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Biomaterials in Orthodontics

Contents         

Introduction Structures and properties of materials Orthodontic wires Orthodontic brackets Etching agents, Adhesive resins & Cements Elastomeric ligatures & Chains Impression materials Prophylactic agents & Miscellaneous

Introduction ď Ž

Knowledge of fundamental principles governing the relationship between composition, structure and properties is central to an understanding of orthodontic materials. Because wide array of metallic, ceramic and polymeric materials are used in the profession, and new materials are continuously being introduced. It is essential that the scientific basis for selection and proper use of materials for clinical practice be thoroughly understood

Structure and properties of materials

Metals Stainless steel:  

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F. Huaptmeyer in 1919 These family of steels contain more than 12% of chromium which owes for its success Relatively high Chromium content in SS→ favors the stability of BCC unit cells of ferrite Ni, Cu, Mn, N→ favors an FCC structure of austenite Other additives are - carbon, Silica, Sulfur, Phosphorus, Manganese Heat treatment of these stainless, which promotes the precipitation of some elements added.

Stainless steels are classified according to the American Iron and Steel Institute Various steels are:       

Austenitic steels (300 series) Martensitic steels (400 series) Ferritic steels Duplex steels Precipitation-hardenable (PH) steels Cobalt containing alloys Manganese containing steels




Austenitic FCC structure is unstable at lower temperature. Austenizing elements (Ni, Mn and N) are added, the highly corrosion resistant solid solution phase can be preserved even at room temperature. The 300 series steels are used for most attachments because of there corrosion resistance.


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In Microstructure of these steels is the same as that of iron at room temperature (BCC). These steels are stronger but less corrosion resistant alloys Such stainless steels should be used only for a short contact with oral environment It is used for sharp instruments and resistant edges

Ferritic steels 

Chromium is substituted for some of the iron atoms in the BCC unit cells Modern “Super ferritics” contain 19% to 30 % chromium and are used in several nickel free brackets. These are highly resistant to chlorides and alloys contain small amounts of aluminum and molybdenum and very little carbon.

Duplex steel 

It consists of an assembly of both austenite and ferrite grains. They also contain molybdenum and chromium and lower nickel content The duplex structure results in improved toughness and ductility.. These steels have been used for the manufacture of one-piece brackets (Eg: Bioline “low nickel” brackets).


These steels can be hardened by heat treatment, which promotes the precipitation of some elements added. PH 17-4 stainless steel is widely used for “mini” brackets. PH 17-7 stainless steel is used to manufacture Edgelock brackets (Ormco)

Titanium 

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The unparalleled tissue tolerance and biocompatibility of titanium have made it the leading metal for dental implants. . Most alloys used in orthodontics contain potentially toxic nickel, chromium, and cobalt. . Titanium alloy are highly corrosion resistant They are strong but not as stiff as stain less steel, hence needed to be compensated by over size It exists in alpha and beta forms; beta form is stable above 1620 deg c . But can be stabilized at lower temperature Titanium oxide film has high affinity which may be the cause for its high frictional resistance. It is used to make intermediate arch wires and can be welded .

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Beta –titanium

Introduced BY BURSTONE AND GOLDBERG Commercial name – TMA (Titanium Molybdenum Alloy) beta-stabilized titanium Composition Titanium – 77.8 % Molybdenum – 11.3 % Zirconium – 6.6 % Tin – 4.3 % A clinical advantage of β - titanium is its excellent formability which is due to the BCC structure of beta stabilized titanium Zirconium and zinc - contribute to increased strength and hardness.

Ceramics 

The first ceramic used was aluminum oxide or alumina, followed by zirconia alumina and zirconia can be found as tridimensional inorganic macromolecules & are esthetically pleasing The ionic crystalline structure accounts for its hardness and compressive strength which exceeds that of the metals, but they have poor flexure strength Alumina is quite stable at normal conditions but zirconia under goes phase transformation from the tetragonal structure to monoclinic structure when cooled through 1100 -1200 range. with a volume change of approximately 3 %that can cause fracture of ceramics

A l 3+

b 2"

A l3+








A l3+

O Al





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A l3+


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b" 2-



b' 2-

Small addition of yttrium oxide and hot isostatic pressing can be employed to achieve very small grain size yttrium oxide partially stabilized zirconia This transformation toughness results in high facture toughness 9- 10mpa.m1/

Organic polymers 

Are natural allies of medicine because they enter the composition of living tissues. To be used in the oral cavity these materials must be nondegradable, stable and should not be mutagenic or carcinogenic The first organic polymer to be used in orthodontics were rubber and its sulfur cross linked derivatives like vulcanite Polymers truly adequate for dental purpose were only discovered in late 1930 s’ Polymethy-methacrylates and polyurethanes by Obeyer in 1937 Polycarbonates and polysulfones has made possible manufacturing esthetic attachment. The discovery of epoxies and cyanoacrylates led to the convenient use of adhesives

Structure and composition  Even though the chemical composition is the same the materials exhibit different properties this is basically determined by the polymer chain length. small chains and residual monomer can be detrimental to their properties  The polymeric chains may be linear branched or three dimensional  with the increase in the side chains the polymer becomes stiff  Polymer shrinkage occurs due to excess monomer

Handling 

The chemical degradation of the polymer and their precursor often takes place well in advance of their delivery Among the most sensitive products are polyurethane elastomers.when these materials are subjected to light high temperature, ph variations, solvents, or even air polyurethanes degrade and become brownish. To control these undesirable effects various additives are need, like the initiators, accelerators ,polymerization inhibitors, plasticizers and uv –stablizers.

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Poly urethanes are further hydrophilic. water and especially saliva hydrolyses them The polymer surface may develop crazes an lodge micro-organism , and become unpleasant Biocompatibility Adhesives, sealants and restorations contain various additives, aromatic amines , peroxide, inhibitors and uv –stabilizers Some of them are potentially toxic, carcinogenic


Implants materials

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The materials commonly used for implants can be divided into 3 categories: Biotolerant - stainless steel, chromium-cobalt alloy. Bioinert - titanium, carbon and Bioactive - vetroceramic apatite hydroxide, ceramic oxidized aluminum.

