Indian Journal of Dental Research

ORIGINAL RESEARCH
Year
: 2019  |  Volume : 30  |  Issue : 5  |  Page : 716--721

Comparing different bar materials for mandibular implant-supported overdenture: Finite-element analysis


Mohamed I El-Anwar1, Ahmad K Aboelfadl2,  
1 Department of Mechanical Engineering, National Research Centre, Giza, Egypt
2 Department of Fixed Prosthodontics, Faculty of Dentistry, Ain Shams University, Cairo, Egypt; Department of Oral Technology, University of Bonn, Bonn, Germany

Correspondence Address:
Dr. Mohamed I El-Anwar
Department of Mechanical Engineering, National Research Centre, 33 El Bohouth St., P.O. Box 12622 Dokki, Giza
Egypt

Abstract

Aim: This study was conducted aiming to optimize the selection of bar material that can minimize stresses on mandibular bone. Subjects and Methods: One finite-element model was created under ANSYS environment to evaluate the use of different materials as a bar-manufacturing material in mandibular implant-supported overdenture (OD). Model components were created on engineering computer-aided design software and then assembled under the finite-element package. A force of 200 N was unilaterally and vertically applied on the left second premolar area. Results: Within these study conditions, the polyether ether ketone bar produced the lowest Von Mises stress on OD and the maximum value of deformation. Stainless steel bar produced the maximum OD total deformation. Conclusions: Cortical and spongy bones are not sensitive to the bar material. Increasing bar material stiffness increases Von Mises stresses in the bar itself and reduces its total deformation, in what is called overconstrained system.



How to cite this article:
El-Anwar MI, Aboelfadl AK. Comparing different bar materials for mandibular implant-supported overdenture: Finite-element analysis.Indian J Dent Res 2019;30:716-721


How to cite this URL:
El-Anwar MI, Aboelfadl AK. Comparing different bar materials for mandibular implant-supported overdenture: Finite-element analysis. Indian J Dent Res [serial online] 2019 [cited 2020 Sep 20 ];30:716-721
Available from: http://www.ijdr.in/text.asp?2019/30/5/716/273442


Full Text



 Introduction



Many edentulous patients are satisfied with their mandibular complete dentures; however, others encounter critical issues such as insufficient retention of the prosthesis, impaired chewing efficiency, masticatory problems, and denture instability. One common solution is a combination of implants together with an overlying prosthesis aiming to minimize such problems.[1],[2],[3] The number and position of implants supporting the prosthesis together with the masticatory forces, interarch distance, and occlusion scheme affect the stress and strain distribution on the implants involved, surrounding bone, and the superstructure complex. Increased forces on the implant unfavorably stimulate bone reduction in the surrounding area causing fibrointegration, which is possibly followed by implant loss.[4],[5]

Retention of the mandibular implant-supported overdentures (ODs) could be achieved by magnetic attachments, ball attachments, or clip on bar connecting the implants. The bar and clip attachments are probably the most commonly used due to higher wear resistance and greater mechanical stability. Furthermore, short distal extensions from rigid bars can be achieved, which help in the prevention of the denture shifting and contribute to its stabilization.[6]

Low-profile attachments connected with an intervening bar are frequently used as a solution for restricted space for prosthetic components in complete edentulous cases with limited interarch space. Favorable transmission of stresses between implants is a major advantage of bar attachment due to load distribution, primary splinting capacity, and minimal postinsertion maintenance.[7],[8]

Many materials have been advocated by authors for bar and framework fabrication, and it has been suggested that the material choice is of critical importance regarding their biomechanical behavior. They affect the bone stress distribution around the implants and restoration serviceability.[9] Titanium and titanium alloys have been used for a long time as a material of choice for bar fabrication; titanium is four times lighter than semi-precious alloys. It offers excellent biocompatibility, corrosion resistance, and very good mechanical properties; however, there are some concerns regarding potential hypersensitivity to titanium.[10]

The recent demands for esthetic and biomimetic materials together with the increased allergies of some patients to certain metals have promoted the development of new metal-free materials.

