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DOI: 10.1016/j.jmrt.2018.07.016
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Mechanical and clinical properties of titanium and titanium-based alloys (Ti G2, Ti G4 cold worked nanostructured and Ti G5) for biomedical applications
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Carlos Nelson Elias
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elias@ime.b.br

Corresponding author.
, Daniel Jogaib Fernandes, Francielly Moura de Souza, Emília dos Santos Monteiro, Ronaldo Sérgio de Biasi
Instituto Militar de Engenharia, Rio de Janeiro, RJ, Brazil
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Received 28 March 2018, Accepted 31 July 2018
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Tables (3)
Table 1. Roughness parameter (μm) of dental implants made of Ti G4 and Ti G4 Hard after acid etching.
Table 2. Vickers hardness and tensile properties (E: Young's modulus, σR: tensile strength, σe: yield strength, El: elongation) of Ti G2, Ti G4, Ti G5 and Ti G4 Hard.
Table 3. Number of samples, mass, dimensions and elastic modulus measured using the impulse excitation technique.
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Abstract

Commercially pure titanium (Ti G2 and Ti G4) and the Ti–6Al–4V (Ti G5) alloy have limitations for biomedical applications, due to either low mechanical strength (Ti G2, Ti G4) or the possible release of toxic ions (Ti G5). Since Ti alloys have a low hardening coefficient, it is very difficult to improve their mechanical properties by work hardening. The purpose of this work was to compare the mechanical and clinical properties of Ti G4 nanostructured after severe plastic deformation by ECAP (Ti G4 Hard) with those of Ti G2, Ti G4 and Ti G5. Bars, disks and dental implants made with Ti G2, Ti G4, Ti G5 and Ti G4 Hard were tested. Mechanical tests (tension, compression, hardness, elastic modulus, fatigue and torque) and roughness measurements were performed. The results of the mechanical tests showed that Ti G4 Hard has a higher mechanical strength and a lower elastic modulus than Ti G2, Ti G4 and Ti G5. Scanning electron microscopy and roughness measurements results showed that acid etched Ti G4 Hard nanostructured has better surface morphological features than Ti G2, Ti G4 and Ti G5. The clinical performances of Ti G4 and Ti G4 Hard were similar. The high mechanical strength of Ti G4 Hard means that it can be used to replace Ti G5 in several clinical applications, with the advantage of not releasing toxic ions. The Ti G4 Hard dental implants have adequate mechanical properties and can be inserted in areas with low bone volume.

Keywords:
Titanium
High strength cp Ti
Strain hardening
Ti nanostructured
Dental implant
ECAP
Full Text
1Introduction

The selection of materials for medical devices is based on mechanical properties, chemical composition and biocompatibility [1]. Technical Standard ASTM F-67 classifies commercially pure titanium (cp Ti) for medical applications in four grades, G1 to G4. Technical Standard ASTM F-136 covers the mechanical properties and chemical composition requirements for Ti grade 5 (Ti–6Al–4V ELI, extra low interstitial) alloy to be used in the manufacture of surgical implants. Currently, dental implant manufacturers use commercially pure titanium (Ti G2 and Ti G4) and Ti–6Al–4V alloy (Ti G5) with a surface treatment in order to optimize the contact between bone cells and the device [2]. This interaction between bone cells and dental implants surfaces is called osseointegration. However, Ti G2 and Ti G4 are not used in medical applications that involve high stresses, such as orthopedic prostheses and narrow dental implants. For orthopedic applications, Cr–Co, stainless steel and Ti G5 (Ti–6Al–4V alloy) are the preferred choice due to a high mechanical resistance. These alloys ensure load transmission to bone tissues over a long time, which is necessary when damaged hard tissues are replaced by prostheses. Ti–6Al–4V ELI alloy has good mechanical properties, but exhibits a possibly toxic effect from released vanadium and aluminum [3]. For this reason, vanadium and aluminum free Ti based alloys have been proposed for biomedical applications.

The disadvantages of Ti G2 and Ti G4 for biomedical applications include higher Young modulus, relatively low mechanical strength, poor wear resistance [4] and a difficulty to improve the mechanical properties without reducing biocompatibility. The mechanical properties of unalloyed Ti are determined by the levels of interstitial solutes (N, O and C) and substitutional atoms (Fe). Although the addition of interstitial solutes increases the strength of pure Ti, it decreases toughness.

