Histological Layering Technique for Composites

INTRODUCTION
A composite restoration may be the one item that patients will pay a large sum for, with the hope that no one ever notices that they have it; putting their trust in you, the dental professional, to ensure that their investment remains discrete. The expectation is that you will choose a shade and implement a technique that allows the composite to create a natural-looking and almost perfect match. Achieving this match is paramount to your patients’ satisfaction, and in turn to supporting their self-esteem. Because natural teeth are so highly variable in color, historically dentists have attempted to achieve a good match by using multiple shades of existing body materials, or concocting mixtures of various opaque and translucent porcelain materials, in order to create a more lifelike restoration. Achieving depth, and accurately mimicking the slight nuances of the color and translucency of natural dentition, can still be an art form; however, modern materials and new layering techniques have made creating beautiful composite restorations achievable for any dental professional.

Strides in Materials Advancement
It was not until the 1990s that aesthetic materials and techniques were expanded, and the “direct composite artists” of the past paired up with dental manufacturers to develop newer direct restorative systems based on the principles of layering ceramic porcelains. While many brands previously offered only body shades mimicking the VITA Shade Guide (Vident), contemporary materials appearing around the turn of the century offered expanded choices. These choices included shades with varying opacities that were ideal for layering direct restorations in what were commonly termed “master’s kits.”

These contemporary product lines (such as EsthetX [DENTSPLY Caulk], Vit-l-escence [Ultradent Products], and Filtek Supreme Dental Restorative [3M ESPE]) were some of the early direct resin materials, specifically designed for layering and providing options that mimicked the optical properties and opacity of each histological layer of natural teeth. Prior to the introduction of these materials, more artistically-inclined dentists created the gradation of color and saturation of teeth with a single opacity material. This was done by placing a darker shade in the gingival third, a progressively lighter material in the middle third, and ultimately an even lighter material in the incisal third. While this now dated technique did provide a more polychromatic restoration, as compared to those created from a single shade, the single opacity available simply could not provide the depth, vitality, or degree of translucency found in natural teeth.

Nanotechnology Allows Enhanced Opacity and Shade Range
As composite restoration materials continued to evolve, nanotechnology emerged on the scene as a burgeoning science that was gaining high acclaim in multiple disciplines of manufacturing including paints, lacquers, and transparent films. Filtek Supreme (3M ESPE), one of the newer (released in 2002) and more successful composite materials, offers 4 opacities and 28 shades, and was the first to utilize nanotechnology in the composite resin chemistry. The incorporation of nanoclusters created high filler loading, excellent physical properties, and a sustainable polish that rivaled early microfill composite materials. Additionally, the highly favorable physical properties of this restorative material made it well-suited for both anterior and posterior direct restorations.

Figure 1. A cross section of a natural incisor, demonstrating the contours of dentin and enamel. The simplicity of nature serves as the foundation for the histological layering technique (HLT).	Figure 2. The preoperative defective restoration did not match the adjacent tooth in color, depth, or opacity.
Figure 3. The VITA shade guide (Vident) was used to determine body or “base” shade.	Figure 4. The Shade Wheel provided with the Filtek Supreme Ultra Universal Restorative kit (3M ESPE). The wheel is turned such that the selected base shade (A2) appears in the center-most window. The suggested recipe for a 2 opacity (dentin and enamel) restoration is then utilized for the HLT.

Other early products on the market also found commercial success, and although they were, in essence, traditional microhybrids, each represented newer proprietary formulations that permitted multiple opacities and favorable handling characteristics with a reasonable level of polishability. However, a sustainable polish remained elusive for microhybrid materials due to a phenomenon called particle plucking. The resin matrix would wear away during extended function, and the larger filler particles would be exposed and ultimately lost, leaving voids and an irregular surface that scattered light rather than reflecting it. Of significant merit, EsthetX was the first advanced composite system to provide a dedicated layering shade guide that provided a “recipe” for a layered 3-opacity polychromatic restoration.