Polyether ether ketone (PEEK) is a metal-free high-density thermoplastic polymer with a linear aromatic semi-crystalline structure that can be fabricated using computer-aided design/computer-aided manufacturing (CAD/CAM). Regarding the load-cushioning capacity of the prosthetic elements, the use of PEEK as a prosthetic structure on implants has been recently increased. PEEK components seem a viable alternative to obtaining a similar modulus to that of cortical bone where bone could be stimulated, favoring remodeling without overload.[11]

In a clinical case report by Parmigiani-Izquierdo et al.,[12] it was concluded that PEEK restorations provide excellent elasticity and resemblance to natural teeth that allows their successful use as restorations over implants.

Three-dimensional (3D) finite-element analysis (FEA) has been widely used for the quantitative evaluation of such stresses on the implant and its surrounding bone, especially when detailed stress information is required where 3D modeling is considered necessary.

This study aimed to compare the stress distribution of the implant-supported mandibular OD according to the bar materials using the 3D FEA.

 Subjects and Methods



Low-profile attachments for implant-retained mandibular OD were used for completely edentulous patients with limited interarch space (the estimated interarch space required for an implant-retained OD measured from the implant shoulder to the incisal edge is approximately 12–14 mm).

The current study simulated a clinical situation where two threaded dental implants with two low-profile attachments (OT Equators square head; Rhein83 srl, Bologna, Italy) compatible with the implants were used. A bar was also used to connect the two low-profile attachments. Four different materials with a wide range of stiffness were tested to be used for bar manufacturing: titanium, stainless steel (St. St.), PEEK, and PEEK with 30% carbon fiber reinforced. All materials were assumed as homogeneous and isotropic materials, these materials' properties are tabulated in [Table 1].{Table 1}

A threaded root-form implant (Dentium Superline-Dentium Inc., Samsung-dong, Gangnam-gu, Seoul, Korea) was selected as a basis for the implant-supported OD model with platform diameter of 3.7 mm, a length of 12 mm, and internal hex with body diameter of 3.4 mm.

A 3D model was then constructed simulating supported OD with two implants and bar, where the model components were modeled in 3D on commercial general-purpose CAD/CAM software “AutoDesk Inventor” ver. 8.0 (Autodesk Inc., San Rafael, CA, USA). These components were exported as SAT file format and then imported into the finite-element package. Meshing and assembly of model components were done under ANSYS software (ANSYS Inc., Canonsburg, PA, USA) environment as illustrated in [Figure 1], where different colors represent different materials as ANSYS screenshots. The meshing software was ANSYS version 14.0 and the used element in meshing all the 3D models was 8-node brick element (SOLID 185), which has three degrees of freedom[13] (translations in the global directions); the mesh density is tabulated in [Table 2].{Figure 1}{Table 2}

The solid modeling and FEA were performed on a personal computer (Intel Core i7, processor 3.2 GHz, 6.0 GB RAM). Eight runs were carried out, two runs with each bar material. The lowest area of the cortical bone was set to be fixed in place as boundary condition, while unilaterally 200 N was applied vertically at the second premolar.

 Results



Four analyses were done (one per bar material), and results were compared. [Figure 2], [Figure 3], [Figure 4], [Figure 5] demonstrate the behavior of the model components.{Figure 2}{Figure 3}{Figure 4}{Figure 5}

OD showed maximum Von Mises stress near the closest implant to the load application point. PEEK bar produced the lowest Von Mises stress on OD and the maximum value of deformation of order 470 μ (about 10% more than using St. St. bar that produced minimum OD total deformation).

As presented in [Figure 3], increasing bar material stiffness increases its Von Mises stress and reduces its total deformation, in what is called overconstrained system. In general, Von Mises stress of implants was slightly increased with increasing bar stiffness that indicated an overconstrained system. [Figure 4]c depicts that total deformation of the implant is insensitive to the bar material (maximum difference of order 3 μ).

Von Mises stress of cortical bone and total deformation distributions were the same for all bar materials. It was noticed that cortical bone is insensitive to bar material change as presented in [Figure 5], where the Von Mises stress showed maximum difference of order 0.2 MPa and maximum difference in total deformation of <1 μ.

