|Year : 2021 | Volume
| Issue : 3 | Page : 141-145
Tantalum: A transmogrifying material in dental implant
Meenakshi Akshaya Lingam, Ishwarya Balasubramanian
Department of Prosthodontics, Tamil Nadu Government Dental College and Hospital, Chennai, Tamil Nadu, India
|Date of Submission||22-Oct-2020|
|Date of Acceptance||21-Jun-2021|
|Date of Web Publication||17-Sep-2021|
Dr. Meenakshi Akshaya Lingam
Tamil Nadu Government Dental College and Hospital, Muthusamy Salai, Chennai - 600 003, Tamil Nadu
Prosthodontic rehabilitation with implants aims at enhancing patient's quality of life and prolongation of survival of prosthesis. Porous tantalum trabecular metal (PTTM ) – incorporated titanium (Ti) is used extensively in orthopedics, but clinical researches and reports in the applications of oral cavity are limited. Therefore, prospective clinical trials of PTTM-incorporated Ti implants are essential for future use in dentistry. In this article, corrosion resistance and biocompatibility of tantalum is discussed elaborately and the usage of PTTM along with Ti in several studies is reported.
Keywords: Implant biomaterial, porous tantalum trabecular metal, solid tantalum, titanium
|How to cite this article:|
Lingam MA, Balasubramanian I. Tantalum: A transmogrifying material in dental implant. SRM J Res Dent Sci 2021;12:141-5
| Introduction|| |
The prosthetic rehabilitation of edentulous individuals in the contemporary world is focused on dental implants. Due to excellent biocompatibility and good mechanical and physical properties of titanium (Ti), it is used as a gold standard biomaterial in the field of oral implantology. However, day-to-day researches focus on different biomaterials to find an alternative that shows characteristics equal to or greater than Ti. One such biomaterial is tantalum (Ta), especially porous Ta rather than solid Ta in the field of orthopedics. Therefore, many studies and clinical trials are in progress to be used as dental implants. This article reviews about properties of Ta and clinical situations where it is used as dental implant biomaterial.
| Chemistry of Tantalum|| |
Ta is a lustrous transition metal which is highly corrosion resistant in nature. When Ta was immersed in acids, early chemists detected the “tantalizing property” of Ta. In 1802, Ta was discovered by the Swedish chemist, Anders Gustaf Ekebereg. Columbium – an oxide form of Ta, comprising columbite and tantalite and Ta, was usually found in this form in the early years of discovery. With greater corrosion resistance, Ta acquires higher attention toward the use as a new biomedical implant. A study by Starikov et al. revealed that the thickness of Ta pentoxide about 0.13–0.25 μm is essential for inhibition of corrosion process. Furthermore, they found that except in hydrofluoric acid and acids comprising fluoride and sulfur trioxide, Ta was highly unreactive or inert in almost all acids and this inertness behavior makes it the most suitable material for orthopedic implants.
| Corrosion Resistance of Tantalum|| |
Ta is considered to have excellent corrosion resistance similar to Ti, various mechanisms lie behind this property of anticorrosion. The factors include:
- Surface passivation oxide film
- Electrochemical corrosion behavior
- Host tissue reaction.
Surface passivation oxide film
The oxide film formed on the surface of the metal acts as a passivating layer, thereby preventing the leakage of metal ions from the metal substrate which further prevents the unfavorable reaction to host cells. The composition of surface passivation film includes air: Ta2O5, thickness: 3.0 mm; simulated body fluid (SBF): Ta oxide and hydroxide; and revised-SBF (R-SBF): mainly Ta2O5.
Ta exists as two forms of oxide Ta pentoxide +5 (Ta2O5) and +4 (TaO2). The stability of the +5 valence state is found to be related to its crystal structure and octahedral shape. A study by Pérez-Walton et al. revealed that metastable structure of Ta2O5 was stabilized by impurities and defects in the crystal structure. A study by Mei et al. has shown that the addition of Ta2O5 to various materials forms the protective coating over the material and improved the physical and biological properties of that particular material used. For instance, properties such as surface roughness, surface energy, hydrophilicity, and protein absorption are enhanced by incorporating Ta2O5 into polyether ether ketone. Since the carbon scaffold making internal skeleton can be altered, unlike solid Ta, several designs of porous Ta trabecular metal can be constructed. In short, Ta possesses strong and stable passivating oxide film which contributes to its corrosion resistance property.
Electrochemical corrosion behavior
The values of indexes such as open-circuit potential (OCP), potentiodynamic polarization test, and electrochemical impedance spectroscopy (EIS) denote the electrochemical corrosion behavior of implant biomaterial which, in turn, contributes to corrosion resistance of the implanting material. In 2015, Li et al. studied electrochemical corrosion behavior of pure Ta, pure Nb, Nb-60Ta-2Zr in the R-SBF), which revealed that EIS index values of pure Ta were greater than Nb-60Ta-2Zr and pure Nb. Pure Ta has a greater polarization resistance and a stronger passivating oxide film. The OCP values of pure Ta moved to positive side which indicates it has a greater electrochemical corrosion behavior. In a study of corrosion behavior of various metals, by Kim and Johnson in 1999, it was shown that even though the potential for pitting corrosion for pure Ta reaches higher values, there was no local corrosion which, in turn, reveals that high resistance of pure Ta impedes pitting corrosion. In essence, pure Ta has a greater electrochemical corrosion behavior, which suggests pure Ta has greater corrosion resistance.
