Due to its sophisticated results regarding long-term stability and retention, dental implant had received a considerable attention by researchers who tried to enhance the implant body design and study all types of factors that facilitate or impede its behavior while inserted. In practical situations, many factors may affect osseointegration between bone and metal implant and consequently the state of interface stresses. These may include implant shape, diameter, length, angulation, type of loading, ¹ , ² , ³ , ⁴ , ⁵ surface treatment, bone quality, and surgical technique. ⁶ , ⁷ In particular, implant body designs with threaded features influence the value or kind of forces applied to the bone–implant interface. A cylinder implant design is much effective in transmitting more shear forces to the bone than tapered design, while on the contrary, the V or tapered design apply more compressive force than cylindrical implants. This indicates that for the cylindrical implant, the bone is strongest under compressive forces, weaker under tensile loads, and much weaker to shear forces. ⁸ To understand the relationship between affecting factors and resulted bone–implant interface stresses, a multiple implant designs were tested in dental practice and many important geometry features were considered such as implant shape, length, and thread type and specifications. ⁹ , ¹⁰ In addition to the aforementioned parameters, the total contact area between the implant and bone may be considered as one of the major design parameters that affect osseointegration strength of implant–bone interface. The osseointegration strength is also influenced by the implant surface treatment and implant thread pitch, depth, and width. ¹¹ , ¹² , ¹³ , ¹⁴ The density of the bone plays another important role in the outcome of the implant treatment in which high density increases bonding properties of the interface region.

In addition to the experimental methods, theoretical analysis methods play a major role in investigating the effect of design parameters on stresses distribution and consequently on the quality of osseointegration in bone–implant configuration. The theoretical method was less expensive, and its results were acceptable since it offers both reliability and accuracy. Among these methods is the finite element method (FEM) which is considered as an effective research tool for dentistry since 1977. ¹⁵ Using FEM complicated geometric structures such as mandible and maxilla bones are simply converted into meshes in a computer set. The simulation models consist of discretization features such as elements, nodes, and predefined boundary conditions. The resulted displacement and stresses which are mostly caused by loading on a part of the proposed structure or total volume can be calculated by a computer program.

For several years, considerable effort has been devoted to the study the influence of implant geometry parameters on stress distribution in mandible bone. The researches show that implant diameter and shape, thread shape, insertion depth, and loading angle were significant factors influencing the stress created in the bone. ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ , ²⁰ The deformations and compression stresses emanated in bone and implant due to different bite forces will increase proportionally with the increase of the force whether it is applied horizontally, vertically, or in oblique directions. ²¹ This increase will be accompanied by a decrease in implant lifetime and the neck zone of the implant system will be the most critical region.

It is well known that most research works done with this aspect used either a simplified geometry or a localized zone of the bone to predict local stress distribution in implant–bone combination. The current paper conducts a three-dimensional (3D)-FE study to investigate the precise situation of the resultant stresses in implant-mandible bone configuration system due to applying a 250 N compressive force. The designed implant was immersed in two different regions of the mandible bone (interior [Type I model] and posterior [Type II model]). Two different shaft designs (cylindrical and tapered) were considered in addition to optimizing three different thread depths (0.25, 0.35, and 0.45 mm) to identify both optimal and worst design parameters that may affect life and stability of the proposed implant design. The proposed null hypothesis for this work was that there is no difference in stress distribution emanated during implant loading when using different implant designs with different thread depth.

Creation of mandible-implant three-dimensional models

To implement the FE analysis (FEA) of the implant-mandible bone whole structure, a computer-aided design (CAD) model was first created with the assistance of using computerized tomography Scanner, SolidWorks, and ANSYS Workbench 14 software (Ansys Inc.; Canonsburg, PA, USA). In this work, the computed tomography (CT) offered a wide versatility in capturing the accurate details and properties of the living complex structures that are going to be anatomized first and later analyzed using FEA. In addition, CT has the ability of material properties inclusion according to bone density, this feature simplified differentiating between cortical and cancellous bones. To account for an accurate FE model and resembling the real situation, a real jaw bone picture for an adult human donator of 65-years-old completely edentulous patient was first created through using CT skull images. A total of 209 CT skull slices with a pixel size of 0.488 mm and slice increment of 1.0 mm were generated and used in the form of (Digital Imaging and Communications in Medicine [DICOM]) file data. Then, the DICOM files were exported to a 3D-slicer software (Ultimaker B.V., geldermalsen, Netherlands) and converted to 3D model. The software has great features such as segmentation and separation which help a lot in the extraction process of the mandible. To distinguish between cortical and spongy bones as two different materials, the software features thresholding, segmentation, and erasing were applied again to separate the spongy bone from the compact alveolar bone. The completed 3D CAD-model file of the mandible was exported as series of stereolithography (STL) files for use by other software and/or rapid prototype manufacturing technologies. The next step was to transfer the STL 3D models of the cortical and spongy bone of mandible into Solidworks software (Dassault Systèmes SOLIDWORKS Corp, Waltham, Massachusetts, USA) to create 3D solid models (SAT files) and then export these files into ANSYS software to implement FE analysis.

