rus
Issue #5/2020
V. P. Veiko, Yu. Yu. Karlagina, V. V. Romanov, R. M. Yatsuk, E. E. Egorova, E. A. Zernitskaya, A. I. Yaremenko, G. N. Chernenko, S. G. Gorny, G. V. Odintsova
Laser Technology for Structuring the Surface of Dental Titanium Implants. Part 1
Laser Technology for Structuring the Surface of Dental Titanium Implants. Part 1
DOI: 10.22184/1993-7296.FRos.2020.14.5.462.472
We have developed a method for the laser formation of a biocompatible surface morphology of titanium dental implants, providing a hydrophilic surface structure that has both micro- and nanorelief. We present this work in two parts. In this first part, we substantiate the physical and functional properties of biocompatible implant surface. Using laser structuring on the surface of titanium dental implants, superhydrophilic reliefs of micro- and nanoscale were formed. The period of structures in the form of holes was 50 μm, and 30 μm in the form of grooves. The results of a study of the physical and chemical properties of biocompatible surface morphology are presented.
We have developed a method for the laser formation of a biocompatible surface morphology of titanium dental implants, providing a hydrophilic surface structure that has both micro- and nanorelief. We present this work in two parts. In this first part, we substantiate the physical and functional properties of biocompatible implant surface. Using laser structuring on the surface of titanium dental implants, superhydrophilic reliefs of micro- and nanoscale were formed. The period of structures in the form of holes was 50 μm, and 30 μm in the form of grooves. The results of a study of the physical and chemical properties of biocompatible surface morphology are presented.
Теги: dental implants implant success in vitro in vivo laser surface treatment preclinical studies wall-to-wall production дентальные имплантаты доклинические исследования лазерная обработка поверхности полный цикл производства приживаемость
Laser Technology
for Structuring the Surface of Dental Titanium Implants
Part 1
V. P. Veiko1, Yu. Yu. Karlagina1, V. V. Romanov1, R. M. Yatsuk1, E. E. Egorova1, E. A. Zernitskaya2, A. I. Yaremenko2, G. N. Chernenko3, S. G. Gorny4, G. V. Odintsova1
ITMO University, St. Petersburg, Russia.
First Saint Petersburg State Medical University n. a. I. P. Pavlov, Saint Petersburg, Russia
St. Petersburg Dental Milling Center and Plant for the Production of Prosthetic Components “Lenmiriot”, St. Petersburg, Russia
Laser Center LLC, St. Petersburg, Russia
We have developed a method for the laser formation of a biocompatible surface morphology of titanium dental implants, providing a hydrophilic surface structure that has both micro- and nanorelief. We present this work in two parts. In this first part, we substantiate the physical and functional properties of biocompatible implant surface. Using laser structuring on the surface of titanium dental implants, superhydrophilic reliefs of micro- and nanoscale were formed. The period of structures in the form of holes was 50 μm, and 30 μm in the form of grooves. The results of a study of the physical and chemical properties of biocompatible surface morphology are presented.
Key words: dental implants, implant success, laser surface treatment, preclinical studies, in vitro, in vivo, wall-to-wall production
Received on: 04.06.2020
Accepted on: 24.06.2020
INTRODUCTION
Is dental implantation successful today
In modern dentistry, one of the most popular, versatile and effective methods of treatment is dental implantation. It allows you to solve not only the functional problems of the oral cavity, but also to return the original or give a more aesthetic appearance to patients. According to the marketing report [1], as of 2015, the market volume of dental implants in Russia amounted to about 540 thousand pieces, and the expected market size by 2020 will amount to 0.8–1 million pieces. However, not all of the installed implants take root. According to statistics, 95% [2] of the installed implants successfully function in the body for the entire service life – from 10 to 15 years, with strict adherence to all the doctor’s recommendations. On the one hand, the implant success is quite high, but this also means that five out of a hundred patients will have to re-operate with the installation of a new implant or, even worse, face unforeseen complications such as infection, bone loss, etc. accompanying unsuccessful implantation. The main factors determining the success of the functioning of the implant are: the quality of the implantation performed, oral hygiene during the operation of the implant, as well as the characteristics of the implant itself (material, shape, structure of its surface). If the first two factors are determined by the qualifications of the attending physician and the patient’s responsible approach, then it is up to the implant manufacturers to improve the characteristics of its surface. Thus, today there is a global goal – to increase the degree of biocompatibility of titanium dental implants and their service life.
The aim of this work is to develop a technology for laser formation of the surface morphology of titanium dental implants.
What implant surface should be like
The success of the integration of the implant into the patient’s body largely depends on the design of the implant and the structure of its surface that will directly contact the bone. As a material for dental implants, titanium alloys are most often used due to their high strength and corrosion characteristics, as well as the hypoallergenicity of titanium itself [3]. Moreover, titanium is a reactive material and spontaneously forms a stable dense oxide film on its surface, which increases the biocompatibility of the implant [4]. As for the relief of the implant surface, scientists have unequivocally established that structured implants have better adhesion to the bone tissue (due to an increase in the contacting surface area) than polished ones [5]. The rough surface of the implant activates the growth of bone tissue into it, the microrelief facilitates the adhesion of bone tissue cells to the surface, and the nanorelief promotes adhesion of proteins to the surface [6]. It is also important that for the successful interaction of an implant with cellular elements and biological fluids, especially at the early stages of osseointegration, its surface must be hydrophilic [7].
