Open access peer-reviewed chapter
Abstract
The introduction of composites and dental materials in the implantology field has shown an important increase in the past years. The restorative approaches using dental implants are currently a desirable option for edentulous patients. Since their introduction in dentistry, dental implants have proven to be a reliable option for restabling the functions and esthetics of certain areas. Characteristics such as high biocompatibility, nontoxicity, and high corrosion resistance have been key factors for their worldwide acceptance. In time, researchers aimed to improve their qualities by manufacturing the implants using various materials that could improve the interaction between the bone and implant. Although, until now, dental implant materials were limited to the use of single or coated metals, there are certain limitations that current studies aimed to overcome by introducing a new category, the composite dental implants. With this new category, the mechanical characteristics can be designed in order for their integration and further functions to have a positive outcome. This chapter describes the use of composite dental implants as a restorative prosthetic option, their advantages, and physicochemical and osteointegration properties as future approaches for restorative prosthetic rehabilitation.
Keywords
- composite dental implants
- prosthetic restorations
- osteointegration
- biocompatibility
- restorative dentistry
1. Introduction
Dental implants have increased in popularity and are nowadays among the first options for prosthetic restoration of partially edentulous or total edentulous patients. They have been used as artificial tooth roots that can support prosthetic restorations, starting from fixed single crowns and ending with removable dentures.
Their clinical use has been for over 30 years [1], providing important progress for dental and maxillofacial surgery as further prosthetic treatment approaches, as their use can improve the local restoration or represent a support for several orthodontic appliances.
From the research results provided by Brånemark and others [2, 3, 4, 5, 6, 7] regarding the process of osteointegration of dental implants, the election material so far was titanium and titanium alloys due to their high biocompatibility, osteointegration induction, high resistance to corrosion, and other excellent mechanical properties. Hence, these are implied for correct substitution of the dental root. Until recently, this type of material was considered the gold standard for the manufacture of dental implants, having the main target to achieve a direct connection between the bone and the implant in order to ensure long-term resistance, fulfilling the future functions of a prosthesis [8]. The results of using titanium dental implants have been quantified, and the success rate is high, reporting no differences between the followed clinical protocol [9]. However, the implant failure cases that implied the removal of the dental implant were mainly the consequence of peri-implantitis, caused by the colonization of anaerobic micro
As currently patients require fast treatment that includes quick implant placement and healing, followed by an immediate load for a functional prosthetic outcome, for the titanium implant in order to fulfill these requirements, their osteointegration and the entire dynamic process require at least 3 to 6 months for local healing and inducing the cellular process for osteointegration [11]. Also, reports show that
The research focusing on the development of materials, especially biomaterials and composites, has provided important information related to their use and applicability in dentistry and implantology. By introducing these classes of biomaterials such as ceramics, glass ceramics, hydroxyapatite, or polyetheretherketone (PEEK), their advantages for clinical applications have quickly transformed them into an alternative to the use of titanium implants. Improving the quality of the dental implants and influencing the future of implant-prosthetic restorations, this chapter aims to provide valuable information related to the use of composite implants as a treatment option.
2. Dental implants—biomechanical properties and osteointegration
2.1 Biomechanical properties of dental implants
A dental implant is considered a medical device with the main role of replacing a missing tooth. As a consequence, the physical, chemical, and biological characteristics of an implant should be correctly adjusted in order to stimulate and sustain the osteointegration process, as well as the future forces that will be directly absorbed and distributed in the area of the bone-implant interface [16]. While aiming to restore an edentation with the help of an implant prosthetic restoration, the clinician must take into consideration the biomechanical aspects of the two components of an implant: the screw and the abutment (Figure 1). Being a foreign dispositive that will be in contact with the human bone (the screw) and the oral soft tissue (the abutment), it is a desiderate to be integrated and accepted in order to be functional [17].
Figure 1.
Basic implant components.
The main goal in implantology is to achieve the osteointegration process of the placed implant, in order to become stable and retained in the alveolar bone, to form a new bone-implant interface for further functional forces to be properly distributed.
