Dental materials, Dental amalgam alloys, noble metals and base metals used in dentistry and alloys for Dental implants, complications of Dental implants.

Dental materials, Dental amalgam alloys, noble metals and base metals used in dentistry and alloys for Dental implants, complications of Dental implants By: Dr. Murtaza Najabat Ali (CEng MIMechE P.E.)

Biomaterials for dental Applications

Caries---a destructive process of decalcification of the tooth enamel and leading to continued destruction of enamel and dentin, and cavitation of the tooth

These materials must withstand forces during either fabrication or mastication, Retain their strength and toughness, and be resistant to Corrosion, Friction and Wear Similar to implantable devices for non-dental applications, they must also be biocompatible Biocompatibility is defined as the ability of a material to elicit an appropriate biological response in a given application in the body

Biocompatibility is defined as the ability of a material to elicit an appropriate biological response in a given application in the body Inherent in this definition is the concept that a single material will not be biologically acceptable in all applications For example, a material that is acceptable as a full cast crown may not be acceptable as a dental implant In a bone implant, the material is expected to allow the bone to integrate with the implant. Therefore, an appropriate biological response for the implant is Osseointegration (i.e. close approximation of bone to the implant material) Whereas in a full cast crown, the material is expected to not cause inflammation of pulpal or periodontal tissues. Osseointegration, however, is not expected

Types of Dental Materials
Three main groups, which are Metals Polymers Ceramics

Classification of Dental Materials
Dental biomaterials fall into three classes, which are Preventive Dental Materials Restorative Dental Materials Auxiliary Dental Materials

Preventive Dental Materials Pit and Fissure Sealants Sealing agents that prevent leakage Materials used primarily for their antibacterial effects Liners, bases, cements and restorative materials that are used primarily because they release fluoride Chlorhexidine (antiseptic/antimicrobial agent) or other therapeutic agents ( such as tooth paste, mouth wash (anti-caries)) used to prevent or inhibit the progression of tooth decay

Restorative Dental Materials Dental restorative materials are specially fabricated materials, designed for use as dental restorations (fillings), which are used to restore tooth structure loss, usually resulting from dental caries (dental cavities) which include bonding agents, liners, cement bases, amalgams, resin-based composites, cast metals, metal-ceramics, ceramics, and denture polymers. Direct restorative materials The chemistry of the setting reaction for direct restorative materials is designed to be more biologically compatible. Heat and byproducts generated cannot damage the tooth or patient, since the reaction needs to take place while in contact with the tooth during restoration Used intraorally to fabricate restoration or prosthetic devices directly on the teeth Indirect restorative materials Indirect restorations are fabricated outside of the mouth and therefore made extraorally in which the materials are formed indirectly on the teeth or tissues Temporary restorative materials

Auxiliary Dental Materials Substances used in the construction of dental prostheses and appliances but do not become part of these devices. Impression materials, casting investments, gypsum cast, and model materials, dental waxes, acrylic resins for impression, bleaching trays, mouth guards, and occlusion aids, finishing and polishing abrasives. Gypsum cast Mouth guard (braces) Dental impression materials Dental wax model

Dental Biomaterials Dental Amalgam Alloys
Dental Amalgam is a metallic restoration made by mixing mercury and a powdered alloy containing silver, tin and copper and sometimes zinc, palladium, indium and selenium The amalgamation reaction produces a solidified alloy over a short time range, allowing the dentist to manipulate the pasty amalgam in tooth cavity Dental amalgam is used to restore chewing surfaces and is subject to heavy forces The aim of the amalgam is to develop an acceptable compressive strength in the restoration within several hours The dimensional stability of the amalgam during setup is important for its clinical application Amalgamator is used to mix and to make a pliable mass of Dental amalgam alloy. It mixes the liquid and powder components and the reaction begins after mixing Amalgam Capsules; one capsule contains powdered amalgam alloy and the other capsule contains liquid mercury

Dental Biomaterials Dental Amalgam Alloys
When a tooth cavity is restored by using an amalgam, there is no adhesion between the amalgam and the tooth material. This can cause marginal leakage of the restoration (filling) The dimensional stability of the amalgam during setup and its ability to reproduce the convolutions of the cavity walls, along with the technique of the dentist in mixing and applying the amalgam, affect leakage Dental amalgam is brittle, which can lead to failure in tension or creep. Dentists avoid this by feathering edges and minimizing chances for high tensile loads

