Luting agents for fixed prosthodontics/ orthodontic course by indian dental academy

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Luting agents for fixed prosthodontics/ orthodontic course by indian dental academy

  1. 1. LUTING AGENTS FOR FIXED PROSTHODONTICS Introduction Numerous dental treatments necessitate attachment of indirect restorations and appliances to the teeth by means of a cement. These include metal, resin, metal-resin, metal-ceramic, and ceramic restorations; provisional or interim restorations; laminate veneers for anterior teeth; orthodontic appliances; and pins and posts used for retention of restorations. The long-term clinical outcome of fixed prosthodontic treatment depends, in part, on the use of adhesives that can provide an impervious seal between the restoration and the tooth. Schwartz et al in 1970 found that loss of crown retention was the second leading cause of failure of traditional crowns and fixed partial dentures. Therefore the clinical success of these luting agents depends on the cementation procedure and clinical handling of these materials. The word ‘luting’ is often used to describe the use of a moldable substance to seal a space or to cement two components together. There are several types of available luting agents, each possessing unique properties and handling characteristics. No one product is ideal for every type of restoration; some of them requiring multiple technique sensitive steps. Although the establishment of optimal resistance and retention forms are obtained from proper tooth preparation, luting agents should essentially serve the following purposes: a) Act as a barrier against microbial and the restoration. b) Seal the interface between the tooth and the restoration. 1
  2. 2. c) Hold the restoration and tooth together through some form of surface attachment. This attachment may be mechanical, chemical or a combination of both methods. Ideal requirements of luting agents: i) Should provide a durable bond between dissimilar materials. ii) Should possess favourable compressive and tensile strengths. iii) Should have sufficient fracture toughness to prevent dislodgement as a result of interfacial or cohesive failures. iv) Should be able to wet the tooth and the restoration. v) Should exhibit adequate film thickness and viscosity to ensure complete sealing. vi) Should be resistant to disintegration in the oral cavity. vii) Should be tissue compatible. viii) Should demonstrate adequate working and setting times. Presently there are cements used for the temporary or permanent cementation of fixed prosthesis. They are: 1) Zinc phosphate. 2) Zinc oxide-eugenol. 3) Zinc oxide-non-eugenol. 4) Zinc polycarboxylate. 5) Glass ionomer Type I. 6) Resin composite cements or compomers. 7) Resin-modified glass ionomer or hybrid ionomers. 2
  3. 3. 1) Zinc phosphate cement ADA specification No. 8 in 1935 defines the properties and requirements for zinc phosphate cement. Powder Weight (%) Zinc oxide (ZnO) principal ingredient. 90.2 Magnesium oxide (MgO) reduces the temperature of the calcination process. 8.2 Silicon dioxide (SiO2) inactive filler and aids in the calcination process 1.4 Bismuth trioxide (Bi2O3) imparts a smoothness to the freshly mixed cement in large amounts it may also lengthen the setting time. 0.1 Barium oxide (BaO), Barium sulphate (Ba2SO4) Calcium oxide (CaO) 0.1 Tannin-fluoride may be added in some commercial products. The ingredients of the powder are heated and sintered at temperatures between 1000°C and 1400°C into a calcined mass, that is subsequently pulverized to a fine powder which is sieved to recover selected particle sizes. 3
  4. 4. Liquid Weight (%) Phosphoric acid (H3PO4) (free acid) 38.2 Phosphoric acid combined with aluminium and zinc Al and Zn partially neutralize the acid, temper its reactivity and act as buffering agents which helps in establishing a smooth, nongranular, workable cement mass. 16.2 Aluminium (Al) 2.5 Zinc (Zn) 7.1 Water H2O 36.0 Controls the setting time and mechanical properties The water content controls the ionization of the acid and influences the rate of setting reaction. This is important to the clinician because an uncapped liquid bottle will permit loss of water resulting in retarded set. Setting reaction When powder particles are wet by the liquid, phosphoric acid attacks the surface of the particles and releases zinc ions into the liquid. The resultant mass yields a hydrated, amorphous network of zinc aluminophosphate gel on the surface of the remaining portion of the particle. The set cement is a cored structure consisting primarily of unreacted zinc oxide particles embedded in a cohesive amorphous matrix of zincaluminophosphate. In presence of excess moisture formation of crystalline hopeite (Zn3 (PO4)2 4H2O) takes place. 4
