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Title: An assessment of fracture resistance and the influence of surface treatment of titanium elements on shear bond strength using different luting cement types and zirconia as applicable in dentistry.
Author and affiliation
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The research sought to assess fracture resistance and the influence of surface treatment of titanium elements on shear bond strength using various luting cement types and zirconia. The loss of teeth can occur due to several reasons, but the major causes are related to their normal functioning, structure, aesthetic and the capacity of the tooth. There are multiple ways of restoring a missing tooth and among them is implant restoration. Abutments are made from a variety of materials. Each material has different physical, chemical and mechanical properties that may have an impact on the surrounding structures. The purpose of the project lies on the abutments made from zirconium and titanium biomaterials.
The results from the study revealed that zirconia and titanium abutments have a relatively strong resistance to fracture, hence reliable for use in dentistry for tooth restoration. The findings also revealed that the various luting cement types have different effects on the shear bond strength and should be selected based on their characteristic features.
Shear Bond Strength, fracture resistance, dental restoration, Zirconium oxide (zirconia), Titanium
The increase in aesthetic demand within developed populations has led to different research on the fabrication of metal-free restorations and widespread use of ceramic materials due to its features of biocompatibility and aesthetics. Several studies show that zirconia abutments provide good results at all levels, but its effectiveness requires further studies and evaluation. The effectiveness of the implantation process depends on the effectiveness of the use of luting cement. There are insufficient and vivid guidelines regarding the use of luting cement. The cement ought to bond adhesively with the abutments and be radiopaque. The excess of the cement should be easy to remove and be retrievable (Śmielak, Gołębiowski & Klimek, 2015). The various types of cement used have been assessed for mechanical properties as well as retention capabilities. The varying information on luting cement types for implant-based restorations and the challenges of choosing the best option forms the basis of the research. The bond strengths of three most commonly used temporary cement types and permanent cement were selected for use in the research. Titanium disks were used and bonded to elements comprising of zirconia (3 Y-TPZ) using one of cement types after treatment. The need to find an optimum cementation method helps to avoid complications associated with the use of titanium abutments. It was also necessary to review on the fracture resistance properties of zirconia abutments as compared to Titanium based abutments.
Tooth replacement by the use of dental implants has been well-documented and scientifically accepted as a treatment modality. The most commonly used material for the implant is titanium. Titanium is a metal that suffers from corrosion which is a gradual degradation of materials due to electrochemical attack. It becomes a concern when a metallic implant is subjected to a hostile electrolytic environment by the human body. As Gahlert et al. (2009) explain a major disadvantage from an aesthetic perspective is the gray color of titanium that becomes a problem when used for thin soft tissue coverage. The implant head is also likely to become visible after usage for some time due to peri-implant recesses that coincide with bone resorption, despite being fully covered by bone and soft tissue. Titanium has had a dominant position as an abutment and implant material, and the long-term clinical studies demonstrate a predictable outcome. The demand for esthetic restorations has raised that led to the introduction of tooth-colored ceramic implant abutments of alumina. Abutments made of titanium and alumina develops similar peri-implant mucosa, having junctional epithelium and connective tissue attachment.
Clinical studies demonstrate stable soft-tissues around alumina abutments. Alumina abutments have a tooth-like color, which is a more esthetic outcome compared to using titanium abutments. Alumina implant abutments perform well biologically and aesthetically, and it is apparent that they have a fracture risk both in laboratory work and after abutment connection (Shenoy & Shenoy, 2010). A proposed approach to overcome the challenge is the use of a different implant material with a more natural color. Zirconia implants have become common for use as dental implants and an alternative to titanium since it looks tooth-like in color. The shortcomings in mechanical properties led to the introduction of yttrium oxide-zirconia as an alternative material for implant abutments other than alumina as the preferred ceramic abutment material. Zirconia has improved mechanical strength and reliability due to its stress-induced transformation toughening mechanism. It led to increased use of Zirconia as a ceramic biomaterial both in medicine and dentistry (Apratim, et al., 2015).
On their part, zirconia ceramics offer several advantages in comparison to other ceramics. The most important of them is the fact that they have high fracture toughness and bending strength. Previous studies have reported promising results for the osseous integration of zirconia implants for both loaded and unloaded conditions without any implant failure. The implant also has reduced plaque affinity thereby limiting the risk of inflammatory changes in the nearby soft tissues. From the existing literature, zirconia ceramics have been shown to be suitable for dental implants. Mechanical, biocompatibility and micro-structural properties of zirconia have been considered for clinical applications (Shenoy & Shenoy, 2010).
