Luting cements
Retention of crowns and bridges is dependant on the proper and successful usage of the luting cements and in this section, the various cements available, their properties, manipulation, precautions and uses are dealt with.
1. What is the basic difference between the types of zinc phosphate cement?
The difference is in the particle size and film thickness. In type I, the powder is finely grained and film thickness is 25 micrometer or less. In type II, the film thickness is 40 micrometer.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,480
2. What is Working time and Setting time?
Working time is the time during which the consistency of the mix is such that it flows under pressure readily to form a thin film. Setting time is the time elapsing from the start of mixing until the point of the Gillmore needle no longer penetrates the surface of the cement when the needle is lowered on to it.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,482,483
3. While mixing zinc phosphate cement, why the powder is added in small increments to the liquid?
This will increase the working time and setting time and allows incorporation of more powder into the mix, thereby increasing the strength of the cement.
Ref: Science of Dental Materials, Skinner, Prism books private limited, 9th Edn, 483
4. Why do highly polished surfaces not exhibit great retention of zinc phosphate cement?
In zinc phosphate cement, there is a mechanical interlocking of the cement. Slight roughness or irregularities on the tooth structure and casting helps the cement to flow and create undercuts which in turn helps for the retention.
Ref: Science of Dental Materials, Skinner, Prism books private limited, 9th Edn, 486
5. Why polycarboxylate cements are called pseudoplastic?
The polycarboxylate mix is pseudoplastic because it undergoes thinning with increased spatulation and seating of the casting.
Ref: Science of Dental Materials, Skinner, Prism books private limited, 9th Edn, 491
6. Why is polycarboxylate cement more biocompatible than zinc phosphate cement?
The size of the polyacrylic acid molecule when compared with that of the phosphoric acid molecule is larger and hence the polycarboxylate molecules cannot diffuse through the dentinal tubules. So it is said to be more biocompatible than the zinc phosphate cement and can be used in deeper cavities.
Ref: Science of Dental Materials, Skinner, Prism books private limited, 9th Edn, 492
7. Why do polycarboxylate cement fail at the cement metal interface?
Polycarboxylate cement is adhesive to the tooth structure but cohesive to metal. The wettability of the cement should be improved with the metal to achieve a metallic bond at the cement metal interface, failing which can lead to a weak bond at the cement metal interface.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,493
8. What are the points to be considered when the liquid of polycarboxylate cement is dispensed for mixing?
The liquid should not be stored in a refrigerator and it should not be dispensed prior to the time of mixing since both can lead to an increase in the viscosity of the liquid. Also, liquid dispensed earlier to mixing can lead to loss of water to the atmosphere.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,493
9. What are Resin cements?
Resin cements are basically composite resin filling materials. They are insoluble in oral fluids and have higher mechanical properties. They are irritating to the pulp and hence need pulp protection for deeper preparations.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,498,499
10. Why is removal of flash critical when resin cements are used for luting?
The time of flash removal is critical because when it is removed in a rubbery state, it may pull the cement from beneath the margins of the restoration which will leave voids, increasing the risk of secondary caries and periodontal disease. When the cement is polymerized, it is difficult to be removed. So it is best removed immediately after the indirect restoration is seated.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,499
11. How do dual cure cements cure?
The dual cure cements have 2 components, chemical and light cured components. It is mixed as done for the chemically activated systems. The chemical activation is slow and so the working time is more and when exposed to light, it solidifies fast. Over a period of time, it gains strength by chemically activated polymerization.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,499
12. What properties of zinc phosphate cement make it a universal luting agent?
It has good handling characteristics and resistance to solubility and disintegration in the oral cavity. These properties make it a universal luting agent though it is a pulpal irritant, not adhesive to tooth and not anti cariogenic.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,501
13. What are the two main properties of polycarboxylate that differs from zinc phosphate?
The polycarboxylate cement is not a pulpal irritant and it forms an adhesive bond to the tooth structure via the carboxyl groups.
Ref: Science of Dental Materials, Skinner, Prism books private limited,9th Edn,501
14. What are liners and bases?
Liners and bases are materials placed between the dentin and the restoration to provide pulpal protection or pulpal response.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;170
15. What are liners?
Liners are thin layers of material used to provide a barrier to protect the dentin from residual reactants diffusing out of the restoration or oral fluids that may penetrate the leaky tooth restoration interfaces.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;170