Composite resins used in dentistry have undergone countless changes. Material chemistry and filler-load tweakage have resulted in the improvement of numerous characteristics, from improved handling and depth of cure, to enhanced polishability and wear. But one of the most critical characteristics receiving attention is that of polymerization shrinkage.
“Shrinkage is a huge problem for all resin composite materials,‘‘ says John Comisi, DDS, a well-known lecturer, author and assistant professor at the Medical University of South Carolina, College of Dental Medicine in Charleston, South Carolina. “All resins shrink, whether they are self-curing or light cured.‘‘
Composite resins used in dentistry are typically composed of bisphenol A-glycidyl (Bis-GMA) and methacrylate monomers, and contain a coupling agent. Monomers are molecules of low molecular weight, which form chains to become polymers during polymerization. Polymerization, or curing, is the process that transforms the material from a plastic (soft) state to a hard state. Polymerization is often initiated through exposure of a photoinitiator ingredient, such as camphoroquinone, to a light source.
During curing, when monomers form a covalent bond with one another, resulting in a polymer chain, distance between them is reduced, just as when two people, who are walking side by side begin walking arm in arm. This results in volumetric shrinkage. Unfortunately, this shrinkage can do a number on whatever surface the composite is bonded to.
Conversion rate: The rate at which monomers are converted into polymers.
Covalent bond: Molecular bond.
Dimer: A type of oligomer.
Modulus of elasticity: A measure of stiffness in a material.
Oligomer: Formed by two identical molecules.
Silorane: The basis for a type of resin that’s composed of two molecules: Siloxane to make the material hydrophobic and and oxirane, which permits polymerization.
SHRINKAGE AND SHRINKAGE STRESS
But Middleburg, Virginia-based practitioner Ron Jackson, DDS, a well-known author and educator, explains that, although related, shrinkage, which is usually measured as volumetric shrinkage, and shrinkage stress are two different things. “The stress on the bond to the tooth (caused only in part by the shrinkage of composite), is, and always has been, an issue since we started using composite resins, especially light-cured ones. Auto- or self-cure resins (since they polymerize slowly) less so.‘‘
Jackson refers to Hook‘s Law, a mathematical formula used to calculate shrinkage stress. “It states that shrinkage stress is equal to shrinkage multiplied by elastic modulus,‘‘ he says. “Shrinkage is easy to understand because everyone knows when a shirt shrinks, it gets smaller (volumetric shrinkage). Elastic modulus, also called flexural modulus, basically refers to the ability to absorb stress. An example of something with a low modulus would be a rubber band. It stretches easily and returns to its original shape as long as it hasn‘t been stretched beyond its limit. In other words, it absorbs the stretching force without any permanent change or damage. An item with a high modulus would be a paper clip. It takes a lot more force to pull it apart, but when that happens, it doesn‘t return to its original shape. Returning to the formula, a composite material with high shrinkage or high elastic modulus will have higher shrinkage stress than a material that has both low shrinkage and low elastic modulus.‘‘
Stress may also be influenced by external factors such as temperature, humidity, etc. But, regardless of the cause, even incremental shrinking can have major consequences, such as marginal leakage and more. Says Comisi, “The degree and extent can be somewhat different, but all resins shrink. Hence the problem develops with the circumstance of microleakage, bacterial invasion and secondary decay. As long as we use resins as we currently know them, and these materials are passive, secondary decay will always be a problem.‘‘
And therein lies the crux of the issue.
Point of Sale | Antishrink Material
- The lower the polymerization shrinkage rate, the more likely the restoration will be a success.
- All composite resins shrink during polymerization, but the higher the fill load, the lower the shrinkage rate will be.
- The best fillers in today’s composite resins are a mix of nano-sizes and shapes, as they are able to pack more tightly together.
- Innovations in nonmethacrylate resin formulations are emerging in other kinds of monomers that do not link as closely, which is what causes volumetric shrinkage.
