The entire groups of possible alloplastic implant materials, regardless of their clinical applications fall into the following categories:
1) Metals and metal alloys.
2) Ceramics and carbons
3) Synthetic Polymers
Materials used for ridge augmentation which includes materials of natural origin will be discussed in the unit on bone augmentation.
Metals and Alloys
Most of the dental implant systems are made of metals or their alloys. Currently commercially pure titanium and its alloy Ti-6Al-4V are most popular. Earlier alloys such as stainless steel, Tantulum, the Co-Cr alloys e.g. Co-Cr-Mo or Co-Cr-W-Ni were used. Of these, only the Co-Cr alloys with Mo are cast, the remainders are used in the form of prefabricated structures. Cobalt chromium alloys containing Berylium are not suitable for implantation as the ions of this metal cause an extremely toxic reaction into tissue.
Martensitic and Ferritic stainless steels are disregarded because of their magnetic property which is undesirable. Austenitic stainless steels are non-magnetic non heat-treatable steels, have excellent corrosion and heat resistance with good mechanical properties over a wide range of temperatures. Of these, the chromium-nickel steels are the most widely used steels and are also known as 18-8(Cr-Ni) steels. However,stainless steel undergoes more corrosion than Co-Cr alloys when embedded in the tissues. Pitting, galvanic corrosion, crevicular and stress corrosion are all observed in metallic implants. This corrosion is enhanced by the piezoelectric properties of bone which causes the release of metallic ions into the surrounding tissue. A tissue reaction occurs when the ion concentration reaches a high value.Tantalum and Niobium: Tantalum has been used successfully in needle type, screw- type and double-bladed implants. However, both are difficult to cast because of high melting points .Tantalum is considered mechanically inferior and even susceptible to corrosion therefore was replaced by titanium. Niobium is also not considered suitable implant material because of its complicated processing.
Titanium and Its Alloys: These have been used primarily for their corrosion resistance and biocompatibility. Titanium exists in nature as a pure element with an atomic number 22 and atomic weight of 47.9. Titanium composes about 0.6 % of the earths crust and is a million times more abundant than gold. This metal exists as rutile (TiO 2 ) or Ilmenite (FeTiO 3 ) and requires special extraction methods to be recovered in the elemental state.Titanium has several favourable properties like low specific gravity with a density of 4.5 gm/cm 3 , high heat resistance and high strength comparable with that of stainless steel. Ti is also very resistant to corrosion as a result of the passivating action afforded by a thin layer of titanium oxide that is formed on the surface. It has the ability to form an oxide layer 10 angstrom in thickness within a millisecond and is generally self healing. If left unchecked, it becomes 100 angstrom thick within a minute. Pure Ti has the ability to form several oxides including TiO, TiO 2 and Ti 2 O 3 . Of these, TiO 2 is considered the most stable and is used more often under physiological conditions.
1) Metals and metal alloys.
2) Ceramics and carbons
3) Synthetic Polymers
Materials used for ridge augmentation which includes materials of natural origin will be discussed in the unit on bone augmentation.
Metals and Alloys
Most of the dental implant systems are made of metals or their alloys. Currently commercially pure titanium and its alloy Ti-6Al-4V are most popular. Earlier alloys such as stainless steel, Tantulum, the Co-Cr alloys e.g. Co-Cr-Mo or Co-Cr-W-Ni were used. Of these, only the Co-Cr alloys with Mo are cast, the remainders are used in the form of prefabricated structures. Cobalt chromium alloys containing Berylium are not suitable for implantation as the ions of this metal cause an extremely toxic reaction into tissue.
Martensitic and Ferritic stainless steels are disregarded because of their magnetic property which is undesirable. Austenitic stainless steels are non-magnetic non heat-treatable steels, have excellent corrosion and heat resistance with good mechanical properties over a wide range of temperatures. Of these, the chromium-nickel steels are the most widely used steels and are also known as 18-8(Cr-Ni) steels. However,stainless steel undergoes more corrosion than Co-Cr alloys when embedded in the tissues. Pitting, galvanic corrosion, crevicular and stress corrosion are all observed in metallic implants. This corrosion is enhanced by the piezoelectric properties of bone which causes the release of metallic ions into the surrounding tissue. A tissue reaction occurs when the ion concentration reaches a high value.Tantalum and Niobium: Tantalum has been used successfully in needle type, screw- type and double-bladed implants. However, both are difficult to cast because of high melting points .Tantalum is considered mechanically inferior and even susceptible to corrosion therefore was replaced by titanium. Niobium is also not considered suitable implant material because of its complicated processing.
