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ADHESIVE CEMENTS CONTENTS Introduction to cements Adhesion Zinc poly-carboxylate cements Glass Ionomer Cements and its advances Resin Cements Recent Advances of resin cements Conclusion What is a dental cement? Dental cements are hard, brittle materials formed by mixing powder and liquid together. They are either resin cements or acid-base cements. In the latter the powder is a basic metal oxide or silicate and the liquid is acidic. An acid base reaction occurs with the formation of a metal salt which acts as the cementing matrix. Applications of Dental Cements: Cementing agent for permanent restorations. Temporary restorations. Liners and bases. Cementing agents for orthodontic appliances. Root canal fillers and sealers. Pulp capping agents. Restorations. Requirement of Dental Cements
Adequate mechanical properties. Non-irritating & non-toxic. Insoluble. Insulating the pulp from thermal, electrical and chemical irritants. Adhesive properties. Esthetic properties. Easy to manipulate. Form a strong bond with enamel and dentin. Provide good marginal sealing to prevent marginal leakage. Be resistant to dissolution in saliva, or in any oral fluid. Classification Cements are classified on the basis of their components. Generally, they can be classified into categories: Water-based acid-base cements: Zinc phosphate (Zn3(PO4)2), Zinc Polyacrylate(Polycarboxylate) Glass ionomer (GIC)
Non-aqueous acid-base cements: Zinc oxide eugenol Non-eugenol zinc oxide Resin-based: Acrylate or methacrylate resin cements, including the latest generation of selfadhesive resin cements which contain silicate or other types of fillers in an organic resin matrix.
Adhesive dentistry Adhesion Adhesion is a process by which two bodies are held together at an interface by ionic or covalent bonding between molecules on either side of the interface. Adhesive dentistry is a branch of dentistry which mainly deals with adhesion or bonding of the adhesive material or cements to the natural substance of teeth, enamel and dentin. Dental Bonding Dental bonding is the process in which tooth colored materials are adhered to the tooth. This is a procedure that can be used to repair or improve the appearance of a tooth that has been badly stained, broken or chipped. Tooth bonding techniques have various clinical applications including operative dentistry and preventive dentistry as well as cosmetic and pediatric dentistry, prosthodontics, and orthodontics. In modern science, adhesive dentistry studies the nature and strength of the adhesion to dental hard tissues, properties of adhesive materials, causes and mechanisms of failure of the bonds, clinical techniques for bonding and newer applications for bonding such as bonding to the soft tissue. There's also direct composite bonding which uses tooth-colored direct dental composites to repair various tooth damages such as cracks or gaps.
Zinc Polycarboxylate Cement (Zinc Polyacrylate Cement)Composition
Powder-Zinc Oxide. -Magnesium Oxide.
Liquid-Polyacrylic acid.-Sodium hydroxide. -Tartaric acid .
-Alumina, Silica, Fluoride & Stainless steel fibers.
Manipulation The proper powder /liquid ratio mixed on paper bad (1:1 or 2:1) Dispensing of the liquid should be done immediately before mixing. Cooled glass slap to extend working time. The consistency of the mix is creamy . The Viscosity decreases as the shear rate increase,i.e.the flow increases as the spatulation increases Should be used only as long as it still has a glossy surface.
Setting Reaction of Zinc Polycarboxylate Cement ZnO + Polyacrylic Acid Zinc polacrylate+ ZnO
Setting Reaction Unlike zinc phosphate cement, the setting reaction of polycarboxylate cement produces little heat. This has made it a material of choice. Manipulation is simpler, and trauma due to thermal shock to the pulp is reduced.
