Loss of teeth can affect an individual’s functions, speech, and esthetics. There is a need for prosthetic rehabilitation to improve quality of life. Patients always prefer fixed restoration so that the implant supports fixed restoration. The use of osseointegrated implant support esthetical clinical covering material become insistent demand; this enables fabrication of metal-free restorations that lead to better results in esthetically compromised areas. Indeed, the development of new high-strength ceramic abutments can enhance the overall esthetic outcome of an implant supported prosthesis.
The occlusal loading effect, that influencing bone resorption, due to the stress transmitted to surrounding bony tissues by the implant structure has been reported 1.
All dental implants support forces in vivo; thus, the biomechanics plays an important role in implant design. In all incidences of clinical loading, occlusal forces are first introduced to the prosthesis and then the bone–implant interface through the implant–abutment interface. Therefore, logically focusing on each of these steps of force transfer can lead to insights into the biomechanical effect.
Mechanical stress of occlusal forces can have both positive and negative consequences for bone tissue and thereby for maintenance of osseointegration of an oral implant. However, it is difficult clinically to quantify the magnitude and direction of naturally occurring occlusal forces. The occlusal forces may exceed the mechanical or biological load-bearing capacity of the osseointegrated oral implants or the prosthesis, causing either a mechanical failure or failure in the osseointegration. If this occurs, the load can be classified as an ‘overload’ 2.
Sahin et al. 3 reported that several factors influence load distribution on implants such as geometry, number, length, diameter, and angulations of implants, and location of implant(s) in the arch. It is important to gain an understanding of the relation between implant–abutment complex design and load distribution at the bone–implant interface 4.
The type and geometry of the prosthesis, prosthesis material, superstructures’ fit, location, direction and magnitude of occlusal forces applied on the prosthesis, condition of the opposing arch (prosthesis vs. natural dentition) mandibular deformation, bone density, age, and sex of the patient are factors that may affect bone–implant interface.
In addition, many factors affect the load transfer at the bone–implant interface such as the type of loading, material properties of the implant and prosthesis, implant geometry, surface structure, implant design (diameter and length) quality and quantity of the surrounding bone, and nature of the bone–implant interface 5.
Çaglar et al. 6 compared the von Mises (Svon), compressive (S3), and tensile (S1) stresses occurring on implants, abutments, and surrounding bone using three-dimensional (3D) finite element analysis (FEA) in three simulations in the anterior maxilla: a single titanium implant with a titanium abutment, a single titanium implant with a zirconia abutment, and a single one-piece zirconia implant. It was found that lower stresses occurred on the zirconia implant (WS), except for tensile stress under oblique loading. The zirconia implant generated the lowest stresses in cortical bone, and the zirconia abutment resulted in lower von Mises and compressive stresses than the titanium abutment in implant and cortical bone 6.
Also, a controlled clinical study was carried out to determine whether abutments made of zirconium dioxide are suitable for the posterior region; it was found that after 12 month of wear, no mechanical failure was registered in any of the all-ceramic abutments. On clinical investigation, the peri-implant hard and soft tissues were generally healthy and devoid of inflammation 7.
Sailer et al.8 studied whether or not customized zirconia abutments show the same survival rates in canine and posterior regions as titanium abutments and compared the esthetic result of the two abutment types. After a 1-year clinical follow-up, zirconia abutments showed the same survival and a similar esthetic outcome as titanium abutments.
Also, Zembic et al.9 tested whether or not zirconia abutments show the same survival and technical/biological outcome as titanium abutments. Eighteen patients with 18 zirconia and 10 titanium abutments were examined at a mean follow-up of 36 months. No fracture of an abutment or loss of a reconstruction was found. Hence, both showed 100% survival. After 3 years, zirconia and titanium abutments showed the same survival and technical, biological, and esthetical outcomes.
A systematic review evaluated available evidence for differences in the stability of preimplant tissues between titanium abutments versus gold alloy, zirconium oxide, or aluminum oxide abutments. The studies included showed that titanium abutments did not maintain a higher bone level in comparison with gold alloy, aluminum oxide, or zirconium oxide abutments. However, there is a lack of information on the clinical performance of zirconium oxide and gold alloy abutments as compared with titanium abutments 10.
