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INVITED REVIEW
Year : 2015  |  Volume : 7  |  Issue : 3  |  Page : 148-159

Trends in prosthetic biomaterials in implant dentistry


Department of Prosthodontics, Faculty of Dentistry, Jamia Millia Islamia, New Delhi, India

Date of Web Publication31-Dec-2015

Correspondence Address:
Saranjit Singh Bhasin
Department of Prosthodontics, Faculty of Dentistry, Jamia Millia Islamia, New Delhi - 110 025
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/2231-0754.172936

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   Abstract 

The most important criterion for the success of dental implants is the selection of a suitable implant biomaterial. To improve the biologic performance of an implant, it is necessary to select a material that does not elicit any negative biological response and at the same time maintains adequate function. It is mandatory for a dentist to have a comprehensive knowledge of various biomaterials used for dental implants. The material of choice for fabrication of the dental implant till date is titanium. With the advancements in the field of implants, zirconia seems to be propitious in the future. However, more advanced in vitro and in vivo studies are required before reaching any such conclusion. To increase the success of zirconia implants, care should be taken to reduce the incidence of mechanical failures. Such failures can be taken care of by having a thorough technical knowledge of implant designing and manufacturing defects. This article attempts to compare the advantages and disadvantages of various dental implant biomaterials. Focus is placed on the recent advances in this field with the recently introduced zirconia and its comparison to conventional titanium.

Keywords: Biomaterial, ceramic, dental implant, titanium, zirconia


How to cite this article:
Bhasin SS, Perwez E, Sachdeva S, Mallick R. Trends in prosthetic biomaterials in implant dentistry. J Int Clin Dent Res Organ 2015;7, Suppl S1:148-59

How to cite this URL:
Bhasin SS, Perwez E, Sachdeva S, Mallick R. Trends in prosthetic biomaterials in implant dentistry. J Int Clin Dent Res Organ [serial online] 2015 [cited 2022 Aug 7];7, Suppl S1:148-59. Available from: https://www.jicdro.org/text.asp?2015/7/3/148/172936


   Introduction Top


Dental implants are today a viable and mainstream alternative to the conventional removable and fixed dental prostheses. In line with the growth and mainstreaming of this treatment form, the science of biomaterials, specifically implant materials is also evolving rapidly. The primary aim of the research and development in this branch of science is to improve the osseointegration properties of these materials.[1]

As per the European Society for Biomaterials, a biomaterial is “a non-viable material used in a medical device, intended to interact with biological system.” Biocompatibility of a material is governed by a number of other material properties including physical properties, surface configuration, corrosivity within the tissue environment, potential for triggering inflammation or rejection, and tissue induction. For a material to be successfully implanted within vital bone tissue, it should possess an interface surface on which viable tissues can proliferate.

The behavior of an implant biomaterial in situ and its functional performance are affected by a number of biomechanical aspects. These biomechanical aspects relate to the macroscopic implant shape as well as microscopic transfer of stress and strain from the biomaterial to the tissue interfaces. On the other hand, the microscopic strain distribution at a more localized level is controlled more by the basic properties of the biomaterials including surface chemistry, microtopography, modulus of elasticity, and whether the biomaterial surface is chemically bonded to the adjacent tissues. These are some of the important considerations that govern the application of a biomaterial as an implant device.

The quest to develop new and improved implant materials that are biocompatible, support rapid healing of interface tissues, are dimensionally stable, and are easy to shape and work with remains an ongoing area of research interest globally.[2] This paper attempts to examine relevant properties of prosthetic biomaterials and through a comparative review, delineate some of the key properties that may govern the successful application of these materials in dental implantology.


   History of Dental Implant Biomaterials Top


The history of dental implants dates back to prehistoric times. Implant designs are traceable to the early Egyptians and South Central American cultures.[3] The earliest dental implants, as cited in the archaeological records of China and Egypt, were of stone and ivory. The Common Era saw the use of carved seashells and stones as implant materials for the replacement of missing teeth.[4] The eighteenth century witnessed the emergence of endosseous oral implantology as a science. In the modern era, materials derived naturally were replaced by synthetic polymers, ceramics, and metal alloys. These materials performed better as implant materials and had more predictable results than the natural ones.

Another milestone in dental implantology was achieved in 1957 when a Swedish orthopedic surgeon, Per-Ingvar Brånemark, while studying bone healing and regeneration, discovered that bone could grow in proximity with titanium (Ti), and that it could effectively adhere to the metal without being rejected.[5] This phenomenon was termed “osseointegration” and became a breakthrough in the predictability of success rates in dental implantology. Titanium was recommended as a dental implant material by the United States (US) Food and Drug Administration in 1982. Since the 1990s, there has been tremendous development in the area of modern ceramics and from thereon, dental implant manufacturing companies have extensively focused on the surface treatments of ceramics and ceramic-like elements with the intent to enhance osseointegration and thus, improve the overall implant success.[5],[6]


   Types of Prosthetic Implant Biomaterials Top


Materials used in dental implantology can be broadly categorized in two different ways [Figure 1].[2],[7]
Figure 1: classification of implant biomaterials

Click here to view


  1. Based on the chemical composition of the material; and
  2. Based on the biological response of the material.


From a chemical composition standpoint, the relevant materials may be classified as:

  • Metals are elements that are crystalline when solid; many metals are characterized by opacity, ductility, conductivity, and a unique luster.
  • Ceramics are hard, brittle, and heat and corrosion-resistant materials made typically of metallic elements combined with oxygen or with carbon, nitrogen, or sulfur.
  • Polymers are compounds of high molecular weight derived either by the addition of many smaller molecules or by the condensation of many smaller molecules with the elimination of water, alcohol, or the like. These materials are relatively inert to biodegradation and have certain properties that are similar to soft tissues.


