• Glass ceramics are the material that are initially formed as glass, and then transformed into ceramic usually by a controlled heat treatment.

• The heat induces partial devitrification which increases the strength as well as improves aesthetics by making it less transparent and more teeth like.

• The two glass ceramics used in dentistry are Castable glass ceramics and pressable glass ceramics.

• Advantages :- Ease of fabrication, Good aesthetics, improved strength and fracture toughness, Good marginal fit, very low processing shrinkage, low abrasion of opposing teeth.

• Disadvantages :- Inadequate strength for posterior teeth.

Castable glass ceramics

•It’s properties are more closer to that of glass and it’s construction is quite different. This is only porcelain restoration that is made by centrifuging casting technique.

• The subsequent Ceramming process is also quite unique to this porcelain Ceramming will enhance the growth of mica crystals within the ceramic.

• It is first commercially available glass ceramic for dental use was Dicor.

• Composition :- Dicor glass ceramic contains 55% volume of Tetrasilicic fluoromica crystals.

• features :- The dicor glass ceramic crowns are very aesthetic. This is because of greater translucency (unlike the other porcelain which have more opaque core). It also picks up some of the color from adjacent teeth (chaemelon effect) as well as from underlying cement.

Uses:- inlays , onlays , veneers

Fabrication of a dicor crown:-

• wax pattern is constructed first and then invested with a refractory material.

• After burnout of the wax ,nuggets of the dicor glass are melted and cast into the mould in centrifuging casting machine.

• The glass casting is carefully recovered from the investment by sandblasting and the sprues are gently cut away.

•The glass restoration is then covered with embedment material to prepare it for next stage that is Ceramming.

• Ceramming is the heat treatment process by which the glass is strengthened. Ceramming results in the development of microscopic crystals of mica which

– improves the strength and toughness of glass

– improves the aesthetics of the restoration ( it reduces the transparency of the glass making it more opaque and less glass like)

• The cerammed glass can be built up with special veneering porcelain and fired to complete the restoration. Surface stains may be applied to improve the aesthetics.

Reference :- Manapalil text book of dental materials

•Our denture base is hard not flexible.So polyamide is added in flexible dentures to make them flexible and enhance the properties.

• Allergic reaction with conventional denture bases is because of free monomer and due to polymethyl methacrylate.

• Main properties of using flexible denture bases over the conventional ones are to avoid allergies to acrylic and metal and to improve retention.

• To improve aesthetic and make them more acceptable to patient.

• Flexible dentures help to avoid some kind of pain associated with old style denture models.

• Flexible denture bases helps to achieve greater stability and comfort.

• In addition to these benefits flexible denture are also designed to be porous and to breathe better.

• This helps to prevent the build up of bacteria on the denture.

• Nowadays it has become selective treatment of option. No more ugly metal wires.No more broken dentures.

• It is indicated in full dentures , partial dentures. Unique features of flexible denture Base are flexibility, strength, transparency, high impact resistance and high solvent resistance.

• Commercial names of flexible denture base are valplast, proflex, sunflex, unbreakable flexit plus dentures.

Reference :- Self notes

Reference :- Manapalil text book of dental materials

Let’s know why was it introduced..

Bonding agents were introduced to improve or to create the bond between the tooth and the Restorative material.

Which was the first bonding agent ??

It’s sevriton cavity seal. It’s based on glycerophosphoric acid dimethacrylate. It had limitations like high polymerization shrinkage and high thermal expansion.

Composition of a dentin bonding agent:-

Primer + Etchants + adhesive , solvents, initiators, fillers particles and others ingredients like polyalkenoic copolymer

• Use of primer it’s to wet the surface properly or to reduce the contact angle. They maintain an expanded collagen network to allow filtration of hydrophobic monomer as well. Ex:- HEMA( Hydroxy ethyl methacrylate ) and TEGDMA ( Tri ethanol glycol dimethacrylate ).

• Conditioners are also called as Etchants. 10% Malic acid for 1 minute , and 37% phosphoric acid for 15 sec and if the concentration of phosphoric acid exceeds more than 50% then it forms monocalcium phosphate monohydrate which inhibits dissolution. Etching is done for both enamel and dentin.

• Mode of action of etchant on enamel :- by selective dissolution of enamel rods at the centre or peripheries or both. They act by increasing the surface energy and increases the surface area which creates microporosities on the surface of enamel.

• Mode of action of etchant on dentin :-It involves removal of smear layer and opening of dentinal tubules

Smear layer :- During tooth preparation the cut material along with water forms a thin film on the floor of the cavities known as smear or debris layer

Smear layer is it desirable? ?

