CS2-24. Conventional Cements and Cementation of Fixed Prosthodontic Restorations.pdf

DTC200 2. Class Theoretical Committee CS2 Fixed Prosthodontics Subcommittee Conventional Cements and Cementation of Fixed Prosthodontic Restorations Muhammad. Saleh, DDS, PhD Near East University faculty Dentistry- Department of Prosthodontic dentistry The Learning Goals Student will have the abi...

DTC200 2. Class Theoretical Committee CS2 Fixed Prosthodontics Subcommittee Conventional Cements and Cementation of Fixed Prosthodontic Restorations Muhammad. Saleh, DDS, PhD Near East University faculty Dentistry- Department of Prosthodontic dentistry The Learning Goals Student will have the ability to mention The definition and classification of dental cements. Student will be able to explain the advantages and disadvantage of every cements. Student will be able to mention the indications of uses depending on the physical and biological properties of the conventional cements Definition A cement is a substance that hardens to act as a base, liner, filling, or adhesive to bind prosthesis to the tooth structure. Philips’ science of dental materials Classifications and Requirements According to Donovan: The ideal requirements of dental cements: Conventional cements: 1- Zinc phosphate 2- Zinc Polycarboxylate 3- Zinc Oxide Eugenol 4- Glass Ionomer Contemporary Cements: 1-Resin Modified GIC 2- Resin Cements 1- Non- toxic, Nonirritant to pulp and tissue. 2- Insoluble. 3- Mechanical Properties. 3- Adhesion to enamel and dentine. 4- Bacteriostatic. 5- Obtundent effect on pulp. 6- Thermal, chemical and electrical insulation. 7- Optical Properties. Zinc Phosphate Cement Oldest cement and acts as the gold standard by which newer materials are compared. The material is generally supplied as zinc oxide powder and phosphoric acid liquid. The setting reaction of zinc phosphate cements is a two-stage process. 1- The zinc oxide in the powder reacts with the phosphoric acid in the liquid to form zinc phosphate and water. 2- This newly formed zinc phosphate reacts with more zinc oxide, forming hopeite. This compound is hydrated zinc phosphate. P:L ratio of 1.4g: 0.5ml The chemical reaction of zinc phosphate cement is the most exothermic of all the dental cements and so there is a potential risk that the heat produced during the setting reaction could cause pulpal inflammation. There are two methods by which manufacturers attempt to reduce this: (1) By including buffers in the liquid component, (2) Treating the powder by heating it to above 1000°C. The cement shrinks slightly during the setting reaction. This matrix is almost insoluble, but the set cement is porous, making it highly permeable as water which was not consumed in the reaction forms globules within the material. Type of Zinc phosphate according to lute thickness Type I: Film thickness less than 25μm Type II: Film thickness 40 μm Zinc phosphate cements do not adhere to either tooth tissue or restorative materials. They function by grouting the potential space between the cast and tooth preparation. The cement forms tags between the micro-irregularities on the two surfaces being luted. For this reason, these surfaces should not be polished. In fact, sandblasting the fitting surface of the cast restoration prior to luting will increase retention. The set cement starts to erode in an acid medium when the pH drops below 4.5, and the erosion becomes more marked with increasing acidity. In the clinical scenario, the set cement exposed to the oral environment will be subjected to cyclical changes in pH, which frequently falls below 4,5. Cario-static Properties Inclusion of fluoride into the cement may impart some cariostatic properties. This is borne out clinically as little caries is seen under inlays when the cement has washed out over time. The addition of the fluoride salt does, however weaken the cement because it is present as inclusions which disrupt the matrix. Mechanical Properties The mechanical properties of the final cement are dependent on the powder/liquid ratio used. The mechanical properties increase with increased powder content until a point is reached where the material will not mix to a homogeneous mass. The more powder that is incorporated, the thicker the cement lute that is produced. The consistency of the mixed cement produced will depend on the clinical indication. Acidity Phosphoric acid is highly acidic and for this reason there has been concern that its acidity may have detrimental effects on the pulp, such as pulpal inflammation and possibly pulpal death. Clinically this would manifest as postoperative sensitivity or pain. The more liquid that is used, the greater