MATERIALS USED FOR MATERIALS USED FOR DENTAL IMPLANTDENTAL IMPLANT

Dr.Sampath Kumar Dept. of Prosthodontics SDMCDS

Contents: Introduction History Physical And Mechanical Properties Corrosion And Biodegradation Classification of dental implant materials Metals and alloys used : Ti-6Al-4V Cobalt-Chromium-Molybdenum based alloys Iron-Chromium-Nickel-based alloys

OTHER MATERIALS USED

Ceramics and carbon Polymers and composites Surface characteristics Tissue interactions Surface energy Passivation and Chemical cleaning Sterilization Summary

INTRODUCTIONINTRODUCTION

The physical,mechanical,chemical and electrical properties of the basic material components must always be fully evaluated for any biomaterial application,as these properties provide key inputs into the biomechanical and biologic analysis of function.

Therefore the desire to positively influence tissue responses and to minimize biodegradation often places restrictions on which materials can be safely used within the oral and tissue environment

History History Ancient Implants:

Implanted animal & Carved ivory teeth cited in ancient Egyptian writings are the oldest examples of primitive implantology.

Attempts to replace lost teeth with endosteal implants have been traced to early Egyptian & South American civilizations.

A skull from Pre-Columbian era in Peabody museum of Harvard Univ in which an artificial tooth carved from dark stone replaced a lower left lateral incisor.

The earliest dental implants were of stone & ivory in 16th century.

Root replacement was by allogenic tooth transplantation which became popular in 17th century.

Metal implant devices of gold, lead, irridium, tantalum, stainless steel were developed in 20th century.

Co-Cr, molybdenum sub-periosteal & Ti blade implants were introduced in 1940’s.

Non metal bio-materials such as Vitreous & pyrolytic carbon, aluminum oxide, HA were introduced in 1970’s.

PHYSICAL AND MECHANICAL PROPERTIES

The fatigue limit of metallic implant materials reaches 50% of their ultimate tensile strength.However this relationship is only applicable to metallic and polymeric systems.

Ceramic materials are weak under shear forces due to the combination of lack of fracture toughness and no ductility which can lead to brittle fracture.According to ASTM metals should have minimum ductility of 8% to minimize brittle fracture.

CORROSION & BIODEGRADATIONCorrosion is a special concern for metallic

materials in dental implantalogy because implants protrude into the oral cavity where electrolyte and oxygen compositions differ from those of tissue fluids.In addition the pH can change significantly in areas below plaque and within the oral cavity.

Plenk and Zitter (1996) stated that galvanic corrosion can be greater for dental implants than for orthopedic implants

Galvanic process depends on the passivity of oxide layers which is only a few nanometers thick and is usually made up of oxides or hydroxides of the metallic elements that have greatest affinity for oxygen.

According to Williams three types of corrosion are most relevant to dental implants:Stress corrosion cracking Galvanic corrosion crackingFretting corrosion cracking

Even ceramic oxide materials are not fully degradation resistant.The corrosion resistance of synthetic polymers on the other hand depends not only on their composition but also on their degree of polymerization.

CLASSIFICATIONBiodynamic

Activity

Biotolerant GoldCo-Cr alloysStainless SteelZirconium

PolyethylenePolyamidePolymethylmethacrylatePolytetrafluoroethylene

Bioinert Commercially Pure TiTi alloy(Ti-6AI-4V)

Al2O3,Zr2O3

Bioactive HA,Ca3PO4,FA,Brushite, Bioglass

Metals Ceramics Polymers

Chemical Composition

BIOTOLERANT:Biotolerant materials are those that are not necessarily rejected when implanted into the living tissue,but are surrounded by a fibrous layer in the form of capsule.

BIOINERT:These allow close apposition of bone on their surface,leading to contact osteogenesis.

BIOACTIVE:These materials also allow the formation of new bone onto their surface,but ion exchange with host tissue leads to the formation of a chemical bond along the interface(bonding osteogenesis).

TITANIUM AND Ti-6Al-4V

This reactive group of metals and alloys form tenacious oxides in air or oxygenated solutions.

This passivated surface minimizes the bio-corrosion phenomenon.

In situations where the implant would be placed within a closely fitting receptor site in bone,areas scratched or abraded during placement would re-passivate in vivo.

Titanium shows relatively low modulus of elasticity and tensile strength when compared with most other alloys.Yet its modulus of elasticity is 5 times greater than that of the compact bone,and this property places emphasis on the importance of design in the proper distribution of mechanical stress transfer.

