3. Introduction

Dental implantation as a method of treatment of secondary edentulousness has been growing in popularity since its invention to the present day.

The process of osseointegration, as a direct structural and functional connection between the structured living bone and the implant surface, is the basis of the global popularity of this technology. This biological phenomenon gives a foreign body the ability to exist stably in a living organism, withstand heavy mechanical loads, have long-term clinical success and, as a result, patient satisfaction. Since implant surface features have long been studied and characterized as an important factor in promoting osseointegration [1,2], current research is focused on optimizing's the potential of the implant surface for osseointegration.

Currently, the most widely used dental implants are those made by machining a titanium rod with subsequent modification of the product surface: sandblasting [3], acid etching [4,5], anodising [6,7], and discrete deposition of calcium phosphate crystals [8].

These surface treatment methods are aimed to improve stability and enhance osseointegration [1-9]. Extensive clinical experience with dental implants has demonstrated that the topography of the implant surface plays a key role in many cellular and molecular mechanisms. Rough surfaces demonstrate better adsorption of biomolecules from biological fluids, which start a cascade of biological events that regenerate bone, improve the initial cellular response, including cytoskeleton organisation and cell differentiation [1-3,8,9].

Histologically, rough surfaces have been shown to be more effective in improving the speed and quality of osseointegration compared to untreated surfaces.

There are researches that clearly demonstrate significantly higher success rates of dental implantation of samples with a rough surface [5,7,10,12,13]. According to modern knowledge of osseointegration, it has become known that the surface roughness is optimal for bone ingrowth when the distance between the peaks of the material structure and the pore size are in the range of 200 - 400 𝜇m [11]. As osseointegration is a key factor in the success of dental implants, it may be biologically justified to use porous implants, expanding their functions by promoting osseointegration throughout the product body.

Improved fixation can be achieved by bone growth into or through the porous structure of the implant, connecting not only to the rough surface of the implant but to the entire implant body.

However, from a mechanical point of view, such an implant-bone connection must be sufficiently rigid to withstand the mechanical pressure exerted on it by the orthopaedic structure.

It is known that classical methods cannot produce a porous structure with a fully controlled design of the external shape and internal architecture. Producers have proposed methods of additive production of dental implants as a way to overcome this problem [14, 15]. This approach of directly generating physical objects with a distinctive structure and shape based on three-dimensional modelling has opened up a wide area for the study of the processes of osseointegration of a printed implant in a living organism.

4. Goal

To study the experience of using dental implants made by direct laser sintering (DMLS) after the surgical stage of dental implantation in comparison with classical milled implants.

6. Materials and methods of the research.

22 people aged 22-54 years, both sexes, took part in the research, who simultaneously, side by side, were implanted with at least two implants of the same size, where the first one was made by the classical method of machining a titanium rod with the further modification of the product surface by sandblasting and acid etching, and the second implant was printed in layers by selective laser melting.

The last ones, received by additive manufacturing, were made using laser fusion technology in a powder layer with different consecutive layers growing in the vertical direction - the direction of load application. For the production of the samples, 30 nm thick layers of Grade-4 titanium powder with an average particle size of 45±10 µm were used, fused by two fibre lasers with a power of 200 W each (weld length 1070 nm).

The samples were formed with the Trumpf TruPrint 1000 device. Samples of Alpha Dent Superior Active (21 samples) and Alpha Dent Cell (21 samples) implants were used. The process of implant surface treatment of the studied samples is identical (wet SLA surface). The initial stability of all implants (ISQ) and the end face were measured. The implant torque was measured with a wrench during implant placement. ISQ was determined using a Penguin RFA device at the time of implantation and at 15, 30, and 60 days.

Some implants were immediately fitted with gingival moulders. Intraoral contact radiography was done using a Pluto super HD visiograph. Radiological control was done at the time of implantation, at 30 and 60 days.

6.1. Results of the study

Different types of bone in patients of different age groups and sexes led to the heterogeneity of the data obtained, but it should be noted that in almost all patients, under identical conditions, the torque value is higher for printed samples (Cell implants), indicating a larger total implant surface area. In the upper jaw, the torque value was relatively lower than in the lower jaw, which is due to the biological characteristics of the jaw bone structure.

The only drawback of torque testing is the impossibility, for obvious reasons, of further measurements at different stages of osseointegration, but nevertheless, this method gives a comprehensive picture of the interaction of the implant with the bone at the time of implantation.

From 14 to 30 days, a recession of indicators is fixed, which may signal the activation of osteoclasts and active restructuring of the bone matrix around the implant. From 30 to 60 days, we can observe a dynamic increase in implant stability. It should be noted that the speed of this stage most likely confirms the quality of interaction between the implant surface and osteoblasts, which in these terms transform the constructed protein matrix into immature bone tissue. When comparing the numerical data of the studied implants, a rapid increase in values in the period from 20 to 60 days for implants of the Cell type is noted (Table 3).

3. Radiological picture of dental implants made by direct laser sintering and those made by the classical method at the stages of osseointegration.