From a biological response standpoint, the relevant materials may be classified as:

  • Biotolerant materials are those that are not necessarily rejected when implants are placed into living tissues but are surrounded by a fibrous layer in the form of a capsule.
  • Bioinert materials allow apposition of bone on their surface leading to contact osteogenesis.
  • Bioactive materials also allow the formation of new bone onto their surface but ion exchange with host tissues leads to the formation of a chemical bond, along with the interface (bonding osteogenesis). A recent development is the development of “biomimetic” materials, which are new tissue engineered materials designed to mimic specific biologic processes and help optimize the healing or regenerative responses of the host microenvironment. These materials can be any combination of the chemical and biodynamic activity categories, depending on the therapeutic strategy and the type of host tissues.[2]



   Properties of Biomaterials Top
[2],[8],[9]

The primary property that a biomaterial needs to have is its acceptability by the human body. Further, an implant biomaterial should have very high corrosion and wear resistance and should also possess high mechanical strength so that it is not susceptible to fatigue failure or fracture under varying load conditions.

A discussion on the most relevant properties of implant biomaterials and associated physiologic implications in the context of dental implantology and the biomechanics involved is presented here under.

Mechanical properties

The relevant mechanical properties of biomaterials in the context of their application in dental implant fabrication include:

  • Modulus of elasticity.
  • Strength (tensile, compressive, shear, yield).
  • Other mechanical properties [Table 1].


Modulus of elasticity

Modulus of elasticity is a property that characterizes a material's elastic response to mechanical stress such as externally applied forces associated with any muscle action or occlusive activity. To enable uniform distribution of stresses at the implant surface and to minimize relative movement at the implant-bone interface, implant biomaterials should have a modulus that is as close as possible to that of the host bone. Depending on the type of the bone and the measurement direction, elastic modulus of the human bone can have a variance of 4-30 Gigapascals (GPA). Stainless steel has an elastic modulus of about 200 GPa; for titanium it is around 117 GPa, for titanium alloys it is around 110 GPa, and for alumina, it is around 400 GPa. A very significant variance between the modulus of the implant material and the host bone leads to aseptic loosening since the material tends to “stress shield” the underlying bone, taking most of the load and over time it loses its biological contact with neighboring tissues; this condition would directly result in failure of the implant.[10]
Table 1: Engineering properties of metals and alloys used for surgical implants

Click here to view


Strength

Tensile or compressive strength characterizes the dimensional stability of a material when a force is applied. A biomaterial being used for dental implant applications should have high tensile and compressive strength to ensure functional stability over time and to be resistant to fractures or deformation. Another relevant property is referred to as “elongation at break” or “elongation at fracture,” which represents the capability of a material to resist changes of shape without fracturing or crack formation. Shear strength is characterized by the material's ability to transfer stress from the implant to the underlying bone; the overall stresses acting on an implant reduce as interfacial shear strength increases. Also, a material being considered for use in dental implant needs to have high yield strength and fatigue strength to minimize the risk of brittle fractures occurring under long-term cyclic loading.[8],[9]

Other mechanical properties

Another relevant property is the ductility of the material, which affects the ability of the material to be shaped or contoured. Ductility values of approximately 8% or more are typically required for dental implant applications so that the implant can be shaped to suit the host site. Hardness and toughness are also relevant properties for a biomaterial being considered for dental implant applications; the implant typically becomes more wear-resistant as its hardness increases and with increase in toughness of the implant material, its resistance to fractures improves. For metallic implant biomaterials, an exquisite balance among the above properties governs the materials' amenability toward “coining,” a process of shaping in a die or a mold, which allows precise and planned modifications of the metallic grain orientation and size distribution, thereby resulting in reduced metal fatigue associated with cyclic loading in the long term.[8],[9]

Biocompatibility[11],[12],[13]

Biocompatibility is the property of a material to sustainably exist within a surrounding biological environment without having toxic or injurious effects on the biological functions. This property is possibly the most significant consideration while considering a material for implant applications, given that the implant is intended for long-term placement within the human oral cavity, crossing the protective epithelial zones and embedded into the host bone.

One of the key factors that govern overall biocompatibility of a material is corrosion. Corrosion is the loss of metallic ions from the surface of a metal to the surrounding environment. Given that electrolyte and oxygen composition within the oral cavity varies when compared to the same within tissue fluids, and given that pH values within the oral cavity also vary significantly as compared to the same in areas below plaque, it may be concluded that a dental implant is exposed to a significantly corrosive environment in the long term. Therefore, corrosion resistance is a key property required for implant biomaterials, particularly for the metallic implant materials. In addition to intrinsic corrosion resistance properties, the surface conditions of metallic implant materials also impact long-term corrosion resistance of the implant.

