Exploring the diverse nanomaterials employed in dental prosthesis and implant techniques: An overview

3.1.1 Ti and its alloys in dental applications

In the ever-evolving realm of dentistry, materials play an indispensable role in determining the success of treatments, especially in restorative procedures and dental implants. Two significant materials, namely, Ti and its alloys, and Co–Cr alloys, have historically marked their territory in this domain due to their unparalleled properties. This article seeks to provide a brief overview of these materials, emphasizing their advantages, limitations, and potential future directions in dental applications. The multifaceted world of dentistry is perpetually in flux, with continuous advancements and innovations driving it forward. Central to this progression is the role of materials, serving as the very foundation upon which dental procedures are built. Their choice directly influences the outcomes, longevity, and overall efficacy of treatments, especially when it comes to restorative procedures and dental implants. Delving into this expansive material landscape, two metallic substances distinctly stand out due to their unique attributes: Ti and its alloys, along with Co–Cr alloys. This exposition offers a succinct insight into these materials, delineating their virtues, inherent challenges, and their potential trajectory in future dental applications.

3.1.2 Ti alloys: The Au standard in dental implants

Ti and its alloys have unequivocally become the Au standard in the realm of dental implants, revolutionizing the approach to mitigating tooth loss and revitalizing oral aesthetics and function. The inception of Ti as the cornerstone material for dental implants can be traced back to its exemplary biocompatibility, mechanical strength, and resistance to corrosion, characteristics that are instrumental in the realms of durability and long-term success. Diving into the heart of Ti’s biocompatibility, it becomes evident that this metallic element fosters an intimate relationship with human biology, particularly the bone tissue [83]. Ti implants remarkably encourage osseointegration, a natural process where the bone intimately binds with the implanted material, cultivating a robust foundation for the artificial tooth. This facilitates a symbiotic relationship where the implant not only replaces the lost tooth but also promotes the preservation and health of the jawbone, forestalling bone resorption and maintaining the structural integrity of the facial features.

Mechanical prowess is another accolade in Ti’s repository of attributes. The inherent strength and resilience of Ti and its alloys mimic the natural tenacity of a tooth root, ensuring that the implant can withstand the vicissitudes of masticatory pressures and diverse oral functionalities. This mechanical integrity is harmonized with a relative lightness, ensuring that the implant does not become a burdensome presence in the oral cavity but rather integrates seamlessly in terms of function and feel [84]. Corrosion resistance further elevates Ti’s stature in dental implantology. In the intricate environment of the mouth, where variables such as saliva, pH levels, and various biochemical interactions are constantly in flux, Ti demonstrates remarkable resilience. Its immunity to corrosive influences ensures that the integrity of the implant is steadfastly maintained, safeguarding both the aesthetics and the functional viability of the implanted structure over substantial periods.

3.1.3 Co–Cr alloys: bridging strength and aesthetics

Co–Cr alloys have ascended to prominence within the dental sciences sector, serving as pivotal materials that harmonize robust mechanical attributes with aesthetic qualities. Historically, a diverse array of metals and alloys has been utilized for the fabrication of dental prostheses, including crowns and bridges, designed to mimic the functional and visual characteristics of natural dentition. The pursuit of an archetypal dental alloy, characterized by its exemplary mechanical strength, heightened biocompatibility, and superior aesthetic integration, has catalyzed the adoption and advancement of Co–Cr alloys in dental applications [97]. The selection of materials for dental prosthetics is paramount, as they must withstand the dynamically harsh conditions of the oral milieu, characterized by fluctuating temperatures, varying pH levels, and the substantial mechanical loads imparted during mastication. Co–Cr alloys are particularly notable in these rigorous environments due to their exceptional mechanical robustness and superior resistance to both wear and corrosion. Cobalt, an inherently strong and durable element, synergistically combines with chromium to forge alloys that manifest outstanding hardness and resilience. These properties are critical in coping with the considerable pressures and forces encountered within the oral cavity. Consequently, the enhanced mechanical stability of Co–Cr alloys contributes significantly to the longevity and reliability of dental restorations, rendering them an efficacious solution for dental prosthetic applications [98].

Aesthetics is another paramount concern in dental prosthetics [99]. People seek dental restorations that not only restore functionality but also blend seamlessly with their natural teeth, enhancing their appearance and confidence. Co–Cr alloys cater to this need by allowing for the creation of thin yet robust dental appliances, offering a more refined and less bulky appearance. The ability to fabricate thinner restorations also facilitates better gum health and easier cleaning, contributing to the overall aesthetic appeal and maintenance of the dental work. Moreover, Co–Cr alloys exhibit excellent biocompatibility, ensuring that they interact favorably with the biological tissues within the oral cavity. The absence of elements like nickel (Ni) [100], which could cause allergic reactions in some individuals, makes Co–Cr alloys safer and more comfortable for a broader spectrum of patients. This biocompatibility contributes to the overall success and acceptance of Co–Cr-based dental restorations, ensuring that patients can wear them with comfort and confidence over extended periods.