As an implant material, we decided on the titanium alloy Ti – 6A1–4V, which is widely used in the production of dental implants. To ensure the biocompatibility of the implant, the task was set to obtain a hydrophilic surface structure with a hierarchical micro- and nanorelief.
What methods are used to create a biocompatible coating
Today, a popular method of structuring implants is sandblasting [8], the essence of which is to create a disordered rough relief by bombarding the surface with a powder jet directed under pressure. For these purposes, powders of hydroxyapatite, aluminum oxide, etc. are usually used. In most cases, sandblasting is followed by acid etching of the treated surface to remove powder residues [9]. Thus, this structuring method does not exclude residual contamination on the surface of the treated implant.
Also, quite recently, the Swiss company Nobel BiocareTM introduced on the market a new design of the implant surface, which improves its osseointegration by forming zones with different morphologies: an abutment with nanoporous (nanostructure size of 69 ± 48 nm) smooth (roughness Sa = 0.13 ± 0.02 μm; Sdr = 1.7 ± 1%) with an oxide coating (the thickness of the oxide layer is 153 ± 5 nm); a neck with surface characteristics similar to the abutment (Sa 0.49 ± 0.03 μm, Sdr 2.1 ± 1.0%, nanostructure size 43 ± 21 nm, oxide layer thickness 142 ± 17 nm); transition zone with roughness varying to the apex of the implant (from Sa = 0.92 ± 0.16 μm and Sdr = 107.2 ± 31.5% to Sa = 1.49 ± 0.19 μm and Sdr = 172.7 ± 18, 0%) and the thickness of the oxide layer of the transition zone is 7.2 ± 0.3 μm and the vertices are 9.9 ± 1.3, the size of the micropores of the transition zone: 1.1 ± 0.5 μm, the vertices: 1.7 ± 1, 1 μm [10].
As the authors note, the oxide layer on the surface of the implant neck and abutment provides additional bactericidal properties to the implant during the period of its survival in the body.
Regulation of the roughness value, pore size, nanostructures, as well as the chemical composition of the implant surface is carried out by fine-tuning the anodizing modes, selecting the current value and a suitable electrolyte. The listed values of structural elements and roughness values are taken from the source [10]. It should be said that often, during the anodizing process, strong acids are used, for example, H2SO4, H3PO4, HF, HNO3, which can remain in the pores on the surface even after sterilization of the implant [11], which negatively affects biocompatibility. In work [12], it is noted that phosphorus content is observed on an anodized titanium implant from Nobel BiocareTM. It can be assumed that this fact indicates the use of phosphorus-containing acid as an electrolyte in this technology.
Among the modern manufacturers of dental implants, the Korean company CSM should be noted, which introduced implants with a surface modified by laser treatment to the market. As a result of exposure to solid-state Nd: YAG laser radiation, in one technological stage, without the use of chemical reagents, an ordered microrelief in the form of holes and grooves is formed on the surface of the implant, due to which the implants demonstrate excellent osseointegration ability and functional stability [13]. It is also known that the structures induced by laser action in air have good wear resistance due to the content of titanium oxynitrides [14].
We also believe that laser-based methods are the most promising for creating a biocompatible implant surface. The indisputable advantage of laser treatment is that the surface relief is formed due to the evaporation of the material itself, without the use of third-party materials for treatment, such as corundum particles Al2O3, and chemical reagents, for example, HCl and H2SO4 acids, which reduces the risk of implant rejection due to residual pollution. In addition, laser treatment opens up great opportunities for obtaining complex multilevel surface morphology with a given chemical composition.
As a tool for treatment, we chose the domestic laser complex MiniMarkerTM 2, widely used in industry [15], based on an ytterbium pulsed fiber laser. Metals absorb the radiation of this laser with a wavelength of 1.06 microns quite well. The working range of power densities I = [6.9–63] ∙ 107 W / cm2 makes it possible to reach the evaporation temperature of titanium, and the scanning system (galvanometric mirrors) together with a focusing system (F‑theta lens with a reverse focal length of 216.1 mm) provides the ability to form structures with complex morphology.
FORMATION OF A BIOCOMPATIBLE STRUCTURE ON THE IMPLANT SURFACE
So far, there is no univocal opinion in the scientific community about the most suitable surface for a titanium dental implant [16]. It is obvious that the rough surface relief is not the only criterion for optimal osseointegration. The question remains, which type of relief (ordered or disordered, consisting of holes, or grooves, or some other morphology) will be the most biocompatible.
The relief in the form of parallel microgrooves is of particular interest. In comparison with the disordered relief, formed, for example, by sandblasting, the micro-grooved surface influences the behavior of cells in such a way that the cells do not grow chaotically on the surface, but “line up” along the grooves of the grooves [17–19]. Important parameters for creating such a relief are the width, depth and period of the grooves [20]. The ratio of these values determines whether a given relief will have an impact, and which one, on the behavior of cells, in this case, neural stem cells. In the case when the width of the grooves is much less or much more than the size of the cells, and the depth of the grooves is less than 5 μm, other interaction mechanisms are activated. Then the behavior of cells largely depends on nanoscale structures, in particular, on their orientation in space. In addition, nanoscale structures affect the adhesion of proteins to the implant surface at the early stages of osseointegration [21, 22], which, in turn, determine the final formation of newly formed bone tissue.