The research aimed at assessing fracture resistance and effect of surface treatment of titanium elements on the shear bond strength using various luting cement types and zirconia. The various approaches used for cementation makes it challenging to have best practices for implant-based restorations with appropriate bond strengths. Titanium used as implant materials has been shown to be prone to corrosion, is not tooth like, and has poor mechanical properties. However, zirconia has been reported to be a good alternative since it is more tooth-like, not prone to corrosion, is biocompatible and its mechanical and micro-structural properties are appropriate for clinical applications. Despite the differences, it is important to have an optimum method of cementation to ensure that appropriate bond strengths are realized. Fracture resistance is an important aspect of research since it determines the longevity of using ceramic biomaterials in implant-supported tooth abutments.
The objectives of the research are as outlined below:
1. To assess the shear bond strength of zirconia on a modified titanium surface by use of various luting cement methods
2. To identify the fracture resistance properties of zirconia abutments as compared to Titanium based abutments
Dental implants tend to improve chewing functions and patient satisfaction. There are various materials used for dental restoration, but their effectiveness varies as discussed in the review of the literature. The focus of the research is evaluating fracture resistance and the influence of surface treatment of titanium elements on the shear bond strength using various luting cement types and zirconia.
According to Nakamura, Kanno, Milleding, and Ortengren (2010), Zirconia is a polymorphic material that has four varying crystalline structures. At room temperature, pure zirconia has a monoclinic form and addition of stabilizing oxides results to a stabilized zirconia with a stress-induced transformation toughening mechanism. Despite the favorable mechanical properties, Zirconia is found to accumulate dental plaque less than titanium. Ceramics have been used as abutment materials in dentistry for several years, but only a few articles on ceramic abutments have been reviewed and published. The increased application of zirconia as an abutment material requires a systematic review of the existing data on zirconia and is effectiveness as a dental implant.
In a systematic review of the literature by Velázquez-Cayón et al. (2012), the mechanical resistance of Zirconium implant abutments was identified and discussed. The researchers reported that abutments show favorable clinical behavior in the rehabilitation of single implants in the anterior area and zirconium abutments are not recommended for used in the molar area. The advantages of the use of ceramic materials in implant-prosthetic rehabilitation were summarized as taking advantage of their light transmission properties, good aesthetic qualities having colors that resemble those of the teeth and excellent compatibility with the soft tissues. The disadvantages identified in the use of ceramic materials include less resistance to fatigue. Research shows that stabilized tetragonal zirconia ceramics have better properties to alumina due to their microstructural differences and polymorphic mechanism. Yttrium stabilization of the zirconia structure makes it more superior than the alumina materials.
Zirconium oxide (zirconia) has similar mechanical properties to metals and a color that resembles that of teeth. The partially stabilized zirconia shows the best properties for dental restoration and does not suffer from corrosion. Throughout decades, materials been used for dental treatments were made of metal. The purpose was to offer mechanical strength and augment the longevity of treatment. Scientific research led to the improvement of ceramic’s mechanical strength and had become the most important due to excellent esthetic properties. Ceramics are inert, non-corrosive, and non-allergic, hence more beneficial than other dental materials.
As Hagi (2015) reports, the main goal of tooth replacement is providing patients with long lasting tooth-like restoration which ought to be strong, easy to maintain, biologically compatible, and aesthetically similar to the natural teeth. The use of titanium in dental implants is widely accepted due to its long-term functionality. However, the late complications like soft tissue deficiencies are common and also problems in tooth contour that have led to the use of white zirconia abutments. Tooth replacement concept focuses on the use of one piece, zirconia all-ceramic dental implants in treating partial edentulous. The concept developed due to the need of simplifying the restorative protocol and enhancing biological stability and simplifying profile formation. The tooth replacement concept involves a roughened and tooth-shaped zirconia implant which shows high degrees of clinical success. Zirconia is a strong ceramic that demonstrates high rates of osseointegration and its CeraRoot system has various implant shapes with specific tooth replacement indications.
A research conducted by Daou (2014) focused on the strengths and weaknesses of Zirconia Ceramic and revealed important findings. The major factors that influence the choice of material used in tooth restoration are esthetics and the strength of the prostheses. Traditional ceramics and aluminum oxide reinforced ceramics had various problems, especially in the molar section. Transformation-toughened zirconia is a likely successful alternative in various clinical situations as compared to other all-ceramic systems. The survival rates for all ceramic crowns and metal-ceramic for anterior teeth have been reported to be five years, and when used for premolars and molars, their success decreased to 90.4% and 84.4 % respectively. The controlled clinical studies on zirconia-based crowns reported low complication rates.
