Abstract
Background. Customized root-analogue implants (RAIs) with a press-fit design, inserted immediately after tooth extraction, have garnered attention from the researchers and dentists due to their ability to generate frictional force within the tooth extraction socket.
Objectives. The aim of this study was to evaluate the stress distribution and microdisplacement of 2 designs of customized root-analogue zirconia implants in the maxillary and mandibular posterior regions using finite element analysis (FEA).
Material and methods. Four computer-aided design (CAD) models of maxillary and mandibular bone with standard density were constructed based on standard tooth dimensions. The models featured 2 distinct designs, namely fin and bulb designs of RAIs, with 2 models designated for the maxillary first molar and 2 models for the mandibular first molar. All three-dimensional models were converted into finite element models using Altair® HyperMesh® software. Thereafter, loads of 300 N and 100 N were applied in the axial direction to analyze the stress distribution and microdisplacement on peri-implant bone areas using FEA.
Results. The customized root-analogue zirconia implant with the bulb design showed better stress distribution in the surrounding bone when compared to the RAI with the fin design. The micromotion values of the fin design were found to be lower than those of the bulb design, indicating that the former exhibits superior primary stability. The stress distribution of both designs demonstrated reduced stress values in the maxillary posterior region compared to the mandibular posterior region.
Conclusions. The customized root-analogue zirconia implant with added press-fit geometry, i.e, fin or bulb design, has a positive effect on stress distribution and provides enhanced primary stability.
Keywords: finite element analysis, zirconia, customized root-analogue implants, computer-aided design (CAD), targeted press-fit design
Introduction
From antiquity to the present day, the major affliction that troubles the individuals of all ages is tooth loss, as it affects the masticatory function and aesthetics. Among the various treatment options available, dental implants have gained popularity due to their ability to provide a natural appearance and comfortable fit, durability and reliability, a high success rate, and improvement in chewing and eating abilities, as well as preservation of facial and bone features.1 However, the success of conventional dental implants is contingent upon numerous factors and can be compromised due to post-treatment complications such as crestal bone loss, implant screw loosening and peri-implantitis. The presence of these issues could be attributed to elevated levels of pro-inflammatory cells like interleukin (IL)-1β, IL-6 and matrix metalloproteinase-8 (MMP-8) in peri-implant support tissues compared to the periodontal support tissues.2
In 1950, Per-Ingvar Brånemark revolutionized the field of dental implantology by discovering, however inadvertently, that biocompatible materials like titanium-alloy implants inserted into the alveolar process can fuse with bone, a process he called osseointegration.3 However, there are drawbacks associated with the utilization of titanium implants, including allergic reactions, galvanic current formation, cellular sensitization, and the visibility of black metallic components through the mucosa in cases of soft tissue recession.3 To address these limitations, the ceramic implants were developed in the 1960s, with zirconia emerging as a promising alternative to conventional titanium-based implant systems for oral rehabilitation due to its superior biological, aesthetic, mechanical, and optical properties.4
The commercially available dental implants offer a limited range of parameters with regard to length, diameter and threads, and cannot fully meet the requirements for all oral conditions. A study performed by Ragucci et al. demonstrated successful osseointegration in immediately placed and loaded dental implants into the fresh extraction socket with high survival and success rates and minimal marginal bone loss.5 However, a void still remained between the implant and the alveolar socket. While searching for a new solution, researchers posited that if the implant’s design were to mirror that of the extracted root and conform to the socket geometry, no residual voids would remain, thus resulting in better healing and aesthetic outcomes. This concept led to the development of customized root-analogue implants (RAIs), which are designed to fit each patient’s unique anatomy. These implants preserve the remaining hard tissues by maintaining the buccolingual and apico-coronal dimensions of the alveolar ridge immediately after extraction. The preservation of soft tissues is facilitated by the maintenance of the emergence profile and the remaining attached gingiva. In addition, RAIs majorly reduce the time required for implant placement and rehabilitation, offering a promising perspective for the field of implant dentistry.5
In recent years, customized RAI systems have been introduced and studied using computer-aided design/computer-aided manufacturing (CAD/CAM) technology. This development has led to the immediate replacement of teeth after extraction through the implementation of copy milling or a rapid prototyping technology, a process in which a computed tomography (CT) scan is processed and converted into an implant. These contemporary techniques have reduced the duration of surgery and simplified implant operations.6 However, these methods have not been explored extensively and are not well established.