Titanium Implants Advantages of titanium 

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Commercially pure titanium is the material most often used in implant logy. It consists of 99.5% titanium, and the remaining 0.5% is other elements, such as carbon, oxygen, nitrogen, and hydrogen. Osseo integration no allergic or immunological reactions Mechanical characteristics -very light weight, excellent resistance to traction and breaking .

Orthodontic wires

Orthodontic wires , which generate the bio-mechanical forces communicate through brackets for tooth movement , are central to the orthodontic practice. In the rational selection of wires for particular treatment, the orthodontist should consider a variety of factors.

Low stiffness, good spring back and produce light forces

highly formable and Ability and ease of joining.

Low coefficient of friction Corrosion resistance Cost and biocompatibility

Stainless steel wires – Austenitic stainless steel  excellent formability & corrosion resistance  Stainless steel alloy used for orthodontic wires are 18-8 austenitic type, containing approximately 18 % chromium and 8 % nickel and less than 0.20 percent carbon.Nickel,  Heat treatment of 400 – 500 degree c ……residual stress, heat treatments above 650 degrees c ……..precipitation of chromium carbide at the grain boundaries  The free hand soldering should be done rapidly with a well controlled torch and use of flux .  spot welding often causes localized loss of wrought microstructure

Cobalt chromium nickel wires  

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Elgiloy 1950 . It is available in four tempers that are color coded – blue (soft) , yellow (ductile) , green (semi resilient) and red (resilent) Composition is 40 % Co, 20% Cr, 15 % Ni, 7 % Mo, and 16% Fe. Corrosion resistance & lower elastic force delivery The eigiloy blue as received wire can easily manipulated into desired shapes and then heat treated to achieve considerable strength and resilience. Elgiloy has been used for making fixed lingual quad helix appliance

Has excellent formability.

Spring characteristics are similar to those of stainless steel.

Can be soldered, but technique is demanding.

Corrosion resistance of the wire is excellent.

Beta titanium  

TMA by Burstone and Goldberg highest friction owing to substantial cold welding or mechanical abrasion. Ion-implantation - causes surface hardening and can decrease frictional force by as much as 70% and improve compressive strength, fatigue resistance and ductility of the wires Katherine Kula and proffit in AJO 1998 concluded that there was no significant difference when ion implanted TMA wire when compared to unimplanted TMA wire in sliding mechanics clinically.

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Properties Corrosion resistant and biocompatible Deflection 2 ( S.S) = TMA Beta titanium is ductile Allows direct welding of auxiliaries to an arch wire without reinforcement by soldering. Beta titanium wires are the most expensive of all the orthodontic wire alloys but the increased cost is offset by its combined advantageous properties.

Nickel titanium wires   

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Introduction Andreasen and Hillman in 1971. Nitinol- Buehler. Naval Ordinance Laboratory, its place of origin. NiTi, Nitinol, Orthonol, Sentinol and Titanal Good spring back and flexibility large deflections but low forces

Composition ď Ž

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Nitinol is approximately 52 percent nickel, 45 percent titanium, and 3 percent cobalt. With proper heat treatment, the alloy demonstrates significant changes in mechanical properties and crystallographic arrangement. Have a stabilized martensitic phase formed by cold welding, were the shape memory effect has been suppressed.

Two major NiTi phases are: 1. Austenitic Niti - a ordered BCC structure occurs at high temperatures / low stress. 2.Martensitic NiTi- distorted monoclinic, triclinic or hexagonal structure and forms at low temperatures / high stress. shape memory effect is associated with a reversible martensite to austenite transformation, which occurs rapidly by crystallographic twinning When these alloys are subjected to high temperatures, detwinning occurs, and the alloy reverts to the original shape or size - shape memory effect.

Kusy has classified nickel titanium wires as  

Martensite stabilised alloys- Nitinol. Martensite active alloys- employ the thermoelastic effect for shape memory. alloys such as Neo-Sentalloy and Copper Ni-Ti Austenitic active alloys (SIM) These alloys are the super elastic wires that do not possess thermoelastic shape memory at the temperature of the oral environment such as Nitinol SE

Shape memory effect

Hurst and Nanda in AJO 1990 -specific TTR depends on the chemical composition of the alloy and its processing history. Memory is set in the material by holding it in the desired shape while annealing it at 450° F to 500° F for 10 minutes Once a certain shape is set, the alloy can then be plastically deformed at temperatures below its TTR. On heating through the TTR, the original shape of the alloy is restored. To obtain maximum shape recovery, the deformation should be limited to 7% or 8% , below TTR . Buehler and Cross- shape-memory phenomenon was related to the inherent capability of a nickel-titanium alloy to alter its atomic bonding as a function of temperature

Superelasticity / Pseudoelasticity 

In response to temperature variations, the crystal structure undergoes deformations On activation, the wire undergoes a transformation from austenitic to martensitic form due to stress The different loading and unloading curves produce the remarkable effect the force delivered by the austenitic NiTi wire can be changed during clinical use by merely releasing the wire and retying it. Deflection generates a local martensitic transformation and produces stress-induced martensite (SIM). In orthodontic clinical applications, SIM forms where the wire is tied to brackets on malaligned teeth so that the wire becomes noticeably pliable in the deflected areas, with seemingly permanent deformation

Clinical usage The high springback of nitinol is useful in circumstances that require large deflections but low forces This results in increased clinical efficiency of nitinol wires since fewer arch wire changes or activations are required. for a given amount of activation, wires made of titanium alloys produce more constant forces on teeth than stainless steel wires. A distinct advantage of nitinol is realized when a rectangular wire is inserted early in treatment. This accomplishes simultaneous leveling, torquing, and correction of rotations. Andreasen and Morrow - fewer arch wire changes, less chairside time, reduction in time required to accomplish rotations and leveling, and less patient discomfort. Since hooks cannot be bent or attached to nitinol, crimpable hooks and stops are recommended for use. Cinch-backs distal to molar buccal tubes can be achieved

Chinese Ni Ti wire ď Ž

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Chinese NiTi wire is applicable in situations where large deflections are required Used in conditions were teeth are badly malaligned and in appliances designed to deliver constant forces. There is a force difference if the appliance is left in place throughout the deactivation or if it is removed and retied. If the force levels have dropped too low for a given type of tooth movement, then the simple act of untying and retying can increase the magnitude of the force.

