Introduction

Developing materials that are physically and biologically compatible with alveolar bone remains a challenge in dental implant design. Titanium is commonly used in dental implant manufacturing due to their proper physical properties. Titanium oxide forms a dense, protective, and strongly adherent layer over dental implants, which is called the passive film, and is excellent in resisting corrosion [1,2,3,4]. Under optimal conditions, bone differentiation occurs directly adjacent to the implanted material. This process is called osseointegration and is the direct structural and functional connection between living bone and load-carrying dental implant surfaces and is the main requirement for long-term success of dental implants [5, 6]. Surface topography, chemistry, degree and scale of roughness, and wettability can modify cellular behaviors such as adhesion, proliferation, differentiation, and migration during the osseous healing period and can promote osseointegration [7, 8].

Although titanium dental implants have high clinical success rates [9], multiple approaches have been developed during the last few decades to enhance physical and chemical aspects of osseointegration and to reduce the duration of its formation. Surface-roughened implants and ceramic coatings are well-established practices while three-dimensional (3D) printing remains an experimental technique [6, 7, 10,11,12,13,14,15]. Several studies have suggested that the roughness of titanium dental implants can promote cytocompatibility, enhance surface area of implants adjacent to bone cells, and increase biochemical interaction of implants with bone osteoblasts [14, 16,17,18]. Compared to 2D surface roughness, porous implants decrease stress shielding and increase bone-implant interlocking such that a high porosity implant that mimics natural tissue is capable of stimulating osteoblast differentiation [19, 20]. Gradient porosity is another important issue in producing dental implants. It confers reactive properties to the implant surface and decreases its modulus of elasticity to match that of bone [21]. Three-dimensional printing gives the chance to directly produce dental implants with different shapes, textures, and gradient of porosity and also minimize post-processing requirements [22].

Nowadays, investigators use laser irradiation for surface or structural modification of dental implants. Applications of laser are versatile and laser can be used to heat, melt, or vaporize materials based on the type of laser used [1]. Laser application for various purposes depends on properties such as direction, divergence, wavelength, and frequency of laser beam, which can be adjusted by the laser components [13]. Considering the importance of cell-implant interaction on the success rate of dental implants, the effect of laser on biological properties of dental implants has gained interest. The aim of this review was to analyze the influence of laser treatments of uncontaminated titanium implant surfaces on in vitro behavior of cells (cellular morphology, cell adhesion, viability, proliferation, and differentiation). Furthermore, the in vitro biological responses as the outcome of the laser application in surface coating were evaluated. Lastly, special attention was focused onto the laser application in additive manufacturing of titanium dental implants to determine whether this new technology is as good as or better than other tools for supporting cell growth.

Materials and methods

Search strategy

We searched PubMed, ISI Web, and Scopus databases to find relevant articles published between 2000 and 2016 using the following keywords: laser, AND modification (OR processing, melting, coating), AND titanium dental implants (OR titanium surface), AND biocompatibility (OR in vitro bioresponse OR cell activity). Subsequently, each article’s references were reviewed to identify other relevant articles.

This systematic review assessed whether laser enhanced the in vitro biological response of titanium dental implants. Table 1 outlines the questions that were addressed with reference to participants or population (P); intervention (I); comparison, control, or comparator (C); outcome (O); and study design (S) (PICOS elements).

Table 1 PICOS format of the question used in this systematic review
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Inclusion criteria

Titanium dental implants which had been treated/coated by use of laser or processed using laser were included. There was no distinction made with regard to the grade of titanium, type of laser, or parameters of laser. In vitro studies which reported some measures of biological responses as an outcome were included. Interventions based on combinations of laser and other modalities such as ceramic coating or growth factors were also considered for review.

Exclusion criteria

Studies carried out on animal models and studies other than in vitro’s were excluded. Publications in languages other than English were excluded. In vitro studies on contaminated implants in which disinfection of the implant surface was stimulated by laser were excluded. Similarly, studies on zirconia implants, orthopedic implants, and those on abutments were excluded.