  5. 5. Manipulation a) No definite P/L ratio and maximum amount of powder should be incorporated into the liquid. b) A cool mixing slab should be employed. The cool slab prolongs the working and setting times. c) The liquid should not be dispensed until the mixing is to be initiated to prevent loss of water. d) Mixing is initiated by incorporation of small portions of powder into the liquid over a wide area to minimize the heat and effectively dissipate it. e) Spatulate each increment for 15 seconds before adding another increment. f) Completion of the mix usually requires approximately 1 minute 30 seconds. g) The casting must be seated immediately with a vibratory action before matrix formation occurs. h) After the casting has been seated, it should be held under pressure until the cement sets to minimize air inclusion. i) The procedure should be carried out in a dry, clean environment. j) Excessive cement should be removed after it has set and a layer of varnish should be applied to the margin to decrease the initial dissolution. k) Frozen Glass slab Method In this method a glass slab cooled at 6°C or at –10°C is used. Around 50% to 75% more amount of powder can be incorporated into the 5
  6. 6. liquid. The working and setting times are prolonged with little difference in physical and mechanical properties. Properties Working and setting times Working time is the time from the start of mixing during which the viscosity of the mix is low enough to flow readily under pressure to form a thin film. Setting time mean that matrix formation has reached a point where external physical disturbance will not cause permanent dimensional changes. It is defined as the elapsed time from the start of mixing no longer penetrates the cement as the needle is lowered onto the surface. Net setting time is 2.5 to 8.0 minutes at 37°C and 100% humidity it varies from 5-9 minutes. Factors influencing the setting time: Those controlled by manufacturer: - Powder composition. - Degree of powder calcinations. - Particle size. - Buffering of liquid. - Water content of liquid. 6
  7. 7. Those controlled by the operator and their influence on selected properties: Manipulative variables Compressive strength Film thicknes s Solubility Initial acidity Setting time Decreased P/L ratio Decrease Decrease Increase Increase Lengthen Increased rate of powder incorporation Decrease Increase Increase Increase Shorten Increased mixing temperature Decrease Increase Increase Increase Shorten Water contamination Decrease Increase Increase Increase Shorten Physical properties When properly manipulated, the set cement exhibits a compressive strength of 104MPa, diametral tensile strength of 5.5MPa, modulus of elasticity of 13GPa. Thus it is quite stiff and resistant to elastic deformation even when it is used for cementation of restorations in high stress-bearing areas. The strength is influenced by P/L ratio, composition, manner of mixing and handling of the cement. Solubility and disintegration The solubility in water in 24 hours is 0.2%. Solubility depends on initial exposure to water of the incompletely set cement resulting in increased dissolution. Greater resistance to solubility can be obtained by increasing the P/L ratio. 7
  8. 8. Consistency and film thickness Two consistencies are used i.e. luting and base. The luting consistency is tenacious and provides a mechanical interlocking between the surface irregularities of the tooth and the restoration. The maximum film thickness is 29µm. It depends on the consistency and seating pressure. Viscosity Viscosity increases with increased P/L ratio, mixing time and higher temperature. Increased viscosity can result in increased film thickness and incomplete seating. Dimensional stability It exhibits shrinkage on hardening ranging from 0.04% to 0.06% in 7 days. Thermal and Electrical conductivity It is an effective thermal insulator and protects against thermal trauma to the pulp. Acidity The acidity of the cement is quite high at the time of cementation of a prosthesis. Two minutes after the start of mixing the pH is approximately 2. It increases rapidly but still is only about 5.5 at 24 hours. The pH remains relatively low for long durations. Zinc phosphate cement does not chemically bond to tooth structure and provides a retentive seal by mechanical means only. Thus, the taper, length and surface area of the tooth preparation are critical to its success. 8
  9. 9. Microleakage, aggravated by dehydration in oral fluids and an initial low setting pH may affect its biocompatibility in clinical use. Applications Permanent luting of well-fitting, prefabricated and cast posts, metal inlays, onlays, crowns, FPDs, and aluminous all-ceramic crowns to tooth structure, amalgam, composite, or glass ionomer core build ups. 2) Zinc-oxide Eugenol and 3) Non-eugenol cements Composition of Type I luting agent Powder Weight (%) Zinc oxide 69.0 White resin reduced brittleness of the set cement 29.3 Zinc state plasticizer 1.0 Zinc acetate improves strength 0.7 Liquid Eugenol 85.0 Olive oil 15.0 To increase the strength of the cement for luting purposes, two modifications have been made  Type II luting agents: a) Methyl methacrylate polymer is added to powder (20% by weight) (Kalzinol). b) Alumina (Al2O3) (30% by weight) is added to powder and ethoxybenzoic acid (EBA) is added to liquids (62.5% ortho EBA by weight). 9