Titanium and Its Alloys: These have been used primarily for their corrosion resistance and biocompatibility. Titanium exists in nature as a pure element with an atomic number 22 and atomic weight of 47.9. Titanium composes about 0.6 % of the earths crust and is a million times more abundant than gold. This metal exists as rutile (TiO 2 ) or Ilmenite (FeTiO 3 ) and requires special extraction methods to be recovered in the elemental state.Titanium has several favourable properties like low specific gravity with a density of 4.5 gm/cm 3 , high heat resistance and high strength comparable with that of stainless steel. Ti is also very resistant to corrosion as a result of the passivating action afforded by a thin layer of titanium oxide that is formed on the surface. It has the ability to form an oxide layer 10 angstrom in thickness within a millisecond and is generally self healing. If left unchecked, it becomes 100 angstrom thick within a minute. Pure Ti has the ability to form several oxides including TiO, TiO 2 and Ti 2 O 3 . Of these, TiO 2 is considered the most stable and is used more often under physiological conditions.
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| Composition of CPTi and Alloys (Weight Percent Minimum values) |
Pure Ti undergoes a transformation from h.c.p. alpha phase to a b.c.c. beta phase at 883 degree Celsius. Alloying elements are added to stabilize either phase. Ti-6Al-4V is the most commonly used alloy. Aluminum acts as an alpha stabilizer for the purpose of increasing strength and decreasing the mass. Vanadium, Copper and palladium are beta stabilizers which are used to minimize the formation of TiAl 3 to approximately 6 % or less to decrease its susceptibility to corrosion.Extra low Interstitial (ELI) contains low levels of oxygen dissolved in interstitial sites in the metal. Lower amounts of oxygen and iron improve the ductility of the ELI alloy. Newer Ti alloys include Ti-13Nb- 13 Zr and Ti-15Mo-2.8 Nb. These alloys utilize other phase stabilizers than Al and V and may be more corrosion resistant.The most commonly used alloys are Ti-6Al-4V and Ti-6Al-4V extra low interstitial (ELI). Commercially pure Titanium comes in different grades from CP Ti grade I to IV. The strength of CPTi is less than that of Ti-6Al-4V alloy, though the modulus of elasticity values is comparable. Ti alloys are able to maintain that fine balance between sufficient strength to resist fracture under occlusal forces and a lower modulus of elasticity for a more uniform stress distribution across the bone implant interface.It is generally accepted that pure titanium has little cytotoxicity and it is suggested that the plaque induced inflammation may be due to creation of damaged surface due to injudicious prophylactic procedures or due to corrosive action of acidic fluoride prophylactic agents.
Apart from the properties of the intrinsic bulk of the material, its surface composition dictates the events governing cellular response, attachment and spreading – the dynamics of protein adsorption and interfacial reaction, the release and generation of particulate material and leaching of components. The surface composition is not only dependent on the overall composition but also an the fabrication, processing,sterilization, and handling of the implant.Most of the corrosion resistant metals or alloys used for implants have a layer of oxides on their surface. The thin (approximately 5 nm) layer on the surface of titanium is amorphous, becoming crystalline if further oxidation and thickening occurs. Ti-6Al-4V has similar thickness, mixed oxide amorphous films in which the composition depends on the treatment (e.g., V is only visible in surface spectra of passivated Ti alloy specimens, whereas in non passivated specimens the surface layer is Al rich), in addition to the main elements, impurities typically present include Ca, P, Si, Cl, S, Na and F.When a biomaterial is introduced into the host, in addition to a host response there is also a material response. Immediately on implantation, there is an adsorption of proteins. These proteins come from the blood and tissue fluids at the wound site and later from cellular activity in the interfacial region. Once on the surface, the proteins (undenatured or denatured, intact or fragmented) can desorb or remain to mediate tissue implant interactions.
The biological activity of TiO 2 probably also influences the protein adsorption of Ti.The surface characteristics of TiO 2 probably change from an anionic to a cationic stage by the adsorption of calcium to its surface. This will subsequently increase its ability to adsorb acidic macromolecules such as albumin.Surface oxides quickly form on the surface of non noble metals or alloys. As the
chemical properties of oxides are generally much different from those of the corresponding metal, the biocompatibility of oxides is the relevant biomaterial parameter in implant dentistry. However the oxides sometimes exist in more than one crystal form. For titanium, there are three forms of oxide crystals- Anatase,Rutile and Brookite- of which rutile is the most stable.