Selection of studies and quality assessment

Two trained reviewers (A.H. and F.T.) performed independent searches, assessed publication validity, and extracted the data in duplicate. Disagreements were resolved by discussion, rereading, and consultation with the third member of the research team (F.F.) when necessary. All citations were imported into an electronic database (EndNote). The quality rating of studies was based on comprehensiveness and reproducibility of the methodology, the use of standard methods to appraise the biological response, and the absence of apparent bias in results.

Data extraction

We extracted data from different scenarios: (1) studies reporting surface modification or treatment of dental implants with laser, (2) studies reporting laser-assisted titanium coating, and (3) studies reporting laser-based manufacturing of titanium dental implants. Given the heterogeneity of the dose, type of laser, and type of cells, no statistical analysis was used to synthesize the data.

Results

The initial search identified 332 articles, 136 of which were chosen after screening their titles and abstracts. After retrieving the articles’ full texts, 28 were included in this systematic review. Sixteen articles studied surface modification or treatment using laser, while six articles studied laser-assisted coating, and another six were different articles that studied using laser for manufacturing titanium dental implants. All 29 articles were in vitro studies published between 2000 and 2016. Tables 2, 3, and 4 summarize the results according to the technique that was utilized.

Table 2 Studies on surface modification of dental implants with laser
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Table 3 Studies on laser-assisted coating of dental implants
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Table 4 Studies on manufacturing dental implants using laser
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Discussion

Today, biocompatibility is a grand area of concern in dental biomaterial properties. Most dental implants support cell attachment by conferring suitable areas for cell adhesion [48]. Laser offers a high energy that can be applied to modify surfaces made of different materials and to produce three-dimensional nano- and microstructures. It is used in different surface modification techniques because of its ability to rapidly and effectively induce physical and/or chemical changes such as surface roughness and deformation and assist coating of biomaterial surfaces [49]. Some advantages of laser include generation of complex features with high resolution, high degree of purity [50], suitability for selective changes in implant surfaces, and its precision [51, 52]. Regulation agencies such as the Food and Drug Administration in the USA require biocompatibility testing per ISO 10993 (International Standard Organization: Standard for Biological Evaluation of Medical Devices) or ASTM F748 (American Society for Testing of Materials: Standard Practice for Selecting Generic Biological Test Methods for Materials and Devices) prior to device approval. Consequently, there is a need to carry out biocompatibility testing for any new material or processing method [53].

In vitro experiments are the first step in biocompatibility testing of new materials by the observation of viability and biofunctionality of cells on a material surface; therefore, in this review, we focused on the in vitro biological responses of dental implants processed with laser as a new processing method. Based on our review, the most common in vitro assays were MTT assays, cell attachment, proliferation, and cell counting.

This review was limited to the study of titanium and its alloys since titanium is used commonly in dental implants, and there are many studies of the long-term outcomes of titanium dental implants. Most of the studies reviewed in this article used Ti6Al4V alloy, and only nine studies used Cp Ti. It has been demonstrated that bone cell interactions are mainly determined by the chemistry of the substrate, the structure of the implanted material, and the production method. However, the topography of the surface is more important in cell behavior than the chemistry of implant material or the processing method [54], although it should be noted that these effects are difficult to separate as they are interrelated [48].

Surface modification of titanium with laser can promote micron-level surface texturing, increase the surface area, and significantly enhance micromechanical properties of titanium dental implants [13, 55]. Laser can also modify the surface roughness as well as the physical and chemical properties and the biocompatibility of the titanium surfaces compared with a smooth surface [34, 55,56,57], depending on the type of laser and the parameters used [58, 59], as well as the contamination control [60]. Laser adjusts the titanium oxide layer and improves biocompatibility [12, 13, 33, 36, 61]. Assessment of surfaces for roughness, microhardness, and phase development after melting with laser showed titanium oxide formation, which has a sterilizing effect and provides a contaminant-free surface that can effectively enhance biocompatibility [33, 56].

