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Dentin and Enamel--Chemistry and Histology

 

The recent advances in adhesion technology have substantially reduced the need for extensive tooth preparations.1 The acid-etch technique has changed in predictability from its beginning.2 The ability of clinicians to bond resin materials to enamel and dentin has changed the concepts of cavity preparation, orthodontic treatment, caries prevention, and cementation of fixed prostheses, including tooth-colored carbon-fiber posts. Procedures used for aesthetic enhancement, such as porcelain laminates, (Figures 1 and 2) rely on new adhesive materials and techniques. Improved marginal sealing around bonded restorations has reduced the frequency of unfavorable postoperative responses. Improvements in adhesive materials have expanded the indications for tooth-colored restorations from anterior to posterior teeth (Figures 3 and 4).

The word “adhesion” is derived from the latin term adhaerere (to stick to). Adhaerere itself is composed of ad (to) and haerere (to stick).3 Adhesion is defined by specification D907 of the American Society for Testing and Materials as “the state in which two surfaces are held together by interfacial forces, which may consist of valence forces or interlocking forces or both.”3 

The adhesive, frequently a fluid, joins two substrates and solidifies. The adherend is the material or initial substrate to which the adhesive is applied. Adhesion or bonding is the process of forming an adhesive joint,3 consisting of two substrates joined together. Most adhesive joints involve only two interfaces; a bonded composite restoration is an example of a more complex adhesive joint

Contemporary adhesive techniques allow dentists to restrict operative procedures to removal of diseased tooth tissue, thereby preserving sound tissue. The short lifetime of restorations, frequently assessed by methods not based on clinical evidence,4,5 requires clinicians to replace restorations recurrently.6-8 Each time the restoration is replaced, part of the remaining sound tooth structure is unavoidably removed, and a more complex restoration is required.9 Increasing the lifetime of a restoration is one of the primary goals of current research.

 

DENTIN AND ENAMEL--CHEMISTRY AND HISTOLOGY 

Human enamel is composed by weight of 96% hydroxyapatite crystals (Figures 5-6-7-8), being the remainder of the tissue formed by an organic matrix consisting of proteins (ie, amelogenins and enamelins) and trace amounts of water.10 The uniqueness of dentin as a histologic entity makes this hard tissue a challenge when trying to adhere restorative materials. Dentin is formed by the interaction between the ectodermal and ectomesenchymal components that induce odontoblast differentiation and dentinogenesis.11 As odontoblasts fabricate dentin, they leave a track throughout the dentin forming the dentinal tubules (Figures 9 and 10).11 The dentin tubules have different density and orientation in distinct locations of the tooth.12 

The tubular structure of dentin is responsible for its intrinsic hydration, due to the communication with the pulp tissue, which is under vascular pressure.13,14 The intertubular dentin is made of a complex lattice of collagen fibers with hydroxyapatite crystals filling up the spaces between fibers and surrounding the tubular or peritubular dentin (Figures 11 and 12).14 

The scaffold of all the tubules and intertubular dentin is represented by the collagen fibrils produced by the odontoblasts, while the hydroxyapatite precipitates in-between the fibers and inside the nanospaces of each fiber during dentinogenesis. Mineral, in the form of carbonate-rich apatite, constitutes approximately 50% of the dentin volume.13 The precipitation of mineral substance on the collagen fibrils during dentinogenesis results in the final mineralized structure.

Bonding to dentin has been one of the most challenging issues since the introduction of the acid-etch technique nearly 50 years ago.2 Acid etching transforms the smooth enamel into an irregular surface (Figures 10-11-12-13A-13B). A mixture of resin monomers or adhesives is applied on the enamel surface (Figure 14). The resin is drawn into the microporosities of the enamel surface by capillary action. The monomers in the fluid resin polymerize and become mechanically interlocked with the enamel structure. The formation of resin microtags (Figure 14) within the enamel structure has been considered the essential mechanism of adhesion of resin to enamel.15,16 

When tooth structure is prepared with a bur or other instrument, residual organic and inorganic components form a “smear layer” of debris on the surface.17 The smear layer obstructs the entrance of dentin tubules (Figure 15), decreasing dentin permeability by up to 86%.18 Submicron porosity of the smear layer still allows flow of dentinal fluid.19 Although the smear layer acts as a physical obstacle that decreases the permeability of dentin, it can also be considered a barrier that must be removed or made permeable, so that monomers can contact and interact with the dentin surface. Manufacturers of dentin/enamel adhesives use one of two major strategies for overcoming the barrier (Figure 16):