In Figure 4, oral rehabilitation for partial edentulism (PEd) involves the use of RPD or assembled prosthetic works (APWs). APWs consist of fixed (Pa) and removable (Pb) components, enhancing the ease of cleaning and chewing [101]. Typically, these prosthetics are crafted from Co–Cr alloys. To maintain research consistency, the design of the Pa framework was standardized across five Co–Cr alloys (0-A, 5-A, 10-A, 15-A, 16.4-A). The study examined three fabrication methods: refractory duplicate (RD, Pb), direct construction (DC, Pb⁻), and casting over metal (CoM, Pb⁺). Notably, the CoM method eliminated the need for Pa-Pb⁺ alignment and the adaptation process, resulting in a 91.7% improvement in component joining precision over RD and an 80.62% improvement over DC. The precision efficiency ranking of these methods is as follows: CoM, DC, and RD.

Figure 4

Diagram of the transition from framework P (individual framework) to the series production of Pa frameworks by using the Ma framework wax model. Reproduced from the study of Uriciuc et al. [101], published by MDPI, 2021.

Technological innovations have significantly contributed to the heightened utility of Co–Cr alloys within dental contexts. Contemporary fabrication methodologies, such as CAD/CAM, have transformed the production landscape for dental prosthetics, facilitating unprecedented precision and customization capabilities. These advancements permit the synthesis of Co–Cr alloy-based prosthetics that boast complex geometries and superior fit and finish, thereby enhancing both their aesthetic and functional attributes. In summation, Co–Cr alloys embody the requisite characteristics for advanced dental prosthetics, adeptly harmonizing robust mechanical properties with aesthetic finesse. Their outstanding durability, biocompatibility, and seamless integration with cutting-edge manufacturing technologies render them an essential component in the progressive development of dental restoration materials. Consequently, Co–Cr alloys epitomize the synthesis of scientific rigor and artistic precision in dental applications, marking a transformative phase in the materials and methodologies employed in dental prosthetics.

3.1.4 Co–Cr alloys in dental prosthetics

Co–Cr alloys have become indispensable in the field of dental prosthetics, merging functionality with biocompatibility to meet the diverse needs of patients and dental professionals alike. This composition of metals, primarily involving cobalt (Co) and chromium (Cr), is skillfully manipulated to create a variety of dental prosthetics, such as crowns, bridges, and partial dentures, due to their commendable mechanical properties, corrosion resistance, and affordability. Initially, the core tenet behind the utilization of Co–Cr alloys resides in their physical and mechanical properties. They possess a significant level of hardness and strength, which is instrumental in bearing the substantial masticatory forces exerted during chewing and biting. These alloys are resilient and offer exceptional durability and wear resistance, thereby ensuring the longevity of dental restorations. This is particularly crucial in the high-load bearing areas of the oral cavity, where the prosthetic materials are subjected to continuous stress and strain [102].

Co–Cr alloys are distinguished by their superior biocompatibility, an essential characteristic of materials used in dental prosthetics. They demonstrate exceptional affinity with the biological tissues of the oral cavity, ensuring minimal adverse biologic interactions and optimal tissue integration. The inherent corrosion resistance of Co–Cr alloys enhances their biocompatibility by reducing the release of metallic ions into the oral environment, which could potentially trigger toxic responses or hypersensitivity in susceptible individuals. These attributes render Co–Cr alloys a reliable choice for promoting oral health and safeguarding the integrity of dental tissues adjacent to prosthetic devices. Moreover, Co–Cr alloys excel in the aesthetic domain, a critical aspect of dental prosthetics. They facilitate the fabrication of slender, intricately detailed prosthetic structures that preserve a low profile while effectively restoring both the function and aesthetics of teeth. Furthermore, Co–Cr alloys can be skillfully employed as a framework over which ceramic or other visually appealing materials may be layered, thereby merging structural strength with aesthetic appeal. This combination ensures that the prosthetics not only meet functional requirements but also blend seamlessly with the natural dentition, enhancing the overall visual harmony.