Thus, we hypothesize that the optimal surface morphology for titanium implants will be a structure with elements of supracellular size (20–40 µm). Because it provides a certain mobility (motor activity) of cells and is equipped with a substructure of a smaller (nanometric) size, the function of which is to ensure the possibility of metabolism: air access and removal of soluble waste products (which may require hydrophobic channels). An example of such a structure on titanium is the structure formed during the deposition of ablation and oxidation products of titanium in air (Fig. 1) on the initial titanium surface.
Even if we accept the above hypothesis, the question still arises as to what type of relief will be optimal: extended grooves or individual dimples periodically located on the surface, or something else. In studies with neurons [23], it was found that different types of reliefs are capable of influencing cell behavior. Only an extended relief in the form of grooves promotes contact guidance of cells. Studies of the differentiation of mesenchymal stem cells (MSCs) into the osteogenic group, for example, into osteocytes, are not found in the literature data similar to this work [23].
In this study, we formed structures of two types: dimples (L‑structure) and grooves (K‑structure). The period, width and depth of the structures were from 20 to 40 μm. Both types of structures were formed with an X‑axis pulse overlap of about 95%.
An ordered L‑structure is a set of wells in which cells should be placed. The L‑structure was obtained using laser radiation with a power density of 6.9 x 107 W / cm2 in a two-pass treatment mode. The dimples are located evenly over the entire surface of the sample, and their diameter is about 40 μm. The period of the structure is 50 μm. Since this relief is formed when the surface is heated above the evaporation threshold, then during the first pass of laser radiation over the sample surface, the substance is carried out and the microrelief is instantly formed in the form of grooves. The second pass forms the same grooves, but in the orthogonal direction, in the same way with the removal of the substance. In those areas of the surface where the radiation pulse from the second pass hit the pulse from the first one, depressions in the form of dimples were formed. Between the dimples is the area that was modified during the first laser pass. Thus, a pronounced network structure is observed on the surface of the titanium alloy (Fig. 1c).
To form a grooved K‑structure, a multipass mode with a power density of 63 ∙ 107 W / cm2 was used. As mentioned above, cells of the same type can be of different sizes and shapes within the same range of values, which should be taken into account when modeling the relief structure. Based on this, the grooved structure was created in three passes of laser radiation with the formation of parallel grooves with a step of 30 μm from the beginning of the previous groove.
This mode of action was applied in order to obtain grooves of different widths. Since this type of structure was also obtained in modes above the evaporation threshold, after the first pass, a relief in the form of grooves appears on the sample surface. After the second pass, a new groove was formed, which partially overlapped the groove from the first pass. The groove formed after the third pass overlapped the groove formed by the previous one. Thus, structuring of the gaps between the grooves took place and the surface was completely filled with structures, without gaps between the grooves (Fig. 1b).
To study the modified surface, a scanning electron microscopy (SEM) analysis was carried out using a Zeiss Merlin microscope with additional Oxford Instruments INCAx-act attachments and an Oxford Instruments CHANNEL5 electron backscattered diffraction (EBSD) recording system for X‑ray microanalysis. In fig. Figure 1 shows SEM images of the initial (untreated) surface (P‑structure) of titanium implants (Fig.1a) and implants after laser treatment: K‑structure (Fig.1b) and L‑structure (Fig.1c). SEM images demonstrate the presence of reliefs of various scales on the titanium surface: micro- and nanoreliefs.
Energy dispersive analysis showed the presence of oxygen on structured surfaces (Table 1). This indicates the presence of titanium oxide on the surface of the implant. Specialists know that it also has good biocompatibility.
The contact angle of the surface of all structures was measured by illuminating the droplet with an LED illumination source with a total power of 1 W and a ToupCam high-resolution CCD camera. Distilled water was used as a test liquid; the droplet volume was 0.1 μL. The Digimizer software was used to determine the contact angle value [27]. Measurements were carried out on three samples of each structure. Photographs of deposited drops on surfaces are shown in Fig. 2. According to the results of measurements, the wetting angle of the titanium surface before laser treatment was 70° (Fig. 2a). It is not possible to measure the angle of wetting of surfaces after laser treatment, because the surface has changed from hydrophilic to superhydrophilic (i. e., a drop, hitting it, instantly spreads and permeates the structure).
The kinetics of changes in the maximum diameter dmax of the spreading droplet with time t is shown in the graph (Fig. 2 b). It should be noted that each structure corresponds to a different character of droplet spreading: the L‑structure corresponds to a symmetric oval spreading region (long axis = 1.6 ± 0.1 cm, short axis = 1.26 ± 0.1 cm), and K‑structure – asymmetrical elongated along the grooves of the grooves and large in area (long axis = 1.5 ± 0.1 cm, short axis = 3.9 ± 0.1 cm). As is known, wettability plays an important role in the adsorption of protein and cells on the surface, and superhydrophilic surfaces contribute to this [24].
