1. Liners and Bases Alaa Sabrah BDS, MSD, PhD March 9, 2015

2. Definition
They are materials placed between dentin (sometimes pulp) and the restoration to provide pulpal protection or pulpal response.
The characteristics of the liner or base selected are determined largely by the purpose it is expected to serve. Because they share similar objectives, liners and bases are not fully distinguishable in all cases, but some generalizations can be made.
Roberson, Theodore, Heymann, Harold O., and Swift, Edward J.. Sturdevant's Art and Science of Operative Dentistry (5th Edition). St. Louis, MO, USA: Elsevier Health Sciences, ProQuest ebrary. Web. 8 March 2015.
Copyright © Elsevier Health Sciences. All rights reserved.

3. Pulpal Protection
Chemical protection (residual reactants that diffusing out of the restoration, oral fluids that may penetrate leaky restoration).
Electrical protection (new amalgam restoration).
Thermal protection
Mechanical protection
Pulpal medication
Protective needs vary depending on the extent and location of the preparation and the restorative material to be used.
Pulpal protection requires consideration of (1) chemical protection, (2) electrical protection, (3) thermal protection, (4) pulpal medication, and (5) mechanical protection

4. 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.

5. 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.
Theory of thermal shock
There are two theories about the cause of thermal sensitivity (usually to cold) following restoration placement and, consequently, two philosophies about how to best address the problem. The first theory states that sensitivity is the result of direct thermal shock to the pulp via temperature changes transferred from the oral cavity through the restorative material,61,62 especially when remaining dentin is thin. Protection from this insult would then be provided by an adequate thickness of an insulating material with low thermal diffusivity.62,63 It has been noted that resin composite exhibits such low thermal diffusivity that a thermal insulating base should be unnecessary in conjunction with resin composite restorations.63,64 Use of an insulating base for thermal protection would therefore be limited to metallic restorative materials that exhibit higher rates of temperature transfer.
When a base is used to provide insulation to counter thermal sensitivity in amalgam restorations, the thickness of the material must be minimized in areas subject to occlusal loading. Research has shown that, as the thickness of the base increases, the fracture resistance of the overlying amalgam decreases.65,66 Because temperature diffusion through amalgam to the floor of the cavity preparation is effectively reduced by 0.50 to 0.75 mm of basing material, if a base is used, its thickness should be restricted to no more than 0.75 mm.63 Modulus of elasticity is the key property that determines how effectively a base or liner will support an amalgam restoration; a high modulus of elasticity indicates stiffness, while a low modulus of elasticity indicates flexibility. As the modulus of elasticity of a basing material decreases, the resistance to fracture of overlying amalgam decreases.65-67
Theory of pulpal hydrodynamics
The more widely accepted theory of thermal sensitivity holds that temperature sensitivity is based on pulpal hydrodynamics. Most restorations have a gap between the wall of the preparation and the restorative material that allows the slow outward movement of dentinal fluid (Fig 6-1 b). Cold temperatures cause a sudden contraction of this fluid, resulting in a rapid increase in the flow, which is perceived by the patient as pain.50 As dentin nears the pulp, tubule density and diameter increase,1,68 as does permeability,69 thus increasing both the volume and the flow of pulpal fluid susceptible to the hydrodynamic effects of cold temperatures. This may explain why deeper restorations are sometimes associated with more problems of sensitivity.17
According to this theory, if the tubules can be occluded, fluid flow is prevented and a cold temperature does not induce pain. The operative factor in reducing sensitivity to thermal change thus becomes effective sealing of dentinal tubules rather than placement of an insulating material of a certain thickness.50 Scanning electron microscopic observations have revealed significantly higher numbers of open tubule orifices in hypersensitive dentin, lending credence to this theory.70,71
Table 6-1. Tooth-restoration interface: Materials and clinical failures*
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.