In Figure 5, Co–Cr alloys, known for their robust mechanical properties, have transitioned from traditional casting to newer techniques like milling, laser melting, and presintered milling for dental and implant constructions [103]. This study explores these alloys’ hardness, yield strength, elastic modulus, and microstructure, categorized by manufacturing method, and examines the influence of heat treatment. Five Co–Cr alloys in various shapes are evaluated: cast, milled, laser melted, and presintered milled. Comparison is made with pure Ti grade 4 and Ti–6Al–4V (vanadium) ELI. Results show that laser-melted and presintered Co–Cr alloys have the highest mechanical properties and finer grain sizes. Ti–6Al–4V (vanadium) ELI exhibits superior hardness and yield strength over pure Ti grade 4. No significant differences are found after heat treatment. In summary, laser melting and presintered milling offer superior mechanical properties for Co–Cr alloys compared to casting and milling.

Figure 5

The backscatter micrographs of heat-treated specimens: (a) Cast specimen with large grains, dendrites (darker gray), and white secondary phase particles (lighter gray), grain size >1 mm. (b) Milled specimen with secondary phase particles (white) along glide planes, possibly due to prior material deformation. (c) Laser melted specimens with ordered grain morphology typical for the process, grain size 20–100 mm, represented by channeling and/or strain contrast. (d) Presintered milled specimens showing many pores (black), grain size 10–100 mm, and twin boundaries with twinned grains (orange arrow) due to channeling contrast. Original magnification: ×1,000 ((a), (b), (d)), ×2,000 (c). Reproduced from the study of Kassapidou et al. [52], published by Elsevier, 2023.

Economic considerations further elevate the status of Co–Cr alloys in dental prosthetics. They offer a cost-effective alternative to other high-end materials such as Au and Ti, making dental restoration more accessible to a broader range of patients. This affordability does not compromise the quality and performance of the prosthetics, as Co–Cr alloys maintain a commendable standard of durability, functionality, and aesthetics. However, despite their numerous benefits, it is also essential to consider the challenges associated with the use of Co–Cr alloys. Allergic reactions, while rare, can still occur in some patients, necessitating careful consideration and patient history evaluation before opting for these materials [104]. In addition, the manipulation and casting of Co–Cr alloys require specialized knowledge and technical expertise, underscoring the need for dental professionals to be proficient in their use and handling to maximize their benefits and minimize potential complications.

In conclusion, Co–Cr alloys stand as a pillar in the landscape of dental prosthetics due to their multifaceted benefits ranging from exceptional mechanical properties to excellent biocompatibility. Their role in enhancing the durability, functionality, and aesthetic appeal of dental restorations is instrumental, providing a versatile and economical solution that caters to the diverse and dynamic needs of modern dentistry. Their thoughtful application, combined with a nuanced understanding of their properties and potential challenges, is crucial in leveraging their capabilities to foster optimal oral health outcomes in dental prosthetics.

3.1.5 Art of alloying: Enhancing utility with Ni–Cr alloy

The world of dental prosthetics has witnessed numerous advancements over the years, driven by a relentless quest for materials that combine strength, durability, and biocompatibility. In this intricate journey, the science of alloying emerges as an essential component. Alloying, the art of combining two or more metals to achieve enhanced properties, has led to the creation of a myriad of materials tailored to meet the diverse needs of various industries, including the medical and dental fields. Among these, the Ni–Cr alloy stands as a testament to the triumph of this science, particularly in the realm of dental prosthetics. Ni–Cr alloys are renowned for their outstanding mechanical properties, exemplary of the achievements possible through the meticulous manipulation of elements [105]. The amalgamation of Ni and Cr results in a material characterized by high tensile strength and a commendable level of hardness [106]. These attributes render it an excellent choice for dental prosthetics, where the need for a robust and durable material is not just preferred but is an absolute necessity. After all, the oral environment is a complex, dynamic system – a battlefield where materials are subjected to a variety of forces and elements, necessitating not only strength but resilience.

One of the defining characteristics of the Ni–Cr alloy is its extraordinary resistance to corrosion [107]. In the context of dental prosthetics, this is a golden attribute. The oral cavity is a highly corrosive environment, harboring a plethora of chemical substances, including acids and enzymes that can wage war against materials. Corrosion resistance is not just about maintaining the structural integrity of the prosthetic; it is intrinsically linked to the biocompatibility of the material. A corroded material can release ions into the oral environment, leading to toxic reactions and allergies [108]. Ni, when alloyed with Cr, exhibits an enhanced ability to withstand the corrosive influences of the oral environment. Cr, with its remarkable knack for forming a passive oxide layer on the surface of the alloy, acts as a shield. This invisible, yet potent, barrier is resistant to the corrosive substances found in the mouth, ensuring that the alloy remains unscathed, its structural integrity unimpaired, and its biocompatibility unchallenged.