Copper Ni – Ti wires 

In 1994 copper Ni –Ti wires were introduced by the ormco corporation. It is available in three temperature variants: 270 C, 350 C and 400 C corresponding to the austenite finish temperatures Shape memory behaviors is reported to occur for each variant at temperatures exceeding the specified temperature. The addition of copper to nickel titanium enhances the thermal- reactive properties of the wire, thereby enabling the clinician to provide optimal forces for consistent tooth movement.

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Composition They are composed of Nickel – 44% Titanium – 51% Copper – less than 5% Chromium – 0.2 – 0.3% Kusy - wire contains nominally 5-6 wt% of copper and 0.2-0.3 % of chromium. The 27deg C variant contains 0.5% of chromium to compensate for the effect of copper in raising the Af above that of the oral environment. The addition of copper to Ni-Ti not only modifies the shape memory , but also increases the stability of transformation and also helped to control hysteresis width and corrosion resistance.

Uses of copper Ni - Ti wires 27°C Copper Ni-Ti generates forces in the high range of physiological force limits and produces constant unloading forces that can result in rapid tooth movement. Engagement force is lower than with other superelastic wires. This variant would be useful in mouth breathers. 35°C Copper Ni-Ti generates mid-range constant force levels when the wire reaches mouth temperature. Early ligation is easier with full-size archwires due to the lower loading forces. When earlier engagement of full-size wires and sustained unloading forces at body temperature are desired, 35°C Copper Ni-Ti is the ideal wire. This variant is activated at normal body temperature. 40°C Copper Ni-Ti provides intermittent forces that are activated when the mouth temperature exceeds 40°C. It is useful as an initial wire and can be used to engage severely malaligned teeth (such as high cuspids) without creating damaging or painful levels of force or unwanted side effects. It is also the wire of choice for patients scheduled for long intervals between visits when control of tooth movement is a concern. This variant would provide activation only after consuming hot food and beverages.

Japanese Ni-Ti wires Classic NiTi alloy wire used in clinical orthodontics is the work-hardened type wire called Nitinol. ď Ž The Japanese NiTi alloy wire possesses excellent springback property, shape memory, and super-elasticity. ď Ž Super-elasticity is especially desirable because it delivers a relatively constant force for a long period of time, which is considered a physiologically desirable force for tooth movement

Orthodontic brackets

Orthodontic brackets bonded to enamel provide the means to transfer the force applied by the activated arch wire to the tooth.

TYPES OF BRACKETS A) METAL BRACKETS 1) Stainless steel brackets 2) Gold-coated brackets 3) Platinum-coated brackets 4) Titanium brackets B) PLASTIC BRACKETS 1) Polycarbonate brackets 2) Polyurethane-composite brackets 3) Thermoplastic-polyurethane brackets C) CERAMIC BRACKETS 1) Monocrystalline alumina (Sapphire) 2) Polycrystalline alumina 3) Polycrystalline Zirconia (YPSZ)

Stainless steel brackets 

The bracket material used for metal brackets is of Aisi type 316L austentic stainless steel ,to stabilize the austentic structure at room temperature nickel is added This Ni leaches out in the oral environment leading to biocompatibility problems. These brackets have 16 -18 Cr ,10-14 Ni ,2-3 Mo and a maximum of 0.03 C. The content of Cr is comparative less and Cro does not form that effective film compared to Tio . A 2205 stainless steel alloy than contains half the amount of nickel found in 316L alloy has recently been proposed by Oshida and colleagues. The 2205 stainless steel alloy has a duplex microstructure consisting of martensitic and deltaferrectic phases, and is harder then the 316L alloy. Moreover, the 2205 alloy demonstrates substantially less corrosion then 316L alloy

Titanium brackets Pure titanium bracket (Rematitan-DENTAURUM) is a one-piece construction requires no brazing layer, and thus it is a solder- and nickel-free bracket. ď Ž These brackets appear grey and have greater coefficient of friction then stainless steel ď Ž According to Hamula et al in JCO 1996, the problems of nickel sensitivity, corrosion, and inadequate retention of SS brackets has been solved with the introduction of new, pure titanium bracket (Rematitan).

A computer-aided laser (CAL) cutting process generates micro- and macroundercuts, making it possible to design an “ideal” adhesive pattern for each tooth. Single-piece construction allows the lowest possible bracket height, This makes the miniaturized appliance even less conspicuous A low bracket profile can be helpful in assessing lip balance during treatment, especially in cases of lip insufficiency and protrusion.

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Titanium also has low thermal conductivity, and thus alleviates the sensitivity to extreme temperature changes often experienced by patients wearing metal appliances. It imparts none of the metallic taste of stainless steel brackets. Such brackets may provide an alternative to SS brackets for those who are concerned with nickel toxicity

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Composition A commercially pure (cp) medical grade 4 Ti (designation DIN 17851-German standards) is used as the basis for the manufacture of titanium brackets. Composition is Titanium - over 99% Iron - < 0.30% Oxygen - < 0.35% Nitrogen - < 0.35% Carbon - 0.05% Hydrogen - 0.06%

Surface characteristics ď Ž

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The surface texture of the Ti brackets is much rougher than that of the SS brackets. These are the reasons for significantly more plaque accumulation and a more marked change of color with titanium brackets. Titanium brackets are a suitable alternative to conventional metal brackets in many aspects. Their biocompatibility, absence of nickel, good corrosion resistance, superior dimensional stability, comparable frictional characteristics and decreased conspicuousness along with low thermal conductivity make these brackets a suitable alternative to conventional S.S brackets specially in nickel sensitive patients.