In dentin the smear layer becomes burnished into the underlying dentinal tubules and it lowers the dentin permeability which has a protective effect.

Developing a bonding agent for enamel was easier than those of dentin, What would be the reason ?

Dentin contains water and due to inhomogeneous composition and presence smear layer will be a problem, whereas for enamel it contains more inorganic content that is calcium hydroxyapatite crystals.

• The purpose of using solvents is in order to increase the diffusion of primers and adhesive into micro retentive tooth surface and most common solvents used are ethanol and acetone.

• Adhesives in 1st generation are GPDM, 2nd generation BISGMA, TEGDMA, 3rd generation NPG- GMA,4th generation NPG-GMA, HEMA, 5th generation PENTA , Methacrylated phosphonates, 6th and 7th generation methacrylated phosphonates in water

• The etchant generally was 37% phosphoric acid almost for all generations; and primer was HEMA.

• Initial generations had all the components primer+ etchant + adhesive separately; as time progressed new generations evolved with all in one bonding agent.

• 2 step method :- 5th and 6th generation; 1 step method :- 7th generation ( all in one system )

• Indications for use of bonding agent:- When composite resin is used as Restorative material; And even when the porcelain veneers are bonded and when the exposed dentin is to be desensitized.

• Conditions that satisfy the true adhesion of Restorative material with tooth structure are sound tooth structure must be conserved; optimal retention must be achieved, microleakage should be prevented.

Hybrid layer :- Structure formed in dental hard tissues by deminerlization of surface followed by infiltration of monomers into collagen mesh and subsequent polymerization is called hybrid layer. It was reported by Nakabayshi in 1982.

Reference :- Philips and Manapalil text book of dental materials

Thermal properties

The arrangement of atoms and molecules in materials is influenced by the temperature; as a result, thermal techniques are important in understanding the properties of dental materials

Thermal conductivity : thermal conductivity of a substance is the quantity of heat in calories or joules per second passing through a body 1cm thick with a cross section of 1 sq.cm when the temperature difference is 1°C.

Eg – a large amalgam filling or gold crown in proximity to the pulp may cause the patient discomfort when hot or cold foods produce temperature changes, this effect is mitigated when adequate tooth tissue remains or cavity liners are placed between the tooth and filling for insulation.

Specific heat : specific heat of a substance is the quantity of heat needed to raise the temperature of 1g of the substance by 1°C.

Eg – during the melting and casting process, the specific heat of the metal or alloy is important because of the total amount of heat that must be applied to the mass to raise the temperature to the melting point.

Thermal diffusivity : it is a measure of transient heat flow and is defined as the thermal conductivity, divided by the product of the specific heat, times the density.

Eg- for a gold crown or a dental amalgam, the low specific heat combined with the high thermal conductivity creates a thermal shock more readily than normal tooth structure does.

Coefficient of thermal expansion : the change in length per unit length of a material for a 1°C change in temperature is called the linear coefficient of thermal expansion.

Although the coefficient is a material constant, it doesn’t remain constant over wide temperature ranges. For eg, the linear coefficient of thermal expansion of a dental wax may be an average value of 300×10-6/°C upto 40°C, whereas it may have an average value of 500×10-6/°C from 40-50°C.

The coefficient of thermal expansion of a polymer changes as the polymer goes from a glassy state to a softer, rubbery material. This change in the coefficient corresponds to the glass transition temperature.

It is obvious that with the reduction in temperature, there is a contraction of a substance as much as of expansion tht occured during heating. Accordingly, tooth structure and restortive materials expand when warmed by hot food or beverages and contract when exposed to cold substances. Such expansions and contractions may break the marginal seal of a filling in tooth particularly when the difference between coefficient of thermal expansion of tooth and restorative material is too large.

Electrical properties

The ability of a material to conduct an electric current may be stated as conductivity or conversely as the specific resistance or resistivity. The conductivity by materials used to replace tooth tissues is of concern in restorative dentistry.

Dielectric constant : a material that provides electrical insulation is known as dielectric. The dielectric constant of a dental cement generally decreases as the material hardens. This decrease reflects a change from a paste that is relatively ionic and polar to one that is less.

Electromotive force : the electromotive series is a listing of electrode potentials of metals according to the order of their decreasing tendency to oxidise in solution. Those metals with a large negative electrode potential are more resistant to tarnish than those with a high positive electrode potential.

Galvanism : the presence of metallic restorations in the mouth may cause a phenomenon called galvnic action where saliva or bone fluids like electrolytes make up an electric cell.

