The biomechanical behavior of an osseointegrated dental implant plays an important role in its functional longevity inside the bone. Finite element analysis (FEA) has been used extensively to predict the biomechanical performance of various dental implant designs and the effect of clinical factors on implant success. By understanding the basic theory, method, application, and limitations of FEA in implant dentistry, the clinician will be better equipped to interpret results of FEA studies and extrapolate these results to clinical situations 1.
FEA is a technique for obtaining a solution to a complex mechanical problem by dividing the problem domain into a collection of much smaller and simpler domains or elements in which the field variables can be interpolated using shape functions. In recent times, image-based approaches combined with FEA have allowed effective stress–strain investigations in dental implantology. Dental implants can be virtually positioned within realistic models of human jaws reproduced from high-definition CT image data with respect to the anatomical-physiological structures of bones.
The use of dental implants has rapidly evolved since the advent of osseointegration, progressively replacing removable dentures in the treatment of partially or completely edentulous patients. A fundamental prerequisite for the clinical success in dental implant surgery is the fast and stable implant osseointegration 2. Osseointegration, which histologically is defined as ‘direct bone-to-implant contact’, is believed to provide rigid fixation of a dental implant within the alveolar bone and may promote the long-term success of dental implants 3,4. The processes of osseointegration involve an initial interlocking between the alveolar bone and the implant body (primary implant stability), and later, biological fixation through continuous bone apposition (contact osteogenesis) and remodeling toward the implant (secondary implant stability) 5.
Worldwide, scientists focused on the topic of endosseous implants, aiming to improve their success. A common goal is to understand the key factors of osseointegration processes after implant surgeries. Some researchers investigated microdisplacements occurring at the bone-implant interface, whereas others considered the load transfer at the interface to be more important in determining the correct mechanical stimulation of the osteoblasts, which are assumed to be responsible for bone tissue regeneration and the consequent osseointegration of the implant. Generally, trabecular microstructures of bones are modeled as homogeneous entities with particular mechanical properties and contiguity assumed at implant-bone interfaces. The contiguity conditions do not allow relative motions between the parts generating a continuum of stress distribution at the interface, where stresses are generally concentrated 2.
Various methods for the evaluation of stress around the dental implant system include photoelastic study, FEA and strain measurement on the bone surface. The finite element method offers several advantages, including accurate representation of complex geometries, easy model modification and representation of the internal state of stress and other mechanical qualities 6. Analytical studies were performed to predict mechanical behaviors such as stress and strain distributions generated in the alveolar bone and the muscles that surround the teeth, mainly for evaluation of the applicability of implants. Therefore, most studies presented FEA 7.
FEAs were performed for various shapes of dental implants to study their effects on stress distribution generated in the surrounding jaw bone and to determine an optimal thread shape for stress distribution 8. Once the systems that were the object of the study (e.g. an implant inserted in a mandible and bearing an abutment) were drawn by computer aided design (CAD), the FEA discretized solids composing the system in many infinitesimal little elementary solids defined as finite elements. This led to mesh formation, where the finite elements were connected by nodes. It is then possible to perform a simulation by applying forces to the system and detect the stress distribution at each single element. It has been hypothesized that marginal bone resorption may result from microdamage accumulation in the bone. In light of this, a dental implant should be designed such that the peak stresses arising in the bone are minimized. The load on an implant can be divided into its vertical and horizontal components. In earlier studies, it was found that the peak bone stresses resulting from vertical load components and those resulting from horizontal load components arise at the top of the marginal bone, and that they coincide spatially. These peak stresses added together produce a risk of stress-induced bone resorption 9.