  10. 10. The non-eugenol cements contain an aromatic oil and zinc oxide. Other ingredients may include olive oil, petroleum jelly, oleic acid, and beeswax. Setting reaction The cement sets by chelation reaction to form eugenolate and water. The presence of moisture is essential for setting to occur. Manipulation A paper mixing pad is used. A P/L ratio of 4-6:1 is employed. The bulk of the powder is incorporated in the initial step, the mix is thoroughly spatulated, and then a series of smaller amounts is added until the mix is complete. Mixing time required is usually 90 seconds. The reinforced cements are kneaded for 30 seconds and then stopped for 60 seconds to develop a creamy consistency. Properties Setting time ranges from 4 to 10 minutes. For reinforced cements, since the P/L ratio increases, the setting time decreases. Setting time depends on composition of powder, particle size, P/L ratio, accelerator and temperature. Physical properties Type I luting cement has a compressive strength of 2.0-14MPa, non- eugenol cements have values of 2.7-4.8MPa, polymer modified has a strength of 37MPa with EBA-alumina having the highest strength 64MPa. Elastic modulus ranges from  0.22 for Type I, 2.7 for Kalzinol and 5.4 for EBA-alumina. 10
  11. 11. Solubility and disintegration Due to the bleaching of eugenol, solubility is high and ranges between 1.5 to 2.5%. addition of additives decreases the solubility. The solubility in water (%) in 24 hours for polymer modified cements is 0.08 and EBA-alumina is 0.02-0.04. Film thickness Film thickness of polymer modified cement is 2.5µm and EBA alumina is 25-35µm. Dimensional stability Shows a shrinkage of 0.9-2.5% on setting. Biologic properties It has a pH of 7-8. It does not cause any harm to the pulp but due to leaching of eugenol, it is an irritant. Therefore non-eugenol cements are used for some patients. Highly compatible with the pulp and has an obtundant effect. It also has an antibacterial action. Its disadvantages such as decreased strength, high solubility, irritant to soft tissues, poor retention and difficulty in manipulation limit its use for temporary cementation purposes. It should not be used for temporary luting purposes when the permanent luting agent is likely to be a resin cement as the eugenol inhibits polymerization. 11
  12. 12. Applications It is used primarily for temporary luting of restorations. 3) Sdadsds 4) Zinc polycarboxylate cement (Zinc polycarboxylate cement) Composition Powder Weight (%) Zinc oxide 85 Magnesium oxide or stannic oxide 10 Stannous fluoride traces of silica dioxide, bismuth, aluminium and colour pigments. 4-5 The powder is sintered and fused to reduce the reactivity of zinc oxide. Liquid Aqueous solution of polyacrylic acid or copolymers of acrylic acid in the range of 30-40%. Molecular weight of 25,000 to 50,000. Tannic acid and malleic acid 10-45%. Tartaric acid prevents gelation on storage 5%. Water settable cements – Here the mixing liquid is water – polyacrylic acid is frozen, dried, powdered and mixed with the original P/L ratio of these cements is very high. 12
  13. 13. Setting reaction and adhesion to tooth structure: When powder and liquid are mixed, a fast acid-base reaction occurs as the powders are rapidly incorporated into a viscous solution of high molecular weight polyacrylic acid. The powder particles are attacked by the acid and zinc, magnesium and tin ions are released. Zinc ions react with the carboxyl group of polyacrylic acid of the same chain and the adjacent chain to cause cross linking. The calcium ions of the tooth structure react with the free carboxyl groups of acid to form a metallic ionic bond. The bond between cement and dentin is 3.4 MPa. Under ideal conditions the adhesion of polycarboxylate cement to a clean, dry surface of the tooth is greater than any other cement. The cement adheres better to a smooth surface than to a rough one. It does not adhere well to gold and porcelain. The failure is at the cement-metal interface. Cement cannot bond to the metal in chemically dirty or pickled condition. Surfaces of the metal have to be sandblasted or electrolytically etched to achieve optimum bonding. Adhesion with stainless steel is excellent. 13
  14. 14. Manipulation P/L ration is 15:1. A glass slate is used for mixing to prevent absorption of liquid. Firstly a meticulously clean surface is essential to intimate contact and interaction between the cement and the tooth. 10% polyacrylic acid solution is used to clean the tooth surface for 10-15 seconds followed by rinsing with water for removal of smear layer. After cleansing, isolate the tooth to prevent further contamination by oral fluids. Blot the surface before cementation. The powder is rapidly incorporated into the liquid in large quantities for a period of 30 to 60 seconds. Mixing on a cooled glass slab prolongs the working time. The cement must be placed on the inner surface of casting and on tooth surface before it loses its glossy appearance. Loss of gloss indicates decreased availability of carboxyl groups, poor bonding, poor wettability due to stringiness and increased film thickness causing incomplete seating of the casting. Precautions Do not refrigerate the liquid dispense the liquid just before mixing. Mixing should be rapid. Use only on cleaned surfaces. Use before glossiness disappears. Removal of excess cement During setting, the cement passes through a rubbery stage. During this stage, excess cement should not be pulled away from the margins as it can leave voids at the interface. Remove excess cement only after it 14
