The New Liner Lineup
Understanding the important role liners play as an interface between the tooth and the restoration can help to improve clinical outcomes.
Liners have been placed under restorations for years to protect or isolate the pulp from the transmission of noxious stimuli created by the placement of a restoration.1 With the advent of restorative materials that bond to tooth structure, liners also serve the function of coupling the restorative material to the underlying tooth structure. Liners generally flow easily and adapt well to the pulpal floor, giving a better seal.2 Flowable composite resin liners are less heavily filled than traditional composite resins. They exhibit a low modulus of elasticity that allow the material to absorb polymerization shrinkage stress as well as occlusal stress.3-4 Liners reduce the risk of gap formation, microleakage, and bacterial invasion.5 An understanding of the role and mechanisms of cavity liners allows clinicians to improve their overall results.6 New materials have recently been added that expand, improve, and simplify the placement of liners (Figure 1).
Calcium Hydroxide Liners
Studies have shown that the placement of calcium hydroxide over an exposed pulp reduces the necessity of endodontic root canal therapy.7 Calcium hydroxide has been demonstrated to be useful for direct pulp capping due to its stimulation of reparative dentin formation. Mineral trioxide aggregate (MTA) has also shown similar ability to stimulate dentin bridge formation and retain pulp vitality.8 Newer-generation calcium hydroxide resin liners adhere better to the surrounding dentin and are light-cured for ease of use. Disinfection prior to placement improves results.9 In documenting more than 100 direct pulpal exposures in the last 18 years, it has been the author's experience that several factors help to predict the success of placing a direct pulp cap over an exposed pulp. The first determining factor has been whether the tooth was sensitive prior to treatment. Symptomatic teeth invariably result in irreversible pulpitis. The second factor is the ability to establish hemostasis. Inability to control bleeding results in poor success. The third factor is the number and size of the pulpal exposure(s). More than one exposure or an exposure larger than a pinhead usually results in failure.
Because exposures can occur at any time and are usually unplanned, the dentist must be prepared. It is recommended that each operatory contain a plastic cup pre-loaded with chlorhexidine and calcium hydroxide applicators (Figure 2). Upon exposure, preparation is immediately stopped (Figure 3). If no rubber dam had been previously placed, the area must be isolated to prevent contamination of the exposure site by oral fluids. The exposure site is flooded with chlorhexidine (eg, Consepsis®, Ultradent, http://www.ultradent.com) and the excess blown away. This process is repeated until bleeding stops. Light-cured calcium hydroxide (eg, Ultrablend™ XT, Ultradent) is placed over the exposure and light-cured before the remaining decay is removed10 (Figure 4). The preparation is completed and a resin-modified glass-ionomer liner/base is placed over all exposed dentin and over the calcium hydroxide (Figure 5). It is not unusual for the calcium hydroxide to become dislodged, making it necessary to reapply the liner. The resin-modified glass ionomer helps to cover and protect the calcium hydroxide during subsequent bonding procedures. The final restoration can then be placed (Figure 6).
Resin-Modified Glass-Ionomer Liners
Resin-modified glass-ionomer liners are being rediscovered by many dentists and, as a result, their use has dramatically increased. Phosphoric-acid conditioner used with the total-etch bonding technique exposes the enamel rods and opens up the dentinal tubules. If the bonding agent is not properly applied, the dentinal tubules may not be adequately sealed, resulting in sensitivity. Resin-modified glass ionomers do not open up the dentinal tubules and have the added benefit of releasing fluoride. The result is a liner that creates zones of inhibition which reduce the amount of mineral loss when subjected to acid attack, a significant benefit under composite restorations that may eventually leak.11 For the dentist who prefers the total-etch bonding technique, resin-modified glass-ionomer liners allow an uncompromised bond to enamel, reduction of postoperative sensitivity, and the benefit of fluoride release.12
Resin-modified glass ionomers are available in several forms. Vitrebond™ (3M ESPE, http://www.3mespe.com) is packaged in a powder/liquid form. The viscosity allows the material to adapt readily to the floor of the preparation, and no primer is necessary prior to placement. GC Fuji™ LINING LC (GC America, http://www.gcamerica.com) is an easy-to-use paste/paste resin-modified glass-ionomer liner. The dentin should be conditioned for 10 seconds and then thoroughly washed, with excess moisture removed before application.