In a clinical study 11 carried out to compare the clinical application of alumina and zirconia ceramic implant abutments in the anterior region, 23 consecutive patients requiring single-tooth implants in the anterior region were randomized to receive either an alumina ceramic abutment or a zirconia ceramic abutment. All-ceramic crowns were then fabricated and cemented over the abutments with composite cement. At the 1-year follow-up, the mean marginal bone loss around implants was 1.2±0.5 mm. The researchers concluded that both ceramic abutments showed a favorable biological response and esthetic outcome; however, zirconia abutments, with their superior mechanical properties, might be better for supporting single-tooth implant restorations in the esthetic zone 11.
Andersson et al.12 compared results after 1 and 3 years when single crowns supported by ceramic abutments or titanium abutments were loaded. Almost no marginal bone loss was recorded, indicating a stable bone situation both at ceramic and at titanium abutments on single-tooth implants. The results also showed that the esthetic possibilities and the safety of single-tooth replacement were acceptable. The tested abutments worked well, although the fractured CerAdapt abutments indicated that ceramic abutments are more sensitive to handling procedures than the titanium abutments 12.
In a recent clinical study 13 carried out to determine the outcome variables of all-ceramic and metal-ceramic implant-supported, single-tooth restorations, the researchers found that after 3 years, the survival rate was 100% for implants and 97% for abutments and crowns. Significantly more marginal bone loss was registered for the gold alloy compared with zirconia abutments. It was concluded that the biological outcomes with the zirconia and metal abutments were comparable 13.
In a review by Guess et al. 14, to present the current knowledge on zirconia utilized as the framework material for implant-borne restorations and implant abutments, laboratory tests and developments, and clinical performance, it was found that the latest applications of zirconia in implant dentistry include implant abutments, high biocompatibility, low bacterial surface adhesion as well as favorable chemical properties of zirconia ceramics.
More evaluations of the clinical outcome of custom-made zirconia abutments for implant-supported single-tooth restorations up to 5 years after insertion were performed. There were no significant differences in changes in any of the soft tissue registrations or the peri-implant marginal bone level between the conventional two-piece abutment-crown restoration and the one-piece restoration. The peri-implant bone-level changes from placement to the clinical examination 3–5 years later were small.
Zirconia abutments for single-implant crowns seem to show good short-term technical and biological results 15.
More evaluations of clinical performances of ceramic and titanium abutments for an implant-supported all-ceramic single crown related to mucosal bleeding and marginal bone loss of each crown were performed at 6, 12, 18, and 24 months after restoration, respectively. No negative influence of abutments on peri-implant tissue was observed during short-term restoration for ceramic and titanium abutments 16.
Some authors carried out investigations to quantify the fracture load of implant-supported Al2O3 and ZrO2 abutments restored with glass-ceramic crowns; it was concluded that both all-ceramic abutments exceeded the established values for maximum incisal forces reported in the literature (90–370 N). The ZrO2 abutments were more than twice as resistant to fracture as the Al2O3 abutments 17.
Vertical or horizontal misalignment imposes extra load on the different restoration components, the implant, and the bone, causing loosening of the zirconia implant abutments, loss of retention, abutment fractures, bone microfractures, lost of crestal bone, and loss of osteointegration.
Although ceramic and titanium abutments are used widely in clinical practice, the mechanical characterization of the implant–abutment interface of ceramic abutments has not been carried out after dynamic loading. The aim of this study was to assess the implant–abutment interface after the dynamic loading of titanium, alumina, and zirconia abutments. After dynamic loading, the titanium abutment control group showed an increased microgap (3.47 μm) than zirconia (1.45 μm) and alumina (1.82 μm) groups at the palatinal site (P<0.05). Owing to their comparable microgap values at the implant–abutment interface after the dynamic loading, ceramic abutments can withstand functional forces similar to conventional titanium abutments 18.
Vigolo et al.19 studied the rotational freedom of Procera abutments made of different materials: titanium, alumina, and zirconia. The values registered for the three types of abutments were consistently found to be inferior to 3°. Nevertheless, the titanium and zirconia groups did not show significant differences, their values being significantly inferior to those of the group of alumina abutments.