Mercury Dose from Amalgam
Dental Biomaterials Dental Amalgam Alloys Mercury Dose from Amalgam Person with average number of fillings would absorb ~1.6 µg/day Person with a moderately high number of fillings would absorb ~3 µg/day According to EPA, absorbed dose of mercury from food, water, and air is 5.7 µg/day

Dental Biomaterials Noble and Base Metals
In addition to the Amalgam alloy, dental alloys can be placed into two broad categories, i.e. Noble Metals Base Metals Noble Metals are elements with a good metallic surface that retain their surface in dry air. Their resistance to oxidation, tarnish and corrosion during heating, casting, soldering and use in the mouth is very good. The noble metals include: Gold Platinum Palladium Iridium Rhodium etc.

Gold, in either pure or alloyed form, is the most commonly used Noble metal used for dental restorations Gold casting alloys are typically used for crowns and bridgework and ceramic-metal restorations Some Palladium-base alloys are also used for similar applications Although many in the metallurgical field also consider silver a Noble metal, it is not considered a Noble metal in dentistry because it corrodes considerably in the oral cavity

Base (Non-Noble) Metals ---- and their alloys for dental restorations include; Cast Cobalt-chromium alloys used for partial dentures and porcelain-metal restorations Cast Nickel-chromium alloys used for partial dentures, crowns and bridges, and porcelain-metal restorations Cast Titanium and titanium alloys used for implants, crowns and bridges, and orthodontic wires Stainless steels used for dental instruments, orthodontic wires and brackets, and reformed crowns Dental Cast Partial Denture Titanium crowns Orthodontic Bridges and Wires

Because of the significant increases in the price of noble metals during the past few decades, alloys with considerably less noble metal content have been developed In addition, base metal alloys have replaced noble metals systems in many applications The metals and alloys used as substitutes for Gold alloys in dental applications must have the following fundamental characteristics, such as The chemical nature of the alloy should not produce harmful toxicologic or allergic effects in the patient The chemical properties of the appliance should provide resistance to corrosion and physical changes when in the oral environment The base metals and alloys for fabrication should be plentiful, relatively inexpensive and readily available The physical and mechanical properties such as Thermal Conductivity, Coefficient of Thermal Expansion and Strength, should all be satisfactory and meet specified values of the application

Dental Biomaterials Porcelain-Fused-to-Metal (PFM) Alloys
All-ceramic anterior (front teeth) restorations can appear very natural Unfortunately, the ceramics used in these restorations are brittle and subject to fracture Conversely, All-metal restorations are strong and tough, but from an aesthetic viewpoint are acceptable only for posterior restorations Fortunately, the aesthetic qualities of ceramic materials can be combined with the strength and toughness of metals to produce restorations that have both a natural tooth like appearance and very good mechanical properties The ceramic used for these restorations are porcelains, hence it is named as Porcelain-Fused-to-Metal (PFM) restorations

Dental Biomaterials Fabrication of PFM Restorations
PFM restorations consist of a cast pre-oxidized metallic coping on which at least two layers of ceramic are baked The first layer applied is the opaque layer, consisting of a ceramic rich in opacifying oxides Its role is to mask the darkness of the oxidized metal core to achieve adequate aesthetics, and as a first layer it also provides a ceramic-metal bond The next step is the buildup of mostly translucent dentin and enamel ceramics to obtain an aesthetic appearance similar to the natural tooth After building up the porcelain powders, the ceramic-metal crown is sintered in a porcelain furnace

Dental Biomaterials Fabrication of PFM Restorations

Dental Biomaterials Types of PFM Alloys
Both noble and base metals and alloys are used for PFM restorations There are five types of Noble Metal Alloys for PFM restorations. In chronological order, they are: Gold-platinum-palladium (Au-Pt-Pd) alloys Gold-palladium-silver (Au-Pd-Ag) alloys Palladium-silver (Pd-Ag) Gold-palladium (Au-Pd) alloys Palladium-copper (Pd-Cu) alloys These alloys have noble metal contents ranging from about 50% to nearly 100%

Dental Biomaterials Types of PFM Alloys
In Base Metal PFM Alloys for PFM restorations, there is a range of compositions available of Base metal alloys for ceramic-metal restorations, which are Nickel-chromium Cobalt-chromium Titanium etc.