The bone is an important tissue structure that is responsible for movement, protection, and support in the entire body. It has an organic component represented by collagen (types I, III, and IV) and fibrillin and an inorganic component represented by hydroxyapatite [18]. The organic part offers flexibility to the entire structure, and the inorganic part is responsible for strength, summing their action during the presence of the tooth or implant in the alveolar bone (Figure 2). The entire architecture of the bone is responsible for the biological, chemical, and mechanical characteristics [19].
Figure 2.
Representation of the alveolar bone and its composition.
The bite forces exhibited by humans were reported to reach approximately 800 N in the molar, 600 N in the premolar, and 500 N in the canine region [20]. The mechanical forces and biological mechanisms are essential factors in order to induce and maintain the osteointegration process [21]. There are two different areas of the bone: the compact bone/cortical bone and the trabecular bone [22]. There are mechanical and biological differences between these areas, exhibiting different stiffness, creep and fatigue moduli, and tensile strength due to their different bone composition. Also, these properties can vary based on the anatomical site of the alveolar bone, patient’s age, bone characteristics, and associated systemic conditions [23].
For an implant to be functional, the mechanical and biocompatibility properties are crucial for its good integration and functionality. The described mechanical properties of an implant are represented by their ability to resist the applied overdenture and added distributed forces, and these properties are divided into toughness, strength, stiffness, and ductility [24]. Related to stress resistance, there are several types that are in relation to the material and shape of the dental implant, the tensile, sheer, and compressive stress [25]. Also, the yield strength refers to the property of a material to resist stress without suffering from a plastic strain. The elastic modulus is represented by the rigidity of a material that has an implication in the strength and fatigue properties as well. The fatigue strength is a consequence of the composition and the suffered thermomechanical procedures. A higher fatigue strength has been associated with a long-term survival and functionality of the implants [26].
The implications of the design of the implant upon its mechanical properties and the osteointegration process are important aspects to be taken into consideration during the manufacturing stage.
The primary stability or the mechanical stability represents the result of the first interaction between the bone and the implant, without the implication of the biological process responsible for osteointegration [27]. During the osteointegration process, once the implant becomes biomechanically stable, the transition from the primary stability to the secondary stability is the key moment for prosthetic overload [28]. The further exhibited mechanical forces upon the implant need to correctly be distributed along the implant and the bone for the occurrence of bone resorption, due to progressive marginal bone loss. The results of an analysis outline two important aspects: When using a rigid material, the transfer of the resulted mechanical stress and deformation to the alveolar bone will be minimum, while the use of a material with a high elastic modulus determined the transfer of the stress to the surrounding alveolar bone [29].
Although titanium and its alloys have been used in dentistry, and especially implantology, their mechanical properties are not similar to the alveolar’s bone, resulting in an imbalance of the stress distribution, leading eventually to bone resorption and secondary implant failure [30]. A solution would be the addition of other materials or manufacturing dental implants from other materials, in order to balance the mechanical forces and their distribution to the surrounding tissue.
2.2 Osteointegration of dental implants
During the placement of a dental implant, a series of changes occur in the soft and hard tissue, damaging and causing a local inflammatory response [31]. This response determines the further cellular interactions that will promote the local repair process. Particularly at this stage, the interaction between the mechanical forces and the biological local changes is the key for the osteointegration of the dental implant [21]. Studies have revealed that, in time, the interaction between the dental implant and the bone will form an effective biomechanical relationship with an important implication in the future stability and load forces once the implant is charged [32].
The individualized implant-prosthetic treatment should consider the interactions and changes that characterize the primary and secondary stability stages of an implant. While the primary stability is dependent on the surgical protocol, the shape, material, design, and bone characteristics, the secondary stability is based on the formation of new bone, a step that starts once the wound healing process begins. The understanding of these cellular and mechanical actions and their involvement in the osteointegration process is essential for the clinician to decide the proper time for loading the implant.
Aiming for efficient secondary stability, the new bone should form at the interface of the implant with the alveolar bone. Immediately after the surgical insertion of the implant, the first step of the osteointegration starts by supplying the area with blood and eventually filling it with a blood clot [33]. The componence of the blood clot contributes to the local formation of granulation tissue due to the interaction of the mesenchymal stem cells and the newly formed vascularization [34] (Figure 3). The final stability of the dental implant is a result of the primary stability that decreases in the first 3 to 4 weeks and the secondary stability that increases by time [35].