The rate of setting is affected by the powder-liquid ratio, the reactivity of the zinc oxide, the particle size, the presence of additives, and the molecular weight and concentration of the polyacrylic acid. The strength can be increased by additives such as alumina and fluoride. The zinc oxide reacts with the polyacrylic acid forming a cross-linked structure of zinc polyacrylate. The set cement consists of residual zinc oxide bonded together by a gel-like matrix. Zinc Polycarboxylate Cement Uses : Used for cementation of crowns and inlays. Used as base under restorations . Primarily for luting permanent alloy restorations
Properties of Zinc Polycorboxylate Cements
1-Consistency and Film thickness : 25 48 m Poly-carboxylate cement mixed with white portland cement at concentrations of 1.8% and 2.4% has reduced setting time and increased flowability of cement. [Wongo Kornchaowalit n, Lert chirakarn U] [J Endod 2011 March] 2-Strength : Compressive strength : 57 99 MPa Tensile strength : 3.6 -6.3 Mpa In an invitro study,retentive strength of zinc phosphate and GIC is higher than that of zinc polycarboxylate cement. [Raghunath Reddy, Subba Reddy VV] [J Indian Pedod Prev Dent - 2010 Oct] 3- Bonding to tooth structure : It bonds chemically to the tooth structure. The polyacrylic acid is believed to react with calcium ions via carboxyl groups on the surface of enamel or dentin. Thus bond strength to enamel is greater than that to dentin. 4-Biological properties : Pulp response is mild when compared with silicate cement. The pH of Zinc polycarboxylate cement is higher than that of a zinc phosphate cement. Its mild irritation is due to that, the larger size of the polyacrylic acid molecule compared with phosphoric acid molecule may limit its diffusion through the dentinal tubules. Zinc poly-carboxylate cement is similar to GIC in controlled release of active organic compounds. [Ali MN, Edwards M]
[J Mater Sci Mater Med 2010 April] Advantages : 1. Adhesion to tooth structure. 2. Less irritation. 3. Easy manipulation. 4. Strength. 5. Film thickness properties. Disadvantages: 1. Critical proportioning. 2. Lower compressive strengths. 3. Requires clean surface.
Glass Ionomer CementCompositionPowder-Silica, Alumina, Aluminium fluoride, Calcium fluoride , Sodium fluoride, Aluminium phosphate.
Liquid -Polyacrylic acidcopolymer with itaconic, maleic, or tricarboxylic acid. -Tartaric acid. -Water
-Lanthanum, Strontium, Barium or zinc oxide.
Types of GIC :
Type I : Luting cement for crowns and inlays.
Type II : Restorative material
TYPE III: Liner or Base Material
MANIPULATION 1.Preparation of tooth surface :The enamel & dentin are first cleaned with pumice slurry followed by swabbing with polyacrylic acid for 5 sec. After conditioning & rinsing ,tooth surface should isolate & dry. 2.Proportioning & mixing : Powder & liquid ratio is 3:1 bywt. Powder & liquid is dispensed just prior to mixing.
First increment is incorporated rapidly to produce a homogenous milky consistency. Mixing done in folding method preserves gel structure. Finished mix should have a glossy surface. 3. Protection of cement during setting :Glass ionomer cement is extremely sensitive to air & water during setting.
Immediately after placement into cavity, preshaped matrix is applied to it. 4. Finishing : Excess material should be trimmed from margins. Hand instruments are preferred to rotary tools to avoid ditching. Further finishing is done after 24hrs. 5.Protection of cement after setting : Before dismissing the patient ,restoration is again coated with the protective agent to protect trimmed area. Failure to protect for first 24hrs from moisture results in weaken cement. In an invitro study, it has been proved that early water exposure does not affect the strength of glass ionomer restoratives. [XY Wang, AUJ Yap] [operative Dentistry 2006] SETTING REACTION
When the powder & liquid are mixed, Surface of glass particles are attacked by acid. then Ca, Al, sodium, & fluoride ions are leached into aqueous medium. Calcium poly salts are formed first, then followed by aluminum poly salts which cross link with poly anion chain. Set cement consist of unreacted powder particle surrounded by silica gel in amorphous matrix of hydrated calcium & aluminum poly salts. Calcium poly salts are responsible for initial set. Aluminum poly salts form the dominant phase.
Water plays an important role in structure of cement. After hardening, fresh cement is extremely prone to the cracking & crazing, due to drying of loosely bound water . Hence these cements must be protected by application of varnish. The structure of the set GIC
SETTING TIME Type I Type II 4 - 5 minutes 2.5-6 minutes
PROPERTIES Compressive strength - 150 mpa
Tensile strength Hardness
- 6.6 mpa. - 49 KHN.
Solubility & Disintegration:Initial solubility is high due to leaching of intermediate products. The complete setting reaction takes place in 24 hrs, cement should be protected from saliva during this period. Film Thickness The film thickness of GICs is similar to or less than that of zinc phosphate cement and is suitable for cementation and luting. Luting : 15m Restorative : 50 m Adhesion By bonding a restorative material to tooth structure, the cavity is theoretically sealed, protecting the pulp, eliminating secondary caries and preventing leakage at the margins. This also allows cavity forms to be more conservative and, to some extent, reinforces the remaining tooth by integrating restorative material with the tooth structures. Bonding between the cement and dental hard tissues is achieved through an ionic exchange at the interface. Polyalkenoate chains enter the molecular surface of dental apatite, replacing phosphate ions. Calcium ions are displaced equally with the phosphate ions so as to maintain electrical equilibrium. This