During the last 60 years commercially pure Ti (cp Ti) is the main biomaterial used for dental application. The use of cp Ti was adequate until now because the dental implants had diameters larger than 3.75mm. The most recent dental implants and new dentistry surgery techniques use narrower implants (<3.0mm) and cp Ti does not have enough mechanical strength to support oral loading in all jaw sites. Pure Ti can be submitted to severe plastic deformation (SPD) at room temperature up to a 90% reduction in thickness without cracking. Such extensive deformability is unusual for HCP metals and is related to the low c/a ratio of Ti [5]. HCP metals, including Ti, have three independent slip systems, which are insufficient to deform only by slip. Ti deformation twinning should be accompanied by slip for the HCP metals to sustain large deformation without cracking. Ahn et al. [6] analyzed the effect of deformation twinning on the strain hardening behavior of Ti. The strain hardening rate of titanium can be divided in three stages. In the first stage, the strain hardening rate decreases as the strain increases due to easy glide. In the second stage, a sudden increase in the strain hardening rate is observed due to deformation twinning. In the third stage, the strain hardening rate decreases again due to dynamic recovery [6].

Although the mechanical strength of Ti implants is important, they must also present adequate stiffness to avoid bone stress shielding. The stress shielding phenomenon results from the fact that the human body tends to reduce or eliminate their own parts when they are not used. The muscle mass, for instance, is increased by exercise; if we do not exercise, the muscle is gradually lost. Stress shielding occurs when the forces exerted on bone by prosthesis are different from the forces exerted by a natural body component. This difference induces the loss of bone density at the site (osteopenia), leading to bone atrophy. Materials with lower elastic moduli have better stress distribution at the implant-bone interface and lead to less bone atrophy [7,8]. A common site for stress shielding is the proximal femoral diaphysis after placement of a femoral prosthesis. The more tightly the stem of the prosthesis fits into the distal medullar canal, the greater the shift of body weight to the prosthetic stem from the proximal femoral cortex. This causes loss of the normal remodeling forces above the level at which the stem is fixated against the endostea surface of the medullar canal resulting in osteopenia of the proximal femoral diaphysis [9]. This can potentially lead to bone loss in the long term and eventual loosening of the device, requiring an early revision surgery.

Ti G2 and Ti G4 do not have enough mechanical strength for some applications where osseointegration and high loading are a priority, such as dental implants inserted in the posterior mandible, and Ti G5 has toxic effects due to release of aluminum and vanadium. In the present work, a modified Ti G4 material is proposed for narrow dental implant application. A new version of Ti G4, called Ti G4 Hard, was hardened by the ECAP process and nanograin size was developed by severe plastic deformation (SPD). More details of ECAP are available elsewhere [10]. Ti Hard combines the excellent mechanical strength of Ti G5 with the corrosion resistance and biocompatibility of Ti G4 and Ti G2.

The purpose of the present work was to compare the mechanical properties (tensile strength, compression strength, Vickers hardness, elastic modulus, plastic deformation and fatigue resistance) and clinical behavior of implants made of Ti G2, Ti G4, Ti G4 Hard and Ti G5.

2Materials and methods

In the present work the mechanical properties, surface morphology, surface roughness and clinical performance of Ti and Ti alloys were investigated. Since the samples were not labeled, all tests were single-blinded.

Standardized samples for tensile testing (ASTM E8) made with Ti G2, Ti G4, Ti G4 Hard and Ti G5 were used. Dental implants made with Ti G2, Ti G4, Ti G4 Hard and Ti G5 were used for roughness measurements. Ti G4 Hard was produced by ECAP process.

Acid etched screw-shaped dental implants made with Ti G4 and Ti G4 Hard were submitted to static compression and fatigue tests. The dental implants were of the external and internal hexagon type (Fig. 1). The model with nominal size of 3.5×8.5mm was chosen for the compression and fatigue tests. This model is considered the critical size, because it has the smallest commercialized dental implant wall thickness. The Technical Standards (ISO 14801: Dentistry – Fatigue test for endosseous dental implants) recommend that the mechanical tests be performed with the smallest implants (critical dimension). Three conventional Ti G4 dental implants lots and three Ti G4 Hard lots were tested. Each lot had 10 implants for static tests and 12 implants for fatigue tests.