Thus, with the help of laser structuring, we formed superhydrophilic reliefs on the surface of titanium dental implants, which simultaneously consist of micro- and nanostructures.
The effectiveness of biocompatibility of reliefs formed by laser exposure was revealed in the course of preclinical in vitro and in vivo tests. Their results will be presented in the second part of the work. The algorithm and technological stages of technology based on this method will be described. The technology has been introduced into the wall-to-wall production of dental milling center and a plant that is part of the ORTOS group of companies.
SEM studies of the surface of implants were carried out at the St. Petersburg State University at the Interdisciplinary Resource Center in the direction of “Nanotechnology” (St. Petersburg).
The authors of the work express their gratitude to the research team of the NRU “BelGU” (Belgorod) under the leadership of Yu. R. Kolobov for help in researching the physicochemical characteristics of laser-induced structures.
In vitro experiments and experimental protocols were approved by the Research Ethics Council of the Nizhny Novgorod State Medical Academy (Privolzhsky Research Medical University, Nizhny Novgorod) and comply with the principles of the Declaration of Helsinki.
The committee of the St. Petersburg State Medical University n. a. I. P. Pavlov carries out its activities in accordance with the Constitution of the Russian Federation, laws and other legal acts of the Russian Federation and St. Petersburg, the Declaration of Helsinki by the World Medical Association of 1964, amended in 1975, 1983, 1989, 1996, 2000 and 2013., ICH Harmonized Tripartite Guideline for Good Clinical Practice (ICH GCP), industry standard OST 42–511–99 “Rules for conducting high-quality clinical trials in the Russian Federation”, which came into force on January 1, 1999. Recommendations of the Ethics Committees conducting the examination of biomedical research by the WHO, the Charter of the St. Petersburg State Medical University n. a. I. P. Pavlov and the Regulations on the Ethics Committee of the St. Petersburg State Medical University n. a. I. P. Pavlov. The “In vivo study of the integration processes of titanium dental implants with a laser-modified surface” was approved (excerpt from Minutes No. 208 of the meeting of the Ethics Committee of the St. Petersburg State Medical University n. a. I. P. Pavlov dated June 25, 2018).
The authors of the work express their gratitude to the research team of the Federal State Budgetary Educational Institution of Higher Education “PRMU” of the Ministry of Health of Russia (Nizhny Novgorod), consisting of Daria Kuznetsova, Vadim Elagin and Elena Zagainova, for the study of cell biointegration on the laser-induced surface of VT6 titanium, and the staff of the Center for Shared Use of Scientific Equipment “Cellular and molecular technologies for studying plants and fungi” of the Botanical Institute n. a. V. L. Komarov of RAS (St. Petersburg), Zernitsky A. Yu. and Zotov P. A. for conducting histological and histomorphometric studies.
The study was supported by a grant from the Russian Science Foundation (project No. 20–62–46045).
Contribution of members
of the creative team to the project
All members of the team of authors took part in the project: setting the task and providing resources – G. N. Chernenko, S. G. Gorny; concept, study design and project management – V. P. Veiko, G. V. Odintsova; experiments on laser structuring of titanium surface – Yu. Yu. Karlagina, V. V. Romanov, R. M. Yatsuk; concept of in vitro and in vivo studies – A. I. Yaremenko; conducting and analyzing in vivo studies – E. A. Zernitskaya; analysis of the results of in vitro and in vivo studies – Yu. Yu. Karlagina, E. E. Egorova.
Conflict of interest
The authors declare no conflicts of interest.
ABOUT AUTHORS
V. P. Veiko (vadim.veiko@mail.ru), full professor, Doctor of Science (Technical), Head of the International Laboratory «Laser Micro-and Nanotechnologies», Faculty of Laser Photonics and Optoelectronics, ITMO University, Saint-Petersburg, Russia.
ORCID: 0000-0001-6071-3449
Yu. Yu. Karlagina (jujukarlagina@itmo.ru), engineer, International Laboratory «Laser Micro-and Nanotechnologies», postgrad. student, ITMO University, Saint-Petersburg, Russia.
ORCID: 0000-0002-6927-9551
V. V. Romanov (ionhcik@rambler.ru), engineer, postgrad. Student, faculty of laser photonics and optoelectronics, ITMO University, Saint-Petersburg, Russia.
ORCID: 0000-0003-1468-9438
R. M. Yatsuk (yatsuk.roman@mail.ru), engineer, ITMO University, Saint-Petersburg, Russia.
ORCID: 0000-0003-2502-7501
G. V. Odintsova (gvodintsova@itmo.ru), Cand. of Science (Technical), Research Associate, Laboratory «Laser Micro-and Nanotechnologies», Faculty of Laser Photonics and Optoelectronics, ITMO University, Saint-Petersburg, Russia.
ORCID: 0000-0001-9581-4290
E. E. Egorova (elena1998959@gmail.com), student, ITMO University, Saint-Petersburg, Russia.
ORCID: 0000-0002-1461-0673
Е. А. Zernitskaya (zernitskaya_ekaterina@mail.ru), postgrad. Student, Pavlov First Saint Petersburg State Medical University, Saint-Petersburg, Russia.