The strength values for wrought soft and ductile metallurgic condition are approximately 1.5 times greater than the strength of compact bone.

In most designs where the bulk dimensions and shapes are simple strength of this magnitude is adequate.

Sharp corners or thin sections must be avoided for regions loaded under tension or shear conditions.

Classification of commercially pure titanium:Grade I –0.18% OxygenGrade IV-0.4% Oxygen

Traces of other elements such as nitrogen,carbon,hydrogen,iron and vanadium added for stability or improvement of mechanical and physicochemical properties.

Pure titanium is mechanically much more ductile than Ti alloys.

The need for adjustment or bending to provide parallel abutments for prosthetic treatment has caused manufacturers to optimize microstructure and residual strain conditions.

Classification of commercially pure titanium:

Grade I –0.18% Oxygen

Grade IV-0.4% Oxygen

Traces of other elements such as nitrogen,carbon,hydrogen,iron and vanadium added for stability or improvement of mechanical and physicochemical properties.

Commercially pure Ti is mechanically much more ductile than Ti alloy.

The need for adjustment or bending to provide parallel abutments for prosthetic treatments has caused manufacturers to optimize microstructure and residual strain conditions.

However if an implant abutment is bent at the time of implantation, the metal is strained locally at the neck region and the local strain is both cumulative and dependent on the total amount of deformation introduced during the procedure.

COBALT CHROMIUM MOLYBDENUM BASED ALLOY

COMPOSITION:

CobaltChromiumMolybdenumCarbonNickelManganese

IRON CHROMIUM NICKEL BASED ALLOYS

This alloy as with Ti systems is used most often in wrought and heat treated metallurgic conditions,which results in a high strength and high ductility alloy.

The ramus blade,ramus frame,stabilizer fins and some mucosal insert systems have been made from the iron based alloy.

Because this alloy contains nickel as major elements,use in patients allergic or hypersensitive to nickel should be avoided.

If a stainless steel implant is modified before surgery, procedures for repassivation to obtain oxidized surface is recommended.

Iron based alloys have galvanic potentials

and corrosion characteristics that could result in

concerns about galvanic coupling and bio-

corrosion if interconnected with Ti,Co,Zr or C

implant biomaterials.

OTHER METALS AND ALLOYS

Tantalum,Platinum,Iridium,Gold,Palladium and alloys of these metals.More recently devices made from Zirconium,Hafnium and Tungsten have been evaluated.

Gold,Platinum and Palladium are metals of relatively low strength which places limits on implant design. But still Gold is used because of nobility and availability.

CERAMICS AND CARBON

Ceramics are inorganic non metallic, non polymeric materials manufactured by compacting and sintering at elevated temperatures.

Because of their inertness to biodegradation,high strength, physical characteristics such as color and minimal thermal and electrical conductivity,and a wide range material specific elastic properties they are in use.

However the low ductility or inherent brittleness has resulted in limitations.Ceramics have been used in bulk forms and more recently as coatings on metals and alloys.

ALUMINA,TITANIUM & ZIRCONIUM OXIDES

High ceramics from aluminum,titanium and zirconium oxides have been used for root form,endosteal plate form and pin type dental implants.

The compressive,tensile and bending strength exceed the strength of the compact bone by 3-5 times.These properties combined with high moduli of elasticity and especially with fatigue and fracture strength have resulted in specialized design requirements.

The Al, Ti and Zr oxide ceramics have a clear white,cream or light gray color which is beneficial for applications in anterior root form devices.

In early studies of dental and orthopedic devices in laboratory animals and humans ceramics have exhibited direct interphases with bone similar to an Osseo integrated condition with Ti.

One series of root form and plate form devices used during the 1970s resulted in intra oral fractures after several years of function.The fractures were initiated by fatigue cycling where biomechanical stresses were along regions of localized bending and tensile loading.

BIOACTIVE AND BIODEGRADABLE CERAMICS BASED ON CALCIUM

PHOSPHATEThe CaPO4 ceramics used in dental

reconstructive surgery include a wide range of implant types and thereby a wide range of clinical implications .The laboratory and clinical results for CaPO4 particulates were most promising and led to expansions for implant application.

In general these classes of bio-ceramics have lower strength,hardness and modulus of elasticity than the more chemically inert forms previously discussed .

Calcium aluminates,sodium lithium inert glasses with CaPO4 and glass ceramics provide a wide range of properties and have found extended applications.