The narrative surrounding Ni–Cr alloys in dental prosthetics encompasses a complex interplay of advanced materials science and biocompatibility issues. Nickel, recognized as a potent allergen, has elicited concerns from both dental professionals and patients. Although adverse reactions are uncommon, their occurrence necessitates a rigorous evaluation of the alloy’s use in clinical applications. This scenario epitomizes the delicate balance between leveraging the superior mechanical and corrosion-resistant properties of Ni–Cr alloys and mitigating allergenic risks, highlighting the critical role of tailored patient assessments and the relentless pursuit of alloy refinement. At the forefront of innovation within the domain of Ni–Cr alloys, research endeavors are robust, marked by a relentless drive to reduce the allergenic properties of nickel while enhancing the alloy’s intrinsic benefits. Advanced coating technologies, precision alloying techniques, and the investigation of novel substitute materials are at the cutting edge of strategies aiming to redefine Ni–Cr not merely as a viable option but as the premier choice for dental prosthetics. These scientific advancements underscore a broader commitment to optimizing material performance and patient safety in the field of dental materials science [109,110].

The evolution of Ni–Cr in dental prosthetics is a narrative of triumphant innovation, of the unyielding human spirit that seeks to conquer nature’s limitations. It is a journey characterized by the harmonious blend of art and science the art of alloying, where metals unite to form an entity greater than the sum of its parts, and the science of engineering, where every atom, every molecule, and every alloy is meticulously crafted to meet the stringent demands of the human body [111]. As we stand on the threshold of new discoveries, with technologies such as additive manufacturing and nanotechnology transforming the landscape, the story of Ni–Cr is still being written. Every challenge is an opportunity, and every limitation a stepping stone to new horizons. The art of alloying continues to evolve, not just in the quest for the perfect material but in the undying human aspiration for excellence – where science, art, and the human spirit unite to transform not just metals, but lives. In the world of dental prosthetics, Ni–Cr is not just an alloy; it is a symbol of human ingenuity, resilience, and the unyielding pursuit of perfection [111,112].

3.1.6 Advanced coatings for enhanced biocompatibility and durability of magnesium implants

Magnesium alloys rank among the less toxic, one of the most biocompatible, and the most biodegradable biomaterials, thus making them strong candidates for use as orthopedic and dental implants. Nevertheless, their short maturation time in physiological samples questions their stability and efficiency. This is being addressed with advanced coating technologies which revolutionizes their use in medical and sports injury applications, improving their durability and functionality [114].

3.1.7 Aesthetic appeal through alloy fusion

In the dynamic world of dentistry, aesthetics often holds as much importance as functionality. As patients increasingly demand dental restorations that do not just serve their primary purpose but also look good, dental professionals are on a continual quest to find materials that meet both criteria. Among the myriad of options available, the fusion of Ni and Cr stands out. This alloy combination, often referred to as Ni–Cr, has proven to be both durable and aesthetically pleasing, making it a popular choice for various dental applications. The use of metals in dental restoration is not new. Au, for instance, has been used for centuries because of its durability and malleability. However, with the rising cost of Au and the demand for more cost-effective materials, dentists started exploring other metal alloys. By the twentieth century, the fusion of Ni and Cr emerged as a favorable choice. Its aesthetic silver-like appearance combined with its cost-effectiveness made it a compelling alternative to Au.

Ni–Cr alloys, commonly composed of approximately 80% nickel and 20% chromium – though variations in this ratio exist exhibit superior corrosion resistance, primarily attributable to the chromium content. Chromium contributes to the formation of a coherent oxide layer on the alloy’s surface upon exposure to oxygen. This oxide layer acts as a barrier, inhibiting oxidation and material degradation. The protective layer not only prolongs the functional lifespan of dental restorations but also preserves their aesthetic quality over extended periods. In addition, Ni–Cr alloys are noted for their high biocompatibility, a critical property in dental applications where materials are in persistent interaction with the oral environment. These materials must not elicit adverse biological responses or trigger allergic reactions. Through meticulous alloying and processing, Ni–Cr alloys minimize the likelihood of allergic responses or negative reactions in the majority of patients [119,120].