ORCID: 0000-0002-3819-693X
А. I. Yaremenko (ayaremenko@me.com), Doctor of Medical Sciences, Professor, Head of the Department of Surgical Dentistry and Oral and Maxillofacial Surgery, Director of the Clinic for Oral and Maxillofacial Surgery, Vice-Rector for Academic Affairs, Pavlov First Saint Petersburg State Medical University, Saint-Petersburg, Russia.
ORCID: 0000-0002-7700-7724
G. N. Chernenko (office@ortos.biz), Director General, Lenmiriot Dental Implant Prosthetics Manufacture, Saint-Petersburg, Russia.
S. G. Gorny (info@newlaser.ru), Cand. of Science (Eng), «Laser Center», Saint-Petersburg, Russia.
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for Structuring the Surface of Dental Titanium Implants
Part 1
V. P. Veiko1, Yu. Yu. Karlagina1, V. V. Romanov1, R. M. Yatsuk1, E. E. Egorova1, E. A. Zernitskaya2, A. I. Yaremenko2, G. N. Chernenko3, S. G. Gorny4, G. V. Odintsova1
ITMO University, St. Petersburg, Russia.
First Saint Petersburg State Medical University n. a. I. P. Pavlov, Saint Petersburg, Russia
St. Petersburg Dental Milling Center and Plant for the Production of Prosthetic Components “Lenmiriot”, St. Petersburg, Russia
Laser Center LLC, St. Petersburg, Russia
We have developed a method for the laser formation of a biocompatible surface morphology of titanium dental implants, providing a hydrophilic surface structure that has both micro- and nanorelief. We present this work in two parts. In this first part, we substantiate the physical and functional properties of biocompatible implant surface. Using laser structuring on the surface of titanium dental implants, superhydrophilic reliefs of micro- and nanoscale were formed. The period of structures in the form of holes was 50 μm, and 30 μm in the form of grooves. The results of a study of the physical and chemical properties of biocompatible surface morphology are presented.
Key words: dental implants, implant success, laser surface treatment, preclinical studies, in vitro, in vivo, wall-to-wall production
Received on: 04.06.2020
Accepted on: 24.06.2020
INTRODUCTION
Is dental implantation successful today
In modern dentistry, one of the most popular, versatile and effective methods of treatment is dental implantation. It allows you to solve not only the functional problems of the oral cavity, but also to return the original or give a more aesthetic appearance to patients. According to the marketing report [1], as of 2015, the market volume of dental implants in Russia amounted to about 540 thousand pieces, and the expected market size by 2020 will amount to 0.8–1 million pieces. However, not all of the installed implants take root. According to statistics, 95% [2] of the installed implants successfully function in the body for the entire service life – from 10 to 15 years, with strict adherence to all the doctor’s recommendations. On the one hand, the implant success is quite high, but this also means that five out of a hundred patients will have to re-operate with the installation of a new implant or, even worse, face unforeseen complications such as infection, bone loss, etc. accompanying unsuccessful implantation. The main factors determining the success of the functioning of the implant are: the quality of the implantation performed, oral hygiene during the operation of the implant, as well as the characteristics of the implant itself (material, shape, structure of its surface). If the first two factors are determined by the qualifications of the attending physician and the patient’s responsible approach, then it is up to the implant manufacturers to improve the characteristics of its surface. Thus, today there is a global goal – to increase the degree of biocompatibility of titanium dental implants and their service life.
The aim of this work is to develop a technology for laser formation of the surface morphology of titanium dental implants.
What implant surface should be like
The success of the integration of the implant into the patient’s body largely depends on the design of the implant and the structure of its surface that will directly contact the bone. As a material for dental implants, titanium alloys are most often used due to their high strength and corrosion characteristics, as well as the hypoallergenicity of titanium itself [3]. Moreover, titanium is a reactive material and spontaneously forms a stable dense oxide film on its surface, which increases the biocompatibility of the implant [4]. As for the relief of the implant surface, scientists have unequivocally established that structured implants have better adhesion to the bone tissue (due to an increase in the contacting surface area) than polished ones [5]. The rough surface of the implant activates the growth of bone tissue into it, the microrelief facilitates the adhesion of bone tissue cells to the surface, and the nanorelief promotes adhesion of proteins to the surface [6]. It is also important that for the successful interaction of an implant with cellular elements and biological fluids, especially at the early stages of osseointegration, its surface must be hydrophilic [7].
As an implant material, we decided on the titanium alloy Ti – 6A1–4V, which is widely used in the production of dental implants. To ensure the biocompatibility of the implant, the task was set to obtain a hydrophilic surface structure with a hierarchical micro- and nanorelief.
What methods are used to create a biocompatible coating
Today, a popular method of structuring implants is sandblasting [8], the essence of which is to create a disordered rough relief by bombarding the surface with a powder jet directed under pressure. For these purposes, powders of hydroxyapatite, aluminum oxide, etc. are usually used. In most cases, sandblasting is followed by acid etching of the treated surface to remove powder residues [9]. Thus, this structuring method does not exclude residual contamination on the surface of the treated implant.