Mechanical properties of zirconia allowed them to be used in dental implants as an alternative to the conventional materials. In-room pressure, temperature influences the crystallographic form of unalloyed zirconia. Upon heating to 1170 degrees (°C), the structure is monoclinic. Between 1170 and 2370 °C, the structure of zirconia is tetragonal and is cubic above 2370 °C to its melting point (Apratim, et al., 2015). Upon cooling, the transformation from a tetragonal phase to monoclinic phase induces a substantial increase in volume leading to failure. An addition of oxides (CaO, MgO, Y2O3 or CeO2) to zirconia alloys allows the retention of the tetragonal structure at ambient temperatures. It helps to control the stress-inducing transformation. Other factors that affect the meta-stability of the transformation include composition, grain size, shape, type and amount of stabilizing oxides, and interaction of zirconia with other phases.
The mechanical strength of Zirconia framework is more than three times higher than all ceramic. It withstands physiological occlusal forces applicable in the posterior region, and framework fractures in all-cerum fixed partial dentures (FPDs) were reported in the connector region. The major problem encountered is porcelain cracking though the difficulties are material-specific. Zirconia-porcelain interface can be involved in crazing and chipping during the function. The stresses are related to surface property since bulk thermal expansion and contraction mismatches do not appear to be the cause of crazing and chipping. Chipping is a typical failure on contact loadings and arises when a crack generator by contact loads deflects due to a free surface nearby (Apratim, et al., 2015).
Zirconia has a thermo-conductivity significantly lower than that of other framework materials. The low thermal conductivity limits ceramic cooling rate at the interface. It generates thermal residual stress and induces thermal cycling delamination. The effect of varying cooling rates on the bond strength between the layers of porcelain and zirconia ceramics has shown that prolonged cooling phases tend to reduce stress and chipping.
Color and aesthetics
Tooth Enamel comprises of 97% hydroxyapatite mineral is translucent and transmits up to 70 % of light. The esthetic dilemma for the metal-ceramic restorations is the opaque porcelain needed to mask a metal substrate. It reflects light and decreases translucency. Zirconia framework is esthetically better than a metallic framework and remains clinically white and opaque. Manufacturers introduced colored zirconia framework to enhance the overall matching color. The studies reviewed showed that bond strength of the colored type zirconia is lower than non-colored zirconia (Apratim, et al., 2015). When a framework gets a color by dipping in pigment solution, the pigmentation concentrates on the outer surface and tend to crystallize thereby weakening the bond with the ceramic.
Mussano, Genova, Munaron, Faga, and Carossa (2016) provide a discussion on the use of ceramic biomaterials for the dental implants at present and the future perspectives. According to the information presented in the book, titanium implants have the longest traceable record of predictable performance and the largest diffusion in the market. However, certain shortcomings to its use have been reported in the literature. Titanium is not fully bioinert since it elicits allergenic reactions and can diffuse within the nearby tissues and also systemically. Ceramic materials have been in use for more than 40 years in the oral application, and the material of choice at present is yttria-stabilized tetragonal zirconia. It has excellent mechanical, biological, and biocompatibility properties. Concerns have been raised on the long-term durability of Zirconia due to the report of in vivo failures caused by low-temperature degradation of zirconia. In addressing the issue, research has helped to improve oxide-based materials like alumina-zirconia and non-oxidic ceramics (Mussano, Genova, Munaron, Faga & Carossa, 2016).
Modern oral implantology is based on titanium due to its cumulative success rate of 98.8 % for the last few decades. Until recently, the use of titanium was widespread before the manufacturing of high-purity aluminum oxide which was chosen for excellent corrosion resistance, biocompatibility, wear resistance, and high strength. However, the features do not make alumina a good alternative to titanium since it is brittle and prone to fracture under unfavorable load. The introduction of yttria-partially stabilized tetragonal zirconia polycrystals (Y-TZP) helped to advance the research on the use of ceramics in dental implants (Mussano, Genova, Munaron, Faga & Carossa, 2016).
Zirconia (ZrO2) is a crystalline dioxide of zirconium and is a good choice for preparing dental implants. However, reports of failures have questioned the long-term stability of the material. Low-temperature degradation of the material known as the aging process has a significant role in limiting its excellent properties. The effects of the aging process include surface degradation with grain pullout, micro cracking, and strength degradation. In the control of aging phenomenon, several factors are considered including the use of stabilizers and modulation of residue stress. Also, the adjustment of the crystal size and removal of impurities during manufacturing is good for anti-aging practices (Apratim, et al., 2015).
