Stresses in lithium disilicate crowns and zirconia implants in patients with bruxism: An in silico study

The aim of this study was to analyze by finite element analysis the influence of the use occlusal splints in rehabilitation with zirconia implant under oblique and vertical masticatory loads. Four models were developed to simulate a clinical of absence of a premolar (element 45) replaced by zirconia implant and lithium disilicate crown. Four groups were created, SP-V without occlusal splint and vertical load; CP-V with occlusal splint and vertical load; SP-O without occlusal splint and oblique load; CP-O with occlusal splint and oblique load. The four models were built using a software (SolidWorks, SolidWorks Corporation). A load of 300N to 45o (oblique) and 90o (vertical) applied to the long axis of the whole structure. The maximum principal stress (tensile) and minimum principal stress (compression), as well as the total deformation in the implant, occlusal splint, crown and bone tissue were evaluated quantitatively and qualitatively. The CP-V and CP-O groups presented the lowest stress intensities, which were homogeneously distributed in all structures analyzed. On other hand, SP-V and SP-O groups presented highest stress distributed in a heterogeneous way. Groups with occlusal splint (CP-V and CP-O) also showed lower deformation than groups without occlusal splint (SP-V and SP-O). It ́s concluded the use of occlusal splint minimizes the stresses and deformation promoted by oblique and vertical occlusal loads of up 300N in implanted lithium disilicate crown supported by zirconia implants.


Introduction
The titanium implants have elevated clinic success rates (Galvão et al., 2016) However, the main titanium's implant disadvantage is its color, which can be seen either through soft tissues or gingival recession, therefore it compromises the restoration's esthetic. The zirconia-based ceramics are restored esthetics materials that can be used as an alternative material to titanium (Hashim et al., 2016). These ceramics have both optical and mechanical properties similar to the natural tooth (Andreiotelli et al., 2009). In addition, they present elevated physical properties, such as being resistant to bending, fracture toughness and hardness (Lee et al., 2013).
The elevated physical properties of zirconia are related to the presence of yttrium oxide (yttria). Yttria-stabilized polycrystalline tetragonal zirconia (Y-TZP) presents a volumetric expansion of its tetragonal crystals under high loads. This expansion causes stress compression that opposes: the applied load, the possible crack formation, and propagation (Hannink et al., 2000).
Patients with bruxism are constantly with dental and prosthetic elements under high occlusal load (Boulad et al., 2019). Thus, bruxism is an etiological oral rehabilitations failure factor (Zhou et al., 2016). The manufacture and usage of the occlusal splint are simple procedures, safe, and non-invasive to the bruxism treatment. The splint sets the temporomandibular joint (TMJ) in a stable joint position by protecting the teeth from excessive chewing loads enhanced by bruxism (Boulad et al., 2019).
Although bruxism is a deleterious habit that affects about 80% of the population (Zhou et al., 2016), only few data are available in the literature on how the elevated stress caused by bruxism affects zirconia implant-supported prostheses and the adjacent bone tissue. Furthermore, due to the increasing of ceramic implant systems commercially available and the increasing demand for highly esthetical restorations, studieshas shown substantial interest for the dental surgeon to assess the prognosis of the rehabilitative treatment.
Thus, the aim of this study is to evaluate the influence of the occlusal plate used in a rehabilitated mandible with zirconia implant under oblique and vertical masticatory loads by using the finite element method. The tested hypothesis were that the use of an occlusal plate would decrease the stresses caused on the implant, crown, and adjacent bone tissue.

