Mechanical properties, surface morphology and stability of a modified commercially pure high strength titanium alloy for dental implants

Abstract

Objective

Commercially pure titanium (cp Ti) and Ti–6Al–4V (Ti G5) alloy have limitations for biomedical application, due to lower mechanical strength and the possibility of ion release, respectively. The purpose of this work was to compare the properties of a modified cp Ti grade 4 (Ti G4 Hard) with those of available cp Ti and Ti G5 alloys.

Methods

Bars, discs and dental implants made with Ti G2, G4, G5 and G4 Hard were used. Mechanical tests (tension, compression, hardness and torque) and roughness measurements were performed. Clinical trials were used to evaluate the biological behavior of dental implants made with Ti G4 Hard and Ti G4.

Results

The results of the mechanical tests showed that the mechanical strength of modified Ti G4 is higher than that of Ti G2, G4 and G5. Scanning electron microscopy analysis showed that modified Ti G4 after etching has better surface morphological features than conventional cp Ti and Ti G5. The clinical performances of Ti G4 and Ti G4 Hard were similar.

Significance

The improvement of the mechanical properties of modified Ti G4 means that Ti G5 can be safely replaced by Ti G4 Hard without compromising the fracture resistance, with the advantage of not releasing toxic ions.

Introduction

The selection of materials for dental implants is based on mechanical properties, chemical properties and biocompatibility. Regardless of the role and place of application of dental implants, the materials should have good corrosion resistance, biocompatibility and be free of toxic elements. Currently, dental implant manufacturers use commercially pure titanium and titanium alloys with a treated surface in order to optimize the contact between alveolar bone and the device surface. This histologic interaction is called osseointegration .

Technical Standard ASTM F67 classifies cp Ti for medical applications in four grades, G1–G4 (see Table 1 ). However, cp Ti is not used in medical applications that involve high stresses, such as orthopedic prostheses. In these cases, Ti G5 (a Ti–6Al–4V alloy) is the preferred choice due to its high mechanical resistance, which ensures load transmission to bone tissues over a long time, which is necessary when damaged hard tissues are replaced by prostheses.

Table 1
Standard ASTM number, chemical requirements (max mass%), tensile strength (TS), yield strength (YS), modulus of elasticity ( E ) and Brinell hardness (HB) of cp Ti, Ti alloy G5 and bone properties. Based on Technical Standards ASTM F67 and ASTM F136.
Ti grade ASTM O Fe H C N Ti TS (MPa) a YS (MPa) E (GPa) HB
Ti grade 1 F67 0.18 0.20 0.015 0.08 0.03 Balance 240 170 100 120
Ti grade 2 F67 0.25 0.30 0.015 0.08 0.03 Balance 345 275 100 160
Ti grade 3 F67 0.35 0.30 0.015 0.08 0.05 Balance 450 380 100 200
Ti grade 4 F67 0.40 0.50 0.015 0.08 0.05 Balance 550 483 102 250
Ti grade 5 b F136 0.13 0.25 0.012 0.08 0.05 Balance 860 795 115
Cortical bone 186 5–30 c

a Mechanical properties requirements for annealed wire with diameter higher than 3.18 mm.

b The compositional requirement Ti G5 shall meet the following: Al: 5.5–6.5 and V: 3.5–4.5.

c The bone modulus depending on the type of the bone and the direction of measurement.

The ASTM F136 Standard specifies the requirements of Ti G5 (Ti–6Al–4V) for biomedical applications. This alloy has good mechanical properties, but exhibits a possible toxic effect from released vanadium and aluminum . For this reason, vanadium and aluminum free Ti alloys have been proposed for biomaterials applications. All materials listed in Table 1 are used in dental implants. The disadvantage of Ti grade s 1–4 (cp Ti) for dental implants include higher Young modulus, relatively low mechanical strength, poor wear resistance and 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). Although the interstitial solutes increase the strength of Ti, they are deleterious to toughness. When high toughness is desired, the unalloyed Ti is produced with extra-low interstitial (ELI). ELI titanium alloy containing small amounts of oxygen, carbon, nitrogen and hydrogen as interstitial solutes ( Table 1 ). Pure Ti can be cold-rolled at room temperature until 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 . HCP metals including cp Ti have three independent slip systems, which is 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. analysed the effect of deformation twinning on the strain hardening behavior of cp Ti. The strain hardening rate of titanium can be divided into three stages. In the first stage, the strain hardening rate decreases as the strain increases due to from easy glide. Following the first stage, however, a sudden increase in the strain hardening rate is observed in the second stage. The second stage results from the generation of deformation twinning. The strain hardening rate decreases again as the strain increases in the third stage due the dynamic recovery .