The corrosion resistance properties of a metallic implant can also be affected by way of contamination from other metals. Contamination of titanium implants can occur by coming into contact with other metals or alloys; such contaminant particles get embedded within the implant surface and can cause corrosion or adverse reactions in the surrounding tissues. Therefore, extreme care should be taken during implant insertion as well as implant storage, handling, sterilization, etc., to prevent any metallic transfer and a titanium implant should be handled using only titanium or titanium-tipped tools and instruments.

However, some ongoing corrosion of metallic implants is inevitable, given that all metals ionize to some extent. Therefore, the cytotoxicity of corrosion products, specifically the primary biodegradation products, is also of concern while considering the overall biocompatibility of an implant biomaterial. The more toxic the corrosion products are and the more soluble they are, higher is the concern. Any significant leaching of metallic ions in situ may induce potentially osteolytic cytokines into tissues; this may further lead to tissue discoloration, implant loosening, and allergic reactions or hypersensitivity. However, so far, only a few instances of tumors and malignancies associated with leaching/corrosion of implants has been reported in the literature.[11],[13]

Surface properties

The factors that govern the mechanical properties and influence the primary stability and force distribution of an implant include its dimensions, shape, and type of threads, and thread design and implant-prosthesis connection, among others. However, some of the other significant factors that affect osseointegration and therefore, long-term implant viability and performance are implant surface morphology, the macrostructure, microstructure, and nanostructure of the implant surface (including associated chemical and physical properties) and homogeneity of the implant surface.

Various surface modifications have been attempted to enhance the osseointegration of implants. These surface modifications have focused on improving biological interactions between the implant surface and the host tissue, especially in relation to adsorption of proteins, adhesion, and differentiation of cells as well as in terms of tissue integration. These have included the designing of mechanical interlocking sites between implant and bone at the macrolevel using threads, fenestrations, pores, grooves, steps, or other surface irregularities; microscopic level interventions involving surface coatings and modification of surface coatings and modification of surface topography to enhance bone implant integration; and controlled formation of nanometer-scale features on implant surfaces aimed at improving the physicochemical behavior of the implant surface in terms of bone bonding and also improved the biochemical behavior in terms of protein adsorption and cell adhesion.[2],[14]


   Biomaterials Being Used in Implant Dentistry Top


Metals and alloys as dental implant materials

Most of the dental implant systems are constructed from metals or alloys. Metals are bioinert and biotolerant and have favorable mechanical properties, which make them desirable as implant material. In addition to possessing these desirable properties, metals are easy to process and have an excellent finish. Metallic implants can be sterilized easily by the routine sterilization procedures, which make them convenient to use.

The major groups of implantable materials for dentistry are titanium and its alloys, cobalt chromium alloys, austenitic Fe-Cr-Ni-Mo steel, tantalum, niobium, and zirconium alloys. With the ongoing improvement in the field of implantology, the conventional metals (gold, stainless steel, and cobalt-chromium) materials have now become outdated and are now replaced by titanium (Ti) and its alloys (mainly Ti-6Al-4V).

At the beginning, gold was used as dental implant material. It has an advantage of corrosion resistance and good biocompatibility. However, its high cost and lower mechanical strength were the major limitations, which precluded its widespread use as a dental implant material and gave way to the base metal alloys. These nonprecious alloys presented with superior mechanical properties and aesthetics for a majority of the dental applications. As far as the dental implants are concerned, they became the material of choice because of their ability to osseointegrate.[15]

Stainless steel alloys are used for orthopedic and implant devices. They have high strength and high ductility. Iron-based alloys are used as implants in the form of ramus frame, ramus blade, and stabilizer pins, etc. The alloy is susceptible to pitting corrosion and it should be used cautiously to retain the passivated (oxide) surface condition because of the presence of nickel as a major element. The use of this metal should be refrained in the patient who is sensitive to it. It possesses high galvanic potential and corrosion resistance. These alloys have lower yield strengths (255-730 MPa) than noble metal alloys. They also have higher elastic moduli (150-210 GPA) and are much harder than noble alloys. This property is helpful in casting thinner copings and frameworks. They have much lower densities (7-8 g/cm 3) and generally higher casting temperatures (1300-1450° C). Acceptable casting compensation is a problem at times, as is the fit of the coping.