  15. 15. becomes hard. Apply petroleum jelly to the outer surfaces of the prosthesis and soft tissues to prevent cement from adhering to them. Properties Setting time is 6-9 minutes. Working time 2.5 – 3.5 minutes. Viscosity The set mix is pseudoplastic in nature. The cement seems viscous, but during cementation pressure, excess flows out from under the margins of the restoration. Physical and Mechanical properties The compressive strength of 24 hour set cement is 57-99 MPa; tensile strength is 3.6-6.3MPa and elastic modulus is 4.0-4.7GPa. Bond strength to dentin is 2.1MPa, to enamel is 3.4-13MPa. Film thickness of polyacrylate cements is 25-48 µm. Solubility of cement in water is low (<0.05%) but when it is exposed to organic acids with a pH of 4.5 or less, the solubility increases markedly. Reduction of P/L ratio also increases solubility and disintegration in the oral cavity. Dimensional stability They show a linear contraction when setting at 37°C. Biologic properties The pH of the cement liquid is 1.7, but the liquid is rapidly neutralized by the powder. PH increases rapidly as the cement sets. 15
  16. 16. Despite the initial acidic nature, these cements produce minimal irritation to the pulp because of quick neutralization and the lack of tubular penetration of the sized polyacrylic acid molecules. This excellent biocompatibility with the pulp is one of the strongest clinical merits of this cement. Advantages  Low level of irritation and increased biocompatibility with the pulp.  Adhesion to tooth structure.  Easy manipulation.  Anticariogenic.  Thermal insultor  Hydrophilic and capable of wetting dentinal surfaces. Disadvantages  If acute proportioning is not done, properties are affected i.e. solubility and disintegration increases.  Working time is short.  Clean surface is required for adhesion.  Absorbs water and softens into a gel.  Increased solubility in acids.  Difficult to remove flash. Failures occurs at cement-metal interface. After hardening, polycarboxylate cements exhibit significantly greater plastic deformation; 16
  17. 17. thus the cement is not well suited for use in regions of high masticatory stress or in the cementation of long-span prosthesis. 17
  18. 18. Application Used for the cementation of single metal units in low stress areas on sensitive teeth. 5) Type I Glass Ionomer Cement / ASPA or Aluminosilicate polycrylate / Alkeneate Definition Akinmade and Nicholson in 1993 defined glass ionomer cement as “a water based cement wherein, following mixing, the glass powder and the polyalkenoic acid undergo an acid base setting reaction”. Mclean and Nicholson defined GIC as “a cement that consists of a basic glass and an acidic polymer which sets by an acid base reaction between these components”. Composition Powder The basic component of a glass ionomer cement powder is a calcium fuoroalumino silicate glass with a formula of: SiO2-Al2O3-CaF2-Na3 AlF6-AlPO4 The nominal composition of the glass is listed below: Chemical Weight (%) SiO2 Al2O3 CaF2 Na3AlF6 AlF3 AlPO4 29.0 16.6 34.3 5.0 5.3 9.8 18
  19. 19. The raw materials are fused together to a uniform flass by heating them to a temperature of 1100°C. The glass is then ground to a powder having particles in the range of 20 to 50µm. glasses high in silica are transparent, whereas glasses high in calcium fluoride or alumina are opaque. Fluoride is an essential constituent of GIC: - It lowers the fusion temperature. - Improves the working characteristics. - Increases the strength of the set cement. - Contributes to anticarcinogenic property perhaps the rationale for using GIC as a luting agent is based on its ability to release fluoride ions into the underlying dentin. This helps prevent secondary caries which is the most cause of failure. The powder is described as an ion-bleachable glass that is susceptible to acid attack when the Si/Al atomic ratio is less than 2:1. Cryolite is added to supplement the flexing action of calcium fluoride and to increase the translucency. Aluminium phosphate improves translucency and adds body to the cement paste. Barium glass may be added to provide radiopacity. Liquid The liquid typically is 4.75% solution of 2:1 polyacrylic acid / itaconic acid copolymer (average molecular weight 10,000) in water. The acid is a polyelectrolyte, which is a homopolymer or copolymer of unsaturated carboxylic acid known as alkenoic acids. 19