Apart from the properties of the intrinsic bulk of the material, its surface composition dictates the events governing cellular response, attachment and spreading – the dynamics of protein adsorption and interfacial reaction, the release and generation of particulate material and leaching of components. The surface composition is not only dependent on the overall composition but also an the fabrication, processing,sterilization, and handling of the implant.Most of the corrosion resistant metals or alloys used for implants have a layer of oxides on their surface. The thin (approximately 5 nm) layer on the surface of titanium is amorphous, becoming crystalline if further oxidation and thickening occurs. Ti-6Al-4V has similar thickness, mixed oxide amorphous films in which the composition depends on the treatment (e.g., V is only visible in surface spectra of passivated Ti alloy specimens, whereas in non passivated specimens the surface layer is Al rich), in addition to the main elements, impurities typically present include Ca, P, Si, Cl, S, Na and F.When a biomaterial is introduced into the host, in addition to a host response there is also a material response. Immediately on implantation, there is an adsorption of proteins. These proteins come from the blood and tissue fluids at the wound site and later from cellular activity in the interfacial region. Once on the surface, the proteins (undenatured or denatured, intact or fragmented) can desorb or remain to mediate tissue implant interactions.
The biological activity of TiO 2 probably also influences the protein adsorption of Ti.The surface characteristics of TiO 2 probably change from an anionic to a cationic stage by the adsorption of calcium to its surface. This will subsequently increase its ability to adsorb acidic macromolecules such as albumin.Surface oxides quickly form on the surface of non noble metals or alloys. As the
chemical properties of oxides are generally much different from those of the corresponding metal, the biocompatibility of oxides is the relevant biomaterial parameter in implant dentistry. However the oxides sometimes exist in more than one crystal form. For titanium, there are three forms of oxide crystals- Anatase,Rutile and Brookite- of which rutile is the most stable.
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| Mechanical Properties of Ti and Ti Alloy as Compared to Bone |
Titanium has been demonstrated to be suitable under normal clinical conditions.TiO 2 not only has exceptionally high corrosion resistance against chemical attacks,but also has a dielectric constant that is high at a level to promote strong bonds with attaching tissues. Because of this high dielectric constant the conformational changes of adsorbed proteins are relatively small on TiO 2 surfaces.Metal levels of upto 21 ppm titanium, 10.5 ppm aluminum, and 1 ppm vanadium around Ti-6Al-4V and upto 2 ppm Co , 12.5 ppm, Cr and 1.5 pp,Mo around Co-Cr-Mo have been measured in the fibrous membrane surrounding hip implants. It is unclear whether such effects are seen with oral and maxillofacial implants, which in general have much less surface area in tissues than orthopedic implants.Titanium tends to seize when in sliding contact with itself or other metal. Titanium based alloys have a high co-efficient of friction which can cause problems. Wear particles are formed in a piece of bone if a piece of bone rubs against the implant or if two parts of an implant rub against one another. Therefore , abutment screws are sometimes coated with other materials.
Titanium alloy is 4 times stronger than Grade I CPTi and almost 2 times stronger than Grade IV CPTi. Ultimate strength and fatigue strength are primary considerations given the ramifications of dental implant body and component fractures. The modulus of elasticity of the four different grades of CPTi is similar (103 GPa) and is only slightly higher in Ti-6Al-4V (113 GPa). Hence, although a significant difference exists in the strength between grades of CPTi and its alloy, the elastic modulus is similar which is about 6 times that of cortical bone (the closest between all biomaterials).Titanium alloy represents the best compromised solution (given current biomaterials technology) between biomechanical strength, biocompatibility and the potential for relative motion (from modulus mismatch) at the bone implant interface and prevents the stress shielding which leads to disuse atrophy of crestal bone as seen with more rigid biomaterials
Ceramics
A kind of material that is “bone like” based on the specific ratios of calcium and phosphorous in particular crystalline structures. Ceramics are inorganic, nonmetallic,nonpolymeric materials manufactured by compacting and sintering at elevated temperatures. They are divided into metallic oxides or other compounds. Oxide ceramics are introduced for surgical implant devices as it offers high strength, inertness to biodegradation, low thermal and electrical conductivity and an elastic modulussimilar to bone but tend to be more brittle.Whilst their tissue compatibility is considered good, some ceramics released soluble silicates which caused adverse tissue reactions. Unfortunately ceramic materials have low tensile and transverse strengths, low impact resistance and are not easy to mould.Synthetic hydroxyapatite is converted into a ceramic material by sintering. The crystalline lattice structure is not changed on firing but there is partial dehydration and transformation from apatite to alpha Whitlockite. The resulting calcium phosphate ceramic appears similar to tooth enamel and is a possibility as an implant material. Porosity of the apatite is achieved by the addition of cellulose before sintering
Polymers
In comparison to metals and ceramics, polymers are weak and flexible. These are not popular as dental implant materials. Examples of polymeric materials include PMMA, silicone rubber, polyethylene, Poly-Lactic Acid, polysulfone and poly tetrafluoroethylene. The polymer most commonly used is polymethyl methacrylate (PMMA). Initially, solid polymers were used but they appear to have only moderate attachment and are eventually exfoliated. A porous PMMA structure is formed by sandblasting the outer surface. Polyhydroxyethylmethacrylate (HEMA) is manufactured as a porous sponge which, on implantation, becomes invaded with fibrous tissue.Polymers are chosen mainly as additive for beneficial secondary purposes such as structural isolation or introduction of shock absorbing qualities in load bearing metallic implants. Their most frequent use is still for non-load bearing applications in maxillofacial reconstruction. Certain “bioresorbable” polymers (Poly Lactic Acid) show potential as temporary implant materials that induce healing and bony ingrowth.