Cobalt is the main constituent of cobalt-based metal-ceramic alloys, with chromium added for strength and corrosion resistance by means of passivation. The most preferred substitute for patients allergic to nickel is cobalt chromium alloys. The melting ranges of Co-Cr alloys are significantly higher among the other casting alloys, which make its manipulation complex in the laboratory. They are difficult to cast and finish in the laboratory but have less ductility as compared to the Ni-Cr alloys, which permits their use as metal frameworks. It is possible to fabricate the implant as custom designs such as subperiosteal frames.[9],[16]

As mentioned earlier, titanium (Ti) and its alloys (mainly Ti6 Al4V) are currently the most widely used dental implant materials. After heat treatment, these alloys possess many favorable physical and mechanical properties that make them excellent implant materials as they are light, strong, and highly resistant to fatigue and corrosion. They are about six times stronger than compact bone and thereby afford more opportunities for designs with thinner selections (e.g., plateaus thin interconnecting regions, rectangular scaffolds, porosities). Compared with Co-Cr-Mo alloys, titanium alloys are twice as strong and have half the elastic modulus. In fact, titanium shows a relatively low modulus of elasticity and tensile strength when compared with most other alloys.

Ceramics as dental implant material

Various ceramics used as dental implant materials are: aluminum oxide (Al2O3), zirconium oxide, hydroxapatite (HA), tricalcium phosphate, tetracalcium phosphate, calcium pyrophosphate, fluorapatite, brushite, bioglass, etc.

Merits of ceramics as a dental implant material

Ceramics are inert to biodegradation; they possess high strength and other physical characteristics suitable for implant application.

Aluminum, titanium, and zirconium oxide: These ceramics have a clear, white cream or light gray color that is beneficial for application on anterior root form devices. Minimum thermal and electric conductivity, biodegradation, and reaction to bone, soft tissue, and oral environment are also considered to be beneficial when compared with other types of synthetic biomaterials.

Calcium phosphate (CaPO4) materials such as tricalcium phosphate (TCP) and glass ceramics have excellent biocompatibility, no local or systemic toxicity, minimal thermal and electrical conductivity, no alteration to natural mineralization process of bone, and lower mechanical, tensile, and shear and fatigue strength. Apart from the use as a bone substitute, calcium phosphates have been considered as a good option for implant coatings that may promote accelerated bone healing around implants.[17],[18]

The latest ceramic to be used as dental implant is zirconia (zirconium dioxide). It is found to possess good mechanical properties owing to its multiphase structure. The metastable tetragonal phase stabilized zirconia will display a stress-induced transformation toughening mechanism. The strength and toughness of zirconia can be accounted for by its toughening mechanisms such as crack deflection, zone shielding, contact shielding, and crack bridging. Prevention of crack propagation is of critical importance in high-fatigue situations such as mastication and parafunction. This combination of favorable mechanical properties makes zirconia a unique and stable material for use in high-load situations. Zirconia is radioopaque and clearly visible on radiographs. Its ivory color is similar to the color of natural teeth and is especially critical in the aesthetic zone with high lip line smiles. Zirconia is also proposed to accumulate lesser plaque than titanium.[19] Furthermore, with the development of dental computer-aided design (CAD) computer-aided manufacturing (CAM) systems, this high strength ceramic is gradually becoming a satisfactory implant biomaterial.

Demerits of ceramics as a dental implant material

Ceramics are chemically inert; therefore, care must be taken in the handling and replacement due to its low ductility and inherent brittleness, which has resulted in limitations to its application as a dental implant material.

The compressive tensile and bending strengths of ceramics exceed the strength of compact bone by three to five times. These properties, combined with a high modulus of elasticity, especially with fatigue and fracture strength, have set apart these biomaterials for peculiar/special design needs.

Polymers and composites as biomaterials [19],[20],[21]

Polymeric implants in form of polymethyl methacrylate (PMMA) and polytetrafluoroethylene (PTFE) were first used in the 1930s. Other types of polymers, which were used subsequently as dental implant material included polyamide, polyethylene (PE), polyurethane (PU), polypropylene (PP), polydimethylsiloxane, polysulfone (PS), and silicone rubber. In general, the polymers have lower strengths and elastic moduli and higher elongation to fracture compared with other classes of biomaterials. Most polymers have shown elastic modulus with magnitudes closer to soft tissues.

Advantages of polymers as a biomaterial

  1. The physical characteristics of the polymers can be altered based on their use as their composition is easy to change. They can be changed to a more porous or softer form.
  2. They can be manipulated easily and allow better reproduction.
  3. They do not generate microwaves or electrolytic current as do metals.
  4. They show fibrous connective tissue attachment.
  5. They can be easily microscopically evaluated as compared with metals.
  6. They are more aesthetically pleasing.
  7. They have a low modulus of elasticity, which is close to soft tissues.


Disadvantages of polymers as a biomaterial

  1. Polymers have inferior mechanical properties compared to other biomaterial classes.
  2. They have poor adhesion to living tissues.
  3. They can have adverse immunologic reactions.
  4. Some polymers (e.g., PMMA) possess relatively low values of cold flow characteristics, creep, and fatigue strength.
  5. Polymers and composites of polymers are especially sensitive to sterilization and handling techniques. If intended for implant use, most cannot be sterilized by steam or ethylene oxide.