  20. 20. The itaconic acid reduces the viscosity of the liquid and inhibits gelation caused by intermolecular hydrogen bonding. Intermolecular hydrogen bonding Tartaric acid is present in 5%, as an optically active isomer and serves as an accelerator by facilitating the extraction of ions from the glass powder. Also tartaric acid prolongs the working time, improves handling characteristics, enables fluoride contact of glass to be reduced and helps in the production of bear glasses. Water is the basic reaction medium and plays a role in hydrating reaction products, that is metal polyalkenoate salts and silica gel. Water settable GIC To extend the working time, one GI formulation consists of freeze dried acid powder and glass powder in one bottle and liquid components in another. The chemical reaction procedures in the same way except that these cements have a longer working time and a shorter setting time. Setting reaction and adhesion to tooth structure The cement sets hydroacid base reaction and consists of 2 stages. The first occurs during the initial 5 minutes when the reaction between the powder and the liquid forms a silaceous hydrogel. The second stage requires about 24 hours and occurs when a polysalt matrix completely surrounds all of the initial reaction products. When the powder and liquid are mixed the following sequence of events take place: i) Polyacid attacks the glass to release calcium, aluminium, sodium and fluoride ions. 20
  21. 21. ii) These ions react with the polyanions to form a salt gel matrix. iii) The polyacrylic acid chains are cross-linked by Ca++ in the first 3 hours. iv) Subsequently aluminium ions react for atleast 48 hours. v) The fluorides and phosphates form insoluble salts and complexes. vi) The sodium ions form a silica gel. vii) Some of the sodium ions replace, the hydrogen ions of the carboxyl groups and the rest combine with fluorine ions. viii) The cross-linked phase is hydrated by water. ix) The unreacted portion of glass particles are sheathed by silica gel. x) Thus, the set cement consists of an agglomeration of unreacted powder particles surrounded a silica gel in an amorphous matrix of hydrated calcium and aluminium polysalts. xi) The glass ionomer chemically bonds to enamel and dentin. It seems that bonding involves an ionic interaction with calcium and/or phosphate ions from the surface of the tooth structure. This results in chelation of carboxyl groups of the polyacids with the calcium in the apatite of the enamel and dentin. 21
  22. 22. xii) Role of water in the setting process Water hydrates the cross-linked matrix, thereby increasing the material strength. During the initial reaction period, this water can be readily removed by dessication and is called loosely bound water. Also at this stage the GI readily absorbs moisture into the glossy matrix resulting in a compromised material. Therefore, any contact with saliva or oral fluids has to be prevented for the first 24 hours to prevent early disintegration and dissolution. As the setting continues the water becomes tightly bound and cannot be removed. This hydration is critical in yielding a stable gel structure and building the strength of the cement. Manipulation The prepared tooth structure and the inner surface of the casting are cleaned. The tooth surface is cleaned with a slurry of pumice, rinsed and then dried but not dehydrated. Undue dessication opens up the dentinal tubules, enhancing penetration of the acidic liquid. A glass slab or a paper pad is used for mixing. A plastic spatula should be used. Use of a metal spatula, causes abrasion by the glass particles of the metal surfaces resulting in discoloration of the set cement. P/L ratio for GIC Type I is 1.3 : 1. The powder is introduced into the liquid in large increments and spatulated rapidly for 30 to 45 seconds. Encapsulated products typically are mixed for 10 seconds in a mechanical mixer and dispensed directly. Hand mixed cements often contain bubbles of larger diameter, which may contribute to a decrease in strength. 22
  23. 23. The cement must be used before it loses its glossy apperance. The field must be isolated completely. Once the cement has achieved its initial set (7 minutes), the cement margins should be coated with a varnish. Precautions:  Tooth should be conditioned.  Should be protected from moisture and drying during setting.  Should be used before loss of glossy appearance.  Flash should be removed only after cement hardens. Properties : Setting time of GIC is within 6-8 minutes from the start of mixing. Physical and mechanical properties. The 24 hour compressive strength of GIC ranges from 93-226MPa, tensile strength being 4.2-5.3MPa and elastic modulus of 3.5-6.4 GPa. Strength increases between 24 hours and 1 year and is significantly increased by initial protection from moisture. Low values of elastic modulus make them susceptible for elastic deformation increase of high masticatory stress. The bond strength of GIC to dentin is 3-5MPa. The bond strength can be improved by treatment of the dentin with an acidic leaching agent followed by an application of a dilute aqueous solution of FeCl3. The GIC bond well to enamel, stainless steel, tin-oxide plated platinum and gold alloy. 23