During the past 10 years, new composite systems designed for layering have been introduced, and many of the early products have also been improved. My material of choice was later advanced to Filtek Supreme Plus Universal Restorative (3M ESPE) with minor revisions such as increasing the opacity of the body shades to make them most useful for single-shade restorations like Class III and Class IV (due to less show-through), and more significantly, an advanced “Shade Wheel” that could be rotated based on the selected base shade desired. It provided recipes for simplified to advanced multilayered restorations. Most recently, Filtek Supreme Ultra Universal Restorative (3M ESPE) was released, and represents even further advancements in nanotechnology by purposely making the agglomerated nanoclusters more friable such that during abrasion, only portions of these clusters are exposed or lost rather than the entire cluster. This advance has led to significant increases in long-term surface gloss, along with improvements in handling, radiopacity, and fluorescence.

Understanding Morphology Leads to a Better Match
There are many variables that determine how a particular dentist will carry out a direct composite layering technique, as the methods used depend greatly upon the dentist’s bias, skill, and desired result. However, it can be said with relative certainty that one of the most important elements in the creation of a beautiful restoration is an understanding of the basic elements that produce the aesthetics of a natural tooth and how the histological layers of natural dentition impact their overall appearance.

We all know that a human tooth contains 4 major tissues: dentin, enamel, cementum, and the dental pulp. The technique that is used in composite layering is based largely on the thickness and coloration of dentin and enamel. By looking at a cross-section of a typical human incisor, it can be observed that the dentin has the greatest influence on the overall color of the tooth (Figure 1). While the degree of saturation is somewhat dependent on age and the depth of the chroma itself, without any interference from the environment, the dentin layer would typically be monochromatic in hue and with a unified opacity.¹ The enamel layer is most often translucent and displays varied thickness from cervical to incisal regions, modulating the chroma of the dentin underneath by simply scattering light at certain wavelengths.² The saturation of color appearing in the cervical third intensified due to a thin layer of translucent enamel. As the enamel becomes thicker toward the middle and incisal two thirds of the tooth, it tends to desaturate the underlying dentin until it ends in the incisal third and becomes entirely enamel.

Figure 5. The defective restoration was removed and the enamel beveled.	Figure 6. The tooth was isolated from the adjacent teeth with dead-soft foil and total-etched with phosphoric acid for 15 seconds, rinsed, and left moist.
Figure 7. Multiple coats of adhesive (ScotchBond Universal Adhesive [3M ESPE]) were applied for 20 seconds. The solvents were volatilized and the primer/adhesive was cured for 20 seconds.	Figure 8. An ample amount of Filtek Supreme Ultra Restorative Shade A3D was applied and shaped with a composite spatula.

The morphology of dentin, as it tapers and comes to an end in the facial plane, is highly variable. It is also often marked with lobes and valleys. This is important to note because the valleys are devoid of dentin and allow the darker oral cavity behind the tooth to show through, showing up as bluish to gray zones. Another morphological characteristic to note is the common incisal “halo” caused by the light passing into the translucent enamel layer at the edge, refracting some wavelengths and scattering or absorbing others depending on the angle of wear on the incisal edge. It is important to understand what causes the variations in the aesthetics of a natural tooth in order to formulate the best plan of action to match a composite restoration.