Fig. 1.
(0.17MB).

Model of dental implant subjected to compression and fatigue tests. Courtesy of Conexão Sistema de Próteses. Dental implant model Torq®.

2.1Surface analysis

Disks and implants made with Ti G2, Ti G4, Ti G4 Hard and Ti G5 were submitted to a surface acid treatment with mixture of HCl and H2SO4 using the same concentration, temperature and time interval than for the available Porous® dental implant surface (Conexão Sistemas de Prótese, Brazil). The disks were polished before surface acid treatment.

The surface morphology (two implants and two disks from each group) was observed on a scanning electron microscope Field Emission Gun FEI QUANTA FEG 250 (FEI Corporate, Hillsboro, Oregon, USA) with energy dispersive spectroscopy (EDS) for semi-quantitative chemical analysis. Some works use only disks or dental implants for surface analysis. In the present work we used both and compared surface morphologies and roughness.

2.2Roughness measurement

The surface roughness was measured in dental implants and disks after acid etching. Three samples from each group were used. For characterizing and quantifying the surface roughness, step height, critical dimensions, and other topographical features the NewView™ 7100 (Zygo Company, Middlefield, CT 06455, USA) light interferometer (profilometer) was used. The NewView™ 7100 provides affordable versatility in non-contact optical 3D surface profiling. The parameters for numerically roughness characterization were the following: arithmetic mean of the absolute values of roughness (Ra), peak-to-valley roughness (Rz), the root square value of average roughness (Rms or Rq), the largest valley depth value (PV), and the average height of the three highest local maximums plus the average height of the three lowest local minimums (R3z).

2.3Hardness vickers, tensile, elastic modulus, compression and fatigue tests

Standard microindentation Vickers hardness tests were performed in disks according to ASTM E384 (E384: Standard Test Method for Knoop and Vickers Hardness of Materials). For optimum accuracy of the measurements, the tests were performed on flat disks bases with polished surfaces.

Tensile tests of Ti G2, Ti G4, Ti G4 Hard and Ti G5 were performed using a Universal testing machine EMIC DL10000 (Emic, Brazil) according to ASTM E8M standard (Standard Test Methods for Tension Testing of Metallic Materials). Round specimens with 4.5mm diameter and 50.0mm gauge length were used. In order to minimize the effects of surface irregularities, the gauge section of the machined test specimens was polished with progressively finer grades of silicon carbide impregnated emery paper (320, 400, 600 and 1200 grit) to remove all circumferential scratches and any residual machine marks.

The samples used for elastic modulus tests were cylinders and their mass, dimensions and elastic modulus were measured five times. An Ohaus Explorer analytical balance was used to determine the mass of the samples and a Mitutoyo caliper with a resolution of 0.05mm and a Mitutoyo micrometer with a resolution of 0.01mm were used to determine the sample dimensions. The elastic modulus of the samples was measured using the impulse excitation technique (ASTM E 1876-0, 2010). In this technique, the sample undergoes an impact of short duration and responds with vibration in its natural vibration frequencies and according to the boundary conditions imposed. The used system is operated by Sonelastic® software version 2.2, from ATCP Physical Engineering.

The static compression and fatigue tests of dental implants followed the recommendation of technical standard ISO 14801: 2012 – Dentistry–Implants–Dynamic test to fatigue endosseous dental implants. Fig. 2 shows the assembly for mechanical tests. Notice the 30° angle of the implants inserted into holes previously prepared a polyacetal block (E=3.0GPa). For mechanical compression tests we used a Universal testing machine EMIC DL10000 with 1.000N load cell and a displacement rate of 1.0mm/min.

Fig. 2.
(0.05MB).

Device used for static compression and fatigue tests.

The standard ISO 14801: 2012 recommends that fatigue tests should be started with the application of forces on the order of 80% of the static compression resistance. If fracture occurs after less than 5.0×106 cycles, the load should be reduced. The fatigue tests were repeated, reducing the load, until at least three (3) samples resisted 5.0×106 cycles. The fatigue tests were performed at room temperature in a dry environment. The tests were performed using a MTS Bionix® servohydraulic system (Eden Prairie, MN-USA) with 370.02 load frame model and Flextest® 40 software. The load varied sinusoidally between a nominal peak value and 10% of this value and frequency of load application was 10Hz.

