Composite resins have been a significant aesthetic alternative to amalgam restorations for more than sixty years.1 A hindrance to this method of restoration is the phenomenon of microleakage. Microleakage results from an imperfect bond, leaving a microscopic gap, allowing molecules and ions to flow between the tooth and restoration interface.2 Lack of margin integrity has been implicated in patient sensitivity complaints, secondary caries formation, corrosion of dental materials, and aesthetically displeasing staining at the margin.2 This paper will discuss the basis and discovery of microleakage, causes and effects of the phenomenon including polymerization shrinkage, teeth whitening agents and certain etching techniques, as well as prevention methods the operator can employ to minimize microleakage in posterior composite restorations.
The study of microleakage dates back to 1912 when Dr. WE Harper first researched the phenomenon.3 Natural teeth were simulated in the study by using steel dies with class II preparations built into them. A tube was then attached through the pulpal floor of the dies and air pressure ranging from 3 to 30 psi was applied. The emergence of air bubbles from the margins indicated microleakage had taken place.
MATERIALS AND METHODS
A comparison of various composite resins
A research study comparing 17 different flowable and non-flowable composite resins was performed.4 The measured parameters include: shrinkage (vol%), contraction stress (Mpa), and tensile modulus (GPa). Figure 1 describes the materials tested. Shrinkage was measured by mercury dilatometry. Contraction stress measurements were obtained using a tensilometer apparatus as shown in Figure 2. Each composite resin was placed between the glass plate and the flat bolt head and pressure was applied. The composites were light-cured through the glass plate and the tensile modules was calculated 30 minutes after the curing process using the formula E = σ/ε, where σ is the stress (F/A) and ε, the strain (Δl/lο).
Two composite resins, Z100 and Tetric Ceram were used to study how different light curing intensities affect margin adaptation.5 A small amount of each composite material was placed between two translucent strips and pressure was applied to give a thin strip of the composite materials. Four different methods of light curing were then applied to the composite resins: I. Standard mode, full intensity 750 mW/cm2 for 40 s; II. Two-step mode, 10 s at low intensity 200 mW/cm2 plus 30 s at full intensity 750 mW/cm2; III. Low intensity 200 mW/cm2 for 40 s; IV. Low intensity 200 mW/cm2 for 10 s. To measure polymerization shrinkage, the bonded-disk method developed by Watts and Cash was used.
Carbamide peroxide and resin coatings
To study the effectiveness of resin coatings as preventive treatment against microleakage caused by teeth whitening agents, forty caries-free extracted molars were used for...