4. bases.pdf

Dental Bases Salma Abuelgasim Mohamed 2022 update 2023 Objective By the end of this session the participants should be able to Deﬁne the bases Demonstrate of Indications Illustrate the different types of bases Dental Base A material that is placed on the ﬂoor of the cavity preparation in a relatively thick layer. Its purpose is to protect the pulp by providing thermal insulation due to temperature changes and absorbing occlusal forces Deﬁnitions A material used to replace missing dentin, and, therefore, is thicker than a liner. It should be able to withstand the forces of condensation immediately after being placed. It is used under a permanent restoration to encourage the recovery of the injured pulp and to protect the pulp from thermal and galvanic shock, as well as mechanical trauma. Additionally, these materials can be shaped and contoured prior to placement of the ﬁnal restoration. Bases Cement bases are 1 to 2 mm thickness They are used to provide Thermal protection. Mechanical support for the restoration by distributing local stresses from the restoration across the underlying dentin surface. Bases The thermal insulating capacity is dependent on: Thickness of the base placed Thermal conductivity of the set base Density of the base placed Commonly used bases include: 1. Zinc phosphate cement 2. Zinc polycarboxylate cement 3. Zinc silico phosphate cement 3. ZOE 4. Glass ionomer cement 5. "Hard setting" calcium hydroxide materials thicker than when used as a liner Zinc phosphate cement Commonly used bases include: 1. Zinc phosphate cement Introduced in 1879 Components: Powder Zinc oxide, magensium oxide, silicone dioxide Liquid phosphoric acid Zinc phosphate cement 2 Types Type 1 ﬁne grain forms ﬁlm thickness of 25µ used for cementation Type 2 medium sized particles forms ﬁlm thickness of 40µ, used as temprorary restorations and as bases Zinc phosphate cement Setting reaction: Exothermic Setting time 4-7 min Powder liquid ratio for ﬁlling 2.5:1 & for luting 1.5:1 Manipulation: Stainless steel spatula on a cool glass slab Zinc phosphate cement Factors inﬂuencing setting time and working time Powder liquid ratio: more powder accelerates setting time & setting reaction Rate of powder incorporation: if the powder is added at one time the reaction is rapid so gradual addition of the powder should be done Zinc phosphate cement Factors inﬂuencing setting time and working time Mixing temperature: cool temp. increases working time Spatulation time: increasing spatulation time destroys the matrix Water contamination or water loss: Water addition increases setting time and water loss decreases setting time. Zinc phosphate cement Properties Compressive strength is 10000 psi and tensile strength 800 psi Zinc phosphate exhibits slight initial expansion due to water sorption and is followed by shrinkage in 7 days Presents good thermal & electrical insulation Zinc phosphate cement Properties Solubility does not exceed 0.3% Highly acidic PH= 3.5 but is neutrilized between 24- 48 hrs Zinc polycarboxylate cement Introduced in 1968 Have strength comparable to zinc phosphate & are less irritant. Has adhesive properties and is anti cariogenic Zinc polycarboxylate cement Composition Powder: modiﬁed zinc oxide similar to that of zinc phosphate Liquid: poly acrylic acid + copolymer Zinc polycarboxylate cement Setting reaction: Acid – base reaction Powder liquid ratio 1:1 for restorative purposes and 1:2 for luting purposes Zinc polycarboxylate cement Properties Initial more viscous than zinc phosphate cement Setting time 7-9 min Film thickness is higher than for zinc phospahte cement Zinc polycarboxylate cement Properties Compressive strength is lower than zinc phosphate but the tensile strength is higher Shows more contraction than zinc phosphate Bonds to enamel & dentine because the carboxyl group chelates with calcium ions in tooth Zinc polycarboxylate cement Properties Solubility is 0.1- 0.25%, so are the most soluble of all cements Ph is 1.5 but is rapidly neutralized so rises to 5.5 fastly compared to the slow rise of zinc phosphate cement so is less irritant. Zinc polycarboxylate cement Properties The large size of the polyaccrylic acid molecule limits its diffusion through the dentinal tubules & polyacrylic acid is less irritating than zinc phosphate cement. Glass Ionomer Cement (GIC) Glass Ionomer Cement Classiﬁcation Chemical Application classiﬁcation classiﬁcation Glass ionomers are water-based, self-adhesive restorative materials in which the ﬁller is a reactive glass called ﬂuoroaluminosilicate glass and the matrix is polymer or copolymer of carboxylic acids. The setting reaction of these materials involves an acid-base reaction. They are used as ﬁlling materials in clinical situations when isolation is not a problem and ﬂuoride release is desirable for the patient. Components Polycarboxylic acid Fluoroaluminosilicate (FAS) glass Water Tartaric acid The polymeric matrix of most glass ionomers is a copolymer of acrylic acid and itaconic acid or maleic acid. itaconic acids improved both stability and shelf life In most cases this is formulated as a concentrated aqueous liquid. Tartaric acid is added to control the working and setting characteristics of the material. The powder consists of an acid-reactive comminuted FAS glass and has ions such as calcium, strontium, and lanthanum. When heavy metal ions are used, the set material is radiopaque to x-rays. When the powder and liquid are mixed, an acid-base