Since there is a direct relation between interstitial content and mechanical strength, Ti G1 has the lowest mechanical strength, while Ti G4 has the highest strength ( Table 1 ). Table 1 shows that the tensile strength and elastic modulus of cp Ti are significantly lower than those of Ti G5; however it is still high when compared to bone (10–30 GPa) and may be about 3–6 times higher than those of cortical bone. Finite element simulations show that materials with lower elastic moduli have better stress distribution at the implant–bone interface and lead to less bone atrophy. A high difference between the moduli of elasticity of the implant material and bone can induce stress shielding, i.e. insufficient transfer of stress to the bone due the high modulus of the prosthesis .

Although the mechanical strength of implants is important, they must also present adequate stiffness to avoid shielding the bones from stress. To understand the stress shielding phenomenon, it is necessary to understand 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; when we do not exercise, the muscle is gradually lost. Stress shielding is a process that occurs when the forces exerted on a member with prosthesis are different from the forces applied to a normal limb. This difference induces the loss of bone density at the site (osteopenia), leading to bone atrophy. 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 medullary 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 endosteal surface of the medullary canal resulting in osteopenia of the proximal femoral diaphysis . This can potentially lead to bone loss in the long term and eventual loosening of the device, requiring an early revision surgery.

Although the G5 titanium alloy is stronger than cp Ti, it can release aluminum and vanadium ions. Some manufacturers use Ti G5 for dental implants, but the implants must have a surface treatment in order to improve the corrosion and reduce ion release. Considering that cp Ti is still chosen for demands where corrosion resistance is a priority (e.g. dental implants), and toxic effects of the dissolution of aluminum and vanadium due to corrosion wear of TiG5 are reason for concern, a modified cp Ti grade 4 alloy is proposed. This modified alloy hardened by cold working (Ti G4 Hard) was developed in an effort to merge the excellent mechanical strength of Ti G5 with the corrosion resistance of cp Ti G4.

The purpose of the present work is to compare the mechanical properties and the surface morphology of dental implant and discs samples after acid etching of modified Ti G4 (Ti G4 Hard) with those of Ti G2, Ti G4 and Ti G5. The surface morphology and mechanical properties of experimental and available commercial dental implants were analyzed. The surface morphology characteristics were quantified by roughness measurements.

Materials and methods

In the present work the following features of the material were investigated: surface morphology, surface roughness, tensile strength, compression strength, hardness, plastic deformation under torsion loading and clinical performance. All tests were single-blinded since the samples were not labeled.

Commercial screw-shaped dental implants made with cp Ti G4 and modified Ti G4 (Ti Hard) were used. The main is to compare the performance between Ti G4 and Ti G4 Hard for dental implant application with the same surface treatment. Ti G5 was not used for dental implant without surface treatment and the acid etching used for Ti G4 does not improve Ti G5 biocompatibility.

Standardized samples for tensile testing made with cp Ti ASTM G2, G4, G5 and G4 Hard were used. Discs made with cp Ti G4, G5 and G4 Hard were used for roughness measurements. Although dental implants made with cp Ti G5 are relatively scarce in the market, discs made with G5 were submitted to acid etching in order to study the surface morphology.

Discs and implants made with cp Ti were submitted to the following surface treatment: (a) acid etching (HCl and H 2 SO 4 solution) with the same concentration, temperature and time interval used for the Porous ® dental implants (Conexão Sistemas de Prótese, Brazil) and (b) anodizing with the same procedure used for the Actives ® implants (Conexão Sistemas de Prótese, Brazil).

Surface and microstructural analysis

The surface morphology (two implants and two discs from each group) was observed on a scanning electron microscope Field Emission Gun FEI QUANTA FEG 250 (FEI Corporate, Hillsboro, OR, USA) with energy dispersive spectroscopy (EDS) for qualitative chemical analysis.