Corrosion : the corrosion of gamma, gamma 1, gamma 2 phases in amalgam has been studied by electrochemical analysis. The dental amalgam specimens become pitted at the boundaries between the phases or in gamma 2 phase. The addition of copper to amalgam alloys to form copper-tin compounds during hardening has improved the resistance of amalgam to chloride and galvanic corrosion.

Tarnish : the process of steam sterilization of surgical instruments has long presented a serious problem of tenish and corrosion. Many non metallic materials such as cements and composites have shown a tendency to discolor in service because the colored substances penetrate the materials and continue chemical reactions in the composites.

Source : Craig’s textbook of restorative materials

After qualifying intermediate, we enthusiastically dream about our dental subjects but land up in the same boring physics of dental materials. Let us know why is it important to understand various properties…

Mechanical properties :

Stress – the force per unit area acting on millions of atoms or molecules in a given plane of material. Stress is the internal resistance of a material to an external load applied on that material.

• Residual stress is caused within the material during the manufacturing process. Eg – during welding
• Structural stress is produced in the structure during function. Eg – in abutments of fixed partial dentures
• Pressure stress is induced in vessels containing pressurized materials. Eg – in dentures during processing under pressure and heat
• Flow stress is produced when force of liquid strikes against the wall acting as load. Eg – molten metal alloy striking the walls of the mould during casting
• Thermal stress is produced by material which is subjected to internal stress due to different temperatures causing varying expansions in the material. Eg – materials that undergo thermal stress such as inlay wax, soldering and welding alloys.
• Fatigue stress is produced due to cyclic rotation of a material. Eg – rotatory instruments undergo rotational or cyclic fatigue.

Strain – it is defined as the change in length per unit original length and it may be elastic or plastic or a combination of both. Elastic strain is reversible i.e it disappears when force is removed. Plastic strain represents permanent deformation of the material which never recovers when the force is removed.

Young’s modulus : it is the stiffness of a material that is calculated as the ratio of the elastic stress to elastic strain i.e a stiff material will have a high modulus of elasticity while a flexible material will have a low modulus of elasticity.

Eg – principle of elastic recovery – burnishing of an open metal margin, where a dental abrasive stone is rotated against the metal margin to close the marginal gap as a result of elastic and plastic strain

Eg – impression material

The impression materials should have a low modulus of elasticity to enable it to be removed from the undercut areas in mouth. Modulus of elasticity should not be too low that the material cannot withstand tearing.

Hooke’s law : within the limits of elasticity the strain produced by a stress is proportional to the stress

Dentin is capable of sustainable significant plastic deformation under a compressive load before it fractures. Enamel – more stiffer and brittle than dentin. But dentin is more flexible and tougher.

Flexibility – defined as the flexural strain that occurs when the material is stressed to.its proportional limit. Materials used to fabricate dental appliances and restoratiots, a high value for the elastic limit is a necessary requirement. This is because the structure is expected to return to it’s origi al shape after it has been stressed and the force removed.

There are instances where a large strain or deformation may be needed with a moderate or slight stress such as in an orthodontic appliance. Here a spring is often bent a considerable distance under the influence of a small stress. In yhis case, the structure is said to possess the property of flexibility.

Resilience – the amount of energy absorbed within a unit volume of a structure when it is stressed to its proportional limit. When a dental restoration is deformed during mastication, it absorbs energy. If induced stress is not greater than proportional limit, the restoration is not permanently deformed i.e only elastic energy is stored in it. So restorative material should exhibit a moderately high elastic modulus and relatively low resilience.

Proportional limit – defined as the magnitude of elastic stress above which plastic deformation occurs. Below the proportional limit, there is no permanent deformation in a structure. Materials like cobalt/chromium alloy which has high proportional limit is widely used for the fabrication of connectors because they can withstand high stresses without permanent deformation.

Yield strength – defined as the stress at which a test specimen exhibits a specific amount of plastic strain. It is a property often used to describe the stress at which the material begins to function in a plastic manner. In the process of shaping an orthodontic appliance or adjusting the clasp of a removable partial denture it is necessary to apply a stress into the structure in excess of yield strength of the material is to be permanently bent or adapted.

Flexural strength – defined as the force per unit area at the instant of fracture in a test specimen subjected to flexural loading. Also known as modulus of rupture. Most prosthesis and restoration fractures develop progressively over many stress cycles after initiation of a crack from a critical flaw and subsequently by propagation of the crack until a sudden, unexpected fracture occurs.

Conclusion – while designing a dental appliance or a restorative material, it should have adequate mechanical properties to withstand the stress and strain caused by the forces of mastication. All the methods must be employed to minimize stress concentration so that the restorative material or the appliance is in harmony with the different types of forces occuring in the oral cavity.

Source : Phillip’s and Craig’s restorative dental materials