setting reaction begins between the FAS glass and the polycarboxylic acid. Chemical Classiﬁcation:- GICs are commonly classiﬁed into 3 principal types: i. Conventional GIC ii. Resin Modiﬁed GIC (Conventional with addition of HEMA) iii. Metal-reinforced GICs i. Conventional GIC Was ﬁrst developed in England by Wilson & Kent in 1972. A Glass Ionomer Cement is a dental restorative material used for ﬁlling teeth & as luting cements. i. Conventional GIC Form: Supplied as powder & liquid or capsules i. Conventional GIC Composition:- Powder Calcium ﬂuroalumino- silicate glass Lanthanum, Strontium, Barium & Zinc Oxide additives provide radiopacity. Liquid 47.5% solution of 2:l polyacrylic acid/itaconic acid copolymer. Schematic illustration of an aqueous polyacrylic acid copolymer liquid with thousands of carboxyl groups (MW = 56,000) reacting with the glass-ionomer powder ii. Resin Modiﬁed GIC Resin Modiﬁed Glass Ionomer Cements are conventional glass ionomer cements with addition of HEMA & light activation catalyst. iii. Metal-reinforced GIC ceremtes Were ﬁrst introduced in 1977. Formed by the addition of silver-amalgam alloy powder to conventional materials to improve physical strength of the cement and provided radiodensity called ceremtes Application classiﬁcation:- Type I – For luting cements Type II – For restorations Type III – Liners and bases Type IV – Fissure sealants Type V – Orthodontic Cements Type VI – Core build up Type VII- Fluoride releasing Type VIII- ART(a traumatic restorative technique) Type IX- Deciduous teeth Setting reaction:- The setting reaction is an acid-base reaction between the acidic polyelectrolyte and the aluminosilicate glass. The setting process involve three overlapping phases Dissolution. Gelation. Hardening. Setting reaction:- Dissolution. Initial acid-base reaction between the glass powder and polyacid liquid components of glass ionomers only the surface of each particle is attacked by acid, releasing Ca, Al, and F ions. When a layer become depleted in Al, Ca,Na and ﬂuorine ions so that only silica gel remains. Setting reaction:- Dissolution. Poly acid liquid Hydrogen polyions ion Ca Al and FL powder Ca and Al ions Glass core poly salts Silica gel Gelation During the initial setting, calcium ions are more rapidly bound to the polyacrylate chains. The carboxylate ions from the polymer react with these metallic ions to form a salt bridge, resulting in gelation phase of setting reaction. Binding to the aluminum ions occurs at a later stage Hardening Binding to the aluminum ions occurs at a later stage so the Al ions provide ﬁnal strength to material as they are responsible for the introduction of cross link. the trivalent nature of al ions ensure that high degree of cross linking of the polymer molecules takes place A fully set and mature glass ionomer is characterized by Ca and Al chains condensed onto the acidic polymer to form a rigid matrix surrounding the partially dissolved particles. matrix phase The release of ﬂuoride ions from the glass particles results in the matrix phase of the set material becoming a reservoir for ﬂuoride After setting the matrix is able to release this ﬂuoride into the surrounding environment or to absorb ﬂuoride from the surroundings when the ambient ﬂuoride concentration is high (e.g. from a ﬂuoride containing toothpaste). Properties Like many dental cements the properties of glass ionomers are critically dependent upon the powder/liquid ratio. Unfortunately hand mixing at optimal powder/liquid ratios may result in a dry and apparently crumbly mix which dentists do not like. Hence there is a tendency for dentists to add too much liquid to give a wetter consistency with a deleterious effect on the physical properties of the material. This problem is surmounted by the use of encapsulation and mechanical mixing. The powder/liquid ratio should be high in order to optimize strength and solubility. The materials are often difﬁcult to mix at the ratios recommended (typically around 3 : 1 by weight). The powder/water materials tend to be easier to handle than the powder/liquid products. Setting time GlC sets within 6–8 minutes from the start of mixing, setting time is lesser for type I materials than Type II materials. The setting can be slowed when the cement is mixed on a cold slab but this technique has an adverse effect on strength. GIC TYPE 1 – 5–7minutes GIC TYPE 2 – within 10 minutes Aesthetics Conventional GIC are tooth-coloured and available in different shades, however they rather opaque and not as esthetic as composite-resins. Surface ﬁnish is not as good as composite Increased discolouration & surface staining because of their hydrophilic monomers and incomplete polymerization. Water sensitivity, solubility & disintegration initial solubility is high (0.4 %) GIC are also more resistant to attack by organic acids The complete setting reaction takes place in 24 hours there the cement should be protected from saliva in mouth during this period. so the ﬁlling should be protected by varnish or dentin bonding agent. Water sensitivity, solubility & disintegration Conventional glass ionomer restorations are diﬃcult to manipulate because they are sensitive to moisture imbibitions during the early setting reaction and to desiccation as the materials begin to harden. Adhesion Bonding between the cement and dental hard tissues is by 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 leads to the development of an ion-enriched layer of cement that