Modifying an Implant: A Mini-review of Dental Implant Biomaterials

Biomaterials are crucial for modern medical applications, and in the last several decades, these materials have been refined and improved for applications seen in dentistry, orthopedics, drug delivery, and cardiology. Candidate materials include polymers, metals (and their alloys), ceramics, composites (e.g. carbon fiber), and glass. For clinical use, the material must not cause any adverse or damaging effects to the patient [1]. This concept is known as biocompatibility and was described by Williams as, “the ability of a biomaterial to perform its desired function with respect to a medical therapy, without eliciting any undesirable local or systemic effects in the recipient or beneficiary of that therapy, but generating the most appropriate beneficial cellular or tissue response to that specific situation, and optimizing the clinically relevant performance of that therapy” [2]. The application of biomaterials as a dental implant is shown in Figure 1A; an endosseous fixture is used to replace the root and support a dental prosthesis. Dental implants have become an evidence-based treatment for the replacement of missing teeth. Selecting a biomaterial for a dental implant requires additional considerations for biological, mechanical, and aesthetic properties. Modern dental implants need to feature effective osseointegration and maintain long-term stability of their favorable properties to maintain both the implant’s structure and the integrity of surrounding hard and soft tissue [3, 4]. This review aims to assess the current state of biomaterials (Ti, zirconia, and novel materials) used in dental implants. Materials are assessed on their attributes, 1School of Life Sciences, Faculty of Science, University of Technology Sydney, Ultimo, NSW, Australia


Introduction
Biomaterials are crucial for modern medical applications, and in the last several decades, these materials have been refined and improved for applications seen in dentistry, orthopedics, drug delivery, and cardiology. Candidate materials include polymers, metals (and their alloys), ceramics, composites (e.g. carbon fiber), and glass. For clinical use, the material must not cause any adverse or damaging effects to the patient [1]. This concept is known as biocompatibility and was described by Williams as, "the ability of a biomaterial to perform its desired function with respect to a medical therapy, without eliciting any undesirable local or systemic effects in the recipient or beneficiary of that therapy, but generating the most appropriate beneficial cellular or tissue response to that specific situation, and optimizing the clinically relevant performance of that therapy" [2].
The application of biomaterials as a dental implant is shown in Figure 1A; an endosseous fixture is used to replace the root and support a dental prosthesis. Dental implants have become an evidence-based treatment for the replacement of missing teeth. Selecting a biomaterial for a dental implant requires additional considerations for biological, mechanical, and aesthetic properties. Modern dental implants need to feature effective osseointegration and maintain long-term stability of their favorable properties to maintain both the implant's structure and the integrity of surrounding hard and soft tissue [3,4].
This review aims to assess the current state of biomaterials (Ti, zirconia, and novel materials) used in dental implants. Materials are assessed on their attributes, characteristics, comparison of strengths and weaknesses, biocompatibility, and new advancements in addressing any issues or concerns.

Titanium
Ti is widely used for medical applications and is currently the most commonly used material for dental implants. The popularity is in part due to the combination of its tensile strength, biocompatibility, ability to osseointegrate, density, corrosion resistance, and inert properties. Pure Ti, while being safe for most clinical use, has been observed to form an accumulation of Ti ions around the implant site. While no material is entirely bioinert, such ions released from an implant that fails to osseointegrate may induce proinflammatory responses from sensitization. Such failure occurs when there is a lack of biomechanical stability due to inadequate osseointegration. Stability can be promoted by modifying the implant's physiochemical properties and surface topography. Observations from failed Ti implants show an increased lymph node Ti ion concentration by 7.4-9 times and lung Ti ion concentration by 2.2-3.8 times when compared to a successfully integrated implant [5,6] (Table 1). Ti dental implants have been occasionally observed to result in an allergic reaction. However, the exact mechanism for Ti ion release and systemic effects remains unclear [7]. Surface modifications of Ti implants are performed to improve osseointegration and modulate cell adhesion. The largest post-operative issue with such implants is biofilm-induced mucositis, inflammation of mucous membranes, which can further develop into peri-implantitis, the inflammation of the gum and the surrounding bone structure. The first generation of surface modification was developed in the 1960s, where machined Ti (smooth surface) was used to promote osseointegration by leaving a small degree of roughness. Second generation surface modifications have been widely available since the 1990s and improve on the first generation by creating a microscopically rough surface topography. Finally, the current third generation (also known as bioactive surfaces) maintains similar roughness to the second generation and are modified chemically or topographically to promote osseointegration [8]. Most surface topographies minimize vertical deviation to 1 μm or less, effectively deterring biofilm formation. Current surface topographies include anodization, sandblasting, and polishing, however, conclusive evidence on which specific surface topographies are optimal and why remains undetermined [9].