is the initial acidity of the cement. In very runny mixes the pH could be as low as 3. It is known that the pH approaches neutrality with time. However, the more liquid used the longer it takes for the pH to rise. Generally, the pH is higher than 4 at 60 minutes after mixing and returns to neutral after 24 hours. The moisture in dentine can have a buffering effect. Advantages 1- Easy to mix 2- Fast set 3- Acceptable physical properties 4- Cheap 5- Long successful record Disadvantages 1- Irritant to pulp 2- Does not bond to tooth tissue or restorative materials 3- Brittle 4- No antibacterial effects 5- Soluble in the mouth 6- Opaque which can compromise the aesthetic result. Indications Definitive cementation of inlays, metal-based crowns, metal-based bridges. Contraindications Definitive cementation of all ceramic crowns and bridges, When in very close proximity to the pulp without another intermediate material such as calcium hydroxide Zinc Polycarboxylate Cement The polycarboxylate cements were invented in 1968 The first cement system with adhesion to tooth structure The liquid is a polyacrylic acid solution (to improve its resistance to solubility in oral fluids). When the zinc oxide and poly(acrylic acid) come into contact, a salt (zinc polyacrylate) matrix is formed only on the surface of the zinc oxide particles. Poly(acrylic acid) chains cross-link through the zinc ions of the zinc oxide. The set material has cores of zinc oxide within the zinc polyacrylate matrix binding the unreacted zinc oxide cores together. Mixing polycarboxylate cement on a cooled glass slab enables extension of the working time. It reaches 80% of its final setting in one hour. You shouldn’t store the liquid in the fridge as this will cause it to gel. Lute Thickness Film thickness has been reported to vary from 20 to 100 μm but this is dependent on the plasticity of the cement at the time of placement. The longer the material has been mixed, the lower the plasticity and hence the thicker the cement lute. The minimum cement lute thickness is governed by the particle size of the cement powder. Mode of Retention Because of the polyacrylic acid molecules, there is some form of chemical adhesion to the tooth structure. When zinc polycarboxylate cement is used, the bond occurs between the carboxylic acid groups in the liquid polyacrylic acid and the calcium in the tooth structure. Main retention is supplied by means of mechanical. Erosion The cement is prone to erosion as the pH of the mouth becomes more acidic. In fact, this type of material exhibits less resistant to erosion than the zinc phosphate cements. Cario-static Properties There is no cariostatic effetc. Mechanical Properties The compressive strength of zinc polycarboxylate is less than that of zinc phosphate cement. Once set, the cement has a lower modulus and is therefore slightly more elastic and less likely to fracture under heavy load. While it appears thick after mixing, the cement is more fluid and it exhibits shear thinning under load allowing restorations to be seated completely under pressure. Acidity Poly(acrylic acid) is a weak acid with a relatively high molecular weight. As such it will not diffuse readily along the dentinal tubules and is rapidly immobilized in them. The cement’s pH returns to neutral within a short period of time after mixing and the cement is less acidic than zinc phosphate. The biological compatibility of the cement with the pulp is regarded as being excellent. There is some release of zinc fluoride and poly(acrylic acid) but these do not appear to affect the tissues in clinical use. This cement continues to maintain some presence in the marketplace because it offers good biocompatibility with pulp tissue. Advantages 1- Does not release fluoride, although some materials have added properties which allow fluoride release 2- Bonds to enamel, dentine and alloys 3- Low irritation 4- Easy manipulation Disadvantages 1- Short mixing/working times 2- Sensitive to manipulation techniques 3- Lower compressive strength when compared to zinc phosphate Indications Cementation of metal crowns and bridges (also suitable for porcelain fused to metal crowns and bridges) Contraindications Does not bond well to untreated gold restorations Glass Ionomer Cement (GIC) Acquired its name from its composition of glass particles and an ionomer that contains carboxylic acid. The GIC provided in two forms: 1- Powder and liquid 2- Pre-proportioned capsules Working time and setting time: It sets rapidly in the mouth that is within 3-5 min and harden to form a body having translucency that matches enamel Setting time for GIC 7-10 Min Consistency and film thickness: