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Liners and Bases Alaa Sabrah BDS, MSD, PhD March 9, 2015 Definition They are materials placed between dentin (sometimes pulp) and the restoration to provide pulpal protection or pulpal response. Pulpal Protection 1. Chemical protection (residual reactants that diffusing out of the restoration, oral fluids that may penetrate leaky restoration). 2. Electrical protection (new amalgam restoration). 3. Thermal protection 4. Mechanical protection 5. Pulpal medication Protective needs vary depending on the extent and location of the preparation and the restorative material to be used. Thermal protection • Mostly required with the use of metallic restorative materials. • Thermal insulation is proportional to the thickness of the material. • 2mm of dentin or an equivalent thickness of material should be present to protect the pulp. • Because this thickness is not always present a layer of 1 to 1.5 mm is practically acceptable. Thermal protection • Theory of thermal shock: sensitivity is the result of direct thermal shock to the pulp via temperature changes transferred from the oral cavity through the restorative material, especially when remaining dentin is thin. Thermal protection • Theory of pulpal hydrodynamics: temperature sensitivity is based on pulpal hydrodynamics. Dentin Vs linear and bases • Conservation of remaining tooth structure is more important to pulpal health than is replacement of lost tooth structure with a cavity liner or base. • The remaining dentinal thickness (RDT), from the depth of the cavity preparation to the pulp, is the single most important factor in protecting the pulp from insult. • 0.5-mm thickness of dentin reduces the effect of toxic substances on the pulp by 75%. • 1.0-mm thickness reduces the effect of toxins by 90%. • Little pulpal reaction occurs when there is an RDT of 2 mm or more. • The greatest impact on the pulp occurs when the RDT is no more than 0.25 to 0.30 mm. Pulpal Medication Two important aspects of pulpal medication are required: a. Relief of pulpal inflammation b. Facilitate dentinal bridging for physiologic protection. The materials eugenol and calcium hydroxide are usually used. Current status…. • The theory of pulpal hydrodynamics has gained general acceptance in recent years and has changed the direction of restorative procedures away from thermal insulation and toward dentinal sealing. Thus, there is increasing emphasis on the integrity of the interface between restorative material and prepared tooth. • The terms varnish, sealer, liner, and base, used to describe a variety of materials, have been a source of confusion in dental literature. • Liners: cement or resin coating of minimal thickness (usually less than 0.5 mm) usually applied only to dentin cavity walls that are near the pulp to achieve 1. a physical barrier to bacteria and their products 2. to provide a therapeutic effect, such as an antibacterial or pulpal anodyne effect. 3. They also contribute initial electrical insulation. 4. Generate some thermal protection. McCoy RB. Bases, liners and varnishes update. Oper Dent 1995;20:216. Indication for using liners In pulpally extended metallic restorations that are not well bonded to tooth structure and that are not insulating such as amalgam and cast gold, or with other indirect restorations. Thin film liners 1-50 um can be subdivided into: 1. Solution liners (varnishes 2-5um) 2. Suspension liners (20-25 um). 3. Cement liners (200-1000um) selected for pulp medication and thermal protection. 1. Solution Liners (varnishes) • Liner ingredients (copal or other resin 10%) are dissolved in a volatile non-aqueous solvent (ether, alcohol and acetone). • The resin content is kept intentionally low to produce a thin film on drying (they are flexible and dry quickly). • Most solvent loss occurs in 8-10 seconds and does not require forced air assistance. 1. Solution Liners (varnishes) • A thin film of 2-5um is formed over the smear layer. • Some moisture is present in the smear layer and varnishes are hydrophobic so a single layer is not enough to cover the dentin surface. • Varnish has commonly been used under amalgam restorations and before cementation of indirect restorations with zinc phosphate cement. 2. Suspension Liners • Liners based on water have many of the constituents suspended instead of dissolved and are called suspension liners. • Produce the same effect as solution liners. • They dry more slowly and produce thicker films. Eugenol • A parasubstituted phenolic compound that is slightly acidic. • It produces palliative or obtundent actions on the pulp when used in low concentrations. They alleviate discomfort resulting from mild to moderate pulpal inflamation. • High concentrations can be irritating. Eugenol • Several cements, bases and liners result from the reaction between zinc oxide and eugenol. • In liners small amount of eugenol is released over a period of several days. For this reason these materials were used in relatively deep preparations. Calcium Hydroxide • They are based on the reaction between calcium ions from calcium hydroxide particles and phenolic moieties on mono-functional or multi-functional molecules. • They are formulated to undergo a chemical setting reaction but allow minor amounts of calcium hydroxide to be released from the liner surface to produce the desirable effect (reparative dentin formation). • Used in the deepest