Click here to enlarge imageThe first brand to appear is Filtek™ LS Low Shrink Posterior Restorative System from 3M ESPE (photo at right). This resin uses silorane chemistry instead of the well-known bis-GMA or urethane dimethacrylate chemistry that has been used in almost all resin-based composite dental restorative resins. The new resin restorative material has increased working time even when in the operating light. This characteristic will be an advantage in some clinical situations.
A special adhesive system is necessary for use with Filtek LS, the LS System Adhesive Self-Etch Primer, which is used before the LS Bond. The need for a specific bonding system may be a disadvantage for the product, since most clinicians use more than one brand of resin restorative material. The company has aimed this radiopaque product at typical posterior tooth restoration needs. There are four colors available in capsules or syringes.
The obvious question is what advantages will a typical dental clinician receive when using a low shrinkage resin? Many scientific studies have been aimed at the real and proposed problems with resin shrinkage during polymerization. In my opinion, most of them appear to have minimal clinical significance.
The two most potentially problematic situations relative to excessive polymerization shrinkage are: stress in the tooth preparation created by the shrinkage with alleged cracks in the tooth, or continuing ongoing stress that could potentially relate to subsequent tooth or restoration failure; and the development of an opening of restoration margins during finishing procedures.
Clinicians are familiar with the development of a “white line” around restorations during finishing. Often, the resin looks excellent when initially placed, and then the white line occurs as finishing is accomplished. This phenomenon occurs commonly as burs that are nonconcentric are used on margins, or rotary discs are used too aggressively on margins. However, in my experience, use of new, sharp, concentric burs and/or nonaggressive use of disks during finishing can prevent the development of the white line phenomenon.
Is the white line a clinical problem? If the white line is located on a margin that is visible when the patient smiles, it may be a slight esthetic problem. More research is needed to determine if the white line is more conducive to future dental caries. Since the white line is a microscopic crevice between tooth structure and the restorative material, logical thinking would conclude that the tooth would be more susceptible to recurrent dental caries in the white line area.
The conclusive answer to your question is not easy at this time. Low polymerization shrinkage resin-based composites have the potential to reduce stress in teeth and to create fewer minute cracks and subsequent restoration failures. In my opinion, based on personal placement of many resin-based composite restorations and research observation of the “white line” under significant magnification, the main clinical difference clinicians will observe is fewer white lines during finishing. The profession should welcome the advent of “low shrink” resin-based composites, and research will continue to show their advantages. At this time, their initial-use characteristics look very promising.
Dr. Christensen is a practicing prosthodontist in Provo, Utah, and Dean of the Scottsdale Center for Dentistry. He is the founder and director of Practical Clinical Courses, an international continuing-education organization initiated in 1981 for dental professionals. Dr. Christensen is a co-founder (with his wife, Rella) and senior consultant of Clinicians Report (formerly Clinical Research Associates), which since 1976 has conducted research in all areas of dentistry and publishes its findings to the dental profession in the “Gordon J. Christensen Clinicians Report” monthly newsletter. Dr. Christensen has educational videos and hands-on courses on the above topics available through Practical Clinical Courses. Call (800) 223-6569 or (801) 226-6569.