The success of the dental implant is the manner in which stresses are transferred to the surrounding bone. Bone response on an implant surface depends on the reaction of the cells and the matrix towards the material surface as well as to the mechanical constraints in the vicinity of the implant. The maintenance of bone and its adaptation to external loads is based on a complex strain-driven regulatory process of cells and matrix components. Outside-in mechanical tension exerts direct effects on cell behavior by activating biochemical signaling pathways and regulating gene expression through focal adhesions 10. Load transfer from implants to the surrounding bone depends on the type of loading, the bone to implant interface, the length and diameter of the implants, the shape and characteristics of the implant surface, the prosthesis type, and the quantity and the quality of the surrounding bone 11. From a bioengineering perspective, an important issue is to design the implant with a geometry that will minimize the peak bone stress caused by standard loading 12.
In recent years, alternative materials and designs have been developed to suit different clinical situations 5. The increasing popularity in all-ceramic restorations is due to their superior esthetic appearance and metal-free structure. This feature has drawn attention to enhancing the strength and reliability of ceramic systems 13.
In this study, three different crown materials were investigated. In addition, different implant materials were tested simultaneously in six case studies. The first implant tested is commercially pure titanium, which according to the American Society of Testing Materials is available in four different grades (grade I–IV), which is based on the amounts of oxygen, nitrogen, hydrogen, iron and carbon during purification procedures, where each grade has different physical and mechanical properties. Grades I and II are the most commonly used from the commercially pure titanium types for the production of metal-ceramic FDPs. In oral implants and implant-supported FPDs, titanium and its alloys exhibit remarkable advantages due to their excellent biocompatibility, corrosion resistance, high strength and low modulus of elasticity 2. Recent studies are attempting to make dental biomaterials more biomimetic from both biomechanical and biological perspectives. Also, titanium oxidizes easily forming titanium oxide; the passive oxide layer formed on the surface of titanium is also considered responsible for its good biological performance. The implant surface is susceptible to infection because of two main reasons, namely formation of a surface biofilm and compromised immune ability at the implant/tissue interface. The biocompatibility of the titanium implant can be attributed to a surface protein layer formed under physiological conditions. This protein layer actually makes the surface suitable for bacterial colonization and biofilm formation, which might lead to failure. The second implant used is ZrO2TZP-A (bioceramic), which is used due to the very high strength of the material, excellent esthetics and great biocompatibility with surrounding tissues; the material is bioinert, and therefore no chemical reaction of living tissue is expected. It is highly recommended in patients with metal allergies. It supposes to promote quick and easy healing; patients with zirconium implants have less periodontic and soft-tissue problems and lower plaque around the implants than patients with metal implants. Zirconium implants show very good physical and mechanical properties, which might indicate significant success of implants made from this kind of materials, and furthermore, might be the material of choice for dental implants in the future, However, long-term clinical data are required before making a final decision concerning the recommendation of using one-piece zirconium implants as a better alternative to other commonly used materials such as titanium implants.
Three crown materials were tested in this study: gold, E-Max (lithium disilicate) and zirconium core. Gold alloy is usually used for its excellent durability and because gold has a thermal coefficient of expansion similar to enamel. Gold crowns do not wear or abrade the opposing teeth like porcelain crowns. Gold alloys used in different restorations such as inlays and crowns have the same wear rates as that of the tooth structure; it is soft enough that it will wear slightly over time by the same rate as natural teeth. Finally, gold restorations are considered to be the best restoration concerning the longevity as it usually stays for several decades if constructed properly. The second crown material is E-max press (lithium disilicate), which has significant biocompatibility, great esthetics and very good physical properties; dental ceramics have nearly no or minimal sensitivity and minimal amount of irritation when compared with other dental materials as it does not produce negative response when it comes in contact with oral tissues. The third crown is the zirconium core, which has significant biocompatibility, is nonallergic and has a high compressive strength, a high tensile stress and good esthetics. Zirconium is used mainly in high stress areas such as the molar region due to its superior mechanical properties 14.
The idea of this research is to determine as to which crown and implant combination is better in minimizing the amount of stresses transferred to the surrounding bone to ensure a high success rate of osseointegration. The stress distribution after loading the implant by average masticatory force of 650 N was computed by FEA.
