Influence of Different Soft Liners on Stress Distribution in Peri-Implant Bone Tissue During Healing Period. A 3D Finite Element Analysis

Since their introduction, dental implants have become a successful restorative modality in clinical dentistry, with a reported success rate of over 90%.¹  Nowadays, oral rehabilitation with dental implants has become the best choice for treatment because it solves many problems of treatment with conventional dentures,²  exceeding the pretreatment expectations for both esthetics and function.³ 

The first studies related to implantology advocated that after the placement of an implant, it has to be maintained undisturbed for 3 to 6 months of healing to obtain osseointegration without major problems⁴ ; this is because the micromotion caused by loading it earlier may induce fibrous encapsulation instead of a direct bone-implant interface.^5,6  At this time, it is safe to say that completely undisturbed healing of the implant-bone interface is not necessary for successful osseointegration to occur,⁷  and according to an experimental work, the healing of peri-implant bone under a load seems to be beneficial.⁸  However, these findings do not imply that the protocol of delayed loading is no longer needed⁹  since the literature reports that the success of immediately loaded implants is dependent on several factors. Among them, primary stability is the key factor: micromotions at the bone-implant interface beyond 150 μm may result in fibrous encapsulation instead of osseointegration.¹⁰  Thus, when primary stability cannot be achieved (30 N/cm),¹¹  it is recommended to opt for adequate healing time before loading.⁹ 

During the healing time, the patient needs to wear a conventional prosthesis on a provisional basis to maintain proper function and esthetics until the placement of the implant-supported prostheses. Using this temporary prosthesis, implants, even when submerged, receive transmission of forces from both chewing and parafunctional habits,¹²  which may affect osseointegration. Relining this prosthesis with soft liners is usually recommended to avoid overloading the implants. Even when relined, it is possible that excessive load will occur on the implants, leading to implant exposure, marginal bone loss, and/or failure in osseointegration.¹¹ 

A recent study¹²  evaluated the influence of the height of healing caps and the use of soft liner materials on the stress distribution in peri-implant bone during masticatory function in conventional complete dentures using finite element analysis. The most reliable method to evaluate the biomechanical response of a given situation is a clinical evaluation; however, the study of intraosseous structures is impractical because of ethical and methodologic issues.^13–15  Thus, finite element analysis becomes a powerful tool to investigate the forces that occur in the bone similarly to what happens in vivo, giving accurate and reliable information about the biomechanics involved in a given situation.^16–20 

The purpose of the present study was to verify, using a three-dimensional (3D) finite element analysis (FEA), the stress concentration in the bone near submerged implants in a fully edentulous mandible when force was applied in the posterior or anterior teeth of conventional complete dentures relined or not relined with soft liner materials. The following null hypotheses were set: (1) the use of soft liners in complete lower dentures during the healing time does not affect the stress concentration in the bone adjacent to the implants, and (2) there is no difference in the stress concentration in the bone near implants when liners are used with different hardness properties.

Three-dimensional finite element models reproducing a severely resorbed jaw with 2 and 4 submerged titanium implants (4.0-mm diameter × 10-mm length) in the anterior region and conventional complete dentures seated on the mucosa were modeled as standard models using specific 3D modeling software (SolidWorks 2010, SolidWorks Corp, Concord, Mass). The implant thread was removed because, after convergence tests, they were found to be irrelevant to the analysis and caused a relevant reduction in elements.

Finite element models were obtained by importing the solid model into mechanical simulation software (ANSYS Workbench 11, Ansys Inc, Canonsburg, Pa). The models were divided into 2 groups according to the number of implants and then in 4 subgroups each, varying among a control (with the denture base formed only by acrylic resin), and the others presenting a 3-mm thick layer of different soft liner materials, as shown in Table 1.

All materials used in the models were considered to be isotropic, homogeneous, and linearly elastic. The elastic properties used were taken from the literature (Table 2).^21–24 Model stability was ensured to obtain a reliable model that was regarded as relevant with respect to engineering and clinical aspects.¹⁴  The total number of elements generated in the FEA models was 354 417 for the control group with 2 implants, 76 115 for the control group with 4 implants, and 355 140 and 371 666 for the relined denture models with 2 and 4 implants, respectively. The shape of the element was tetrahedral with 10 nodes. The investigated models showed the configurations presented in Figure 1. The stability of the model was checked, and particular attention was paid to the refinement of the mesh resulting from the convergence tests at the bone/implant interface.

The base of the mandible was set to be the fixed support, and loads were applied separately at the right inferior canine (35 N) and the inferior right first molar (50 N), as observed in a clinical study that evaluated the bite force of complete denture wearers.²⁵  Data for the maximum principal stresses and microstrain were produced numerically, color-coded, and compared among the models.

The maximum principal stresses that occurred in the different groups in the mandible with 2 and 4 implants when the load was applied to the canine are presented in Figures 2 and 3, respectively. The highest values of maximum principal stresses and microstrains for each situation in the mandible with 2 and 4 implants are presented in Tables 3 and 4, respectively.

All of the models and situations showed stress concentration in the cortical bone corresponding to the cervical part of the implant. The different hardness of the materials was shown to be relevant in the stress distribution bone adjacent to the implants.

