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COMPOSITE TECHNIQUE    SMILE DESIGN     COMPANIES/MATERIALS    MATERIAL SCIENCE

 

                                    CLASS 5

Loss of tooth structure on facial or lingual surfaces and within the gingival third are referred to as class 5 defects.  Caries and toothbrush abrasion often combined with abfraction cause loss of tooth structure.  Abfraction is bending of tooth structure caused by force which is most commonly tooth grinding.  Wear facets are common on these teeth.  Bending in the gingival third is thought to accelerate toothbrush abrasion. 

Color, shape and depth of a defect are important for restoration strength and color.  Class 5 defects have tooth backing when viewed from the facial.  Defect color shows through composite so thickness and translucency of composite material determines restoration color.  It is difficult to create proper color restoring a shallow, dark colored defect because there is little composite thickness to block out underlying color.  A deep or shallow, naturally colored defect is the easiest match color because replication of normal layers produces a perfect result.  A deep, dark colored defect provides room of layering techniques of opaque and translucent composite or placement of opaquers underneath.

Defect shape is important to resist forces of abfraction.  In cross section, a rounded defect resists forces well but an angled defect (two straight walls, one on the gingival and one above, forming an angle) does not.  Rounding a defect by tooth preparation with diamond burs is important.  Bending of tooth structure places force on a restoration and interfaces for composite, bonding materials and tooth structure.  Composite and bonding agent must compensate for bending by having a modulus of elasticity similar to tooth structure or a restoration will fall out.  A large restoration receives increased force and might require mechanical retention from retention walls, grooves or dimples depending on restoration materials and size verses amount of force.

Gingival retraction is required when a restoration is placed subgingival.  Caries is excavated with slow speed round burs and spoon excavators to maintain tooth structure and minimize trauma to pulpal tissues.  Diamond burs are used to prepare tooth structure.  Dentin is roughened to improve retention and remove dentin that is hard to bond to from years of abrasive wear.  Enamel margins are beveled to achieve cross section of enamel rods for improved bond strength.  Long bevels create improved strength and show through of tooth color for composite color matching.

Enamel and dentin bonding is done to maximize interface strength and to minimize microleakage and sensitivity.  Acid is placed on enamel for 15 to 30 seconds.  Acidity is neutralized so only 10 microns of enamel is removed.  Inorganic structure dissolves quicker than organic structure and a porous, honeycomb surface is created.  Acid is placed on dentin for 10 to 15 seconds to remove the smear layer created by debris build up during tooth preparation.  Long exposure to acid removes collagen fibers and leaves a bare dentin surface.  The role of collagen fibers to assist or hinder bonding requires further study.  Areas of dentin bonding are precured prior to placement of composite so collagen is not pushed flat.

Composite is placed and cured in increments to minimize stresses of shrinkage during curing.  A final, outer, translucent enamel layer is placed, trimmed with burs and polished with sandpaper disks, rubber wheels, cups, points and polishing pastes.

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