Alternative alloys
It is known that Ti can form alloys to solve reactivity concerns [1]. Vanadium (V) and aluminum (Al) are common alloying elements for Ti (seen in Ti6Al4V alloy). Individually each element has shown potential adverse effects in high concentrations, causing carcinogenicity and neurotoxicity [10][11][12]. However, reports on V also demonstrate antidiabetic effects and resulted in reduced weight gain and gastrointestinal discomfort [1,13]. Attempts are being made to replace these alloying elements with less toxic elements such as niobium (Nb), tantalum (Ta), and palladium (Pd), to improve biocompatibility; however, Ti6Al4V remains a common choice [14]. When comparing a Ti alloy to surgical stainless steel and cobalt alloys, the superior quality of Ti is its corrosion resistance. The ability for Ti alloys to withstand corrosive environments and develop an oxide layer with a shorter repassivation time compared to other metal alloys, aids in preventing a release of ions which can potentially be toxic (cobalt) or cause allergic reactions (nickel, found in stainless steel) [15].
A unique alloy, TiZr has shown clinical success in the past decade. Under the trade name Roxolid, Institut Straumann AG, Basel, Switzerland (a 15% Zr, 85% Ti composition), the TiZr alloy was designed for dental implants, becoming a viable alternative to zirconia and other Ti-based implants [16]. TiZr offers enhanced osseointegration compared to Ti6Al4V and pure Ti and avoids the use of toxic alloying elements. Additionally, corrosion resistance can potentially reduce problems associated with peripheral Ti ion concentrations [17][18][19]. A recent study observed that TiZr retains the ability for surface enhancement without compromising the mechanical strength of the implant [20]. While TiZr dental implants do not offer the aesthetic qualities of zirconia, they demonstrate highly preferable qualities among clinically available Ti alloy options.
TiNb is another alloy with favorable properties and provides a low elastic modulus similar to human cortical bone. It is clinically significant as it reduces the issue of stress concentration between the implant and bone, thus   [79] In vitro Investigating the blood interaction, and resulting osseointegration, at the bone-implant interface of microstructure Ti and zirconia implants. Thrombogenicity was the highest on zirconia surfaces compared to Ti. It was suggested that a higher thrombogenicity promotes osseointegration, however, similarly micro-structured Ti maintained good osseointegration potential Zeller et al. (2020) [80] In vivo Investigation of biofilm formation on gold and silver metal alloys, PEEK, zirconia, and TiZr alloy implants. PEEK, Zirconia and TiZr materials demonstrated similar biofilm formation on commercially available implants. Only gold and silver-based materials showed reduced biofilm growth Herrmann et al.
(2020) [81] In vivo Examining early and mature biofilms on Ti and zirconia implants at different roughness. Zirconia showed significantly higher total bacterial cell counts, but could be due to surface roughness. Biofilm formation and total bacterial cell counts could be assumed to be caused by surface roughness Kubasiewicz-Ross et al. (2018) [82] In vivo: pig model Studying the osseointegration of commercially available Ti and zirconia implants Overall zirconia displayed similar levels of osseointegration, measured through BIC. Differences between implant material yielded no statistically important differences

BIOI 2021
Mini Review promoting osseointegration. This alloy improves on other functional aspects compared to pure Ti, possessing higher toughness, hardness, and corrosion resistance. TiNb has been proposed as a suitable alternative for replacing failed hard tissue [21,22].

