Digital Photographic Procedure for Comprehensive Two-Dimensional Tooth Shade Analysis

Shade matching between natural teeth and composite materials is critical when creating esthetic restorations.^1-6 Hue, chroma, and value are basic color parameters that influence the esthetic outcome of a tooth-colored restoration.^7-13 The shade of contemporary enamel and dentin composite materials should, and most likely do, accurately mimic the shade of enamel^14-17 and dentin.^18,19 Unfortunately, commercially available composite materials, even of the same shade, vary in fundamental properties, such as opalescence,^20-24 fluorescence,^25-33 translucency^34-48 and metamerism^49,50, between different brands and shades. Clinicians should be able to determine the variations of these properties and either correct any mismatch in the course of the layering technique^51-58 or at least keep a record of non-correctable differences. The application of appropriate digital photography techniques is the procedure of choice to control and document the esthetic properties of current materials.

Digital cameras have undergone impressive evolution since their development in the early 2000s^59,60 incorporating emerging technologies to make photography highly accurate, simple, and economic. Digital photography^61-65 has long been a necessary tool in dentistry for recording, diagnosing, communicating, presenting, informing, and documenting^66,67 the oral status of the patient before and after treatment. A digital single lens reflex (DSLR) system should be considered standard equipment for dental photography. A DSLR camera body using an 80-mm to 105-mm macro lens and a ring flash or a twin flash system is the basic setup for capturing detailed extraoral and intraoral images. Most current digital camera bodies are efficient enough in terms of resolution, aperture, and exposure settings. ISO configuration is considered an important and sensitive parameter to be investigated, especially for advanced fluorescence, red-orange opalescence, and translucency recording. Macro lenses of 80 mm to 105 mm provide an ideal combination of proper magnification within convenient working distance for intraoral close-up photography. A life-size reproduction of the photographed object is referred to as 1:1 magnification and, in frontal dental photography, comprises the four maxillary incisors. Electronic flashlight is necessary for proper illumination of the dark areas within the intraoral environment. The two main types of flash systems for dental purposes are a ring flash and a twin-light flash. The light of a ring flash eliminates shadows in the oral cavity but may produce undesirable specular reflections. The illumination by a twin flash produces a soft and 3-dimensional lighting effect with prominent and detailed surface morphology.

Evolved capturing techniques^68-70 enable clinicians to observe the physical characteristics and properties of natural dentition. Surface details, such as macro- and micromorphology, texture, luster, gloss, enamel cracks and striations, chromatic mapping, and dentinal architecture, can be revealed and recorded with direct reflective lighting techniques using different illumination angles. In addition, photographic applications for daily clinical practice have recently been developed to address the refinement or diagnosis of material properties, such as opalescence, fluorescence, translucency, and metamerism.

The purpose of this article is to explore through the eye of the lens these optical properties, while demystifying and describing the respective photographic applications from a clinical and technical point of view.

A DSLR Canon 550D camera (Canon U.S.A., Inc., usa.canon.com) with a Canon 100-mm macro lens and a Canon MT-24EX twin-light flash were used in the photographic protocol presented. A custom-fabricated plastic o-ring, with four metallic screws at 0o, 90o, 180o, and 270o, was fixed with silicone to the original flash framework, ready to receive the interchangeable add-on filters (Figure 1 and Figure 2).

The full excitation wavelength range of the xenon flash lamps is between 300 nm and 800 nm. When the original protective plastic diffusers cover the flash lamps, the range is limited within the visible light of 400 nm to 700 nm. In the case of fluorescence, the appropriate excitation wavelength is in the ultraviolet (UV) range, more precisely at 365 nm. Both plastic diffusers were removed from the flash and attached to the interchangeable plastic framework (Figure 3 and Figure 4) to protect mechanically both flash lamps and to protect the polarizing membrane (described later) from the increased output of the lamps. For the cross-polarizing add-on filter, the framework with the diffusers and the framework with the polarizing membranes were combined (Figure 5). Additional caution was taken by placing clear plastics in place of plastic diffusers.

Three interchangeable plastic frameworks with different filters were fabricated and connected to the underlying flash by four magnets, either separately or combined together, in front of the lens and the flash lamps. The first contained the diffusers (Figure 6), the second the polarizing membranes (Figure 7), and the third the 365-nm UV glass filters (Figure 8). Removing the plastic diffusers from the flash may lead to flash warranty loss. Because the purpose of this article is to present the principal technical aspects of the applications, the warranty was not taken into consideration.