For nonliving structures (implants), the current study treats them as an isotropic, so two independent material constants are required, these are Young's modulus and Poisson's ratio. In the present study, the thickness of cortical bones was evaluated using CT, and it was found that it is minimum thickness approximately equal to 1.2 mm. Six different dental implants were constructed by SolidWorks and then inserted into the mandible bone. Later, the implants were grouped into two design groups (cylindrical and tapered), with standard length of 10 mm, diameter 4 mm, pitch 0.8 mm, and standard V-shape threads with different thread depth (0.25 mm, 0.35 mm, 0.45 mm) as show in Figure 1. Six different cases were analyzed for each location (i.e., anterior and posterior) which mean an overall of 12 cases were considered in this study. For each implant, a conical shape suprastructure of (5 mm) was added. It is worthy to know that the current study assumes a state of optimal osseointegration was achieved (i.e., 100% of interface was satisfied), which means cortical and trabecular bone were assumed to be perfectly bonded to the implant. Figure 1

Illustration shows Group A (cylinder shaft) implants and Group B (taper shaft) implants.

Fine mesh of the 3D element model was generated, the number of nodes and elements generated by the 3D FE models are listed in Table 2). Figure 4shows the values of maximum EQV stresses values of supporting bone in type I models in mandible anterior region resulting from application of 250 N vertical load on the implant. It is clear that a ring-type stresses surrounded the implant neck and the highest EQV von Mises stress was found in tapered shaft implant of (0.25 mm) thread depth (T1), followed by tapered shaft implant of (0.45 mm) thread depth (T3), while lowest stress value was in cylinder shaft implant of 0.35 mm thread depth (C2).{Table 2} Figure 4

Chart shows maximum equivalent stresses in Type I models (anterior mandible bone) with different thread depth implants.

The effect of implant body design parameters variation such as thread depth, implant body shapes (cylindrical and tapered), and standard V-shape threads on stress distribution status in two different regions (interior and posterior) of the mandible bone was considered as the aim point of the current study. The null hypothesis states that implant shape design has a direct effect on the bone stresses during loading process. The screw shape of the implant body satisfies a wide contact area between implant and bone, which increases stability, reduces the shear stress in the bone–implant interface, reduces the stress concentration in the cervical region, and relieves stress concentration. ²⁹ The FEM represented by ANSYS Workbench simulation module was used to define the environmental loading conditions of the current problem and calculate bone stresses resulting due to applying a vertical load. The FEM is a numerical method of analysis in which the structure is broken up into set of elements interconnected by nodes, the loads are usually applied on either elements or nodes to calculate for stresses and deformations in structures of any given geometry. The completion of modeling process with high percentage of success depends on some prior issues such as accuracy in modeling the geometry and surface structure of the implant, the material characteristics of the implant and jawbone, and the loading and boundary conditions in addition to the biomechanical implant jawbone interface. In the work, a nondestructive method was used to create the study model with the assistance of CT and magnetic resonance imaging. While most of the previous studies used nonspiral dental implant models for the purpose of simplicity, ¹⁰ the current study is not and high accuracy in both modeling and resulted obtained was achieved. The previous FE simulations showed that an implant in the form of threaded shape is more likely to transmit axial tensile or compressive loads better than the cylindrical type implant. ³⁰ This transmission occurs due to high retention after implant placement. The results obtained throughout this study showed that all models had a peak stress centered in the crestal region of the cortical bone and this agrees well with the results of the previous studies. ²⁵ , ³¹ The most likely explanation for this observation is the irregular geometry the study is dealing with, which in turn emphasize on the necessity of accomplishing an optimization study for both neck area of implant and crestal cortical zone of the bone which will enhance osseointegration between implant and bone and consequently decrease stress in the surrounding bone tissues. For the tapered implants, the peak von Mises stresses on cortical bone were higher than that of cylindrical implants due to high-stress concentration that occurs at sharp line angles located at the sides. In addition, it should not be neglected that thread implants showed great ability to dissipate interfacial stresses of bone. Although threads have multiple useful functions such as maximize initial contact, enhance functional surface area and facilitate dissipations of stress at the interfacial area, it should be realized that threads are classified as of the main reasons that cause implant failure. ³² Considering cylindrical implants, the most likely explanation for why the cylindrical implants exhibit less stresses than tapered is the fact that cylindrical implants generate less lateral force in spongy bone than the tapered implants. The results also show that increasing thread depth will decrease and improve distribution of the maximum value of von Mises EQV stress in both types of implant. Biomechanically, this will result in maximizing the necessary area for osseointegration and providing good initial stability.

From the total results of both models, a conclusion may be drawn here that suggests von Mises stresses are inversely proportional to thread depth (i.e., von Mises stresses decreases as thread depth increases), and this relationship is valid for both cylindrical and tapered implants.

From the simulation analysis that has been done using FEM, the following conclusions can be drawn concerning the effect of both thread depth and implant shape design on implant-mandible bone interface stress distribution:

The results obtained throughout this work showed that taper body implant had a higher peak of von Mises stress than that of cylinder body implants in all types of bones. The maximum von Mises stresses were recorded in the distal side of implants in all models

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Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or nonfinancial in this article.