  • Self-etching: Adhesives that treat the dentin and enamel surface with a non-rinsing solution of acidic monomers in water. These bonding systems do not remove the smear layer (Figures 17 and 18), but make it permeable to the monomers subsequently applied.
  • Total-etch: Adhesives that include an acid gel to treat dentin and enamel for 15 to 30 seconds. The acid dissolves the smear layer and the top 1 µm to 6 µm of hydroxyapatite (Figure 19). Total-etch has led to significant improvements in the in vitro bond strengths of resins to dental substrates.20,21 

It is clear that dentin and enamel etching is moving forward before our eyes. By studying the methods used in the past, dentists will be better suited to deal with the methods that will ultimately be used in the future. The ability of dentists to bond material to both enamel and dentin has reshaped the field of dental adhesion and the potential of restorative care.

References: 

  1. Swift EJ, Perdigão J, Heymann HO. Bonding to enamel and dentin: A brief history and state of the art, 1995. Quintessence Int 1995;26(2):95-110.
  2. Buonocore MG. A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces. J Dent Res 1955;34(6):849-853.
  3. Packham DE. Adhesion. In: Packham DE, ed. Handbook of Adhesion. Essex: John Wiley & Sons; 1992:18-20.
  4. Cardoso M, Baratieri LN, Ritter AV. The effect of finishing and polishing on the decision to replace existing amalgam restorations. Quintessence Int 1999;30(6):413-418.
  5. Pink FE, Minden NJ, Simmonds S. Decisions of practitioners regarding placement of amalgam and composite restorations in general practice settings. Oper Dent 1994;19(4):127-132.
  6. Burke FJ, Cheung SW, Mjör IA, Wilson NH. Restoration longevity and analysis of reasons for the placement and replacement of restorations provided by vocational dental practitioners and their trainers in the United Kingdom. Quintessence Int 1999;30(4):234-242.
  7. Forss H, Widstrom E. Factors influencing the selection of restorative materials in dental care in Finland. J Dent 1996;24(4):257-262.
  8. Mjör IA. The reasons for replacement and the age of failed restorations in general dental practice. Acta Odontol Scand 1997;55(1):58-63
  9. Gordan VV. In vitro evaluation of margins of replaced resin-based composite restorations. J Esthet Dent 2000;12(4):209-215.
  10. Termine JD, Torchia DA, Conn KM. Enamel matrix: Structural proteins. J Dent Res 1979;58(Spec Issue B):773-781.
  11. Ten Cate AR. An analysis of Tomes’ granular layer. Anat Rec 1972;172(2):137-147.
  12. Mjör IA, Nordahl I. The density and branching of dentinal tubules in human teeth. Arch Oral Biol 1996;41(5):401-412.
  13. Marshall GW, Marshall SJ, Kinney JH, Balooch M. The dentin substrate: Structure and properties related to bonding. J Dent 1997;25(6):441-458.
  14. Pashley DH. Dentin: A dynamic substrate---A review. Scan Microsc 1989;3(1):161-174.
  15. Buonocore MG, Matsui A, Gwinnett AJ. Penetration of resin into enamel surfaces with reference to bonding. Arch Oral Biol 1968;13(1):61-70.
  16. Gwinnett AJ, Matsui A. A study of enamel adhesives. The physical relationship between enamel and adhesive. Arch Oral Biol 1967;12(12):1615-1620.
  17. Bowen RL, Eick JD, Henderson DA, Anderson DW. Smear layer: Removal and bonding considerations. Oper Dent Suppl 1984;3:30-34.
  18. Pashley DH, Livingston MJ, Greenhill JD. Regional resistances to fluid flow in human dentin in vitro. Arch Oral Biol 1978;23(9):807-810.
  19. Pashley DH, Horner JA, Brewer PD. Interactions of conditioners on the dentin surface. Oper Dent 1992;Suppl 5:137-150.
  20. Kanca J. Resin bonding to wet substrate. 1. Bonding to dentin. Quintessence Int 1992;23(1):39-41.
  21. Triolo PT, Swift EJ. Shear bond strengths of ten dentin adhesive systems. Dent Mater 1992;8(6):370-374.
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