Dental Cements Lecture 3 PDF

Zinc oxide powder-based dental cements Faculty of Dentistry ILOs ⚫ Be familiar with zinc phosphate and Polycarboxylate cements. ⚫ Know their composition. ⚫ Understand the setting reaction of both cements. ⚫ How to manipulate them. ⚫ U...

Zinc oxide powder-based dental cements Faculty of Dentistry ILOs ⚫ Be familiar with zinc phosphate and Polycarboxylate cements. ⚫ Know their composition. ⚫ Understand the setting reaction of both cements. ⚫ How to manipulate them. ⚫ Understand properties and applications of both cements. Faculty of Dentistry Classification of dental cements They can be classified by the nature of the cement powder into: 1. Zinc oxide (ZOE, Zinc Phosphate, Zinc polycarboxylate cements). 2. Ion leachable glasses (alumino-silicate glass) Silicate and glass ionomer cements. Faculty of Dentistry Zinc phosphate cements Faculty of Dentistry Zinc phosphate cement It is a water-based cement, packaged as a powder and liquid system. ⚫ Powder: - Zinc oxide (90%) - Magnesium oxide - Other oxides ⚫ Liquid - Aqueous solution of phosphoric acid (30-40% H2O) - Zinc or aluminum phosphate Zinc phosphate cement Role of water ⚫ Controls ionization of the acid. ⚫ Influences the rate of reaction. ⚫ Additional water increases the rate of ionization which accelerates the reaction (shorten working and setting time). Faculty of Dentistry Uses ⚫ Type I: used for luting (permanent cementation) of alloy restorations. ⚫ TypeII: used for other purposes like bases. Faculty of Dentistry Setting reaction (crystallization) ⚫ The surface of the zinc oxide particles reacts with phosphoric acid to give an insoluble phosphate. ⚫ The final set mass is heterogeneous consisting of a core of unreacted powder in a matrix of zinc phosphate. ⚫ Shrinkage occurs. ⚫ It is exothermic reaction (produces heat). Setting reaction (exothermic reaction) Zinc oxide + Phosphoric acid Zinc phosphate + Heat Set mass of zinc phosphate cement Manipulation Proportioning (Powder/liquid ration) It depends on the needed consistency (base or luting). Mixing - Carried out on a cool thick glass slab using a cement spatula to dissipate the heat of reaction. - Adding powder to the liquid in small increments, to prolong the working time. Manipulation ⚫ A small portion of the powder is added first to the whole liquid to neutralize the acidity and minimize heat liberation. ⚫ Mixing is done over large area. ⚫ Large amounts of powder can be incorporated to saturate the mix. ⚫ Finally a small increment of powder is incorporated to control consistency of the mix. Dispensing powder and liquid Mixing and luting consistency Mixing powder with liquid in increments Consistency of the mix (for cementation) Consistency of the mix Precautions ⚫ The liquid should be kept in a stoppered bottle to avoid evaporation of water. ⚫ If the liquid becomes cloudy in the bottle, it should be discarded. ⚫ Mixing should be done on a wide area of thick glass slab to dissipate the resulting heat. Properties of zinc phosphate cement ⚫ Working time 1-1.5 min ⚫ Setting time 5-9 min Factors affecting setting time ⚫ Powder - Finer particles react more quickly. - Higher heat during powder preparation by manufacturer reduces the reactivity. ⚫ Liquid Adding buffering slowdown the rate of reaction. Faculty of Dentistry Factors affecting setting time ⚫ Manipulative variables The following four factors causes acceleration of the reaction: 1. A high powder-liquid ratio. 2. Fast rate of addition of powder to liquid. 3. The presence of moisture. 4. Higher temperature. Properties ⚫ Biological properties - Freshly mixed cement has a pH between 1.6 and 3.6. - So cavity liner should be used underneath it to protect the pulp in deep cavities. - It approaches neutrality within one to two days. - Thin mix is more acidic than thick mix. Properties ⚫ Retention depends on mechanical interlocking between the surface irregularities of the tooth and restorations ⚫ Stronger than zinc oxide eugenol cement. ⚫ The set cement is opaque. ⚫ Low film thickness (less than 25 micrometer). Copper and silver modified zinc phosphate cements ⚫ They are similar to phosphate cements except that the powder contains copper or silver. ⚫ They are considered bactericidal. ⚫ Used mainly for cementation of orthodontic appliances. Faculty of Dentistry Ploycarboxylate cements Carboxylate-based cements Faculty of Dentistry Ploycarboxylate cement ⚫ It is water-based cement. ⚫ It is the first cement that produces true chemical adhesion with tooth structure. Composition ⚫ Powder/Liquid system It is composed of a powder like zinc phosphate cement with some fluoride. The liquid is a polyacrylic acid (PAA) copolymer in water and tartaric acid to reduce viscosity. ⚫ Single bottle system The liquid can be freeze-dried and added to the powder. This form is mixed with water. (less viscous). Packaging of polycarboxylate cement Packaging of polycarboxylate cement Bonding with tooth structure ⚫ This occurs through an ionic interaction between negatively charged molecules in the cement with positively charged atoms, such as calcium, in the tooth structure. ⚫ The carboxylic group in PAA reacts with Ca ion in tooth structures. ⚫ Ionic bond formed between PAA and Ca ions in hydroxyapatite. ⚫ This bond is greater with enamel than dentin. Setting reaction (gelation) and manipulation ⚫ Zinc oxide + polyacrylic acid → zinc polycaboxylate ⚫ The powder is rapidly incorporated in the liquid in large quantities. ⚫ Mix should be done rapidly in 30 to 40 sec. ⚫ The mix should be used while it is still glossy. ⚫ Working time is 2.5 to 3.5 min. Properties ⚫ Zinc polycarboxylate cement combines the strength properties of phosphate cement with the biologic acceptability of ZOE cement. ⚫ It is weaker than zinc phosphate and stronger than ZOE. Faculty of Dentistry Properties ⚫ Biological properties Although polycarboxylate cement is acidic, it is not irritant as zinc phosphate cement. This is for two reasons: 1. Ployacrylic acid is weaker. 2. Polyacid chains are too large and lack the mobility (high viscosity) to penetrate dentinal tubules. Properties ⚫ Higher solubility than zinc phosphate cement, silicophosphate and glass ionomer. ⚫ Low irritation. ⚫ Easy manipulation. ⚫ Comparable film thickness with zinc phosphate. ⚫ More difficult to mix than other cement because of its high viscosity. Applications ⚫ Permanent cementation of metallic restorations (common) ⚫ Cavity liner ⚫ Base Faculty of Dentistry Filling the fitting surface of the bridge with the cement Removal of excess cement