portions of the preparation or when pulp exposure is suspected. Calcium Hydroxide • It encourages dentinal bridging. • Reparative dentin formation is assisted, rather than stimulated due to the antibacterial action of calcium hydroxide, which reduces or eliminates the inflammatory effects of bacteria and their byproducts on the pulp. • calcium hydroxide may release growth factors from dentin that can assist in pulpal healing. Calcium Hydroxide • They may degrade severely over a long period of time so that they no longer provide the mechanical support for the overlying restoration. • Unfavorable physical properties restrict calcium hydroxide use to application over the smallest area that would suffice to aid in the formation of reparative dentin when a known or suspected pulp exposure exists. Current status…… • Newer liners place less emphasis on pulpal medication and more on chemical protection by sealing, adhesion and mechanical protection. • Sealing is the most important property. • Ceramic and or polymeric materials provide excellent thermal insulation. • Newer compositions rely on mechanically strong acrylic resin matrix and this makes the release of eugenol or calcium hydroxide ions almost impossible. Bases • • 1 2 • Materials to replace missing dentin, used for bulk buildup and/or for blocking out undercuts in preparations for indirect restorations. Cement bases typically 1-2mm. They are used to: Provide thermal protection for the pulp. Supplement mechanical support for the restoration by distributing the stresses from the restoration across the underlying dentin surface. This mechanical support provides resistance against disruption of the thin dentin layer over the pulp during condensation of amalgam or cementation of indirect restorations. Bases Zinc phosphate cement and resin reinforced zinc oxide eugenol were widely used for bases in the 1960, then poly carboxylate cements became popular. Later they started using glass ionomer cements. Highly modified forms of glass ionomer s provide chemical adhesion, good mechanical strength, and rapid achievement of strength. • Previously in a deep preparation a calcium hydroxide liner was used then a base would be added to provide mechanical support and stress distribution. Then the base and the walls of the preparation would be covered with varnish (except when using zinc phosphate cement varnish will be applied before the base. • Nowadays both light cured calcium hydroxide and glass ionomer are used to line and base the cavity. Clinical considerations • The need for specific types of liners and bases depends on: 1. the remaining dentin thickness. 2. Consideration of the adhesive material. 3. Type of restorative material being used. • Various liners and bases may be combined in a single preparation and the dimension between the pulp and the restoration may be a combination between natural dentin, liner, and base. Clinical considerations In a shallow tooth excavation: • there is no need for pulpal protection, other than in terms of chemical protection. • For an amalgam restoration dentin is coated with two thin layers of varnish, a single coat of dentin sealer, or a dentin bonding system. • For a composite restoration the prep is treated with the bonding system. Clinical considerations In a moderately deep tooth excavation: • For an amalgam restoration that includes some extension towards the pulp we apply zinc oxide eugenol (provides some thermal insulation and releases some amount of eugenol that is obtundent to the pulp) or calcium hydroxide to provide pulpal medication. • For a composite tooth preparation eugenol is not used as it has some potential of inhibiting polymerization of layers of bonding agent or composite that are in contact with it. So calcium hydroxide is used if a liner is indicated. Clinical consideration • If the RDT is very small or if pulp exposure has taken place calcium hydroxide is used with a layer of 0.5-1mm to treat a near or actual pulp exposure. • If extensive dentin is lost because of caries and tooth excavation extends close to the pulp a cement base should be applied over the calcium hydroxide liner. Liners and bases under composite restorations • Materials include RMGIs, compomers, flowable composites. • Proponents of this approach do not promote these materials for pulp protection in the traditional way but as materials that provide a better seal for composite restorations when extended to the root surface. • These materials provide: 1. Seal that protects the pulp. 2. Stress breakers to resist polymerization shrinkage stresses. Survival of liners and bases under restorations • Varnishes are relatively thin and brittle and may only provide chemical protection for a couple of days to weeks • Sealers maintain their integrity better than varnishes. • Bonding agents survive many years. • Liners and bases may be sufficiently intact to limit the extent of tooth re-preparation to only the outline necessary for removal of the old restorative material. • Calcium hydroxide may continue to dissolve and may loose 10-30% of their volume over 10 or more years • It may be a good practice to remove all liners and bases during the re-restoration procedure as long term changes are not well characterized References • The art and science of operative dentistry Chapter 4 pp: 170-177 Chapter 11 p:499 • SUMMITT'S FUNDAMENTALS OF OPERATIVE DENTISTRY: A CONTEMPORARY APPROACH - 4th ed. (2013) Chapter 6.