Gold coated brackets 

Recently gold-coated steel brackets have been introduced and rapidly gained considerable popularity. Brackets are now available with 24 karat gold plating, plated with 300 micro inches of gold. Gold-coated brackets may be regarded as an esthetic improvement over stainless steel attachments, and they are clean and thus more hygienic than ceramic. Patient acceptance of gold-coated attachments is generally positive. Significant side effects in the form of corrosion or allergic reactions have not been observed

Platinum coated brackets 

The result is a bracket with greater abrasion resistance than gold. A smoother, harder surface than stainless steel for reduced friction and improved sliding mechanics is achieved. By combining platinum metal and an exclusive implantation process, a barrier has been created against the diffusion of nickel, cobalt, and chromium. Platinum has been found to be superior to all other known metals

Nickel free brackets 

Made of Cobalt chromium (CoCr) dental alloy One-piece construction (without solder) by metal injection molding technique Laser structured bracket base for retention

Plastic brackets   

Unfilled polycarbonate 1970 s. creep deformation, discoloration ceramic reinforced, fiberglass and metal slotreinforced polycarbonate brackets were introduced. while metal slot reinforced polycarbonate brackets reported problems with the integrity of the slot periphery. The beneficial effect of these brackets due to their low modulus of elasticity they tend to peel of during debonding like the metal brackets.

Various plastic brackets were:  

Polycarbonate brackets (E.g.Elation) Reinforced polycarbonate brackets ( D B fibre ) Polyurethane-composite brackets (E.g.Envision) Thermoplastic-polyurethane brackets (E.g.Value line)

Polycarbonate brackets 

Various reinforced polycarbonate brackets:     

Polymer fiber reinforced polycarbonate brackets Fiberglass reinforced polycarbonate brackets Ceramic reinforced polycarbonate brackets Metal slot reinforced polycarbonate brackets Metal slot and ceramic reinforced polycarbonate brackets

Bonding mechanism of plastic brackets is mainly mechanical retention


Polycarbonate brackets undergo creep deformation when transferring torque loads generated by arch wires to the teeth Discoloration of first generation unfilled polycarbonate brackets during clinical aging. They absorb water to a slight extent and tend to weaken in the course of about one year (Newman 1973).

Ceramic brackets 

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Ceramics used for the manufacturing of ceramic brackets were Alumina and Zirconia. Both can be found as tridimensional inorganic macromolecules. Types of ceramic brackets Monocrystalline (Sapphire)-Inspire ,Starfire TMB Polycrystalline Alumina-Allure, Mxi, Clarity Polycrystalline Zirconia (YPSZ) ) Hi-Brace

Bonding mechanisms : 

Mechanical retention employing large recesses. Chemical adhesion facilitated by the use of a silane layer. Micromechanical retention through the utilization of a number of configurations, including protruding crystals, grooves, a porous surface, and spherical glass particles

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Strength of the brackets can be increased by eliminating the surface flaws that can serve as the sites of stress concentration and fracture initiation. Decreasing the grain size also increases the strength of polycrystalline brackets. The polycrystalline zirconia brackets manufactured by injection molding technique followed by isostatic pressing in partially stabilized zirconia 5 %wt yttrium oxide


Several in-office bracket-reconditioning methods have been introduced since 1980,

Grinding - Wright and Powers (1985) Sandblasting - Millet et al (1993), Sonis (1996) Direct flaming Buchman method - Buchman (1980) BigJane machine method - Buchman (1980)

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Buchman (1980) concluded that as temperatures are increased in thermal treatment, the hardness and tensile strength are decreased and the microstructures illustrate corresponding susceptibility to metallic intergranular corrosion. Matasa et al (1989) described that heating method used for reconditioning metal brackets causes intergranular corrosion. He also enumerated the effects of heat on brackets like, structural metal weakening, vertical slot obstruction, corrosion and base clogging

Enamel Etching

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Traditionally slots were welded to the bands and cemented to the crowns, before the introduction of etching agents and adhesive resins, which demanded extra arch width ,time of the clinician, compromised oral hygiene and esthetics With the introduction of acid etching (Bonocore) opened new vents for bonding brackets to the teeth. Etching of enamel creates microporosities with in the enamel and reduces the surface tension that allows resins to penetrate and polymerize with in the etched enamel rods (mechanical bonding)

Enamel etching with acids 

Type and concentration of acid

Orthodontic bonding of brackets to teeth does not require high bond strength (6-8 mpa), as need in restorative dentistry. Various studies have shown that etching with 10%-37% phosphoric acid has provided adequate bond strength. Some studies have also shown that even treating the enamel with 2% phosphoric acid have been able to provide adequate bond strength. The use of 10% maleic acid for etching results in lower bond strength.

Duration of etching

No difference in bond strength was detected between 15 second and 60 second etching with 37% phosphoric acid However shorter etching time results in decreased bond strength (0 – 5 seconds) Scanning electron microscopy showed that etching with 37% phosphoric acid for atleast 30 seconds produces more optimal etchin pattern than etching for 15 seconds.

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Resin composite does not bond well to un etched enamel; however, hybrid inomer orthodontic cements have a bond strength ranging from 8 to 25 mpa . Hybrid inomer cements have better bond strength to enamel than sand blasted metal bracket base. And these cements lack cohesive strength Use of pumice before etching to clean the enamel of surface deposits has shown no alteration in bond strength

Iatrogenic effect of etching 

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Fracture or cracking of the enamel during debonding Porosities caused by etching may cause staining of tooth Loss of enamel. (10- 20 um) Resin tags retained in the enamel after debonding may get discolored. Alternate methods for etching enamel

Acidic primer Composition  Acid(phenyl-p)  HEMA  And dimethacrylate 

Although they are expensive ,comparable bond strength can be attained It also reduces the chair time

Air abrasion 

Also known as micro etching is a tecnique in which particles of aluminum oxide are propelled against the surface of enamel causes abrasion of the enamel surface Micro etching metals is an effective way of increasing bond strength of brackets Microetching of enamel produces only 50 % of the bond strength to that of acid etching

Laser etching 

The application of laser energy on the enamel surface causes localized melting and ablation. Removal of enamel results primarily by micro explosion of entrapped water in the enamel Laser etching is done by neodymium-yttriumaluminum garnet Laser typically produces low bond strength compared to acid etching