2.4Clinical testing

In order to evaluate the performance of Ti G4 Hard dental implants a clinical test with humans was performed. Dental implants made of Ti G4 and Ti G4 Hard were inserted into the patient. Dental prostheses on the implants were installed 3–5 months after surgery. A radiograph was taken 3 years after surgery.

Five hundred implants were installed in patients. All patients were healthy and were not taking medications that could affect bone metabolism or bone calcification. The implant primary stability was estimated from the insertion torque, which was 50–60Ncm.

3Results3.1Surface morphology

No morphological difference was observed among the disks and dental implants. It was possible to observe that the prepared surface morphologies were similar to some commercial dental implants available from some companies (Porous: Conexão Sistemas e Protese; SLA: Institut Straumann AG, Waldenburg, CH).

Fig. 3 shows the surface morphologies of the dental implants after acid etching. After the acid etching treatment, the dental implants made with Ti G5 showed a smooth surface which is inadequate for osseointegration. Implants made with Ti G4 Hard has more nanometric features than the implants made with Ti G4. The nanocavities facilitate the implant osseointegration mechanisms [11].

Fig. 3.
(0.95MB).

Surface morphology of dental implants after acid etching. (A) Ti G2; (B) Ti G4; (C) Ti G4 Hard; (D) Ti G5.

3.2Roughness

The mean and SD (Standard Deviation) of the titanium dental implant surface roughness parameters after etching are displayed in Table 1. The surface treatment significantly increases the roughness and improves osseointegration. Considering that the disk and implant surfaces had the same characteristics, no significant difference was observed in the roughness between disks and implants. Fig. 4 shows a representative surface morphology of Ti G4 Hard obtained by interferometry.

Table 1.

Roughness parameter (μm) of dental implants made of Ti G4 and Ti G4 Hard after acid etching.

  Ra  Rz  Rms  PV  R3z 
Ti G4  0.92  2.72  0.859  2.39  2.70 
Ti G4 Hard  0.89  1.98  0.772  2.54  2.81 
Fig. 4.
(0.56MB).

Surface morphology of dental implants: (A) Ti G4; (B) Ti G4 Hard. Light interferometer.

3.3Mechanical properties

Table 2 shows the Vickers hardness and tensile properties of Ti G4, Ti G4 Hard and Ti G5. The results of the mass, size and elastic modulus measured by impulsive excitation technique of the received samples are shown in Table 3.

Table 2.

Vickers hardness and tensile properties (E: Young's modulus, σR: tensile strength, σe: yield strength, El: elongation) of Ti G2, Ti G4, Ti G5 and Ti G4 Hard.

  Vickers  E (GPa)  σR (MPa)  σe (MPa)  El (%) 
Ti G2  171.3 (14.1)  108  362.6 (12.9)  310.4 (11.4)  19.2 (1.6) 
Ti G4  267.8 (10.4)  109  547.8 (18.5)  489.7 (14.3)  18.4 (2.5) 
Ti G4 Hard  453.2 (11.9)  110  970.7 (25.3)  812.6 (21.4)  18.5 (2.3) 
4Titude      1150 (26.7)  932 (25.3)  17.0 (3.2) 
Ti G5  435.7 (12.7)  115  871.9 (27.3)  797.2 (20.9)  10.1 (2.6) 
Table 3.

Number of samples, mass, dimensions and elastic modulus measured using the impulse excitation technique.

  N  Weight (g)  Length (mm)  Diameter (mm)  E (GPa) 
Ti G2  32.023  40.53  4.76  112.3 
Ti G4  42.787  53.92  6.35  144.3 
Ti Hard  32.117  40.60  4.76  105.7 
4Titude  50.902  40.44  6.00  107.7 
Ti G5  41.453  40.05  5.50  109.7 

Fig. 5 shows a Ti G4 Hard implant during compression loading. The compression tests of the implants were interrupted by the fracture of any part. It can be seen that the implant undergoes plastic deformation, showing that this alloy has good plasticity. Fig. 6 shows the results of compression tests of implants from each group.

Fig. 5.
(0.05MB).

Implant during the compression test.

Fig. 6.
(0.04MB).

Mean peak force applied in the compressive test of three groups of Ti G4 Hard and Ti G4 dental implants.

The statistical analysis using the null hypothesis showed that the compression resistances of Ti G4 Hard and Ti G4 are significantly different.