Host tissue reaction
Once the material is implanted, metal ions are released into the environment. When the quantity of metal ions exceeds the threshold limit of surrounding host tissue, the following reaction occurs:
- Proinflammatory reactions triggered
- Intracellular signaling pathways activated
- Release of cytokines
- Reactive oxygen species (ROS)
- Oxidative stress.
A study by Sevost'yanov et al. revealed that reduction in the formation of ROS is noticed if the surface layer is rich in Ti or Ta when compared with nitinol. In 2017, Kang et al. studied about effects of Ta nanoparticles (Ta–NPs) on mouse osteoblasts. Their study revealed that from the surface of porous Ta which has deposited Ta–NPs, during loading shows a spontaneous release of Ta–NPs and they approach the osteoblasts surrounding the implant. They concluded that Ta–NPs enhance the osteoblast proliferation and promote autophagy which, in turn, showed enhanced cell proliferation within autophagy threshold of Ta–NPs which ranges from 10 to 20 μg\ml. In brief, host tissue reaction is less when Ta is implanted as dental implant biomaterial.
| Biocompatibility of Tantalum|| |
Ta exhibits excellent biocompatibility similar to Ti and recent studies conducted various experiments to validate this essential property to be used as dental implant biomaterial. Those experiments include the following:
- Cytological experiment
- Molecular biology experiment
- Protein adsorption experiment
- Hematology experiment.
Experiment involves exposing the cells to the Ta metal substrate and evaluating the biocompatibility by assessing the degree of cytotoxicity. Factors that help in assessing include in vitro effects of metal substrate on cell diffusion, cell growth, and proliferation. Commonly used cell types are bone marrow mesenchymal stem cells, osteoblasts, fibroblasts, normal tissue cells, and blood tissue cells, etc.; Ta exhibits excellent cellular compatibility though it is bio inert in nature. A study by Qiao et al. has shown that after 5 days of cell culture, surface coating on the nitinol alloy containing Ti and Ta had the ability to form a fused monolayer of cells thereby demonstrating environment created by these metals facilitate cellular diffusion. In 2013, an in vitro study by Hofstetter et al. concluded that surface coating of Ta facilitated cell proliferation and osseointegration more efficiently than Ti and zirconium surfaces.
Molecular biology experiment
In 2013, both in vitro and in vivo studies conducted by Tang et al. suggested that when Ta coatings over the Ti substrate were used, cellular (human bone marrow mesenchymal stem cells) adhesion, proliferation, and osteogenic differentiation were improved when compared to Ti used without Ta coatings. In 2018, in a study on intrinsic surface effects of Ta and Ti on integrin-mediated osteogenic differentiation in bone marrow stem cells, they concluded that Ta possessed greater potential than Ti in activating integrin signaling cascade which takes part a significant role in regulating osteoblastic differentiation. Therefore, this study indicates that Ta exhibits superior osteoinductive performance than Ti.
Protein adsorption experiment
Metallic implants are open to complicated biological environment that usually comprises various high molecular compounds, chiefly proteins. The effect of protein adsorption on biocompatibility is massive because depending on the type, properties, and concentration of protein, they can involve in inhibition or promotion of metal degradation, thereby indicating the level of biocompatibility of the used metal as implant. A study by An et al. revealed that if nano-lamellar Ta surfaces are used rather than bulk Ta, it creates a favorable environment for adsorption of proteins which attributes to greater cell surface interaction that would thus enhance osteoblast adhesion.
Since metallic implant comes in contact with blood, it is essential for evaluation for blood compatibility. Factors that help in assessing the hemocompatibility of biomaterials used are hemolysis test, clotting time, and platelet adhesion and activation.
In vitro and in vivo experiments conducted by Chen et al. in 2002 revealed that if Ta covers the surface of other metals, it enhanced the hemocompatibility of metals. They also added that Ta added on Ti forms Ti (Ta +5) O2 which is antithrombogenic in nature and they possessed attractive hemocompatibility.
| Porous Tantalum Trabecular Metal|| |
Even though Ta is advantageous due to their properties such as inertness, greater biocompatibility, and corrosion resistant, it was complicated in handling solid Ta due to its increased stiffness and density. In addition to these two factors, it has a very high melting point which makes its casting procedure very difficult, thus led to the concept of production of porous Ta trabecular metal (PTTM). Porous Ta possesses various properties which include high frictional characteristics, greater volumetric porosity, and low modulus of elasticity.