Methodology
The entire finite element analysis procedure consisted of three stages: preprocessing, processing and postprocessing for analysis results.
On the preprocessing, the fabrication of the geometric model was carried out and information regarding the material's mechanical properties used in the model construction was added too. The structure of the model was split in finite number of elements, which were interconnected through nodal points that lie in the three-dimensional coordinate system. The resulting set was called mesh.
Four three-dimensional models were developed, reproducing a clinical situation with the absence of a mandibular premolar (element 45). This tooth was replaced by a zirconia implant and lithium disilicate crown on implants in order to stimulate both oblique and vertical occlusal loading in bone-integrated implants.
With the aim of making three-dimensional maxilla bone model, imagens of the artificial toothed mandible were obtained through a computed tomography (I-CAT Cone Beam 3D Dental Imaging System, Imaging Sciences International, Hatfield, PA, USA). In addition, finite elements were developed with a software aid (SolidWorks, SolidWorks Corporation, Concord, MA, USA). Lastly, an IPS emax lithium disilicate prosthetic crown (Ivoclar, Vivadent) was virtually reproduced to all groups using an image previously obtained by computed microtomography. The models were distributed to the respective study groups (Figure 1  The materials were considered isotropic, homogeneous, and linearly elastic. The total of elasticity modulus and Poisson's ratio described in Table 1 were transferred to the software and all connections were considered closely bonded (without friction).  Vieriu et al. (2015).
Some measures were taken to avoid stress concentration errors existing in the meshes, therefore, improving its efficiency such as the refinement at the interfaces between solids with a maximum element size of 0.2 mm, element growth factor of 1.2, and elements being 10-node tetrahedra. The meshes for the finite element analyses were produced and their integrity related to the quality of the elements in the interested regions were evaluated.
Afterwards, the processing was computationally performed simulating the application of a 300 N load at 45° (oblique) and 90° (vertical) to the extended axis of the entire structure.
Finally, the post-processing consisted of analysing the model and comparing them quantitatively and qualitatively using the obtained and evaluated results through maximum principal stress (tension), minimum principal stress (compression), and total deformation in the implant, plate, crown, and bone tissue.

Results
The Figure    Research, Society and Development, v. 10, n. 5, e29710515099, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i5.15099 6 However, the minimum principal stress in CP-O group was centred in the alveolar bone crest and apical bone region (0.12 MPa). Regarding the implant, the minimum principal stress was centred in every thread and mainly in the apical implant region, with a maximum intensity of 3.0 MPa (Figure 4).      Research, Society andDevelopment, v. 10, n. 5, e29710515099, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i5.15099 11 The SP-O group had the highest minimum principal stress concentration on the alveolar bone crest regarding to both buccal and lingual surfaces with a maximum intensity of 12.0 MPa in the lingual side. On the implant, the stress dispersed throughout the structure with a maximum intensity of 32,7 MPa on the implant cervical region regarding the lingual side ( Figure 10). Source: Authors.
The Figure 11 show the distributed stresses (arrow) in the group without plate and vertical load (SP-V). As in the SP-O group, these stresses distribute itself heterogeneously, concentrating in the crown occlusal third of tooth 45. However, the total maximum strain is equal to ~0.006 MPa. On the bone, the maximum principal stress was also concentrated in the alveolar bone crest, covering the lingual face as the SP-O group. However, the maximum intensity of this stress was 3.9 MPa. The stress on the implant was dispersed through all threads, concentrating a piece more in the apical third (last thread), in a maximum intensity of 16.0 MPa ( Figure   12). Research, Society andDevelopment, v. 10, n. 5, e29710515099, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i5.15099 13 The SP-V group showed the minimum principal stress concentrated in the alveolar bone crest on the buccal and lingual surfaces, in a maximum intensity of 0.13 MPa in relation to the lingual side. On the implant, the stress was dispersed through all the structure in a maximum intensity of 3.2 MPa (Figure 13).