Dental Biomaterials Alloys for Dental Implants
Dental implants are intended to support various dental appliances in the mouth Dental implant designs can be separated into two categories called Endosteal (endosseous), which enter the bone tissue Subperiosteal systems, which contact the exterior bone surfaces

Dental Biomaterials Alloys for Dental Implants
The Endosteal implant designs such as cylinders, screws, blades etc., are placed into the bone In contrast, the Subperiosteal devices are fitted to the bone surface and fixed with Endosteal screws

Introduction to ceramics, their structure, tissue attachment mechanisms, classification of ceramics and non-absorbable or relatively bioinert bioceramics

Ceramics Ceramics are inorganic materials composed of non-directional ionic bonds between electron-donating and electron-accepting elements Ceramics may contain crystals like metals, or may be non- crystalline (amorphous glasses) Ceramics are very hard and brittle because of the nature of ionic bonds Articulating surfaces in several implants

Ceramics Due to the similarity between the chemistry of ceramics and that of native bone, ceramics are most often used as a part of orthopedic implants or as dental materials Their aesthetic quality and appearance is closer to natural bone/tooth These materials are attractive as biological implants because bone bonds well to them, and they exhibit minimum foreign body reaction (implying inertness within the body), high stiffness, and low friction/wear as articulating surfaces Implantable skull fixators

Ceramics Their main drawback is their brittle nature and resultant low impact resistance Although dozens of compositions have been explored in the past, relatively few have achieved human clinical application Ceramics due to their non-ductile nature, are very susceptible to notches or microcracks because instead of undergoing plastic deformation (or yield) they will fracture elastically on initiation of a crack Excellent Aesthetics

Ceramics In order to be classified as a Bioceramic, the ceramic material must meet or exceed the following properties: If a ceramic is flawless, it is very strong even when subjected to tension Flawless glass fibers have twice the tensile strengths of high strength steel Non-toxic Non-carcinogenic Non-allergenic Non-inflammatory Biofunctional for its lifetime in the host

Ceramics Structure of Ceramics
Since the bonds in ceramics are partially to totally ionic in nature (i.e. Pure ionic bonding cannot exist: all ionic compounds have some degree of covalent bonding. Thus, an ionic bond is considered a bond where the ionic character is greater than the covalent character) Crystal structures in ceramic materials are thought of as being composed of ions rather than atoms The variety of chemical compositions of ceramic materials results in a wider range of crystal structures than with metals Ceramic crystal structure is affected by two parameters that are not concerns in metallic structures; (i) the magnitude of the electrical charge on the constituent ions, and (ii) the particle size of these ions

Ceramics Ceramics Ceramics Structure of Ceramics
The magnitude of the electrical charge is important because the crystal must remain electrically neutral (i.e. Sum of cation and anion charges in unit cell should be zero) The second characteristic requires the knowledge of the radii of both the cations (rc) and anions (ra) composing a ceramic material Cations are generally smaller, positively charged and usually metals Anions are usually O, C, N, larger and negative charge For an optimally stable structure, cations prefer to contact the maximum allowable number of anions (and vice versa for the anions) Certain ratios does not allow close contact between cations and anions and thus produce unstable structures

Ceramics Ceramics Ceramics Structure of Ceramics

Ceramics Structure of Ceramics
In this case, ion’s coordination number refers to the number of nearest neighbors with opposite charge and it depends on the rc/ra ratio This ratio of ionic radii dictates the coordination number of anions around each cation As the ratio gets larger, the coordination number gets larger For example, for a coordination number of 4, the cation is found at the center of a tetrahedron, with anions at each corner The most common coordination numbers for ceramics are 4,6 and 8 Silicate Structure

Ceramics Structure of Ceramics AX Crystal Structures
For ceramics in which both the cation and anion have the same charge, an equal number of each is required for a stable crystal structure These are called AX crystals, with A representing the cation and X representing the anion The most common AX structure is the Sodium Chloride (NaCl) structure, and the coordination number for both cations (Na+) and anions (Cl-) is 6 The NaCl structure can be thought of as two interpenetrating FCC-type crystals, one composed of anions and the other of cations

