|Year : 2018 | Volume
| Issue : 3 | Page : 143-147
A finite element study on effect of cement type and preparation angle on mandibular molar crown restorations' stresses
Rami M Galal1, Salah A Yossef2, Mawadda Adel Alsairafi3, Tariq Muhammad Alkhashem3
1 Department of Fixed and Removable Prosthodontics, National Research Centre, Cairo; Department of Fixed Prosthodontics, Al Nahda University (NUB), Beni Suef; Department of Fixed Prosthodontics, Al Ahram Canadian University (ACU), Giza, Egypt
2 Department of Crown and Bridge, Faculty of Dental Medicine, Al Azhar University, Cairo, Egypt; Department of Restorative Dentistry, Al-Farabi Dental College, Jeddah, Saudi Arabia
3 Afarabi Dental College, Jeddah, Saudi Arabia
|Date of Web Publication||14-Jun-2018|
Dr. Rami M Galal
59 4th, Touristic District, 6th of October City, Giza
Source of Support: None, Conflict of Interest: None
Aim: The goal is to assess the efficacy of cement type, preparation angle regarding stresses generated on a crown used for covering mandibular first molar under maximum compressive masticatory force. Materials and Methods: Two three-dimensional (3D) finite element models have been created in the study where molar roots and preparation were modeled by engineering CAD/CAM and crown was 3D scanned. The model components were assembled in ANSYS environment with simplified bone geometry. Four analyses were done on the two models to test cement types (Glass ionomer and resin cement) and preparation angles (10° and 18°). Results: Resin cement showed less von Mises stress by about 4% than the glass ionomer at high preparation angle (18°). With preparation angle of 10°, resin cement showed superior behavior by receiving about 30% less von Mises stress in comparison to glass ionomer. Conclusions: Thin cement layer of 40 μm thickness has no effect on the underneath structures. The E-max crown above resin cement and 18° preparation angle may be better than the same combination with 10° preparation angle for roots and bone. On the other hand, 10° preparation angle reduced the crown stresses dramatically.
Keywords: Cement, crown, E-max, finite element analysis, molar, preparation
|How to cite this article:|
Galal RM, Yossef SA, Alsairafi MA, Alkhashem TM. A finite element study on effect of cement type and preparation angle on mandibular molar crown restorations' stresses. J Int Oral Health 2018;10:143-7
|How to cite this URL:|
Galal RM, Yossef SA, Alsairafi MA, Alkhashem TM. A finite element study on effect of cement type and preparation angle on mandibular molar crown restorations' stresses. J Int Oral Health [serial online] 2018 [cited 2021 May 18];10:143-7. Available from: https://www.jioh.org/text.asp?2018/10/3/143/234523
| Introduction|| |
The crowns are one of the most common prosthetic options for covering and protecting teeth, with many variables affecting its success. Tolerance of stresses and masticatory forces are from the important points affecting crowns success. Nowadays, there is increased demand for high esthetic results by most of the patients. Lithium disilicate (E-max) is one of the materials used for fabrication of crowns with high esthetic quality. It can be made by two techniques, either staining or veneering technique. It is composed of glass ceramic reinforced with lithium disilicate crystals. This ceramic can be used for single crowns and short span three units bridges anteriorly with possibility reach until the second premolar region. Ceramic crowns are complex structure, and their strength and survival depend on several factors such as their shape, thickness of the core and veneering material, technique of fabrication, modulus of elasticity of the materials used, design of the restoration framework, tooth preparation, and residual tooth structure after preparation. The ability of the prosthesis to withstand loads depends on the fracture resistance of the material as well as preparation of suitable design and enough material thickness. Each step in the tooth preparation is critical to have proper retention and resistance form in the end with proper structural durability of the restoration. Proper preparation surely has an effect on the ability of the prosthesis to withstand loads. Furthermore, the design of how we prepare the tooth affects the stress state in the ceramic crown - tooth complex. Some studies stated that it is better to have minimal preparation amounts, but enough amount of preparation decreases the stresses falling on the prosthesis due to its increased thickness hence. Furthermore, it was found that bonding of crowns with resin cement increases its withstanding before failure twice as much as nonbonded crowns. The cement thickness is an important point, a finite element analysis by Proos et al. stated that there was just slight increase of stresses (only 2%) between the adjustment of cement thickness either 50 μ or 100 μ.