BIOACTIVE CERAMIC PROPERTIES

Physical properties are specific to surface area or form of the product,porosity and crystallinity.Chemical properties are related to CaPO4 ratio,composition and elemental impurities such as carbonate ionic substitution in atomic structure forms can exist with exposure to water.this has caused some confusion in the literature,in that some CaPO4 ceramics have been steam autoclaved for sterilization purposes before surgical implantation.

Steam or water autoclaving can significantly change the basic structure and properties of CaPO4 ceramics & thereby provide an unknown biomaterial condition at the time of implantation.

Forms,Microstructure & Mechanical Properties.

Hydroxyapatite provided in a non-porous form as a spherical or angular shaped particles, is an example of crystalline, high purity HA biomaterial.

These particles can have relatively high compressive strength up to 500mPa,with tensile strength in the range of 50-70mPa.

Ceramics are brittle materials and exhibit high compressive strength compared with tensile strength.However,less resistance to tensile and shear stresses limit their application as dental implants because of mechanical constraints of implant form & volume.

Non-resorbable, bio inert ceramics exhibiting satisfactory load bearing capacity are limited to dense mono and poly crystalline Al, Zr & Ti oxide ceramics.

These same mechanical characteristics exist for the solid portions of several porous HA particulates and blocks.

The macroporous or microporous particulates have an increased surface area/unit vol. This provides more surface area for solution and cell ,mediated resorption under static conditions and a significant reduction in compressive & tensile strength.

The porous materials also provide additional regions for tissue in growth & integration & thereby minimization of interfacial motion and dynamic interfacial breakdown.

Bulk form implant designs made from CaPO4 ceramics,which were shown to be contraindicated for some implant designs because of poor mechanical performance, have found a wide range of indications as coatings of stronger implant materials.

These coatings for the most part are applied by Plasma spraying, have average thickness between 50 & 70micrometers are mixtures of crystalline and amorphous phases and have a variable micro structure compared with the solid portions of the particulate forms of HA and Ca3PO4

Density, Conductivity & Solubility Bioactive ceramics are especially interesting for

implant dentistry because the inorganic portion of the recipient bone is likely to grow next to a more a chemically similar material.

Under the bioactive categorization:CaPO4 materials such as TCP, HA, CaCO3 ,Ca2SO4 & ceramics are included.

Their limitations have been associated wit material forms that have lower strength.

Dissolution characteristics of bioactive ceramics have been determined for both particulates and coatings.

In general, solubility is greater for TCP than for HA.The solubility profiles depends on the environment.

The larger the particle size, the longer the material will remain at the site.

The porosity of the product also impacts the resorption rate.

The porous or the holes are regions where blood components and organic materials can reside when placed within the bone and represent the regions where living material existed before the exploration and processing of implant material.

The greater the porosity, the more rapid the resorption of the graft material.

The hard & soft tissues of the body are more able to degrade the components & resorb the amorphous forms of grafting materials.

Thus, crystalline forms of HA are found to be very stable over a long term under normal conditions, whereas, amorphous structures are more likely to exhibit resorption and susceptibility to enzymes or cell mediated breakdown.

The pH in the region in which the bone substitutes are placed also affects the rate of resorption. As the pH decreases the components of living bone primarily the CaPO4 resorb by solution mediated activity.

Since CaPO4 is a non conductor of heat, it can be used as a coating for implants when mixture of conductive materials are included in the overall prosthetic reconstruction.

In addition to its color, the above mentioned properties are considered to be advantageous.

Carbon & Carbon Silicon Compounds Carbon compounds are often classified under

ceramics because of their chemical inertness & absence of ductility,however they are conductors of heat & electricity.

Ceramic & carbonatic substances continue to be used as coatings on metallic & ceramic materials.

Advantages: Tissue attachment Opportunities for the attachment of active bio-

molecules/synthetic compounds.

Disadvantage: Lack of mechanical strength properties

Polymers & Composites

Advantages of Fiber reinforced polymers:

Can be designed to match tissue properties

Can be anisotropic wrt mechanical characteristics

Can be coated for attachment to tissues &

Can be fabricated at relatively low cost

Structural Biomaterial Polymers

In general, polymers have low strength & elastic moduli and higher elongations to fracture compared with other classes of biomaterials

They are thermal & electrical insulators. They are relatively resistant to bio degradation Polymers have been fabricated in porous and

solid forms for tissue attachment, replacement, augmentation & as coatings for force transfer to soft & hard tissue region

Cold flow characteristics,creep & fatigue strengths are relatively low for some classes of polymers.

ex: SR PMMA In contrast some are extremely tough and fatigue

cycle resistant.ex:PP UHMW-PE PTFE

The indications for PTFE have grown exponentially in the last decade because of the development of membranes for guided tissue regeneration technique.