Two pieces of a polarized plastic membrane were placed in parallel on both sides of the plastic framework. Another piece was placed in the center, perpendicularly to the lateral pieces (Figure 9). The cross-polarized filter may not be used directly on the flash lamps because the membrane might be burned by the energy of the flashes (Figure 10). For this reason, as mentioned previously, the cross-polarized filters were placed on top of the add-on frame with the plastic diffusers.

The fluorescence filters were composed of two 365-nm UV glass filters placed on both sides of the plastic framework to cover the flash lamps. No additional filter was required in front of the lens (Figure 11).

For metamerism diagnosis, a device with continuous illumination from two different light-emitting diodes (LEDs) (Rite Lite 2™, AdDent, Inc., addent.com) generating three different illumination qualities, such as 5500°K simulating day light, 3200°K simulating incandescent light, and 3900°K simulating fluorescent tube light, was luted to a plastic o-ring with four magnets (Figure 12). In this way, it was possible to attach it in front of the lens for static images (Figure 13) without flash.

The color rendering index (CRI) is described as the relative ability of a light source to replicate and is reported as a number between 0 and 100. A CRI score of 100 would accurately reproduce the colors on a sunny day at noon. This lighting condition is considered the ideal illumination environment for shade matching in restorative dentistry, even though it is rarely present during shade matching.

Rite Lite 2 has LEDs with high CRI values, varying from 87 to 92, thus approaching the ideal illumination conditions (CRI 100, 5000°K to 6000°K).

Fluorescence^71-76 is a variation of luminescence. The more fluorescent a material is, the more bright and ‘vital’ it appears. Fluorescence is defined as the ability of a natural or artificial substance to emit visible light spontaneously when irradiated by UVA illumination. The excitation spectrum of dentin has a center wavelength of 365 nm, and the fluorescence emission peak is observed at 440 nm with a full width at half maximum of 20 nm. Enamel presents a much less intense fluorescence peak at 450 nm, which slowly decreases up to 680 nm. Thus a wide, but not intense, band of fluorescence spectrum is present in enamel. Studies show that dentin is three times more fluorescent than enamel, and that dentin fluorescence intensity increases over time because dentin has higher quantities of minerals, pyrimidine, pyridinoline, tryptophan, and hydroxypyridium.^77-82

Fluorescence can be captured with two photographic applications using continuous or flash lighting. The traditional method of continuous lighting is quite complicated. It requires a continuous UV light source and a dark room to avoid any other artificial lighting source in the operatory field to make the light visible because the intensity of the fluorescence is very low. In addition, the photographic recording necessitates long exposure times of several seconds and increased ISO sensitivity of the sensor of the DSLR camera with the disadvantage of increased picture noise, the need for stabilization of the camera on a tripod, and significant irradiation of the patient by UV light (Figure 14).

Recently, the authors proposed a novel setup that avoids all these practical disadvantages and minimizes the exposure of the patient to UVA light. This setup consists of an interchangeable UVA 365-nm excitation filter placed in front of the commercial macro flash lamps after removal of their plastic diffusers, together with a DSLR camera with a macro lens. This allows for fluorescence documentation under normal dental office conditions in the same way as standard clinical photography using a macro lens and a flash does, without the need of a dark room or extended exposure times (Figure 15).

Fluorescence is considered a clinically significant optical property in esthetic restorations because, under fluorescence, teeth appear more vital, whiter, and brighter. Under black lighting, such as in nightclubs, where UV-coated lamps emit the appropriate excitation light to induce fluorescence, restorations should not be distinguishable from the natural dentition. A perfectly integrated esthetic restoration should exhibit a similar fluorescence to that of the natural dentition, and the practitioner must be able to check on this property in his or her routine clinical setting. Enamel and dentin composite materials should closely mimic the fluorescence emission levels of enamel and dentin. Unfortunately, currently commercially available composite materials vary in their fluorescence, depending on brand and shade.^83-85

Opalescence^86,87 is defined as the optical property observed in natural tissues or artificial substances to appear bluish-greyish in reflective illumination (opalescent halo) and yellowish-orange-brown under transmitted illumination (counter opalescence). The enamel of natural teeth is opalescent. This optical phenomenon is based on the difference in refractive indexes of the enamel components, which are hydroxyapatite crystals and water. Moreover, the specific dimension and diverse orientation of the hydroxyapatite crystals scatter light within the visible spectrum, more in short wavelengths for bluish effect and less in long wavelengths for yellowish-red effect. These effects are clearly visible, especially at the incisal enamel edge and also on the border between enamel and dentinal lobes.

