LUTING AGENT I. 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, orthodontic appliances; and pins and posts used for retention of restorations. The term LUTING is often used in textbooks to describe the use of a moldable substance to seal space or to cement two components together. These different applications make varying demands on manipulative properties, working and setting times, resistance to mechanical breakdown, and to dissolution. Thus some materials are better suited to some applications than others. Because one type of cement is unlikely to perform satisfactorily under all conditions, specific cements must be selected and developed for each applications. II. Basic considerations: The discrepancies of fit arising during the fabrication process for the inlay or crown, the preparation of the tooth leaves a rough and debris covered surfaces. The cement lute then must have the ability to wet the tooth and restoration, flow into the irregularities on the surfaces it is joining and fill in and seal the gaps between the restorations and the tooth. Because an exposed cement line at the restorations margin is inevitable â€“ especially with todays restorative materials the cement must
also have adequate resistance to dissolution in the oral environment. It must also develop an adequately strong bond through mechanic interlocking and adhesion. High strength in tension, shear, and compression are required, as well as good fracture toughness to resist stresses at the restoration tooth interface. Good manipulation properties including adequate working and setting time are essential for successful use. The manipulation, including dispensation of the ingredients, should allow for some margin of error in practice. The material must be biologically acceptable. As the literature says the oldest established and most widely used types of dental cements. 1. Zinc phosphate and 2. Zinc oxide-Eugenol were developed in the late nineteenth century and early twentieth century. Although they have undergone considerable technical improvement, in principle they have remained almost unchanged for 50 years. most clinical techniques and evaluation criteria are based on long experience with these materials significant research on new cements has been carried out only in the last 25 years. The advent of acrylic resins led to the development of fine grained cold curing polymethyl methacrylate cements in the Mid 1950â€™s. Such materials did not become popular for routine cementation. More recently, cements based on the BIS-GMA monomer of Bowen have become available in powder to liquid (filler monomer) form and two paste forms. All these materials set by polymerization mechanisms, so the handling
qualities are different and generally less satisfactory than conventional cements. Another problem with resin cements that has limited their use is the potential tissue reaction to residual monomers in the set material. None of the foregoing cements shows significant adhesion to clean enamel and dentin. The need for good wetting and bonding and low toxicity led to the development of cements based on the reaction of polymerize organic acids and metal ions in the mid 1960s by Smith. The early carboxylate (or polycarboxylate) cements were based on zinc oxide and an aqueous solution of polyacrylic acid or its co-polymers later work by Wilson and cowoskers resulted in the glass ionomer cements that utilize non-leachable glasses rather than zinc oxide. Development in this area still proceeding. As a result of the research of the last few years there are now available cements of four basic types, classified according to the matrix forming species. 1. Phosphate bonded. 2. Phenolate bonded 3. Carboxylate bonded and 4. Methacrylate (resin) bonded Within each category are several classes and this multiplicity together with the choice of several brands of material in each class, has lead to confusion among clinicians as to which type of cement is most suitable to a given situation. Although there are national (ADA, ANSI, BSI, ASA) and international (ISO, FDI) standards for cements, these are of limited value in predicting clinical durability. Another difficulty is that
reports in the literature are often based on the testing of one or two brands of a cement type, and the results are then assumed to apply to all such cements. It is therefore appropriate to briefly review the characteristics of the available cements. III. Types of cement 1. Phosphate based cements. a. Zinc phosphate cement: It is the oldest of the cementation agents having a widest range of application and terms is the one that has the longest track record. It serves as a standard by which newer systems can be compared. It consists of powder and liquid in two separate bottles. Composition and chemistry: the main ingredients of the powder are: a. Zinc oxide (90%). b. Magnesium oxide (10%). The ingredients of powder are sintered at temperature between 1000Â°C and 1400Â°C into a cake that is subsequently ground into five powders. The powder particle size influences setting rate. Generally, the smaller the particle size, the faster the set of the cement. The liquid, contain phosphoric acid, water aluminium phosphate and in some instances, zinc phosphate. The water content of most liquids is 33%Âą5%. The water controls the ionization of the acid, which in terms influences the rate of the liquid powder (acid base) reaction. It is obvious that because water is critical to the reaction, the composition of the liquid should be preserved to ensure a consistent