Most of the inert polymers have been combined with particulate or fibers of carbon, Aluminum oxide, HA & glass ceramics.

In some cases, bio degradable polymers such as Poly Vinyl Alcohol,Poly Lactides or Glycolides,Cyanoacralates or other Hydra table forms have been combined with bio degradable CaPO4 particulate or fibers.

These are intended as structural scaffolds, plates, screws or other such applications.

COMPOSITES

Surface characteristics The biomaterial characteristics can be separated

into 2 categories associated either with Surface Bulk properties

In general, the biomaterial surface chemistry, topography & type of tissue integration can be correlated with shorter & longer term in vivo host responses.

The host environment has been shown to directly influence the biomaterial to tissue inter facial zone specific to the local biochemical & biomechanical circumstances of healing and long term clinical aspects of load bearing function.

Surface Characterization & Tissue Interaction-Metal & Alloy Surfaces

There is a formation of thin oxide due to a reaction with oxygen or other mechanisms such as oxygen or metal ions diffusion from and to the metallic surface, especially for Ti.

This thin layer of amorphous oxide will rapidly reform if removed mechanically.

Surface properties are due to this oxide layer and differ fundamentally from metallic substrate.

Therefore the oxidation parameters such as temperature, type & concentration of oxidizing elements & eventual contaminants- all influence the physical & chemical properties of the final implant product.

The surgical implant is primarily amorphous in atomic structure if formed in normal temperature, air or tissue fluid environments and is very adherent & thin in thickness (<20nm)

In contrast, if unalloyed Ti substrates are processed at elevated temperatures, the oxide forms a crystalline atomic structure & can be 10-100 time thicker.

Low temperatures thermal oxides are relatively homogenous & dense.

The role of alloying elements in Ti alloys & how these elemental compositions may influence oxide properties & host tissue compatibility—is dependent on the amount of ions available to the tissues & relative rates of ion transfers

Tissue Interactions Oxide modifications during in vivo exposure has

been shown to result in increased Ti oxide layer thickness of up to 200nm.

The highest growth area corresponded to a bone marrow site while the lowest growth was associated with Ti, in contact with cortical regions of bone.

Increased levels of Ca & Phosphorous were found in oxide surface layers and seemed to indicate an active exchange of ions at the interface.

The surface bio interaction process maybe slow or activated by local reactions and may cause ion release and oxide alteration of the substrate

Especially high rates of ion release were observed in EDTA & sodium citrate solutions & varied as a function of the corroding media.

Integration with Titanium Although Ti is known to exhibit better corrosion

resistance, independent of the surface preparation,in vivo & in vitro, studies have shown that Ti may interact with recipient living tissues over several years.

Bundy (1993) exposed implant alloys simultaneously to tensile stress & corrosive environments. In vivo, stainless & Ti alloy demonstrated cracks when loaded to yield stress & re-implanted under lab conditions for 8 weeks. Crack like features were also seen in stainless steel & TI alloys loaded to or beyond the yield stress & subsequently electrochemically polarized for 38 weeks in the in-vitro part of the study None of the samples actually failed by completely cracking but the author presumed that it would have occurred with a longer exposure time as previously suggested.

Lemons (1977) studied single staged solid

implants modified by bending or cutting &

showed that damage could increase corrosion.

Cohen & Burdairon (1978) showed that

odontogenic fluoride gels which create an

acidic environment can lead to the degradation

of the Ti oxide layer & possibly inhibit the

Osseo-integration process.

Cobalt & Iron Alloys The alloys of Co & Fe exhibit oxides of chromium under

normal implant surface finishing conditions after acid or electro-chemical passivation.

These chromium oxides as with Ti alloys result in significant reduction in chemical activity & environmental iron transfer.

In general, if stainless steel implant surfaces are mechanically altered during implantation or if the construct induces an interface that is subjected to biomechanical fretting, the iron alloy will biodegrade.

However, in the absence of surface damage, the chromium oxide on stainless steel biomaterials have shown excellent resistance to breakdown & multiple examples of tissue & host compatibility have been shown for implants removed after long term implantation.

Ceramics Aluminum oxide ceramics have been extensively

investigated related to surface properties and how those properties relate to bone and soft tissue integration.

Ceramic coatings have been shown to enhance the corrosion resistance & biocompatibility of metal implants,in particular, surgical stainless steel, ni-cr, co-cr alloys.

However, studies in orthopedics cautioned that the Al2O3 may cause demineralization phenomenon caused by a high local concentration of substrate ions in the presence of metallic bone disease.