Carbon Based Materials
These include Pyrolytic carbon, polycrystalline (Vitreous Carbon), and carbon/silicon interstitial combination.Vitreous Carbon -Solid or porous vitreous carbon has also been used as an implant material. Being pure carbon, this material is highly resistant to oral and tissue fluids and unlike polymers, contains no stabilizers, plasticizers, etc. It has relatively low impact strength. The shape of the vitreous carbon structure is related to the shape of the polymer from which it arose, thus making it possible to produce a variety of shapes. The manufacturing of vitreous carbon structures requires specialized equipment.
Titanium alloy is 4 times stronger than Grade I CPTi and almost 2 times stronger than Grade IV CPTi. Ultimate strength and fatigue strength are primary considerations given the ramifications of dental implant body and component fractures. The modulus of elasticity of the four different grades of CPTi is similar (103 GPa) and is only slightly higher in Ti-6Al-4V (113 GPa). Hence, although a significant difference exists in the strength between grades of CPTi and its alloy, the elastic modulus is similar which is about 6 times that of cortical bone (the closest between all biomaterials).Titanium alloy represents the best compromised solution (given current biomaterials technology) between biomechanical strength, biocompatibility and the potential for relative motion (from modulus mismatch) at the bone implant interface and prevents the stress shielding which leads to disuse atrophy of crestal bone as seen with more rigid biomaterials
Ceramics
A kind of material that is “bone like” based on the specific ratios of calcium and phosphorous in particular crystalline structures. Ceramics are inorganic, nonmetallic,nonpolymeric materials manufactured by compacting and sintering at elevated temperatures. They are divided into metallic oxides or other compounds. Oxide ceramics are introduced for surgical implant devices as it offers high strength, inertness to biodegradation, low thermal and electrical conductivity and an elastic modulussimilar to bone but tend to be more brittle.Whilst their tissue compatibility is considered good, some ceramics released soluble silicates which caused adverse tissue reactions. Unfortunately ceramic materials have low tensile and transverse strengths, low impact resistance and are not easy to mould.Synthetic hydroxyapatite is converted into a ceramic material by sintering. The crystalline lattice structure is not changed on firing but there is partial dehydration and transformation from apatite to alpha Whitlockite. The resulting calcium phosphate ceramic appears similar to tooth enamel and is a possibility as an implant material. Porosity of the apatite is achieved by the addition of cellulose before sintering
Polymers
In comparison to metals and ceramics, polymers are weak and flexible. These are not popular as dental implant materials. Examples of polymeric materials include PMMA, silicone rubber, polyethylene, Poly-Lactic Acid, polysulfone and poly tetrafluoroethylene. The polymer most commonly used is polymethyl methacrylate (PMMA). Initially, solid polymers were used but they appear to have only moderate attachment and are eventually exfoliated. A porous PMMA structure is formed by sandblasting the outer surface. Polyhydroxyethylmethacrylate (HEMA) is manufactured as a porous sponge which, on implantation, becomes invaded with fibrous tissue.Polymers are chosen mainly as additive for beneficial secondary purposes such as structural isolation or introduction of shock absorbing qualities in load bearing metallic implants. Their most frequent use is still for non-load bearing applications in maxillofacial reconstruction. Certain “bioresorbable” polymers (Poly Lactic Acid) show potential as temporary implant materials that induce healing and bony ingrowth.
Carbon Based Materials
These include Pyrolytic carbon, polycrystalline (Vitreous Carbon), and carbon/silicon interstitial combination.Vitreous Carbon -Solid or porous vitreous carbon has also been used as an implant material. Being pure carbon, this material is highly resistant to oral and tissue fluids and unlike polymers, contains no stabilizers, plasticizers, etc. It has relatively low impact strength. The shape of the vitreous carbon structure is related to the shape of the polymer from which it arose, thus making it possible to produce a variety of shapes. The manufacturing of vitreous carbon structures requires specialized equipment.
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