Ceramics Structure of Ceramics AmXp Crystal Structures
Ceramic materials are often composed of cations and anions that do not have equal charges, leading to compounds with the formula AmXp A common example is found in Fluorite (CaF2) The coordination number for Ca2+ is 8, and the system exhibits a cubic coordination geometry The cations are at the center of the cube, with the anions at the corners

Ceramics Structure of Ceramics Carbon-based Materials
Although carbon-based materials do not neatly fall into any of the classes of materials (metals, ceramic or polymers) One common form, Graphite, is sometimes considered a ceramic Even though it does not possess a standard unit cell, Graphite is crystalline Crystal structure of Graphite

The structure consists of planes of hexagonally arranged carbon atoms Within the planes, each carbon atom is bonded covalently to three neighbors While the fourth valence electron participates in van der Waals interactions with plane above it

A property of graphite that is important to the biomaterialist is its ability to adsorb gases This is used in the formation of Pyrolytic carbon, in which carbon in the gaseous state is deposited onto another material (such as graphite) Pyrolytic carbon has been used in a number of cardiovascular devices, including replacement heart valves

An additional synthetic form of carbon can be found in Single-walled nanotubes (SWNT) and Multi- walled nanotubes (MWNT) A single-walled nanotube can be visualized as a single sheet of graphite rolled to form a tube Similarly, a multi-walled nanotube can be visualized as a tube rolled from multiple layers of graphite sheets These carbon nanotubes are generally a few nanometers in diameter and on the order of a micron in length

Ceramics Tissue Attachment Mechanisms
No one material is suitable for all biomaterial applications As a class of biomaterials, ceramics, glasses and glass- ceramics are generally used for repair or replacement of musculoskeletal hard connective tissues Their use depends on achieving a stable attachment to connective tissue Carbon-base ceramics are also used for replacement heart valves, where resistance to blood clotting and mechanical fatigue are essential characteristics Diamond-like carbon (DLC) coated

Ceramics Tissue Attachment Mechanisms Implant Material Characteristics
The mechanism of tissue attachment is directly related to the type of tissue response at the implant interface No material implanted in living tissues is practically inert; all materials elicit a response from living tissues Four types of possible tissue responses to biomedical implants, given below Implant Material Characteristics Tissue Response Toxic Surrounding tissue dies Nontoxic, biologically inactive Fibrous tissue of variable thickness forms Nontoxic, bioactive Interfacial bond forms Nontoxic, dissolves Surrounding tissue replaces material

Ceramics Tissue Attachment Mechanisms Type of Attachment Example
These types of tissue responses (mentioned in the previous slide) allow four different means of achieving attachment of prostheses to the musculoskeletal system Therefore, tissue attachment mechanisms for bioceramic implants, are Type of Attachment Example Dense, nonporous, nearly inert ceramics attached by bone growth into surface irregularities by cementing the device into the tissues, by press-fitting into a defect, or attachment via a sewing ring (Morphological fixation) Alumina (Aluminium oxide Al2O3) Pyrolytic Carbon For porous inert implants, bone ingrowth occurs, which mechanically attaches the bone to the materials (Biological fixation) Hydroxyapatite-coated porous metals Dense, nonporous, surface-reactive ceramics, glasses and glass ceramics attach directly by chemical bonding with the bone (Bioactive fixation) Bioactive glasses Hydroxyapatite Dense, nonporous (or porous), resorbable ceramics are designed to be slowly replaced by bone Calcium sulfate (Plaster of Paris) Tricalcium Phosphate

Ceramics Tissue Attachment Mechanisms
When biomaterials are nearly inert and the interface between an implant and bone is not chemically or biologically bonded There is relative movement and progressive development of a fibrous capsule in soft and hard tissues The presence of movement at the biomaterial-tissue interface eventually leads to deterioration in function of the implant or the tissue at the interface or both