Manufacturers have long been moving heaven and earth to develop formulations that do not shrink — or don’t shrink as much. And the quest to minimize shrinkage continues. One remedy has to do with filler content. Filler particles are often made of glasses or ceramics. It’s also not uncommon to find contemporary composites using prepolymerized fillers, commonly made of ground-up resins. Fillers are known to increase a wide range of beneficial properties in restorative materials. And they do not shrink. Instead, they take up space, increasing viscosity. Their significance can be seen in the fact that unfilled methacrylate monomer-based materials are reported to shrink about 25% overall during polymerization. With the addition of fillers, that percentage drops to around 7%.1
A 7% shrinkage rate is still not clinically acceptable, but as the wonders of microscopy have allowed us to utilize particles at the nano-sized level, shrinkage is being further reduced. This is said to be especially true as particle sizes and shapes are mixed, which tends to take up more space. Although one manufacturer is using what it calls “spherical technology,‘‘ in which particles, all in the size range of 200 nm, move freely and smoothly around each other during polymerization. This is said to limit shrinkage stress while also enhancing esthetics.
As Robert Lowe, DDS, a well-known Charlotte, North Carolina-based practitioner, author and educator, notes, “The more space you can take up by packing filler into a matrix, the less shrinkage there will be.‘‘ For instance, flowable composites are available in light-bodied and heavy-bodied formulations. Lowe explains that the less viscous light-bodied flowables are typically used as liners, while the more viscous heavy-bodied flowables are used for filling the majority of a cavity. Says Lowe, “The heavy-bodied flowables offer more strength and less shrinkage.‘‘ He adds that bulk-fills are giving clinicians the added advantage of filling cavities in fewer layers — or even a single layer, which can cut down on time and the potential for operator error.
Traditionally, techniques, such as layering composite resin material and curing incrementally, have been seen as offering the best solution to polymerization shrinkage. But incremental placement also offers multiple opportunities for operator error, such as the introduction of voids. Incremental placement can also be extremely time consuming and tedious work, especially in hard-to-reach posterior areas. With the introduction of bulk-fill materials, this is becoming less of an issue.
Jackson suggests, the best way to counteract polymerization shrinkage stress is to use bulk-fill materials for posterior restoration. Says Jackson, “I think the problem is that dental schools have been teaching for years that shrinkage stress can only be controlled, and depth of cure achieved, by placing in 2 mm increments or less. This was true up until bulk fills were introduced. But many dentists trained prior to that (the majority in practice) still don‘t believe you can do this. At least I can‘t think of any other reason unless they want to use the same material in the posterior as they use in the anterior and put up with the aggravation of placing multiple layers.‘‘
According to Jackson, manufacturers have had to conquer two things in order for their bulk fills to be used. “They needed to develop materials with lower shrinkage stress and higher depth of cure so that their products could be placed at 4 or 5 mm. And they have achieved that. I could drown you in the amount of research that proves this for all the ones in the market today.‘‘
POTENTIAL POLYMERIZATION SHRINKAGE PROBLEMS
Polymerization shrinkage is one of the banes of a clinician’s existence, and marginal leakage tops the list of resulting woes for many. This occurs when a composite resin shrinks ever so slightly as it cures, breaking the tooth-to-material bond, and leaving a slight gap. But it’s enough to allow the entry of bacteria into the far reaches of a cavity — enough to set the stage for secondary infection.
But leakage isn’t the only problem. If material does not pull away from the margins, and the bond is maintained, polymerization stress can result. This refers to the torsion that tooth structure undergoes when the bonded material shrinks. This kind of stress can result in cracks and fractures of enamel.
Excessive polymerization shrinkage and polymerization shrinkage stress are reported to cause a sort of deformation known as “cuspal deflection.‘‘ Robert Lowe, DDS, of Charlotte, North Carolina explains, “Cuspal deflection occurs when the composite shrinks pulling the cusps closer together. This can result in sensitivity upon chewing.‘‘
In fact, post-operative sensitivity, as well as marginal staining, debonding and the formation of internal gaps between the material and cavity walls are additional issues that are linked to polymerization shrinkage and stress.2
In addition to exploring the use of fillers, dental material scientists continue to evaluate the resin matrix itself. “Researchers are always looking at different polymers or monomers that are not as sensitive to light,‘‘ says Lowe, adding that new chemistries are being introduced aimed at tackling polymerization shrinkage and polymerization shrinkage stress as well as other related issues. He reports that the search for polymers that won’t shorten or shrink during photo polymerization has been an important goal of dental material scientists. For instance, monomers featuring higher molecular weights could be good choices for incorporation into composite materials as they don’t shrink as much as their lighter weight counterparts.3
Such materials, some of which are already available, are reportedly engineered sans the kinds of monomers used in most traditional composite chemistries. One, using a nanotechnology based on silicon oxide, is said to reduce polymerization shrinkage and stress by up to 50%. Another is based on nano and dimer acid chemistry, using monomers that become oligomers rather than polymers, to lessen shrinkage and stress while offering a high conversion rate for a deep cure.4 A third example offers a proprietary monomer technology combined with pre-polymerized fillers, said to offer reduced volumetric shrinkage, as low as 0.85%.