reaction. Changes in composition and reaction rates may occur either because of self degradation or by water evaporation from the liquid. This means that changes in the composition can affect the reaction. Self degradation effects are best detected as a clouding of liquid over time. Properties: the long persistence of zinc phosphate cements in clinical practice indicates that reasonable performance is obtained. Although the properties are far from ideal they are usually regarded as a standard against which to compare newer cements. The principal reasons for their satisfactory performance under routine conditions are that they can be easily manipulated and that they set sharply to a relatively strong mass from a fluid consistency. At standard luting consistency the powder to liquid ratio is 2.5 to 3.5 (g per ml). The cementing mix flows readily under pressure to a film thickness between 20 and 40 mm, which is adequate to seat most types of restorations as in practice the space between the restoration and the tooth may range upto as much as 100 mm. The film thickness achieved in specific clinical situations is a function of the rheology of the cement and the geometry of the surface being cemented. At the recommended powder to liquid ratio, the compressive strength of the set zinc phosphate cement is 80+0110 MPa after 24 hours. The strength is strongly and almost linearly dependent on powder to liquid ratio. The tensile strength is much lower than the compressive strength, 5 to 7 MPa and the cement shows brittle characteristics. The modulus of elasticity (stiffness) is about 13 GPa. According to the standard method, the solubility and disintegration in distilled water
after 23 hours may range from 0.04% to 3.3%, for inferior material the standard limit is 0.2%. The comparative evaluation a cement solubility under clinical conditions has shown significant loss, but conflicting results. Dissolution contributes to marginal leakage around restorations and bacterial penetration. At room temperature (21 to 27째C) the working time for most brands at luting consistency is 3 to 6 minutes the setting time is 5 to 14 achieved by use of a cold (frozen) mixing slab, which permits upto an approximately 50% increase in the amount of powder, improving both strength and resistance to dissolution. The cement has been found to contract about 0.5% linear giving rise to slits at the tooth cement and cement restoration interface. Retention: Setting of the zinc phosphate cement does not involve any reaction with surrounding hard tissues or other restorative materials. Therefore primary bonding occurs by mechanical interlocking at interfaces and not by chemical interactions. Prologic effects: the freshly mixed zinc phosphate is highly acidic with a pH of 1.6 two minutes after mixing. Even after setting at 1 hr the pH may still be below 4. after 24 hrs the pH reaches 6 to 7. One material that has a low acid content and incorporates calcium hydroxide has little effect on the pulp when used as a lining. Very thin mixes will also lead to etching of the enamel. The etching may assist mechanical interlocking to the adjacent substrates.
Advantages and disadvantages: The main advantages of the zinc phosphate cements are that they can be mixed easily and that they set sharply to a relatively strong mass from a fluid consistency. unless the mix is extremely thin (for instance, with a very low powder to liquid ratio) the set cement has a strength that is adequate for clinical service, so their manipulation is less critical than with other cements. However, then distinct disadvantages include pulp irritation, lack of antibacterial action, brittleness, lack of adhesion, and solubility in acid fluids.
Modified zinc phosphate cements Fluoridated cements: Some phosphate cements contains fluoride in the form of stannous or other fluorides. -
Which have low strength and higher solubility rate.
They used in orthodontic bracket cementation.
Copper cements: they come improves oxide (red) or cupric oxide (oxides) or copper salts added to the zinc oxide powder. -
They have germicidal action.
Silicophosphate cements: These materials that are combination of zinc phosphate and silicate cements have been available for many years. The principal applications have been for the cementation of fixed restorations especially porcelain, because of their translucence.
They are germicidal as they contain little amount of mercury or silver compounds.
Composition and setting: the powder in these materials consists of a combination of silicate glass and zinc oxide, the silicate glass contains 13 to 25% fluoride. The liquid is similar to silicate liquids containing about 50% H3PO4; 4% Zn and 2% Al the set cement seems likely to consist of unreacted and zinc oxide particles bonded together by an alumino phosphate gel containing zinc, Ca, Aluminium and Flouride ions. Properties: Powder to liquid ratio is 2.1 to 3.2g per ml. Flow properties of the mix are not as good as per Zinc Phosphate contents, leading to a higher film thickness in practice. Compressive strength to set cement is 135 to 175 Mpa better than Zinc phosphate. Tensile strength is 7 Mpa. These materials appear to be tougher and more abrasion resistant from phosphate cements. Solubility in distilled water after 7 days is higher than for Zinc phosphate cements, but under clinical conditions it is less so. Biological effects: The set cement is much more translucent than the opaque Zinc phosphate. Thus it has been used for the cementation of porcelain restorations.
Because of the acidity of the mix and the prolonged low PH (410.5) after setting. Hence pulp protection is necessary on all vital reduced teeth.
Advantages and Disadvantages: The silicophosphate cements have better strength and toughness properties than the Zinc phospate cements, show considerable flouride release, transulcence and, under clinical conditions, lower solubility and better bording. Disadvantages include less satisfactory mixing and rheologic properties, leading to higher film thickness in practice and greater potential for pupal irritation. They are but suited to cementation of orthodontic bands and restorations on non vital teeth. Inauguration PHENOLATE – BASED CEMENTS There are three main types of cements under this classification: 1) The simple Zinc oxide – Eugenol. 2) Reinforced Zinc oxide – Eugenol. 3) EBA cements. 1) Zinc oxide – Eugenol cement: Compositon and setting: The basic combination of Zinc oxide and Eugenol finds it principal applications in the temperory filling of teeth, and as a cavity lining in deep davities.
