Stress Distribution Along the Implant-Bone Interface: A Pilot Study using Finite Element Analysis to Compare Tilted and Non-Tilted Implants under Different Loads

Introduction

Over the past few decades, dental implants have risen in popu-larity due to their success in rehabilitating both completely and partially edentulous areas [1]. However, their placement is con-strained by many factors, including anatomy and bone value. The loss of posterior teeth at an early age, for instance, prohibits the placement of implants in related regions due to bone loss, which leads to reduced bone value. In addition, the alveolar nerve canal and the mental nerve loop limit implant placement in the mandibular posterior regions of the mouth. To overcome these issues, bone grafting and short implants have been devel-oped, in addition to inferior alveolar nerve lateral transposition. The use of tilted implants provides another option; it allows for maximum use of existing bone, and placement of posterior fixed restorations with reduced cantilevers. In addition, it circumvents the mandibular nerve [2, 3].

Unlike the mandible, implant stability is restricted in maxilla due to bone resorption, especially in the posterior region. Hence, bone grafting is often indicated in maxillary posterior regions. Another concern for maxillary implant placement is pneuma-tization, in which there is inferior expansion of the maxillary sinus in relation to fixed anatomic landmarks. This condition de¬velops after the extraction of the posterior maxillary teeth [2-6].

To address it, maxillary sinus elevation or bone grafts have been proposed. Pterygomaxillary and zygomatic implants are also op¬tions, but are considered surgically complex [2, 6]. Tilted implants offer an alternative with less morbidity and low¬er financial costs in comparison with other procedures. They can also provide a more postsurgical comfort, as reported in previ¬ous studies [7]. The influence of tilt to the survival and success¬ful rate of dental implants has been studied, revealing that one of key factors of a successful implant is stress distribution at the bone-implant interface [8-11]. However, less is known about the stress distribution of tilted implants.

The finite element method (FEM) is one of the most common methods in stress analysis across a number of scientific fields [12]. Otherwise known as Finite Element Analysis (FEA), this technique has been commonly applied in the quantitative three-dimensional (3D) evaluation of stress distribution along dental implants and surrounding bone [13, 14]. Our research used FEM analysis to evaluate the stress of implants placed at different angles, under a variety of occlusal loads. The null hy-pothesis was that tilted and non-tilted implants would exhibit similar stress distribution along the implant-bone interface, re-gardless of variance in occlusal loads.

Methods

CAD Model and finite element modeling of elements This in vitro study was approved by the University at Buffalo Institutional Review Board (UBIRB).

Four CAD models of implant and bone were manufactured via three-dimensional (3D) optical scanner (Dentium Rainbow Scanner, Dentium) and point cloud data extraction, including implants placed at 1) 0o (used as the control); 2) 15o; 3) 30o; and 4) 45o. A Straumann tissue-level implant (Ø3.3mm RN x 10mm [SLA; Institut Straumann AG, CH-4002 Basel, Switzer¬land]) was scanned with a 3D scanner (Rainbow™, Dentium, USA), and a mandible bone model was extracted from a CBCT DICOM file. The 3D model of the implants in the mandible was constructed via aCAD program (SolidWorks 2010 [Das-sault Systѐmes]). A 3D model of a section of mandible miss¬ing second molar, and its superstructures, was extracted from CBCT. The mandibular bone model was selected as previously described.15 Trabecular bone was modeled as a solid structure encapsulated in cortical bone. A bone block with dimensions of 20mm x 14mm x 35mm, representing the second molar region of the mandible, was modeled. It consisted of a cancellous bone center surrounded by 2mm thick cortical bone. The CAD mod¬el objects had fully bonded contact surfaces (Figure 1). It was assumed that the implant was fully osseointegrated, providing immediate stability after implant placement.

Figure 1: Mandible 3D bone model

FEA Loads

The FEA were carried out using Solidworks® Simulation (Das-sault Systèmes) (Figure 2). The physical interactions at the im-plant-bone interfaces during loading were considered. Static loadings, with an axial force of 150N and 300N, were applied to the implant-bone interface surface. The von Mises stress values were used to measure stress levels and evaluate the stress distri¬bution at the implant-bone interface.