Combination of polymers and other categories of synthetic biomaterials (HA, Al2O3, glass ceramics) have been used in porous or solid forms for tissue attachment, replacement, and augmentation and as coatings to transfer force to soft and hard tissue regions. Biodegradable polymers such as polyvinyl alcohol, polylactides or glycosides, and cyanoacrylates or other hydrated forms have been combined with biodegradable CaPO4 for use as structured scaffolds, plates, screws, or other applications such as bone augmentation and periimplant bone defect repairs. The use of polymers for osseointegrated implant is primarily in form of components placed between the prosthesis and the implant for shock absorption and to better simulate the biomechanical function of natural tooth.[2] A recent addition to the polymers being used as dental implant biomaterial is polyether ether ketone (PEEK). The major advantage of this material over titanium and zirconium is its elastic modulus (3.6 Gpa), which is closer to bone. Further, this material is being reinforced with carbon fiber so as to achieve a modulus of elasticity of 17.4 Gpa, which is close to cortical bone. Also, this material has better aesthetic properties as it allows transmission of light and is favorable in patients who are potentially allergic to titanium.[22]

However, from the above biomaterials, two of the most popular and promising implant biomaterials are hereafter being selected for a more in-depth, comparative review and these are:

  • Titanium and titanium alloys and
  • Zirconia (zirconium dioxide).



   Comparative Review of Select Biomaterials Top


Titanium and titanium alloys

Pure titanium metal can exist as a dark gray, shiny metal or as a dark gray powder. It has a melting point of 1677° C and a boiling point of 3277° C. Its density is 4.6 g per cubic centimeter. Titanium metal is brittle when cold and can break easily at room temperature. At higher temperatures, it becomes malleable and ductile. Titanium has a high strength-to-weight ratio and small amounts of oxygen or nitrogen make it much stronger.[23] There are six different types of titanium used in dentistry as implant biomaterials. These are.

Four grades of commercially pure titanium (CpTi) -Grade I, Grade II, Grade III, and Grade IV; and two titanium alloys — Ti-6Al-4V and Ti-6Al-4V-ELI (extra low interstitial alloy).

Commercially pure titanium is also referred to as unalloyed titanium. Based on the oxygen residuals in the metal, the mechanical and physical properties of CpTi differ from each other. The trace elements of carbon, oxygen, nitrogen, and iron improve the mechanical properties of pure titanium. These trace elements are present in increasing concentrations from Grade I to Grade IV. The modulus of elasticity of Cp Grade I titanium to Cp Grade IV titanium has a range of 102-104 GPa (a change of only 2%). The modulus of elasticity of Cp titanium is about five times greater than the compact bone and this property places emphasis on the importance of design in the proper distribution of mechanical stress transfer.

Titanium alloys exists in three different structural forms: alpha (α), beta (β), and alpha-beta (α + β). These phases are obtained when pure titanium is mixed with elements like aluminum and vanadium, which act as phase condition stabilizers. Aluminum increases the strength of the alloy and is alpha-phase stabilizer while vanadium is the beta phase stabilizer. The alpha alloys have a hexagonal closely packed (hcp) crystallographic structure while the beta alloys have a body-centered cubic (bcc) form. Both the alpha and beta forms may coexist, which depends largely on composition and heat treatment.

For the fabrication of dental Implants, the alpha-beta combination alloy is the most commonly used consisting of 6% aluminum and 4% vanadium (Ti-6Al-4V). The strength of these alloys is further increased by heat treatment. They are highly resistant to fatigue and corrosion, whereas they have a relatively low density. The modulus of elasticity is low and closer to bone than any other implant material [Table 2].
Table 2: Mechanical properties of commercially pure titanium and its alloys

Click here to view


The strength characteristics of titanium alloys when compared to Cp titanium are:

  • The elastic modulus of titanium alloys is slightly higher (113 GPa compared with 104 GPa of CP Grade IV titanium).
  • The yield strength of titanium alloys increases over 60% to 795 GPa compared with 483 GPa for Cp titanium.


The modulus of elasticity of titanium alloys is slightly greater than that of Cp titanium, being about five to six times than that of compact bone. Coining, stamping, or forging followed by controlled annealing treatments are routinely used during metallurgical processing for manufacturing titanium implants in order to avoid bending at the time of insertion due to local strains.[24],[25],[26]

Failure modes of titanium and titanium alloy based implants

A perusal of the available literature suggests that failure of titanium implants is a rare occurrence, with a reported incidence ranging from 0% to 6%.[27],[28],[29],[30] Based on the same, the possible causative factors of titanium implant failure appear to be:

  • Implant design.
  • Bending overload.
  • Manufacturing imperfections.
  • Restoration design.
  • Accuracy of fit of restoration.
  • Implant numbers, dimensions and positioning.
  • Marginal bone loss.
  • Occlusion and parafunctional habits.
  • Chemical factors.
  • Allergy to titanium.


A review of the actual mode of failure accompanying the reported failure cases reveals the following primary failure modes for titanium and titanium alloy-based implants:

Metal fatigue, leading to fracture: Metal fatigue from high cyclic occlusal loading is the most common failure mode, observed mostly in 3.75-mm diameter implants made from Cp Grade I titanium.[31] Scanning electron microscope (SEM) analysis of fractured titanium implants reveals consistent uniformity of the microstructure with no indications that major inclusions or porosities are present and refutes the possibility of implant failure due to manufacturing errors.[7] All of the fractures were compatible with signs of fatigue failure that appeared to initiate from the stress concentration associated with the thread and typically initiated in the section of the implant that was not internally supported by the abutment screw. The crack typically follows the thread, and when the crack has almost completely encircled the implant, a tearing-like fracture occurs, linking the cracks on adjacent threads.