Hydroxy Apatite In addition to bulk, aluminum oxide

biomaterials, CaPO4 based ceramic or ceramic like coatings have been added to Ti & Co alloy substrates to enhance tissue integration.

These coatings for the most part are applied by plasma spraying small size particles of crystalline HA ceramic powders.

Surface roughening by particulate blasting can be achieved by different media.

Sandblasting provides irregular, rough surface of 10microns.

Ti implants maybe etched with a solution of Nitric acid and Hydrochloric acid.

The acids very rapidly attack metals other than Ti & these processes are electrochemical in nature.

Proponents of this technique argue that implants treated by sandblasting and acid-etch provide superior radiographic bone densities along implant inter phases compared with TPS

Titanium Plasma Spray Porous or rough Ti surfaces have been fabricated by a

plasma spraying a powder form of molten droplets at high temperatures.

At temperatures of 15000 deg centigrade an argon plasma is associated with a nozzle to provide very high velocity 600m/s partially molten particles of Ti powder projected onto a metal or an alloy substrate.

Schroeder et al in animal experiments and histologic studies, concluded that the rough and porous surfaces showed a 3D interconnected configuration likely to achieve bone-implant attachment for stable anchorage.

Another animal study showed that attachment is enhanced by increasing ionic interactions introduced a dual physical & chemical anchor system.

Hydroxy Apatite Coating HA coating by plasma spray was brought to

the dental profession by DeGroot. Key et al showed with SEM & Spectrographic

analysis that plasma sprayed HA coating could be crystalline and could offer chemical & mechanical properties compatible with dental implant applications.

Cook et al measured the HA coating thickness after retrieval from specimens inserted in animals for 32weeks & showed a consistent thickness of 50microns which is in the range of advocated for manufacturing.

Other Surface Modifications

Methods include controlled clinical reactions with nitrogen or other elements or surface ion implantation procedures.

The reaction of Nitrogen with Ti alloys at elevated temperatures results in Ti-Nitride compounds being formed along the surface.

Most Ti-Nitride surfaces are Gold in color & this process has been extensively used for enhancing the surface properties of industrial & surgical instruments.

Electrochemically the Ti-Nitrides are similar to the oxides and no adverse electrochemical behavior has been noted if the Nitride is lost regionally.

The Ti substrate re-oxidizes when the surface layer of nitride is removed.

Nitrogen implantation & carbon doped layer deposition have been recommended to improve the physical properties of stainless steel without affecting its bio compatibility.

Surface Energy Measurements of surface property, values of

an implant’s ability to integrate within bone include contact angle with fluids, local PH and surface topography.

An intrinsically high surface energy is said to be most desirable. High surface energy implants showed a 3fold increase in fibroblast adhesion and higher surface energy surfaces such as metals, alloys & ceramics are best suited to achieve cell adhesion.

Surface tension values of 40 dyne/cm & higher are characteristic of very clean surfaces and excellent biologic integration conditions.

The shift in contact angle is related to contamination of the surface by hydrophobic contaminants & decreases the surface tension parameters.

A spontaneously deposited host dependent conditioning film is pre-requisite to the adhesion of any biologic element

It is suggested that the wetting of the surface by blood at the time of placement can be a good indication of the high surface energy of the implant.

Passivation & Chemical Cleaning

The ASTM specifications for final surface

treatment of surgical Ti implants require pickling

and descaling with molten alkaline base salts.

This is often followed by treatment with a

solution of nitric or hydroflouric acid to decrease

& eliminate contaminants such as iron.

Sterilization Manipulation with bare fingers, powdered

gloves, tap water can contaminate implant surfaces.

Today, most of the implants are available in pre-sterilized form.

Proteinaceous deposits & their action as films can be best eliminated by RFGDT

UV light sterilization after further evaluation may be good alternative.

Adequate sterilization of clean, pre-packed dental implants & related surgical components had resulted in an ever expanding use of Gamma Radiation procedures.

Summary & Conclusion Dental Implantology in the 1960’s was judged

to be rather disorganized. Treatments provided were said to be often not as successful as in other specialty procedures like Orthopedic & Cardiovascular Surgeries.

This discipline evolved in 1970’s. The basis for this was the successful use of synthetic bio materials.

Hence, the synthetic biomaterials have evolved & are now constituted, fabricated & provided to health care professionals as mechanically & chemically clean devices that have high predictability of success when used appropriately within surgical disciplines.

References:

Carl E Misch: Implant dentistry ( 2nd edition)

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