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Cavity Preparations for Posterior Composite Resins Karl F. Leinfelder DDS, MS

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The first attempt to substitute composite resins for amalgam restorations came nearly forty years ago. Two basic problems were identified. These included an unacceptable rate of wear and a higher incidence of caries as compared to amalgam. Furthermore the progression of caries under composites was appreciably greater than with amalgam. While the exact reason for the caries progression differential has not been identified it is interesting to note that the amalgam releases a number of ionic elements (silver, copper, tin, mercury) that might retard the process of caries progression. Composite resins on the other hand possess no such attribute.

Fortunately the problem of wear or loss of anatomic form has been essentially resolved. By reducing in part, the dimension of the filler particle and increasing the load rate, the wear of composite resins is essentially the same as amalgam which is approximately five microns per year. The problem of secondary caries still exists. The primary reason can be attributed to a less than ideal adaptation of the restoration to the margins of the preparation. The use of dentin bonding agents as well as flowable composites has contributed appreciably to a reduction in this clinical problem.

In addition to all of these changes, modification of the cavity preparation has been most contributive to a successful posterior composite resin restoration. When composites were first used as a posterior restorative material the preparation most commonly used was designed for amalgam. Developed by G.V. Black the preparation was engineered to automatically incorporate most of the proximal region of the tooth containing the highest plaque concentration and bacteria count. Buccal and lingual extension of the preparation was limited so as not to considerably reduce the strength of the tooth itself. Extension of the preparation bucally and lingually would ensure that the margins of the restoration be positioned into an area of relatively low bacterial count. This is an important consideration since the margin of the amalgam at the time of placement averages 10 to 12 microns. Such an interfacial gap is great enough to allow the invasion of caries-producing micro-organisms. In the case of composite resins there is no such interface. The use of dentin bonding agents causes the restoration to become an integral part of the cavity preparation along the entire length of the margin. As a result, a well-placed posterior composite resin technically contains no gap or interface for microbes to enter.

When caries was present in the mesial and distal pits of a maxillary premolar, Black recommended that the entire central fissure, as well as the two pits be included in the preparation. It was argued that since the caries rate was so high (100 years ago) restoration of the two pits only would be followed by caries in the central fissure. Today caries are less frequent than it was during the time of G.V. Black. Furthermore, oral hygiene (through education) is generally much improved over that of several decades ago.

Incidentally the more conservative preparation (treatment of mesial and distal pits) involves far less removal of tooth structure than the more aggressive approach. Calculation of the amount of tooth structure removed under both conditions shows that the more conservative technique results in about 400 percent less tooth structure.

In the case of the Class II cavity preparation it is also possible to save considerably more tooth structure by using a conservative approach. In general the proximal box of the preparation is narrower than that associated with conventional amalgam cavity preparations. Furthermore, the presence of caries does not dictate the proximal margins be extended into a contact-free zone of the adjacent tooth. Also the proximal box is not extended onto the occlusal surface by more than 2 to 2.5mm beyond the location of the proximal marginal ridge. Finally, the gingival margin should be at least 2mm from the cervical line. In the case of amalgam, the gingival margin is extended until the tine of an explorer passes through the space between the margin and the adjacent tooth. These reductions in dimensions of the cavity preparation amount approximately to a 200 percent savings of sound tooth structure. Reducing the size of the restoration is clinically important since there is a strong relationship between dimension and clinical longevity. The smaller the dimension of the restoration, the greater the potential for extended longevity.

The proximal box of the cavity preparation needs special attention to prevent the potential for secondary caries. It can be identified as the Achilles' Heel of the Class II preparation. Occlusal stresses on or near the marginal ridge during mastication tend to force the proximal aspect of the restoration into the interproximal space. Release of the occlusal stress results in a return to the original location. The amount of displacement depends upon the modulus of elasticity of the composite resin which is twice as great as it is for amalgam (and therefore twice the deformation of amalgam). Displacement is also dependent upon how well the restoration is bonded to the floor of the proximal box. The deeper the proximal box, the less the amount of enamel along the gingival margin. While the immediate bond strength of dentin bonding agents is similar for dentin and enamel, those for dentin tend to decrease over a period of time.

As the thickness of the enamel decreases along the gingival margin, a special technique has been suggested to resolve the problem. Based upon the recommendation of Professor Qvist from Norway, a glass ionomer liner about 2ml in thickness is placed over the gingival floor. Procedurally the entire preparation is bonded with a dentin bonding agent. At this point a glass ionomer such as Fuji II LC is placed over the gingival box. Fuji IX is also recommended but since it is self-curing it will take longer to set.

Glass ionomers are excellent auxiliary restorative materials for a number of reasons. First of all they release fluoride ions from their surfaces. The glass ionomer fluoride ions are not only absorbed into the adjacent tooth structure but they kill microbes in the immediate vicinity. Secondly the glass ionomers effectively resist microleakage. This interesting clinical property is the result of a matched coefficient of thermal expansion between the glass ionomer and surface to which it is bonded. When the glass ionomer is completely surrounded by tooth structure and restorative material it is identified as a "closed sandwich." When one of the surfaces is exposed to the oral cavity (such as a glass ionomer on the gingival box) it is classified as an "open sandwich."

One of the major differences between an amalgam and composite resin restoration is the location of the pulpal floor in the case of mesial and distal pit caries (i.e. maxillary premolar). In the case of amalgam, the floor of the preparation must consist of dentin. Retention is achieved by convergence of the preparation to the occlusal surface as well as micro-mechanical retention of the prepared tooth structure. Consider the amalgam as a free-floating restoration with well-defined (microscopic) spaces at the restoration/tooth interface. As the masticatory force is introduced to the surface of the restoration, the energy is transferred though the amalgam and onto the floor of the preparation. If the floor of the preparation consists of enamel the energy will be retransferred to the occlusal surface. Constant recycling of this energy could result in premature cracking and fracturing of the restoration.

In the case of composite resin restorations, the depth of the preparation can be stopped short of the dentinal-enamel junction if the caries process also stops before the dentin is reached. In such a case the dentinal surface (floor of the preparation) acts an absorber of the masticatory energy thereby causing it to dissipate. Furthermore since the restoration is bonded to the enamel walls of the preparation, the entire tooth will serve to absorb the energy as well.

An appreciable difference exists between the preparations for composite and amalgam restorations. Almost without exception those for composites are considerably more conservative than those recommended for amalgam. Based upon years of research and clinical use it can be stated that the greater the degree of conservatism associated with the composite, the greater the longevity. Interestingly some of the conservatism associated with the composite has been transferred to the amalgam preparation A comparison of illustrations depicted in the original text by Dr. Black with some of the more current publications on cavity designs make this finding quite apparent.
Author’s Bio
Dr. Karl F. Leinfelder earned both his Doctor of Dental Surgery and Master of Science (dental materials) degrees from Marquette University. In 1983, he joined the School of Dentistry at the University of Alabama and is the recipient of the Joseph Volker Chair. He also served as Chairman of the Department of Biomaterials until 1994. Presently he holds positions at both universities; adjunct professor at University of North Carolina and professor emeritus at the University of Alabama. Dr. Leinfelder has published more than 275 papers on restorative materials, authored more than 150 scientific presentations, two textbooks on restorative systems and has lectured nationally and internationally on clinical biomaterials.
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