The laser parameters play an important role in determining bioresponses [62]. The main parameters related to processing include laser power and peak power for continuous wave (CW) and pulsed lasers, respectively, as well as laser spot diameter [63]. The main advantage of pulsed lasers compared with CW lasers is the ability to deliver high peak power in a short pulse length, resulting in effective melting with a small heat-affected zone [64]. In contrast, evaluation of the processing window for pulsed lasers is more troublesome because peak power, pulse width, and frequency need to be optimized. One study showed that the best parameters for using selective laser melting with pulsed laser were a scan speed of 6 mm/s, laser peak power of 1 kW, and hatching pitch of 0.4 mm, yielding a tensile strength of 300 MPa and torsional fatigue strength of 100 MPa [63]. As we summarized in tables, different laser devices were used in different studies. It seems that the power or energy used depends on the desired effect (melting vs surface texturing or coating).

The growth and differentiation of osteoblasts are essential for the regeneration of bone around dental implants. This may explain the greater use of osteoblast-like cells in biocompatibility investigation. However, it appears that cell type does not play a great role in determining the biological response of laser-processed dental implants as most of the articles showed that biofunctionalization with laser led to higher levels of cell bioactivity, proliferation, and attachment to titanium surfaces [33, 65]. Observation of Alamar Blue proliferation assay measurements showed positive cellular metabolic activity [37] while MTT assays showed an increase in the number and viability of cells [27, 31,32,33]. Also, creation of hybrid nano- and microscale titanium surface roughness by laser treatment allows stimulation of osteoblast differentiation and bioactivity to form mineralized zones [15] and improves the bone response to the laser-modified titanium surfaces [66, 67]. The main mechanism governing the cell adhesion on the laser-treated groups could be change of the wettability characteristics [25]. However, Györgyey et al. did not find any significant differences in cell bioactivity or attachment between laser-treated and controlled groups. In their study, they used osteogenic sarcoma cells (SaOs-2) treated with Er:YAG laser (60–100-mJ pulse energy) [32]. Ayobian-Markazi et al., also found that the difference between laser-treated and controlled groups in the mean MTT score did not reach statistical significance [31]. Variations in the irradiation protocols could be the cause of such discrepancies. It seems that surface features dimensionally closer to the cell dimensions are able to positively affect the viability and spreading of cells [35]. Alkaline phosphatase activity and gene expression were assays used in six studies to investigate osteogenic differentiation of cells.

In order to enhance the integration of titanium into living tissues, researchers have used laser to coat implant with materials such as bioactive ceramics that imitate bone [41, 42, 68]. Surface modification with laser, in association with biomimetic coating, shortens implant healing period by increasing bone implant interaction [39]. Cell adhesion assays indicate that laser Ti surface CaP biofunctionalization enhances cell adhesion to the surface and provides osteoconductivity [42]. Also, Nd:YAG laser-assisted nitride titanium surface (TiN) treatment appears to support tissue growth on the surface of dental implants [23]. The Q-switched Nd:YAG laser titanium surface microscale patterning plays a significant role in enhancing metal-ceramic bond strength and is a promising method for manufacturing dental implant biomaterial with high osteoconductivity, cell growth, and differentiation and better adherence to bone surfaces compared to oxidized titanium surfaces [69]. Commercially available dental implants coated by pulsed laser deposition demonstrate uniform coating thickness around the corners and sidewalls of implants [42, 70, 71].

Surface treatment and coating of dental implants could be further customized with additive manufacturing. Some additive manufacturing techniques use laser as energy source for printing with high accuracy and maneuverability [21, 43, 72,73,74]. Laser additive manufacturing is a scalable manufacturing method that can create complex structures with high dimensional accuracy and controllable density and reduce material waste. It also provides the ability to produce costume made dental implants with enhanced osseointegration [73]. Titanium dental implants are made by laser-forming techniques such as laser sintering and laser melting. Laser sintering is an efficient method, which meets the required micromechanical and surface criteria for dental implant biomaterials. It is better adapted to the elastic properties of bone and minimizes stress-shielding effects while improving long-term performance [21, 43, 75]. One advantage of laser melting is the ability to fabricate parts with controlled porosity. Implants manufactured by this technique have a porous surface structure that increases bone osseointegration and a compact core that enhances mechanical strength [76]. Three-dimensional laser synthesized porous titanium constructs have improved cell bioactivity and stimulate osteoblast differentiation and maturation. Osteoblasts retain their structural proliferative activity activated by high-porosity laser additive manufacturing [22, 46, 77]. Bone shows active growth into the intricate porous structure of titanium implant surface with no signs of inflammation, indicating high compatibility of the titanium implants [63, 77]. Enhanced bone growth and osseointegration into the surface with adequate micro- to nanoroughness support osteoblastic differentiation and increase the production of local factors important for creating an osteogenic environment [78].