The powder is Zinc oxide, with additives such as silica, may be present. Upto 1% Zinc acitate, chloride, sulfate, or other salts may be present in accelerate the setting.
The liquid is purified Eugenol in some cases, oil of cloves (85% Eugenol).
It may contain about 1% of acetic acid or alcohol to accelerate setting together with small amounts of water.
The cement sits by a chelation reaction between two basic components involving the formation of Zinc eugenolate. However, the reaction is reversible, the Zinc Eugenolate being easily hydrolyzed by moisture Eugenol and Zinc hydroxide. Thus the cement disintegrates rapidly when exposed to oral conditions.
Properties: The material is easy to mix but requires a long spatulation time at least 90 seconds.
Because of work nature of binding agent, the compressive strength is low, ranging from 7 Mpa (luting consistency) to 40 Mpa (filling consistencyA).
Tensile strength is much lower.
The solubility of the set cement in distilled water is high when exposed directly to oral conditions, the material maintains good sealing characteristics despite a volumetric shrinkage of 0.9% and a thermal expansion of 35x10-6/degree C.
Biologic effects: The presence of Eugenol in the set cement under clinical conditions appears to lead to an anodyne and abundant effect on the pulp in deep cavities.
The seating capacity and antibacterial action appears to facilitate pulpal healing.
Reinforced Zinc oxide – Eugenol cements Composition and setting: These materials contain 10 to 40% of finely divided natural or synthetic resins added to or coated on to the powder particles. Additional accelerator (Zinc acetate, chloride, or acetic acid) may be present as well as antimicrobial agents such as thymol or 8hydroxyghinoline. Properties: •
Working time is about 5 mts. and setting 7 to 9 mts. Lly to Zinc phosphate.
Compressive strength – 35 to 55 mpa and
Tensile strength – 4 MPa and
Modulus of elasticity is – 2 to 3000 Gpa.
The mechanical properties are reduced by impression in water, resulting in loss of Eugenol.
This tendency seems less
pronounced with the polymer – reinforced materials. Biological Effects: There may be irritation to connective tissue. Advantages and disadvantages: Advantages – Main advantage is the minimum reaction to the pulp. • Good sealing properties
• The strength is adequate as a lining material and for luting single restorations and retains with good retention form. Disadvantages - Main is hydrolic break down when exposed to oral fluids. • The inflammatory reaction is soft tissues and potential allergic response. EBA and other chelate cements
In order to further improve on the basic Zinc-oxide
Eugenol system. Many
mixtues of zinc and other oxides with other liquid chelating agents. Composition and setting: The Zinc oxide contains 20 to 30% Aluminium oxide or other mineral fillers; polymeric reinforcing agents, such as polymethyl methacrycate. -
The liquid consists of 50 to 60% EBA with the reminder Eugenol.
In order to obtain optimal properties it is important to use as high a powder – liquid ratio as possible i.e., 3.5g per ml. Properties: The working and setting times range between 7 and 15 mts. The film thickness is in the range 40-70mm. Compressive strength is 50-70 Mpa. Tensile strength is 6 to 7 Mpa. Modulus of elasticity – 5 Gpa.
The EBA cements show viscoelastic properties with very low strength, and large plastic deformation at slow (0.1 mm/ mint) rates of deformation and at mouth temperature (37°C). This says its retention values for crown is low than Zinc phosphate cements.
When exposed to moisture, greater oral dissolution occurs than for other cements.
Advantages and Disadvantages: Advantages: • The principal advantages of the EBA cements are their easy mixing. • Long working time. • Good flow and low irritation to the pulp. Disadvantages: -
Main is critical proportioning.
Hydrolic break down in oral fluids.
Liability to plastic deformation.
Poorer retention than Zinc phospate cement.
POLYCARBOXYLATE – BASED CEMENTS Zinc polycarboxylate cements: These cements were developed in late 1960’s as an adhesive dental cement in the search for a material that would combine the strength properties of the phosphate system in the Biologic acceptability of Zinc oxide Eugenol materials. These materials have gone
through several stages of development since their acception and progress is continuing. Compositon and chemistry: The polycarboxylate cements are liquid systems.
The liquid is an aqueous solution of polyacrylic acid or a copolymer of acrylic acid with other unsaturated carboxylic acids, such as itaconic acid.
