Base and liner 1 l Operative MCQs

Base and liner ; SILICATE CEMENT , GLASS IONOMER CEMENT (describing composition and prosperities of each cement)   l Operative MCQs (multiple choice question) for dental students

Dental Base and liner

Introduction

• Most dental cements set by acid-base reaction except calcium hydroxide and resin cement. Resins set by polymerization reaction.

• Restorative cements can be classified as temporary or short-term (for days to weeks), intermediate term ( for weeks to months) and permanent or long term (for years).

• Fluoride releasing cements are silicate, GIC, Silicophosphate and polycarboxylate (least).

• Fluoride contributes to the anticariogenic property by inhibiting demineralization and enhancing remineralization of carious enamel.

• Among the fluoride releasing cements the amount of fluoride release and longevity of release are more in GIC.

• Among the cements ZOE cement is the most soluble and resin cement is the least soluble in oral fluids.

• Silicate cement has most irritant effect on pulp while ZOE cement has least irritant effect on pulp.

• Among all the cements, silicate cement has highest compressive strength while resin cement has the highest tensile strength.

• GIC and polycarboxylate are the cements that produce a truly adhesive (chemical) bond to tooth structure.

• The bonding of resin cements to tooth occurs by micro-mechanical retention.

 

Factors increasing the setting time:

1. Cooling the mixing slab
2. Decreased powder liquid ratio
3. Prolonged mixing time
4. Loss of water from the liquid

SILICATE CEMENT

Composition:

Powder

1. Silica-40%
2. Alumina-30%
3. NaF , Cryolite , CaF2 : 19%
4. Calcium phosphate

 

Liquid

1. Phosphoric acid- 52%
2. Aluminum phosphate- 2%
3. Magnesium phosphate- 6%
4. Water-6%

• The purpose of fluoride salts is to lower the fusion temperature of the glass. Thus they are called as ‘ ceramic fluxes':
• Silicate cement was the first translucent filling material by fletcher in england (AP-2013) in 1878.

Properties:

1. Silicate is the strongest of all dental cements. It has a compressive strength of 180 Mpa.
2. Setting time is 3-8 min.
3. Its hardness (70 KHN) and coefficient of thermal expansion is closer to dentin.
4. It is a severe irritant to the pulp. At the time of insertion it has a pH of 2.8 and even after one month, it remains below 7.
5. Silicate has high solubility and disintegrates readily in oral fluids. They become stained over a period of time.
6. It has ant cariogenic property due to release of fluoride and so the incidence of secondary caries and contact caries is less.
7. Its refractive index is similar to enamel and dentin.
8. It is used as an anterior esthetic restorative material.
9. It is contraindicated in mouth breathers, as its surface becomes rough and opaque when allowed to dry.
10. Finishing and polishing should be delayed for several days. The surface should be protected by coating with varnish.

GLASS IONOMER CEMENT

Other names:

1. Polyalkenoate cement
2. Man made dentin
3. Dentin substitute
4. Alumino silicate poly acrylic cement (ASPA)

Composition of GLASS IONOMER CEMENT:

Powder of GLASS IONOMER CEMENT:

• The powder is referred to as 'ion leachable glass'

1. Silica－35-50%
2. Alumina －20-30%
3. NaF －3-6%
4. AlF3－1.5-2.5%
5. Aluminum phosphate-4-12%
6. Traces of barium, Strontium radiopacity

Liquid

1. Polyacrylic acid-45%
2. Itaconic acid, Maleic acid, Tricarballylic acid－5%(Decreases viscosity)
3. Tartaric acid-increases working time
4. Water -50%

Classification of GIC:

1. Type I:  Luting cement
2. Type II : Restorative cement
3. Type III:Liner
4. Type IV: Fissure sealant
5. Type V: Orthodontic cement
6. Type VI: Core build up cement
7. Type VIII: Posterior packable GIC for
8. Type IX: atraumatic restorations

 

• Among the first three types i.e, type I, type II and type III (conventional and light cure) types the highest cumulative release of fluoride after 30 days is from glass ionomer liner (light cure)[KCET-08]

 

Light polymerization of GLASS IONOMER CEMENT:

• The powder contains initiators for light curing and liquid component is modified with hydroxy ethyl methacrylate (HEMA).
• The polymerization starts when exposed to light and subsequently followed by acid base reactions. This is called "DUAL CURE" GIC.

Properties:

1. GIC has low fracture toughness and wear resistance.
2. Very sensitive to moisture, especially during initial setting reaction. During this period, absorption of water leads to weak cement and over drying will lead to cracks in the cement. Therefore, the surface of cement should be protected by coating with varnish or cocoa butter during setting.
3. Bonds chemically to tooth structure:  The bond of enamel is always higher than that of dentin.
4. Relatively biocompatible-The pulpal reaction is greater than ZOE but less than zinc phosphate cement.
5. Anti cariogenic property is due to fluoride release.

 

Important points in Glass ionomer cement:

1. GIC was introduced as a potential replacement for silicate cement. It has been evolved as a hybrid from the silicate and polycarboxylate cement.
2. Powder liquid ratio is 3:1 by weight. Mixing should be done by agate or plastic spatula.
3. 10% polyacrylic acid should be used for conditioning the cavity surface before insertion of the cement.
4. Final finishing is done after 24 hours of insertion.

 

 

Modifications of GIC:

Miracle Mix (or) silver cermet:

• Silver-Tin alloy powder is added to GIC powder.
• None of the properties were improved and it gave a gray or blackish color to the cement. It is also called as silver alloy admix.

Glass cermet or cermet:

• Glass and metal (Silver-tin-titanium) powders were sintered at high temperature and made to react with liquid.
• It improved the fracture toughness and wear resistance and at the same time maintained the esthetics.

Resin modified Glass Ionomer Cement:

• BisGMA, TEGDMA are added to powder and HEMA to the liquid.
• With exposure of light polymerization is initiated along the methacrylate groups. After that the liquid reacts with glass particles through acid base reaction.
• It improved the wear resistance and decreased the sensitivity to water attack.

Compomer (Polyacid modified composite resins):

• It is a combination of composite and GIC. Glass particles are partially silanated (for bonding
• with the matrix) and are added as fillers in the composite resin.
• There is no water in the reaction. The properties were inferior to composites but superior to GIC and resin modified GIC.

Bilayered or Sandwich Restorations:

• In this technique GIC is used as a liner under composites restorations.
• It increases the retention form as GIC bonds both the tooth and composite and the fluoride content reduces secondary caries.

Tunneling restorations:

• Joining the occlusal lesion with the proximal lesion by means of a prepared tunnel under the involved marginal ridge.
• The marginal ridge remains intact. GIC is used as the restorative material in this technique.

Atraumatic Restoration (ART)

• Involves removal of affected tooth structure with hand instruments, followed by restoring with GIC material (GC Fuji VIII).

High viscosity Glass Ionomer Cement:

• Used for Atraumatic restorative treatment.
• They contain small practice sizes and a high P/L ratio, yielding greater compressive strength and excellent packability.
• Also used for core buildups, primary tooth fillings and intermediate restorations.

Calcium aluminate Glass Ionomer Cement:

• It is a hybrid product of calcium aluminate and GIC. The GIC components are responsible for early properties (i.e., setting time, viscosity and strength).
• The calcium aluminate contributes to basic pH, biocompatibility and reduction in micro leakage. Also called as hydraulic cement.