The aesthetic appeal of Ni–Cr is one of its standout features. Unlike some other metals that may discolor/decolorize or tarnish over time, Ni–Cr maintains its lustrous appearance [116]. Its natural silver-like hue closely resembles the color of natural teeth, especially when compared to the yellowish tint of Au. This makes it easier for dentists to achieve a natural look, especially when Ni–Cr is used in conjunction with porcelain or ceramic materials. Further, with advancements in dental technology, Ni–Cr can be manipulated to achieve varying degrees of translucency. This allows for even more accurate matching with the patient’s natural teeth, ensuring a seamless integration of the restoration into the oral environment.

Ni–Cr alloy finds application in various dental restorations, including crowns, bridges, and dentures. Crowns made of Ni–Cr alloy, often layered with porcelain, offer both strength and beauty. The underlying metal provides the necessary strength to withstand chewing forces, while the porcelain overlay imparts a lifelike appearance. Moreover, Ni–Cr’s ability to bond well with dental ceramics makes it an ideal choice for fixed dental bridges. These bridges are designed to replace missing teeth, and the aesthetic compatibility between Ni–Cr and ceramics ensures a natural-looking result. While Ni–Cr offers many advantages, it is essential to acknowledge its limitations. Some patients might have a Ni allergy, making it imperative for dental professionals to screen for such allergies before opting for Ni-Cr restorations. In addition, while Ni–Cr is resistant to corrosion, poor oral hygiene can still lead to issues like plaque accumulation around the restoration, which can compromise aesthetics over time.

In the domain of dental materials science, Ni–Cr alloys have emerged as a pivotal material due to their robust integration of mechanical strength, corrosion resistance, and aesthetic qualities. These properties render Ni–Cr alloys particularly advantageous for dental applications, aligning durability with visual harmony, thus garnering preference among dental professionals and patients. The intrinsic qualities of Ni–Cr, including its formidable structural integrity and excellent biocompatibility when alloyed, enable the creation of dental restorations that not only fulfill functional requisites but also aesthetically complement the patient’s dentition. Moreover, Ni–Cr alloys’ compatibility with advanced ceramic materials enhances their application in crafting prosthetic devices that are both efficacious and visually appealing. This synergy between metal and ceramic is instrumental in producing restorations that mimic the natural luster and translucency of tooth enamel, thereby elevating the overall aesthetic outcome.

In the broader context of dental technology, the strategic utilization of Ni–Cr alloys is integral to developing customized solutions that address individual patient profiles, particularly considering variability in oral biomechanics and potential metal sensitivities. Emphasizing a tailored approach in the deployment of these materials is crucial to optimizing therapeutic outcomes. Thus, the integration of Ni–Cr in dental restorative procedures not only marks a significant advancement in dental materials science but also enhances patient satisfaction through improved functional and aesthetic results, affirming its role as a transformative element in contemporary dental practice.

3.1.8 The revolution in dental aesthetics

The inclusion of Ni–Cr alloys in dentistry marked a turning point in the quest for natural-looking dental restorations. The fusion of this alloy with porcelain paved the way for dental work that was not only functional but also indistinguishable from natural teeth. As dental technologies continue to evolve, and patient expectations rise, materials like Ni–Cr will play an instrumental role in bridging the gap between functionality and aesthetics. In conclusion, the aesthetic appeal of Ni–Cr in dental applications has elevated the standard of dental restorations, offering a harmonious blend of strength, durability, and natural appearance. As dentistry continues its journey toward merging health, function, and beauty, the role of materials like Ni–Cr, with their intrinsic blend of science and art, becomes ever more critical.

3.2.1 Zirconia and alumina ceramics

Zirconia and alumina ceramics have carved a significant niche in the dental domain, owing to their superior mechanical and aesthetic properties. Dentistry, a field that combines both the art of aesthetics and the robustness of mechanical engineering, has welcomed these materials as solutions for a myriad of applications, ranging from dental implants to crowns and bridges. Zirconia, scientifically known as zirconium dioxide (ZrO₂), is renowned for its strength, durability, and biocompatibility. These characteristics make it an ideal choice for applications where strength is paramount, such as in the creation of dental implants and crowns. Zirconia’s color and translucency can be tailored to match the natural teeth, offering an aesthetic appeal that is often superior to other restorative materials.

Polymorphic zirconia structure is found in three different crystal forms: monoclinic (M), cubic (C), and tetragonal (T). At ambient temperature, zirconia takes on a monoclinic structure [126]. At 1,170°C, it transforms into a tetragonal phase, and at 2,370°C, it becomes a cubic phase. These phases are unstable at ambient temperature and fragment upon cooling. By adding CaO, MgO, and Y₂O₃ (Yttrium) to pure zirconia, the C-phase can be stabilized [127]. This produces a multiphase material known as partially stabilized zirconia, which combines the cubic, monoclinic, and tetragonal phases in the order of significance. Yttrium can be added to tetragonal zirconia polycrystals (TZP) at room temperature to produce a material that exclusively contains the tetragonal phase [128]. Yttria-stabilized titanium dioxide (TZP) exhibits favorable properties for biomedical applications, including low porosity, high density, strong bending, and compression strength.