  24. 24. Solubility and disintegration A 24 hours solubility for GIC in H2O is 0.4-1.5% solubility is less in acidic solutions and also depends on initial exposure to water. Film thickness for GIC is 22-24µm. Biologic properties: i) Resistance to microleakage The cement bonds adhesively to tooth structure and prevents ingress of fluids at the interface. This is probably because the coefficient of thermal expansion of GIC is similar to that of the adjacent tooth structure particularly the dentin. ii) Anticariogenic due to the release of fluoride ions. iii) Post cementation sensitivity This is related to the pH and the length of time that this acidity persists. The pH of the mix at 2 minutes after mixing is 2.33 and it increases upto 5.67 in 24 hours but never reaches neutral pH. Also if the tooth is excessively dehydrated before cementation, the tubules open up allowing acids to seen through. If the crown is overfilled, the excessive hydraulic pressure required to remove excess cement caused sensitivity. Advantages i) Chemical adhesion to tooth structure. ii) Anticariogenic. iii) Esthetic properties. iv) Ease of manipulation. 24
  25. 25. v) They possess low film thickness and maintain relatively constant viscosity for a short time after mixing. This results in improved seating of cast restorations. Disadvantages i) Low film thickness can cause inhomogenous distribution of curing stresses and microcracks resulting in cementation failure. ii) Low elastic modulus iii) Susceptibility to moisture attack and subsequent solubility if exposed to water during the initial setting period. iv) Early exposure to moisture and saliva decreases the ultimate strength. v) Susceptibility to dehydration and cohesive failure due to microcracks. vi) Post cementation sensitivity. Applications Used as permanent luting agent for cast posts, metal inlays, onlays, crowns, FPDs and all-ceramic crowns to tooth structure, amalgam, composite core build ups. 6) Resin Composite Cements Resin cements are variations of filled BIS-GMA resin and other methacrylates. Composition The early resin cements were primarily poly(methylmethacrylate) powder with inorganic fillers and methyl methacrylate liquid. The 25
  26. 26. composition of most modern resin based cements is similar to that of composite resin materials. Because most of the prepared tooth surface is dentin, monomers with functional groups that have been used to induce bonding to dentin are often incorporated in these resin cements. They include organophosphonates, hydroxyethyl methacrylate (HEMA) and the 4-methacryloxyethyl trimellitic anhydrite (4-META) system. The phosphonate cements also contain a silanated quartz filler. The phosphonate is very sensitive to oxygen, so the margins of the casting have to be protected until setting has occurred. The phosphate end of the phosphonate reacts with calcium of the tooth or with a metal oxide. The 4-META cement is formulated with methyl methacrylate monomer and acrylic resin filler and is catalyzed by tri-butyl borax. They polymerize through conventional peroxide amine induction systems (chemically initiated polymerization) or by photoinitiation or a combination of both (dual-cure systems). Manipulation The chemically activated systems are available in powder-liquid system or as two paste systems. The peroxide initiator is in one component and the amine actiator is contained in the other. The components are mixed on a paper pad for 20-30 seconds. The restorations should be promptly seated and excess cement should be removed immediately. Light activated systems are single component systems. The time of exposure to light needed for polymerization of the resin cement is dependent on the light transmitted through the ceramic restoration. It should never be less than 40 seconds. 26
  27. 27. The dual cure systems are 2-component systems. The part of the cement that does not come in contact with the light source is cured chemically. Adhesion to tooth structure i) Application of an acid or dentin conditioner to remove the smear layer and smear plus. ii) The tubules are opened and widen with demineralization of the top 2 to 5µm of dentin. iii) The acid dissolves and extracts the apatite mineral phase that normally covers the collagen fibres of the dentin matrix and opens 20 to 30nm channels around the collagen fibres. These channels provide an opportunity to achieve mechanical retention of subsequently placed hydrophilic adhesive monomers. If application of the conditioner exceeds 15 seconds, a deeper demineralized zone results which resists subsequent resin infiltration. If complete infiltration of the collagen by the primer does not occur, the collagen at the deeper demineralized zone will be left unprotected and subjected to future hydrolysis and final breakdown. iv) After demineralization, the primer, a wetting agent such as HEMA is applied. The agent is bifunctional, in that it is both hydrophilic, which enables a bond to dentin, and hydrophobic, which enables a bond to the adhesive. The primer is applied in multiple coats to a moist dental surface. Multiple coats are required to replace the water in the damp dentin with the resin monomers and to carry the adhesive material into the tubules. The 27