The “Histological Layering Technique” for Highly Aesthetic Results
Being aware of the thickness, color, and morphology of both natural dentin and enamel allows for the replication of these histological tissues in composite by using formulations that are optically similar to each layer, and sculpting these materials to mimic the morphology of each area of the tooth. Throughout the years, several elaborate but highly effective layering techniques have been published that at least in theory, are based on replicating the natural histological layers found in natural teeth.^3-7 These complex layering techniques advocate the placement of a highly translucent enamel material, usually in conjunction with a silicone index created from a mock-up, to replace the lingual enamel layer and serve as scaffolding on which subsequent and more opacious layers are placed. A properly fabricated lingual index does aid in creating lingual contours, lingual and incisal embrasures, and incisal edge position, which are certainly valuable. While these techniques can generate highly aesthetic results, some general practice clinicians find them overly complex, arduous, and difficult to master and impractical for everyday dentistry. To address these key concerns, the author proposes the novel Histological Layering Technique (HLT) as a simplified, highly intuitive, and easily-mastered solution. Based on the inherent simplicity of natural teeth, this technique employs 2 to 3 opacities of composite material and is adaptable to every class of anterior and posterior direct composite restorations. It is very well-suited for creating extremely vital direct composite veneers. What may be most attractive about the HLT is that with minimal artistic skill, any clinician can create highly aesthetic restorations with any contemporary composite material in minutes, not hours.⁷

The foundational principle of the HLT is simple—the dentist replaces the layer being restored with the closest matching shade and opacity of composite material. If the restoration is contained within enamel, only the enamel material is used. If the restoration involves dentin, the dentin layer is replaced with dentin material to the dentin enamel junction, and the enamel layer is replaced with enamel material.

Figure 9. Incisal view showing dentin layer tapering back toward the lingual, mimicking the lost layer of dentin with sufficient space for the subsequent enamel layer.	Figure 10. Thin composite instrument was used to create thin and varied grooves in the composite material which serve as “receptacles” for the white resin stain to replicate the hypocalcified striations found in the residual tooth structure and adjacent teeth.
Figure 11. Incisal view of the sculpted dentin layer, after light-curing.	Figure 12. White resin stain was brushed into the grooves place in the dentin layer with a sable artist’s brush.

When the HLT is utilized to place direct composite veneers, a subtle (but obvious) variation in layering exists. Since direct composite veneers are often placed with little or no preparation of tooth structure, technically no anatomic layer is being “replaced.” To create a stratified veneer onto the facial surface of natural teeth, clinicians must merely synthetically synthesize a façade of both a thin layer of dentin and enamel with the same biomimetic principles inherent in the HLT protocol. Since dentin is the most chromagenic tissue in natural teeth and imparts the majority of the overall shade, the first step is to determine the desired base shade of the veneer and place a thin layer of corresponding dentin material sculpted to mimic the contours of dentin found in natural teeth. The dentin material is built to the final facial surface in the gingival one third of the veneer, and then tapered back to the lingual as it approaches the incisal two thirds. Kept a few millimeters shy of the incisal edge, the natural scalloped and highly variable dentin “mammelons” are created and the dentin material is light-cured. The corresponding shade of enamel material is placed to histologically mimic the form of the enamel layer found in natural teeth. The enamel material is kept thin to nonexistent in the gingival one third, allowing for increased saturation of chroma of the underlying dentin material to show through. The enamel material thickens as it is layered into the incisal two thirds and ultimately extending a few millimeters beyond the dentin material to create the incisal edge. Since natural enamel is basically a translucent histological tissue, the darker oral cavity behind the restoration shows through the sculpted dentin mammelons and creates the vibrant translucent incisal “edge effects” often found in healthy, natural teeth. Most natural, unworn incisors display a thin, glowing incisal edge denoted as an “incisal halo,” and this can easily be replicated by either beveling the incisal edge slightly toward the lingual, or more dramatically by placing a thin ribbon of bright, bleached enamel material onto the incisal edge and sculpting it into the mesial and distal incisal embrasures. When finished to a fine edge, the glowing effect of the incisal halo seen in natural teeth can be easily achieved.⁷

CASE REPORT
A 25-year-old female patient presented in the office seeking replacement of an 8-year-old stained and failing Class IV composite restoration on tooth No. 9 (Figure 2). Upon observation, the residual natural tooth structure as well as the adjacent anterior teeth possessed a highly variable pattern of white hypomineralized zones as well as pronounced incisal edge translucency which automatically mandate a polychromatic layering technique to assure proper integration of the restoration. Looking at the existing restoration, one can easily see the severe limitations of using a single opacity composite material. Note in Figure 2 that while the existing restoration is technically the correct shade, no effort was made to mimic the subtle variations of color and translucency found in the residual tooth structure and surrounding dentition. By utilizing the author’s HLT, these subtle intricacies in color and translucency could easily be replicated with minimal effort and artistic talent.