Recent trends
Recent trends in the field point toward the utility of novel ideas such as: (1) the development of implant coatings, and (2) the incorporation of alternative elements (e.g. iodine (I) and strontium (Sr)) to support Ti implants. The next generation of therapeutic implants may incorporate nano-engineered surface topographies. The use of anodized titania (titanium dioxide, TiO 2 ) nanotubes (TNT) on the surface of Ti implants shows promising results. These NTs offer advantages in topography, mechano-transduction, drug release, and mediating toxicity. The TNTs mimic the topographical scale of surrounding native tissues to significantly upregulate protein absorption. The bone remodeling caused by mechanical strain stimulates cells at the bone-implant interface to promote initial attachment and continued adhesion. TNTs that are capable of local drug release potentially allow for direct targeting of the implant site and a reduction in toxicity compared to systemic administration. Together these factors contribute to influence cellular functions and immunomodulatory responses towards dental implants [23,24]. Compared to conventional Ti implants, TNTs have been observed to orchestrate osteogenesis [25]. A recent development in surface modifications is plasma immersion ion implantation which provides a uniform application over complex surface geometries, while reducing overall cost and processing time. The in vitro testing of an oxygen plasma immersion ion implantation (OPIII) to a Ti implant identified improvements in protein expression and corrosion resistance [26]. The use of OPIII is capable of producing a bioactive oxide film which was determined to improve in vitro cell differentiation and in vivo osseointegration. It is undetermined to what degree the process optimizes the implant's surface properties (topography, wettability, and roughness), however, the most current research has observed no significant alteration [27]. In its current state, OPIII has a good potential to become a viable surface modification through further research.
Additionally, an in vivo study demonstrated the ability to use macrophage polarization via NTs to augment osseointegration [28]. The property of local drug release is another significant advantage of this technology. While some attempts have been made to incorporate antibiotics into NTs, these traditional or organic antibiotics pose the risk of resistance and are limited by selective toxicity. This has resulted in attention towards inorganic antibacterial agents, such as silver. Using silver as a coating has shown strong antimicrobial effects, however, it poses biocompatibility issues. Studies are testing the effect of loading silver nanoparticles onto TNTs. These TNTs use a reducing agent to disperse silver nanoparticles from the coating and can aid in patient care during the initial postoperative risk of infection. Clinical safety and molecular mechanism still require further studies [29]. Long-term studies are required to translate this technology for clinical application, and cost will also need to be considered for commercial implementation.
Continued research in coatings has led to novel improvements on hydroxyapatite (HAp) for Ti6Al4V implants. In situ synthesis of HAp-based nanocomposites including chitosan and polycaprolactone/fluoride (PCL/F) substitution aim to utilize the advantages of HAp with promoting the formation of bone-like apatite at the bone-implant interface, while minimizing the brittleness of HAp. A sol-gel method and alkali-treated Ti is used to provide a homogenous, crackfree coating that does not suffer from any delamination. Both nanocomposites required a minimum 10 wt.% of HAp to provide the desired results with the respective composite [30,31]. Research for chitosan/HAp identified improved cellular response at lower HAp content (10 wt.%), with higher content (50 and 60 wt.%) providing improved surface adhesion strength, roughness, and hydrophilicity [30]. It was noted with PCL/F that HAp content up to 30 wt.% improved similar surface properties, and higher corrosion resistance [31]. Both coatings meet the required performance of bioactive coatings and show potential for future clinical application.
An alternative material to Ti is Ta. Ta is a useful biomaterial due to being highly bioactive and holds similar properties to Ti regarding corrosion resistance and biocompatibility. The improvement over Ti is the optimization of the osseointegration process. However, it is a high-density material which is expensive to manufacture. Therefore, combining the properties of Ti and Ta, research has focused on developing Ta oxide films. Preliminary research observed promising results, improving wear resistance to microbial and chemical degradation. Improved osseointegration was also observed with increased osteoblastic activity and morphology. Finally, the study did not indicate any increased bacterial cell adhesion when compared to conventional Ti implants [32].
Adding alternative elements is not a new concept, and continuing with the idea of surface treatment, the addition of Sr is a possibility. Historically Sr is biologically known for promoting osteoblast activity and inhibiting osteoclast activity. Oral administration of Sr may cause adverse effects; therefore, surface incorporation (on Ti implants) has been preliminarily observed to increase early bone osseointegration [33]. Another potential element is I, an essential element with antiseptic properties and may be a useful biomaterial for dental implants. A previous study indicated that I-supported Ti implants in orthopedic applications substantially reduced the incidence of inflammation. The enhanced implants exhibited cytocompatibility and increased antimicrobial activity [34].
Another approach to modification is altering the implant design; the use of porous Ti has been researched to improve integration into hard tissue. The sponge-like framework also allows for antibacterial agents to be directly incorporated into the implant surface or via degradable drug carriers [35]. Currently challenges around optimization of the structure and high manufacturing cost are a barrier to implementation of this technology in the clinic [36].
A recent study has improved on the capability of porous Ti through using micropore channels in Ti implants to preserve alveolar height without additional growth factors. The canine in vivo model provides evidence in minimizing alveolar bone loss, while promoting cell viability, proliferation, and osteogenic differentiation [37]. Natural bone has periodontal ligament fibers which apply the required mechanical stimulation to maintain the correct height of alveolar bone. Traditional dental implants lack this function due to a direct osseointegration at the bone-implant interface [38]. The horizontal traction of the micropores simulates the periodontal ligament fibers which provide a potential clinical opportunity for higher quality dental implants. Overall, Ti offers many unique opportunities to be alloyed and altered. Thus, novel permutations are continually being invented and designed to improve the material for more effective clinical use.

