Modern Cavity Preparations

Engineering principles for composite restorations built to last.

Despite composite resin being in service for over 60 years, there is still much confusion regarding ideal cavity preparations for the material. This is due in part to dental schools throughout the United States teaching standardized, retentive cavity preparations ala G.V. Black which are convenient to teach and grade due to their prescriptive shapes and dimensions. While these cavity preparations are often used interchangeably for both amalgam and composite (particularly in the early pre-clinical years of dental school), they are mostly ideal for the mechanical retention of amalgam and inappropriate for use with bonded composite resin restorative materials.

Cavity preparation design remains both a subjective and sometimes controversial topic today, but it shouldn’t be. In fact, researchers like Alton M. Lacy and Hakon Nordbo had been looking into more compatible cavity preparations for composite resin restorative materials since as early as 1987. Their curiosity was sparked by observations of “a high rate of occlusal wear, a high incidence of microleakage and recur­rent caries, difficulties in obtaining suit­able proximal contacts with adjacent teeth, isthmus fracture in two- and three-surface restorations, post-operative sensitivity, and lack of radiopacity” with posterior composite restorations (Lacy, 1987).

Comparison of two Class II cavity preparations. Retentive amalgam preparation with sharp cavosurface and internal line angles prepared with a #56 carbide bur (left) and non-retentive composite resin preparation with beveled cavosurface and rounded internal line angles prepared with a super coarse chamfer diamond bur (right). Note the dentin crack in the amalgam preparation (left) resulting from the use of an aggressive carbide bur.

Lacy and Nordbo highlighted in detail the most common issues with early composite resin restorations, and thanks to their efforts along with findings from researchers like Urabe, Unterbrink, Davidson, Fusayama, Tagami, Van Meerbeek, Alleman, Belli, Sadr, Magne, and more, we are now enlightened. Adhesive dentistry has come a long way since the 1980s, yet many general dentists express an unwarranted lack of trust in dental adhesion and still resort to resistance and retention engineering principles when using composite.

Today, tooth substrates are far more understood than ever before through advanced imaging, structural testing, and computer modeling. Modern carbide and diamond rotary instruments along with air-particle abrasion systems are available, caries detecting dye use is more widespread, adhesive systems are far more advanced, flowable and paste composite resins are optimized for better flow and reduced shrinkage, and polyethylene fiber is now available to minimize polymerization shrinkage stresses and facilitate a secure bond at the tooth-restoration interface (Sadr et al, 2019).

Use of these modern advancements or technologies, however, proves futile without first considering a well-designed and engineered cavity preparation. In this article, we’ll discuss enamel and dentin as highly bondable tooth substrates, highlight the concept of hierarchy of bondability, and explore modern cavity preparation considerations to facilitate direct composite tooth restorations built to last.