Figure 3.
Osteointegration of dental implants.
Differences in the osteointegration process can occur due to the changes of the elastic modulus of the used materials. Studies have discussed that differences in the elastic modulus can have a negative effect upon the bone, determining peri-implant bone resorption [36]. Results have highlighted that the manufacture of dental implants with materials that exhibit a low elastic modulus has osteoconductive properties.
3. Composite dental implants
3.1 Polyetheretherketone (PEEK) dental implants
In implantology, the mechanical properties of the used dental materials are an important aspect to be taken into consideration. Currently, there are attempts in the development of new materials with modifications in their composition, in order to improve the existent inconveniences regarding the use of titanium and titanium-based alloys. Among the discussed mechanical changes are the fatigue and density properties that have direct repercussions upon the newly formed surrounding bone [37].
Among the developed materials are the polymeric ones, whose properties can be modified accordingly to their clinical purpose. Their mechanical behavior and structure can be designed to mimic the dental and bone structures, in order to achieve a desirable osteointegration process [38]. Polymers used for the manufacture of dental implants exhibited high fatigue and hardness levels, with an accurate deformation modulus that makes them an appealing approach for this restorative option [39].
PEEK is an engineered plastic that provides good strength, high biocompatibility, and good chemical stability in most of the environments [40]. Nevertheless, this type of polymer exhibits good mechanical properties and corrosion resistance, overcoming the reported disadvantages of titanium implants. Research studies outlined that PEEK implants compared to the titanium ones do not release particles and they do not determine an immune response from the host [40]. Another reported advantage was the fact that this type of polymer can be processed into different shapes and dimensions without influencing its properties, fitting the needs required for the dental implants. The high esthetic requirements of different areas of the dental arch can be fulfilled by using this type of implants, as their color is similar to the one of the alveolar bones and it does not influence the imagistic examination that uses magnetic resonance [41]. Nevertheless, regarding the elastic modulus that has an important impact upon the long-term stability of the dental implants, PEEK implants have a similar elastic modulus to the alveolar bone, limiting the stress transfer to the surrounding alveolar bone [42]. This type of polymer was approved by the Food and Drug Administration starting 1980 and has been successfully used also for orthopedic purposes [43].
Researchers have aimed to further improve the characteristics of this material by adding supplementary fillers such as glass and carbon fibers or hydroxyapatite in the PEEK matrix for maximizing the biomechanical properties [44, 45, 46]. The PEEK composites used for dental implants provided a high biocompatibility, having as well an important osteogenic activity and antimicrobial action [47]. By encapsulating the fillers in the matrix, their surface bioactive properties are still low, this being a further target for researchers [48]. By loading the PEEK composites with biomolecules, an improvement of the biochemical and biological characteristics was achieved, by developing a local environment that promotes cell growth and differentiation [49]. Liu et al. [50] in their study aimed to coat the surface of PEEK implants with titanium oxide (TiO2) and activated by methacrylate hyaluronic acid in order to improve its properties. The results showed that the modified PEEK composites had higher hydrophilicity and promoted migration, adhesion, and proliferation of the human mesenchymal stem cells [50].
The osteointegration of the dental implants depends on the activity of the cellular growth factors. The fact that PEEK alone is an inert and hydrophobic material determines a low adherence of these growth factors that are the key for the new bone formation. In case of the PEEK coated with TiO2, the adhesion of BMP-2 (bone morphogenic protein-2) was higher compared to PEEK implants alone, facilitating through a porous structure the adherence of BMP-2 to the surface of the implants [51]. In their study, Sun et al. [52] added and immobilized BMP-2 on the surface of PEEK implants, the results revealing a slow release of these proteins for the next 28 days. On the other hand, Guillot et al. [53], in the study they conducted, coated the PEEK implants with hyaluronic acid and loaded them with BMP-2. After their implantation in rabbits, they observed that, in case of the PEEK implants coated with BMP-2, the new bone formation was lower compared to the cases of using only PEEK implants. The explanation would be that the presence of local BMP-2 in high quantity determined the activation of osteoclasts and lower osteogenic activity. The conclusion was that the delivery of high levels of BMP-2 in the local environment should be avoided, having an initial antiosteogenic activity.