The interest in the use of zirconia has increased for its superior biocompatibility and biomechanical properties. However, conventional adhesive techniques do not produce high bond strength to substrates. Researchers continue to conduct an extensive study to establish reliable, reproducible and commercially viable composite bonding protocol for zirconia. Grinding and abrasion to create rough surfaces for micromechanical interlocking introduce flaws that compromise the strength and reliability of the material. The use of silica-coating is reported to increase bond strength, but can also decrease long term storage in vitro. Recent research studies have reported that combining mechanical and chemical pretreatments is important to obtain durable bonding for zirconia (Li, Chow & Matinlinna, 2014).
Clinical Studies and reports on Zirconia implants
A recent study focused on analyzing the survival and success rates of zirconia dental implants and based its findings on the available clinical data from other studies. The different studies, except one, reported the use of one-piece zirconia implants. The observation period after intervention ranged from one to five years and had a survival rate of 74% to 98% and success rates of between 79.6% and 91.6% of prosthetic restoration (Materials, 2015). The authors asserted that there exists limited evidence to support the use of zirconia dental implants due to a short period of observation and number of participants in the studies. The authors emphasized on the need of well-documented, long-term, randomized controlled trials to establish an evidence-based utilization of zirconia implants as an alternative to titanium implants.
Several case reports analyzed reported the use of one-piece zirconia implants with a rough surface for replacing the missing single or multiple rooted teeth in the jaw with excellent aesthetic and functional outcomes after a follow-up of one to three years. The findings of prospective case series on single-piece zirconia implants restoring single tooth gaps revealed comparable results to the restored titanium implants after 24 months of clinical functionality. The success rate of one-piece zirconium implants with different surface treatments was evaluated in other studies. The findings showed overall success rates of between 92% and 95% after a follow-up period of between 2.5 and five years (Materials, 2015). There was also excellent aesthetic, and functional results and no mechanical complications were reported. The authors suggested that zirconia implants can be a viable alternative to titanium implants during tooth replacement.
The findings of a cohort study assessing the clinical and radiographic outcomes of one-piece zirconia oral implant for single tooth replacement failed to confirm the previous findings. A one-year follow up revealed a comparable survival rate for ceramic and titanium implants. Increased radiographic bone loss exceeding 2 mm around ceramic implants limited its recommendation for clinical use. The results of a recent prospective case revealed increased radiographic bone loss after one year (Materials, 2015).
In a different randomized clinical trial as reported in Materials (2015), immediate occlusal and non-occlusal loading of single zirconia implants was evaluated but did not provide a conclusive answer. The conclusion made was that immediately loaded zirconia implants had higher failure rates than implants placed in healed sites.
1. Titanium disks (Tritan Cp Ti grade 2) having a diameter of 21 mm and thickness of 5 mm
2. Sintered zirconia disks (Ceramill Zi) having a diameter of 8 mm and thickness of 5 mm
4. Rotary grinder
5. Permanent and temporary cement types
7. Dental probe
8. Ultrasonic cleaning bath
9. Loading frame
10. Electron microscope
12. Screws and nuts
A. Evaluating the shear bond strength
Titanium and sintered zirconia disks were used in the experiment. To establish the parameters of the treatment, the roughness of the implant abutment was assessed using a profilometer. The process involved initial milling and was observed that grinding with SiC abrasive paper having a grit size of 180 is subject to causing roughness compared to milling. The materials for use in the experiment were ground using a rotary grinder cooled by water and then washed with water. They were later dried using compressed air after grinding.
There were two groups of Titanium disks which were treated using grinding or grinding and sandblasting with aluminum oxide. It had a particle size of 60 μm by use of Mikroblast Duo under 0.4 Mpa pressure and glancing angles of 45 degrees and a separation distance of 20 mm for 20 seconds. Titanium disks were then cleaned using pressurized steam and washed in deionized water using an ultrasonic cleaning bath for 8 minutes. They were then dried with compressed air and bonded onto zirconia disks using one of the four selected cement types. They cement comprised of one permanent (composite cement, Panavia F 2.0) and three temporaries (polycarboxylate cement, Adhesion Carbofine; zinc oxide eugenol cement, TempBond; and resin cement, Premier Implant Cement). All the cement specimens underwent preparation procedures in respect to the instructions provided by the manufacturer. Any excess material was discarded using a curette and a dental probe.
Measurement of the shear bond strength was done to determine the strength of the bonds between titanium, cement, and zirconia. The material specimens were mounted onto a crosshead with a speed of 2 mm per minute until the failure of the titanium-cement-zirconia bond. It was appropriate to record the maximum forces as was useful in calculating the shear bond strength. The collected data and the diagram were entered into a computer program and integrated with a testing device.