The molecular net of the polyacids ranges from 30,00 to 50,000. The acid concentration may vary to some degree from one cement to another but usually is about 40%.
The composition and manufacturing procedure for the powder are similar to those of Zinc phosphate cement. The powders mainly zinc oxide with some Magnesium oxide. Stannic oxide may be substituted for magnesium oxide. Other oxides, such as bismuth and Aluminium, can be added. The powder may also contain small quantities of stannous flourides, which modify setting time and enhance manipulative properties. It is an important additive because it increases strength. However, the flouride released from this cement is only a fraction (15% to 70%) of the amount released from Silicophosphate and glass ionomer cements.
The setting reaction of this cement involves particle surface dissolution by the acid that releases zinc magnesium, and tin ions, which bind to the polymer chain via the carboxyl groups, as given below. These ions react with carboxyl groups, as adjacent polyacid chains so that a cross-linked salt is formed as the cement sets. The
hardened cement consists of an amorphous gel matrix in which unreacted particles are disposed. The microstructure resembles that of Zinc phosphate cement in appearance. STRUCTURE OF THE CHEMICAL The role of carboxylate functional groups in polycarboxylate cements: A ďƒ Yielding matrix through cross linking by zinc ions. B ďƒ Bonding to tooth structure through Calcium hydroxide). -
Water settable versions of this cement are available, the polyacid is a freeze dried powder that is then mixed with the cement powder. The liquid is water or a weak solution of NaH 2PO4. However, the setting reaction is the same whether the polyacid is freeze dried and subsequently mixed with water or if the conventional aqueous solution of polyacid is used as the liquid.
Bonding to tooth structure: The outstanding characteristics of this cement are that it bonds chemically to the tooth structure. The mechanism is not clear but as shown in above diagrams, the polyacrylic acid is believed to react via the carboxyl groups with calcium of Hydroxyapitite. (In reference to GIC, the inorganic component and the homogeneity of enamel are greater than those of dentin. Thus, the bond strength to enamel is greater than that to dentin. Properties: For luting consistency the recommended powder to liquid ratio is 1.5 1(2t/wt). About the film thickness, the freshly mixed mix is in spatulation and seating of a restoration, it exhibits shear thinking. Thus, contrary to the subjective impression that the correct mix for a Zinc polycarbohydrate cement is much thicker than a luting zinc phosphate mix.
Under presssure mix tends to thicken more quickly than the zinc polycarboxylate mix.
polycarboxylate cements is to make a mix that appears to be a fluid as a zinc phospate mix; this will result in the use of low powder-to-liquid ratio with consequent poor properties in the cement. Measuring devices for these material will ensure correct proportions.
The working time is 2.5 to 3.5 minutes at room temperature and the setting time is 6 to 9 mts at 37C, the water mix materials tending to give slightly longer setting times. As with other cements, working time can be substantially increased by mixing the material on a cold slab and by refrigerating the powder. The liquid should not be chilled as this encourages gelation due to hydrogen bonding.
At cement consistency the compressive strength from 55 to 85 MPa.
Tensile strength 8 to 12 MPa. In general these cements have somewhat lower compressive
strengths than zinc phosphate cements but are significantly stronger in tension. The cement gains strength rapidly after the initial setting period; the strength at 1 hr. is about 80% of the 24 hr. value. These data indicate a slow continuance of the setting reaction tending towards greater rigidity and more brittle behaviour. However, the cement remains much less brittle and is tougher than silicate, since phosphate, or glass ionomer cement, through less so than a resin cement.
The solubility of the present day cements in distilled water, when determined by a specification weight loss method, ranges from less than 0.1% to 0.6%. The latter high value relates particularly to cements that contain stannous fluoride.
Effective fluoride release can be obtained
without substantial effects on the mechanical properties of the cement. Significant fluoride intake by neighbouring enamel occurs. As with zinc phosphate cements, the solubility is much higher in organic acid solutions, especially at lower PH and if the acid has chelating powers. Few recent clinical studies of solubility gave conflicting results, two studies conducted by Mitchem and Osborne in 1978 respectively showing lower results than zinc phosphate and the other the reverse. Both studies agreed in finding the zinc silicophosphate both the least loss of the cement tested. In vivo evaluation of marginal leakage showed similar results for the two types of cement and whose results for an EBA alumina cement. Thus these all suggests that polycarboxylate cement has adequate clinic performance. The polycarboxylate cements display good adhesion to enamel, and to a lesser extend, to dentin as well as to the various alloys. Adequate fluidity of the mix and sufficient available carboxyl groups are necessary for interfacial interaction as well as a surface free of contaminants and void defects. Bonding to both or alloy surface is reduced if contaminated with saliva. Biologic effects: The effect of Zinc polycarboxylate cements on soft and calcified tissues found to be molding Macrons investigators like Smita D.C. in 1971. The effect on the pulp is less than Zinc oxide Eugenol. The general biocompatibility of these materials seems excellent this appears to
be primarily due to the low intrinsic toxicity the mild effect on the pulp and other tissues is also due to the rapid rise of the PH of the cement towards neutrality; localization of the polyacrylic acid and limitation diffusion try its molecular size and acid ion bonding to dentinal fluid ca and proteins; and the minimal movement of fluid in the dential tubules in response to the cement the presence of stannous fluoride does not appear to affect the mild respnse. -It gives anticariogenic properties in fluorides containing cements. Advantages and Disadvantages: •
The main advantages of these materials are the low irritancy adhesion to tooth substance and alloys.
