Material science has made tremendous advancements, offering dentists materials that accomplish more and are less complicated to use. However, with all of the new materials on the market today, it appears as though the compatibility of different materials coupled with the technique sensitivity of placing these materials have dentists' heads spinning. One category of materials that has seen huge technological advances is direct composites. These advancements afford dentists the opportunity to do more with composites than ever before; however, the dentist must be aware of what to look for in a new composite and then how to utilize the material effectively.
The current "buzz word" for new composites and liners on the market today is "polymerization shrinkage." Why is this so important? How does the new composite technology with polymerization shrinkage differ from past composites? And why do you need to be concerned about it?
This article will simplify the term polymerization shrinkage and will suggest strategies for placement of a new direct composite resin. The techniques introduced herein will increase the ease of placement, and improve the expected longevity and predictability of the restoration placed as well.
Polymerization shrinkage is a measure of the change in dimension as monomers are cross-linked during polymerization (initial cure). The more shrinkage that a resin has at initial cure, the more stress created within the tooth over the life of the composite. This causes clinical concern because it introduces residual stresses in restored teeth. These stresses can lead to microleakage, cause postoperative sensitivity, and propagate enamel cracks. The degree of polymerization shrinkage stress does not depend solely on how much a composite resin contracts, but also on the elastic modulus ("stiffness") of the composite, the shape of the cavity, the established bond between the tooth and the restoration, etc.1
Stresses within a tooth associated with direct resin restorations placement can never be eliminated completely. However, the dentist must be aware of a number of factors that have been shown to increase stresses within a restoration.2 These factors include the monomer systems within the composite used, the most common being bis-GMA based. Other factors include, but are not limited to, the volume of material being placed and the depth of cure. Most manufacturers recommend placing composite in increments of no more than 2.0 mm in order to eliminate the risk of increased stresses. An inadequate depth of cure can result in uncured composite, incomplete bonding, and a decrease in marginal seal of the restoration, resulting in compromised restorative longevity.
STRESS REDUCTION STRATEGIES
Preparation Shape Optimization
Decreasing stresses within a direct resin restoration is a critical factor in reducing the amount of microleakage over time, thus increasing the longevity of the restoration.3 Stress reduction within a restoration does not rest solely on material science. Research supports and validates that preparation shape optimization can assist in increasing the debonding resistance of restored teeth by reducing the stresses between the tooth and the restoration while under occlusal loads.4-6 A recent study (Li et al4) decreased maximum stress value by more than 50%, suggesting shape optimization is an effective and efficient means of reducing stresses in restored teeth with the potential of increasing the longevity of the restoration.
|Figure 1. The modified (optimized) cavity |
preparation (Step 1).
|Figure 2. Modified T shaped cavity preparation.|
|Figure 3. A round-ended bur (KOMET No. SC850 [KOMET USA]) was used to help create a rounded cavosurface.||Figure 4. Note the diverging axial walls and line angles.|
The following are suggested preparation-shape modifications, which have been shown to decrease internal stresses:2
• Modifying the profiles of the internal line angles of cavity preparations making them more obtuse/rounded instead of acute/sharp (Figure 1).
• Keeping all internal and external line angles rounded, and creating a T-shaped preparation (Figure 2).
• Utilize a round-ended bur to help create a rounded cavosurface line angle (photographed, Midwest 856L) (Figure 3).
• Internal axial walls and line angles should diverge rather than following the traditional convergence profile with no undercutting (Figure 4).
Most conventional composites shrink between 2% and 4% during polymerization when volumetric shrinkage is measured.7 Introduction of various newer monomer systems may also help improve shrinkage stress issues associated with resin-based composites.
