Assembling the Proper Armamentarium
Composite resin restorations are performed daily in virtually every dental office in America. Between composite materials, matrix systems, and curing lights, creating a posterior contact on a Class II restoration can be a challenging situation unless the right armamentarium is used.
With the variety of composite resins, matrix systems, and curing lights available to clinicians today, selecting the right products for a given restoration can be a challenging task. Creating a posterior contact on a Class II restoration, for example, requires the proper armamentarium, and the success or failure of such a restoration depends on several factors.
After applying and light-curing adhesive, composite resin is placed in the preparation. Upon polymerization of the composite, stresses may ensue due to contraction of the material. Shrinkage stress occurs when the contraction is obstructed and the material is rigid enough to resist sufficient plastic flow to compensate for the original volume.1 The magnitude of contraction stresses is highly dependent on the visco-elastic properties of the material.1 Clinically, these stresses may be transferred to the margins of the restoration, possibly affecting marginal quality.2 When marginal quality is not adequate, problems like leakage, recurrent caries, and pulpal irritation may occur.3,4 While an absolutely perfect marginal seal may not be achievable clinically, a good marginal quality should be the main objective for clinicians.3 Enamel margins always provide a better seal, but this is not always possible.
Restoration placement techniques are widely recognized as a major factor in the modification of shrinkage stresses.5 To avoid the clinical consequences of polymerization shrinkage, incremental filling techniques are usually preferred over the bulk-filling method to obtain effective marginal seal.6,7 Incremental techniques have been suggested to compensate the polymerization shrinkage of composites8,9 by reducing the stresses developed within the tooth restoration system.10 In some studies, better marginal quality with incremental methods of composite placement was observed.11,12 However, other studies found no significant differences in marginal quality using different stratification methods.13-16 Although incremental technique may be important for adequate light penetration, its disadvantages are the possibility of trapping voids between layers and the time required to place the restoration. Bulk application technique is simpler and faster, with a reduced number of clinical steps.13 As one can see from the literature, different studies provide different outcomes.
In clinical practice, the author still builds posterior restorations incrementally, limiting the maximum thickness per increment to 2 mm and preferring to use a conventional nanofilled composite that can be highly polished and has great handling properties; each increment is light-cured for 20 seconds. In the past few years manufacturers have developed materials that can be bulk-filled and cured to a depth of 5 mm or more. These materials need to be somewhat translucent for the light to penetrate that deep. Some bulk-fill materials are flowable and need to be covered with a material that is more highly filled; others are more filled and can be placed like a conventional composite. Sonic-activated, bulk-fill composite, such as SonicFill™ (Kerr Corporation, www.kerrdental.com), is a hot topic. Many of the author’s colleagues have been using this system for a couple of years with excellent results. They like the ease of use and the ability to be efficient.
This leads to another clinical problem that can frustrate dentists: the difficulty in achieving an interproximal contact when performing a posterior Class II composite restoration. The sectional matrix has existed since the early 1970s but was originally developed for amalgam restorations, as composite was not yet used for posterior restorations. A problem, however, was when the amalgam condensed and was allowed to set, upon removal of the matrix band the amalgam would fracture due to the band’s contour and the ideal size contact point. This product was brought back to market in the early 1980s when composite resin started being used frequently for posterior restorations. Composite could be cured and the band removed without fracturing the restoration.
Over the past several years, sectional matrix systems have been perfected and bitine ring systems have been dramatically improved, making it easier to achieve a tight and ideal interproximal contact. Separation of the teeth is easily achieved with the newer designs, and the products last longer after autoclaving, too.
Newer light-emitting diode (LED) type lights offer high output with a longer life compared to older halogen versions. Curing composite restorations can seem like a simple procedure; however, dentists can easily undercure their composites without even realizing it. When the light is placed on the restoration it is difficult to hold the light tip flat on the tooth. This is due in part to anatomical concerns, the patient’s ability to open wide enough, and the design of the light tip. This leads to the light source being held off the tooth a few millimeters, and what most dentists don’t realize is the distance furthest from the light is not getting enough curing. In turn, this can lead to undercuring of interproximal areas, especially on the gingival seat. Undercured composite can lead to decay, sensitivity, and a failed restoration. While some materials are said to be able to be light-cured in 5-mm or more increments with minimal exposure time, most curing lights have an output that is not uniform throughout the whole surface of the tip. If the clinician holds the light in one area, and another is not exposed properly, the restoration will be undercured. Also, with high-powered lights, clinicians may believe that because the output is greater, the curing time can be shorter, which may not be the case.
Regarding composite color, the darker the color and more opacious the material, the longer the curing must be. The reason bulk-fill materials are much more translucent is so the light source can get to the bottom of the restoration and fully cure it. In order for a composite material to polymerize there must be photoinitiators in the material that will start free radicals to initiate curing once a light source is used. The problem with some of the newer composite systems is the incorporation of different photoinitiators. Camphorquinone (CQ) has been the most popular photoinitiator in composite resins for decades. However, CQ is yellow and photobleaches only slightly upon exposure using a curing light for clinically relevant times.17 This can create color changes when CQ is used in translucent or very light shades. To overcome this problem, some resin manufacturers use alternative photoinitiators that are not as chromogenic as CQ.18 These photoinitiators have peak absorbance values below 420 nm. Thus, these initiators are not efficiently activated by single-peak LED units that deliver blue light mostly in the 440-nm to 470-nm range. Unfortunately, manufacturers do not commonly list all of the photoinitiators used in their products.19 This lack of information can make it difficult for clinicians to predict the performance of a narrowband, single-peak LED unit on a specific brand of composite. To compensate for this limitation, third-generation polywave LED curing lights have been introduced and are said to cure all resin-based restorations.20 These lights use a combination of different wavelength LED chips to produce a broader spectral output that covers both the wavelengths needed by CQ and the shorter wavelengths below 420 nm required by the alternative initiators.21 The output is different based on where the LED is placed in the tip, thus creating a problem in curing the whole restoration if the tip is not moved over the composite. Unfortunately, most dentists do not pay attention to this detail. This can lead to premature failure of the restoration without the dentist knowing why. Manufacturers need to be more prudent in stating the actual output of the lights and which products the light may work best with.
Restorative success—or failure—hinges on using the proper composite resin, matrix system, and curing light, and dedicating meticulous attention to each aspect of technique.
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ABOUT THE AUTHOR
Robert Margeas, DDS
Adjunct Professor, Department of Operative Dentistry, University of Iowa College of Dentistry, Iowa City, Iowa; Private Practice, Des Moines, Iowa