Also, quite recently, the Swiss company Nobel BiocareTM introduced on the market a new design of the implant surface, which improves its osseointegration by forming zones with different morphologies: an abutment with nanoporous (nanostructure size of 69 ± 48 nm) smooth (roughness Sa = 0.13 ± 0.02 μm; Sdr = 1.7 ± 1%) with an oxide coating (the thickness of the oxide layer is 153 ± 5 nm); a neck with surface characteristics similar to the abutment (Sa 0.49 ± 0.03 μm, Sdr 2.1 ± 1.0%, nanostructure size 43 ± 21 nm, oxide layer thickness 142 ± 17 nm); transition zone with roughness varying to the apex of the implant (from Sa = 0.92 ± 0.16 μm and Sdr = 107.2 ± 31.5% to Sa = 1.49 ± 0.19 μm and Sdr = 172.7 ± 18, 0%) and the thickness of the oxide layer of the transition zone is 7.2 ± 0.3 μm and the vertices are 9.9 ± 1.3, the size of the micropores of the transition zone: 1.1 ± 0.5 μm, the vertices: 1.7 ± 1, 1 μm [10].
As the authors note, the oxide layer on the surface of the implant neck and abutment provides additional bactericidal properties to the implant during the period of its survival in the body.
Regulation of the roughness value, pore size, nanostructures, as well as the chemical composition of the implant surface is carried out by fine-tuning the anodizing modes, selecting the current value and a suitable electrolyte. The listed values of structural elements and roughness values are taken from the source [10]. It should be said that often, during the anodizing process, strong acids are used, for example, H2SO4, H3PO4, HF, HNO3, which can remain in the pores on the surface even after sterilization of the implant [11], which negatively affects biocompatibility. In work [12], it is noted that phosphorus content is observed on an anodized titanium implant from Nobel BiocareTM. It can be assumed that this fact indicates the use of phosphorus-containing acid as an electrolyte in this technology.
Among the modern manufacturers of dental implants, the Korean company CSM should be noted, which introduced implants with a surface modified by laser treatment to the market. As a result of exposure to solid-state Nd: YAG laser radiation, in one technological stage, without the use of chemical reagents, an ordered microrelief in the form of holes and grooves is formed on the surface of the implant, due to which the implants demonstrate excellent osseointegration ability and functional stability [13]. It is also known that the structures induced by laser action in air have good wear resistance due to the content of titanium oxynitrides [14].
We also believe that laser-based methods are the most promising for creating a biocompatible implant surface. The indisputable advantage of laser treatment is that the surface relief is formed due to the evaporation of the material itself, without the use of third-party materials for treatment, such as corundum particles Al2O3, and chemical reagents, for example, HCl and H2SO4 acids, which reduces the risk of implant rejection due to residual pollution. In addition, laser treatment opens up great opportunities for obtaining complex multilevel surface morphology with a given chemical composition.
As a tool for treatment, we chose the domestic laser complex MiniMarkerTM 2, widely used in industry [15], based on an ytterbium pulsed fiber laser. Metals absorb the radiation of this laser with a wavelength of 1.06 microns quite well. The working range of power densities I = [6.9–63] ∙ 107 W / cm2 makes it possible to reach the evaporation temperature of titanium, and the scanning system (galvanometric mirrors) together with a focusing system (F‑theta lens with a reverse focal length of 216.1 mm) provides the ability to form structures with complex morphology.
FORMATION OF A BIOCOMPATIBLE STRUCTURE ON THE IMPLANT SURFACE
So far, there is no univocal opinion in the scientific community about the most suitable surface for a titanium dental implant [16]. It is obvious that the rough surface relief is not the only criterion for optimal osseointegration. The question remains, which type of relief (ordered or disordered, consisting of holes, or grooves, or some other morphology) will be the most biocompatible.
The relief in the form of parallel microgrooves is of particular interest. In comparison with the disordered relief, formed, for example, by sandblasting, the micro-grooved surface influences the behavior of cells in such a way that the cells do not grow chaotically on the surface, but “line up” along the grooves of the grooves [17–19]. Important parameters for creating such a relief are the width, depth and period of the grooves [20]. The ratio of these values determines whether a given relief will have an impact, and which one, on the behavior of cells, in this case, neural stem cells. In the case when the width of the grooves is much less or much more than the size of the cells, and the depth of the grooves is less than 5 μm, other interaction mechanisms are activated. Then the behavior of cells largely depends on nanoscale structures, in particular, on their orientation in space. In addition, nanoscale structures affect the adhesion of proteins to the implant surface at the early stages of osseointegration [21, 22], which, in turn, determine the final formation of newly formed bone tissue.
Thus, we hypothesize that the optimal surface morphology for titanium implants will be a structure with elements of supracellular size (20–40 µm). Because it provides a certain mobility (motor activity) of cells and is equipped with a substructure of a smaller (nanometric) size, the function of which is to ensure the possibility of metabolism: air access and removal of soluble waste products (which may require hydrophobic channels). An example of such a structure on titanium is the structure formed during the deposition of ablation and oxidation products of titanium in air (Fig. 1) on the initial titanium surface.