The bond strength was determined using the formula:
Rt = F/S, where Rt is the shear force (Pa),
F is the force that acts on the specimen (N),
And S is the surface area of the specimen (meter square)
Figure 1: Loading frame in use to perform the shear strength test
The obtained results from the experiment were then analyzed statistically by use of R statistical package. The basis of the analysis was a two-way analysis of variance and Tukey post hoc test for comparison purposes. After performing the strength measurements, cross sections of the fractured specimens were then analyzed using an electron scanning microscope. It assisted in determining the state of the fractures formed in the shearing process and the sites where the specimens showed fractures. It was also necessary to determine the chemical composition of the fractures by use of energy-dispersive X-ray spectroscopy using an instrument that was compatible with the scanning electron microscope. The energy values of characteristic X-rays emitted from the specimens after excitation using a primary beam were measured by use of a semiconductor detector. The obtained spectra were useful for the evaluation of the chemical elements available in the specimens.
B. Evaluating fracture resistance
Several analogous (implants) representing missed root of the anterior premolar were used for the study. The implants were organized into two groups by the type of abutment: Group A; titanium abutments, and Group B; readymade zirconia abutments.
All the abutments were of standard measurement, and a glass-infiltrated ceramic crown system used. The abutments were fixed onto the analogs through titanium screw. The specimens were embedded in special holders using an auto polymerizing acrylic resin vertically to a flat plane as a simulation of the clinical conditions. The opening of the abutment was obstructed using sticky wax and the core placed on the abutment and adjusted carefully. The separating material was applied to the core. Each of the specimens was then burnt at 250 °C and cast at 1250 °C. The metal form was polished and both halves repositioned and fastened by screws and nuts to assume a Slip Counter model. The separating medium was applied to the internal surface of the metal abutment and entered to the internal surface of slip counter model. After cooling to room temperature, abutment was removed and the screws opened.
Cementing the crowns was achieved by etching the internal surface with 4.5 % hydrofluoric acid for 60 seconds, cleaning with water, and drying by air. Silane was smeared to the treated ceramic surface after which the abutments were sandblasted for one minute. Rely-X- ARC was mixed and applied to the fitting surface of the crown and cemented to the abutment. Any excess luting material was discarded. The specimens were stored in a bottle having distilled water for 24 hours before applying thermal and mechanical loading. The specimen was subjected to a maximum vertical load of 10 kg with a constant cyclic frequency equivalent to 12 months of clinical usage. All the samples were mounted on a computer controlled the testing machine with a load and the data recorded using software. Tin foil sheet was placed between the loading tip and the occlusal surface of crown samples to attain a constant stress distribution. The samples were compressed until fracturing, and the failure was shown by a sudden drop on the load-deflection curve recorded with computer software. The fractured crown was coated with gold made as a conductor for electron beam, and the specimens were scanned by an electron microscope to evaluate the failure mode. Data analysis was performed through a series of steps. First was descriptive statistics for fracture resistance results involving the mean and standard deviation. Next was the comparison of fracture resistance results using ANOVA test and t-test was done to quantify the significance between the groups having P values ≤0.05.
Results and Discussion
From the first experimental procedure of evaluating the shear bond strength, the statistical analysis of the results as shown in the table revealed that the treatment method used is a highly significant factor that affects shear bond strength.
Table 1: Shear bond strength at the titanium/cement/zirconia interface MPa
Type of cement Grinding Grinding and sandblasting
Mean Standard deviation Coefficient of variation Standard error Mean Standard deviation Coefficient of variation Standard error
TempBond 0.50 0.27 55.08 0.10 2.67 0.60 22.58 0.25
Adhesor 3.45 1.46 42.17 0.55 2.67 1.73 65.01 0.66
Premier Implant Cement 1.99 0.67 33.74 0.25 3,57 1,10 30.98 0.42
Panavia F 2.0 13.25 1.93 14.56 0.73 18,18 2.53 13.94 0.96
The results were high when grinding is done together with sandblasting compared to applying it in isolation. The type of cement used also showed highly significant results, and it affected the shear bond strength. Significant differences were evident between the cement types. The lowest shear bond strengths were recorded for Temp Bond, and next was Premier Implant Cement after grinding and lastly was Adhesion after grinding and sandblasting. The highest average shear bond strength result was obtained from Panavia after grinding was combined with sandblasting. The use of grinding alone with Panavia recorded significantly lower results than the former but was still higher than other cement combinations.