Furthermore, zirconia has an edge due to its crack-resistant property and low thermal conductivity, ensuring both the comfort and safety of dental patients. The material’s ability to withstand wear and tear not only elevates its functional efficacy but also extends the longevity of the dental restorations. Enhanced by its capability to be milled to precise dimensions, zirconia has become a favorite for CAD/CAM systems in modern dentistry. On the other hand, alumina, or aluminum oxide ceramics brings their unique set of advantages to the table. Renowned for their excellent biocompatibility and hardness, alumina ceramics are resistant to wear and corrosion. Their inert nature makes them a suitable option for patients with allergies to metals, ensuring that adverse reactions are kept at bay. Although less translucent than zirconia, alumina is still capable of delivering satisfactory aesthetic outcomes, especially when high strength is a priority.

The advent of technology has further augmented the properties of these ceramic materials. Modifications and improvements in their microstructure have led to enhanced mechanical properties, including increased strength and toughness. For instance, the transformation toughened zirconia exhibits a metamorphosis in its crystal structure under stress, enhancing its toughness and resistance to crack propagation. In the realm of dentistry, the role of zirconia and alumina ceramics is not limited to their mechanical prowess. The aesthetic outcomes, a crucial aspect of dental restorations, are profoundly influenced by the optical properties of these materials. The ability to customize their color and translucency to impeccably mimic the natural dental structures has rendered these ceramics as preferred choices for restorations that are both functional and visually appealing.

Moreover, the integration of digital technology in dentistry, including intraoral scanners and CAD/CAM systems, has streamlined the utilization of zirconia and alumina. Dentists can now achieve precise fittings and optimal aesthetics with reduced chair-time, enhancing the patient experience. The digital workflow, from the diagnosis and treatment planning to the fabrication and placement of restorations, has been revolutionized, with zirconia and alumina ceramics at the forefront [129]. Their compatibility with various bonding systems, including adhesive cementation and mechanical retention, adds another feather to their cap. Zirconia, particularly, exhibits excellent bonding with resin cements, ensuring the stability and longevity of restorations. The ease of modification and adaptability of these materials have facilitated the development of restorative solutions tailored to individual patient’s needs and preferences.

Despite their myriad advantages, it is essential to consider the limitations and challenges associated with zirconia and alumina ceramics. Like any material, they are not devoid of drawbacks. Understanding these limitations, such as the potential for low-temperature degradation in zirconia and the less aesthetic appeal of alumina compared to zirconia, is pivotal for maximizing their clinical efficacy. In conclusion, zirconia and alumina ceramics have emerged as quintessential materials in contemporary dentistry. Balancing mechanical strength, biocompatibility, and aesthetics, they offer a comprehensive solution for a range of dental applications. Their evolution, propelled by technological advancements and extensive research, is a testament to the ongoing quest for excellence in delivering optimal oral health care [130]. As technology and research progress, we can anticipate further refinements and innovations in the application of zirconia and alumina ceramics in the dental domain, fostering a synergy of functionality, durability, and aesthetics.

3.2.2 Advantages of zirconia–alumina ceramics in dental domain

• High strength and toughness: Zirconia, especially Y-TZP, demonstrates high strength and fracture toughness, making it suitable for load-bearing dental applications like crowns and bridges [131].

• Biocompatibility: Both zirconia and alumina ceramics exhibit excellent biocompatibility, causing minimal inflammatory reactions when in contact with oral tissues.

• Esthetics: Zirconia’s optical properties can be tailored to closely resemble that of natural teeth, making restorations almost indistinguishable.

• Low wear: Zirconia and alumina are wear-resistant materials, which lead to less wear of the opposing natural tooth structure.

• No metal infrastructure: There is no metal framework beneath, eliminating any possible metal allergies or grayish discoloration at the gum line.

• Cementation flexibility: Zirconia restorations can be cemented using traditional cementation methods or adhesive bonding, providing flexibility for the dentist.

• Low thermal conductivity: Zirconia and alumina have low thermal conductivity, which provides insulation against temperature changes, thus reducing tooth sensitivity.

• Resistance to corrosion: Unlike metals, these ceramics resist corrosion in the oral environment, ensuring longevity and maintaining esthetics.

