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Optimizing the Design of Zirconia Implant Abutments: A Finite Element Analysis

Grant Bullis, MBA, Vaheh Golestanian, MSc, David Leeson, MSc and Weihan Zhang, Ph.D

Overview

Implant abutments milled from zirconia are esthetic and strong; however, as a ceramic material that exhibits fracture rather than deformation when its strength tolerances are exceeded, zirconia requires unique geometrical design optimizations in order to withstand functional stresses. Using finite element analysis (FEA), manufacturing engineers at Glidewell Laboratories evaluated many combinations of prosthetic connection geometry and abutment screw parameters for maximum performance under occlusal loading, in an effort to produce an all-zirconia abutment equal or superior in strength to titanium materials traditionally used.

Grant Bullis, MBA

bullisGrant Bullis, director of implant R&D and digital manufacturing at Glidewell Laboratories, began his dental industry career at Steri-Oss (now a subsidiary of Nobel Biocare) in 1997. Since joining the lab in 2007, Grant has been integral in obtaining FDA 510(k) clearances for the company's Inclusive Custom Implant Abutments. In 2010, he was promoted to director and now oversees all aspects of CAD/CAM, implant product development and manufacturing. Grant has a degree in mechanical CAD/CAM from Irvine Valley College and an MBA from the Keller Graduate School of Management. Contact him at inclusivemagazine@glidewelldental.com.

Vaheh Golestanian, MSc

golestianVaheh Golestanian received a master's degree in biomedical engineering at Iran University of Science and Technology in Tehran. In 2008, he joined Glidewell Laboratories' Implant R&D and Digital Manufacturing department as a manufacturing engineer. Vaheh has eight years' experience as a mechanical engineer focused on finite element analysis and CNC programming, and is a member of the Society of Manufacturing Engineers. Contact him at inclusivemagazine@glidewelldental.com.

David Leeson, MSc

leesonDavid Leeson received a first class honors degree in manufacturing engineering from England's Loughborough University, followed by a Master of Science in advanced automation and design from Cranfield University. After graduation, he worked in the motorsports industry, creating manufacturing processes for high-precision racing engines. David is currently senior manufacturing engineer in Glidewell Laboratories' Implant R&D and Digital Manufacturing department. After joining Glidewell in January 2007, he used his engineering background to launch many new products, including titanium and zirconia abutments, as well as implant bars. He also led the development of automated machining capability. Contact him at inclusivemagazine@glidewelldental.com.

Weihan Zhang, Ph.D

zhangWeihan Zhang graduated from the University of Missouri-Rolla (now Missouri University of Science and Technology) in 2008 with a Ph.D in mechanical engineering. Since joining Glidewell Laboratories in 2008, Weihan has been actively involved in software development, prototyping machine and system development projects as a manufacturing software engineer. He has also published more than 20 articles on geometric modeling and manufacturing engineering. Contact him at inclusivemagazine@glidewelldental.com.

Optimizing the Design of Zirconia Implant Abutments:
A Finite Element Analysis

Introduction

Esthetic concerns are essential for anterior tooth restorations. Zirconia is a widely used engineering ceramic considered to have high load strength and fracture toughness compared to other ceramic materials used in dentistry. It is also a highly biocompatible ceramic material less prone to discoloring the cervical soft tissues than metal abutments.1 Zirconia, like other ceramics, is sensitive to tensile stresses and extreme care must be taken to design the prosthetic connection to the implant so that tensile stresses are minimized. Manufacturing small, high-precision components from zirconia is challenging, and it requires rigorous attention to detail in every aspect of the manufacturing process. Small defects introduced during manufacturing can lead to fractures, and poor tolerance control can lead to rotational play between abutment and implant, which can result in loosening of the abutment's retaining screw.2,3

For proper prosthetic function, zirconia abutments must exhibit performance characteristics comparable to those of traditional titanium abutments.4 Zirconia has a higher compressive strength compared to titanium alloy. However, under cyclic loading situations, zirconia abutments will fracture when overloaded, while a titanium abutment will undergo plastic deformation. ISO 14801:2007 is the fatigue testing standard for endosseous dental implants.5 With this procedure, the implant and abutment assembly is cyclically loaded to determine its fatigue limit. The fatigue limit will be reached at lower loads if the prosthetic connection geometries of zirconia abutments are left identical to titanium, rather than being optimized for the intrinsic material properties of zirconia.

Methods and Materials

The initial abutment design used in this study was Glidewell Laboratories' titanium abutment CAD model. A three-dimensional finite element model was generated from the abutment and implant by importing these CAD models into COSMOSWorks software (Fig. 1). For simplicity, the implant threads were removed and the abutment screw was modeled by beam elements.

The implant material was defined as commercially pure titanium, grade 4. The abutment screw material was defined as Ti-6AL-4V titanium alloy, and was tightened to 35 Ncm torque.

Since repeated cyclic loading of implants induces plastic deformation, it is normally recommended to conduct a nonlinear finite element analysis.6 However, in this study, a linear analysis was performed as a simplification. This assumption is justified by the following:

  1. The maximum applied force (250 N) produces a very small region of plastic deformation on the implant, so the deviation between linear and nonlinear analysis would be relatively small and adequate for design optimization, but not an absolute indication of the strength of the abutment.
  2. Contact analysis is possible in linear mode with COSMOSWorks. It was assumed that the relative movement between abutment and implant would be negligible and would not significantly affect the analysis results.