Easy manipulation and strength, solubility and film thickness properties comparable to those of zinc phosphate cements.
The need for accurate proportioning for optimal properties and thus more critical manipulation.
The lower compressive strength and greater viscoelasticity from zinc phosphate cements, the short working time of some materials and the need for clean surfaces to utilize the adhesion potential.
Removal of excess cement During setting, the polycarboxylate cement passes through a rubbery stage that makes the removal of the excess cement quite difficult. The excess cement that has extruded beyond the margins of the casting should not be removed while the cement is in this stage, because there is
danger that some of the cement may be pulled out from beneath the margins, leaving a void. The excess should not be removed until the cement becomes hard. The outer surface of the prosthesis must be coated carefully with a separating medium such as petroleum jelly to prevent excess cement from adhering. -
Care should be taken not to allow the medium to touch the margin of the prosthesis. Another approach is to start removing excess cement as soon as seating is completed. The goal of these two method is to avoid removing the excess during th rubbery stage.
GLASS IONOMER CEMENT: Type I GIC is designed for cementation of castings. Compositon and setting: These materials were formulated by bringing together the silicate and poly acrylate system. Originally the use of silicate glasses in the zinc polycarboxylate cements was envisaged that the available material were insufficiently reactive. Wilson and Kent and their coworkers in 1975 developed glasses that were ion â€“ leachable by aqueous polyacrylic acid and its acid copolymers. The powder in these materials is a fine ground calcium aluminium fluoro-silicate glass with a particle sized of around 40 cm for the filling materials and less than 25cm for the luting materials. The liquid is a 50% aqueous solution of a polyacrylic â€“ Itaconic acid or other poly carboxylic acid copolymer containing about 5% tartaric acid. On mixing the acids react with the glass leaching ca and aluminium ions from the surface which cross-link the polyacid molecules into a set. A recent material has the polyacid contained in the powder and the liquid is a solution of the tartaric acid. This contributes to easier mixing and better stability.
Properties: The powder to liquid ratio for luting is about 1:3:1 for the conventional types of glass ionomes cement. Best results on a chilled seas. The slow ratio of hardening initially during formation of the calcium polysalt before al cross linking becomes effective means that the cement is sensitive to moisture and more soluble during the early stages of its hardening. The gel can also craze if allowed to dry out. Thus, it is essential to protect exposed margins until sufficient strength has developed •
The setting time is 8 to 9 mts. somewhat shorter than with zinc phosphate cements.
The film thickness less than 30 cm was also comparable and was adequate to seat castings satisfactorily.
Over 24 hours the compressive strength increased to 900 to 1400 Mpa.
Tensile strength to 60 to 80 Mpa. -
The modulus of elasticity was about 7 Mpa.
A glass ionomer cement showed superior retention of gold inlays and onlays compared with a phosphate and a silicophospate cement. -
The solubility of the cements in water was about 1% and this was increased in artificial saliva and lactic acid.
Good resistance to dissolution was observed under clinical conditions. However, the initial slow set and moisture sensitivity may contribute to leakage. Varnish protection is desirable.
These cements have potential for adhesion to enamel, dentin and alloys in a similar manner to the polycarboxylate. In vitro the adhesion is variable and affected by surface conditions. Slight and variable marginal leakage in tests of cemented restorations has been reported. Biologic effects: Evidence fro in vitro testing and clinical experience with the restorative form of the glass ionomer cements suggest the tissue response would be similar to the zinc polycarboxylate cements. However, there is only limited data on the luting cements. Paterson and Watts observed pulp necrosis in rat molars after application to exposures. However parmajer et. al. found little pulp irritation from one commercial cement in cavities in monkey teeth after 3 months. Likewise Reisbick in 1980, in a clinical study, found in slight sensitivity on cementing but no evidence of hypersensitivity after 6 months. However, some cases o postoperative sensitivity have been reported and this may be due to mismanipulation and marginal leakage of bacteria.