Studies reported that PEEK does not exhibit an antimicrobial property, offering an appealing environment for the plaque and bacterial adherence, with important consequences on the peri-implant tissue and long-term stability [54]. By adding PDA coating with silver-ion, having a controlled release of the silver ions, the results showed a long-term antimicrobial activity [55]. Another approach to overcome this disadvantage of PEEK was the coating with sodium alginate hydrogel loaded with chlorogenic acid, exhibiting a significant antimicrobial activity upon Gram-positive and Gram-negative populations [56].
There have been intensive studies regarding the treatment of the surface of PEEK implants in order to achieve a bioactive product. One option presented by Khoury et al. [57] described the treatment of the surface using accelerated atom beams and reported an increase in the osteointegration of these implants without modifying their chemical structure. Another approach was presented by Poulsson et al. [58] by treating the surface of the implants with oxygen plasma for better osteointegration. A similar method was performed by Hassan et al. [59] that treated the surface with nitrogen plasma. Their results outlined a higher osteointegration rate compared to the one reported by using standard PEEK implants.
PEEK exhibits excellent mechanical and biological properties, and by adding filler materials, these properties can be improved in order to overcome the potential disadvantages that could influence the stability and osteointegration of these implants.
3.2 Bis-GMA and TEGDMA composite dental implants
Bis-GMA (bisphenol A-glycidyl methacrylate) and TEGDMA (triethylene glycol dimethacrylate) are categories of composites that have been recently introduced as potential materials for dental implants. Based on the properties they provide, although research has not focused on them related to this field, they can become a viable alternative to dental implants.
Resins are currently being used in dentistry for their high esthetic properties, decent strength, lower cost compared to ceramics, and capacity to form a bond with the structures of the teeth [60]. They have been classified as thermo-materials that can be reinforced with different fibers, such as glass fibers, in order for them to be successfully introduced in the implantology field.
Bis-GMA is a polymeric resin that has the property to penetrate easily into the printed samples, transforming into a strong material after polymerization. This characteristic makes them suitable for their integration as a matrix in the manufacture of dental implants [60].
Further
Chen et al. [62] reported that, by reinforcing the Bis-GMA composite with HA fibers, their properties can be improved by sintering methods, modifying their elastic modulus in order to be more similar to one of the alveolar bones.
The use of TEGDMA composite was assessed by combining it with Bis-GMA and bioactive glasses for the manufacture of dental implants in order to use two thermosets based on light-induced copolymerization. Abdulmajeed et al. [63] in their study aimed to evaluate the local response of the fibroblasts in relationship with these types of implants. The results reveal a higher adhesion of the fibroblasts on the surface of the implants manufactured from the combination of TEGDMA, Bis-GMA, and bioactive glasses.
3.3 Chitosan
Chitosan is a natural polymer, characterized by good biocompatibility, being biodegradable, and having the capacity to penetrate solid structures. In order to fulfill its purposes in the dentistry field, it is often associated with composites based on calcium phosphate, being reported an increase in the biomechanical properties, without interfering with the activity of the osteoblasts [64]. An
One of the main advantages of this natural composite is its chemical composition due to the presence of hydrogen bonds that provide an increased resistance to heat [66]. This property is important when chitosan is combined with poly methyl-methacrylate, needing a lower curing temperature [66]. Also, as studies observed, in time, the porous structure increases in dimension, outlining the biodegradable characteristic. The adherence of osteoblasts was also observed when coating the titanium implants with chitosan, suggesting the implication of coating in the osteointegration process of the standard used dental implants [64]. These properties provide an important perspective for the use of this type of material for the manufacture of dental implants.
4. Conclusions
Composite dental implants represent a viable option for the prosthetic rehabilitation of edentulous patients. Being described as biocompatible, osteoconductive, and with similar mechanical properties similar to the alveolar bone, composite dental implants represent a progress in the implantology field, overcoming the reported disadvantages of the standard titanium dental implants.
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