Ceramics Tissue Attachment Mechanisms

Ceramics Classification of Ceramics
Ceramics used in fabricating implants can be classified as Nonabsorbable (relatively inert) Bioactive or Surface Reactive (semi-inert) Biodegradable or Resorbable (non-inert) Calcium phosphate Putty for coating/filling purpose of various Grafts (which enables excellent cell infiltration, vascularization and resorption) Alumina, zirconia, silicone nitrides and carbons are inert bioceramics Certain glass ceramics and dense hydroxyapatite are semi-inert (bioreactive/bioactive) Calcium phosphates and calcium aluminates are resorbable ceramics Collagen ceramic osteoconductive scaffolds are engineered to mimic the composition and pore structure of natural bone

Ceramics Classification of Ceramics

Ceramics Nonabsorbable (Relatively inert) Ceramics
Relatively bioinert ceramics maintain their physical and mechanical properties while in the host They resist corrosion and wear have properties, such as Non-toxic Non-carcinogenic Non-allergenic Non-inflammatory Biofunctional for its lifetime in the host Examples of relatively bioinert ceramics are dense and porous Alumina (Aluminium oxides) and Zirconia (Zirconium dioxides) etc. Relatively bioinert ceramics are typically used as structural-support implants, such as bone plates, bone screws and femoral heads Examples of non-structural support uses are ventilation tubes, sterilization devices and drug delivery devices

Composition (weight %)
Ceramics Nonabsorbable (Relatively inert) Ceramics Aluminium oxide Al2O3 (Alumina) The main source of high purity Alumina is Bauxite and Corundum The chemical composition of commercially pure Alumina are given below Bauxite rock Chemicals Composition (weight %) Al2O3 99.6 SiO2 0.12 Fe2O3 0.03 Na2O 0.04 The American Society for Testing and Materials (ASTM) specifies that Alumina for implant use should contain 99.5% pure Alumina and less than 0.1% combined SiO2 and alkali oxides (mostly Na2O) Corundum rock

Polycrystalline structure
Ceramics Nonabsorbable (Relatively inert) Ceramics Aluminium oxide Al2O3 (Alumina) The single crystal form of Alumina has been used successfully to make implants The strength of polycrystalline Alumina depends on its grain size and porosity Generally, the smaller the grains, the lower the porosity and the higher the strength The ASTM standards requires Elastic Modulus of around 380GPa and a flexural Strength greater than 500MPa Structure of Single Crystal Polycrystalline structure

Aluminium oxide Al2O3 (Alumina) Single crystal Alumina has been used in orthopedics and dental surgery for almost 20 years Alumina is usually a quite hard material, its hardness varies from 20 to 30 GPa The high hardness is accompanied by low friction and wear and inertness to the in vivo environment. These properties make alumina an ideal material for use in joint replacements

Zirconium dioxide ZrO2 (Zirconia) Zirconium dioxide (ZrO2), sometimes known as zirconia, is a white crystalline oxide of zirconium Zirconium is taken from the mineral zircon (the most important source of zirconium). It is a lustrous, grey-white, strong transition metal that resembles titanium Zirconia (ZrO2) is an oxidized form of the zirconium metal, just as alumina (Al2O3) is an oxidized form of aluminum metal. Zirconium Metal

Zirconium dioxide ZrO2 (Zirconia) Zirconia may exist in several crystal types (phases), depending on the addition of minor components such as calcia (CaO), magnesia (MgO), yttria (Y2O3), or ceria (CeO2) Pure zirconia exists in 3 crystal phases at different temperatures, i.e. Cubic, Tetragonal and Monoclinic. The transformation between phases occurs during temperature change and can lead to structural failure in the material. A number of different oxides can be dissolved into the zirconia crystal structure to prevent or control these structural changes. Y2O3, MgO and CeO2 are the most common.

Zirconium dioxide ZrO2 (Zirconia) These phases are said be stabilized at room temperature by the minor components If the right amount of component is added, one can produce a fully stabilized Cubic phase If smaller amounts are added, 3 wt% to 5wt%, a partially stabilized zirconia is produced. The tetragonal zirconia phase is stabilized, but under stress, the phase may change to monoclinic, with a subsequent 3% volumetric size increase. This dimensional change takes energy away from the crack and can stop it in its tracks. This is called “transformation toughening”

Dental materials, Dental amalgam alloys, noble metals and base metals used in dentistry and alloys for Dental implants, complications of Dental implants.

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Dental materials, Dental amalgam alloys, noble metals and base metals used in dentistry and alloys for Dental implants, complications of Dental implants.

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