Finite element analysis (FEA) studies concerning commercial dental implants in bone are extant in literature. However, to the best of our knowledge, no FEA studies have been conducted to evaluate the biomechanical response of RAIs in the posterior region. Thus, this study aimed to evaluate the stress distribution in the peri-implant bone area with 2 different zirconia implant designs in the maxillary and mandibular posterior regions. Additionally, the primary stability of customized RAIs was evaluated immediately after placement.
Material and methods
An in vitro study was performed to determine the stress distribution patterns in the maxillary and mandibular first molar regions with 2 different customized RAI designs, namely the fin design and the bulb design. Four models were prepared, and each model was subjected to loads of 300 N and 100 N in a vertical direction (90° angulations). Subsequently, three-dimensional FEA was employed for the assessment of the models.
Instruments used for the study
The personal computer used for the design process featured a configuration comprising an Intel Core® i5-760 processor (2.80 GHz) with 8 GB of RAM memory and a 2 GB graphic card (NVIDIA, Santa Clara, USA). The software utilized for the study included SolidWorks (Dassault Systèmes SolidWorks Corporation, Waltham, USA), Inventor (Autodesk, San Francisco, USA), Altair® HyperMesh®, v. 13.0 (Altair Engineering, Troy, USA), and ANSYS, v. 12.1 (Ansys, Inc., Canonsburg, USA).
Study design
Geometry modeling
Three-dimensional CAD models of maxillary and mandibular bone with standard density were constructed by means of organic modeling using SolidWorks software. The construction of 4 CAD models with 2 RAI designs was based on standard tooth dimensions. Two models were constructed on the maxillary first molar and 2 on the mandibular first molar. The standard abutment shape of the RAIs was designed to mirror the morphology of the original tooth crown. The dimensions of the all-ceramic restoration were set to include a 1-mm width shoulder finish line and a smooth surface on the abutment. The shape of the root was modeled after the ideal natural tooth. The root was solid in structure, with the outer surface designed with fin and bulb designs, sparing 2 mm of cortical bone at the coronal region and 2 mm at the apical region to avoid trauma while placing implants. Subsequently, all CAD models were converted and saved in the standardized triangulation language (STL) file format. The design of the study is presented in Figure 1.
Dimensions of the CAD model
The external designs of maxillary and mandibular RAIs are presented in Figure 2. They were constructed according to the ideal tooth dimensions provided by Wheelers et al.7 The dimensions of the RAIs and their external designs8 are outlined in Table 1 and Table 2.
The bone model was based on the ideal adult maxillary and mandibular bone.7
Conversion of the geometric model to the finite element model
The second step was the discretization of the geometric model into finite discrete elements. Discretization entails the division of the geometric model into several small elements and connecting them at the nodes. The process of connecting the elements and eliminating duplicate nodes is known as meshing. Using Altair® HyperMesh® software, the CAD model of the maxilla, mandible and 2 implants in the STL file format was meshed with quadratic tetrahedral solid elements. Refinements were made based on the convergence analysis to obtain 4 finite element models of implants and bone. Table 3 provides a summary of the number of elements and nodes for each model.
Material properties
The different structures involved in the study were cortical bone, cancellous bone, teeth, and zirconia. Each material possesses distinct properties. Hence, the representation of material properties was necessary to ensure a comprehensive comparison of the stresses. To simulate a clinical scenario, 2 material properties were utilized, i.e., Young modulus (elastic modulus) and Poisson’s ratio. It was assumed that all the materials were homogenous, linearly elastic and isotropic. Table 4 provides a summary of the physical and mechanical properties of the biomaterials used in the study.9, 10
Boundary conditions
When a model is constructed in a computer and force is applied, it will act freely and move in different directions, leading to rotation or translation, or both, without undergoing any strain or deformation. Hence, to study strain and deformation in bone after load application, some areas should be restricted in their degrees of freedom (movement of the node in each direction – x, y and z). These constraints are known as boundary conditions. In this study, the bottom and sides of the maxillary and mandibular bone were fixed during load applications.
Bone–implant contact interface
To simulate immediate implant placement after tooth extraction (non-osseo-integrated contact), non-linear surface-to-surface frictional contact conditions were employed at the implant and bone interface to allow for sliding behaviors.
Loading conditions
The models were loaded at the uppermost surface of the implant as a static load to simulate centric occlusion at an axial direction (90°), which were directed parallel to the long axis of the implant. Two loads were applied: one at 300 N to observe the maximum implant stress at the peri-implant bone; and at 100 N to observe the masticatory response during normal mastication.
Linear static analysis
The assembled finite element models of implants with maxillary and mandibular bone were imported into ANSYS software for linear static analysis. This analysis was performed after the application of 2 different loads (300 N and 100 N) to determine maximum principal stress, equivalent von Mises stress on cortical bone, cancellous bone and zirconia implants, and total deformation.