Zirconia
Dental implants made from the bioceramic zirconia (zirconium dioxide; ZrO 2 ) are a non-metal alternative to Ti implants that address aesthetic concerns. Widely available as partially stabilized zirconia (PSZ), it provides a transformative toughening of the biomaterial [39]. Additionally, zirconia advantageously displays a significant reduction in plaque affinity and proinflammatory responses in surrounding soft tissue [40].
Compared to Ti implants, similar or better results from zirconia have been observed in in vitro cell culture experiments. These results however are not directly comparable, probably due to inconsistencies in influential variables of cell type, incubation conditions, and surface properties. Surface modifications for both implant materials are well characterized and contribute a significant effect on the bone-implant interface. Nanoporous zirconia surfaces have indicated promise for improved cell growth as observed by larger cell size and higher cell count compared to polished zirconia and surface modified Ti implants (acid etched and sandblasted) [41,42]. Assessing the biocompatibility of zirconia, the cytotoxicity is comparable to Ti nanoparticles. High concentrations of zirconia nanoparticles inhibit osteogenic differentiation and may induce necrotic or apoptotic effects on tissue. Proinflammatory effects for ziconia have not been reported. Research is thus limited and requires further study to rule out potential biocompatibility issues [43,44].
The most significant flaw of zirconia is an aging phenomenon known as low-temperature degradation (LTD) which transforms the tetragonal surface back to the initial monoclinic structure in the presence of water or a humid environment [45,46]. Zirconia surfaces have shown lower bacterial adhesion compared to Ti; however, these bacteria are still able to cause deterioration of zirconia surfaces [47]. Currently, no research has been able to adequately study the effect of oral bacteria on LTD. One study attempted to identify a correlative link between oral bacteria and mechanical fatigue; however, it was unable to arrive at any conclusive evidence [48]. The majority of LTD comes from manufacturing flaws. Further studies are required to observe the role of oral temperature and pH fluctuation on zirconia implants.
The success of an implant is reliant on the degree of biomaterial osseointegration. Much research in surface modifications has been conducted to enhance implant stability and bone apposition, however few optimal alterations have been identified. The use of surface topography for Ti plays a similar role in zirconia implants; however, producing modified surfaces is more complicated than in Ti due to a potential compromise of biomechanical stability. Furthermore, the current developments in surface modifications for zirconia do not have clinical applications as seen in novel surface topographies of Ti dental implants. Many clinicians remain skeptical of the osseointegrative properties of zirconia. Improvement of surface properties has taken two main approaches; (1) via optimization of surface roughness, and (2) through the application of a coating. Traditional surface modifications follow the first approach, which include air particle abrasion, acid etching, and polishing. However, these methods introduce risk of surface and internal damage, possibly leading to fracturing. An effective roughening process would make zirconia implants more viable, as optimal roughness demonstrates a similar capacity for osseointegration compared to roughened Ti implants [3,49].