Glauser et al. 20 evaluated an experimental implant abutment made of densely sintered zirconia with respect to peri-implant hard and soft tissue reaction as well as fracture resistance over time. At the 1-year and 4-year examinations, reconstructions were evaluated for technical problems (fracture of abutment or crown, loosening of abutment screw). Modified plaque and simplified gingival indices were recorded at implants and neighboring teeth, and peri-implant bone levels were determined radiographically. The research team found that no abutment fractures occurred and the mean marginal bone loss measured 1.2 mm after 4 years of functional loading and it was concluded that zirconia abutments offered sufficient stability to support implant-supported single-tooth reconstructions in anterior and premolar regions. The soft and hard tissue reaction toward zirconia was favorable 20.
The aim of this study was to evaluate, by FEA, the effect of abutment material type on force transmission to the bones supporting implant when force is applied at different points in the vertical direction and also to evaluate stresses and deformations generated on the crown and implant to determine a general trend for the selection of abutment material.
Materials and methods
FEA model is an engineering method that allows investigators to assess stresses and strains within a solid body; this model allows simulated force application to specific points in the system and it provides the resultant forces in the surrounding structures, especially for implant. The resultant forces on the implants, implant–abutment connection, or underlying bone can be evaluated easily once a detailed 3D model has been created 21.
Bone geometry was simplified and simulated as cylinders that consist of two coaxial cylinders. The inner one represents the spongy bone (diameter 14 mm and height 22 mm) that fills the internal space of the other cylinder (shell of 1 mm thickness), which represents cortical bone (diameter 16 mm and height 24 mm). The implant–abutment complex was modeled in 3D using the commercial general-purpose CAD/CAM software ‘AutoDesk Inventor’ version 8.0 (Autodesk Inc., San Rafael, California). These parts are regular and symmetric, and their dimensions can be simply measured with their full details. However, the ‘crown’ has a very complicated geometry; therefore, a 3D scanner was used for this purpose, Roland MDX-15 (Roland DG Corporation of Hamamatsu, Japan), to produce a cloud of points or triangulations to be trimmed before use in any other application (Fig. 1).
A second phase of difficulty appeared in solving the engineering problem of importing two different parts: one scanned and one modeled or drawn. Most of the CAD/CAM and graphics packages deal with parts as shells (outer surface only). However, the stress analysis required in this study is based on the volume of different materials. Therefore, a set of Boolean operations such as cutting volumes by the imported set of surfaces in addition to scaling, adding, and subtracting volumes can ensure the required volumes representing implant–abutment assembly, and porcelain fused to metal crown 22 as shown in Fig. 2, respectively, after meshing each part.
A threaded titanium dental implant (Zimmer Dental Inc., Carlsbad, California), the root form dental implant, had a nominal diameter of 3.7 mm, a length of 13 mm, and the shape of an internal hex, with a hex width of 3.5 mm. Three different material types of abutment material were examined: zirconium, alumina, and titanium. The base of the finite element model was assumed to be fixed, which defined the boundary condition 3. Loading was applied vertically (z-axis) as a compressing load on five selected nodes on top of the coating as follows:
- 135 N, at the distolingual marginal ridge.
- 110 N, at the central fossa.
- 90 N, at the lingual cusp height.
- 70 N, at the distolingual slope.
- 45 N, at the mesiolingual marginal ridge.
This type of loading may represent the functional and parafunctional occlusion.
A linear static analysis was carried out. Solid modeling and FEA were carried out on a personal computer, Intel Pentium Core to Duo, processor 3.2 GHz, 4.0 GB RAM. The meshing software was ANSYS version 9.0 (Canonsburg, Pennsylvania) and the element used in meshing all 3D models was an eight-node Brick element (SOLID-45), which has three degrees of freedom 23 (translations in the global directions). Table 1 lists the number of nodes and elements of each part of the model and the material properties used in this study are listed in Table 2.
Three runs were carried out by the described numerical model. Each run was performed with a different abutment material. Deformations in global directions, principal, and von Mises stresses on each part of the studied system were obtained. A sample of the results obtained and a comparative graphical representation of von Mises stresses generated on cortical and spongy bones under different abutment materials are presented in Figs 3–5.