Even if we accept the above hypothesis, the question still arises as to what type of relief will be optimal: extended grooves or individual dimples periodically located on the surface, or something else. In studies with neurons [23], it was found that different types of reliefs are capable of influencing cell behavior. Only an extended relief in the form of grooves promotes contact guidance of cells. Studies of the differentiation of mesenchymal stem cells (MSCs) into the osteogenic group, for example, into osteocytes, are not found in the literature data similar to this work [23].
In this study, we formed structures of two types: dimples (L‑structure) and grooves (K‑structure). The period, width and depth of the structures were from 20 to 40 μm. Both types of structures were formed with an X‑axis pulse overlap of about 95%.
An ordered L‑structure is a set of wells in which cells should be placed. The L‑structure was obtained using laser radiation with a power density of 6.9 x 107 W / cm2 in a two-pass treatment mode. The dimples are located evenly over the entire surface of the sample, and their diameter is about 40 μm. The period of the structure is 50 μm. Since this relief is formed when the surface is heated above the evaporation threshold, then during the first pass of laser radiation over the sample surface, the substance is carried out and the microrelief is instantly formed in the form of grooves. The second pass forms the same grooves, but in the orthogonal direction, in the same way with the removal of the substance. In those areas of the surface where the radiation pulse from the second pass hit the pulse from the first one, depressions in the form of dimples were formed. Between the dimples is the area that was modified during the first laser pass. Thus, a pronounced network structure is observed on the surface of the titanium alloy (Fig. 1c).
To form a grooved K‑structure, a multipass mode with a power density of 63 ∙ 107 W / cm2 was used. As mentioned above, cells of the same type can be of different sizes and shapes within the same range of values, which should be taken into account when modeling the relief structure. Based on this, the grooved structure was created in three passes of laser radiation with the formation of parallel grooves with a step of 30 μm from the beginning of the previous groove.
This mode of action was applied in order to obtain grooves of different widths. Since this type of structure was also obtained in modes above the evaporation threshold, after the first pass, a relief in the form of grooves appears on the sample surface. After the second pass, a new groove was formed, which partially overlapped the groove from the first pass. The groove formed after the third pass overlapped the groove formed by the previous one. Thus, structuring of the gaps between the grooves took place and the surface was completely filled with structures, without gaps between the grooves (Fig. 1b).
To study the modified surface, a scanning electron microscopy (SEM) analysis was carried out using a Zeiss Merlin microscope with additional Oxford Instruments INCAx-act attachments and an Oxford Instruments CHANNEL5 electron backscattered diffraction (EBSD) recording system for X‑ray microanalysis. In fig. Figure 1 shows SEM images of the initial (untreated) surface (P‑structure) of titanium implants (Fig.1a) and implants after laser treatment: K‑structure (Fig.1b) and L‑structure (Fig.1c). SEM images demonstrate the presence of reliefs of various scales on the titanium surface: micro- and nanoreliefs.
Energy dispersive analysis showed the presence of oxygen on structured surfaces (Table 1). This indicates the presence of titanium oxide on the surface of the implant. Specialists know that it also has good biocompatibility.
The contact angle of the surface of all structures was measured by illuminating the droplet with an LED illumination source with a total power of 1 W and a ToupCam high-resolution CCD camera. Distilled water was used as a test liquid; the droplet volume was 0.1 μL. The Digimizer software was used to determine the contact angle value [27]. Measurements were carried out on three samples of each structure. Photographs of deposited drops on surfaces are shown in Fig. 2. According to the results of measurements, the wetting angle of the titanium surface before laser treatment was 70° (Fig. 2a). It is not possible to measure the angle of wetting of surfaces after laser treatment, because the surface has changed from hydrophilic to superhydrophilic (i. e., a drop, hitting it, instantly spreads and permeates the structure).
The kinetics of changes in the maximum diameter dmax of the spreading droplet with time t is shown in the graph (Fig. 2 b). It should be noted that each structure corresponds to a different character of droplet spreading: the L‑structure corresponds to a symmetric oval spreading region (long axis = 1.6 ± 0.1 cm, short axis = 1.26 ± 0.1 cm), and K‑structure – asymmetrical elongated along the grooves of the grooves and large in area (long axis = 1.5 ± 0.1 cm, short axis = 3.9 ± 0.1 cm). As is known, wettability plays an important role in the adsorption of protein and cells on the surface, and superhydrophilic surfaces contribute to this [24].
Thus, with the help of laser structuring, we formed superhydrophilic reliefs on the surface of titanium dental implants, which simultaneously consist of micro- and nanostructures.
The effectiveness of biocompatibility of reliefs formed by laser exposure was revealed in the course of preclinical in vitro and in vivo tests. Their results will be presented in the second part of the work. The algorithm and technological stages of technology based on this method will be described. The technology has been introduced into the wall-to-wall production of dental milling center and a plant that is part of the ORTOS group of companies.
SEM studies of the surface of implants were carried out at the St. Petersburg State University at the Interdisciplinary Resource Center in the direction of “Nanotechnology” (St. Petersburg).
The authors of the work express their gratitude to the research team of the NRU “BelGU” (Belgorod) under the leadership of Yu. R. Kolobov for help in researching the physicochemical characteristics of laser-induced structures.