Figure 2. The von Mises stress (MPa) for osseointegration configuration

Results

Figures 3 and 4 show the von Mises stress (MPa) for osseointe-gration configurations under 150N and 300N, respectively. The maximum von Mises stress of the implants under different load¬ings are listed in Table 1. The amount of stress along the angu- lated implants was greater than that on the control group implant under both 150N and 300N loads. There was no significant dif¬ference in stress level among the angled implants. In addition, the von Mises stress under 150N of loading is less than that un¬der 300N of loading, as shown in Table 1.

Figure 3: The von Mises stress (MPa) for osseointegration configurations under 15

Figure 4: The von Mises stress (MPa) for osseointegration configurations under 300N

                            Table 1: von Mises Stress

Stiffness Analysis The displacements of implants at different angulations are shown in Table 2. The displacements along the angulated implant were larger than that of the control group implant under both 150N and 300N loads; additionally, there was no significant difference in displacement among differently angled implants (Table 2).  

                                      Table 2. Displacement

Discussion

FEM is the most common tool for analyzing stress distribution of dental restorations under different loading conditions [16, 17]. Three-dimensional FEA virtual modeling, along with appropri-ate stress loads, has been extensively used for the quantitative evaluation of stresses between the implant and its surrounding bone [13, 18]. FEM simulation results have been shown to pro- vide considerable information which cannot be derived from clinical trials, and this data guide innovative designs [19]. It is best to evaluate dental materials using clinical trials; however, time constraints, financial burden, and the need for adequate sample sizes limit such trials [20, 21]. The accurate prediction of dental implant stability and failure mechanisms using FEA provides help in compensating for these [19]. Principal stresses and von Mises stress are both frequently used for interpreting the results of stress analyses in research [21]. The von Mises stress criterion is used to interpret stresses on ductile materials, which are commonly used for implants (e.g., titanium) [21]. As implant failure occurs when von Mises stress values exceed the yield strength of an implant material, we found it meaningful to eval¬uate the von Mises stress distribution along the implant-bone in¬terface of implants placed at various angulations under different loads [22].

The null hypothesis was rejected, as the stress along the tilted implants was found to be greater on the (non-tilted) control group under both 150N and 300N loads. Additionally, the stress under 300N of loading is greater than that under 150N of load¬ing. These results are in accordance with Watanabe et al’s find¬ings [23]. Watanabe et al used a two-dimensional FE model to analyze the influence of load inclination to the stress distribu¬tion between the bone and implant interface. They found that compressive stress at the bone-implant interface increased with implant inclination, regardless of the location and direction of loading [23].

However, other studies yielded different results. Zampelis found that distal tilting of implants splinted by fixed restorations did not increase bone stress in comparison to vertically placed im¬plants [24]. A 2D model for FEA was used in Zampelis’s study, whereas our research used a 3D model). It is important to note that, at the time of Zampelis’ study, 3D models for simulating implants were flat cylinders, which were found to lead to under-estimation of the stress generated at the bone-implant interface. In our study, a 3D model with threads was simulated and used.

Research by Fazi found that a distribution of four implants, with the distal tilted implants at 340 (such as the all-on-four configu¬ration), leads to a favorable reduction of stresses on the bone and implants, and even the restoration framework [14]. Fazi’s study evaluated stresses at the external cortical bone surface, distal to the terminal implant, and in the cancellous bone along the im¬plant body. The current study analyzed the stresses across the entire implant body.

Satoh et al found that using implants tilted mesially at 10-20° did not result in increased stress on the bone [25]. However, the forces applied in Satoh’s study were parallel to the long axis of the tilted implant; in the current study, all forces applied were parallel to the long axis of the non-tilted implant, which is more similar to intraoral occlusal loading. Additionally, the applied loads in our research were 300N and 150N, which correspond to physiologic occlusal loads during chewing and swallowing in patients [26]. It has been well established that implant function and long-term success are mainly dependent on osseointegra-tion with the surrounding bone [22]. The interfacial stress on implants is focused at the interface between the implants and the surrounding bone. This affects the interface biological reactions, including bone resorption and remodeling. Avoiding implant overloading, and ensuring sufficient initial intraosseous stability, are key in promoting a safe biomechanical environment [27].