The average static compression force for Ti G4 Hard implants was 562.4±69.9N, which is 34.9% higher than Ti G4 (416.7±39.0N). All values for implants made with Ti G4 Hard were higher than for the same implants model manufactured with Ti G4. The maximum compressive moments showed the same behavior as the maximum forces. Basically, dental implants made with Ti G4 Hard are stronger than those made with Ti G4.

Fig. 7 shows the profiles of peak load vs. number of cycles of all implants during fatigue test. In Fig. 7 the number of load cycles endured by each specimen is plotted on a logarithmic scale and the corresponding peak load is plotted on a linear scale. In the diagram, each point represents a different sample.

Fig. 7.
(0.05MB).

Fatigue test results.

In the fatigue test, Ti G4 Hard implants resisted 5×106 cycles for 310.5N loading and conventional Ti G4 implants resisted 191.1N. Based on this result and comparing the results shown in Figs. 6 and 7 it can be seen that the cyclic load to fracture the dental implant is lower than static loading. It is therefore extremely important for manufacturers to conduct the tests not just for static compression but also for cyclic compression in order to determine implant strength under fatigue loading.

3.4Failure mode and fracture surface

After the mechanical tests, the mode of failure of the implants was investigated. In the compressive tests, the commonest failure mode was fracture of the prosthetic screw (Fig. 8). Macroscopic deformation was observed in dental implants with internal connection.

Fig. 8.
(0.54MB).

Surface morphology after mechanic testing. (A) Deformed dental implant after compression test. (B) Fractured dental implant after fatigue test.

No consistent fracture mode was observed in fatigue testing. In the fatigue tests, the fracture of dental implants occurred either on the prosthetic screw or on the implant neck (Fig. 8B). Some implants fractured between the collar and the first thread and some fractured between the second and the third threads.

After the fatigue test the fracture surfaces were flat, suggesting a single failure mechanism. Typical metal fatigue features were identified. Striations were observed over most of the fracture surface, indicating that the cracks propagated under cyclic loads. The fracture surface morphology of implants made of Ti G4 Hard exhibited a transgranular pattern with faceted crystallographic merged with cleavage facets with clear fatigue striations and dimple marking, an indication of ductile failure.

Fig. 9 shows the dental implant after the torque test. It is possible to observe that the torque of 120Ncm deformed the implant Ti G4 more than implant Ti Hard. The highest clinical torque was 60Ncm.

Fig. 9.
(0.53MB).

Morphology of the dental implant after application of the torque of 100Ncm. (A) Ti G4. (B) Ti G4 Hard.

3.5Clinical results

A case of clinical test is shown in Fig. 10, which shows the surgical procedures and a radiograph after 3 years of surgery.

Fig. 10.
(0.14MB).

Example 4 clinical results 3 years after the surgery of dental implant treatment with ultrafine grain size.

Courtesy of Dr Celso Renato de S Resende.

Three years after the surgery, absence of pain, peri-implant infection, and mechanical stability of all implants was observed. All implants were classified as surviving because they present good mechanical stability at the end of the study. The clinical performances of Ti G4 and Ti G4 Hard were similar.

Clinical evaluation showed that the dental implants made with nanostructured cp Ti G4 (Ti Hard) were successful.

4Discussion

The concept of biocompatibility involves not only biological compatibility of the material with the tissue but also its ability to perform a specific function. Therefore, biocompatibility is defined only for a particular application. For example, the biomaterial used in hip prostheses or coronary stents is not adequate for dental implants, even if it displays biological compatibility.

One of the requirements for biocompatibility in a medical device intended for long term contact with the tissues of the human body is that the material will not harm the tissues. However, in the case of dental implants, it is also important that it undergoes osseointegration, The new cp Ti with nanostructured morphology by ECAP process has both properties.

The implant surface characteristics (morphology, roughness, macroporosity and microporosity) are very important for the success of dental implants [11]. The implant topography, chemistry and surface energy play an essential part in cell adhesion on dental implants, especially in terms of osteoblast adhesion. In the present work the acid etched Ti implant has in its surface a layer of titanium oxide, which is biocompatible.

