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 Table of Contents  
Year : 2019  |  Volume : 11  |  Issue : 1  |  Page : 50-53

Monolithic zirconia crown does not increase the peri-implant strain under axial load

1 Department of Dental Materials and Prosthodontics, São Paulo State University, São José, Brazil; Department of Dental Materials Science, Academic Centre for Dentistry Amsterdam, Free University Amsterdam, Amsterdam, Noord-Holland, The Netherlands
2 Department of Dental Materials and Prosthodontics, São Paulo State University, São José, Brazil
3 Department of Dental Materials and Prosthodontics, Faculty of Pindamonhangaba (FUNVIC), Pindamonhangaba, Brazil

Date of Web Publication27-Feb-2019

Correspondence Address:
Dr. Amanda Maria De Oliveira Dal Piva
Department of Dental Materials Science, Academic Centre for Dentistry Amsterdam, Free University Amsterdam, Gustav Mahlerlaan 3004, 1081 LA Amsterdam, Noord-Holland
The Netherlands
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jioh.jioh_307_18

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Aims: This study aimed to evaluate the influence of the crown type on the cervical microstrain around an external hexagon implant. Subjects and Methods: A dental manikin was impressed with addition-polymerizing silicone, and a hemiarch model was obtained with polyurethane resin. Then, a 3.75 mm × 11 mm implant was installed with 40 N/cm of torque in the region of element 36. Two groups were separated according to the type of crown used for rehabilitation: metal-ceramic crown (n = 10) or monolithic zirconia crown (n = 10). All crowns presented similar anatomy, with contact point in elements 35 and 37. Then, the polyurethane model was cleaned with isopropyl alcohol, and four strain gauges were bonded with cyanoacrylate adhesive in different areas (bucccal, lingual, mesial, and distal) around the implant. The crowns were installed with 20 N/cm torque, and an axial load (30 kgf) was applied in the center of the crown. Statistical Analysis: After performing 10 readings in each specimen, the data were analyzed by two-way analysis of variance and Tukey's test, all with α = 5%. Results: The results showed no statistical difference for the microstrain between the analyzed crowns (P = 0.065), and the microstrain values were different according to the area (P = 0.002): buccal (1514.9 ± 233.8) > lingual (1280.8 ± 245.5) > distal (373.2 ± 105.2) > mesial (216.7 ± 111.4). Conclusions: The crown type did not modify the microstrain in the peri-implant tissue.

Keywords: Dental implants, prosthetic dentistry, strain gauge

How to cite this article:
Tribst JP, Dal Piva AM, Riquieri H, Nishioka RS, Bottino MA, Rodrigues VA. Monolithic zirconia crown does not increase the peri-implant strain under axial load. J Int Oral Health 2019;11:50-3

How to cite this URL:
Tribst JP, Dal Piva AM, Riquieri H, Nishioka RS, Bottino MA, Rodrigues VA. Monolithic zirconia crown does not increase the peri-implant strain under axial load. J Int Oral Health [serial online] 2019 [cited 2023 Dec 4];11:50-3. Available from:

  Introduction Top

Several studies have demonstrated that peri-implant microstrain is directly related to the longevity of dental implant osseointegration.[1],[2],[3] The reason is because the juxtaposition of the dental implant with the newly formed bone tissue is not totally static,[4] and bone remodeling also occurs due to the masticatory load dissipation.[5] Therefore, clinicians have concern regarding the correct dissipation of masticatory loads, without inducing undesirable bone remodeling.[6] Factors such as implant inclination,[7] masticatory loading direction,[6],[8] vertical misfit between restoration/implant,[8],[9] type of connection,[9],[10] and restorative material[2] can modify the stress concentration in the system of an implant-supported prosthesis. However, no data is available regarding the effect of crown-type on the stress distribution in an implant-supported prosthesis.

The use of zirconia ceramic in dentistry is directly related to its high elastic modulus (220 GPa) and the high flexural strength of the restoration, which guarantees reliability to the dental surgeon in the resistance to fatigue of this material.[11] Zirconia restoration adhesion to dental cements is still a challenge due to the difficulty of maintaining a stable bond with a purely crystalline material.[12] Thus, the use of zirconia in implant-supported restorations seems to be a way to avoid the adhesive problems of this material because it is possible to make screwed or even cemented restorations on titanium bases with more parallel walls than a dental preparation. The use of monolithic restorations of zirconia onto implants promotes a greater possibility of damage in the prosthetic connection. This damage is caused by the difference in the hardness values of titanium and zirconia.[13] Moreover, mechanical problems are more common in implants with zirconia components than in titanium.[2]