Clinical Protocol
To begin the procedure, a “body” or “base” shade of Filtek Supreme Ultra Universal Restorative was selected, using a manufacturer supplied VITA Shade Guide (Figure 3). It is important to note that the cervical collar or neck of a VITA Shade Tab is tinted and more saturated than the clinical crown portion of the Shade Tab itself. As a result, the author finds it advantageous to remove the gingival neck of the tab for more accurate shade matching. Various Shade Tabs are compared to the middle one third or “body” surface of the tooth to establish the overall “base” dentin shade of the restoration. While the HLT can be utilized with nearly every contemporary composite system available, the Filtek Supreme Ultra Universal Restorative comes with a Shade Wheel which is quite valuable in determining the shading of the various opacities of the final restoration (Figure 4). It is particularly useful when utilizing the HLT in that it provides a 2-Shade recipe that, in most cases, is suitable for merely replacing dentin and enamel. However, the range for Filtek Supreme Ultra Universal Restorative is quite diverse, and additional recipes for more complex layering strategies and conversely a single-shade alternative are provided. The Shade Wheel is used by adjusting the center ring to the base shade selected, and the corresponding shades of both the dentin and enamel materials appear in the 2-shade and multishade windows. In this case, VITA A2 was selected as the base shade and utilizing the HLT, the suggested dentin and enamel shades are A3 Dentin and A2 Enamel of Filtek Supreme Ultra Universal Restorative.

Figure 13. Shade A2E (Filtek Supreme Ultra Universal) was dispensed and sculpted over the dentin material to full-facial contour.	Figure 14. The enamel material was blended onto the beveled cavosurface and contoured to match adjacent tooth anatomy.
Figure 15. Immediate view of the final Class IV restoration.	Figure 16. Final Class IV restoration showing depth, vitality, and an ideal shade match with the simplified HLT.

The failing restoration was removed with a medium grit diamond bur (bur No. 6844 medium [KOMET USA]), and then the cavosurface enamel was beveled to permit ideal adhesion to the enamel rods (Figure 5). Next, the tooth was isolated from the adjacent teeth using dead-soft foil (Dead Soft Foil [DenMat]), and the enamel and exposed dentin were etched with a 37% phosphoric acid liquid-gel etchant (Scotchbond Etchant Gel [3M ESPE]) for 15 seconds (Figure 6), and rinsed leaving the dentin visibly moist (Figure 6). Multiple coats of a primer/adhesive combination (Scotchbond Universal Adhesive [3M ESPE]) were applied (Figure 7), and then the solvents were volatilized with a steady stream of oil- and moisture-free air prior to being light-cured with an LED curing light (Elipar S10 LED Curing Light [3M ESPE]) for 10 seconds.

A small quantity of Filtek Supreme Ultra Universal Restorative Shade A3D was dispensed and applied to the preparation (Figure 8) with a composite placement instrument (CCIB Instrument [Hu-Friedy]). The opacious dentin material was shaped to mimic the natural histological form of dentin. The incisal view, demonstrated in Figure 9, shows that this dentin layer was brought almost to the facial surface in the apical portion of the defect, and then tapered back toward the lingual as the material approached the incisal edge. This permitted a biomimetic thickness of the subsequent layer of enamel material to be placed, and served as the palate for the application of striations of white tint material that would subsequently be used to replicate the erratic zones of hypocalcification. In this case, it is important to note that there was little to no incisal edge translucency in the intact residual tooth structure, or in the contralateral central incisor. In cases where such zones exist, an erratic undulating edge would be sculpted to duplicate these areas. Prior to curing the final dentin layer, a thin composite instrument (Mini 4 [Hu-Friedy]) was used to create thin varied grooves/striations (to serve as “receptacles”) in the composite material, in which white resin stain was then placed to replicate the pattern of hypocalcification seen in both natural incisors (Figure 10). While some clinicians choose to simply paint these white striations on the cured surface of the composite, it has been the author’s experience that these receptacles for stain reduce the likelihood that the stains will inadvertently be removed during finishing and polishing, thus providing additional depth to the final restoration. Figure 11 shows the incisal view of the sculpted dentin layer. The dentin layer was then light-cured for 20 seconds with an LED curing light (Elipar S10 LED Curing Light).