The aim of this study is to have enhanced understanding of restorations through comparing the stress patterns induced by loading E-max mandibular molar crown in two combinations of preparation angles (10° and 18°) and cement types (RelyX and glass ionomer).
| Materials and Methods|| |
Two three-dimensional (3D) finite element models were specially made with ANSYS environment with two different preparation angles. Bone shape has been made simpler, mimicked as two coaxial cylinders. The one inside is equivalent to spongy bone (cross section 14 mm, 22 mm high) in the inner core of the second cylinder (as layer 1 mm thick) representing cortical bone (cross section of 16 mm and 24 mm high).,
First molar was modeled with its roots in 3D in three steps. First, the two roots were modeled on commercial general-purpose CAD/CAM software (Solid Works 2013 – Dassault Systèmes SolidWorks Corporation, Waltham, MA, USA). Second, two prepared tooth geometry were modeled with two different taper angles 10°, and 18°. In addition to the two cement layers, a thin layer of thickness of 40 μm were also modeled on the same software. Finally, the crown geometrical shape has been obtained with 3D contacting probe scanner (Roland Modela, model MDX-15; Roland DG Corporation, Hamamatsu) and graphics software (Roland's Dr PICZA 3) to control Roland Active Piezoelectric Sensor. This scanner produces a file with cloud of points coordinates. Another program was used (Rhino 3.0; McNeel Inc., Seattle, Washington) for trimming new surface and then the crown surface is filled forming its volume; crown geometrical shape is transferred into finite element software in SAT format  [Figure 1].
Each material characteristic involved here has been introduced in [Table 1]. That, each material has been assigned to its part (volume) of the complete model before meshing. The mesh producing program has been ANSYS version 14.5 (ANSYS Inc., Canonsburg, Pennsylvania), elements for mesh production of the 3D models have been ten-node tetrahedral elements (Solid 187), with 3° freedoms (translations through global directions). Meshing density was also an important point. With complex geometrical shape, the more meshing density the more accurate results of discrete model (with more accurate stress degrees from regions with great-stresses), the mesh density for the two geometric models is tabulated in [Table 2], while [Figure 2] illustrated one assembled model and sectional cut showing a model details (each color indicate different material). The bonded one mimics good contact where root with adjacent compact bone well integrated, so no gliding or disruption for root-bone connection can happen.
The crown was loaded vertically with 650 N over the central fossa. While the base of the finite element models was in fixed settings, with definite boundaries as a result.
In solid models, finite element linear static analyzing have been done with a computer, Intel Core 2 Duo, processor 2.1 Gigahertz, 2 Gigabyte RAM. Linear static analysis has been performed in this study two times on each model. Each time cement material properties were changed. Linear static analysis was performed on Workstation HP ProLiant ML150, with Intel Xeon 3.2 GHz processors (1MB L2 cache), 10GB RAM, using ANSYS version 14.5, results of these models were verified against similar studies., In this research, four analyses were done for assessing the efficacy of reduction angle, cement type regarding stresses generated by compressive loading.
| Results|| |
Von Misses 4 runs were performed as a combination of changing the two parameters: (1) preparation angle and (2) cement type. As presented in [Figure 3], although the total deformation is the same in the two types of cement, the RelyX showed less von Mises stress by about 4% than the glass ionomer at high preparation angle (18°). With preparation angle of 10°, RelyX showed superior behavior by receiving about 30% less von Mises stress in comparison to glass ionomer.
The root and the prepared part of the tooth were dealt with as one part, which was insensitive to changing cement material in general. On the other hand, only one difference appeared in the root total deflection with preparation angle of 10°. Rely X cement type had 4% more deflection in comparison to glass ionomer as illustrated in [Figure 4].
Crown deformation is not sensitive to cement layer type, while the preparation angle influenced its von Mises stress levels. Glass ionomer showed better behaviour with a low value of stresses with preparation angle 10°; that its' Von Mises stress was 2.5% less in comparison to Relay X cement type. On the other hand, increasing preparation angle to 18° makes Relay X cement type better than glass ionomer that it showed about 7% less von Mises stress as indicated in [Figure 5].