Advantages and disadvantages: Advantages: The glass ionomer cement materials include easy mixing, high strength and stiffness, leachable fluoride, good resistance to acid dissolution, and potential adhesive characteristics. Disadvantages: It includes initial slow setting and moisture sensitivity, variable adhesive characteristics, radiolucency, and possible pulp sensitivity. Precautions should be taken to protect the pulp when cementing restorations with glass ionomer cements. The priologic considerations take precedence over other matters, such as the potential for adhesion that
ensures a strong bond to tooth structure. The smear layer on the cut surface of the cavity preparation should not be removed but should be left intact to act as a barrier to the penetration of the tubules by the acid component of the cement. All deep areas of the preparation should be protected by a thin layer of a hard setting calcium hydroxide cement. Methacrylate (Resin) based cements: A variety of Resin-based comments have now become available because of the development of the direct filling resin with improved properties, the acid etch technique for attaching resins to enamel, and molecules with a potential to bond to dentin conditioned with organic or inorganic acid. Acrylic cements: For many years powder to liquid cold curing acrylic cements have been available. These materials have been used for the cementation of restorations, of temporary crowns, and also as core materials. The powder in these materials is a finely divided methyl-methacrylate polymer or copolymer containing benzoyl peroxide as initiations. Mineral filler and pigments may also be present. The liquid is a methyl methacrylate monomer containing an amine accelerator. The material sits by polymerization of the monomer, which concurrently dissolves and softens the polymer particles. The set mass consists of the new polymer matrix uniting the undissolved but swollen original larger polymer beads or particles. These cements are stronger and less soluble than other cements but display low rigidity and visco-elastic properties. They have no effective
bond to tooth structure in the presence of moisture and tins permit marginal leakage although they may show better bonding than other cements to resin facings and polycarbonate crowns. Pulp reaction on vital dentin from monomer in the unset material, and residual monomer in the set material are biologic concerns. Other problems include the short working time and the difficulty in removing excess materials from margins. Bis-GMA type cements: The materials of more recent development are based on the BISGMA system and thus are combinations of an aromatic dimethacrylate with other monomers. Such materials have been supplied as two viscous liquid or two pastes. The material that has been widest explanation and investigation is a powder to liquid combination. The powder is a finely divided borosilicate glass of average particle size of 15mm. The particles are silam treated to improve bonding and contain an organic peroxide initiator. The liquid is based on the reaction product of the diglycidyl either of bis-phenol A and methacrylic acid. Which is diluted with a low viscosity monomer such as ethylene glycol demethacrylate. An amine Accelerators is also present. On mixing the polymerization of the monomer mixture occurs, leading to a highly cross linked composite resin structure. The material is easily mixed to a fluid consistency and is used in conjunction with an etching solution of 50% citric acid to clean the tooth surface and promote adaptation and bonding. The mix rapidly increases in viscosity and working time is short. When set, the material has higher bending and compressive strengths than other cements. The modulus of elasticity was found to be less than for zinc phosphate, but the plastic strain at fracture and toughness
much higher. Sections of cemented casting, revealed spaces at the tooth resin interfaces, presumably due to polymerization contraction. Although bonding was improved by citric acid treatment, it appeared to be attributable to penetration of resin into the tubules, a phenomenon that has also been observed by Vongiduklakis and Smith. Although the strength and resistance to dissolution of this type of material is superior to any other type of cement, these biologic and practical questions common to other types of resin cement have limited its use on vital teeth. Poorer retention for full crowns than for other types of cement was observed by Chan et al in 1975. These problems also include short working time, difficulty in seating castings and difficulty in removing excess material.
They may be best suited to long term temporary
concentration of a loose fitting casting when restoration care is delayed. Factors affecting the clinical performance of cements: The correct seating of a restoration is important to occlusal function, esthetics and durability of the cement, especially in relation to securing the thinnest set cement time between restoration and tooth. Another factor that influence the material situation is the taper and marginal geometry of the restoration. Characteristics of abutment â€“ Prosthesis interface: When two relatively flat surfaces are brought into contact, Analogous to a fixed prosthesis being placed on a prepared tooth, a space exists between the substrates on a microscopic scale. As shown is Fig 1 typical prepared surfaces on a microscopic scale are rough that is there are peats and valleys. When two surfaces are placed against each other, there
are only point contacts along the peaks (Fig2). The areas that are not in contact then become open space. The space created is substantial in terms of oral fluid flow and bacterial invasion. One of the main purpose of a cement is to fill this space completely. On can seal the space by placing a soft material, such as an elastomer, between the two surfaces that can conform under pressure to the â€œroughnessâ€?. The current approach is to use the technology of adhesives. Adhesive bonding involves the placement of a third material, often called a cement, that flows within the rough surfaces and set to a solid from within a few minutes (Fig 3). The solid matter not only seals the space but also retains the prosthesis. If the third material is not fluid enough or is incompatible with the surfaces, voids can develop around deep, narrow valleys (Fig 4) and undermine the effectiveness of the cement.