Recent trends in surface modifications
Following the second approach to surface modifications, HAp resembles the chemical composition of bone and teeth, and has been suggested as a bioactive coating for zirconia implants. The advantage is stimulating osseointegration through the supply of calcium and phosphate ions [50]. Research has indicated that HAp-coated metallic implants heal faster and attach more readily and completely to the surrounding tissue [51]. While design of the coating's properties would need to be optimized, transitioning this coating to zirconia may also prove difficult, because of potential bioactivity of calcium phosphate coatings [52][53][54][55]. A novel implementation of etching (selective infiltration via hydrofluoric acid and a molten glass coating) has been found to maintain surface chemistry and alter the roughness for effective osseointegration [56,57]. Other novel surface modifications include biofunctionalization via an adhesive peptide (arginine-glycine-aspartate, fibronectin, or collagen), and laser treatment. Laser-treated zirconia exerted no surface contamination and promoted fibroblast attachment, an effect attributed to improved surface wettability [58,59]. The immobilized peptide in biofunctionalization enhances the biocompatibility of the implant through extracellular matrix regulation, as peptides such as fibronectin encourage cell adhesion and bone formation [60,61]. While being effective for in vitro treatments, these modifications are clinically limited by a lack of supporting research and the question of longevity remains open [58].
A promising approach to improving bioactivity of zirconia without a substantial compromise of mechanical properties is through a gradated design and composite outer layer.
While the calcium phosphate composite has contraindications in the literature, the gradated design aids to ensure no coating detachment after implantation which otherwise may result in implant failure [62].
Bioactive surface coatings, such as HAp, can result in delamination of the coating from the implant after a long loading time. Control of the coating thickness is a strategy to mitigate this issue. Simple and repeatable methods of arc induction or plasma spraying are used to provide accurate thickness control and maintain the required chemical effect. Plasma anodization and fluoridation are alternative methods to control surface energy on zirconia implants. Diode laser application can be used to clean implant surfaces in the cases of peri-implantitis. Erbium: YAG and carbon dioxide lasers cannot be recommended due to their penetrative, and surface altering properties, respectively. Modification from longterm exposure to fluoride has shown enhanced contact of the bone-implant interface [63].
Another interesting surface modification includes selective infiltration etching (SIE), which creates a nanoporous surface to facilitate cell growth and attachment. The technique involves molten glass infiltration to alter the surface grain of the zirconia implant and promote grain reorganization. It is followed by acid etching to dissolve the glass, which results in a few microns of porous surface material. This is advantageous by maintaining structural integrity and creating an ideal surface for osseointegration [64].
The use of plasma immersion ion implantation has also been researched for zirconia implants. Similarly to Ti, the method implanted surface level functional groups without compromising the structural integrity of the material. The nitrogen-containing functional groups promoted early stage osseointegration in vitro. The proliferation and osteogenic properties provide evidence for a foundational method that will require future in vivo investigations [65].
Zirconia remains a viable alternative to Ti implants. While not as mechanically robust, zirconia implants satisfy a need in the consumer market for aesthetic and non-metal alternatives. Current research is developing improvements for the bone-implant interface to improve successful osseointegration and adapt to structural limitations.

