Layering Modern Nanohybrid Composite

Predictable approaches for anterior and posterior restorations

Nikhil Sethi, BDS | Sanjay Sethi, BDS

Esthetic outcomes have become an integral part of the success of dental treatments. Patient expectations have driven material science to a point where a restoration's longevity is no longer the sole determinant of whether it is considered a "good" result. Clinicians must be able to mimic nature with direct and indirect restorations and, more importantly, do so in a predictable manner and within a reasonable time frame. With factors such as dental disease, parafunction, aging, and diet providing significant variables for each presenting clinical situation, this is not an easy task.¹ The dental industry colloquially uses the term "tooth-colored" to describe restorations. This simplified term masks the significant difficulty that clinicians encounter in their daily dentistry as well as the challenge that material scientists face to provide the ideal restorative solution.

Traditionally, the conundrum has involved choosing the appropriate composite for different clinical situations in the anterior and posterior regions. Higher polishing microfill materials hold their esthetics for many more years when compared with stronger microhybrid materials with larger filler particles. However, microhybrid materials offer increased resistance to wear and chipping despite having a reduced ability to hold surface polish over time.²

This debate regarding which composite materials to use in the anterior and posterior regions has largely subsided because the technologies in most modern nanohybrid restoratives provide excellent strength in function as well as good polishability. For esthetically demanding situations, some manufacturers also include a reinforced microfill material in their kits that can be applied as a final "veneering" layer in zones that are free of occlusal stress to enhance the polish. Before applying this microfill material, clinicians must ensure that it is clinically indicated because it can complicate the layering process and risks chipping if it is placed in areas involved in protrusive or lateral guidance.

Compromising on material properties is no longer an issue with state-of-the-art systems; now, the main issues involve handling and understanding the layering protocols to produce predictable, beautiful restorations. For anterior restorations, most systems still contain materials of a higher opacity with more intense chroma to recreate the dentin and a more translucent veneering material to recreate the enamel. Managing the thickness of each layer is critical to establishing a natural-looking result. For a large Class IV restoration, the operator must control the layering of various opacities in four dimensions: interproximal, palatal, incisal, and buccal. Placing an insufficient thickness of opacious material when recreating dentin and a thicker layer of translucent material when recreating enamel can result in a restoration with a gray, lifeless appearance and obvious marginal shine through. Conversely, if there is too little room for the translucent enamel layer, the final restoration can appear creamy and opaque, especially when viewed under natural light. Many products are available to enhance the character of the restoration, including tints. However, a predictable method to control the layers and an understanding of anatomy are key to establishing a natural-looking restoration.

Several brands of composite resins that utilize nanotechnology are available, and each has its own unique mechanical and optical properties. Surprising differences can be seen when comparing the A3 body shades from various manufacturers. Shade guides exist to assist dental professionals in their selection; however, quite often, the material of the shade guide does not correlate to the restorative material that is being used. Blind application of these guides to determine shade can often result in a suboptimal esthetic integration. Creating custom shade guides from each system can be useful, but it is a time-consuming option. Therefore, before undertaking a procedure, it is recommended that clinicians perform a direct trial of several shades of the chosen material system on the teeth (Figure 1) and determine the correct choice from an assessment of clinical photographs. This also helps in the patient consent process as they can be directly involved in the choice of colors, reducing the risk of discontent after completion of the restoration.

For posterior restorations, the focus should be on establishing occlusal form and tight interproximal contacts. Although esthetics are important, the minute details of character come under less scrutiny when compared with anterior composite restorations. Manufacturers have tried to reduce the burden on the operator's skill by blending the opacity of their dentin and enamel shades to produce "universal" shades without compromising on the material properties. The ability of these materials to produce an esthetic outcome using just a single shade has revolutionized the protocol for posterior restorations.

Clinical Case No. 1

A patient in his mid-50s presented with a fractured and heavily carious upper central incisor (Figure 2 and Figure 3). He had short-lasting sensitivity to thermal changes but no other persisting symptoms. After clinical and radiographic testing, it was determined that the pulp was vital. The clinical photos show the intense character effects seen on his natural teeth. Treatment options were discussed, including whitening and direct and indirect rehabilitation of his central incisor. The patient expressed that he had no esthetic concerns with the color and wanted a reasonably priced restoration that would blend in with his current situation. Based on this, the decision was made to treat with a direct composite restorative. After anaesthetizing the area, the carious lesion was cleaned with water spray under magnification. Isolation was achieved with a rubber dam, and small particle air abrasion was performed with 27-μm aluminum oxide (Figure 4).

