Resin composite restorations
The clinical aspects for a successful resin composite restoration need to be taken care of sincerely and this covers all the steps involved in restoring with resin composites.
1. Why acid etching of enamel does not produce uniform etching pattern for all surfaces/teeth?
The dissolution of enamel to acids depends upon the mineral content of the enamel. The enamel contains enamel prisms which is structurally composed of small elongated apatite crystallites of varying shape and size. The shape of the prism in transverse section has round head (body) and a tail section. The orientation of the apatite crystallites in the head section is almost parallel to the rod long axis and is inclined to the prism axis (upto 65 degree) in the tail region. This orientation determines the susceptability of the apatite crystals to acids. The dissolution due to acid occurs more in the head region of the rod and the tail /the periphery of the head region are relatively resistant.
Ref:Theodore.M.Roberson,Harald.O.Heymann,Edward.J.Swift.JR.Sturdevant's Art and Science of Operative Dentistry.Mosby Publications;2002;17-18
2. Why C-factor is an important clinical consideration for composite restoration?
The polymerization shrinkage of composite material, which occurs upon hardening of material is detrimental to the restoration. This polymerization shrinkage depends upon the Configuration factor or C-factor. The C-factor is the ratio of bonded surface to the unbonded or free surfaces in a tooth preparation. The higher the C-factor, more will the detrimental effects of polymerization shrinkage. Class I composite restoration with the C-factor of 5 (five bonded surface and one free surface) is at more risk from detrimental effects than class IV restoration, whose C-factor is 0.25.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;479-480
3. Why properitary prophylatic pastes are not ideal to use for preparing tooth surface before placing a composite restoration?
Conventional prophylatic pastes may contain flavouring agents, glycerine or fluoride, which can act as contaminants and will interfere with the acid etching technique. Hence the tooth preparation should be cleaned with a slurry of pumice to remove plaque, pellicle and superficial stains before placing a composite restoration.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;483
4. Why shade selection for composites should not be done under isolation?
Once isolated, the teeth surface becomes dehydrated and lighter in shade as a result of a decrease in translucency. Hence the shade should be selected before isolation.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry,Mosby publications;2002;483
5. Why should the shade tab be partially covered while selecting the shade?
To create the natural effect of shadows, the shade tab should be partially covered with the patients lip or operator's thumb.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;484
6. Why the shade selection for composites should be done as rapidly as possible?
Beyond 30 seconds, it is difficult for the color receptors in the eyes to distinguish between similar colours.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;484
7. Why the operator should look at blue or violet object, if the shade selection for composites takes more than 30 seconds?
The colour receptors in the eye will not be able to distinguish between similar shades beyond 30 seconds. If more time is taken to select the shade, then by looking at blue or violet object, which are complimentary colours to yellow and orange shades of the teeth, the colour receptors are revitalised and resensitized to perceive the minor variations of the tooth surface.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications; 2002;484
8. Why is the resin dispensed just before it is used, in a light activated system?
The resin material is not dispensed long before it is used because, when it is exposed to the operatory lights for sometime, polymerization of the material occurs since the operatory lights contain some light in the range of 400-500nm.
Ref:Science of Dental Materials,Skinner, Prism books private limited,9th Edn,234
9. What are the specifications to be followed when an increment is light cured?
A high intensity light is used with the light tip placed as close as possible to the surface of the resin for about 40-60 seconds. The thickness of each increment should be 2-2.5 mm. Longer exposure times are required for darker shades and for resins polymerized through the enamel.
Ref:Science of Dental Materials,Skinner, Prism books private limited,9th Edn,235
10. What are the features of etched enamel?
The etched enamel surface shows preferentially dissolved centres of the enamel rods or preferential dissolution of the rod peripheries resulting in microporosity. This microporosity allows the penetration of the resin into them, forming a metallic bond.
Ref:Science of Dental Materials,Skinner, Prism books private limited,9th Edn,236, 237
11. Why the dentin should not be etched in vital teeth?
Dentinal etching does not cause pulpal inflammation since the acids are buffered by the dentin and doesn’t reach the pulp. But the dentinal tubules are opened up allowing for fluid movement and subsequently marginal leakage and the pulp becomes more susceptible to irritation. There is a risk of sensitivity, caries and pulpal irritation and ingress of bacteria and their toxins.
Ref:Preservation and Restoration of tooth structure,Graham.J.Mount,Mosby,1998, 97
12. Why an incremental build is up required for composites?
Incremental build up improves the depth of cure, minimizes shrinkage and directs shrinkage towards the tooth structure.
Ref:Preservation and Restoration of tooth structure,Graham.J.Mount,Mosby,1998, 101
13. Why is proper wedging necessary for a composite restoration?
A strong wedge is essential while restoring a tooth with composite to gain separation between the teeth and to avoid an overhang. It also protects the gingival tissue and stretches the periodontal ligament for separation for placement of matrix.
Ref:Preservation and Restoration of tooth structure,Graham.J.Mount,Mosby,1998, 102