To replicate the white striations found in the residual tooth structure and adjacent teeth, a white resin stain (Artiste Opaque White Stain [Pentron Clinical]) was brushed on with a sable artist’s brush, filling the striations previously placed in the dentin material (Figure 12). Excess stain was wiped away with a dry sable brush, inspected for accuracy, and then light-cured for 20 seconds.

Shade A2E of Filtek Supreme Ultra Universal Restorative was dispensed and sculpted over the cured dentin material and white striations (Figure 13). The material was then blended onto the beveled cavosurface and contoured to replicate the facial profile of the remaining tooth structure in harmony with the anatomy of the contralateral tooth (Figure 14). This final layer of the restoration was light-cured with an LED curing light for 20 seconds.

Finishing and polishing was performed with a series of carbide finishing burs (Q-Finishers [KOMET USA]), sandpaper discs (Sof-Lex Finishing and Polishing Discs [3M ESPE]), and the use of a composite polishing paste (Enamelize Polishing Paste [Cosmedent]) on a felt disc.

The immediate view of the final Class IV restoration is shown in Figures 15 and 16. Note the depth, vitality, and ideal shade match that was created by using the HLT and the Filtek Supreme Ultra Universal Resorative. The more translucent enamel material allowed just enough of the internal white stains to show, replicating the intrinsic coloration of the adjacent natural tooth. The luster achievable with this composite resin restorative material is comparable to microfilled materials. In addition, its excellent physical properties will provide for a long-lasting and durable restoration.

CLOSING COMMENTS
Gone are the days when creating a well-matched composite restoration was relegated only to those who possessed a Picasso-like talent for mixing single-opacity shades. The evolution of composite materials now makes it possible to utilize advanced shades and varied translucencies to mimic the properties of a natural tooth using simple layering and a bit of attention to detail. With the right technique and a solid knowledge of the morphological characteristics of natural dentition, creating beautiful composite restorations using modern materials is within reach for any dentist.

References

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2. ten Bosch JJ, Coops JC. Tooth color and reflectance as related to light scattering and enamel hardness. J Dent Res. 1995;74:374-380.
3. Blank JT. Direct composite veneers in minutes, not hours: a simplified technique and material. Dent Today. 2007;26:72, 74-75.
4. Vanini L. Light and color in anterior composite restorations. Pract Periodontics Aesthet Dent. 1996;8:673-682.
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6. Fahl N Jr. Achieving ultimate anterior esthetics with a new microhybrid composite. Compend Contin Educ Dent Suppl. 2000;(26):4-13.
7. Blank JT. Esthetic anterior composite restorations. September 2011. Penn Well Publications supplement. ineedce.com/courses/2193/PDF/ 1108cei_dentsply_Restorations.pdf. Accessed April 24, 2012.

Dr. Blank maintains a private practice in Fort Mill, SC, where he emphasizes comprehensive restorative and cosmetic reconstruction. He is an adjunct professor of general dentistry at the Medical University of South Carolina and a guest lecturer for graduate and undergraduate studies. He has consulted with dental manufacturers and lectured throughout the United States on the recent advances in adhesion technology and the benefits of various cosmetic techniques. He can be reached at (803) 327-3240 or at carolinasmilecenter.com.

Disclosure: Dr. Blank has received a stipend from 3M ESPE for writing this article.