is ﬁrmly attached to the tooth. Bulk material no need to place in increments. The bond to dentin is hydrogen bond to collagen combined with ionic bond to the apetite. Marginal adaptation & leakage The coeﬃcient of thermal expansion of conventional GIC is close to that of dental hard tissues this is why it has good margin adaptation however micro leakage also occurs. thermal diffusivity The thermal diffusivity value for glass ionomer cements is close to that for dentine. Hence the material has an adequate thermal insulating effect on the pulp and helps to protect it from thermal Trauma. Physical strength The main limitation of GIC is its lack of strength and low resistance to abrasion and wear. Conventional GICs have low tensile strength(week) but high modulus of elasticity(rigid), and are therefore very brittle and prone to bulk fracture. Ant cariogenic effect Fluoride is released from the glass powder at the time of mixing and lies free within the matrix. It can therefore be released without affecting the physical properties of the cement. It can also be taken up into the cement during topical ﬂuoride treatment and released again, the cement may act as a ﬂuoride reservoir over a relatively long period. Manipulation uses Manipulation Uses To restore loss of tooth structure from the roots of teeth either as a consequence of decay or the so-called cervical abrasion cavity. Cavities for glass ionomer cements should be of sufﬁcient depth at their margins to give adequate bulk to the restorative material. a butt joint margin of 0.75 mm depth or greater. Glass ionomer cements are gaining wide acceptance as ﬁlling materials for deciduous teeth. Another suggested use of glass ionomers is as ﬁssure sealants. The material is mixed to a more ﬂuid consistency to allow ﬂow into the depths of the pits and ﬁssures of posterior teeth. ART technique. Dentine surface treatment Dentine surfaces and enamel surfaces that are contaminated with saliva are not receptive to bond formation. Even transient wetting with saliva during cavity preparation will inhibit good bond formation. These surfaces should be prepared to remove the precipitated salivary protein and /or conditioned. Dentine surface treatment 10–15% poly(acrylic) acid. is applied to the tooth surface for 30 seconds then washed off and the tooth dried, but not dessicated to achieve a receptive surface for bonding. Matrix techniques The matrix technique for proximal cavities on anterior teeth is very similar to that for composites, using transparent ﬂexible ﬁlm s made from either cellulose acetate or polyester. The matrix is inserted between the teeth adjacent to the prepared cavity usually before any dentine surface conditioning. Once the material has been placed in the cavity to slight excess, the matrix is drawn round the tooth root and held in place using ﬁrm digital pressure until the material sets. Finishing and polishing Any ﬁnishing that is required will remove this protected layer; consequently early ﬁnishing should be conﬁned to removal of gross excess alone. if gross excess is present it should be removed either with a sharp-bladed hand instrument or, if necessary, a bur in a hand piece. Water cooling should be avoided. protective layer of resin or varnish should be re-applied to the surface of the material. Cementation 1. Surface of the prepared tooth must be clean and dry. 2. The consistency of the mixed cement must allow complete coating of the surface irregularities and complete seating of prostheses. 3.Excess cement must be remove at the appropriate time Cementation 4. The surface must be ﬁnished without excessive drying. 5. Protection of the restoration surface must be ensured to prevent cracking or dissolution Resin modiﬁed GIC component Resin modiﬁed GIC Activators/initiators/stabilizers Resin-modiﬁed glass ionomers. These products are considered by some authorities to be more truly based on a hybrid of the two parent groups of materials. In their simplest form they consist of a powder and liquid which require mixing prior to activation of polymerisation (often by light activation). The most convenient of the restorative type resin-modiﬁ ed glass ionomers are provided in an encapsulated form in which the powder/liquid ratio is determined by the manufacturer and the mixing is carried out mechanically in only a few seconds (1) A methacrylate resin which enables setting to occur by polymerisation. (2) A polyacid which reacts with the ion- leachable glass to bring about setting by an acid–base mechanism. (3) Hydroxyethylmethacrylate (HEMA), a hydro- philic methacrylate which enables both the resin and acid components to co-exist in aqueous solution; the HEMA also takes part in the polymerisation reaction. (4) Water, which is an essential component required to allow ionization of the acid component so that the acid–base reaction can occur. setting reaction The presence of three setting mechanisms may be claimed by the manufacturer: (1) Acid-base setting; (2) Light-activated polymerization; (3) Chemically-activated polymerization properties The addition of resin chemistry has signiﬁcantly improved many of properties. As compared with conventional GIC modiﬁed resin GIC has: Prolonged working time and rapid set. Release of ﬂuoride Bond to enamel and dentin The restoration can be polished immediately. Improved strength and their resistance to desiccation. No need to be etched under composite. Thank you

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