Post-processing
The results were expressed as equivalent von Mises stresses in megapascals [MPa] and displacement in millimeters [mm]. Pictorial and/or tabular representations of the stresses were prepared. The stress flow in each component was plotted using contour plots and color-coded.
Statistical analysis
As the values obtained through FEA of both designs were discrete and remained constant even after multiple analyses, there was no need for statistical analysis. The values obtained were compared directly.
Results
The obtained color plots were analyzed, and the maximum von Mises stress and strain values were noted and graphed for each condition. The unit of stress was defined as the unit of force [N] divided by a unit of area or length squared, commonly expressed as Pascal [Pa]. In the majority of studies, megapascals [MPa] are employed, as was also the case in our study.
Stress distribution in the FEA models was expressed in numerical values and by color coding. Maximum von Mises stress values were denoted by color red, and minimum values were expressed by color blue. The in-between values were represented by bluish green, greenish yellow and yellowish red in the ascending order of stress distribution.
A comparison of maxillary and mandibular fin and bulb designs at 100 N revealed that the bulb design exhibited lower stress values than the fin design, indicating that the stress distribution was more pronounced in the bulb design. Additionally, when the equivalent stress distribution of cortical and cancellous bone was compared, the cortical peri-implant bone exhibited more stress concentration than the trabecular bone in both designs. These findings were replicated at 300 N. The von Mises stress values were measured and averaged at the implant and supporting tissues for both groups (Figure 3,Figure 4).
While comparing maxillary and mandibular RAIs with the fin design under both loads, the overall von Mises stress of the zirconia implant in the maxillary fins was three to four times lower than in the mandibular fins. Similarly, the overall von Mises stress of the bone and zirconia implant in the maxillary bulbs was three to five times lower than that of the mandibular bulbs. This indicates that the stress distribution is more favorable in the maxillary molar region compared to the mandibular molar region.
To ensure primary stability of the implant, the microdisplacement between the bone and the implant interface was evaluated. The lowest levels of contact separation were measured in the fin design of both maxillary and mandibular RAIs under vertical forces of 100 N and 300 N. Figure 4E displays the micromotion values of maxillary and mandibular RAIs with the fin and bulb designs.
Discussion
Dental implants are a widely used treatment option for replacing missing teeth. Pure titanium and its alloys are the most preferred materials for implantation. However, titanium has its limitations in cases where aesthetic demands are high after soft tissue recession, when the black metallic components become visible through the mucosa, and also in instances of allergic reactions to metallic restorations. Conventional implants have not undergone significant changes in form or material over the past 40 years.11, 12 Ceramic restorations are now preferred by many patients. In response to this growing demand, technological advancements in additive manufacturing and CAD/CAM technology have led to the development of enhanced ceramic-based structures for patient-specific implant-supported prostheses.1, 13
Zirconia-based materials are one of the most commonly used ceramic materials. They have the potential to replace titanium implants due to their excellent biocompatibility, improved aesthetic results, high flexural strength, fracture toughness, high chemical resistance, and tooth-like color. The invention of additive manufacturing and CAD/CAM technology has enabled the production of these materials in the shape of tooth roots, facilitating immediate placement in the socket following extraction.14
Conventional zirconia implant designs offer a limited range of diameter and length options, which may not suit the needs of all patients. To overcome this challenge, customized RAIs have been introduced, reducing both hard and soft tissue trauma and enhancing initial stability.15, 16 Additionally, to address the challenges associated with sandblasting and etching of RAIs, as well as to enhance osseointegration, various micro- and macro-retentive modifications have been applied to the root surface of RAIs.17, 18, 19 Other authors have analyzed more extensive root surface modifications and evaluated their biomechanical effects, such as stress distribution, and primary and secondary stability, using FEA as a tool prior to its application in a patient.2, 8, 10, 11, 20
Numerous studies have examined the biomechanical response of standard dental implants. However, literature on RAIs is limited, particularly with regard to the posterior region. Therefore, in this study, we planned to fabricate customized RAIs in the maxillary and mandibular posterior regions. Given the potential limitations of micro-retentions in ensuring implant mechanical stability, we incorporated 2 targeted press-fit macro-retentive features: fins and bulbs. These designs were selected based on the findings of Moin et al., who reported that incorporating targeted press-fit geometry, such as fins or bulbs, into the standard RAI design, enhances stress distribution, reduces the concentration of bone stress, and improves primary stability.8 However, these modifications are novel and have not yet been tested in patients. The present study employed FEA as a tool to investigate bone stress and strain around implants and relative microdisplacement between bone–implant interfaces.
