Subir Banerji and Shamir B. Mehta
For an adhesively retained restoration, cavity preparation is usually limited to access to the defect, removal of diseased/compromised tissue (and/or failed restorative material) and development of a convenience form to facilitate matrix placement and application of the restorative material. All peripheral stain should also be removed to avoid poor aesthetics and to optimise adhesion.
There are a variety of burs available for the preparation and placement of plastic restorations. For a Class 3 restoration, access to the carious lesion is perhaps best approached from a lingual direction (where possible), using a small diamond bur to gain initial access in a high-speed handpiece (such as a No. 2 round bur). Cavity preparation for a posterior carious lesion may be commenced using a flat-fissure bur. The use of round, water-cooled burs of progressively increasing sizes in a contra-angled handpiece can then be used to remove carious dentine.
It is imperative to preserve as much enamel at the cavity margins as possible, particularly at the cervical floors of the boxes, as bonding to enamel is considerably more effective and significantly more predictable in the longer term than that to dentine or cementum. Cavity walls and floors should be finished with rounded internal line angles. For posterior resin restorations, interproximal boxes should be finished just beneath the contact point to permit caries removal and matrix application. It has also been shown that the preparation of a conservative concave cavity with a 4° taper not only lends itself to a minimally invasive design, but significantly improves the strength of the resin composite restoration, regardless of the type of material used.1
The bevelling of enamel margins (by 0.5–1.0 mm) has been advocated for proximal surfaces, as it exposes enamel rods transversely, thereby presenting a greater surface area for etching and bonding; it is also thought to provide a more effective etching pattern. Bevelling is best achieved using a flame-shaped finishing bur or a fine diamond bur.
In the case of a posterior cavity, the gingival cavosurface margin should only be bevelled if it is well above the cemento-enamel junction and if there remains a thin band of enamel after cavity preparation. However, bevelling is not generally recommended for use on occlusal cavosurface margins, because:
- It increases the loss of sound tooth tissue.
- It increases the surface area of the final restorations, increasing predisposition to wear.
- It can result in the formation of a thin area of composite resin that may be vulnerable to fracture.
- A less well-defined peripheral outline is developed, which may not be conducive to attaining a precise finish.2
Thus, a butt-joint margin is preferred occlusally for posterior cavities.
For anterior teeth, there appears to be a variation in opinion as to the size and form of the bevel, with some operators advocating the use of a two-part bevel: an initial, smaller bevel (perhaps less than 1.0 mm) at 45° followed by a wider, diffused scalloped bevel that is less steep but slightly longer in width to further enhance the transition between the resin composite material and the tooth. On the palatal surface, as aesthetics are less critical, the use of a longer bevel has little merit. However, should the placement of a bevel result in the loss of enamel tissue at the cervical margin, then bevel placement in this area should be avoided so as not to compromise the marginal seal (which will be inferior in the absence of enamel tissue). Figures 5.3.1, 5.3.2 and 5.3.3 show the restoration of two central incisors with direct composite restorations using a bevel cavity preparation to achieve an acceptable transition in colour at the margins of the cavity.