Operative Dentistry - Dental liners and bases - Part 2

Dental Liners and Bases

 

Polycarboxylate cement

Was the first adhesive material developed for use in dentistry supplied as:                                                                                      Powder + liquid

- The powder is zinc oxide and magnesium oxide. (Stainless steel fibres + alumina to increase the strength)

- The liquid is a weak solution of polyacrylic acid (30-50%) which is very viscous

Or supplied as: Water hardening cement or water setting

P: zinc oxide or glass powder is mixed with the powdered anhydrous, freeze, dried, polyacrylic acid.

L: water



Properties

  1. - Polycarboxylate cement is not as acidic as ZPC because of the PH of polycarboxylate rises more rapidly than that of ZPC.
  2. - The large size of the polyacrylic acid molecules compared with phosphoric acid may limit its diffusion through the dental tubules and is very biocompatible.
  3. - The carboxylate groups of the polyacrylic acid bond to calcium in tooth structure and the bond to the enamel is stronger than dentin.
  4. - Polycarboxylate cement adheres to gold casting alloy.

Uses

  1. - Intermediate base.
  2. - Luting cement
  3. - Used for direct bonding of orthodontic brackets to teeth

Mixing

  1. The powder and liquid are dispensed and mixed on a paper pad with a cement spatula. The liquid should not be dispensed until one is ready to mix, as water can evaporate and cause an increase in viscosity with a decrease in strength and higher solubility. The powder is rapidly incorporated to the liquid in large quantities; we should complete mixing within 30 -45sec. in order to provide sufficient working time = 2.5-6 min
  2. - The mixed material must be placed while the cement is still glossy which indicate that the liquid still available to bond to the tooth "adhesion depends on unreacted carboxylic acid groups".
  3. - For placing the material as a base the procedure is similar to that of ZPC


(Glass ionomer cement (GIC

 Powder: is an acid soluble, calciu fluro-alumino silicate glass

  Liquid: an aqueous solution of polyacrylic acid that contains carboxyl group - viscous liquid


Uses

  1.  As abase material in deep cavity
  2.  As a luting agent for permanently cement crown and orthodontic band and may
  3.  As a restorative material

Properties

 GIC materials are the strongest and least soluble dental cement

They are also adhesive and release fluorides

They have good biocompatibility

 GIC materials bond to tooth structure; in addition, they bond to stainless steel and alloys ceramometal crown

 Dehydration causes crazing and cracking and produces an opaque restoration

 Water absorption can cause swelling and surface disruption and protection is required for 10- 30 min. after placement


Various improvements in the original material have been made and incorporated into a number of following formulations

1- 

Conventional GIC: This is composed of powder, which is Ca, fluro alumino-silicate glass, and the liquid is polyacrylic acid. Used as liner, base, cement.

2- 

Metal-modified glass ionomers: (used as filling material for class V (permanent teeth), class I and II (primary teeth), bases under composite and amalgam, cores).

A- Miracle mixture (amalgam alloy particle admixed with cement powder).

B- Cermet particle reinforcement (Ag-Pd). Much stronger than unmodified, but had poor esthetic.


-3

Light cured glass ionomers: (liners, bases) hydroxyethyl methacrylate (HEMA) added to liquid component and light cure accelerator. Other powder particles mixed with alumino-silicate glass. It is more desirable under composite materials because this type is acid resistance


- 4 

Hybrid (resin-modified) glass ionomers: cements, restorative filling material, cores

  1.  HEMA and other polymers added to the liquid component
  2.  Polymers added to powder component
  3.  Silicate glass of composites substituted for some powder
  4.  They are light cured, less technique sensitive, stronger
Mixing
- It's critical that GIC is properly mixed and handled.
- If it is not, non-adhesive materials result. The powder is dispensed with a scoop, and the liquid is dispensed as drops. A cement spatula and paper pad are typically used "special mixing pad will keep all the liquid available for the reaction and facilitate spatulation". 
- The powder incorporated into the liquid in one or two portions and the mixing process is much quicker than for ZOE, ZPC, if the mixing procedure is too slow, the resulting mix becomes too thick
- Mixing time = 30-45 sec. S.T. = 7 min. W.T. = 2min


 The best results have been obtained when the material is applied to clean cavity walls that are well isolated in a dry field. The mixed cement should be used only as long as it still appears glossy on the surface

 GIC may be dispensed in disposable capsules that mixed in an amalgamator. Specific directions for mixing are provided by the manufacturer. The capsule has a "spout" where the mixed material is expressed from the capsule with a gun or dispenser

 The use of GIC as an intermediate layer between dentin and resin composite is often referred to as sandwich technique

 GIC use most often in conjunction with class II resin composite restorations are sometimes called the bonded-base techniques


General considerations

In shallow cavity

One to two mm  or more of remaining dentin for amalgam restoration the cavity is coated with 2 thin coats of a varnish and restored. For composite restoration, the cavity is etched, primed coated with single coat of a bonding agent and then restored

  Moderately deep cavity 

Varnish is used first then cement base such as ZPC or modified ZOE cement may be contoured to replace the missing dentin, then amalgam restoration. If we want to use adhesive base cement (GIC, polycarboxylate cement) varnish or bonding agent is not indicated at first because the coating will eliminate the potential for adhesion.

  In deep cavity

  A liner such as Ca (OH)2 on the pulpal and axial wall, overlaid with a base, then restorative materials

 If there is a pulp exposure, calcium hydroxide is used to stimulate reparative dentin

 The best possible base for any restoration is a sound tooth structure=

>>>>>  So you must remember that

Don’t remove sound tooth structure to provide space for a base. Maintaining sound dentin will enhance restoration support and provide maximum dentin  thickness for pulpal protection

.

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