Based on silorane chemistry, a fourth type of composite is composed of monomers that connect via ring openings that extend toward each other instead of closing in on each other. This formulation is reported to shrink less than 1%, while reducing stress by as much as 80% when used with a dedicated primer and bonding agent.
One unique material starts as a high-viscosity material, but as it is injected into a cavity via a sonic handpiece, it is liquefied to the point that it becomes a flowable. In its low-viscosity form, its adaption to cavity anatomy is enhanced, helping to eliminate void formation. It then resumes its more viscous form after a few minutes, which can be sculpted into the final restoration. This material can reportedly be placed in bulk in up to 5 mm increments.
Yet, Comisi remains skeptical. “There are attempts to provide ‘low shrinkage‘ or self-curing materials, but again, the chemistry of all resin materials, when they cure, makes them shrink. You can’t combat it, it will always occur,‘‘ he argues. “A lightly filled resin material will shrink more then a highly filled resin, but there is always shrinkage in resin materials. The best we can ask for is an antimicrobial or bioactive component added to these materials to help reduce the biofilm invasion and secondary decay.‘‘
Indeed, the addition of bioactive components would certainly seem to be valuable as a safeguard against even the most minor marginal gaps. But Lowe contends that many of today’s composite materials only shrink about 2% or less by volume, so he doesn’t believe that polymerization shrinkage is as much of a clinical issue as it was in years past. “It all boils down to clinical significance,“ he says. “Is there a clinically significant problem? Do we really need less shrinkage than we see with current ‘low shrink‘ composite materials? I think we‘re at the point where today‘s materials are really refined, and I’m not sure how clinically significant even less shrinkage would really be at this point. Even flowables are more highly filled.‘‘
OTHER ANTISHRINK STRATEGIES
According to Comisi, even small adjustments, such as those involving curing light technique, can reap big rewards in minimizing polymerization shrinkage. “Research indicates that a delay in photo curing of about 20 to 30 seconds can reduce the shrinkage stress that occurs in materials. Also, the addition of certain types of urethane to resin materials can help make these materials less brittle and more resilient,‘‘ he notes. “The combination of delay in curing, urethanes and use of bioactive calcium- and phosphate-releasing glass offer the best possibilities, at this time, of overcoming the challenges of our traditional composite resin direct restorations.‘‘
Other tactics have involved the use of liners to more evenly distribute stress. There have also been studies addressing material temperature variations, with some concluding that refrigerated materials shrink less than heated materials.
Says Jackson, “One more thing that could help dentists control some of the effects of shrinkage stress is something called the C-Factor (configuration factor). It’s a method whereby dentists place the increments in a certain way. Though, it’s less important now with bulk fills.‘‘
“Dentists sometimes have a difficult time letting go of ‘the old rules,‘ such as that you can‘t place composites using bulk-fill techniques. So that is one of the challenges for modern composite materials,‘‘ says Lowe. “Ideally, we‘d like to have composites with zero shrinkage. But clinically, we‘re at a point where materials are much better. Many of the materials-related issues that we had only a few years ago no longer exist.”
In all likelihood, the quest for zero percent shrinkage will continue. But if we can get restorations that are clinically viable, even though they’re not quite at the zero percent shrinkage mark, that may be good enough to call it a wrap. At least for now.
- Pitel ML. Low-Shrink Composite Resins: A Review Of Their History, Strategies for Managing Shrinkage, and Clinical Significance. Available at: parkell.cdeworld.com/courses/4964-Low-Shrink_Composite_Resins:A_Review_of_Their_History-Strategies_for_Managing_Shrinkage-and_Clinical_Significance. Accessed March 27, 2018.
- Soares CJ, Faria-E-Silva AL, de Paula Rodrigues M, et al. Polymerization shrinkage stress of composite resins and resin cements — what do we need to know? Braz Oral Res. 2017;31(suppl 1):e62.
- Lowe RA. Clinical update on composite restoratives. Decisions in Dentistry. 2017;3(7):37–42.
- Lowe RA. Advances in composite resin materials. Inside Dentistry. 2015;11:45–49.
Featured image by VCHAL/ISTOCK/GETTY IMAGES PLUS
From MENTOR. May 2018;9(5): 16-18,20-21.