  28. 28. primer is gently dried so as not to disturb the collagen network but to remove any remaining organic solvents or water. v) Adhesive resin is then applied to the “primed” surface to stabilize the primer infiltrated demineralized dentin. Retention is achieved by the following means:  Infiltration of resin into etched dentin, producing a micromechanical interlocking with the open tubules forming resin tags; which underlies the hybrid layer of resin interdiffusion zone.  Adhesion to enamel through the micromechanical interlocking of resin to the hydroxyapatite crystals and rods of etched enamel.  Adhesion to dentin, involving penetration of hydrophilic monomers through a collagen layer overlying partially demineralized apatite of etched dentin. vi) The use of dentin bonding agents has somewhat compensated for the polymerization shrinkage evident with all resins. Properties Their properties on dependent on compositional differences, amounts of diluent monomers and filler levels. Setting time  4-5 minutes at 37°C. Compressive strength  52-224MPa. Tensile strength  37-41 MPa. Elastic modulus  1.2-10.7 GPa. Bond strength to dentin  11-24 MPa with bonding agent. 28
  29. 29. Virtually insoluble in oral fluids Film thickness  13-20µm. Exhibits polymerization shrinkage which is an impediment to complete dentinal adhesion. They are pulpal irritants due to the presence of bleachable monomers. Bond strength in Tension MPa Substrate Resin cement Dentin (unetched) Enamel (etched) Ni-Cr-Be alloy Sandblasted Electrolytically etched Type IV Gold alloy Sandblasted Tin-plated 4% 15.0 24.0 27.4 22.0 25.5 Advantages a) Resin cements bond chemically to resin composite restorative materials and to silanated porcelain. They increase the fracture resistance of ceramic materials that can be etched and silanated. b) They demonstrate good bond strengths to sandblasted base metal alloys, the 4-META resin cements show strong adhesion as a result of chemical interaction of the resin with an oxide layer on the metal surface. Noble alloys may be 29
  30. 30. electroplated with tin to increase the surface area for bonding and enable a chemical bond with tin oxide. c) Most resin adhesives are filled, 50% to 70% by weight, with glass or silica due to which they exhibit high compressive strength, resistance to tensile fatigue and virtual insolubility in the oral environment. d) Improved marginal wear resistance. e) Some of the formulations contain ytterbium tori fluoride, other include a barium fluorosilicate filler and have fluoride release and cariostatic potential. f) Offer adequate retention for short, tapered crown preparations. Disadvantages a) High filler content increases viscosity, which reduces flow and increases film thickness and chances of incomplete seating of the restoration. b) polymerization shrinkage. c) Irritant to the pulp. Their ability to adhere to multiple substrates high strength, insolubility and shade matching potential have made them the adhesives of choice for cementation of the following:  Resin composite inlays and onlays.  All-ceramic inlays and onlays.  Veneers, crowns, FPDs. 30
  31. 31.  Fiber reinforced composite restorations.  Luting base metal resin bonded bridges (“Maryland” type). 31
  32. 32. 7) Resin Modified Glass Ionomers / Hybrid Ionomers To overcome inherent drawbacks of GIC such as moisture sensitivity and low early strength, polymerizable functional groups have been added to the formulations to impart additional curing processes and allow the bulk of the material to mature through acid-base reaction. This group of materials are also known as light cured GICs, dual cure GICs (light cure and acid base reaction), tri-cure GICs (dual cure and chemical cure), resin ionomers, compomers and hybrid ionomers. Composition and setting reactions The powder consists of ion-bleachable glass and initiators for light or chemical curing or both. The powder blends is formed of glass, tartaric acid and polyacrylic acid. The liquid component may have only water or polyacrylic acid modified with HEMA monomers and methacrylate monomers. They contain hydroxyl groups that make them water soluble. These are the simplest form of resin ionomers. They are mixed in the same way as conventional GICs and remain workable for 10 or more minutes provided they are not exposed to light. The reaction is dual-setting once exposed to light. a) Acid base reaction : Calcium fluoroalumino silicate glass (base) and polyacrylic acid = calcium and aluminium polysalt hydrogel. b) Free radical or photochemical polymerization HEMA and photochemical initiator / activator 32