Other alternatives Graphene
The highly published carbon allotrope, graphene, has the potential for many bioapplications, including dental implants. Graphene offers reliable functionality through mechanical strength and stability. Early research of graphene identified issues related to long-term biocompatibility in an oral environment and the possibility of oxidative debris which can induce cytotoxicity. Current attempts at applications are limited by manufacturing difficulties and cost effectiveness [66]. However, future applications may deploy graphene as a nanotube coating alternative to the TNTs mentioned in Titanium -Recent Trends.

Bioactive glass
The use of bioactive glass has long been considered for its capability to bond to soft and hard tissue. It is commonly used as a reinforcement material in HAp-coatings on metallic implants to improve mechanical properties and osseointegration [67]. A recent study has proposed the use of bioactive glass as a coating to reduce the incidence of bacterial infections which result in implant failure. The bioglass was synthesized with up to 2% silver (in weight), to aid in antibacterial properties. The addition of ≥0.5% silver prevented the growth of common Gram-negative and positive bacteria (Staphylococcus aureus and Escherichia coli) [68]. Longterm structural stability and additional bacteria types specific to the oral environment, such as Streptococcus salivarius, need to be further examined.

Polymer materials
The use of fiber reinforced composites (FRC) have been used extensively within the broader fields of science and engineering. FRC implants exhibit durable mechanical performance and elasticity, with in vivo studies indicating equal biocompatibility and osseointegration compared to pure Ti. A large benefit over conventional materials is the ability to be used as a fixed implant that is mouldable in situ. The implant creates a strong adhesion to underlaying tooth substrate, transferring stress based on fiber direction. Further novel approaches have been made by embedding the FRC surface with previously mentioned bioactive glass to improve the osseointegration process [69,70].
Another polymer of interest is a thermo plastic, polyetheretherketone (PEEK). PEEK composites can be reinforced with glass fiber or carbon fiber to replicate the biomechanics of human cortical bone, potentially decreasing bone loss and improving osseointegration. Compared to pure Ti implants, PEEK implants have a lower fracture resistance, however, they suffer from decreased stress shielding. Clinical evidence is currently lacking to determine if polymer composites will replace common Ti and zirconia implants [71,72].
A potential biopolymer that can be considered for future directions are collagen hydrogels, which as a coating could promote osseointegration. The use of a naturally occurring polymer may provide favorable properties in terms of biocompatibility and lower cytotoxicity, due to having a low immune response. Collagen hydrogels benefit from being capable of simulating the oral microenvironment and are easy to form through self-assembly [73]. This polymer requires further research in the potential introduction of collagen hydrogels into the bone-implant interface to promote implant osseointegration.

Magnesium composite
Magnesium-based materials are not new to medical applications and often degrade during the healing process [74]. The use of alloyed magnesium raises concerns with ion release from degradation, which may cause systemic toxicity [75,76].
Studies recognize the use of powder metallurgy as a promising approach to the development of magnesium-based biocomposites [83,84]. Metallic or ceramic additions, such as Ti, zirconia, or zinc oxide in the metallurgy process allows for the composite matrix to be reinforced for high mechanical performance [85][86][87][88]. Using fluorapatite, calcium, or rare-earth elements such as dysprosium or gadolinium enhances corrosion resistance [86][87][88][89][90][91][92]. Through utilizing a bioactive, bioinert, and mechanically stable composition, a modern magnesium composite has the potential to become a viable alternative. Studies are still required to investigate if the composite can outperform alternative zirconia forms or Ti alloys.