• Digital workflow integration: Zirconia restorations can be easily integrated into digital dentistry workflows, from digital impressions to CAD/CAM fabrication.

• Variety in translucency: Modern zirconia ceramics come in varying translucencies, from opaque for posterior teeth to highly translucent for anterior restorations.

3.2.3 Limitations of zirconia–alumina ceramics in dental domain

1. Brittleness: Like all ceramics, zirconia and alumina are inherently brittle. Though they have high strength, they lack the flexibility of metals.

2. Technique sensitivity: The success of zirconia restorations can be technique sensitive. Improper preparation or bonding procedures can result in restoration failure.

3. High stiffness: Zirconia’s stiffness, while providing strength, may lead to stress concentration and potential fracturing in certain situations.

4. Aging concerns: Zirconia, especially Y-TZP, may undergo low-temperature degradation over time in moist environments, which might reduce its mechanical properties [132].

5. Limited repairability: If a zirconia restoration chips or fractures after cementation, intraoral repair can be challenging.

6. Cost: The materials and technology associated with zirconia–alumina restorations tend to be more expensive than traditional materials.

7. Need for specialized equipment: Milling and sintering zirconia require specialized equipment, increasing setup costs for dental labs.

8. Wear of opposing dentition: Even though zirconia is wear resistant, if not polished adequately, it can cause wear on the opposing natural teeth.

9. Thickness requirement: Zirconia restorations require a certain thickness for optimal strength, which can necessitate more tooth reduction than some other materials.

10. Bonding challenges: Achieving a reliable bond between zirconia and the tooth or resin cements can be tricky and may require specific primers or treatments.

In conclusion, while zirconia–alumina ceramics offer several advantages in the dental domain, such as strength, esthetics, and biocompatibility, there are also limitations to consider. The choice of material should be based on the individual patient’s needs and the clinical situation.

3.3.1 Advantages and limitations

Biodegradable polymers, PEEK, PMMA, PLA, and PGA have been widely recognized in the field of dentistry for their unique sets of advantages and limitations. Each of these materials brings a distinctive set of properties that makes them suitable for various dental applications. PEEK is renowned for its impressive mechanical strength, biocompatibility, and stability, positioning it as a preferred material for dental implants and prosthetics. Its mechanical properties are comparable to that of bone, promoting osseointegration and ensuring stability of the implant over time. PEEK is also radiolucent, rendering it invisible on X-rays and beneficial for postoperative assessments. The material’s resistance to a wide range of chemicals ensures its stability in the corrosive environment of the mouth. However, PEEK is not without its limitations. The material’s color, a natural off-white, can sometimes be considered aesthetically unpleasing. Furthermore, the cost of PEEK can be a limiting factor, as it is relatively expensive to produce and process.

PMMA, on the other hand, has been a popular choice for dental prosthetics, especially dentures, due to its ease of fabrication, light weight, and aesthetics. Its translucency and ability to be tinted allows for the creation of natural-looking teeth and gums. Moreover, PMMA is relatively inexpensive, making it accessible to a wider patient population. Nevertheless, PMMA is often criticized for its lower mechanical strength compared to materials like PEEK. It can fracture under stress, leading to a need for frequent replacements. Furthermore, some patients might experience allergic reactions to the monomers released from the polymerized material.

Biodegradable polymers like PLA and PGA are gaining traction in dentistry for their eco-friendly and bioresorbable nature. PLA has been employed in the fabrication of screws, plates, and other fixation devices for its high tensile strength and modulus. It is renowned for reducing the need for a second surgery to remove the implant, as it naturally degrades in the body over time. However, the rate of degradation can be a limitation, as it is crucial to balance the degradation rate with the tissue healing rate to ensure structural support during the healing process. The material’s mechanical properties might also be inadequate for load-bearing applications.

PGA, similar to PLA, is appreciated for its biodegradability. It has a faster degradation rate, making it suitable for temporary implants that only need to provide support for a short period. The limitation, however, lies in its rapid loss of mechanical strength due to the quick degradation rate, which might not be suitable for all clinical applications. Both PLA and PGA also have limitations in terms of their thermal processing, which can sometimes affect their mechanical properties and degradation behavior. In the grand scheme of dental applications, the choice between PEEK, PMMA, PLA, and PGA is highly contingent on the specific application, the required mechanical properties, the aesthetic considerations, and the patient’s specific needs and constraints. PEEK is often chosen for its strength and biocompatibility, especially for long-term implants. PMMA is prevalent in prosthetics and dentures for its aesthetics and affordability. Meanwhile, biodegradable polymers like PLA and PGA are carving a niche for themselves for temporary implants and environmentally friendly options.