An in-depth analysis of Figs 3–5, and taking into account the von Mises stresses calculated on all parts of the model presented in Fig. 6, and equivalent displacement (Ut) in Fig. 7, important conclusions can be made from this study.
The titanium abutment shows a high value of stress concentration on cortical and spongy bone when compared with alumina and zirconium types and no significant differences between the other two types.
As the abutment material rigidity increases, the implant total stress and deformation decrease. This indicated less energy absorption in the implant material with increasing abutment material rigidity.
Moreover, the crown was not affected by a change in the abutment material as the stress distribution, but its value reduced with decreased abutment material rigidity. However, crown deformation showed the maximum value with the zirconia abutment and the minimum value with the alumina abutment.
From the energy absorption point of view, use of the titanium abutment (lowest rigidity) results in lower level of energy absorption in the abutment. Thus higher energy will be transferred to the following parts (implant and bone), which is not preferred. In addition, the life of the crown decreases with a reduction in abutment rigidity.
Historically, implant abutments were manufactured from metals. The use of titanium abutments prevents the occurrence of galvanic and corrosive reactions in the implant–abutment interface, which enhances the health of peri-implant soft tissues also because of its high biocompatibility.
Metal abutments only partially solve esthetical, functional, and hygienic questions fundamental to restorations over implant success 24.
In order to consider the ceramic abutments as a viable alternative, it should have mechanical and biological qualities identical or superior to those of universally used titanium abutments. The strength of the abutments will have to be superior to the registered maximum values for the anterior sector, which can fluctuate between 90 and 370 N. In a prospective study of 4 years, with experimental zirconia abutments placed directly on an implant of an external hexagon, abutment fractures were not registered 14.
Butz et al.25 compared the fracture strength, rate of survival, and mode of failure of ceramic abutments. The authors concluded that after being under the mastication simulator and static loads, the strength of the zirconia abutments was comparable with that of titanium (281 vs. 305 N), the rate of fracture also being similar to that of the titanium abutments. Thus, the authors recommend zirconia abutments as an alternative for restoration of unitary implant rehabilitations in the anterior region.
Yildirim et al.18 reported that zirconia abutments obtained values that were more than two times higher than those of the alumina ones. Both materials showed a resistance that could tolerate incisal forces documented in the literature.
Sundh and Sjögren 26 evaluated the bending resistance of implant-supported CAD/CAM-processed restorations made out of zirconia or manually shaped from reinforced alumina. Units of titanium abutment attached to titanium implant fixtures were used as a reference. All the ceramic abutments and copies showed values that were equal or superior to those of the control and exceeded the reported value, up to 300 N, for maximum incisal bite forces.
In 2010, a review was performed 27 to gain more knowledge of the clinical performance of metal and ceramic abutments. After analysis of 129 studies, it was found that the estimated 5-year survival rate of ceramic abutments was 99.1 and 97.4% for metal abutments and the cumulative incidence of biological complications after 5 years was estimated to be 5.2% for ceramic abutments and 7.7% for metal abutments. Finally, esthetic complications tended to be more frequent for metal abutments.
As abutment material rigidity decreased it will absorb less energy and transfers more load to the following parts of the studied model. Moving down with abutment rigidity from alumina, zirconium, to titanium, the crown and implant materials receive less stress. However, the other parts (cortical and spongy bones) have negligible effect as stresses. A similar trend was also found for total deformation on the system parts. These results are in agreement with those of Çaglar et al.6.
Stresses and deformation distributions did not change with a change in the abutment manufacturing material, whereas the stresses and deformation values showed significant changes.
The energy absorbed by each component of the model studied was evaluated in order to determine the effect of changing abutment material.
Titanium implants showed a high value of stress concentration on cortical and spongy bone when compared with alumina and zirconium types. Values of stresses may alter considerably with changing abutment material. Stiffer abutment material absorbs more energy than softer abutment materials. Cortical and spongy bones are not affected markedly by changing abutment materials. More rigid abutment materials are not recommended to reduce stress levels transferred to the crown, implant, and bones.
Conflicts of interest
There are no conflicts of interest.
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