Procedure for cementation of prosthesis: to be effective cement must be fluid and be able to flow into continuous film of 25mm thick or less without fragmentation. The procedure consists of placing the cements on the internal surface of the prosthesis and extending slightly over the margin, seating it on the preparation, and removing the excess cement at an appropriate time. Cementation of a single crown as an example is described with (Fig 5a).
Placement of cement: The cement paste should coat the entire inner surface of the crown and extend slightly beyond the margin. It should fill about half of the interior crown volume (Fig 5b). the clinician should make certain that the occlusal aspect of the tooth preparation is free of voids to ensure that there is no air entrapment in the critical area during the early age of the seating. Seating: The important factors in seating the cemented restoration include the rheology of the cement, the working time, the final film thickness and the geometry of the gap through which the excess cement. They may be suited to longterm temporary cementation of a loose filling casting when restoration care is delayed must escape. The cement should have a fluid consistency and along working time. The mix should also wet tooth and restriction surface readily. In these respects, fluid hydrophilic
characteristics of the cement mix are a function of the pressure and gap size. The correct mixes of zinc phosphate, polycarboxylate, and EBA cements flow on to low film thickness with moderate pressure under practical conditions. The data of Hoard et al using a model full crown die system showed that the most fluid cement (zinc oxide eugenol) generated least hydraulic pressures duirng seating followed by polycarboxylate with zinc phosphate exhibiting greatest peak and residual hydraulic pressure. Both Eames and associates and Hembree and Coworkers have confirmed that venting is a satisfactory method of achieving minimal film thickness under crowns. In addition to venting, provision of a 30mm relief space or etching away the interior of the casting have been suggested.
Eames et al found better seating of full crowns using 10 and 20째 convergence angles and recommended the most satisfactory technique for allowing escape of cement to be a die relief method. Moderate finger pressure should be used to displace excess cement and to seat the crown or other prosthesis on the preparation. An alternatively method is to use a vibrational instrument to facilitate the seating of the prosthesis without creating excess pressure. After the marginal gap area is evaluated for closure with an explorer the patient may be asked to complete the seating by biting on a soft piece of wood which is static method and a round stick rolling on the crown which is called as dynamic method. During this stage, the last increment of excess cement is expelled through the space between the prosthesis and the tooth. As the prosthesis reaches its final position on the preparation. The space for expelling the excess cement becomes smaller, making the seating more difficult (Fig 5c). variable that can facilitate scaling include using a cement of lower viscosity, increasing the taper and decreasing the height of the crown preparation (Fig 5d) vibration, and introducing escape vents on the occlusal aspect of the prosthesis (Fig 5e), increasing the degree of taper can compromise retention, monomer the escape vents can be filled with gold foil or cast gold plugs. If the occlusal surface contacts the axial wall of the tooth during insertion, air pockets may be introduced (Fig 5f). Removal of Excess cement: The excess cement aluminates around the marginal area at the completion of seating. Its removal depends on the properties of the cement used. If the cement sets to a brittle state and does not adhere to the surrounding surfaces, the tooth and the prosthesis, it is best removed after it
sets. This applies to zinc phosphate, silicophasphate, and ZoE cements. For glass ionomer cements, polycarboxylate cements and resin based cements that are potentially capable of adhering both chemically and physically to the surrounding surfaces the protocol of excess cement removal varies. One can coat the surrounding surface with a separating medium such as petroleum jelly, thereby inhibiting the materials adherence to the surfaces, and remove the excess after the cement sets. Another technique involves the removal of excess cement as soon as the seating is completed, thus preventing the material from adhering to the adjacent surfaces. Post cementation Aqueous based cements continue to nature over time well after they have passed the defined setting time. If they are allowed to nature is an isolated environment, that is free of contamination from surrounding moisture and free from loss of water through evaporation, the cements will acquire additional strength and become more resistance to dissolution. It is recommended that coats of varnish or a bonding agent should be placed around the margin before the patient is discharged. Mechanism of retention: A prosthesis can be retained by mechanical or chemical means or a combination of mechanical 6 mechanical factors. As we know the retention of crowns, bridges is a function not only of the mechanical properties of the luting agent but also the design of the tooth preparation and the restoration. There factors influence the stress distribution within the interposed cement layer, the efficiency of bonding of the cement to both of the surfaces being joined, and the durability of the