The main lasers used in metal forming or manufacturing of titanium dental implants include 1054 nm Yb-doped fiber laser system and 1064 nm Nd:YAG, with an average power of 200–300 W [44, 45]. Nd:YAG laser creates efficient constructs with functionally graded complex structures and costume-made dental implants with high chemotaxis for cells that stimulate osteoblast differentiation and maturation and are activated by high porosity laser additive manufacturing [21, 22, 43, 44, 46, 63, 73, 74, 78]. Hollander and colleagues in their study on porous blasted direct metal laser-sintered (DMLS) specimens demonstrated that DMLS-fabricated Ti6Al4V allowed structure-oriented growth of human osteoblasts on its surface. In their study, the biocompatibility of selective laser melting (SLM) Ti-64 material was also studied. Comparisons were made between SLM surfaces and commercially available Thermanox® (Nalge Nunc Int., New York, NY, USA) control and conventional bulk titanium. The authors concluded that the increased metabolic activity of osteoblasts on SLM discs compared to the controls may have been due to the greater surface area of the SLM material, which took longer to be covered by the cells [43].

Mangano et al. seeded human dental pulp stem cells (DPSCs) on direct laser-sintered titanium scaffolds and acid-etched surfaces. They observed that gene expression and protein secretion were faster on laser-sintered scaffolds [79]. These results were confirmed by another study on cell cultures, where rat calvarial osteoblasts were seeded and cultured on disc specimens produced by DMLS. Cell density was similar to that of commercially available rough microtextured surfaces but lower than that of machined and smooth-textured grit-blasted, acid-etched surfaces [45]. Finally, in another in vitro study, human osteoblasts and human DPSCs were cultured either on acid-etched or DMLS titanium surfaces, in order to investigate their osseointegration and clinical applicability of the derived implants. When stem cells were exposed to DMLS titanium surface, osteoblastic differentiation of DPSCs and bone morphogenetic protein production occurred more quickly. These successful results suggest that DMLS titanium surfaces may represent a promising alternative for clinical use in implants [22].

Witek et al. measured bone implant contact and removal torque of dental implants that had a porous layer, which were produced by laser sintering and compared them with sandblasted acid-etched implants (i.e., those with a rough, but not porous, surface) and concluded that porous dental implants produced by laser sintering showed better biocompatibility [80]. It seems that laser engineered net shaping to construct porous structures from Ti6Al4V alloy across the range of 23–32% porosity with low modulus (7–60 GPa), which can be tailored to match human cortical bone [81].

The studies reviewed in this article may have been inconsistent with regard to laser parameters disclosed. This can make the studies heterogeneous and difficult to compare, even if lasers with the same wavelength were used.

In addition, studies of in vitro biocompatibility of laser-processed dental implants are limited in number. More research is needed to investigate the effect of cell type, laser type, and laser power on biocompatibility and functionality of titanium dental implants that are manufactured by a range of additive and advanced manufacturing technologies that are available.

Conclusions

In the present review, an attempt was made to summarize in vitro biological outcome of different applications of laser technology in titanium dental implants (surface treatment, assisted coating, and 3D construction). Based on the obtained results, the following conclusions can be drawn:

  • Almost, all examined surface modifications by laser were as good as or better than other treatments for supporting cell attachment and growth. However, the property of laser (type, wavelength, and time of radiation) might affect the cell proliferation or at least the cell spreading.

  • Laser-assisted coating of Ti dental implants might produce uniform coating thickness around the corners and sidewalls of implants and shorten the healing period.

  • Three-dimensional laser forming of titanium implants is a reasonable and effective technology to produce titanium constructs with controlled porosity, which can be further modified to enhance their biocompatibility.

  • The environment or atmosphere that the titanium surface is modified in or coated in can affect the surface characteristics.

  • Laser type and parameters used in all three applications examined may affect the dental implant’s biocompatibility outcome.

  • Diverse assays and cells have been used by various researchers as biological assessment of laser application in implantology.