Shear bond strength results were similar for Temp Bond, Adhesion, and Premier Implant Cement despite the method of treatment used. The minimum acceptable shear bond strength for all cement tooth restorations ought to be 13-15 Mpa, but only one type of cement attained it (Panavia). Electron scanning microscopy was used for analyzing the cross sections of the fractures as shown for Zirconia and Titanium. The analysis shows that fractures occurred at the titanium/cement interface when composite cement was used or in the cement at the titanium/cement and zirconia/cement interface. The chemical composition of the surfaces could be established through X-ray spectra.
The results from the second experiment that evaluated fracture resistance of zirconia and titanium abutments showed that all the specimens survived the loading machine and thermal cycles. Screws were not loosened, and the lowest fracture resistance value after load application to fracture testing was observed in Zirconia (785.76)) and the highest in Titanium (830.2693 N). The descriptive statistics for the fracture resistance test results are as shown in the table below:
Table 2: Descriptive statistics of fracture resistance results for all groups
Ti ZrO2 Al2O3
Maximum 866.4613 805.13692 510.79951
Minimum 830.2693 785.755521 484.7978612
Mean 844.52 795.56 494.92
Std. Deviation 12.26 6.22 8.99
Std. Error 3.88 1.97 2.84
The results showed that Titanium group had the highest fracture load mean followed by Zirconia and the differences between the fracture resistances mean values for the groups were statistically significant (t=11.26; P < 0.05). The values for the fracture load mean were recorded as 844.52 for Titanium and 795.56 for Zirconia. The study shows that Titanium and Zirconia abutments survived the fracture load test, an observation that would vary if Aluminum oxide was used.
The fractured specimens can have classifications of the fractured component based on favorable fracture (only in the crown especially for Titanium and Zirconia specimens), and unfavorable fracture (in the crown and abutments). A favorable fracture was an indication of good mechanical properties of the material in use for abutments. The unfavorable fracture was a sign that the material used in making the abutment has reasonable mechanical properties.
For the Scanning Electron Microscope (SEM) analysis, the results showed that distortions occurred in the crowns in coping. There was no difference in fracture mode of the crown in all the specimens.
The first experiment sought to evaluate abutment surface treatment and shear Bond strength for Titanium and Zirconia implants developed by luting cement between them. The researcher postulated that surface treatment and type of luting cement used did not influence the bond strength of titanium which was rejected. The experimental results showed that surface treatment has a significant influence on the shear bond strength for many of the luting cement types in use. High shear bond strength was observed in the specimens that were treated with sandblasting and bonded using composite luting cement. The results were as expected since composite cement is mostly used as permanent luting cement. Sandblasting of titanium specimens enhanced the shear bond strength of the composite cement. From the study findings, a suitable long lasting bonding method to titanium was obtained upon the combination of titanium surface with alumina-blast with a methacrylate-phosphate primer and a luting agent. Hydrochloric acid could be used to enhance the adhesive performance of resin to Titanium when applied in a highly concentrated solution. It is crucial to report that Titanium should not have pretreatment with phosphoric acid since the acid inhibits the adsorption of the functional monomers onto Titanium (Śmielak, Gołębiowski & Klimek, 2015).
Low mean values of shear bond strength were recorded for zinc oxide eugenol cement and Temp Bond after grinding, but the values increased after grinding and sandblasting. The increase in shear bond strength values of the composite cement, resin cement, and zinc oxide eugenol cement was due to increased surface expansion after sandblasting and grinding than mere grinding. Panavia F 2.0 is likely to exhibit micromechanical interlocking more than other types.
The microscopic and chemical composition analysis showed that connection was broken at the interface between titanium and cement upon the use of composite cement. For temporary luting cement, the bond broke at the titanium-cement interface and also at the zirconium oxide-cement interface.
In the second experiment, it was evident that both titanium and zirconia based abutments exhibited enhanced fracture resistance. It can be explained by microstructural differences existing between them. Zirconium oxide ceramic is partially stabilized by Yttrium oxide and has a high density of 6 g/cm3 but a small particle size of 0.4 μm. Zirconia ceramic has metastable tetragonal crystalline structure at ambient temperatures and is likely the major reason for the superior fracture strength. The structure provides an efficient mechanism against flaws and has a significant impact on subcritical crack growth. For the current study, zirconia abutment was likely affected by temperature peaks that altered the metastable tetragonal crystalline phase. The abutments should not be subjected to heat producing surface treatments since they produce spots because of slow heat dissipation which raises controversy as to whether it would lead to reduced fracture resistance of the material. Zirconia is sensitive to the alterations in humidity and temperature and extended the exposure of zirconia ceramics to such conditions leads to slow, low-temperature degradation that becomes significant within few years of use.