Spongy bone behavior [Figure 6] is the same and insensitive to cement layer type at the two preparation angles tested in this study. [Figure 7] showed that the cortical bone deformations were insensitive to cement type, while it negligibly showed slightly better behavior with glass ionomer.
| Discussion|| |
Our study question was the assessment of two variables regarding the load-bearing ability of E-max crowns; the type of cement and the preparation taper. The hypothesis that the cement layer share in load energy absorption is proved as there was a controversy regarding this. Resin cement has good physical properties as low solubility, low water sorption, and so better esthetics and function. The strong cohesion forces in the specific net structure of resin cement allow a better stress distribution.
The results of the finite element method  used showed that the RelyX resin cement showed less von Misses stress by about 4% than glass ionomer cement at 18° taper. These tapers were within acceptable values. This was in agreement with May et al. who found that at 50-μm cement thickness, bonding crowns with resin cement lead to withstanding at least twice loading before failure than nonbonded crowns. The ceramic cement bonding is considered now a very important factor in fixed restorations success based on many clinical and in vitro studies. In this study, the increased bond strength of resin cement may explain the increased stresses resistance developed by the resin cement. This was in controversy to the concept that the high compressive and tensile strengths and also the high modulus of elasticity of traditional cement as zinc phosphate cement allows it to withstand higher stresses more than that beard by resin cement. While Nakamura et al. in 2016 found that the compressive strength of self-adhesive resin-based cement, resin-based cement in dual cure, and chemical cure modes were higher than the zinc phosphate and glass ionomer cement; however, the differences between them did not affect the fracture resistance of the crowns cemented by them. They concluded that the cement type has no effect on the fracture resistance of the crown. While Ha et al. in 2016 showed that the cement thickness plays an essential role in the success of monolithic zirconia restorations in terms of reducing cement wash out. The difference in results with our study can be explained by the fact that there is a difference in the flexural strength of lithium disilicate crowns that are studied in our research, which are much more, less than the zirconia crowns studied in Nakamura et al. study.
Clinical significance regarding poor bonding due to improper clinical techniques or the longevity of the bonding is not in the scope of our study. Here, simple crowns are designed by the finite element analysis models are used to investigate the effect of the cement type. In addition, the effect of preparation taper. The effect of preparation designs on prosthesis success was studied in recent researches. Other studies examined the effect of preparation taper on resistance form of fixed partial denture abutments.
The results found in our study regarding the preference of the 10° taper over the 18° regarding the stresses generated was also supported by Zhang et al. in 2016 who found in an extended finite element method study that decreasing the convergence angle and placing an adhesive layer was able to enhance the fracture strength. In addition, this supported and explained the results in our study showing that the stresses falling on the crown were less in case of the 10° taper than with the 18° taper. The taper angle stated in this research is also in accordance with the results of some previous researches in the literature. In a finite element study by weed and Baez a large correlation between the occlusal taper of crown reduction and the fracture resistance was found, they found that the 10° occlusal taper provided the best fracture resistance, while 22° showed inadequate resistance. Certainly, this could not be done easily clinically. Mainly measured taper angles varied from 12° to 27° according to whether it is done in the preclinical laboratory or clinical situation.
The stresses induced over the root and bone were more in case of preparation taper 18°, and this may be explained by more load transfer in direction other than the axial or vertical direction after resolution of the force falling over the converged wall.
This study added to the proof that the preparation design, especially the preparation taper affected the stresses induced over the crown and the root and supporting bone.
| Conclusions|| |
Within the limitations of this study, it may be concluded that cement layer of 40 μm thickness has no effect on the underneath structures, while RelyX shared the load energy absorption with the E-max crown better than the glass ionomer one.
The E-max crown above RelyX cement and 18° preparation angle may be better than the same combination with 10° preparation angle for roots and bone. On the other hand, 10° preparation angle reduced the crown stresses dramatically.
Future research needs to be done to study other preparation angles and other types of cement.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
[Table 1], [Table 2]