Crystal growing solution 

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A proposed alternative to etching for retention of adhesive is to grow crystals on the enamel surface This technique is called crystal bonding The potential advantages of crystal bonding include easy debonding, less residual adhesive left on the tooth, and less enamel damage. It includes application of poly(acrylic acid) solution containing sulfate ions,which cause growth of calcium sulfate dehydrate crystals on the enamel It produces 60-80 %0f bond strength compared to acid etching Acidic primers

Bonding agents

Based on the polymerization initiation mechanism: 

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Chemically activated (self cured ) :two paste or one paste Light cured ( photo cured ) Dual cured (chemically activated and light cured Thermo cured

Chemically activated orthodontic adhesive systems 

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These adhesives employ benzoyl as an initiator and tertiary aromatic amine such as dimethy-p-toluidene as activator Initiation occurs from the mixing of the paste and liquid components of these systems and free radicals are formed by multi step processes Chemically cured two phase systems: Polymerization is initiated by mixing of liquid and paste Clinical handling is laborious; time consuming Properties Increased exposure of the components to the air induces oxygen inhibition Mixing introduces defects due to trapping of air and formation of voids Concise (3M)

Chemically cured one phase system 

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Application of the liquid component on the enamel and on the bracket base. No mixing is involved Clinical handling: - efficient application ;limitation in time requirements Properties Limited data is available on the bond strength and degree of curing Inhomogeneous Patten of curing due to the sandwich involved Enamel bracket sides of the polymer is more polymerized than the middle of the bracket System 1(ormco) Unite (3M)

Visible light cured The photo initiator in these systems is camphoroquinone and a reducing amine  Polymerization is initated by exposure to light curing source Clinical handling  Provides increased working time and bracket placement time.  Curing should be done from the incisal and cervical margins Properties  The degree of cure of a stain less steel brackets bonded with light cured adhesive is comparable to that of a transparent aesthetic bracket 

Dual cure Polymerization is initiation is achieved by through exposure to light and the reaction progress following a chemically cured pattern Clinical handling  Combines the disadvantages of handling both light cured and chemical cured materials.  Most time consuming application Properties  Increased degree of cure and bond strength ,but of questionable clinical significance  Ideal for bonding molar tubes 

Moisture active Polymerization – cyanoacrylate, no liquid component is involved. Polymerization is initiated on exposure to water Clinical handling  One step procedure-intentionally the surface of the tooth must be wetted  One of the study has shown acceptable bond strength  Smart bond (Gestenco) 

Moisture resistant 

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Primer compatible with the use of the adhesive Clinical handling Application of primer on wet enamel surface Trans bond MIP(3M)

Microbial alteration and caries prophylaxis

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Individuals with malocclution have many retion site owing to the irregularities of teeth .more retion sites are introduced when orthodontic appliances are bonded and banded to teeth. Oral hygine is thus markedly more difficult to maintain for orthodontic patients

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Table 6.1

Caries prophylaxis aspect of orthodontic treatment ď Ž ď Ž

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Fluoride is the most caristatic agent know The mechanism of action of fluoride is generally believed to be due to its effect in plaque film around the mineral crystallites, by inhibiting demineralizing and increasing remineralizing of mineral lost during the caries processes Rationale caries prophylactic measures for orthodontic patient is prevention of caries lesion developed during orthodontic treatment

Fluoride tooth pastes Fluoride toothpaste is the basis for all caries prevention. Most tooth pastes contain sodium fluoride, monofluorophosphate, stannous fluoride  The fluoride concentration may vary, but the maximum concentration allowed is 0.15 %.fluoride concentration less than 0.1 % should not be recommended for orthodontic patients  The cariostatic effect of fluoride will improved significantly if oral hygiene is improved  The anionic agent sodium lauryl sulphate is a popular detergent  It increases the permeability of the oral mucosa and increases the nickel sensitivity and has been reported for helping in development of ulcers

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Stannous fluoride has a plaque inhibiting effect in addition to the anticaries action. the stannous ion is responsible for the plaque inhibiting effect. Stannous inhibit the adsorption of plaque bacteria to the enamel by bonding to the phosphate polymer lipoteichoic acid present on the surface of gram positive bacteria. Stannous fluoride also interferes with the acidogenicity of the plaque. It is possible that tin atoms bound to the surface of the bacteria also block the sucrose from entering the cell and there by preventing acid formation Detergents and surface active agents are incorporated in to the tooth paste and mouth rinses to lower the surface tension and loosen and penetrate the deposits and emulsify and suspend the debris

Fluoride supplements 

High and long lasting cariogenic challenge For average orthodontic patient it was found that tooth pastes alone were inadequate to stop caries and there fore recommended the use of fluoride mouth rinses (0.05 % NaF ) daily An improved cariostatic effect can be achieved by use of fluoride in combination with antibacterial agents like chlorhexidine, triclosan and zinc Topical fluoride in the form of varnishes or gels may be recommended Solution of titanium tetra fluoride inhibits the development of lesions associated with fixed appliances more efficiently than other conventional preparation ,its mechanism of action is probably due to retentive, titanium rich. Glaze like coating formed on the treated enamel

Fluoride releasing bonding agents 

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Fluoride reservoir that does not depend on patient co operation .and fluoride is deposited in an area immediately adjacent to the caries susceptible areas. Gass inomer cements, and resin modified glass inomer cements In studies simulating oral environment, it was found that the fluoride availability from glass inomer cements is PH-controlled. glass inomer cements take up fluoride from the oral environment and released . It is seen that much fluoride is released during the first few days to weeks. Short term studies have shown that here is reduction in incidence of caries but long term effect shows there was a significant reduction in the release of fluoride Fluoride releasing sealants are also available s

Elastomeric ligatures and chains

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Elastomeric products are used in orthodontics as ligatures and as continuous modules for the engagement and the retraction. Despite the popularity, there has been some concern about the force degradation exhibited by the elastomeric chains Efforts have also been directed to minimize plaque retention capacity of elastomeric chains. Fluoride releasing elastomeric ligatures has been introduced to minimize the risk of demineralization of enamel margins

Composition and structure 

The elastomeric ligature and chains are polyurethanes, which are thermosetting polymers possessing a –(NH)-(C=O)-O-stural unit formed by condensation polymerization. The cross linking between the chains must be relatively few to facilate large extension with the failure of primary bonds. The glass transition temperature of biomedical polyurethanes range from -50 to -80 deg. C The difference in energy between the rigid and rubber states corresponds to increase in the amount of molecular motion experienced by the polymer after undergoing the glass transition temperature

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Greater the glass transition temperature more rigid is the polymer and generates more force. The two main methods of processing the modules are injection molding technique and die stamping. The die stamping polymers are found to be more consistent in physical properties. It is said that pigments added to the elastomers also effect the physical properties of polymer, however the general studies has shown that there is no difference between the conventional and the colored elastomeric materials.

