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Introduction

Exposure of restorations in extracted teeth to cyclic thermal fluctuations to simulate one of the many factors in the oral environment has been common in many tracer penetration, marginal gap and bond strength laboratory investigations.

Dental cements are used for the anchoring of indirectly prepared cast restorations and are thought to fill and lute the marginal gaps between teeth and machined restorations. The minimum width of the marginal gaps ideally corresponds to the minimum cement layer thickness. Marginal adaption is one of the important criteria used in the clinical evaluation of fixed restorations. A well-fitting restoration reduces the chance for recurrent caries and periodontal disease, whereas the space between a poorly fitting restoration and tooth preparation enables accumulation of bacterial plaque. The presence of marginal discrepancies in the cemented restoration exposes the luting agent to the oral environment. Larger marginal discrepancy and subsequent exposure of the dental luting agent to oral fluids enhance the risk of cement dissolution. Such dissolution in turn can promote shrinkage and microleakage, caused by a lack of adhesion between luting agent and tooth structure on one side and luting agent and restoration on the other side. Mechanical failure of the luting agent can be the consequence. Accordingly, small marginal gaps will minimize plaque accumulation and subsequent disease and will thus protect the luting agents from the oral environment and mechanical forces. Caries as well as crown dislodgment are the most common reasons for failure of indirect restorations: several laboratory fatigue studies have shown that cement microfractures are typically the initial mode of failure that is followed either by dislodgment of the restoration or even by tooth fracture. Clinical studies also have shown that poor marginal adaption of a restoration correlates with increased plaque retention and reduced gingival health , as indicated by a higher Plaque Index (PI), an elevated Gingival Index (GI), and increased pocket depths (PD). Also changes in the subgingival microflora can be attributed to inadequate marginal fitting.

Microleakage is defined as the movement of fluids carrying bacteria and other molecules and ions at the boundary between a restoration and a tooth . Although it is difficult to detect clinically, microleakage is considered to be a major factor influencing the longevity of dental restorations since it may, for instance, lead to staining at the margins of the fillings, hastening damage of the restorations at the marginal areas, recurrent caries at the tooth/restoration interface, hypersensitivity of restored teeth, or development of pulpal pathology. Factors involved in the formation of marginal gaps and subsequent leakage between the cavity wall and restoration include besides operator errors also temperature variations, inadequate moisture control leading to moisture absorption and polymerization shrinkage, as well as masticatory forces. Hygroscopic absorption, i.e. water uptake by the restorative material and incremental insertion of restorative materials can partially compensate for these inadequacies . However, microgaps formed at the margins facilitate bacterial ingress precipitating staining and hence discoloration, defective restorations, postoperative sensitivity, chronic hypersensitivity, secondary caries, and possible pulpal pathosis.

Different methods have been employed to evaluate microleakage around restorations in vivo and in vitro. The latter, i.e. in vitro microleakage testing of dental materials, is a commonly accepted evaluation technique of margin integrity. Although clinical relevance of in vitro leakage tests does not always correlate with the current clinical situation these tests are the most frequently used laboratory examinations to study mechanisms of fluid leakage and hence derive possible measures to minimize or even eliminate microleakage around dental restorations. Such in vitro studies include the use of tracers such as dyes, radioactive isotopes, chemical tracers, and bacteria, as well as marginal percolation of water and subjection to air pressure, neutron activation analysis (NAA), scanning electron microscopy (SEM), and electrical conductivity.

Thermocycling is defined as the in vitro process of subjecting an extracted tooth carrying a restoration to temperature extremes that comply with those found in the oral cavity. In general, thermal stress can be pathogenic in two ways: firstly, mechanical stresses induced by temperature chances can directly induce crack propagation through bonded interfaces. Secondly, the changing gap dimensions are associated with flow of pathogenic oral fluids into and out of the gaps. The linear coefficient of thermal expansion of a material has been noted as an important factor contributing to microleakage. It is defined as the change in length per unit length of a material caused by a variation in temperature by one degree. As already mentioned above, the method of “temperature cycling” has been utilized by many researchers to study occurrence of marginal percolation in restorations in vitro . The technique has been combined with the use of dyes, isotopes, air pressure or bacteria that were employed for the detection of microleakage.

The aim of this investigation was to analyze microleakage data on indirect bonded crowns published in the dental literature until 12/2009, to identify methodological factors that might potentially affect the results of in vitro microleakage tests and to compare the results.