 Discussion



Implant-supported dentures are well established as a prosthetic solution for fully edentulous patients as suggested by clinical studies reporting high success rates. However, bone loss is also highly reported when stresses exceed the physiological limit of bone,[14] which was expected by FEA in a previous study.[5]

Many materials have been attempted for bar attachment fabrication, and the impact of the biomechanical behavior of the material on stress distribution around the prosthesis as well as the implant–bone interface has been proved to be of critical importance.[9] The use of a pure titanium grade 2 bar-sustained OD compared to a fixed rehabilitation proved to be highly advantageous due to the use of one biocompatible metal without soldering issues.[15] Moreover, a 4-year follow-up case report suggested 100% implant survival rate with predictable success when titanium bars were used and engineered with CAD/CAM technology.[16]

Patients allergic to metals, or who dislike the metallic taste, the weight, and the unpleasant metal display of the denture framework are all reasons calling for a new alternative biocompatible material. A modified PEEK material containing 20% ceramic fillers is a high-performance polymer that has been used successfully over the last years in the medical field presenting high mechanical properties, biocompatibility, and high thermal and chemical stability.[17] It is also worth mentioning that resin materials have been recently used in implantology due to their shock-absorbing capacity which allows better stress distribution.[18]

Regarding the influence on the OD, PEEK bar produced the lowest Von Mises stress and the maximum value of deformation of order 470 μ (about 10% more than using stainless steel bar that produced minimum OD total deformation). This finding pointed out that increasing the bar material stiffness decreased the OD deformation, yet increased the stresses on the OD. The maximum difference between the lowest and maximum total deformation was of order 50 μ. This result matched a previous study,[5] concluding that the Von Mises stress of OD is nearly insensitive to bar material; while as the bar material stiffness increased, the total deformation of the OD decreased.

Concerning the influence of the bar material on the bar itself in the current study, Von Mises stress was dramatically increased with titanium in comparison to stainless steel and PEEK that might be referred to the high rigidity of titanium.

On the other hand, total bar deformation was decreased by increasing the bar material's stiffness. This is in accordance to Erkmen et al.[19] who evaluated the biomechanical behavior of 3-unit implant-supported fixed partial dentures, varying the framework (metal- or glass fiber-reinforced resin), where it has been concluded that the resin framework minimizes excessive stresses in the bone–implant interface with a stress-shielding capacity that maintains a normal physiological loading of the surrounding bone with reduced bone loss in comparison to metal rigidity and consequent high stresses directly concentrated to the bone.

Moreover, this goes consistently with a fatigue analysis study by Bonfante et al.,[20] where the resin bar showed satisfactory results similar to conventional metallic structures without accelerating the failure of any group by fatigue, concluding that this material is a feasible alternative to metallic infrastructures in dental prosthesis.

Implants' Von Mises stress was increased by about 4% with increasing bar material Young's modulus from 3 to 195 GPa, which is fairly small. Similarly, regarding the influence of the bar material on the cortical and spongy bones, results of this study showed insignificant difference among different bar materials following the fact that the amount of stress transmitted to the bone is related more to the internal fit between the bar and the superstructure rather than the material itself, causing more stress concentration as the internal misfit increases, which is more related to the fabrication technique.[21],[22],[23]

In addition, these study findings are consistent with the findings of a photo-elastic analysis study[24] of mandibular full-arch implant-supported dentures, where resin bar exhibited better stress distribution and low weight compared to other metal groups including titanium. These findings, along with the tensile strength of such polymer materials being very close to that of zircona while their flexural and compressive strength is comparable to that of base metal alloys, can encourage the use of polymer- and resin-containing materials in oral rehabilitation with implant-supported dentures.

 Conclusions



Within the limitations of thisin vitro study, it could be concluded that:

As the bar materials' stiffness increased, the total deformation of the OD decreasedIncreasing the bar materials' stiffness dramatically increases the stresses received by the bar itself, while the opposite behavior was reported for total deformationImplants' Von Mises stresses were increased by about 4% with increasing bar materials' Young's modulus from 3 to 195 GPaCortical and spongy bones were not sensitive to the bar materials' stiffness.

Acknowledgment

The authors would like to show their gratitude to Dr. Mahmoud ElHomossany, Department of Removable Prosthodontics, Faculty of Dentistry, Ain Shams University, for his support during the course of this research.

.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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