Flowable Resin Liners
The trend in composite materials has been the increased use of nanofill particles and the incorporation of proprietary resins. Nanofill particles improve polishability, wear resistance, and strength.13 Manufacturers are beginning to discontinue the use of bis-GMA resin due to the public's concern about bis Phenol A (known as BPA). Newly patented resins with higher molecular weight and unique rheology that help to reduce polymerization shrinkage and the accompanying stress are now available.14 Venus® Diamond Flow (Heraeus, http://www.heraeus.com) incorporates nanofill filler particles in a proprietary high-molecular-weight resin. The result is a translucent and strong material with reduced shrinkage.
Multiple-Viscosity Flowable Resin Liners
Until recently, most companies offered only one viscosity of flowable composite resin in their product line. Dentists had to experiment with different brands to find the material with the handling characteristics they preferred. Now several companies have introduced multiple viscosities of flowable composites.
Wave, Wave MV, and Wave HV (Figure 7) are three viscosities of flowable resins from the same manufacturer (SDI, http://www.sdi.com.au). The Wave HV (high viscosity) does not appreciably move once placed, while the Wave (low viscosity) flows readily. This allows the dentist to pre-determine the desired optimal handling characteristics for the intended use of the material.
Fluoride-Releasing Flowable Resin Liners
While the addition of fluoride to flowable resins results in significantly less fluoride release than glass ionomers, some feel that it may be of benefit. Flowable composite liners are often placed along the cervical floor of Class II restorations where fluoride release could be beneficial (Figure 8).
Beautifil® Flow (Shofu, http://www.shofu.com) is a glass ionomer containing flowable composite resin. Pre-reacted glass-ionomer cement is ground into very small filler particles and incorporated into the resin. It is available in two viscosities. The Beautifil Flow F02 is designed for cervical Class V restorations, and the F10 is a high flow for use as a liner. Fluoride release is enhanced as a result of the glass-ionomer filler particles, and the material can be recharged with fluoride ions when exposed to the oral environment.
Low-Shrinkage Flowable Resin Liners
A new flowable resin composite, SureFil® SDR® (DENTSPLY Caulk, http://www.caulk.com), has been developed that uses a unique resin with reduced polymerization shrinkage and, more importantly, less polymerization shrinkage stress. Often the most innocuous-looking Class I restoration can be the most problematic due to a high "C factor," the ratio of bonded to unbonded surfaces. Lowered polymerization shrinkage stress should result in less potential for postoperative sensitivity and reduced potential for debonding and/or fractured enamel.
Another benefit of this flowable composite is that it light-cures more efficiently than most composites. Liners are often located several millimeters below the occlusal surface, making it difficult to place the light in close proximity to the resin. Because flowable composites contain more resin and require more light, it may be difficult to adequately cure the resin; therefore, exposure times should be increased (Figure 9). Uncured resin in deep restorations can result in postoperative sensitivity. This material can be placed in bulk up to 4 mm thick, reducing the time required for layered placement (Figure 10). The chief benefits are a resin that is more readily polymerized, exhibits lower shrinkage stress, and reduces the potential for postoperative sensitivity.
Self-Etching Primer/Flowable Resin Liners
Flowable resin liners have required the use of a bonding agent prior to placement. When using the total-etch technique, the dentinal tubules are opened during the etching process, increasing the likelihood of postoperative sensitivity especially in deep restorations. Self-etching primers are viewed by many to reduce postoperative sensitivity but may not bond as well to unprepared enamel.
Recently, a self-etching primer/flowable resin-composite material (Vertise™ Flow, Kerr Corporation, http://www.kerr.com) has been introduced. The material can be used with either the total-etch or the self-etch technique. Bonding first to the dentin with an all-inclusive self-etch primer/flowable resin and then etching the remaining superficial dentin and enamel is a simplified and effective way of optimally bonding to both types of tooth structure. Technique sensitivity is reduced regarding the proper treatment and moisture of the dentin. For those who do not wish to place glass-ionomer liners but prefer the total-etch technique, this may be a viable alternative.
Liners play an important role as an interface between the tooth and the restoration. Their role as a pulp protector is critical. The newly formulated liners simplify and broaden their usage. The ability of some liners to significantly reduce postoperative sensitivity and to reduce recurrent decay under the restoration makes them a useful tool for the restorative dentist.
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About the Author
Daniel H. Ward, DDS