Restorations that are bonded "stick" to the tooth without the aid of undercuts or "lock and key" cementation.  There are four types of bonding used in dentistry today.

• Acid etch enamel conditioning 

This was the first form of bonding used in dentistry.  In this technique, a 10% solution of phosphoric acid is placed on the enamel portions of the tooth and left in place for fifteen seconds.  When it is washed off, the formerly shiny enamel surface now looks like it is chalky, or frosted.  Under a microscope, the surface looks like a ragged landscape of jagged mountains and valleys (see micrograph to the right).  These microscopic irregularities are then filled with a liquid acrylic plastic which hardens in place.  Since the filling material is composed of the same sort of plastic, mixed with glass particles (see filled resins below) it will bond onto the plastic which becomes mechanically adhered to the conditioned enamel.  Click the image to learn more about the structure of enamel.

• Dentinal bonding

  The micrograph on the left shows what dentin looks like when it is sliced perpendicularly to the dentinal tubules.  The tubule openings are clearly visible, but the hard material between them is still fairly smooth and will not bond to a layer of liquid plastic in the same way as it does to etched enamel.  Etching the dentin with 10% phosphoric acid does not create the same type of roughness in dentin that it does when etching enamel.  Instead, the acid completely dissolves a small amount of the hard dentin material around the tubules allowing the strands of collagen that permeate the dentin to project beyond the cut surface.  It also partially opens up the the tubules (image to the right). 

  The first step in adhering a layer of resin to etched dentin involves placing a primer composed of a solution of 2-hydroxyethyl methacrylate (HEMA)--a hydophylic (water soluble) polymer, generally dissolved in acetone--to the conditioned dentin.  Repeated applications of the solvent facilitates the evaporation of the water in the dentin and replaces the water with HEMA.  The primer flows into the tubules and between the exposed collagen fibers and acts as a bridge between the otherwise hydophylic collagen fibers and a subsequent layer of hydrophobic (water insoluble) resin.   Thus the final layer of resin can thoroughly infiltrate between the collagen fibers and into the tubules.  Once the resin hardens, it serves as the basis of dentinal bonding.   Click either image to learn more about the structure of dentin. 

  Note that bonding composite to enamel and dentin always involves three steps: conditioning with acid, priming the dentin with HEMA and application of acrylic resin to the etched enamel and primed dentin.  Any system that utilizes a separate conditioning step, etching both dentin and enamel with phosphoric acid, is called a "total etch system".

  The next generation in bonding agents, (also considered total etch systems), utilizes a separate phosphoric acid conditioning step but combines the HEMA and the resin in a dilute mixture cut with a volatile solvent such as acetone. (Note: acetone is soluble in both water and many hydrophobic materials such as plastic resin.)

  The newest iteration of bonding agents combines all three steps (conditioning with acid, application of HEMA in solvent, and resin) in a single step.  These materials are called self-etching primer systems (SEPs).

   

• Chemical adhesion

  Certain materials such as glass ionomer, and polycarboxylate cements may be applied directly to unconditioned enamel and dentin without conditioning or application of a primer.  They are applied in a thick liquid or paste form, and this liquid is fairly acidic.  Metallic polyalkenoate salts combine with the hydroxyapatite by replacing phosphate ions.  The carboxylic groups of the polyalkenoic chains can chelate (chemically combine with) the calcium of the hydroxyapatite to bond the cement to both dentin and enamel. This cross linking of restorative material and tooth structure gives excellent chemical bonding strength.

    

• Amalgam bonding

  The bonding of a dental amalgam to a tooth involves any or all three of the above mechanisms to bond a filled resin cement to the tooth structure and a mechanical mechanism to bind the amalgam to the resin.  The enamel and dentin are conditioned with 10% phosphoric acid, HEMA is applied to the dentin for dentinal bonding, and a layer of very loose filled resin is applied over the tooth structure.   Dental amalgam is condensed into the tooth while the resin is still unset.  This causes tags of amalgam and filled resin to intermingle at the interface, and when both materials set, they are securely mechanically locked together.  Thus the amalgam is locked to the resin, and the resin is bonded to the tooth. 

How were fillings and crowns retained on teeth before bonding?

Prior to the age of bonding, dental restorations (fillings, crowns, onlays etc.) had to be attached to teeth mechanically.  Even with bonding techniques, mechanical retention remains the major method of retaining crowns and large fillings in teeth.  In the case of most fillings, this is done by the use of undercuts placed inside the cavity preparation (the "hole" in the tooth). The filling material is condensed into the cavity preparation so that it flows into the undercuts.  When hardened, the filling will not be able to dislodge because it is larger at the bottom of the hole than it is at the top.  When placing a cast restoration such as a crown or an inlay, there can be no undercuts.  Otherwise, the casting would not be able to seat.  The vertical walls of the preparation are made nearly parallel, usually slightly tapered.  The space between the restoration and the tooth is filled with a waterproof cement such as zinc phosphate which hardens and "locks" the restoration onto or into the tooth.   The cement flows into the tiny imperfections in the sides of both the preparation and the restoration and acts as a "lock and key" to keep the restoration from sliding out or off the prepared tooth.  

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