6. Thermal protection
Theory of pulpal hydrodynamics: temperature sensitivity is based on pulpal hydrodynamics.
Theory of thermal shock
There are two theories about the cause of thermal sensitivity (usually to cold) following restoration placement and, consequently, two philosophies about how to best address the problem. The first theory states that sensitivity is the result of direct thermal shock to the pulp via temperature changes transferred from the oral cavity through the restorative material,61,62 especially when remaining dentin is thin. Protection from this insult would then be provided by an adequate thickness of an insulating material with low thermal diffusivity.62,63 It has been noted that resin composite exhibits such low thermal diffusivity that a thermal insulating base should be unnecessary in conjunction with resin composite restorations.63,64 Use of an insulating base for thermal protection would therefore be limited to metallic restorative materials that exhibit higher rates of temperature transfer.
When a base is used to provide insulation to counter thermal sensitivity in amalgam restorations, the thickness of the material must be minimized in areas subject to occlusal loading. Research has shown that, as the thickness of the base increases, the fracture resistance of the overlying amalgam decreases.65,66 Because temperature diffusion through amalgam to the floor of the cavity preparation is effectively reduced by 0.50 to 0.75 mm of basing material, if a base is used, its thickness should be restricted to no more than 0.75 mm.63 Modulus of elasticity is the key property that determines how effectively a base or liner will support an amalgam restoration; a high modulus of elasticity indicates stiffness, while a low modulus of elasticity indicates flexibility. As the modulus of elasticity of a basing material decreases, the resistance to fracture of overlying amalgam decreases.65-67
Theory of pulpal hydrodynamics
The more widely accepted theory of thermal sensitivity holds that temperature sensitivity is based on pulpal hydrodynamics. Most restorations have a gap between the wall of the preparation and the restorative material that allows the slow outward movement of dentinal fluid (Fig 6-1 b). Cold temperatures cause a sudden contraction of this fluid, resulting in a rapid increase in the flow, which is perceived by the patient as pain.50 As dentin nears the pulp, tubule density and diameter increase,1,68 as does permeability,69 thus increasing both the volume and the flow of pulpal fluid susceptible to the hydrodynamic effects of cold temperatures. This may explain why deeper restorations are sometimes associated with more problems of sensitivity.17
According to this theory, if the tubules can be occluded, fluid flow is prevented and a cold temperature does not induce pain. The operative factor in reducing sensitivity to thermal change thus becomes effective sealing of dentinal tubules rather than placement of an insulating material of a certain thickness.50 Scanning electron microscopic observations have revealed significantly higher numbers of open tubule orifices in hypersensitive dentin, lending credence to this theory.70,71
Table 6-1. Tooth-restoration interface: Materials and clinical failures*
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.

7. If the insult produces fluid flow, in or out of the dentinal tubules, the pressure change is sensed by mechanoreceptors within the pulp, and the patient experiences sensitivity. If leakage of chemical irritants from biomaterials or bacteria occurs, the pulp complex can become inflamed. To protect against these events, it is paramount to seal the outer ends of the tubules along the dentinal tooth preparation wall.
The dentin smear layer (Fig. 4-46) produces some degree of dentinal tubule sealing, although it is 25% to 30% porous. Flow or microleakage in or out of tubules is proportional to the fourth power of the diameter of the opening (Fig. 4-47). Halving the diameter of the opening produces a 16-fold reduction in flow. 169,209,253 The smear layer is an effective barrier. Because it is partially porous, however, it cannot prevent slow long-term diffusion.

8. 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.

9. Pulpal Medication
Two important aspects of pulpal medication are required:
Relief of pulpal inflammation
Facilitate dentinal bridging for physiologic protection.
The materials eugenol and calcium hydroxide are usually used.

10. 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.

11. The terms varnish, sealer, liner, and base, used to describe a variety of materials, have been a source of confusion in dental literature.

12. a physical barrier to bacteria and their products
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
a physical barrier to bacteria and their products
to provide a therapeutic effect, such as an antibacterial or pulpal anodyne effect.
They also contribute initial electrical insulation.
Generate some thermal protection.
McCoy RB. Bases, liners and varnishes update. Oper Dent 1995;20:216.

