both sides; B model – Two implants in the canine area on the both sides and another in the central area, C model – Two implants in the canine area and two implants in the central areas

The results showed that after applying 100-N force, the amount of von Mises stress in the cortical and cancellous bones around implants under a 30-degree angle in a comparison between the square arches in the SC model was less than all others and SB and SA models, respectively [Figures 1 and 2]. Furthermore, in the ovoid and tapered arches, the results were the same.

A comparison between the two implants (A model) in the three arches shows that the amount of stress created in implants and cortical and cancellous bones around them in the square arch (SA model) and ovoid (OA model) and tapered arches (TA model) increased, respectively. Stresses in the ovoid arch are less than in tapered arch [Charts 1 and 2].

A comparison between the three implants (B model) in the three jaw arches shows that the amount of stress created in the implants and cancellous and cortical bones around them in the tapered arch (TB model) is less than in the ovoid (OB model) and square arches (SB model). In the ovoid arch, stresses are less than in square arch [Charts 1 and 2].

A comparison between the four implants (C model) in the three jaw arches shows that the amount of stress created in the implants and cortical and cancellous bones around them in the tapered arch (TC model) is less than in the ovoid (OC model) and square arches (model SC). In the ovoid arch, stresses are less than in square arch [Charts 1 and 2].

The greatest amount of stress applied to implant is in the neck area of the implants, and the stress levels are reduced by moving towards the apical implant in all models [Charts 1 and 2].

DISCUSSION

This study is the first study that examines the effect of arc shape on the stress around the implant and is the only study that evaluated the reconstruction of the anterior region with finite element analysis. The finite element analysis is a proven theoretical technique that is used to solve engineering problems and can be an alternative for studies of the clinical samples that data collection of in vivo data in them is impossible or scientifically questionable.[29] While trying to simulate any things (e.g., teeth, implant, and restoration) by finite element analysis software, there are limitations such as dimensions of the substructures needed to obtained from real clinical samples, and implant size was taken from commercial sources. Furthermore, the structures were assumed to be homogenized, and isotropic as well as the contact area between the bone and the implant was considered thoroughly as osseointegrated, which is away from reality, but this is accepted;[30] also Mosavar et al. demonstrated that the stress distribution patterns in the supporting bone did not affected using different thread designs and various osseointegration conditions and simulated implant with thoroughly osseointegrated approved.[31]

The force of 100 N was applied under a 30° angle on the longitudinal axis of the implant. According to the results obtained in this study, it was shown that whatever the number of implants in different models of the jaw arch increased, the amount of the von Mises stress created in the implant, veneer, and cancellous and cortical bone around implants reduced. The least stress was created by the force of 100 N in a 30° angle in four implants of tapered jaw model (TC). These findings are parallel to study conducted by Mahshid et al. that showed the level of stress in cancellous bone decreases from two-implant model to the four-implant model, but it increases in the five-implant model. Furthermore, stress on cortical bone of the end implants in two-, three-, and four-implant models was similar. While in five-implant model, the amount of stress on the end implants was dramatically higher in the five-implant model.[32] Another study assessed that the fixed denture restorations supported three- or four-implant structure and showed that the failure rate of three-implant-supported prosthesis was more than four implants. As a result, this type of prosthesis is not structurally recommended because they do not enough support the occlusal forces.[29] The effect of the implant numbers on the biomechanical properties of the implant-supported overdentures were investigated by Liu and colleagues. They concluded that the three- or four-implant models were more stable than two-implant model and less stress transferred to the bone around the implant.[33]

According to the results obtained from the comparison of stress in the cortical and cancellous bones, it was demonstrated that the amount of stress created in the cortical bone around implants in each of the studied models was more than cancellous bone around the same implant, and maximum stress on all the models was accumulated in the coronal area of the bone and the implant. Duyck et al. reported that applying an excessive force to the implants increased loss of bone in the neck area of the implant and the percentage of bone in this area was reduced.[34] Another study that investigated the stress distribution around the implant showed stress in all situations at the coronal area of the implant was more than the apical area.[35] In contrast to our study, Tada et al. showed that the utmost stress created with the applied power in models with low bone density was seen in the bone around the apex of implant due to lower density of bone in the area.[36] In accordance with physical laws, when two substances with different modulus of elasticity were put together without any interstitial substance and one of them is loaded, the stress on the contact increased and this effect will be greater in first-time contact.[37] This result was shown that most stress in the place of contact of implants to the bone was seen in the coronal area of the implant. The results of this study were consistent with the findings of the researchers.[343839]

According to the values of the von Mises stress obtained, it was shown that in any of the models, the amount of stress created in restoration, implants, and cortical and cancellous bones was decreased, respectively, and the least stress was applied to cancellous bone in each model. This finding could be due to the very highest modulus of elasticity of superstructures and implants than cortical and cancellous bones. As a result, more stress was created in an object that had a higher modulus of elasticity.[40]

According to the results obtained from the comparison of three jaw models, it was shown that under the same force in the C model, the amount of tension created in the implants and cancellous and cortical bone around them was less than of the stress created in the A and B models. The most amount of stress was seen in the mesial area of the implant, at the junction with the pontic area. Material study showed that two pontic implant-supported prostheses were bent 8 times more than one pontic implant-supported prosthesis.[41] As a result, a more length of pontic in the A model can be a cause for creating further stress in this model compared with the B and C models, respectively.

The tapered arch with one or two implants in incisor area in comparing ovoid and square arch form more decreases von Mises stress around implant. The central incisor implant would decrease the stress concentration to a third (1:3 ratio) on anterior loading, and then, the alveolar arch form had effect on stress distribution and then should also be considered as an important factor to avoid excessive stress concentration around implant.[42]

In addition, the amount of tension created in the implants and cortical and cancellous bone around them in A model was more than tension created in the B model. It seems that finding was due to the more length of the pontic in the A model. This study emphasized and evaluated similar dimensions of implants, and it may be that the different implant sizes have contradictory results, So we proposed to be investigated in subsequent studies.

CONCLUSIONS

According to this study, following replacement of anterior teeth and the minimum number of implants should be replaced the shape of the arch of the jaw as an intervention factor that should be considered. With the increase in the number of implants, the effect of the jaw arch shape decreases. Therefore, in the anterior of the maxilla, it is recommended that one or two implants be replaced in the incisor area to reduce the effect of arc shape.

As a result, C model with four implants (two implants in the canine areas on both sides and two implants in the central region on the both sides) proposed treatment plan for the reconstruction of six teeth anterior maxillary in each three jaw arches.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.