Materials and methods
A three-dimensional (3D) finite element model was constructed under an ANSYS environment. The bone geometry was simplified and simulated as a cylinder that consists of two coaxial cylinders. The inner one represents the spongy bone (diameter 14 mm and height 22 mm) filling the internal space of the other cylinder (a shell of 1 mm thickness) that represents the cortical bone (diameter 16 mm and height 24 mm) 15,16.
The implant–abutment complex [the design was taken as 12 mm length and 3.7 mm diameter (Zimmer implant one-piece system; Zimmer Dental Inc., Carlsbad, California, USA)] was modeled in 3D on commercial general purpose CAD/CAM software ‘AutoDesk Inventor’ version 8.0 (Autodesk Inc., San Rafael, California, USA). Two materials were investigated for the implant–abutment complex: titanium and zirconium. However, a cement layer of 40 μm of glass ionomer was simulated in this model.
The crown geometry was acquired using a 3D contact scanner (Roland Modela, model MDX-15; Roland DG Corporation, Hamamatsu, Japan) and computer graphics program (Roland’s Dr PICZA 3 software) for controlling Roland Active Piezoelectric Sensor. Such scanners produced a data file containing a cloud of points’ coordinates. An intermediate software was required (Rhino 3.0; McNeel Inc., Seattle, Washington, USA) to trim a newly created surface, finally filling the crown surface to form the crown volume; then the crown geometry was exported to a finite element program in SAT file format 17 (see Fig. 1). Three crown materials, gold alloy, E-Max, and zirconia core, were tested in this study. All material properties used in this study were tabulated in Table 1.
The crown was subjected to a vertical compressive loading of 650 N, at a central fossa location (equally distributed on four close points), which is close to the implant–abutment complex axis. The base of the finite element models were set to be fixed, which defined the boundary condition 15.
Solid modeling and finite element linear static analyses were performed on a personal computer Intel Pentium Core 2 Duo, processor 3.0 GHz, 4.0 GB RAM. The meshing software was ANSYS version 9.0 (ANSYS Inc., Canonsburg, Pennsylvania, USA), and the element used in meshing all 3D models was an eight-node brick element (SOLID 45), which has three degrees of freedom (translations in the global directions) 18. The mesh density is another relevant parameter. As the geometry is complex, increasing the mesh density improves the accuracy of the results for the discrete model (increasing the accuracy of the stress levels obtained in regions of high-stress gradients). Another effect of increasing the number of elements was to reduce sharp angles created artificially by the process of substituting the geometric model by the mesh, reducing artificial peak stresses by improving the representation of the actual geometry; the mesh density is tabulated in Table 2, whereas Fig. 2 illustrates the complete meshed model and section showing part of the crown, implant–abutment and cement layer elements in different colors.
The bonded type simulates perfect osseointegration in which the implant and the surrounding compact bone are fully integrated such that neither sliding nor separation in the implant-bone interface is possible. Analyzing the force transfer at the bone-implant interface is an essential step in the overall analysis of loading, which determines the success or failure of an implant. It has long been recognized that both implant and bone should be stressed within a certain range for physiologic homeostasis. Overload can lead to bone resorption or fatigue failure of the implant, whereas underloading of the bone may cause disuse atrophy and subsequent bone loss.
Linear static analysis was performed in this study, while the equivalent stress did not reach the endurance limit (fatigue failure limit) which governs most cases of dental analysis. In case of having static stresses lower than the endurance limit under worst case of extreme loads, there will be no risk of fatigue failure.
This research does not require ethical approval, and it followed the Helsinki declaration.
Six runs on the constructed model were carried out, simulating the use of three crown materials and the two implant materials prescribed for this study. Graphical comparisons were preferred to show stresses and deformation distributions, whereas tabling the obtained results can indicate extreme values and calculated percentage and differences. Figures 3–5 show samples of different model results. However, different stress distributions and deformation distributions on all parts of the studied model do not change noticeably, but the values were clearly different. Deformations in Figs 3 and 4 indicate that the central fossa location is a little bit shifted from the axis of the implant–abutment complex axis. In contrast, Figs 4 and 5 showed that the maximum equivalent (Von Mises) stress was located at the implant neck and in connection with the cortical cone.