Chemical degradation, leading to fracture: It has been suggested that titanium implants may adsorb hydrogen from the biological environment, consequently becoming more brittle and prone to fracture.[32] Furthermore, galvanic corrosion between nonprecious metal alloy restorations supported on titanium implants might initiate a cytotoxic reaction as well as potentially assist with fatigue crack initiation. However, there is little evidence to support the chemical theory as a major contributor to implant failure.[33] Various studies have reported the presence of titanium ions around dental and orthopedic implants as well as in regional lymph nodes and pulmonary tissue. Discoloration of periimplant tissues along with concentrations of titanium ions between 100 ppm and 300 ppm have been well-documented.[34]

Recent developments related to titanium-based implant biomaterials

Recent trends in the research and development of titanium-based biomaterials show the aim to develop alloys composed of nontoxic and nonallergenic elements with excellent mechanical properties (low modulus and high strength) and workability.[35] Developments are underway aimed at replacing vanadium and aluminum with other nontoxic components such as Nb, Fe, Mo, Ta, Pd, and Zr. These have lower modulus of elasticity (55-85 GPa), which is closer to that of bone (17-28 GPa) and are mainly β alloys. This lower modulus of elasticity is desirable, as it results in a more favorable stress distribution at the bone-implant interface. These alloys are also capable of attaining higher strength and toughness compared to α + β alloys.

A new alloy for manufacturing narrow diameter implants (Roxolid ®, Straumann, Basel, Switzerland) has recently been introduced in dentistry.[36] This alloy is based on the binary formulation of 83-87% titanium and 13-17% zirconium. It has been claimed that this alloy exhibits better mechanical characteristics compared to CpTi and Ti-6Al-4V, with a tensile strength value of 953 MPa and a 40% higher fatigue strength.[37] The addition of zirconia to titanium leads to improved osseointegartion and the alloy of zirconia and titanium is more biocompatible as compared to pure titanium.[38]

Another promising α-titanium alloy used as a surgical implant material is Ti12.5Zr2.5Nb2.5Ta (TZNT). This alloy had a unique advantage of having a modulus of elasticity closer to the human bone (100 Gpa) as compared to the conventional titanium alloys. (120 Gpa) and it also possesses approximately equivalent admission strain (0.65%) with that of human bones (0.67%).[39] The addition of elements such as Zr, Nb, and Ta to the alloy have shown no toxicity or any adverse tissue reactions and displays a better corrosion resistance.[40]

Zirconia

The name zirconium comes from the Arabic word zargon (golden in color), which in turn comes from the two Persian words — zar (gold) and gun (color). Zircon has been well-known as a lustrous gem throughout history.[41]

Zirconium is a lustrous, grayish white, soft, ductile, and malleable metal, which is solid at room temperature though it becomes hard and brittle at lower purities. In powder form, zirconium is highly flammable but the solid form is less prone to ignition. Zirconium is highly resistant to corrosion by alkalis, acids, salt water, and other agents.

Zirconium oxide or zirconia ceramic was first introduced in dentistry in the early 1990s for clinical applications such as frameworks for all-ceramic crowns and fixed partial dentures and abutments.

At room temperature, unalloyed zirconia can assume three crystallographic forms, depending on the temperature:

  • Monoclinic form, between room temperature and upon heating up to 1170° C;
  • Tetragonal form, between 1170° C and 2370° C; and
  • Cubic structure above 2370° C and up to the melting point.


However, this transformation across crystallographic forms also results in crumbling of the material upon cooling. Alloying with lower valance oxides such as CaO, MgO, Y2O3, or CeO allows retention of tetragonal or cubic phases at room temperature, depending on the amount of dopant.[42],[43],[44]

Although many types of zirconia-containing ceramic materials are present, only three types are typically used for dental application. These are:

  • Yttrium stabilized zirconia polycrystals (tetragonal zirconia polycrystals 3Y-TZP).
  • Transformation toughened partially stabilized zirconia with magnesium (Mg-PSZ), and
  • Dispersion toughened ceramics — zirconia toughened alumina (ZTA).


Of these, the yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP) exhibits mechanical properties that make them suitable substrates for the fabrication of dental implants.[44]

Yttrium stabilized zirconia polycrystals

Y-TZP zirconia powder has an ivory color and possesses material density above 6.0 g/cc, maximum working temperature of 2000° C, and 0% porosity. The excellent mechanical properties of Y-TZP zirconia powder are a result of its very small grain size, extremely low porosity, and a unique property referred to as “transformation toughening.”[45]

External stresses such as sandblasting, grinding, and thermal aging can trigger the transformation of partially stabilized tetragonal zirconia polycrystalline ceramics from a tetragonal to a monoclinic state. This transformation is associated with 3-5% volume expansion, which induces compressive stresses, thereby closing the crack tip and preventing further crack propagation. This mechanism is known as transformation toughening and confers Y-TZP with superior fracture strength and fracture toughness compared with other dental ceramics. However, severe transformation to the monoclinic phase can induce a deterioration of materials such as a reduction in the strength and the fracture of the materials. Therefore, transformation toughening can provide either an advantage or disadvantage depending on the degree of transformation.[7],[46],[47],[48]