Silorane,8 a new monomer, is utilized to reduce shrinkage in composites. Filtek LS (3M ESPE) is one of those composites. (This composite system requires a dedicated bonding resin, LS Bond [3M ESPE], to achieve similar bond strengths to enamel and dentin as found with conventional adhesive systems using bis-GMA-based composite resins.) Volumetric polymerization shrinkage of Filtek LS has been shown to be at 1.7%.9 In addition, other "low shrinkage" composites that have volumetric shrinkages of less than 3% include products such as Aelite LS (Bisco) at 1.39%; KALORE (GC America), at 1.72%; N'Durance (Septodont), at 1.4%; and Grandio (VOCO America), at 2.4%.8-12 It should be noted that some clinical studies question whether or not this translates into a clinically significant difference related to the long-term survivability of the restoration.8,13
Bulk Filling Materials
It has been widely accepted by material science and the dental community alike that bulk filling a restoration increases stresses on the tooth, decreases bond strength, and is (overall) a poor technique to ensure the longevity of the restoration.14 The generally accepted depth of placement of a composite is 2 mm for both conventional and flowable composite resin restoratives. However, the challenge comes with the box portion of a Class II cavity preparation; how do you know how much you have placed? For example, when lining with a flowable composite, the dentist must simply estimate the thickness (2.0 mm) of the composite resin placed.
|Figure 5. The flowable composite application tip (Compula Tips [DENTSPLY Caulk]) is designed for operator ease of placement.||Figure 6. The Compula Tip is 6-mm long. This tip is designed to allow the clinician to quickly visualize (at a glance) the depth of box and to gauge the subsequent thickness of the composite being placed.|
To help address this clinical dilemma, a new flowable composite resin material has been introduced (SureFil SDR flow [DENTSPLY Caulk]) that utilizes low-shrinkage (low-stress) chemistry, allowing the clinician to bulk fill (up to 4.0 mm) a restoration. In a recent study (Burgess et al14), this flowable composite resin material, when compared to traditional flowable composite resins, demonstrated lower polymerization shrinkage and associated stress while possessing similar physical properties in terms of wear, surface roughness, gloss, color, stability, and stain resistance, as compared to traditional flowable composites resins. The system's placement tips (Compula Tips [DENTSPLY Caulk) (Figure 5) are designed to allow the operator to see the depth of fill of the restoration at a glance. The metal cannula is 6.0 mm long, allowing one to gain direct access to the floor of the deepest boxes of Class II preparations (Figure 6). Also, limiting the maximum fill to 2.0 mm below the attachment of the Compula's cannula allows the clinician to visually gauge the thickness of the material. Another beneficial characteristic of SureFil SDR flow is the ability of the material to level itself. This, coupled with the cannula design, helps to ensure proper adaptation of the material to the internal walls, crevices, and margins of the preparation, thereby reducing the potential for microleakage.
The following is the suggested clinical protocol for using the SureFil SDR flowable composite resin as a 4.0 mm deep liner to decrease stress within the restoration and to increase restorative longevity:
Step 1 (Refer once again to Figure 1): The composite resin preparation is done, applying shape optimization principles.
Step 2 (Figure 7): Isolate properly and apply a matrix. It is suggested that rubber dam isolation is applied before beginning the preparation phase, and most definitely prior to using any dental adhesives and composite resin systems. Select a matrix system that you are comfortable with, such as a V-ring design (Triodent).
Step 3 (Figures 8a and 8b): The adhesive system is chosen and applied. (XP Bond [DENTSPLY Caulk], a universal total-etch bonding agent, was used here).