The finite element analysis revealed that the von Mises stress and microdisplacement values for both designs in the mandibular molar region exceeded those in the maxillary molar region. This discrepancy can be attributed to the fact that large and wide implants exhibit reduced compressive and tensile principal stresses compared to short and narrow implants. The larger implants exhibit increased bone-to-implant contact area to dissipate the masticatory forces, leading to functional load distribution. Additionally, implants with a wider diameter have superior circumferential bone contact, which reduces stress and the likelihood of implant fracture.21 Maxillary molar teeth possess a larger cross section and buccolingual width compared to mandibular molar teeth, and maximum equivalent stresses occur in the mandibular molar region during mastication.22 Furthermore, maxillary teeth have 3 roots, indicating that the area of bone–implant contact is more pronounced in maxillary implants compared to mandibular implants, which is more significant for stress distribution.23, 24
A comparison of the fin and bulb designs demonstrated that the von Mises stress values for the implant, cortical bone and cancellous bone at both loads (300 N and 100 N) were lower for the bulb design than for the fin design in both the maxillary and mandibular molar regions. Previous studies have reported that the incorporation of targeted press-fit design characteristics into standard RAIs reduces the maximum von Mises stress in the peri-implant bone, leading to more positive load behavior.8, 20, 25 However, the primary stability of the fin design was superior to that of the bulb design in both regions because the bulb design exhibited greater microdisplacement.
A comparison of the equivalent stress in cortical and cancellous bone revealed that the cortical bone exhibited greater stress concentration. Misch stated that the cortical bone had a significantly higher percentage of bone–implant contact than the trabecular bone.26 Thus, inappropriate loading can lead to excessive stress accumulation and subsequent bone loss. To avoid this situation, the diameter of the implant was reduced near the cortical bone, thereby averting crestal bone loss and fracture. This phenomenon has been documented in the studies conducted by Lin et al.11 and Memari et al.24
To the best of our knowledge, the present study is the first to compare the fin and bulb design modification of RAIs in maxillary and mandibular posterior regions using FEA. Previous studies have employed titanium to customize RAIs. However, numerous studies have shown that zirconia is a viable alternative to titanium, with greater soft tissue response, biocompatibility and aesthetics, as well as equivalent osseointegration.9, 27 Hence, zirconia was chosen as a RAI material. The majority of previous studies focused on the anterior and premolar regions.8, 20, 25 Therefore, the present study was designed to test RAIs in the maxillary and mandibular posterior regions.
Limitations
Despite the study’s innovative nature, it had certain limitations. The loads used in the study were static and unidirectional, with amplitudes of 100 N and 300 N. However, it should be noted that changing loads are observed in patient clinical scenarios. The peri-implant bone was modeled as a homogeneous, isotropic, linearly elastic material. Furthermore, the biomechanical behavior of biological tissues is known to be diverse, anisotropic and non-linear. Given the complexity of non-linear contact analysis, the interaction between the bone and the implant was modeled as linear contact. It is also notable that the results of FEA in dentistry should be regarded as a complement to clinical investigations, with the objective of enhancing understanding of the impact of specific variables on the clinical performance of implants.
The present study investigated implant microdisplacement and the overall stress distribution at the bone–implant contact. Because bone possesses both ductile and brittle qualities, maximum (tensile stress) and minimum (compressive stress) principal stress, as well as maximum shear stress should be evaluated in future investigations. Additionally, studies that compare the von Mises stresses, principal stresses and displacement of root-analogue zirconia implants with root-analogue titanium implants are necessary. The clinical condition may not have been fully duplicated since FEA is a computational in vitro study. Furthermore, given the variability inherent in individual clinical situations, long-term in vivo investigations on root-analogue zirconia implants are required.
Conclusions
In the context of the study’s limitations, it can be concluded that the customized root-analogue zirconia implants with the bulb design exhibit superior stress distribution in the surrounding bone compared to the customized RAIs with the fin design. The micromotion values of the fin design were lower than those in the bulb design, indicating enhanced primary stability of the fin design. The stress distribution of both designs demonstrates lower stress values in the maxillary posterior region compared to the mandibular posterior region. However, long-term clinical trials are necessary to determine the optimal design and evaluate its long-term functionality and mechanical resistance.
Ethics approval and consent to participate
Not applicable.
Data availability
The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.
Consent for publication
Not applicable.
Use of AI and AI-assisted technologies
Not applicable.