In conclusion, each of these materials is associated with a distinct combination of advantages and limitations. The evolution of dental materials is an ongoing journey, with continuous research aimed at optimizing the properties of existing materials like PEEK, PMMA, PLA, and PGA, and exploring new materials to overcome the present limitations. Balancing factors such as mechanical strength, biocompatibility, aesthetics, and cost will continue to be central in the advancement of dental materials to meet the diverse and evolving needs of patients and clinicians alike. The optimal material would offer a harmonious blend of strength, biocompatibility, aesthetics, and affordability, tailored to the specific requirements of each dental application. In the continuous quest for perfection, the collaboration between material scientists, engineers, and dental practitioners is pivotal, marrying theoretical knowledge with practical insights to innovate and refine the next generation of dental materials.

3.4.1 Composites materials

3.4.2 Carbon and carbon-silicon based materials

Carbon and carbon-silicon based materials are often classified as ceramics because of their chemical inertness. These are also classified as metal free dental implants. These are good conductors of heat and electricity. Mainly, carbon and carbon-silicon-based materials are used as surface coatings in metallic and ceramic implants [164]. Their main disadvantages are (1) lack of mechanical strength, prone to scratching and (2) biodegradation may adversely affect tissue stability.

3.4.3 Inorganic compounds as filler materials

Several inorganic substances have shown promising outcomes in terms of remineralization and direct and indirect effects on biofilm formation. These include boron nitride, BAG, nanosized amorphous CaP, calcium fluoride (CaF₂), and HAP. The chemicals’ effects differed based to their composition, the percentage or amount incorporated, and the intended clinical use. The remineralizing effects were shown as either indirect-like increasing the pH in the area around the material- or direct-like raising the mineral content of the dental tissue. To enhance the results of remineralization and the antibacterial action against the cariogenic biofilms, certain studies have documented combining inorganic remineralizing chemicals with other bioactive agents, such as quaternary ammonium compounds [165]. The more inert polymers have been combined with particulate or fibers of carbon, Al₂O₃, HAP, and glass ceramics. Some are porous, but others are constituted as solid-composite structural forms. Biodegradable polymers, such as polyvinyl alcohol polylactides or glycolides, cyanoacrylates, or other hydratable forms, have been combined with biodegradable CaPO₄ particulate or fibers.

3.4.4 Fiber-based materials

3.4.5 Glass-based materials

3.4.6 Silicon nitride (SiN)

SiN is an emerging material for dental implant applications. Due to its exceptional mechanical, physical, thermal, structural, and biological qualities, silicon nitride has showed promise for use in medical and dental implants. Among the qualities that are most impressive and clinically significant are the antibacterial surface and the ability of surface changes. In comparison to titanium implants, SiN dental implants have shown mechanical characteristics that approximate bone tissues, lowering stress shielding. Three polymorphic structures of silicon nitride exist: a cubic (γ-Si₃N₄) high-pressure/high-temperature modification with spinel structure, and two hexagonal α- and β-Si₃N₄ modifications. The structures of hexagonal silicon nitride are represented in Figure 8 as somewhat distorted SiN₄ tetrahedra that share corners to form deformed hexagonal rings stacked in layers with ABAB (β-Si₃N₄) and ABCD (α-Si₃N₄) stacking sequences.

Figure 8

Structures of hexagonal β- and trigonal α-silicon nitride. Arrangement of Si and N atoms in hexagonal β- and α-silicon nitride shown projected down the c axis onto the AB plane [178]. With permission from Wiley-VCH. Black circles: silicon atoms, white circles: nitrogen atoms.

3.4.7 Dental resin composites (DRCs)

DRCs are widely used materials for caries restoration. DRCs are tooth-colored restorations that are used to replace tooth structure that has decayed. The principal advantage over traditional dental amalgam is its aesthetic appearance. A resin-based matrix, like bisphenol A-glycidyl methacrylate, plus an inorganic filler, like silica, make up a typical composite resin. The composite’s mechanical properties, wear resistance, and translucency are all enhanced by the filler.

Restoration failures persist despite the development of diverse DRC types with varying properties. The failure of composite restorations has been attributed mostly to bulk fracture and subsequent caries. Numerous fillers with specialized purposes have been introduced and investigated to address these issues. Fillers that release ions, such Ag⁺, Ca²⁺, and F⁻, have been utilized to reduce secondary caries, while fillers with specific morphologies, including whisker, fiber, and nanotube, have been used to improve the mechanical qualities of DRCs. The functionality and longevity of DRCs are enhanced by these fillers [179].