Another positive clinical attribute of SureFil SDR flow is that you can use any methacrylate based bonding system (which most are), so select an adhesive system that you are comfortable using. If you use a self-etch product, the author suggests etching the enamel margins with a conventional etch 37% (Ultra-Etch [Ultradent Products]). Of course, if a total-etch bonding technique is used, all enamel and dentin needs to be properly acid etched.
|Figure 7. (Step 2) The modified cavity preparation was isolated.||Figure 8a. (Step 3) The adhesive system is chosen and applied.|
|Figure 8b. (Step 3) A universal total-etch bonding agent (XP Bond [DENTSPLY Caulk]) was used in this case.||Figure 9. (Step 4) Evaluating depth of box: the length of Compula Tip was marked for reference.|
|Figure 10a. (Step 5) Compula tip in place and used to gauge depth of preparation.||Figure 10b. (Step 5) Use tip of Compula Tip to direct material into corners of prep.|
|Figure 11a. (Step 6) Light-cure material.||Figure 11b. (Step 6) Evaluate the flow into the preparation.|
|Figure 12a. (Step 7) Place composite of choice.||Figure 12b. (Step 7) Postoperative view of molar; the layers are evident after the Surefil SDR flowable and layering composite (Esthet-X HD Composite [DENTSPLY Caulk]) placement.|
Step 4 (Figures 9a and 9b): Evaluate the depth of the preparation.
By visualizing the deepest part of the floor of your preparation you will be able to visualize on the specially designed cannula just how much material to place into the preparation.
Step 5 (Figures 10a and 10b): Place the SureFil SDR flowable composite.
Use the tip of the metal cannula to direct the material into the corners and crevices of the preparation, using it to gauge the depth of your preparation and to measure how much material being extruded.
Step 6 (Figures 11a and 11b): Light-cure the first layer of composite, and then evaluate.
Remember, the self-leveling characteristic of this particular flowable composite will allow for easier placement of the composite layers, discouraging the formation of air pockets within subsequent layers.
Step 7 (Figures 12a and 12b): Place the final composite layer, and then evaluate.
Esthet-X HD Composite (DENTSPLY Caulk) was used here.
It is a great time to be a practicing dentist! Material science offers us new materials that allow us the freedom to provide our patients with more options and to be more productive. If material and technique protocols are carried out properly, our patients have the opportunity to experience better and longer-lasting dentistry. In order to realize the full potential of the most current restorative products, dentists need to educate themselves on new dental materials. They should consider staying up-to-date with regard to the latest materials that offer low (shrinkage) stress technology, learning how to implement new techniques such as preparation-shape optimization.
- Versluis A, Tantbirojn D. Theoretical considerations of contraction stress. Compend Contin Educ Dent Suppl. 1999:S24-S32.
- Malhotra N, Kundabala M, Shashirashmi A. Strategies to overcome polymerization shrinkage—materials and techniques. A review. Dent Update. 2010;37:115-125.
- Rodrigues Junior SA, Pin LF, Machado G, et al. Influence of different restorative techniques on marginal seal of class II composite restorations. J Appl Oral Sci. 2010;18:37-43.
- Li H, Yun X, Li J, et al. Strengthening of a model composite restoration using shape optimization: a numerical and experimental study. Dent Mater. 2010;26:126-134.
- Shi L, Fok AS, Qualtrough A. A two-stage shape optimization process for cavity preparation. Dent Mater. 2008;24:1444-1453.
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- Duarte S Jr, Phark JH, Varjão FM, et al. Nanoleakage, ultramorphological characteristics, and microtensile bond strengths of a new low-shrinkage composite to dentin after artificial aging. Dent Mater. 2009;25:589-600.
- Simon JF, Waldemar G, de Rijk BA. Low-shrink composites. Inside Dentistry. 2009;5:56-60.
- Radz G. New chemistry opening doors. Dental Products Report. September 2009. dentalproductsreport.com/articles/show/dpr0909_360_cosresto. Accessed April 25, 2011.
- Data on file. GC America Corporation, Research & Development; Tokyo, Japan.
- Van Dijken JW, Lindberg A. Clinical effectiveness of a low-shrinkage resin composite: a five-year evaluation. J Adhes Dent. 2009;11:143-148.
- Burgess J, Cakir D. Comparative properties of low-shrinkage composite resins. Compend Contin Educ Dent. 2010;31 Spec No 2:10-15.
Disclosure: Dr. Simos reports no disclosures.