Favorable fracture for all Titanium and Zirconium oxide abutments was observed due to their superior mechanical properties. Titanium has an excellent thermal conductivity and is not affected by thermal aging. The force needed for fracturing of Titanium abutments is higher than that of Zirconia abutments. Titanium is known for its good ductility, malleability, surface hardness and has high bond strength between crystals. As such, cracks and weakening points were not expected on its surface after aging. The use of electron microscope revealed a fracture in the crowns and abutments between crystals and not through them since the force required to break the bonds is less than that needed in breaking the crystals. The surface was shown to have a highly interlocked microstructure and layered crystals thereby reinforcing the strength (El-S'adany, Masoud, Kamel & Korsel, 2013).
Dental restoration is a major procedure applied in the restoration of the oral anatomy, morphology, and function of the missing teeth (Stern, 1996). The process is used to restore the function and integrity of the missing teeth (Nentwig, 2004). Dental restoration involves different ways to replace lost teeth or repair the lost structure and function of teeth. Teeth can be damaged either naturally or due to any accident. Dental restoration entails fillings, crowns, bridges, implants and dentures. Tooth restorations can be both direct and indirect, in which the latter method enhances the strength of the filling, thereby reshaping and resolving the problem of shrinkage and low-quality restorations. In the direct restoration method, a soft material is placed on the prepared tooth, and then the tooth worked on before the material gets hard. Amalgam and composite resin materials are common materials used in the direct restoration of teeth. Amalgam is the most common materials used for dental restoration in different countries. The material has a unique property of ‘creep’ in which it can be deformed when the load is below its proportion (Osborne, 2006; Rho et al., 2013). Amalgam has this unique property due to its low-frequency cyclic stresses. The studies by Goldstein (2010) and Schwass et al. (2013) show that the estimated survival time of amalgam is 22.5 years with an annual failure of 3%.However, rising mercury toxicity in amalgam and aesthetical concerns of patients have reduced its use.
For an indirect restoration, the restoration material is placed over the damaged tooth to provide stability and support. Lower lengths of the prepared tooth reduce the resistance and retention (Fernandes et al., 2015). To promote retention and resistance, the basis of restoration depends on the area and pattern of the tooth. A common practice in the indirect restoration is the usage of gold crowns and inlays. These methods are considered as the gold standard for longevity. Various researchers have assessed the survival rate, which approximates 96% over ten years, 87% over 20 years, and 74% over 30 years with the statistical analysis in the subsequent permanent dentition showing 1.4% to be the mean failure rate (Goldstein, 2010; Schwass et al., 2013).
The use of heat-pressed and reinforced ceramics has been reported in various studies. The studies report that this method has a survival rate of 99% within four years and 95% in 11 years, which is considered to be very effective. In this method, materials of lithium disilicate glass ceramic and zirconium dioxide are used since they are appropriate for compressing. The system is strong and durable and has a success rate of 96% over five years. The use of magnesium, alumina, and zirconium infiltrated variants fall under the category of slip-cast glass infiltrated ceramics which success rate is 100% over five years (Schwass et al., 2013).
The metal oxide ceramics include alumina and zirconia and possess some distinct properties including durability and high resistance to fractures. According to the study by Schwass et al. (2013), the survival rate of Procera Alumina is 98% over five years while zirconia has a survival rate of 94% over ten years. Zirconia has superior material properties and is growing in popularity in dentistry due to its similar color to that of the natural teeth and the mechanical nature of metals.
There is limited evidence on the efficacy of zirconia-based fixed dental prostheses. A literature review conducted by Panadero et al. (2014) discussed the available literature on the topic of zirconium oxide properties. The results revealed that a high rate of fracture for the porcelain-veneered zirconia-based restoration varied between 6% and 15% over three to the five-year period while for the ceramic-metallic restorations, the fracture rate ranged between 4% and 10% over ten years. Zirconium oxide is the main focus of research and clinical trials due to its chemical and dimensional stability, mechanical resistance, hardness, and elastic modulus. The material is biocompatible, has low thermal conductivity, is resistant to corrosion, has a high tenacity, and total crystalline microstructure. In the role of restoration, both the all-ceramic and metal ceramic are evaluated over time for success and survival measurements. Zirconia-based fixed dental prostheses have a high rate of fracture associated with the bond failure and zirconia structure. Zirconia has emerged to be a versatile and promising material due to its biological, mechanical, and optical properties (Panadero et al., 2014).