Our first hypothesis was that the use of soft liners in complete lower dentures during the healing time does not affect the stress concentration in the bone adjacent to submerged implants. However, complete dentures relined with soft materials showed lower stress concentration compared to the control groups when the load was applied to the inferior right canine (above the implant) (Figures 2 and 3). When the load was applied to the inferior right molar, this situation was inverted but with a smaller proportion of values, as conventional complete dentures (control) showed the lowest values of stress, corroborating a previous study.¹²  This hypothesis was rejected because the values of stresses and strains when the load was applied to the molar seem to be not relevant in all materials and, when applied to the canine, the value for stress in the control group was more than double that of all the other groups (Tables 3 and 4).

Our second hypothesis, that there is no difference in the stress concentration in the bone adjacent to submerged implants when liners with different hardness properties are used, was also rejected by our findings (Tables 3 and 4). In the molar, all of the values seem to be irrelevant, and in the canine, the values for the stresses were influenced by the hardness of the materials. The softest material showed less stress concentration in the peri-implant bone, being 4.82 times lower than the control group, while the hardest material was only 2.44 times lower than the control group (Table 3).

Our results also did not find substantial differences among mandibles with 2 or 4 implants since the stresses and microstrain values in the mandible with 4 implants showed a slightly higher average than that of the mandible with 2 implants when the load was applied to the canine and molar, with the exception of the conventional dentures, which presented a bigger difference. One would expect the opposite: more implants for support and, thus, lower stress and strain values. However, the implants were submerged and unsplinted, so each implant received and dissipated the forces alone. It is possible that the position of the implant in the arch is more important than the number of implants when complete dentures are used during the healing period. The stresses and microstrains in all groups were concentrated in the cortical bone around the implant neck. A possible explanation of this finding is that the elastic modulus of cortical bone is higher than that of cancellous bone and that cortical bone is much stronger and more resistant to deformation.^16,20  Usually, the stress levels that actually cause biologic response, such as resorption and remodeling of the bone, are not comprehensively known. Therefore, the data regarding stress provided by the FEA need substantiation by clinical research.^7,15  However, according to Frost's mechanostat theory,²⁶  microstrain values are very useful and can indicate whether there will be no response in the bone, bone formation, or loss of bone.²⁷ 

According to this theory, the peri-implant bone in all of the situations was at threshold range for the disuse mode of bone remodeling (50–100 microstrain; 1–2 MPa).²⁶  However, these values for microstrains are related to bony structures that have already formed. Since this study evaluated the bone during osseointegration, it may be supposed that the avoidance of micromotion, which can be related to the minimal values for stress and microstrain, is more important to the success of osseointegration than Frost's zone of mechanical adaptation.

It has been reported that prostheses used during healing time can cause uncontrolled implant loading, which may lead, in some cases, to failed integration.^5,11  The height of the healing caps also has a direct influence on the stress distribution in the peri-implant bone during the healing period.¹² 

Well-done oral rehabilitations, even conventional ones,²⁸  can restore patients' self-esteem, improving esthetics and function.^2,3  Although the success rates of immediately loaded implants are comparable to those of a staged healing protocol, there are greater risks with this approach.^8,9,11  Screw loosening, prosthesis breakage, overloading, and/or parafunction can lead to significant micromovement of the implant, resulting in failure.^6,10,11  Thus, if unfavorable conditions are present or discovered at the time of surgery, the patient should be treated with the traditional submerged healing approach.

FEA has been widely used in the field of oral implantology to estimate peri-implant stresses and strains¹⁷ ; it is a numerical method of analysis for stresses and deformations in structures of any given geometry; the structures are broken down into many small, simple blocks or elements that can be described with a relatively simple set of equations.⁷  This method allows researchers to overcome some ethical and methodologic limitations and, thus, it enables them to verify how the stresses are transferred to the studied structures. However, like any methodology, it has pros and cons. A common limitation is that the analyses are based on a specific set of input values, assumed to be average or representative values, such as specific occlusal loading directions, bone material properties, and bone dimensions.¹³  The materials were assumed to be homogeneous and isotropic and to possess linear elasticity; however, it is known that some materials, such as the cortical bone of the mandible, are isotropic and inhomogeneous.¹⁹  Also, the type, arrangement, and total number of elements may affect the accuracy of the results.^7,15 

In this study, the contact bone-implant was set at 100%, although this is not what happens in reality. A previous study found that the results based on complete osseointegration and nonlinear frictional bone-implant contacts are very similar.⁷  This study furthers our understanding of the stress distribution of mastication on submerged implants during the healing period. The study suggests that the use of softer reline materials is better in reducing peri-implant stresses and strains than harder reline materials.

Abbreviation

FEA

finite element analysis

The authors thank the Department of Products Development of the Renato Archer's Center of Information Technology, Campinas, SP, Brazil, in the person of Pedro Yoshito Noritomi, for its generous help with the finite element analysis. The authors also thank the National Counsel of Technological and Scientific Development (CNPq) for the support to the PhD Program at Piracicaba Dental School, State University of Campinas, SP, Brazil.