In vitro experiments and experimental protocols were approved by the Research Ethics Council of the Nizhny Novgorod State Medical Academy (Privolzhsky Research Medical University, Nizhny Novgorod) and comply with the principles of the Declaration of Helsinki.
The committee of the St. Petersburg State Medical University n. a. I. P. Pavlov carries out its activities in accordance with the Constitution of the Russian Federation, laws and other legal acts of the Russian Federation and St. Petersburg, the Declaration of Helsinki by the World Medical Association of 1964, amended in 1975, 1983, 1989, 1996, 2000 and 2013., ICH Harmonized Tripartite Guideline for Good Clinical Practice (ICH GCP), industry standard OST 42–511–99 “Rules for conducting high-quality clinical trials in the Russian Federation”, which came into force on January 1, 1999. Recommendations of the Ethics Committees conducting the examination of biomedical research by the WHO, the Charter of the St. Petersburg State Medical University n. a. I. P. Pavlov and the Regulations on the Ethics Committee of the St. Petersburg State Medical University n. a. I. P. Pavlov. The “In vivo study of the integration processes of titanium dental implants with a laser-modified surface” was approved (excerpt from Minutes No. 208 of the meeting of the Ethics Committee of the St. Petersburg State Medical University n. a. I. P. Pavlov dated June 25, 2018).
The authors of the work express their gratitude to the research team of the Federal State Budgetary Educational Institution of Higher Education “PRMU” of the Ministry of Health of Russia (Nizhny Novgorod), consisting of Daria Kuznetsova, Vadim Elagin and Elena Zagainova, for the study of cell biointegration on the laser-induced surface of VT6 titanium, and the staff of the Center for Shared Use of Scientific Equipment “Cellular and molecular technologies for studying plants and fungi” of the Botanical Institute n. a. V. L. Komarov of RAS (St. Petersburg), Zernitsky A. Yu. and Zotov P. A. for conducting histological and histomorphometric studies.
The study was supported by a grant from the Russian Science Foundation (project No. 20–62–46045).
Contribution of members
of the creative team to the project
All members of the team of authors took part in the project: setting the task and providing resources – G. N. Chernenko, S. G. Gorny; concept, study design and project management – V. P. Veiko, G. V. Odintsova; experiments on laser structuring of titanium surface – Yu. Yu. Karlagina, V. V. Romanov, R. M. Yatsuk; concept of in vitro and in vivo studies – A. I. Yaremenko; conducting and analyzing in vivo studies – E. A. Zernitskaya; analysis of the results of in vitro and in vivo studies – Yu. Yu. Karlagina, E. E. Egorova.
Conflict of interest
The authors declare no conflicts of interest.
ABOUT AUTHORS
V. P. Veiko (vadim.veiko@mail.ru), full professor, Doctor of Science (Technical), Head of the International Laboratory «Laser Micro-and Nanotechnologies», Faculty of Laser Photonics and Optoelectronics, ITMO University, Saint-Petersburg, Russia.
ORCID: 0000-0001-6071-3449
Yu. Yu. Karlagina (jujukarlagina@itmo.ru), engineer, International Laboratory «Laser Micro-and Nanotechnologies», postgrad. student, ITMO University, Saint-Petersburg, Russia.
ORCID: 0000-0002-6927-9551
V. V. Romanov (ionhcik@rambler.ru), engineer, postgrad. Student, faculty of laser photonics and optoelectronics, ITMO University, Saint-Petersburg, Russia.
ORCID: 0000-0003-1468-9438
R. M. Yatsuk (yatsuk.roman@mail.ru), engineer, ITMO University, Saint-Petersburg, Russia.
ORCID: 0000-0003-2502-7501
G. V. Odintsova (gvodintsova@itmo.ru), Cand. of Science (Technical), Research Associate, Laboratory «Laser Micro-and Nanotechnologies», Faculty of Laser Photonics and Optoelectronics, ITMO University, Saint-Petersburg, Russia.
ORCID: 0000-0001-9581-4290
E. E. Egorova (elena1998959@gmail.com), student, ITMO University, Saint-Petersburg, Russia.
ORCID: 0000-0002-1461-0673
Е. А. Zernitskaya (zernitskaya_ekaterina@mail.ru), postgrad. Student, Pavlov First Saint Petersburg State Medical University, Saint-Petersburg, Russia.
ORCID: 0000-0002-3819-693X
А. I. Yaremenko (ayaremenko@me.com), Doctor of Medical Sciences, Professor, Head of the Department of Surgical Dentistry and Oral and Maxillofacial Surgery, Director of the Clinic for Oral and Maxillofacial Surgery, Vice-Rector for Academic Affairs, Pavlov First Saint Petersburg State Medical University, Saint-Petersburg, Russia.
ORCID: 0000-0002-7700-7724
G. N. Chernenko (office@ortos.biz), Director General, Lenmiriot Dental Implant Prosthetics Manufacture, Saint-Petersburg, Russia.
S. G. Gorny (info@newlaser.ru), Cand. of Science (Eng), «Laser Center», Saint-Petersburg, Russia.
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