Zirconia implants were introduced into the dental implantology as an alternative to titanium implants. Zirconia seems to be an appropriate implant material due to its teeth-like color, mechanical properties, biocompatibility, and the low plaque affinity. The major advantage of zirconia implant is that it has no prosthetic connections where bacteria can thrive thereby considered to facilitate enhanced gum health. Zirconia implants are 100% white and thus compatible with other teeth and no metal is visible when smiling or communicating. In contrast with titanium, zirconia is a ceramic and does not suffer from corrosion (Manicone, Iommetti & Raffaelli, 2007).
The materials that have been used for the dental treatments are usually made with metal since it provides mechanical strength and prolongs the longevity of the treatment. Over time, research has contributed to the improvement of the ceramic’s strength as a restorative material. Ceramics were introduced because of the aesthetic properties. Ceramics are inert, non-corrosive, and have non-allergic properties; hence, they are more beneficial to the patient than other materials (Manicone, Iommetti & Raffaelli, 2007).
Zirconium is a metal-free option for tooth restoration, and it differs significantly from titanium, which is also commonly used. The ceramic implants have a restorative margin at the gum level, and they are easy to maintain. However, titanium implants have margins at the bone level, which makes brushing difficult and bacteria may grow. Titanium implants are placed deep into the gum to hide the gray color of the metal. In contrast, ceramic implants are white and more similar to the natural teeth. The gums remain healthy around the ceramic materials and are well preserved. In the case of titanium implants, bacteria tend to accumulate more rapidly in the metal surfaces. Also, metals are prone to oxidation and corrosion. Furthermore, the one-piece design of the ceramic implants provides no opportunity for bacteria to accumulate unlike the two-piece design of titanium implants. Bleeding and inflammation around the gums are more frequent in titanium implants than in ceramic implants (Al-Radha et al., 2012).
There are several unique characteristics of titanium and zirconium oxide that make them appropriate for use in dental restoration procedures as described in details below.
Properties of Titanium
Titanium is a unique metal which is very biocompatible with bone and soft tissue. Titanium is resistant to corrosion in sodium chloride solution making it a successful implant material in humans. It has very low density and has the highest strength to weight ratio of any metal. Titanium readily forms oxides which are significant in the mechanism of osseointegration (Wataha, 1996). Titanium has been used as a successful material in implant dentistry for over 50 years, and it continues to be the accepted standard. Titanium exists in two different atomic crystalline forms as described. First, unalloyed or commercially pure titanium has a hexagonal close-packed (HCP) or alpha atomic structure. At high temperatures titanium transforms to the body-centered cubic (BCC) or beta atomic structure, but this can be stable at room temperature by using alloys of elements such as molybdenum or vanadium. Commercially pure titanium exhibits greater corrosion resistance but lower strength than the titanium alloys. Commercially pure titanium (alpha) immediately oxidizes when it is in the presence of air because of its extremely high reactivity. The strength of commercially pure titanium is affected by the interstitial oxygen and nitrogen content (Donachie, 2000).
Beta Titanium has the highest strength values of titanium alloys, but forms fewer oxides and is less biocompatible. These alloys gain their strength from not only the BCC crystalline structure but also the beta-stabilizing elements alloyed with the titanium. Furthermore, beta alloys can be heat treated to obtain even seven higher strengths. Beta alloys are commonly used in industrial and engineering applications due to their high strength and light weight. They are also used in orthodontic applications because of their high strength to elastic modulus ratio. Alpha-beta alloys of titanium contain both crystalline structures and exhibit intermediate properties (Quirynen, et al., 1994). One alpha-beta alloy, Ti-6Al-4V, has found limited use in dentistry for the fabrication of implants and abutments. This alloy shows higher strength values than commercially pure (alpha) titanium. However, it is less biocompatible and is harder to develop a micro-textured surface which is advantageous with the osseointegration of dental implants. The American Society for Testing and Materials (ASTM) recognizes 38 grades of titanium alloy with grades one through four considered being commercially pure. Grade four-commercially-pure titanium (alpha) is commonly used in the fabrication of dental implant and abutments because it has the highest flexural strength of all the commercially pure titanium alloys (550 Mpa) and high biocompatibility. The hardness of grade four-commercially-pure titanium is 258 VH (Quirynen et al. 1994).
Properties of Zirconium oxide (zirconia)
Zirconia (ZrO₂) is an oxide of the transition metal zirconium and has received much attention in the dental industry due to its attractive physical properties. Zirconia is a white, opaque structural ceramic that has the high flexural strength (800-1000 Mpa), high fracture toughness (6 to 8 MPa•m), and high hardness (1600-2000 VH) (Denry& Kelly, 2008). The physical properties of zirconia are attributed to its single phase polycrystalline structure, and it is very small (
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