The structure of PTTM comprises repeating three-dimensional dodecahedron units which resembles the structure of trabecular bone. The construction of PTTM is as follows:
- Foam-like vitreous carbon forming overall scaffold initially but later on turns into skeletal framework of the entire porous metal biomaterial into which Ta is incorporated usually extracted from recycling procedure rather than from nature
- In an air-sealed chamber, scaffold made of vitreous carbon is placed
- Using gases like hydrogen and chlorine, by a process of vapor diffusion Ta is coated into the carbon scaffold. Ta metal is evaporated as TaCl2 and then Ta is deposited into carbon scaffold.
Several designs of porous Ta trabecular metal can be constructed because the carbon scaffold making internal skeleton can be modified. A study by Levine et al. revealed that when porous Ta is used as an orthopedic implant, they reported various superior properties such as greater biocompatibility, osteoconductivity, bone ingrowth and outgrowth, and allowed neovascularization.
| Titanium Implants with Porous Tantalum Trabecular Metal|| |
Ti implants have been considered as gold standard material for dental implant for many decades because of their greater biocompatibility and osseointegration. But in order to enhance their surface properties, various modifications are attempted. The point of interest in modifying surface properties is to enhance the interaction between the implant surface and the host tissue which is influenced by factors such as adsorption of proteins, cellular adhesion, proliferation and differentiation, and hemocompatibilty. The above-mentioned factors are very well exhibited by PTTM rather than solid Ta. Hence, many attempts were made to coat the surface of the Ti to enhance their surface properties. A study by Zardiackas et al. suggested that the structure of PTTM-incorporated Ti implants is formed by the following procedure:
- In the middle part of the multithreaded endosseous Ti implant, PTTM is incorporated
- In the apical and cervical parts of the Ti implants with PTTM, surface roughness is created by blasting with hydroxyapatite particles and screw-type pattern of an implant is retained without incorporating PTTM.
The manufacture of Ti alloy and PTTM was done separately. In addition, apical section of the implant was milled separately, whereas the cervical and middle sections were milled as single pieces. The PTTM sleeve is about 2 mm and it is cylindrical and comprises 2% of vitreous carbon and 98% Ta surface coating. Finally, the sleeve is attached with middle section of Ti alloy and laser welded with an apical section of Ti alloy core material.
| Merits of Porous Tantalum Trabecular Metal|| |
- Rapid endothelial neovascularization at the implant interface is essential for the arrival of osteoblastic precursors which, in turn, causes osteoblastic differentiation, growth and matrix secretion, and subsequent bone ingrowth. This is achieved by repeating units of open-cell dodecahedron in PTTM
- Since the oxide layer of Ta is highly inert in nature, it exhibits greater biocompatibility
- Ta neither possesses cell toxicity nor inhibits local cell growth
- Since the implant interface surface area is increased by a trabecular component of PTTM, it not only enhances osseointegration it also simulates natural bone
- In addition to simulating natural osseous structure, since it possesses elastic modulus same as natural bone, it is considered as superior to other alloys mechanically
- A Study by Harrison et al. suggested that using PTTM as an implant biomaterial in knee, adjacent tibia resorption is prevented indicating that since the stress is distributed evenly there are less chances for bone resorption in the surrounding area.
| Clinical Scenarios|| |
Ti is considered as a standard implant biomaterial and so many efforts have been taken to enhance its properties. One such step is PTTM-incorporated Ti implants, which are used often as orthopedic implants. There are only limited researches that reported the usage of PTTM as dental implant biomaterial.
In the in vivo study conducted by El Chaar and Castaño in 2017 using a flapless technique, it was concluded that immediate PTTM implant placement attained success rates >95% in comparison with conventional delayed loading protocols. A retrospective analysis conducted by Edelmann et al. in 2018 concluded that PTTM implants have less peri-implant bone loss in comparison with conventional Ti implants. In 2017,an experiment conducted by Lee et al., compared PTTM and conventional Ti implants in terms of microarchitecture and bone formation in the implant site and concluded that PTTM has shown faster and stronger secondary implant stability than Ti implants.
| Limitations of Porous Tantalum Trabecular Metal|| |
In 2016, Ma et al. conducted a study in a population of osteonecrosis of femoral head, where they are treated with porous Ta implants. In this study, they observed these implants cannot provide adequate mechanical support for the subchondral bone and limited bone ingrowth into the necrotic area. This study suggested that the use of PTTM is viable only for normal bone and cannot be used in necrotic bone.
| Conclusion|| |
Compared to current techniques enhancing surface properties of Ti as dental implant biomaterial, conceptual knowledge about PTTM-incorporated Ti implant in enhancing osseointegration derived from their usage as orthopedic implants would be major revolutionary approaches in future implant dentistry. Since there are limited studies reported on PTTM, more number of clinical trials and researches on its effects on oral cavity would be essential for efficient use of PTTM as dental implant biomaterial.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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