3Y-TZP ceramics for dental application typically consists of 98% small equiaxed tetragonal grains (0.2-0.5 μm), sometimes combined with a small fraction of the cubic phase, and 3 mol% yttria (Y2O3) as a stabilizer. The mechanical properties of 3Y-TZP depend on the grain size, which is dictated by the sintering temperature. The critical grain size reported in the literature ranges from 0.2 μm to 1 μm depending on the Y2O3 content. Above a grain size of 1 μm, 3Y-TZP is less stable and below grain size of 0.2 μm, transformation toughening is not possible resulting in reduced fracture toughness. It may be noted that higher sintering temperatures and longer sintering times lead to larger grain sizes.[7],[49],[50],[51]

3Y-TZP zirconia powder displays high flexural strength when processed using a hot isostatic pressing (HIP), attaining flexural values of around 900 MPa and 1,400 MPa with hardness is 13-13.5 GPa. The properties of Y-TZP ceramics that make it relevant as an implant biomaterial are high strength and toughness at higher temperatures, corrosion resistance, and lightweight and wear resistance.

Recently, 3Y-TZP has been used as a substrate for the manufacturing of single piece endosseous oral implants. It is done either using fully sintered ceramic blocs (hard machining) or by partially sintered ceramics (soft machining) that are later fully heat treated to ensure adequate sintering.[52] The soft machining sintering prevents the stress-induced transformation from tetragonal to monoclinic and leads to a final sintered surface virtually free of the monoclinic phase unless grinding adjustments are needed or sandblasting is performed. On the other hand, hard machining has been shown to result in a significant amount of monoclinic phase with subsequent surface microcracking and higher susceptibility to low-temperature degradation (LTD). LTD, also known as aging, occurs by a slow surface transformation of the metastable tetragonal crystals to the stable monoclinic structure in the presence of water or water vapor. The aging process depends on several microstructure features such as porosity, residual stresses, grain size, and the stabilizer content of the processed material.[7],[53],[54]

Zirconia is generally susceptible to low temperature degradation and this is relevant for 3Y-TZP dental abutments and implants as well, given the fact that the material is in contact with biological fluids. Low temperature degradation of 3Y-TZP involves microstructural changes such as grain pull-out, microcracking, and increased wear and surface roughening. Changes in the processing technique being used can alter the microstructure of the Y-TZP material and can therefore, have a significant impact on accelerated aging. Therefore, accurate processing techniques are an important factor in avoiding accelerated aging.[7],[47],[55]

Glass-infiltrated zirconia toughened alumina (ZTA)

Zirconia-based ceramics based can be combined with a matrix of alumina (Al2O3) to favorably utilize the stress-induced transformation ability of zirconia and produce a structure known as ZTA (alumina reinforced with zirconia grains). ZTA is developed by addition of 33 vol% of 12 mol% ceria-stabilized zirconia (12Ce-TZP) to In-Ceram alumina. Slip-casting or soft machining is utilized to process In-Ceram zirconia and then it is glass infiltrated. A limited amount of shrinkage upon sintering has been observed using the slip casting technique. As compared to 3Y-TZP dental ceramics, ZTA exhibits lower mechanical properties due to incorporation of residual porosity (8-11%)[7], 53, [56],[57],[58],[59]

Alumina toughened zirconia (ATZ)

ATZ is a composite ceramic material combining of 20 wt% alumina and 80 wt% zirconia containing 3 mol% yttria. Resistance to surface degradation (LTD) of the material has been improved by the addition of alumina as low as 0.25 wt% to TZP ceramics significantly improves the resistance of the material to surface degradation (LTD). ATZ exhibits the highest bending strength known for ceramics, both at room temperature (1,800-2,400 MPa) and at elevated temperatures (>800 MPa at 1000° C) and this property results in a high thermal shock resistance of ATZ [7],[60],[61] [Table 3].
Table 3: Mechanical properties of zirconia ceramics

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Failure modes of zirconia-based implants

Failures of zirconia implants are mainly chemical or mechanical in nature. As with all the implant materials, the primary cause of mechanical failure for zirconia implants is either due to functional overloading or over torquing during the surgical placement of the implant. However, in contrast to titanium implants, manufacturing defects and flaws created during ceramic implant fabrication and subsequent surface treatment have a significant impact on the strength and long-term viability of ceramic implants.[62]

Owing to the brittle nature of ceramics, areas of stress concentration can be a primary trigger for implant failure. Bending forces, when combined with manufacturing flaws such as porosities or microcracks within a ceramic implant, can initiate crack propagation and can result in early implant failure. Bone resorption around the periimplant region can also result in fracture of implants, when the implant is subjected to lateral or bending forces. Sharp, deep, and thin threads, along with sharp line form susceptible areas for stress concentration. Placing implants in dense hard bone and hand torquing during final insertion of the implant may generate bending forces that can result in implant failure.[63],[64]