Fluoride releasing polyurethanes 

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Advances in the field of elastomerics include the introduction of products with fluoride releasing features It was thought a reliable means of long term fluoride releasing areas adjacent to the bracket margins would be paramount significance. Though it was thought such fluoride release is beneficial it is said to hamper the properties of elastomers and early degrading of elastomer

Elastomeric chains 

Significant differences in their force decay characteristics have been reported. These differences may be attributed to several factors , variation in manufacturing techniques, variations in additives incorporated in the basic polyurethane polymer, variation in morphology or dimensional characteristics . Many in vitro studies that have measures the force degradation rate of he elastomerics modules ……the census of these studies is that elastomerics modules experience a steeps decline in force , ranging from 40 to 50 % during the first 24 hrs which continues at a lower rate for nest 2 to 3 weeks.

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Traditionally these modules have been used for retraction of anterior teeth to close extraction spaces as well as midline diastemas with the advent of Super elastic ni ti coil springs which can deliver low constant forces , use of elastomerics has diminished significantly Then there has been a criticism related to lack of mechanical control of teeth Engaged with elastomerics chains , because loss of directional control of moments leads occasionally to undesirable mesio-distal or bucco-lingual rotations as describes previously that the elastometic chains loose almost half the applied force

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very early , some investigators have proposed that this deficiency can be counteracted by application of higher initial force of 3 to 4 times the desired force level .Some other investigators felt that the application of orthodontic force up to 4 times the optimal level for tooth moment may have unpredictable outcomes on biological processes Several studies have also dealt with the use of prestreaching to eliminate the force loss by elastomerics modules . 2 methods have been suggested , one is instantaneous pre stretching technique by young and et el other is the extended time technique by Brantley et al but evidence has also shown that it only eliminates about 10 % of force decay

Cements in orthodontics

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In orthodontics application of cements is limited to luting of appliances, for acceptable performance the luting agent should have a variety of properties : Adequate working time and setting time. high tensile Compressive Shear strength Resistance to dissolution Clinically acceptable bond strength low adhesive remnant index score on debonding, and anti carigenic potential

Zinc phosphate cement ď Ž

Once zinc phosphate cements were widely used for cementation of orthodontic bands.

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These cements are available as hand mixed powder and liquid system although some encapsulated products are marketed

Composition  The principal constituent of cement powder is zinc oxide.  A small quantity of Magnesium oxide which improves mechanical properties and color stability. Small amounts of additives such as Silica or alumina which also improves mechanical properties.  Approximately 10% fluoride in the form of stannous fluoride for anti cariogenic effect.  The liquid is an aqueous solution of phosphoric acid in concentrations from 45% to 64% buffered by 2 to 3% of aluminum phosphate and 1 to 9% of zinc phosphate.  Buffered by a small quaintly of 2-3% of aluminum phosphate .

Properties.  The powder to liquid ratio for the cement strongly affects the working and setting times.  A thin consistency (low viscosity)is essential when the cement is used as a luting agent, to ensure adequate flow during cementation of orthodontic bands.  A reasonable working time for zinc phosphate cements ranges between 3 and 6 minutes, and the setting time should be between 5 and 9 minutes.  For optimum results the powder should be incorporated into the liquid in small Proportions and a relatively slow rate to achieve the desired consistency, the exothermic setting reaction is retarded

In contrast, rapid mixing of the cement powder and liquid causes substantial heat evolution, with considerable decrease in working and setting time. Mixing over a large area of the glass slab also results in a lower temperature increase from the setting reaction Cooled and dried mixing slab retards the reaction rate. Care must be taken not to cool the slab below dew point, since condensation from the air can cause contamination by water. The film thickness should be around 20 um Since adhesion has not been documented to develop between zinc phosphate cements and orthodontic bands, retention of the bands is attained by mechanical interlocking.

Properties  Initial setting takes place at 4-7 minutes it attains up to 50 %of the final strength which is attained at 24 hours. Exhibits compressive strength of 80 – 140 mpa tensile strength of less than 5 mpa  Solubility  During the first 24 hours of cementation significant solubility of the cement has been reported as much as 0.04 -3.3 %by weight and decreases due time .  High powder liquid ratio decreases the solubility of the cement Draw backs  Zinc phosphate is brittle.  It has a relatively high solubility in the mouth and it does not adhere to tooth substance.  Zinc phosphate relies on mechanical interlocking for its retentive effect.  It does not provide any chemical bonding to tooth or metal surfaces.

Zinc polycarboxylate cements:

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These cement were introduced by smith in 1968 and these cements were the 1 st dental materials developed with an adhesive potential to enamel and dentin and it has the desirable properties of zinc phosphate and zinc oxide eugenol

Composition 

The zinc polycarboxylate is available in powder liquid formations, the powder consists of zinc oxide with 10 % of magnesium oxide or tin oxide , silica , alumina , or bismuth salts and small quantities of stannous fluoride may be incorporated in some brands The presence of fluoride in these cements also increases the strength while controlling the setting time, The liquid is an aqueous solution of a homopolimer o acrylic acid or co polymers acrylic with other unsaturated carboxylic acids such as etaconic and maelic acid . The acid concentration is 40 %by wt. it has a relatively high molecular wt which may increase the strength of the material . undesirable effects such as short shelf life and difficulties in manipulation occur because of high viscosity

Mixing of the polycarboxylate cements should be completed rapidly within 30 to 40 seconds. The working time varies from 2 to 5 minutes at room temperature and setting time ranges from 6 to 9 minutes at 37 degree centigrade. The powder should be rapidly incorporated into the liquid in large quantities to optimise the working and setting time. The working time of polycarboxylate cement can be extended by lowering the temperature of the mixing slab and storing the powder in a refrigerator.