There are few in vitro reports that have evaluated the influence of the use of implant-supported zirconia crowns on the peri-implant tissues. Thus, the metal-ceramic crown is considered to be the gold standard in dentistry[14] due to its long history of proven clinical success and well-documented scientific basis, and it is important to compare new restorative modalities with metal-fused ceramic crowns. Therefore, questions about whether the crown influences the distribution of stresses in the peri-implant tissue are raised. For laboratory studies to evaluate stresses and load distribution, the use of a bone tissue simulator material with the same mechanical behavior can guarantee that the system has a reproducible pattern in all specimens and makes the results more concrete for inferences on the influence of the variables in the bone. Polyurethane resin[6],[10] was the chosen material herein due to its modulus of elasticity and scientific validation. One of the bioengineering tools able to demonstrate the mechanical behavior of implant-supported restorations in vitro and in vivo is the strain gauge methodology.[10],[15] An electrical circuit forming the Wheatstone bridge enables interpretation according to the variation in resistance due to a mechanical deformation that will generate an output stress other than zero, representing the strain of the system. Therefore, the goal of this study was to compare the peri-implant strains generated around implant-supported metal-ceramic crowns or monolithic zirconia crowns with the strain gauge method. As the specific purpose, this study aimed to evaluate the influence of crown type on four regions around the implant (buccal, mesial, distal, and palatine). The null hypothesis was that there would be no difference in peri-implant strain according to the crown used.

  Subjects and Methods Top

A half mandible model was constructed using polyurethane resin (Polyurethane F16 Axson, Cercy, France) created in a dental manikin (Manequins Odontológicos Marília Ltda, São Paulo, Brazil). For this, a rigid custom aluminum tray was used to copy the dental manikin using vinyl polysiloxane impression material (Elite H-D Putty and Elite H-D Light Body (Zhermack, Rovigo, Italy). All materials were mixed in standardized proportions according to the manufacturer's recommendations. The model surfaces were regularized with granulated sandpapers of #220 to #600 (3M ESPE, St. Paul, USA), and then one implant (3.75-mm diameter, 10 mm) was installed following a conventional drilling protocol (AS TECHNOLOGY TITANIUM FIX, São Paulo, Brazil).[7] The external hexagon implant presented 0.7 mm of hexagon height and internal torque. The installation torque was 40 N/cm using manual torque wrench. The model surface was cleaned with isopropyl alcohol, and four linear strain gauges (Model KFG-02-120-C1-11, Kyowa Electronic Instruments Co., Ltd., Tokyo, Japan) were attached to polyurethane model with cyanoacrylate adhesive (Super Bonder Loctite, São Paulo, Brazil) in four different areas. Next, the calibration of each strain gauge was performed using a multimeter (Minida ET 2055, São Paulo, Brazil).[7]

Next, a universal castable abutment was positioned on the implant platform, and a first molar crown was made in wax. This crown model was used for manufacturing a ceramic fused to a metal crown (VM13, Vita Zhanfabrik, Bad Säckingen, DE) and a zirconia crown (VITA YZHT, Vita Zhanfabrik, Bad Säckingen, DE) with similar anatomies. Both restorations received a surface finishing with abrasive rubbers and presented direct connection with the implant hexagon without abutments. Each crown was fixed with 20 N/cm. After that, constant static loading (30 kgf, 10 s) was promoted by a device with a 2-mm rounded tip in a universal testing machine (50-kgf load cell). The variations of electrical resistance were converted to microstrain units through an electrical signal (n = 10 and n = 20) conditioning apparatus (Model 5100B Scanner-System 5000-Instruments Division Measurements Group, Inc., Raleigh, North Carolina, USA).[7] Data recording after 10 load applications was performed using StrainSmart software (Instruments Division Measurements Group, Inc., Raleigh, North Carolina, USA), totaling 40 data for each specimen according to the evaluated region. All steps were performed by only one operator to avoid bias and to standardize the samples preparation and analysis. The strain assumed as acceptable respected the principle of Wolf's law bordering up to 3000 μm/μm.[16]

Statistical analysis

Descriptive statistics consisted of means and standard deviations, and inferential statistical analysis consisted of 95% confidence interval and two-way ANOVA analysis of variance followed by the Tukey's test, all with α = 5%, using MINITAB software (version 16.1.0, Minitab, State College, Pensilvânia, USA).[7] The evaluated factors consisted of the type of crown (two levels) and the location of the strain gauges (four levels). [Figure 1] illustrates the crowns, strain gauge positions, and load application.
Figure 1: Strain gauges positioning. (a) Zirconia and metal-ceramic crowns. (b) 30-kgf load was applied in the center of the crown. Four strain gauges were glued to the model to evaluate the microstrain in the peri-implant area (c). (d) One on each region: buccal, mesial, distal, and palatine.

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  Results Top

The results were collected after loading for 10 s. Two-way ANOVA [Table 1] revealed that there was no influence of the crown type (P = 0.065) on the peri-implant microstrains. Therefore, the mean values of microstrain for the zirconia crown (919.2 ± 213.8) were similar to the metal-ceramic crown (823.5 ± 156.9). However, the location of the strain gauges showed significance (P = 0.002). In observing the location of the strain gauges, it is possible to notice that the buccal region showed the higher mean stress values, followed by the lingual, distal, and mesial regions, respectively [Table 2]. Assuming 3000 μ as the baseline strain value[7] for bone resorption, it is possible to assume that both crowns are not able to induce unwanted bone remodeling in any of the four evaluated regions.
Table 1: Two-way analysis of variance results considering the factors “crown type” (two levels) and “strain gauge location” (four levels), α=5%

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Table 2: Microstrain mean values, standard deviation, and Tukey's test according to the factor “strain gauge location” (regardless of the type of crown), with P=0.002

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  Discussion Top

The aim of this study was to evaluate the bone microstrain generated around dental implants rehabilitated with monolithic zirconia crown or metal-ceramic crown under axial load. Based on the results, the null hypothesis was accepted because there was no influence of the type of crowns on the microstrain around the implant.