Tons of pictures can be extracted from the few runs performed on this model; therefore, a sample was presented in this paper just to show different parts and their stresses and deformation distributions. Maximum stresses were located at the central fossa, whereas vertical and total deformations were located at two extreme points on the crown due to the missalignment (minor shift) between the central fossa and the implant–abutment complex vertical axis.
The purpose of this investigation was to analyze different materials of crown and implant to compare their biomechanical behavior. As presented in Fig. 6, the gold crown absorbs more energy than the other tested materials. Although E-Max has a lower Young’s modus of elasticity than gold, which may indicate softer material in 1D analysis, the effect of its low Poisson’s ratio value reduces the overall absorbed energy in 3D in comparison with gold. Gold showed the best behavior, due to absorbing the highest amount of energy introduced by the compressive (applied) load.
Implant–abutment complex material selection is a critical issue as it comes in direct contact with the cortical and spongy bones. Figures 7 and 8 compare the behavior of bones under the two tested materials for the implant–abutment complex. Zirconium implant is clearly stiffer than titanium and it has a higher Young’s modulus and a lower Poisson’s ratio; thus, cortical and spongy bones will receive lower levels of stresses (equivalent stress). This means the patient’s bones are safer with zirconium implant than titanium.
As the expectations regarding esthetics in dentistry are growing, 25 researches in the field of all-ceramic materials for restoration of the natural dentition and dental implants were intensified. Although the crown that covers the implant may be esthetically optimal, the possibility exists that the grayish color of the titanium implant shines through the thin peri-implant mucosa, thus harming the entire esthetic result.
Cylindrical implants produce high shear stress on the bone; increase in diameter increases the load-bearing area by the square of its radius and the bending resistance by the fourth power of the radius. Studies show that stresses reach only a particular distance within the implant 6.
A critical aspect affecting the success or failure of an implant is the manner in which mechanical stresses are transferred from the implant to the bone. It is essential that neither implant nor bone be stressed beyond the long-term fatigue capacity. Any relative motion that can produce abrasion on the bone or progressive loosening of the implant should be avoided. These requirements are met by osseointegrated implants by virtue of close apposition of the bone to implant in the angstrom level. The close apposition of titanium and bone at the angstrom level means that under any subsequent loading, the interface moves as a unit without relative motion of the bone and implant and with the possibility of transferring stress to all parts of the interface 6.
Implant biomaterials should have adequate strength and modulus of elasticity to withstand forces acting on them. Biomaterials are intolerant to such forces, and hence are not preferred as primary implant materials. Conversely, ceramics are avoided despite their strength due to their low modulus of elasticity. In conclusion, titanium alloys (Ti6Al4V), which offer superior strength and comparable modulus of elasticity, are preferable to transfer forces acting on them. An increase in force magnitude is deleterious to osseointegration. Hence, the above factors should be considered when planning treatment so as to minimize force magnitude.
All-ceramic restoration provides superior esthetics, biocompatibility and resistance to masticatory forces. Increased ceramic fracture toughness and flexural strengths (350 MPa) are desirable properties that resist chipping and fracture of the restoration 13.
Within the limitations of this study, the following conclusions can be drawn:
- Using crown materials with lower modulus of elasticity reduces the stresses generated on the jaw bone (cortical and spongy), it absorbs more energy from the applied load, and transfers less energy to the following parts of the system (implant–abutment complex and bones).
- Using more rigid material for the implant–abutment complex may be preferred for weak bones. Bioceramic implants transfer less stresses to surrounding bones than Titanium, and showed close performance to titanium in deformation, whereas their osseointegration behavior was much better. Therefore, bioceramic implants are highly recommended over titanium implant–abutment complex.
Conflicts of interest
There are no conflicts of interest.
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