Refrigeration of liquid is avoided because gelation may occur from formation of hydrogen bonds. The cement mixture should be used while it still has a glossy surface appearance. Although the polycarboxylate cement mixture has a thicker consistency than that for zinc phosphate cement its flows readily when loaded, yielding an appropriate film thickness of 20 μm. The fully set cement at 24 hours after mixing has compressive strength ranging from 48 to 80 Mpa. Tensile strength ranging from 8 to 12 MPa. Modulus of elasticity of zinc polycarboxylate cement is between 3 to 6 GPa.

Solubility 

The solubility of these cements in water is low , ranging between .1 % to .6%by wt. some products incorporating stannous fluoride exhibit higher solubility because of fluoride release . These cements have low resistance to dissolution under acidic conditions such as lactic acid citric acid.depending upon the pH of the environment.

Bonding  These cements are capable of bonding with surfaces of metallic restorations, prostheses and appliances particularly nickelchromium, silver-palladium and stainless steel alloys Disadvantages  High viscosity.  High intraoral solubility  Short setting time.

Glass inomer cement History ď Ž Glass ionomer cements was first introduced by Wilson and kent in 1972 ď Ž It is referred to as polyalkenoate cement ,also known as ASPA(Alumino-silicate poly acrylate)

Glass ionomer cements can be classified based on use as:  Type I or luting cements.Glass particle size is 13 to 19 microns.Powder to liquid ratio is approximately 1.5:1.  Type II or restorative cements.Particle size of upto 50 microns.Powder to liquid ratio is approximately 3:1  Type III. Chemically set liners/bases or pit and fissure forms.  Type IV.Visible light activated liners/bases

Glass ionomer cement was introduced to orthodontics in 1986. Composition  GIC powder is an acid soluble Calcium flouroalumino silicate glass.  Main constituents include silicon dioxide, aluminium oxide,calcium flouride, aluminium phosphate, aluminium flouride and sodium flouride.  The liquid for GIC is an aqueous solutions of polyacrylic acid in a concentration of about 50%.  The liquid was quite viscous and tended to gel over time.  The other acids were in the form of itaconic acid, maleic acid and tartaric acid.These acids tend to increase the reactivity of the liquid, decrease the viscosity and reduce the tendency for gelation.  These acids can be freeze dried into powder and added to glass powder or water, to extend the working time. 

Properties  The luting glass ionomer cements have working times 3 to 5 minutes and setting time 5 to 9 minutes.  The water based glass ionomer cements have longer working and setting times compared to polyacid containing cements.  The margins of setting cement should be protected from moisture contamination with a varnish  Compressive strength ranges between 90 and 140 MPa.  Tensile strength is 6 to 8 MPa.  Modulus of elasticity is 3.5 to 4 GPa.  The flexural strength (9 to 20 MPa) and fracture toughness are higher for glass ionomer cements than for all other of luting cements

The solubility in water of the fully set cement is considerably lower than that of zinc polycarboxylate and zinc phosphate. Their early susceptibility to moisture within 4 to 10 minutes after the start of mixing is very high. Prior to application of glass ionomer ,the enamel surface for bonding may be conditioned with an aqueous solution of polyacrylic acid having a concentration in the range of 10 to 40%. GlC act as a reservoir of fluoride,providing a possible means to minimise the potential of subsurface enamel demineralisation. The initially elevated level of fluoride release is attributed to higher elution occuring before the cement has set. the cement,finally attaining a low constant level

Hybrid resin inomers Since the early 1990's,hybrid resin/glass ionomer products have been introduced for clinical purposes. They may be categorised as:  Resin modified glass ionomers  Compomers (polyacid modified resin composites)  Ionomer modified composite 


The powder of resin modified glass ionomer cements, consists of either the glass composition used for conventional glass ionomer cements barium aluminosilicate glass is also incorporated in some products. Significant alteration have been made in the liquid component of RMGIC's The most prominent changes are the replacement of water by a water-HEMA (Hydroxyethyl methacrylate)mixture and the incorporation of photoinitiators or chemical initiators for free radical polymerisation. In some products,methacrylate based monomers BisGMA, TEGDMA and UDMA are added to the polyacrylic acid solution ; The final hardening and strengthening is enhanced by the formation of polycarboxylate salt matrix.

Properties  Resin modified glass ionomer cements have longer working time and undergo rapid setting after light curing.  The desirable film thickness for luting applications may be obtained with a lower powder to liquid ratio.  The enamel surface should first be pumiced,rinced and dried without desiccation taking place.  The powder should be incorporated into liquid in large portions and rapid spatulation for 10 seconds is suggested; the usual working time is upto 20 seconds.  A more rapid rate of strength development for these materials may be attained by photopolymerisation, which presumably accelerates the setting process.  RMGIC appear to provide significantly higher bond strength than the conventional GIC and a decreased probability for bond failure.

The fluoride release from RMGIC has a relatively constant rate, characterized by a substantially lower initial ion elution compared to other types of glass ionomer cements. Resin modified glass ionomer cement demonstrate superior compressive and tensile strength, fracture toughness and are more resistant to permanent deformation and dissolution in wet environment, even during the early setting stage. A more rapid rate of strength development for these materials may be attained by photopolymerisation, which presumably accelerates the setting process. RMGIC appear to provide significantly higher bond strength than the conventional GIC and a decreased probability for bond failure. The fluoride release from RMGIC has a relatively constant rate, characterized by a substantially lower initial ion elution compared to other types of glass ionomer cements.