A number of studies have examined the effect of restorative materials on implant prostheses.[2],[17],[18] The present study corroborates with reports that dental materials do not modify the distribution of stresses in the bone.[1],[2],[3] This fact may be associated with the restoration presenting an anatomical substrate model, with contact points in the proximal regions (mesial and distal) of the crown. Studies with unitary and absent contact point crowns could present different results because the absence of the contact point is responsible for significantly increasing peri-implant microstrain.[19]

It is noteworthy that linear strain gauge consists of an in vitro method limited to the analysis of the regions whose strain gauges were glued.[6],[15] Therefore, other areas may have been deformed during the analysis; however, these data were not computed. The regions whose strain gauges were glued corresponded to regions of greater strain accumulation reported by previous studies,[1],[2],[7],[20] as well as the area of crestal bone where bone remodeling and insertion loss begin.[21]

In evaluating different regions whose strain gauges were glued, it is possible to notice a greater strain value in the regions where the polyurethane resin volume in the hemimandible model is smaller. Thus, the buccal region is thin, deforming more during an axial load compared to the other regions. This finding is important to elucidate that regardless of the crown material, the anatomy of the supporting tissue may be responsible for greater or lesser cervical bone deformation.

Thus, the results corroborate other studies reporting buccal bone resorption.[21],[22] However, previous investigations using simplified models of substrates such as blocks[8],[10] or cylinders[2] should be carefully evaluated because they do not have the individual variations of each region found in an anatomical bone model. Similarly, the results obtained on bone simulant materials, although validated in the literature,[7],[23] do not necessarily express what occurs in a bone model containing trabeculae, vessels, and nerves. However, a laboratory experiment is still preferable to create and answer hypotheses because it dispenses with the use of animals at that first moment.

Finally, it is worth mentioning that this study was only based on an in vitro methodology whose limitations do not allow direct extrapolation for clinical applications without a comparative and complementary reading with other literature findings.[9] Herein, a crown made in zirconia with indication for monolithic crown was evaluated. However, the authors do not indicate the manufacture of monolithic zirconia restorations directly on the implant platform, even if this restoration induces the same mechanical response around the peri-implant tissue as a metal-ceramic crown. This is because the wear of the prosthetic platform[13] has been reported as problems for this modality of restoration, which was not evaluated in this manuscript.

  Conclusions Top

Despite the limitations of this in vitro study, it is possible to conclude that both crowns presented promising results for the dissipation of masticatory loads in the peri-implant tissue with the buccal region as the most stressed area.

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Conflicts of interest

There are no conflicts of interest.

  References Top

Santiago Junior JF, Pellizzer EP, Verri FR, de Carvalho PS. Stress analysis in bone tissue around single implants with different diameters and veneering materials: A 3-D finite element study. Mater Sci Eng C Mater Biol Appl 2013;33:4700-14.  Back to cited text no. 1
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Wazeh AM, El-Anwar MI, Atia RM, Mahjari RM, Linga SA, Al-Pakistani LM, et al. 3D FEA study on implant threading role on selection of implant and crown materials. Open Access Maced J Med Sci 2018;6:1702-6.  Back to cited text no. 3
Vasconcellos LM, Villaça-Carvalho MF, Prado RF, Santos EL, Regone NN, Pereira MC, et al. A study about cell activity on anodized Ti-6Al-4V by means of pulsed current. J Eng Sci Technol 2017;12:1240-52.  Back to cited text no. 4
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Tribst JP, Rodrigues VA, Dal Piva AO, Borges AL, Nishioka RS. The importance of correct implants positioning and masticatory load direction on a fixed prosthesis. J Clin Exp Dent 2018;10:e81-7.  Back to cited text no. 6
Rodrigues VA, Tribst JP, Santis LR, Borges AL, Nishioka RS. Biomechanical effect of inclined implants in fixed prosthesis: strain and stress analysis. Rev Odontol UNESP 2018;47:237-43.  Back to cited text no. 7
Rodrigues VA, Tribst JP, Santis LR, Lima DR, Nishioka RS. Influence of angulation and vertical misfit in the evaluation of micro-deformations around implants. Braz Dent Sci 2017;20:32-9.  Back to cited text no. 8
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Kondo T, Komine F, Honda J, Takata H, Moriya Y. Effect of veneering materials on fracture loads of implant-supported zirconia molar fixed dental prostheses. J Prosthodont Res 2018. pii: S1883-1958 (18) 30207-X.  Back to cited text no. 18
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Matthys C, Vervaeke S, Jacquet W, De Bruyn H. Impact of crestal bone resorption on quality of life and professional maintenance with conventional dentures or locator-retained mandibular implant overdentures. J Prosthet Dent 2018;120:886-94.  Back to cited text no. 21
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