The cross sections for microstructural analysis were investigated by transmission electron microscopy (TEM). The foils were prepared by grinding (400–3200) to a thickness of 0.1 mm, and electropolished with an electrolyte consisting of 6% percloric acid, 35% butanol and 59% methanol by volume . Analytical investigations of Ti G4 Hard and Ti G4 specimens were executed the microstructures of the two materials were compared.

Roughness measurement

The surface roughness was measured in discs after the same dental implant surface treatments. Three discs from each group were used; the roughness parameter was determined in two directions in each sample ( n = 10). The roughness parameters were measured two-dimensionally with a non-contact method using a contact profilometer Mitutoyo model SURFTEST SJ-400. The parameters for numerically characterizing the roughness were: arithmetic mean of the absolute values of roughness ( R a ), peak-to-valley roughness ( R z ) and the root square value of average roughness ( R q ).

Hardness Vickers and tensile test

Standard microindentation Vickers hardness tests were performed in discs 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 discs bases with polished surfaces. The disk surface was not be etched before indentation.

Tensile tests of Ti G2, G4, G5 and modified G4 Hard 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 4.5 mm diameter and 50.0 mm gage length were used.

Compression tests

The dental implants were submitted to compression tests according to international standard ISO 14801 (14801: Dentistry-Fatigue test for endosseous dental implants). A multi-part endosseous dental implant was tested as assembled according to its use. A multi-part device was assembled by means of screw joints and was tightened to the manufacturer’s recommended torque (32 N cm) using a device that provides torque within ±5% of the recommended value. The tightening sequence was that recommended by the manufacturer. Fig. 1 shows the test setup for the loading force applied by the testing machine during compression testing and a deformed sample after compression testing. Compression testing was carried out with a unidirectional load according to ISO 14801.

Fig. 1
Set up for dental implant compression testing (A) and a sample after compression testing (B).

Torque with key implant insertion

The implants were submitted to simulated clinical insertion torques from 45 to 120 N cm using a stainless steel key insertion. Fig. 2 shows the setup used for the torsion test. The maximum torque level used in the present tests is not recommended for surgical insertion but was done to evaluate the strength of the implant under extreme loading.

Fig. 2
Setup for the torque test. (A) Set up and (B) set up detail.

In order to evaluate possible plastic deformations, the implants were observed in an electron microscope (Quanta FEG 250 (FEI, Germany)) before and after torque application.

Clinical testing

In order to evaluate the performance of Ti G4 Hard dental implants a pilot report of the clinical was done. Dental implants made of Ti G4 Hard were inserted into patients and the surgeon did not receive any information about the type of implant. Results of dental prostheses on the implants were not installed because was not the objective of the present work.

Statistical analysis

The data were subjected to a one way analysis of variance (ANOVA) and Tukey’s HSD test. In order to determine any statistical significance among the values, the data were subjected to an independent sample test. The significance was determined at 95% confidence level.

Materials and methods

In the present work the following features of the material were investigated: surface morphology, surface roughness, tensile strength, compression strength, hardness, plastic deformation under torsion loading and clinical performance. All tests were single-blinded since the samples were not labeled.

Commercial screw-shaped dental implants made with cp Ti G4 and modified Ti G4 (Ti Hard) were used. The main is to compare the performance between Ti G4 and Ti G4 Hard for dental implant application with the same surface treatment. Ti G5 was not used for dental implant without surface treatment and the acid etching used for Ti G4 does not improve Ti G5 biocompatibility.

Standardized samples for tensile testing made with cp Ti ASTM G2, G4, G5 and G4 Hard were used. Discs made with cp Ti G4, G5 and G4 Hard were used for roughness measurements. Although dental implants made with cp Ti G5 are relatively scarce in the market, discs made with G5 were submitted to acid etching in order to study the surface morphology.

Discs and implants made with cp Ti were submitted to the following surface treatment: (a) acid etching (HCl and H 2 SO 4 solution) with the same concentration, temperature and time interval used for the Porous ® dental implants (Conexão Sistemas de Prótese, Brazil) and (b) anodizing with the same procedure used for the Actives ® implants (Conexão Sistemas de Prótese, Brazil).