Compomers 

These are supplied as anhydrous single pastes and contain major ingredients of both resin composites and glass ionomers,except for water. Exclusion of water ensures that initial setting occurs only by polymerisation and is essential in preventing premature setting of the material in the container An acid base reaction may occur later as the material absorbs water invivo.This cannot take place without appreciable water diffusion. By the time this has occured,the self limiting visible light cure generated network will have a sufficient cross link density to suppress extensive reaction, although the water does provide a measure of plasticization. Flouride release is minimal.However strength and ease of handling are

IONOMER MODIFIED COMPOSITES  These set only by a polymerisation mechanism but contain ion leachable glasses in an attempt to achieve flouride release.  Developments involving the use of glass ionomers as luting agents has been the introduction of selfcured hybrid resin/glass ionomer products  These cements have several advantages compared to traditional glass ionomer luting agents  They have greater tensile strength and are less brittle.  In addition,they release at least as much flouride as traditional glass ionomers,are less soluble and are less sensitive to moisture contamination and desiccation.

IONOMER MODIFIED COMPOSITES  These set only by a polymerisation mechanism but contain ion leachable glasses in an attempt to achieve flouride release.  Developments involving the use of glass ionomers as luting agents has been the introduction of self-cured hybrid resin/glass ionomer products  These cements have several advantages compared to traditional glass ionomer luting agents  They have greater tensile strength and are less brittle.  In addition,they release at least as much flouride as traditional glass ionomers,are less soluble and are less sensitive to moisture contamination and desiccation.

Impression materials

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There are a variety of impression materials for general dentistry but orthodontic purposes the routinely used impression materials are alginate less commonly polyvinylsiloxanes

Alginate hydrocolloid Components: Diatomaceous earth K alginate or Ca alginate K sulfate Na phosphate Ammonium salts & CLX Glycol Others

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Functions Filler Alginate gel Plaster setting Retarder Disinfectants Render the powder dust less Provide taste and color

Properties of the impression materials and relation ship to clinical use Properties before insertion into the patient mouth ď Ž

Material cost and shelf life

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Ease of preparation and use

Properties while in the patients mouth      

Biocompatibility Patient acceptance Flow Wetting of oral structures Setting dimensional change Setting time

Properties during the removal from patient mouth   

Flexibility Tear strength Creep compliance

Properties after removal mouth 

Dimensional stability

Immunity and disinfection

Compatibility with die material

Polyvinysiloxane ď Ž

Component Siloxane oligomer Oligomer with terminal vinyl group + Platinum acid crystal + Filler

Function Mech & chem. inertness for cross linking Catalyst Improve the handling

Bonding to non conventional surfaces

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With the ever increasing adult patients undergoing orthodontic treatment situations arise were many of these patients have restored teeth. Although banding can one alternative for bonding these teeth but sometimes it may become necessary to bond these teeth for esthetics and better hygiene. The materials commonly used for restoration teeth are ceramics, cast alloys, composites, amalgam restoration, acrylic resins

Bonding to ceramics ď Ž

A variety of ceramic materials are used to restore teeth, it becomes extremely difficult for the clinician to identify the chemical composition but what becomes important is the external restoration of the ceramic. The principal veneering material used is leucite containing feldspar.. How ever there are other materials like fluorine mica silicate in dicor castable alloy systems .these type of ceramics all the color is obtained from the external layer hence can be problematic. All other ceramic achieve there color from the internal ceramic layer.

The protocol for bonding the ceramic surfaces is as follows 

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The glaze is first removed by sand blasting , using 50 um aluminum oxide for 2 to 4 seconds The ceramic surface is then etched with 9.6% HF acid for two minutes Subsequently followed by two to three layers of silane coupling agent on the etched surface and followed by drying. Then two layers of unfilled resins are applied as thin coating Then the bracket is finally bonded to the prepared ceramic surface with highly filled bisGMA resin. It is said that if the color of ceramic and integrity of the restorations not to be altered and further chances of rebonding are there then it is advised not to follow the above protocol

Alternative surface preparation that have been found to achieve satisfactory results  mechanical roughening with stones and diamond  sand blasting  chemical roughening with hydro fluoric acid  and chemical coupling with the use of saline

Bonding to casting alloys  A proper surface preparation and special adhesives are required for acceptable bonding to the casting alloys. although roughening the surface of the alloy with stone increases the bond strength.  intaoral sand blasters provide better results  Research has shown that tin plating the noble alloys increases the bond strength  In the recent years adhesives that chemically bond to metal surface have been developed. The commercial products superbond C & B and C & B metabond .  Other commercial products like panavia EX and panavia21 and bis-GMA  Intermediate resins are also used

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Bonding to dental amalgam Sand blasting of the surface the restoration followed by use of adhesives4-metabisGMAand intermediate resins improves the bonding to the dental amalgam Bonding to composite resins The uppermost resin composite has to be removed with a diamond bur then the surface is etched with37% phosphoric acid.silination follows before the application of an unfilled resin and bonding

Bonding of acrylic resins

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The surface is first wetted with methyl methacrylate for three minutes then the brackets then can be bonded using a bonding agent and resin composite Or it can be embedded in a Pmma restoration+ Glass ionomer cements

Dental Solder ď Ž

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Brazing : it is the joining of metal parts by a filler metal at temperature below the solidus temparature of the metals being joined and above 450 deg c Soldering :it is the joining of metal parts by a filler metal at temparature below the solidus temparature of metals being joined and below 450 deg c


hard solder

Soft solder 

Lead tin alloys

Gold solder

Not used

Silver solder

Composition Gold solder  Gold:-45 to 81%  Silver:- 8 to 30 %  Copper :- 7 to 20 %  Tin:- 2 to 4 % Silver solder  Silver:- 10 to 80 %  Copper :- 15 to 50 %  Zinc :- 4 to 32%

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Properties These have high melting temperature Greater strength and hardness

Conclusion Whether to surrender to the manufacturer

or drive away the bullâ&#x20AC;Ś..

Thank you

Biomaterials in orthodontics 1/ dental implant courses by Indian dental academy  

The Indian Dental Academy is the Leader in continuing dental education , training dentists in all aspects of dentistry and offering a wide r...

Biomaterials in orthodontics 1/ dental implant courses by Indian dental academy  

The Indian Dental Academy is the Leader in continuing dental education , training dentists in all aspects of dentistry and offering a wide r...