The model was restrained from the implant's body, 3 mm beneath the contact face of the abutment and implant, and a static, oblique force of 250 N was applied at a 30-degree angle to the long axis of the implant. Three-dimensional tetrahedral solid elements were used to mesh the abutment and implant, and a beam element for the abutment screw (Fig. 2). Static analysis was conducted to optimize the design of the abutment.

On the mating surface between the implant and abutment, a surface-to-surface contact condition without penetration was chosen. For the abutment connection, a node-to-surface contact condition was chosen. In both cases, the implant was selected as the source, and the abutment as the target. The abutment was selected as the target because it had a higher modulus of elasticity.7 The mesh was refined in contact areas and around small features.

Results

Several features of the abutment were changed during the design optimization to increase the strength of the zirconia abutment assembly. Some of these features were the abutment screw bore diameter and radius, the distance between the screw seat and contact face of the abutment and implant, the abutment screw shank bore diameter, and the profile of the platform (Fig. 3). A total of 46 iterations of these features were analyzed. Mohr-Colomb yield criterion was used for calculating the safety factor of the abutment, and Von-Mises yield criterion used for calculating the safety factor of the titanium implant.

On the abutment, there were three areas requiring attention to ensure an adequate factor of safety (FOS):

  1. The mating surface between the implant and abutment
  2. The abutment screw head mating surface
  3. The abutment prosthetic connection geometry
Under load, localized areas on the implant's mating surface reach the yield point, upon which this area expands downward. This pattern is similar to what has been observed during previous tests of titanium abutments according to ISO 14801:2007 (Fig. 4).

For comparison, a zirconia abutment with connection geometry not optimized for zirconia was analyzed alongside a zirconia abutment with connection geometry optimized for zirconia. The abutments were identical except for the connection geometry. The results are shown in Figure 5.

Discussion

In this study, the zirconia abutment assemblies were analyzed in static and linear conditions. Figure 5 details the comparison between a zirconia abutment with geometry not optimized for zirconia and an otherwise identical zirconia abutment with connection geometry optimized for zirconia. The mating surface factors of safety for both abutments are nearly identical. However, when the abutment connection geometry contacts the implant connection geometry, the minimum factor of safety drops to 0.903 on the non-optimized zirconia abutment, compared to 2.125 on the Inclusive® All-Zirconia Custom Abutment from Glidewell Laboratories. The Inclusive All-Zirconia Custom Abutment achieves the higher factor of safety by optimizing the connection geometry for the material properties of the zirconia. Specifically, the connection height is reduced to minimize the potential transmission of tensile loads, sharp corners are removed from the connection, and the implant mating surface is undercut to provide clearance.

The shanks of zirconia abutments screws are smaller compared to those used for titanium abutments. The smaller- diameter shanks stretch more and generate higher preload in the screw from the same amount of torque. This increased preload improves the performance of the connection between the implant and abutment, inhibiting risk of the screw loosening over time.

The fracture toughness of fully sintered zirconia is about 13 MPa(m½) compared to titanium alloys, which are about 75 MPa(m½). This makes zirconia vulnerable to cracks and faults. The manufacturing process for zirconia abutments requires very precise, repeatable machinery and a well-controlled sintering process to produce abutments that fit and perform well.

Conclusion

Zirconia is an engineering ceramic with great potential for implant abutments. Zirconia has good esthetics and high compressive strength, but lower tensile strength and fracture toughness relative to titanium. Because of this, zirconia abutments have to be designed with these material properties in mind. The material properties of zirconia require a specially designed prosthetic connection to minimize the potential for breakage.

Finite element analysis (FEA) provides a valuable tool to evaluate and optimize multiple designs prior to manufacturing physical components. Using FEA, engineers can evaluate multiple zirconia abutment and screw designs to minimize tensile loads in the abutment connection geometry and maximize preload of the abutment screw.

References

  1. Butz F, Heydecke G, Okutan M, Strub JR. Survival rate, fracture strength and failure mode of ceramic implant abutments after chewing simulation. J Oral Rehabil. 2005 Nov;32(11):838-43.
  2. Kim SK, Lee JB, Koak JY, Heo SJ, Lee KR, Cho LR, Lee SS. An abutment screw loosening study of a Diamond Like Carbon-coated CP titanium implant. J Oral Rehabil. 2005 May;32(5):346-50.
  3. Garine WN, Funkenbusch PD, Ercoli C, Wodenscheck J, Murphy WC. Measurement of the rotational misfit and implant-abutment gap of all-ceramic abutments. Int J Oral Maxillofac Implants. 2007 Nov-Dec;22(6):928-37.
  4. Gehrke P, Dhom G, Brunner J, Wolf D, Degidi M, Piattelli A. Zirconium implant abutments: fracture strength and influence of cyclic loading on retaining-screw loosening. Quintessence Int. 2006 Jan;37(1):19-26.
  5. ISO 14801:2007. Dentistry – Implants – Dynamic fatigue test for endosseous dental implants.
  6. Cehreli MC, K. Akça K, H. Iplikçioglu. Force transmission of one- and two-piece morse-taper oral implants: a nonlinear finite element analysis. Clin Oral Implants Res. 2004 Aug;15(4):481-89.
  7. SolidWorks Web Help

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