Film thickness should not exceed 20 micrometer (which is less than zinc phosphate) Resin Coating (Protection of cement) Water plays a key role for proper maturation of GIC. Water contamination and rehydration during the initial setting stages can compromise the physical properties of the restoration. It is recommended to strictly excluded water during the vulnerable setting stages, which is reported to last for at least one hour until even two weeks after placement. Petroleum jelly, coco butter, waterproof varnish have been recommended as suitable surface coating agents. Coating are lost by oral masticate wear, but by this time the cements become more resistant to variation in water balance due to their post hardening. Anticariogenic Properties: Fluoride is the most effective agent in caries prevention. The metabolism of the bacteria that cause caries is inhibited and the resistance of enamel and dentine is increased due to the remineralization of porous or softened enamel and dentine. Sustained, long-term fluoride release especially in marginal gaps, between filling material and tooth help prevent secondary caries of dental tissue. For conventional GIC, an initial release of up 10 ppm and constant long-term release of 1 to 3 ppm over 100 month was reported. Aesthetic: Glass ionomer has got a degree of translucency because of its glass filler. Unlike resin, glass ionomer will not be affected by oral fluids. Because of slow hydration reaction glass ionomer cements take at least 24 hrs. to fully mature and develop translucency Adhesion Glass ionomers bond permanently to tooth structure and to other polar substrates such as base metal. Zinc Oxide Eugenol Cements It is available for both temporary cementation and permanent fixation of metallic and metallo-ceramic crowns and bridges. Due to an inhibitory effect of eugenol on polymerization of methacrylate-based resins and luting composites, temporary cements using nonphenolic components are often preferred over conventional formulations. Their popularity is justified by their ease of use, antibacterial action, and anodyne effect on dental pulp. The powder is basically zinc oxide, with up to 8% of other zinc salts (acetate, propionate, or succinate) as accelerators. Rosin (abietic acid) is added to reduce brittleness and increase working time and strength. The liquid contains eugenol (4-allyl-2- methoxy phenol), a weak acid. Acetic acid (up to 2%) is added as accelerator. The reaction of zinc oxide with eugenol results in the formation of a zinc eugenolate chelate, that is, a complex in which one zinc (Zn) atom binds to two eugenolate molecules. Also, as mentioned above, the dissociation constant of eugenol is small. Therefore, reaction rate is increased with the use of more reactive oxides, along with the presence of accelerators. ZOE cements are considered biocompatible because of their neutral pH, antibacterial action, and anodyne effect on hyperemic pulpal tissue. Their antibacterial activity in vitro was shown to be more efficient than those displayed by conventional and resinmodified glass ionomers. That characteristic associated with a good marginal sealing favors the recovery of the pulp. Eugenol released from the salt matrix may contribute to pain relief in preparations with little remaining dentin thickness. However, in high concentrations or when placed directly in contact with connective tissue, it may increase the inflammatory response because of its cytotoxicity. The low strength displayed by ZOE cements makes them a suitable material for temporary cementation. Reinforced ZOE cements EBA-eugenol cements is a general-purpose zinc oxide eugenol cement reinforced with ethoxy benzoic acid (EBA) which may be used in crown cementation, temporary dressing or as a cavity liner. EBA-eugenol cements can be several times stronger than the basic formulation of zinc oxide eugenol (72 MPa versus 26 MPa, in compression at room temperature). EBA-alumina cements can present 20% higher strength compared to EBA-eugenol materials. Even though these values are above the minimum compressive strength required for permanent luting cements, both reinforced ZOE cements are the weakest among luting agents used for permanent cementation. Temporary cements containing eugenol may negatively affect the polymerization of methylmethacrylate used in provisional restorations. I f the final restoration will be bonded to the prepared tooth, the polymerization of both the adhesive system and the resin cement may be inhibited, increasing the risk of debonding, or even fracture in case of low-strength ceramic or indirect composite restorations. In fact, in ZOE cement/ composite interface