Ceramic composites
Combinations of alumina (Al 2 O 3 ) and zirconia (ZrO 2 have been explored due to their high stiffness and toughness, however, they are limited by high cost and complex manufacturing methods. The addition of TiO 2 has been researched to improve production through a less complex sintering process and enhanced bioactivity at the bone-implant interface. The resulting alumina-zirconia-titania ceramic has been optimized into a low-cost manufacturing process with high mechanical strength. Compared to zirconia, aging resistance has been improved, showing no signs of LTD in 40 years of simulated clinical use [93].
The inclusion of titania has promoted studies into the ceramic's biocompatibility. An increased wettability of the ceramic surface produced a positive correlation with cell viability, with no significant cytotoxic effect. While a higher content of titania improved wettability, DNA damage was associated with the composite containing >10wt. % TiO 2 [94]. The proposed implant contains less titania than the genotoxic concentration, providing enhanced surface properties and suitability for future in vivo trialing.

Conclusion and outlook
This review aims to provide an accessible overview of the use of biomaterials in implant dentistry. Focusing primarily on the use of Ti and zirconia, our discussion expands on the properties of these biomaterials and novel modifications that can be implemented to improve their utility. Alternative biomaterials are also introduced; however, analysis is limited due to lack of extensive clinical and laboratory data. This review seeks to provide guidance for continued research into modifying Ti and zirconia-based dental implants. While it takes account of current clinical implications of novel modifications, this is not the focus of the review. Future reviews can address the clinical management of a wider range of dental implants as a complement to the current review.
In modern dentistry there is a growing demand for improvements in mechanical, bioactive, and aesthetic properties of tooth replacement implants. Furthermore, the success of dental implants depends on their capability for osseointegration, corrosion resistance, infection prevention, and durability against degradation. Current commercially available biomaterials exhibit a unanimous capability for osseointegration, however, they suffer from limitations in their clinical application. Ti-based materials offer superior mechanical properties (such as tensile strength and stability) that achieve reduced implant failure and longer lifespan, while zirconia focuses on satisfying aesthetic purposes by mimicking natural tooth-like color. Surface topographies will remain an essential aspect of implants in optimizing cell adhesion for improving osseointegration and reducing biofilm formation [9]. The importance of maintaining mechanical and aesthetic concerns is supported by López-Píriz et al., who also agree that the impact of peri-implantitis from biofilm formation is another significant cause of implant failure in modern dental implants [4]. Bacterial colonization is reduced by novel biomaterials such as a bioactive glass coating, which prevents growth on the implant surface. Additionally, the use of certain coatings induces a similar effect to surface topographies for cell and tissue adhesion, while providing additional mechanical support and corrosion resistance. Corrosion will remain an issue as no material is entirely bioinert. Novel materials and compositions such as graphene and polymers require additional long-term studies to observe potential effects on stability and biocompatibility [66,71]. The utilization of bioactive glass as a coating for polymer and metallic Biocompatible [74] Low corrosion resistance, hydrogen evolution from degradation [75,76] Refining composition of alloy/composite to enhance mechanical strength and bioactivity [83][84][85][86][87][88][89][90][91][92] implants presents an effective improvement for osseointegration [70]. Future directions for implants are likely to retain the use of Ti and zirconia, based on patient preference for mechanical vs aesthetic and metal vs non-metal ( Table 2). Zirconia used in a PSZ form should undergo further development in surface modifications via coatings or innovations in altered topography to maintain its position as a competitive alternative to Ti. Improved surface properties will enhance adhesion to soft and hard tissues and inhibit bacterial colonization [53]. Ti currently offers the most versatility in design for alloying, surface modification, and coating adhesion. A TiZr implant may be the most promising alloy due to the lack of potentially toxic ions (such as V and Al) from the composition, adequate mechanical strength, and surface modification capability [16]. For example, a productive future direction may be an acid-etched TiZr dental implant with a bioactive glass coating. In summary, dental implants are well-established but require continued measurement and optimisation of biocompatibility and material properties to improve the technology.

Conflicts of interest
The authors declare no conflict of interest.