In this case, because of the size of the lesion and the complexity of the patient's tooth shape form, a diagnostic wax-up (Figure 5) was used as a template to create a silicone matrix to build the composite restorations. Having an index of the desired final restoration offers an immediate advantage in managing layers of varying opacities because the incisal and palatal form is set. This control of two of the four previously mentioned dimensions helps to create a more natural tooth appearance, preventing the restoration from being too translucent or opaque.³

A universal composite that comes in a full range of shades (Evanesce^™ Nano-Enhanced Universal Restorative Composite, Clinician's Choice Dental Products Inc.) was selected for its low polymerization shrinkage and very high polishability. With older systems in the past, several different shades were indicated to treat a case such as this; however, during the past decade, the composite layering or stratification process has been largely simplified to include just two or three composite materials, namely an enamel shade, a dentin shade, and an incisal shade.

The first step was to rebuild the palatal shelf by placing an increment of enamel shade A2 into the silicone matrix (Figure 6 and Figure 7), followed by a dentin build using the dentin shade A2 and the application of flowable tints (Paint On Color Kit, Coltene Group) to recreate his intense characteristics (Figure 8). After the final increment was completed with enamel shade A2 and enamel incisal pastes (Figure 9), final polishing was achieved with diamond burs, contouring and polishing discs (Sof-Lex^™ Extra-Thin Discs, 3M), and polishing wheels (A.S.A.P. All Surface Access Polishers, Clinician's Choice Dental Products) to produce a highly polished surface (Figure 10 and Figure 11).

Clinical Case No. 2

A young patient presented for a routine examination, and a clinical and radiographic assessment revealed several small areas of caries on the upper right first molar as well as distal caries on the adjacent bicuspid (Figure 12). In order to preserve as much natural tooth structure as possible around the infected areas, the clinician performed minimally invasive preparation under a microscope. Isolation was achieved with a rubber dam, and small particle air abrasion was performed with 27-μm aluminum oxide (Figure 13). A sectional matrix system was used to ensure excellent contact point rehabilitation (Figure 14).⁴ Because it is difficult for one matrix system to satisfy all clinical situations, mixing the components of various manufacturers can often be very useful in obtaining the ideal result. After completing the adhesive procedure with a 2-step, self-etch adhesive system (OptiBond^™ XTR, Kerr Dental) and light curing for 20 seconds per surface, the preparations were ready for the composite restorative. To lower the surface tension and improve the wettability for the next increment, a small amount of flowable composite (Evanesce^™ Flow Flowable Restorative Composite, Clinician's Choice Dental Products Inc.) was placed on the dentin base and light cured. To achieve an esthetic result with a simplified protocol, a universal composite (Evanesce^™ Nano-Enhanced Universal Restorative Composite, Clinician's Choice Dental Products Inc.) in universal shade A2 was selected and placed using a heated injection molding technique on the bicuspid and conventional layering on the molar. Again, the use of flowable tints helped to secure a natural appearance. At the 6-month follow up, it can be seen that the integration of the material is very satisfactory and that the surface polish has been maintained (Figure 15).

Conclusion

Attaining longevity of composite restorations requires a good volumetric thickness or bulk of the material to reduce the incidence of fractures and chipping.¹ The downside to increasing the thickness is the potential for the optical properties of the materials to be altered or masked. Composite resins do not have the same long-term esthetic performance as ceramics; therefore, it must be stressed from the outset that refurbishment will be required over time to maintain these restorations.² However, composite restorations have excellent shock absorbing abilities in function, and their fractures are easier to repair than those of ceramic restorations.⁵ In addition, when compared with enamel and dentin, composite resin wears at a more favorable rate and it exhibits physiologic wear and adaptation to the dynamic situation in the oral environment,³ which will undergo changes over time. Understanding the limitations, cost benefits, and esthetic possibilities of composite resins allows clinicians to be able to give an honest appraisal of longevity to their patients.

About the Authors

Nikhil Sethi, BDS
Private Practice
London, England

Sanjay Sethi, BDS
Private Practice
London, England

References

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4. Spreafico RC, Krejci I, Dietschi DJ. Clinical performance and marginal adaptation of class II direct and semidirect composite restorations over 3.5 years in vivo. J Dent. 2005;33(6):499-507.

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