  33. 33.  Poly Hema Matrix Thus two matrices are formed; a metal polyacrylate salt and a polymer. The initial set is a result of polymerization of HEMA. The acid base reaction serves only to harden and strengthen the already formed polymer matrix. Class I materials Composition a) Powder component: Calcium fluoroalumino silicate glass, polyacrylic acid and tartaric acid. b) Liquid component (replaces water): Water /HEMA, other difunctional hydroxydimethacrylates (such as ethyleneglycol dimethacrylate) and bis-GMA. c) Initiator / Activator. Chemically polymerized materials: Initiator  Hydrogen peroxide. Activator  Ascorbic acid. Co-activator  Cupric sulphate. Light activated materials Visible light photochemical initiator  Camphorquinone Activator  Sodium p-toluenesulphinate Photoaccelerator  ethyl 4-N n-dimethylaminobenzoate. 33
  34. 34. The two matrices do not interpenetrate but form separate phases. To prevent phase separation. Class II materials (Vitrebond) have been formulated. In this material, polyacrylic acid (PAA) is replaced by modified PAAs. In these modified PAAs, a small percentage of –COOH is converted to pendant unsaturated groups by a condensation process. They are condensed with methcrylate polymers to have terminal –COOH and –CH3 groups. 34
  35. 35. The Class II materials uses an aqueous solution containing 25% to 45% of modified PAA and 21% to 41% HEMA along with initiator system of camphorquinone and diphenliodonuim chloride with a glass of the following percentage composition : SiO2 26.84%, Al2O3 0.80%, P2O5 0.94%, NH4F 3.32%, AlF3 20.66%, Na3AlF6 10.65%, ZnO 20.66%, MgO 2.12%, SrO 12.55%. On mixing and activation by light, the HEMA polymerizes to form poly HEMA. The modified PAA copolymerizes with HEMA; thus poly HEMA will be chemically linked to the polyacrylate matrix and phase separation will not occur. Also the modified PAA further polymerizes to form a cross- linked PAA which increases the strength of the cement. The matrix of such a cement will contain both ionic and covalent crosslinks. Properties Compomers have both advantages and disadvantages compared to conventional GICs. They have improved setting characteristics. There is a longer working time because HEMA slows the acid-base reaction, and yet, they set sharpely once the polymerization reaction is initiated by light. They are also resistant to early contamination by water because of the formation of 35
  36. 36. an organic matrix and so do not require protection by varnish. This combination of properties is clinically appealing. These cements have compressive and diametral tensile strengths greater than zinc phosphate, polycarboxylate and GIC but les than resin composite. 24 hours in MPa Class I Class II Compressive strength 94 53-96 Flexural - 25.5 Tensile 21.9-33.9 11.2-12.4 Adhesion (dentine) 47 6.2-11.3 Their adhesion to enamel and dentin, and their fluoride release pattern is similar to GIC. They also bond to resin composite. They have cariostatic potential and show resistance to marginal leakage. The biggest advantage is ease of mixing and use, because multiple bonding steps are not required. They also have adequately low film thickness (10-22µm). They have a bond strength to dentin of about 10-12MPa without bonding agent and 14-20MPa with bonding agent. A significant disadvantage of the resin ionomers is hydrophilic nature of polyHEMA which results in increased water resorption and subsequent plasticity and hygroscopic expansion. Although initial water sorption may compensate for polymerization shrinkage stress, continual water sorption has deleterious effects. Potential for substantial dimensional change contraindicates their use with all-ceramic feldspathic-type restorations. 36
  37. 37. Their use for cementing posts in non-vital teeth is questionable because of the potential for expansion induced root fracture. They lack translucency and the presence of free monomers in the freshly mixed cement presents concerns regarding biocompatibility. Dimetracrylates may elicit allergic response and therefore careful handling by dental personnel is required during mixing. It is known that eugenol containing materials inhibit the cross- linking of resin adhesives. They should not be used for final cementation when the luting agent for interim restoration has been eugenol containing provisional materials. Applications Luting metal or porcelain fused-to-metal crowns and FPDs to tooth, amalgam, composite resin or glass ionomer core build ups. Summary and Conclusion Luting agents possess varied complex chemistries that affect their physical properties, longevity, and suitability in clinical situations. It appears a single adhesive will not suffice in modern day practice. To date, no adhesive can completely compensate for the shortcomings of preparation retention and resistance forms or ill-fitting, low strength restorations. Practitioners must be aware of the virtues and shortcomings of each cement type and select them appropriately. 37

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