cement that include its long-term resistance to mechanical breakdown and dissolution. Analysis of the stress distribution in the restored tooth indicate that compressive shear and tensile forces are all generated in the cement layer. Craig and Farah recommended that a cement with a high tensile strength should be used for the cementation of crowns as shear stresses in the marginal area can exceed the strength of low strength cements. For the support of restorations, the tensile strength was again found to be important, but the most important property was the elastic modulus of the cements (stiffness). As previously noted, except for the resin cements, zinc polycarboxylates tend to display the greatest tensile strength, but have lower modulus and compressive strength than zinc phosphate cements. The silicophosphate and glass ionomer cements tend to be superior in the latter property to both these cements, but are better materials of lower tensile strength, the EBA and resistance to plastic deformation. Theoretically, chemical bonds can be resist interfacial separation and thus improve retention. Aqueous cement based on polyacrylic acids to provide chemical bonding through the use of acrylic acids. Resin based cements using some specialty functional groups also have exhibited chemical bonding. Cavity varnish reduces retention for all cements. Improved mechanical and adhesives retention is obtained for all cements by careful clearing of the preparation to remove residual temporary cement and all residues including cutting debris. Such cleansing may include mechanical treatment (premier slurry) and chemical agents such as detergent cleaners and EDTA. Such agents have yet to be fully optimized,
however, similarly the interior of restorations should be cleansed by sand blasting or etching. Dislodgement of prosthesis: Fixed prostheses can debond because of biologic or physical reasons or a combination of the two. Recurrent caries results from a biologic origin. Disintegration of the cements can result from fracture or erosion of the cement. For brittle prostheses, such as glass ceramic crowns, fracture of the prosthesis also occurs because of physical factors, including intraoral forces, flaws within the crown surfaces, and voids with in the cement layer. In the oral environment cementation agents are immersed in an aqueous solution. In this environment the cement layer near the margin can dissolve and erode leaving a space (Fig 7). This space can be susceptible to plaque accumulation and recurrent caries; therefore, the margin should be protected with a coating (if possible) to allow continues setting of the cement. There are two basic modes of failure associated with cements. Cohesive fracture of the cement (Fig 8a) and separation along the interface (Fig 8 b). because the cement layer is the weakest link of the entire assembly, one should favor higher strength cements to enhance retention and prevent prosthesis dislodgement by providing a firm support base against applied forces. To summuries the factors of retention of fixed prosthesis. 1.
The film thickness beneath the prosthesis should be be thin. It is believed that a thinner film has fewer internal flows compared with a thicken one.
The cement should have high strength values.
The dimensional changes occurring in the cement during setting should be minimized, hence isolate the cement immediately after removal of the excess.
A cement with the potential of chemically bonding to the tooth and prosthetic surfaces or bond enhancing intermediate layers may be used to reduce the potential of separation at the interface and maximize the effect of the inherent strength on the retention. When a mechanical undercut is the mechanism of retention, the
further often occurs along the interfaces. If chemical bonding is involved, the failure often occurs cohesively through the cement itself. The prosthesis become loose only when the cement fracturer or dissolves. Fig: Failure modes of the interface, A) Clearage through the cement layer. This is unlikely because of the dimension of the cement involved. B) The most likely failure that occurs at the cement prosthesis and cement tooth interfaces. Remnants of the cement often remain on the opposing surfaces.
Summary It is evident that none of the materials available to use at present is free from deficiencies in the required clinical characteristics such as biocompatibility, ease of manipulation, satisfactory sealing and retentive properties, and long term stability. Thus, a proportion of clinical failure is inevitable. This incidence can minimized by proper selection and manipulation of the cement as previously outlined, however, the two principal modes of failure for the cement lute (namely, dissolution, including erosion and disintegration, and mechanical breakdown) are both dependent on the clinical situation as well as on the intrinsic properties of the cement. Factors within the control of the clinician, such as the desing of the preparation, the fit of the restoration the manipulation of the cement, the seating of the restoration and the finishing of the margins are some of the important determinants of success. A more rational approach to cement selection manipulation and cementation procedures can give us improved postoperative results and greater average longevity of the restoration. However the development of new and improved cement systems with higher strength and stiffness and lower oral dissolution is required. Adhesive and anticariogenic properties are also desirable. More research is needed on cement performance in clinical practice for both simple and complex restorative procedure to develop a predicture correlation between laboratory measurements and clinical performance. Improved laboratory and clinical characterization of cements should lead us to the goal of an adhesive biocompatible cement that will last as long as the restoration.
CONTENTS 1. Introduction 2. Basic Consideration 3. Types of cement 4. Factors affecting the clinical performance of cements a. Characteristics of abutment prosthesis interface b. Procedure for concentration of prosthesis c. Placement of cement d. Seating e. Removal of excess cement f. Post cementation g. Mechanism of retention h. Dislodgement of prosthesis 5. Summary