Surface and microstructural analysis

The surface morphology (two implants and two discs from each group) was observed on a scanning electron microscope Field Emission Gun FEI QUANTA FEG 250 (FEI Corporate, Hillsboro, OR, USA) with energy dispersive spectroscopy (EDS) for qualitative chemical analysis.

The cross sections for microstructural analysis were investigated by transmission electron microscopy (TEM). The foils were prepared by grinding (400–3200) to a thickness of 0.1 mm, and electropolished with an electrolyte consisting of 6% percloric acid, 35% butanol and 59% methanol by volume . Analytical investigations of Ti G4 Hard and Ti G4 specimens were executed the microstructures of the two materials were compared.

Roughness measurement

The surface roughness was measured in discs after the same dental implant surface treatments. Three discs from each group were used; the roughness parameter was determined in two directions in each sample ( n = 10). The roughness parameters were measured two-dimensionally with a non-contact method using a contact profilometer Mitutoyo model SURFTEST SJ-400. The parameters for numerically characterizing the roughness were: arithmetic mean of the absolute values of roughness ( R a ), peak-to-valley roughness ( R z ) and the root square value of average roughness ( R q ).

Hardness Vickers and tensile test

Standard microindentation Vickers hardness tests were performed in discs 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 discs bases with polished surfaces. The disk surface was not be etched before indentation.

Tensile tests of Ti G2, G4, G5 and modified G4 Hard 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 4.5 mm diameter and 50.0 mm gage length were used.

Compression tests

The dental implants were submitted to compression tests according to international standard ISO 14801 (14801: Dentistry-Fatigue test for endosseous dental implants). A multi-part endosseous dental implant was tested as assembled according to its use. A multi-part device was assembled by means of screw joints and was tightened to the manufacturer’s recommended torque (32 N cm) using a device that provides torque within ±5% of the recommended value. The tightening sequence was that recommended by the manufacturer. Fig. 1 shows the test setup for the loading force applied by the testing machine during compression testing and a deformed sample after compression testing. Compression testing was carried out with a unidirectional load according to ISO 14801.

Fig. 1
Set up for dental implant compression testing (A) and a sample after compression testing (B).

Torque with key implant insertion

The implants were submitted to simulated clinical insertion torques from 45 to 120 N cm using a stainless steel key insertion. Fig. 2 shows the setup used for the torsion test. The maximum torque level used in the present tests is not recommended for surgical insertion but was done to evaluate the strength of the implant under extreme loading.

Fig. 2
Setup for the torque test. (A) Set up and (B) set up detail.

In order to evaluate possible plastic deformations, the implants were observed in an electron microscope (Quanta FEG 250 (FEI, Germany)) before and after torque application.

Clinical testing

In order to evaluate the performance of Ti G4 Hard dental implants a pilot report of the clinical was done. Dental implants made of Ti G4 Hard were inserted into patients and the surgeon did not receive any information about the type of implant. Results of dental prostheses on the implants were not installed because was not the objective of the present work.

Statistical analysis

The data were subjected to a one way analysis of variance (ANOVA) and Tukey’s HSD test. In order to determine any statistical significance among the values, the data were subjected to an independent sample test. The significance was determined at 95% confidence level.

Results

Fig. 3 shows the surface morphology of Ti discs after acid treatments. Fig. 4 shows the surface morphology of etched and anodized Ti dental implants. No morphological significant differences were observed among the disks and dental implants with the same Ti alloy and surface treatment. This is important because it is thus possible to assume that the roughness parameters measured in the discs are equal to those of implants.

Fig. 3
Surface morphology of Ti discs after etching. (A) Ti alloy G5, (B) cp Ti G4 and (C) cp Ti G4 Hard.

Fig. 4
Surface morphology of dental implants. (A) Acid etching cp Ti G4; (B) the same implant at high magnification; (C) acid etching Ti G4 Hard; (D) the same implant at high magnification; (E) anodized Ti G4; (F) the same implant at high magnification.

Fig. 3 and Table 2 revealed characteristic differences among Ti discs surfaces after acid etching. The Ti G5 has less roughness surface morphology than Ti G4 and Ti G4 Hard, which means that the used acid treatment is inadequate for Ti G5 for dental implant application.

Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Mechanical properties, surface morphology and stability of a modified commercially pure high strength titanium alloy for dental implants
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