Operative Dentistry MCQ Answer 22

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The correct answer is c. (2, 3, 4, 5.)

The polymerization shrinkage occurs outside of the mouth, not in the tooth, which improves the physical properties and resistance to wear. The polymerization shrinkage does not occur in the tooth. This reduces induced stresses and bond failures. This will theoretically reduce the potential for leakage. These resins are also repairable in the mouth and are not as abrasive to opposing tooth structure as ceramic inlays.

Laboratory processing may employ heat (140 C) and pressure (0.6 MPa for 10 minutes). This polymerization under heat and pressure is used for a homogeneous microfilled resin that is claimed to have a higher filler content, less porosity and greater color stability than light-activated resins. In addition, a slightly higher degree of polymerization (degree of conversion) is attained.

The following are additional physical properties of laboratory composites:

• Flexural strength is high-90-150 MPa
• Flexural modulus is variable-4.7-15 GPa
• Compressive strength is 210-280 MPa
• Handling characteristics and properties:
• Advantages-best anatomy and contacts, lower wear
• Disadvantages-lab cost, special equipment, requires resin cement

1. Is incorrect as noted above. The polymerization occurs completely outside of the mouth.

Extra notes to recall about all composites (direct) regarding repair: A restoration that has just been cured and polished may have up to 50%of unreacted methacrylate groups to copolymerize with the newly added material. As the restoration ages, fewer and fewer unreacted methacrylate groups resin, and greater cross-linking reduces the ability for fresh monomer to penetrate into the matrix. Then strength of the bond between the original material and the added composite resin decreases in direct proportion to the time that has elapsed between the original polymerization and the addition of new composite. In addition, it should be noted that the polishing step would expose filler particles that are free from silane coating, which will inhibit a chemical bond to the new composite. The strength of the repair composite is less than ½ of the original material.

References:

Anusavice K: Philips’ Science of Dental Materials, 10^th Ed. WB Saunders, 1996.

Craig RG and Powers JM: Restorative Dental Materials, 11^th Ed. Mosby, 2002.