Intraoral contact radiography allows to objectively assess the bone pattern around the implant and is a leading method in the diagnosis and prognosis of early complications of dental implantation. During the observation period, no complications were observed in any patient. Radiological control was carried out at the time of implantation, on the 30th and 60th day. The radiological picture had a similar uniform character, which changed little even with orthopaedic loading (Fig. 1). The clear borders of the implant profile with no signs of intensity reduction at the border with bone tissue indicate true osseointegration.

7. Conclusions

Based on the analysis of the data presented above, comparing different types of implants, a number of interesting conclusions can be drawn. The values of the dynamometric force of the implants installed in all pairs of the investigated implants were close to each other, which is explained by identical bone supply conditions and the same size of the implants. Their average value ranged from 25-55 N\cm2.

There were no significant statistical differences. But it should still be noted that under identical conditions, a slightly higher torque value can be developed with Cell implants. The ISQ-test demonstrates similar values for the compared implant samples, but special attention should be paid to the dynamic increase in the ISQ value from 20 to 60 days, which indicates the participation of the printed frame of the implant body as a structure with a more developed area of contact with the bone and, as a result, somewhat faster and better osseointegration. This is also confirmed by X-rays. Thus, studying the experience of using dental implants made by direct laser sintering in comparison with classical milled samples, we can conclude that DMLS implants are promising and have clinical advantages.

8. List of the cited literature

1. M. Aljateeli and H.-L. Wang, “Implant microdesigns and their impact on osseointegration,” Implant Dentistry, vol. 22, no. 2, pp. 127–132, 2013.
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3. M. Monjo, C. Petzold, J. M. Ramis, S. P. Lyngstadaas, and J. E. Ellingsen, “In vitro osteogenic properties of two dental implant surfaces,” International Journal of Biomaterials, vol. 2012, Article ID 181024, 14 pages, 2012.
4. C. Mangano, A. Piattelli, F. Mangano, V. Perrotti, and G. Iezzi, “Immediate loading of modified acid etched dental implants in postextraction sockets: a histological and histomorphometrical comparative study in nonhuman primate papio ursinus,” Implant Dentistry, vol. 18, no. 2, pp. 142–150, 2009.
5. N. Sesma, C. M. Pannuti, and G. Cardaropoli, “Retrospective clinical study of 988 dual acid-etched implants placed in grafted and native bone for single-tooth replacement, The International Journal of Oral and Maxillofacial Implants, vol. 27, no. 5, pp. 1243–1248, 2012.
6. J.-Y. Choi, H.-J. Lee, J.-U. Jang, and I.-S. Yeo, “Comparison between bioactive fluoride modified and bioinert anodically oxidized implant surfaces in early bone response using rabbit tibia model,” Implant Dentistry, vol. 21, no. 2, pp. 124–128, 2012.
7. M. Degidi, D. Nardi, and A. Piattelli, “10-year follow-up of immediately loaded implants with TiUnite porous anodized surface,” Clinical Implant Dentistry and Related Research, vol. 14, no. 6, pp. 828–838, 2012.
8. V. C. Mendes, R. Moineddin, and J. E. Davies, “The effect of discrete calcium phosphate nanocrystals on bone-bonding to titanium surfaces,” Biomaterials, vol. 28, no. 32, pp. 4748–4755, 2007.
9. C. N. Elias, P. A. Gravina, C. E. Silva Filho, and P. A. D. P. Nascente, “Preparation of bioactive titanium surfaces via fluoride and fibronectin retention,” International Journal of Biomaterials, vol. 2012, Article ID 290179, 7 pages, 2012.
10. F. Rupp, R. A. Gittens, L. Scheideler et al., “A review on the wettability of dental implant surfaces I: theoretical and experimental aspects,” Acta Biomaterialia, vol. 10, no. 7, pp. 2894– 2906, 2014.
11. C. Larsson Wexell, P. Thomsen, B.-O. Aronsson et al., “Bone response to surface-modified titanium implants: studies on the early tissue response to implants with different surface characteristics,” International Journal of Biomaterials, vol. 2013, Article ID 412482, 10 pages, 2013.
12. C. Mangano, V. Perrotti, M. Raspanti et al., “Human dental implants with a sandblasted, acid-etched surface retrieved after 5 and 10 years: a light and scanning electron microscopy evaluation of two cases,” The International Journal of Oral & Maxillofacial Implants, vol. 28, no. 3, pp. 917–920, 2013.
13. C. Mangano, F. Mangano, J. A. Shibli, M. Ricci, R. L. Sammons, and M. Figliuzzi, “Morse taper connection implants supporting “planned” maxillary and mandibular bar-retained overdentures: a 5-year prospective multicenter study,” Clinical Oral Implants Research, vol. 22, no. 10, pp. 1117–1124, 2011.
14. T. Traini, C. Mangano, R. L. Sammons, F. Mangano, A. Macchi, and A. Piattelli, “Direct laser metal sintering as a new approach to fabrication of an isoelastic functionally graded material for manufacture of porous titanium dental implants,” Dental Materials, vol. 24, no. 11, pp. 1525–1533, 2008.
15. J. A. Shibli, C. Mangano, S. D’Avila et al., “Influence of direct laser fabrication implant topography on type IV bone: a histomorphometric study in humans,” Journal of Biomedical Materials Research Part A, vol. 93, no. 2, pp. 607–614, 2010

9. Other product research