cons sheet 3 -Fadi Salameh

Liners and Bases
Definition: they are materials placed between the dentin or sometimes the pulp –in case of exposure- and the restoration to provide pulpal protection and/or pulpal response.
In this course we will discuss the clinical aspects and applications of these materials, their chemical compositions which we studied in the dental materials course last year are also required, students from previous years confirmed that a few (3-5) questions about the ratios (P/L) and manipulation of the materials were asked.
When we prepare a cavity debris is produced forming either the enamel smear layer or the dentin smear layer. Smear layers are beneficial for pulpal protection, providing chemicals and thermal protection.
We provide protection by blocking dentinal tubules so separate the sensory nerve endings in the tubules from external stimuli, so the role of liners or bases is tubule obstruction.
Pulpal Protection
• Chemical protection: residual reactants diffusing out of the restoration. If the material is set by a chemical reaction, these chemicals may be irritating to the underlying tissues, also in case of a leaky restoration (a restoration with gaps allowing oral fluids to enter the cavity) we get irritation by the oral fluids.
• Electrical protection: mainly needed when we are using metallic restorations (amalgam or gold), liners and bases provide a certain extent of electrical insulation.
• Mechanical protection: if the remaining dentin doesn’t withstand applied forces such as amalgam condensation and may break, we use a base in such a case, and not a liner.
• Thermal protection: thermal insulation is provided by liners and bases especially in the case of restorations with high conductance (amalgam), thermal insulation in proportional to the thickness if the material, the greater the restorations thickness, the greater the amount of thermal insulation needed. 2mm of dentin or an equivalent insulator are recommended for pulpal protection, 1-1.5mm are also acceptable, but 1mm of dentin is considered to be insufficient protection and we need insulation. Materials used for thermal protection must set by chemical reaction and not light cure.
• Pulpal medication: 1- Relief of inflammation –Eugenol in ZnOE
2- Facilitation of dentinal bridging- CaOH


Protective needs vary depending on the extent and location of the preparation and the restorative material to be used.

Liners: thin layers of materials.
Used primarily as a barrier to protect dentin from residual reactants (chemical protection), also to provide initial electrical and thermal insulation, some provide pulpal medication. Notice that we didn’t mention mechanical protection, liners do not provide it.
We rarely use liners nowadays, as they are not needed when the restorative material binds to the tooth structure, like amalgam, composite or GI (Glass Ionomer).
In pulpaly extended metallic restorations that are not well bonded to the tooth structure we need to use liners.
Also used with indirect restorations (inlays and onlays) as a means of cementing the restoration to the tooth.
Inlays and onlays are prepared in the lab, they are not a full crown, thety are large restorations whose impression is taken and then a restoration is made from gold or ceramic. Metallic indirect restorations are made from gold.
A liner is not always needed with inlays or onlays
Classification of liners:
1. Solution liners (varnishes)
2. Suspension liners (20-25µm)
3. Cement liners (CaOH, ZnOE) –mainly used as pulp medication or thermal protection
Varnishes:
Copal or other resin 10%, ether, alcohol or acetone 90%, since there is such a low percentage of resin, material evaporates easily, and there is no need to dry it, we only have to wait for a few seconds, and it dries spontaneously, we usually apply 2 layers of varnish, as 1 layer is not enough for complete obstruction of the dentinal tubules, due to the fact that the material is hydrophobic, if the material was hydrophilic they 1 layer would’ve been sufficient to cover the surface.