Several mechanisms involved in osseointegration depend on surface roughness, since the cells react differently in contact with smooth surfaces and rough surfaces. Fibroblasts and epithelial cells adhere more strongly to smooth surfaces and the ability of osteoblast proliferation and collagen synthesis is greater in surfaces with moderate roughness [12]. Despite the importance of roughness in osseointegration, there is no standard for the roughness of dental implants. Data from the literature [12,13] show that good machined dental implants present Ra between 0.5 and 1.0μm. Machined dental implants without surface treatment were used until during the end of the last century. The Ra roughness parameter of commercially available dental implant surfaces treated with acid is between 0.54 and 1.97μm. In the present work, the Ra values were close to 1.0μm. This roughness is considered adequate for osseointegration.

Fig. 3 shows that the treatment of the implant surface with acid, besides modifying its roughness, makes the surface isotropic. The surface energy expresses the chemical composition, the residual stress, the local spatial arrangement of atoms and the implant bone contact. The surface morphology shown in Fig. 3 is isotropic and presents microvoids with defined borders. This surface type facilitates osteogenic cell retention and allows for cell migration at the implant surface. Some researchers point out that a surface morphology similar to that shown in Fig. 3 induces fibrin retention and facilitates the osseointegration process [14].

During decades, the biocompatibility of materials used in dental implants has been evaluated by studying the reaction between the implant and the bone cells. However, cell adhesion to the surface of implants and osseointegration is not enough to guarantee the success of implants. The mechanical resistance of the material must also be enough to withstand oral loading. In the last few years, new surgical techniques for placement of osseointegrated dental implants have been developed that use implants with smaller diameters. The reduction of the outer implant diameter without decreasing the inner diameter is the most critical, since it reduces the implant wall thickness and increases the probability of fracture. This means that cp Ti G2 and Ti G4 are inadequate for narrow dental implants with an external diameter smaller than 3.0mm and for internal prosthesis components.

One of the limitations of cp Ti G2 and Ti G4 for use in dental implants is the thickness of the implant walls, which must be larger than 0.5mm. Insertion of implants with small outer diameter with internal prosthesis components into the posterior jaw area is particularly critical due the high oral loads in this region.

Based on the values of mechanical properties shown in Table 2 and 3, it can be stated that: (i) one can replace Ti G4 with Ti G4 Hard in order to manufacture dental implants with smaller diameters and internal connections without compromising the mechanical resistance; (ii) the use of Ti G4 Hard instead of Ti G4 increases the mechanical stability of the implant, since its higher hardness prevents deformation of the external hexagon during insertion (Fig. 8).

The possibility of using implants with smaller diameter reduces the need of graft surgery to increase the volume of cortical bone and provides less invasive solutions, especially in cases where the bone site has small thickness and there is limited space between the teeth.

The methodology of ECAP used in this study to increase the mechanical strength of cp Ti without damaging the plasticity of the alloy was to subject Ti G4 to cold plastic deformation in order to create internal defects in the microstructure. The increase of the mechanical strength by cold plastic deformation is called strain hardening. It can be seen in Table 2 that the implant has extensive plastic deformation during the tensile test, indicating that Ti G4 Hard after ECAP processing is ductile. The ductility of Ti G4 Hard was confirmed by the morphology of the fracture surface observed by SEM. Fig. 10 shows a representative fracture surface of Ti G4 Hard implants, where one can see the presence of microcavities characteristic of ductile fracture. Although Ti G4 Hard does not contain alloying elements, it has a higher mechanical strength than Ti G5.

The surface roughness of implants affects osseointegration and the mechanical stability of dental implants. Osseointegration depends on cell contact with the implant surface and cell surface adhesion. The surface roughness and wettability of the implant influences cell behavior. Fig. 3 shows that Ti G4 Hard has a better surface morphology than Ti G4.

5Conclusions

This study led to the following conclusions:

  • a)

    Dental implants made with cp Ti Hard submitted to a severe plastic deformation (ECAP) are more resistant to compression and fatigue load than those made with conventional cp Ti G4;

  • b)

    After acid etching, the surface of Ti G4 Hard implants exhibits morphological features which are suitable to osseointegration;

  • c)

    Clinical tests showed a higher success rate of Ti G4 Hard dental implants.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

The work described in this paper was supported by two grants from CNPq (Processes 472449/20044, 400603/20047 and 500126/20036) and one grant from FAPERJ (Process E26/151.970/2004).

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Journal of Materials Research and Technology

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