Addition of composite to tooth structure increases tooth size and therefore, there is often tooth reduction to make room for composite placement. Amount of reduction is determined by position of original tooth in relation to the desired position and color of tooth structure in relation to the thickness of composite materials required to block out color.
Veneer failure occurs from intracomposite fracture, separation of bonded interfaces or fracture of supporting tooth structure.
Force intensity, duration, frequency and direction verses restoration strength determine success or failure of a restoration. Direction of forces is lateral, rotational, downward, and pulling upward resulting in tensile, shear or compressive failure. Preparation design, material selection and placement technique are critical resist forces.
Removal of caries and previously placed restorations, tooth fracture, or increasing tooth dimensions, either interproximally or incisally, produces areas with no tooth backing. A translucent restoration will show through darkness from the back of the mouth and appear darker. Composite must block out darkness from the back of the mouth. Placement of opaque material or dentin shades covered with translucent material produces a natural looking restoration that is not influenced by this darkness. Blending composite color to tooth color is further achieved by proper composite selection, placement and preparation design.
Caries, defective restorations and weak tooth structure are removed. Diamond burs shape remaining tooth structure to accept composite by leaving a rough surface for improved bond strengths.
Enamel and dentin bonding provides strength to hold composite onto tooth structure, minimize microleakage and sensitivity. Removal of caries often creates areas of mechanical retention. Strength of enamel bonding is increased by beveling across enamel rods. A long enamel bevel or chamfer preparation creates more surface area for strength and provides a long gradual show through of tooth structure for better color transition. A translucent outer layer of composite provides a chameleon effect picking up and showing through surrounding color.
Enamel and dentin bonding is done to maximize interface strength. 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 deposited during tooth preparation. Long exposure to acid removes collagen fibers and leaves a bare dentin surface.
Dentin bonding is precured prior to composite placement to stabilize collagen fibers and avoid fiber collapse. Enamel bonding agent should be thin which is achieved by placing a small amount on a brush or blowing away excess. Enamel bonding resin does not require light curing prior to composite placement and allows easier wetting of the surface by composite.
Restoration of class 3 and class 4 areas that show through darkness from the back of the mouth are done first. If required, a sublayer of composite and/or opaquer is used to block out tooth color. Composite is rolled into a ball. The ball is placed on the facial surface resulting in a point contact that when pressed with a finger or instrument spreads and contacts tooth structure without trapping air. Shaping with instruments using a tapping motion minimizes sticking of the instrument to composite. Composite is shaped as close to final contours as possible and light cured.
Shaping and polishing is done with burs, sandpaper disks, rubber wheels, points, cups, and polishing pastes. Composite shaping, contouring and texturing require different techniques depending on the desired result.