• Film thickness=2-5µm
• Applied over the smear layer
• The tooth should be dry (remember hydrophobic)
• Solution and suspension liners may be used on the cavo-surface margin, and not cement liners because later on dissolution occurs and since cement liners are thick, the gap they leave is too big to be filled with amalgam corrosion products as happens with solution and suspension liners
• Liners are difficult to control, we can’t limit their spread, for example to the pulpal floor or the axial wall like we do with CaOH.
• Good isolation must be provided when working with varnish, either by using a rubber dam, cotton rolls, saliva ejector…. Etc.
• they will spread all over the cavity, due to their low viscosity, so they are applied by a cotton pellet.
Suspension liners:
Produce the same effect as solution liners but are thicker, they have a higher percentage of resin, so we need to dry them by a flow of air, they also may be placed on the cavo-surface margin.
Cement liners:
1. Zinc Oxide Eugenol: releases small amounts of eugenol over time, eugenol is acidic, therefore relieves pulpal inflammation, also it acts as a palliative and reduces pain. Eugenol works most efficiently on pulp when used in small concentrations, a high concentration of eugenol would cause a chemical burn. If the material sets directly, then no eugenol would be slowly released over time, and no effect on the pulp would be achieved.
2. CaOH: based on a reaction between Ca2+ ions from CaOH, it undergoes a chemical setting reaction but allows a minor amount of CaOH to be released to the liner surface, to produce the desirable effect. CaOH is the only material that may be used on zero dentin, ie, pulp exposure. CaOH also encourages dentinal bridging, it may degrade severely over a long period of time, and may no longer provide mechanical support for the overlying restoration. When repairing old restorations, CaOH must be removed and replace because the material deteriorates.

Some other liners: Bonding agents, in composite or amalgam, they don’t depend on relief of symptoms or bridging of dentin, instead they depend on the bonding of the material decreasing micro leakage and providing thermal insulation and protection.
So if we are using composite, there is no need to use CaOH unless there is pulp exposure, because the material is well bonded to the tooth structure.
Bases:
• 1-2µm
• Provide thermal protection for pulp
• Supplement mechanical support for restorations by distributing stresses from restorations across underlying dentin surface, this provides resistance against destruction on thin dentin layers over the pulp during condensation of amalgam, or cementation of indirect restorations.

Metallic restorations may benefit from the seating on sound dentin peripheral to the lined and/or base region.
Usually cement liners (CaOH, ZnOE) ir bases shouldn’t contact the walls of the cavity, not the cavo-surface margin, they are either placed on the pulpal floor, or on the exial wall, because dissolution occurs and a gap forms promoting micro leakage.
Various liners and bases may be combined in a single preparation depending on the size of the preparation:
Shallow: varnish
Moderate: CaOH
Deep: CaOH and base then filling



Arrangement of layers of fillings
Liner applied usually 1st, but when using Zn Phosphate, we place varnish below it, because it is acidic and has small molecules, so it may penetrate the dentinal tubules.
Zn polycarboxylate or ZnOE, we place the liner before the base, then place varnish to block the tubules at the walls of the cavity, then place the filling.
So…. Liner, then base then varnish, then filling
Varnish may be extended to cavo-surface margin
Dr. mentioned that table from the book; she said that we need to know the physical and chemical properties that each material provides.
GI is the only base used under composite, because composite treats GI like tooth structure, so this way we get mechanical between GI and filling and GI and cavity walls.

In a shallow amalgam excavation:
• No need for pulpal protection other than chemical
• For amalgam, dentin is coated by 2 layers of varnish

Shallow composite:
• Only bonding agent which is provided with composite package is needed



Moderate amalgam cavity:
• ZnOE for thermal insulation and palliative effect, IRM is used because it has a faster setting time
• CaOH for pulpal medication (dentinal bridging)
Moderate composite:
• No eugenol used because it stops chemical bonding of composite
• Zn phosphate and polycarboxylate are not used, instead GI is used for better bonding with composite and increased retention
Deep amalgam cavity or pulp exposure:
• CaOH treats pulp exposure and provides adequate resistance for condensation of amalgam
• Base is used to compensate for lost dentin
So for deep amalgam cavities we use CaOH then Base then Varnish then amalgam. Unless were using Zn phosphate, then we would start with liner, then varnish then Zn phosphate then amalgam to avoid post op sensitivity.
Gold inlays and onlays:
Since were using gold (a metal) we need a base if cavity is moderate or deep for protection (mainly mechanical)
Liners and bases are only used if cavity is deep or patient is complaining of sensitivity, materials that may be used are GI or resin modified GI
We may not use ZnOE
Newer generations of composite have reduced polymerization shrinkage, so bases are not a necessity.

Survival of liners and bases below restorations:
Varnishes: undergo dissolution, but the gaps formed are narrow and are filled with corrosion products of amalgam
Liners: longevity variable, so if we are repairing a restoration, we should replace liner.

Conservative dentistry, Dr. Yara
22-2-2011
Sorry for taking so long,
Fadi Salameh