LTD occurs as aging of the material and can eventually lead to failure of ceramic implant. It has been observed that under moist oral environment, along with low temperatures, a phase transformation occurs with yttria-stabilized tetragonal zirconia polycrystalline (Y-TZP), resulting in roughening, microcracking and grain pull-out, and consequently loss of material strength. However, whether the aging of zirconia is of concern is yet to be established clinically.[65],[66]

Recent developments related to zirconia-based implant biomaterials

Significant research is currently underway improving the reliability of ceramics in general and zirconia-based biomaterials in particular, for dental and biomedical applications. There are a number of developments focusing on the application of zirconia/alumina ceramic composites, consisting of either ZTA or ATZ. In general, these advanced composites benefit from the transformation toughening capabilities of zirconia and at the same time, are less vulnerable to low temperature degradation in biological fluids.[7],[67],[68]

Recently, introducing small amounts of alumina to 3Y-TZP has produced ceramic blocks known as TZP-A. The added traces of alumina improve the durability and stability under high temperatures and humid environments. However, this is achieved at the expense of reduced translucency of the ceramic blocks and therefore, represents a degree of aesthetic disadvantage.

Attempts to minimize LTD of 3Y-TZP systems include the addition of small amounts of silica, the use of yttria-coated rather than coprecipitated powder, the reduction of the grain size, and an increase of the stabilizer content or even the formation of composites with aluminum oxide (Al2O3). A composite material processed with 80% tetragonal zirconia polycrystals (ZrO2-TZP) and 20% alumina (Al2O3) is reported to have outstanding mechanical and tribological properties. The addition of alumina to zirconia clearly hinders aging or at least drastically reduces its kinetics, as it changes the grain-boundary chemistry and limits the tetragonal grain growth during sintering, which results in a more stable structure.

Another addition to the advancements in zirconia is the zirconia-based bulk metallic glass (BMG), the recent being Zr61Ti2Cu25Al12 ZT1. This amorphous alloys exhibits a good amalgamation of high strength, high fracture toughness, and lower Young's modulus. Metallic glasses (or amorphous alloys) as compared to the conventional crystalline metals possess substantially uniform microstructure, with no defects such as dislocation and grain boundary. Also, a short range arrangement of atoms takes place in the amorphous solids as compared to a long range order of crystalline solids resulting in many desirable properties such as high yield strength, large elastic strain (~2%), and excellent corrosion resistance. This unique display of properties has favored the use of Zr-based bulk metallic glasses in the field of implantology.[69]

Advancements in zirconia are also being made in terms of enhanced surface topography and modifications, which provides improved osseointegration. Various studies and experiments are being conducted in an attempt to achieve a surface modified zirconia-like sandblasted, sandblasted light grit, and acid etched, plasma-anodized and ceramic-coated zirconia. A stronger bone response to the sand blasted/acid etched zirconia implants surface has been reported.[68],[70] The coated or surface-modified zirconia implants also revealed higher removal torque values than machined zirconia implants. With all the surface enhancement approaches to roughen the zirconia surface, it is has been observed that the surface roughness of zirconia is comparable with that of titanium implants. Though it is difficult to achieve surface modifications for zirconia, with procedures such as CO2 lasers, distinctive surface alterations to zirconia were produced.[71]

Ceria-stabilized zirconia/alumina nanocomposites for dental applications have also been shown to exhibit high flexural strength (1422 ± 60 MPa), high reliability, and an excellent resistance to low temperature degradation. However, further research is needed to evaluate the long-tem in vivo performance of these composites in the oral environment.

It is well-documented that a favorable implant surface and tissue interface is responsible for the successful outcome of a dental implant. Improving the implant topography at the nanoscale level is the key to eliminate rejection and enhance osseoinegration.[72]


   Conclusion Top


The quest for the “perfect” dental implant biomaterial continues. That said, the above review has highlighted the long-term promise that both newer titanium-based alloys and zirconium-based composite ceramics represent in this regard.

Both material categories have a fairly unique set of pros and cons, which are actually complementary in many respects. While zirconia-based ceramics have higher biocompatibility and better aesthetics as compared to titanium-based alloys, titanium-based implants represent significantly better mechanical properties, particularly fracture strength as well as a longer history of application and therefore, established reliability over time.

The authors feel that in addition to the ongoing research and development that are already underway in the fields of newer metal alloys and ceramic composites, the future may see more innovations in a new class of materials-metal-ceramic binary formulations that have highly modulated surface properties. These biomaterials could represent an optimum blend of the properties' strength, predictability, and workability exceeding that of titanium alloys and biocompatibility and aesthetics exceeding that of zirconia-based composites.

Irrespective of the direction that dental implant biomaterials research takes in the future, it is clear that the most significant developments in this space will be at the cutting edge interface of material science and will be informed by innovations in substrate-surface differentiation and ability to easily “custom-create” surface properties, aided by the mainstreaming of nanotechnology and proliferation of three-dimensional (3-D) printing/forming technology.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
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    Figures

  [Figure 1]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]


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