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Cross Polarization and Custom Dyes

I would like to share with you my observations and experiences with utilizing a tool that helps me to select shades for my direct and indirect restorations. As restorative dentists, we are given the task of either replacing or enhancing what is absent or deficient in nature. Innately, that solution is essentially simple.

We have two structures that we are trying to replace, enamel and dentin.

Ideally, we would like to replace dentin with a dentin substitute, and enamel with an enamel substitute. Histological cross-sections of teeth begin to show us that there this is an optical dynamic that is more challenging than what we may initially have considered.

Enamel is an achromatic crystalline structure that refracts and filters light. It is affected by the aging process, which will change those prismatic effects or potentially introduce maverick shades, making the selection of shades more challenging.

Dentin is a very dense, chromatic structure, which gives us much of the perceived color of the tooth. However, dentin will also undergo an aging process and will be influenced by the pupal morphology or sclerosis of the pulp

Why are DSLR cameras more accurate than our eyes?

Camera Calibration for Dental PhotosThe eye can only interpret what the shade is by receiving the available light that is reflected from the surface.  If we were to look at a smile under a variety of lighting conditions, with different spectrums of light, the perceived shade would be limited by the bias of light applied.

Ideally, when selecting shades, we would like to be able to observe the teeth under the fullest spectrum of light possible at 5,500 degrees Kelvin with a color rendering index of 95% or better. In digital photography we utilize a flash to illuminate our subjects. Flash photography is a known quantity of light that bathes the surfaces of the teeth. The software within our DSLR card cameras allow accommodation to this known quantity of light to calibrate our images, typically with the auto white balance setting.

How can grey cards and image management software improve accuracy?

Dental Photography LightingHowever, there still is some influence by ambient lighting conditions and biases in color that may be introduced when we begin to utilize certain filters. If we can reference a known coordinate of color, like an 18% gray card, into the field as we our record of our images, we can then calibrate those images within our image management software, which will result in a more accurate representation of shades. Complete calibration of these images can best be achieved when the images are recorded in a RAW format.

What are the limitations to using flash photography?

Flash photography is a great tool, however, it is wrought with certain limitations. Those limitations are the artifacts that it causes on the surfaces of virtually any reflective surface. Those artifacts are localized distortions that we would refer to as “reflections”, but they don’t really exist in nature. When our eye sees these reflections, three things happen:

  • Our eyes are naturally diverted from that area as a protective reflex.
  • We interpret essentially a white spot, as our brain doesn’t see a reflection.
  • It hides what is occurring underneath of that reflection, and a lot of our data or our perception of the polychromicity in that particular area of the tooth is distorted.

What is the value of cross polarization?

Cross PolarizationReflected light is an advantage when we're trying to describe texture and contours, but a liability, when we're trying to select shade. The introduction of cross polarization filters allows us to remove this spectral reflection, giving us a deeper appreciation of each of the layers of shading.

Cross polarization filters are not new in dentistry. They were first described in an article by Dr. Lorenzo Vanini when he began to use cross-polarization filters as a way to provide a quality control to predictably create a shade recipe for direct restorations. In those situations, a mockup was completed with the selected direct composite materials. It was then photographed in the field to see whether or not it would harmonize with the balance of the smile.

How can lab technicians benefit from cross polarization?

Tooth Shade SelectionWe can use this same technique when we create our indirect restorations. We begin by photographing the shade tabs, in the intraoral field with the same orientation and axial inclination relative to the teeth. We will then repeat that same photographic series with our cross-polarization filters. A final series of images are then recorded with the cross-polarization filters and an 18% gray card. Once our preparations are completed, we will repeat this entire series of images to determine the extent that the underlying tooth structure will potentially influence our final restoration. This is increasingly a critical consideration because modern materials allow us to create restorations in very thin cross section.

How can the lab use these pictures to create custom dyes?

Lab Bench VerificationOur colleagues on the lab bench, typically do not have the luxury of observing what is viewed clinically, yet they are presented with the challenge of matching what they don’t see. To compound this issue, restorations are fabricated either on stone models, blue-stereolithic models, yellow milled models or potentially on no model at all.

The images that are recorded clinically in this protocol are the missing link that provides the critical information necessary for the laboratory technician to be successful. With the understanding of what the underlying tooth structure presents, custom shaded dyes can be utilized to evaluate the success in shade selection. The newly fabricated indirect restorations can be place on the custom shaded dyes with the appropriate try-in paste, there by creating a quality of control check of what can be anticipated clinically. The laboratory technician can then replicate the same series of photographic images that were recorded clinically in a virtual environment on the lab bench. Only then can the restorations be delivered to the clinician with predictable esthetic outcomes related to shade.

How can the shade of a custom dyes be verified?

Once the restorations are received , the clinician will use the same selected try-in pastes from the esthetic cementation system that matches what was simulated on the lab bench. The definitive restorations are seated on the intended surfaces of the teeth and evaluated to determine whether or not our esthetic goals have been achieved.

The restorations are then photographed with and without  the cross-polarization filters. This will aid in the critical evaluation of the harmony of these restorations into the balance of the smile. In this example, this protocol was implemented for the replacement of two central incisors, one with a veneer and one with a crown. Although this protocol involves a few more images to be recorded and the investment in cross polarization filters by both the clinician and the laboratory technician, it will predictably reduce or eliminate the multiple try in process that often plagues indirect restorative treatment when dealing in a limited segment of the esthetic zone. 

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Scott Finlay, D.D.S., FAGD, FAACD is the Resident Expert in Dental Esthetics and Senior Faculty member at the Dawson Academy. Dr. Finlay is one of the world’s most recognized dentists in the area of esthetic dentistry, winning several awards from the AACD. He is also a full-time practicing dentist at his practice in Annapolis, MD.