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Until recent times the recommended cavity preparations for the restoration of posterior teeth has remained unchallenged. As long as amalgam was “king”, there really was no need to change the “status quo”. It is interesting to note that the standard amalgam preparation basically was designed by one individual; Dr. GV Black. While it is true that the addition of retentive grooves and the convergence of the preparation towards the occlusal margins were contribution of others, the basic preparation design was authored by one person.

Black’s configuration was based upon several principles. Perhaps the most interesting related to his understanding of the plaque distribution and zone of greatest concentration of microbial agents.. Acknowledging that the distribution of microbes and plaque resembled a normal distribution curve in the proximal aspects of posterior teeth he designed the proximal aspect of the preparation accordingly. In essence the bucal-lingual dimension of his preparation was extended in such a way that in the general patient, the margins would lie outside of the normal distribution curve (bell-shaped). His reasoning was further enhanced by his apparent knowledge of the relationship between the wall of the preparation and the amalgam restoration. Regardless of the circumstance, an interfacial gap between the prepared wall and the amalgam averages 10 to 12 microns. Under such a condition, there is ample potential for caries producing microbes to enter into this interface. Reducing the distance between the extended walls of the proximal portion of the cavity preparation would automatically increase the potential for positioning these margins in an elevated plaque zone.

In the case of composite resin, there is no incidence of a marginal gap; at lest if the bonding procedure were carried out appropriately. Through acid etching and hybridizing of the dentin, the clinician eliminates the potential for microbial invasion. The location of these proximal margins then is far less important than it is with the corresponding amalgam restoration. For this reason the proximal walls of the preparation are not necessarily extended into a so called “self-cleansing” zone. It is then possible to minimize the dimension of the preparation which in turn increases the potential of an increased longevity.

Another consideration related to the composite resin preparation is the restoration of the central fissure and associated pits. Under normal circumstances, a mesial and/or a distal pit on a premolar are treated individually. In the days of Dr. Black, restoration of one or more pits with an amalgam would also mandate the restoration of the entire central fissure. The differences in procedure between yesteryear and today relate to the incidence of caries. One hundred years ago, the restoration of only the central pits would be followed up by secondary caries in the central fissure in a relatively short period of time. Identified as “extension for prevention” this form of treatment was certainly justifiable. Currently, with the far lower incidence of primary and secondary caries, the clinician can afford to be appreciably more conservative in cavity design. And again; the more conservative the preparation, the greater the longevity. 

As a general rule, the preparation for posterior composite resins is smaller in all three dimensions as compared to that for amalgams. Assume for the moment that a small lesion has occurred on the mesial aspect of the lower right first molar. The extension of the proximal aspect of the preparation should be no more than three millimeters in the bucal-lingual direction. No attempt necessarily needs to be made in breaking contact with the adjacent proximal surface. Next, the proximal portion of the cavity preparation should be extended into the occlusal surface by no more than two millimeters. If caries is present on both proximal surfaces, the clinician should not attempt to include the central fissure unless there is caries in the central fissure. A small pit lesion if present should be treated as a separate entity using a conservative bur such as a No. 243.  Finally, if conditions allow, the distance between the gingival margin and the cervical line should be at least two millimeters.

There is still one more interesting difference between preparations generated for composites as compared to amalgam. If, for instance, the clinician identifies a small pit lesion in the central fissure of a maxillary premolar, how deep does the preparation need to be extended if the defect falls short of the dentino-enamel junction?  If the restoration is to be a composite resin, the extension of the defect need not be extended any further. If on the other hand, the clinician is to place an amalgam the preparation needs to be extended beyond the dentin-enamel junction and into the dentin. In the case of amalgam, the masticatory energy is transmitted through the restoration and into the underlying surface. If the bottom of the preparation consists of enamel, the energy vector is reversed in the opposite direction which then results in an absorption of energy by the amalgam. After a sufficient number of cycles the amalgam undergoes microcracking and eventual failure. If the base of the preparation consists of dentin, the masticatory energy is partially absorbed by the dentin, thereby minimizing the trauma to the restoration thereby extending the longevity of the amalgam. Since the amalgam is bonded to the tooth, the tooth as well as the composite resin absorbs the energy. This principle is far different in the case of the amalgam.

One final point should be made. If an isthmus is to be established, it is important to minimize its width as much as possible. The bucal-lingual dimension of the isthmus determines the rate of wear of the composite resin on the occlusal surface. In the presence of a bolus of food and the energy of mastication, the composite gradually undergoes attrition. Minimizing the width of the preparation reduces the potential for contact by the food bolus. It also reduces the potential for contact by the antagonist cusp. Again, the smaller the dimension of the preparation, the greater the potential for longevity.