To document and photograph yellowish-red opalescence, transmitted continuous light must be used for illumination, without flashlight. An appropriate illumination source for this application is a white LED in the oral cavity directed toward the palatal surface of the tooth to be examined. The exposure time is increased to 1/60 (but not below, to avoid a shaking effect), the aperture is decreased to around f = 10, and the sensitivity of the sensor is increased to a high value (~ 3200). With these settings, the DSLR camera is sufficiently sensitive to capture enough light without any further increase in the exposure time, which would necessitate a tripod to avoid shaking (Figure 16). The LED Microlux™ Transilluminator (AdDent) with a 3-mm microtip glass light guide was used for the specific photographic application.

To capture bluish opalescence, reflective technique is acquired using normal macro photography and flash lighting. The use of a polarizing filter is helpful, eliminating whitish areas of specular reflections and revealing the exact anatomy of the opalescence zone (Figure 17).

Ideal restorative materials should exhibit comparable properties to natural dentition. Unfortunately, commercially available composite materials, as they do for other optical properties, vary in their opalescence. Bluish opalescence, which is more evident than yellowish-red opalescence in social settings, may be determined with the appropriate photographic technique and controlled at the restorative phase.

Translucency is the property of a material that allows for light transmission but also dissipates the light within the material.88-90. As such the material is not completely transparent but has an appearance of a milky glass. In other words, translucency is the relative amount of light transmittance or diffuse reflectance from a material.⁹¹Factors that influence the transmission of light within composite resins are related to structural components, such as the resin matrix and filler contents, the difference of refraction indexes between them, the size and shape of inorganic fillers, and, finally, pigments and other additives. The aging of the material and the polymerization procedure used may also influence the degree of translucency that current composite materials exhibit.

To document and photograph translucency, a similar method to the capturing of yellowish-red opalescence is necessary. Thus, transmitted continuous light for illumination, without flash light but in contact with the palatal surface of the tooth to be examined, is required (Figure 18).

Dental enamel is translucent. Consequently, contemporary composite systems have highlighted the importance of this property. Translucency of enamel materials provides a depth of color for the underlying dentin and contributes to shade matching by enhancing the chameleon effect. Enamel becomes more translucent with age.

The metameric effect leads to different color changes under different lighting conditions.^92-97 Two substances that have the same color appearance under certain lightning conditions may have different color appearances when the lightning conditions change. This fundamental effect in the field of restorative dentistry may be influenced by three parameters: the material, the observer, and the lighting conditions. For example, if the spectrophotometric light emission curve of the material in the visible light range is different from the one of the surrounding tissue, then metamerism is observed. This parameter can only be controlled if the curve of the material is known and compared before its use. Regarding the observer parameter, individual subjective color perception of the observer may be biased. This could be due to deuteranopia, for example. Color vision deficiencies by practitioners, especially men, who are much more likely than women to have deuteranopia, should therefore be considered. Finally, daylight in outdoor environments and room and mixed lighting conditions in various indoor environments may influence the appearance of restorations because of metamerism. If composite restorations exhibit a coherent appearance and shade matching under these lighting conditions, then they are considered to have successfully overcome the effect of metamerism.

Commercially available devices using LED technology can simulate multiple lighting conditions, which may be used for the disclosure of metamerism. One of these devices was adapted in front of the lens of the digital camera to capture images in three different color temperatures (Figure 19 through Figure 21), disclosing shade differences if metameric effect was present. Figure 22 depicts how the restorations appeared under daylight, Figure 23 under incandescent light, and Figure 24 under ambient light.

A successful restorative outcome should be evaluated under varying lighting conditions that are representative of the social environments and activities of the patient. Shade matching procedures, therefore, should be performed under daylight close to a window, as well as under room or ambient light in the operating environment.

Value is one of the three basic color parameters and is related to brightness (ie, to the degree of white and black) (Figure 25).

The color of a cross-polarized photograph is converted to a gray scale image by using a generic software program, such as Adobe Photoshop^®.