13. 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.
Direct composite restorations, indirect composite or ceramic restorations, and resin-modified glass-ionomer restorations routinely are bonded to tooth structure. The insulating nature of these tooth-colored materials and the sealing effects of the bonding agents preclude the need for traditional liners and bases, unless the tooth preparation is extremely close to the pulp, and pulpal medication becomes a concern.
Roberson, Theodore, Heymann, Harold O., and Swift, Edward J.. Sturdevant's Art and Science of Operative Dentistry (5th Edition). St. Louis, MO, USA: Elsevier Health Sciences, ProQuest ebrary. Web. 8 March 2015.
Copyright © Elsevier Health Sciences. All rights reserved.

14. Thin film liners 1-50 um can be subdivided into:
Solution liners (varnishes 2-5um)
Suspension liners (20-25 um).
Cement liners ( um) selected for pulp medication and thermal protection.

15. 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.
To produce a thin film liner, liner ingredients are dissolved in a volatile nonaqueous solvent. The solution is applied to tooth structure and dries to generate a thin film. Any liner based on nonaqueous solvents that rely on evaporation for hardening is designated as a solution liner (or varnish).
Thin films work best because they are flexible and dry rapidly. Thick films tend to trap solvent during rapid superficial drying and become brittle when they finally dry.
Because some moisture is in the smear layer, and varnishes are hydrophobic, the film does not wet the surfaces well. A single coat effectively covers only 55% of the surface (Fig. 4-48). A second thin layer is recommended to produce sealing of 80% to 85% of the surface. Because of the use of bonding systems or desensitizing systems (discussed later) with amalgams,

16. 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.
To produce a thin film liner, liner ingredients are dissolved in a volatile nonaqueous solvent. The solution is applied to tooth structure and dries to generate a thin film. Any liner based on nonaqueous solvents that rely on evaporation for hardening is designated as a solution liner (or varnish).
Thin films work best because they are flexible and dry rapidly. Thick films tend to trap solvent during rapid superficial drying and become brittle when they finally dry.
Because some moisture is in the smear layer, and varnishes are hydrophobic, the film does not wet the surfaces well. A single coat effectively covers only 55% of the surface (Fig. 4-48). A second thin layer is recommended to produce sealing of 80% to 85% of the surface. Because of the use of bonding systems or desensitizing systems (discussed later) with amalgams,

18. 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.
Liners based on water have many of the constituents suspended instead of dissolved and are called suspension liners.
Both types of liner are often extended over the cavo-surface margin of the preparation.
Excess material is not necessary but is difficult to avoid.

19. 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.

20. 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.

21. 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.

22. 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 by-products on the pulp.
calcium hydroxide may release growth factors from dentin that can assist in pulpal healing.

23. 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.

25. 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.

26. Bases
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.
Metallic restorations may benefit from seating (resting) on sound dentin peripheral to the lined or based regions that result from excavating infected dentin (see Fig. 4-45). These seats may help distribute stresses laterally to sound dentin and away from weaker underlying structures.
Roberson, Theodore, Heymann, Harold O., and Swift, Edward J.. Sturdevant's Art and Science of Operative Dentistry (5th Edition). St. Louis, MO, USA: Elsevier Health Sciences, ProQuest ebrary. Web. 8 March 2015.
Copyright © Elsevier Health Sciences. All rights reserved.

27. 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.

28. 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.

30. Clinical considerations
The need for specific types of liners and bases depends on:
the remaining dentin thickness.
Consideration of the adhesive material.
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.

31. 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.

32. 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.

33. 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.

36. 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:
Seal that protects the pulp.
Stress breakers to resist polymerization shrinkage stresses.

37. 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

38. References The art and science of operative dentistry
Chapter 4 pp:
Chapter 11 p:499
SUMMITT'S FUNDAMENTALS OF OPERATIVE DENTISTRY: A CONTEMPORARY APPROACH - 4th ed. (2013)
Chapter 6.