As a principle, implants placed vertically (without tilt) receive compressive (occlusal) and moderate (lateral) shear forces, most of them directed on the apical third. In the case of tilted implants, these normal forces might lead to uneven stress distribution [6]. Hence, it is necessary to evaluate the stress distribution along tilted implants. This study was limited by the assumption of per-fect contact between the implant and the surrounding bone. Clin¬ically, the implant may only be in partial contact with the bone

Discussion

FEM is the most common tool for analyzing stress distribution of dental restorations under different loading conditions [16, 17]. Three-dimensional FEA virtual modeling, along with appropri-ate stress loads, has been extensively used for the quantitative evaluation of stresses between the implant and its surrounding bone [13, 18]. FEM simulation results have been shown to pro- vide considerable information which cannot be derived from clinical trials, and this data guide innovative designs [19]. It is best to evaluate dental materials using clinical trials; however, time constraints, financial burden, and the need for adequate sample sizes limit such trials [20, 21]. The accurate prediction of dental implant stability and failure mechanisms using FEA provides help in compensating for these [19]. Principal stresses and von Mises stress are both frequently used for interpreting the results of stress analyses in research [21]. The von Mises stress criterion is used to interpret stresses on ductile materials, which are commonly used for implants (e.g., titanium) [21]. As implant failure occurs when von Mises stress values exceed the yield strength of an implant material, we found it meaningful to eval¬uate the von Mises stress distribution along the implant-bone in¬terface of implants placed at various angulations under different loads [22].

The null hypothesis was rejected, as the stress along the tilted implants was found to be greater on the (non-tilted) control group under both 150N and 300N loads. Additionally, the stress under 300N of loading is greater than that under 150N of load¬ing. These results are in accordance with Watanabe et al’s find¬ings [23]. Watanabe et al used a two-dimensional FE model to analyze the influence of load inclination to the stress distribu¬tion between the bone and implant interface. They found that compressive stress at the bone-implant interface increased with implant inclination, regardless of the location and direction of loading [23].

However, other studies yielded different results. Zampelis found that distal tilting of implants splinted by fixed restorations did not increase bone stress in comparison to vertically placed im¬plants [24]. A 2D model for FEA was used in Zampelis’s study, whereas our research used a 3D model). It is important to note that, at the time of Zampelis’ study, 3D models for simulating implants were flat cylinders, which were found to lead to under-estimation of the stress generated at the bone-implant interface. In our study, a 3D model with threads was simulated and used.

Research by Fazi found that a distribution of four implants, with the distal tilted implants at 340 (such as the all-on-four configu¬ration), leads to a favorable reduction of stresses on the bone and implants, and even the restoration framework [14]. Fazi’s study evaluated stresses at the external cortical bone surface, distal to the terminal implant, and in the cancellous bone along the im¬plant body. The current study analyzed the stresses across the entire implant body.

Satoh et al found that using implants tilted mesially at 10-20° did not result in increased stress on the bone [25]. However, the forces applied in Satoh’s study were parallel to the long axis of the tilted implant; in the current study, all forces applied were parallel to the long axis of the non-tilted implant, which is more similar to intraoral occlusal loading. Additionally, the applied loads in our research were 300N and 150N, which correspond to physiologic occlusal loads during chewing and swallowing in patients [26]. It has been well established that implant function and long-term success are mainly dependent on osseointegra-tion with the surrounding bone [22]. The interfacial stress on implants is focused at the interface between the implants and the surrounding bone. This affects the interface biological reactions, including bone resorption and remodeling. Avoiding implant overloading, and ensuring sufficient initial intraosseous stability, are key in promoting a safe biomechanical environment [27].

As a principle, implants placed vertically (without tilt) receive compressive (occlusal) and moderate (lateral) shear forces, most of them directed on the apical third. In the case of tilted implants, these normal forces might lead to uneven stress distribution [6]. Hence, it is necessary to evaluate the stress distribution along tilted implants. This study was limited by the assumption of per-fect contact between the implant and the surrounding bone. Clin¬ically, the implant may only be in partial contact with the bone

Conclusion

Within the limitations of the current research, we concluded that stress distribution along the implant-bone interface increased when the loading increased. Implants which were tilted had greater stress distribution in comparison to non-tilted implants. We recommend further study on this topic to better understand stress distribution and dental implants

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