The value of the restoration may be comparatively determined during the shade matching procedure and corrected if necessary within the restorative phase.

Shade matching^98-104 between natural teeth and composite materials is the primary objective when creating direct restorations. This involves shade or chromatic mapping, which aims at identification of shade distribution, revealing important color information in two dimensions. Identifying dentin areas of diverse gradation in hue and chroma, the configuration and architecture of the lobes, bluish opalescence, and regions of intense hue are related to this.

Cross-polarized reflective capturing technique is considered the application of choice for chromatic mapping. The cross-polarized technique was described for both analog and digital cameras many years ago.¹⁰⁵ Polarized photographs eliminate specular reflections of the flash, which makes identifying shade distribution easier. The photographic equipment is the same (camera body, macro lens, macro flash), but an additional polarizing filtering system is needed. A polarizing membrane sheet is placed in front of the flash lamp, covering the surface. For both the ring flash and twin light, the piece of membrane is split in two to cover the left and right flash lamps, always maintaining the orientation of the sheets parallel to each other. Another piece is placed perpendicular to the other sheets in front of the lens. In this way, the pieces are cross-polarized against each other, creating a matte photographic image (Figure 26).

Accurate chromatic mapping is an essential step for the overall successful shade matching between natural teeth and composite materials.

The present clinical report describes a comprehensive approach in documenting the optical properties of both dental restorative materials and natural hard dental tissues. Based on this concept, a simple and straightforward procedure is outlined and may be used during the restorative phase. In this way, documentation can take place in three different yet equally important time intervals: before, during, and after the restorative session. In direct adhesive procedures, such a concept is useful in refining the optical properties of the restorative materials, increasing the esthetic success rate of the restorative procedure.

Clinicians generally are familiar with the equipment and basic photographic techniques and are relatively confident working with digital cameras and dental photography. Updating the indications, optimizing the use of existing equipment, and decreasing overall costs were among the main goals of the authors. The photographic applications are explained in detail and clinicians can use them with any digital camera and flash system. Commercially available polarizing membrane sheets (linear polarizing film sheet) are very inexpensive and ideally suited for the cross-polarizing application. UVA 365-nm glass filters (eg, Schott^® Optical Filters, us.schott.com) of 2-mm thickness are available in the market and are indicated for the recording of fluorescence. LED sources for transillumination (eg, Microlux) and LED illumination devices with different light temperatures (eg, Rite Lite 2) are available in the market and may be used for metamerism, translucency, and red-yellowish opalescence. The latter LED device, even though initially intended for clinical application only, can be easily adjusted in front of the digital camera lens for static no-flash pictures.

The described protocol of comprehensive documentation of the optical properties aims to determine differences between composite materials and natural tissues. Practitioners should be able to expand their knowledge of the shade-matching process and learn to distinguish the color elements that can and cannot be controlled during the restorative phase. Hue, chroma, metamerism, translucency, and red opalescence are inherent properties of restorative materials that may be diagnosed but which may not be influenced during the fabrication of direct restorations when a given brand of a composite material is used. On the other hand, value, bluish opalescence, and fluorescence are all considered inherent properties of the restorative materials that may be diagnosed and refined during the layering of materials in direct restorative procedures.

Within the limits of this clinical report, some conclusions can be drawn. Comparative level of luminosity, fluorescence, opalescence, translucency, and metamerism between natural tissues and dental materials may be recorded in daily practice by any clinician by using low-cost equipment based on a simple DSLR camera combined with inexpensive, commercially available add-on equipment. For direct restorations particularly, color elements that influence the overall restorative outcome, such as luminosity, fluorescence, and bluish opalescence, may be optimized because of correct diagnosis during the layering technique.

Yiannis Brokos, DDS, MSc, Dr. med. dent.
Division of Cariology and Endodontology
University Clinics of Dental Medicine
University of Geneva
Geneva, Switzerland
Private Practice
Rhodes, Greece

Minos Stavridakis, DDS, MSc, Dr. med. dent.
Division of Cariology and Endodontology
University Clinics of Dental Medicine
University of Geneva
Geneva, Switzerland
Private Practice
Athens, Greece

Ivo Krejci, Prof. Dr. med. dent.
Chairman and Professor
Division of Cariology and Endodontology
University Clinics of Dental Medicine
University of Geneva
Geneva, Switzerland