Discussion
The tested hypothesis was accepted due to both groups with occlusal plate (CP-V and CP-O) has shown lower and better stress distribution. The CP-O group showed homogeneous strain result in tooth 44 due to its smaller area when compared to the crowns of teeth 45 and 46. On the same group, the maximum principal stress was concentrated in the bone near the implant cervical area above the first thread. In other words, in the crown-implant, where the most fragile assembly area is found (Lombardo et al., 2019;Bergamo et al., 2021). Concerning the implant, this stress was concentrated in its apical area and the minimum principal stress on this groups was concentrated in the alveolar bone crest and on the apical bone region due to the compression stress. The same situation happened in the implant, which the same stress was concentrated in every thread and mainly in the implant apical area.
Regarding the group CP-O, the quantitative stress results were lower than the CP-V group due to the loading type (oblique). In the CP-V group, the maximum principal stress was also concentrated in the bone, near the cervical implant region, above the first thread, yet with greater intensity. Just as the CP-O group, the stress on the implant was concentrated in its apical area, after the last thread, but in greater intensity too. Regarding the minimum principal stress, the CP-V group had higher concentration in both the alveolar bone crest and apical bone area, similar to the CP-O group, yet in a higher intensity.
The stress on the implant was concentrated in every thread, mainly in the apical implant area.
The stress on the SP-O group was distributed heterogeneously, concentrating itself on the implant (45) in a higher occlusal third tooth crown strain result (45). The result can be explained due to the fact that the implant does not have periodontal ligament, which provides the natural teeth to function under the load chewing pressure (Washio et al., 2018;Moga et al., 2021;Ono et al., 2021). On the bone side, we can see that the maximum principal stress is centred in the alveolar bone crest of the lingual face, which is the opposite side of the load applied. This maximum intensity stress was higher than in the CP-O group. On the other hand, this stress was concentrated in the implant from the cervical third until the third thread. The minimum principal stress in the CP-O group was more concentrated in the alveolar bone crest in the buccal and lingual sides, in a higher result on the lingual side.
As in the SP-O group, but lower strain result, these stresses in the SP-V group were also distributed heterogeneously, concentrating in the occlusal third of the tooth crown 45. As well as the SP-O group, the maximum principal stress on the bone was also concentrated on the alveolar bone crest in the lingual surface, however, the maximum intensity of stress was lower in this group. In the implant, this stress was dispersed through all threads, concentrating a piece more in the apical third (last thread). Contrarily, the minimum principal stress was concentrated in the alveolar bone crest, yet with less intensity than in the SP-O group. The occurrence is due to the fact that the load was vertical and not oblique (Moura et al., 2020;Hong et al., 2020;Sesha et al., 2020). Due to the implant does not have a function in the study, this stress was dispersed through all the structure (Hajizadeh, Panahi, 2019;Brandão de Holanda et al., 2020).
Overall, both the implant and the bone tissue showed high stress and strain on the models that the occlusal plate was not applied/used (SP-O and SP-V), which may lead to bone loss and, consequently an implant loss (Brandão de Holanda et al., 2020;Meijndert et al., 2021;Moga et al., 2021). In regard to the crown, the stresses were higher in the groups without plate (SP-O and SP-V) when compared to the groups with plate (CP-O AND CP-V), which may lead to a fracture in the ceramic crown (Lee et al, 2013;Brandão de Holanda et al., 2020;Hafezeqoran et al., 2020).
Lastly, the best results in the pattern groups with occlusal plate are related to the fact that the occlusal plate absorbs the masticatory loads stress (Domanic et al., 2020;Henrique et al., 2020). In addition, there is a decrease of the muscle activity due to the growth in the patient's vertical dimension, stimulated by occlusal plate use (Boulad et al., 2019). The occlusal plate helps the patient to not clench and/or grind the teeth when sleeping and, as a result, these patients become aware of their occlusal problems when awake (Boulad et al., 2019;Henrique et al., 2020). The groups that used the plate showed the lowest stress results, supporting the studies that proves the occlusal plate decreases the stresses caused on teeth and implants by patient with bruxism (Domanic et al., 2020;Henrique et al., 2020).

Conclusion
The use of occlusal splint minimizes the stresses and deformation promoted by oblique and vertical occlusal loads of up 300N in implanted lithium disilicate crown supported by zirconia implants.