Print Article: Creating the Illusion of a Dilating Pupil
From Journal of Ophthalmic Prosthetics
One of the more challenging aspects of creating a realistic ocular prosthesis is a need for a pupil that appears to respond to changes in available light sources. Although there have been attempts to provide changes in pupil size by chemical, mechanical, and electronic means, in general these attempts have proved to be unsuccessful. I have developed a technique to give the illusion of pupil differentiation and thereby create a more natural looking prosthesis.
The Pacific Northwest area has a large population of people descended from Scandinavian and Northern European ancestry, for whom light eyes are a predominant genetic feature.
My practice is located in Portland, Oregon, where I see many patients with light eyes. It is especially difficult to create a prosthesis for a lighteyed person that gives the illusion of a real eye with changing pupil diameter. In response to varying amounts of available light, the average pupil size of a functioning eye changes frequently throughout the day and evening. Because the pupils of light eyes are much more noticeable, the lack of size change in a prosthesis is also more noticeable.
The average pupil size is approximately 3.5-4.0 mm in diameter (in a median light condition). To create a prosthesis in whi.ch pupil size appears to change, begin by preparing the iris button in white acrylic. Remember that when the clear acrylic is applied to the iris to simulate the cornea and anterior chamber, the iris and pupil will appear approximately 0.5 mm larger. This magnification will also cause the paint to appear as a lighter value and a brighter hue. Thus, the iris should be painted in a darker color measuring 0.5 mm smaller in diameter in order to compensate for the enlargement caused by the application of the clear acrylic.
The Process in Sequence
The first layer of paint is applied to the posterior surface of the iris button in a mid-range tone and hue. At this stage, I am more concerned with laying a foundation for the painting rather than creating details of the iris. With thought of the anatomical relationship of the eye, obviously the. pupil is a hole in the iris that gives the appearance of depth. To facilitate this look, I remove a 5.0-6.0 mm diameter of the painted surface that will represent the pupil when it is painted black. The button is then imbedded in white acrylic, processed and ground down to prepare the surface for the final painting.
I have found that a concave surface creates a better illusion of a real iris by increasing the refractional properties within the simulated anterior chamber. Therefore, when grinding the clear acrylic off the anterior surface of the button, I create a concave surface that is the diameter of the desired iris with the deepest area 0.25-0.5 located just above the center of the pupil.
Next I begin to paint the anterior surface starting with a color that will be the collarette of the iris. If I desire a finished prosthesis with a 3.5 mm pupil, I begin with a pupil size 5.0 mm in diameter. Using a fine quality #000 red sable brush, I paint thin lines 1 mm in length radiating from the outer edge towards the center around the entire circumference of the black pupil area. This will leave a 3.0 mm diameter area that represents the pupil in brighter light. These lines should be painted in thin layers, and there should be thin spaces in between the lines, in order that light may pass through (Figure 1).
After the lines are painted and have dried, I use a #11 blade scalpel to scratch back between the painted lines to make them more distinct. (Be careful not to scratch too deeply, as this may damage the surface and could leave small light lines in the finished prosthesis). If the paint is too thick or the blade is dull, the paint will have a tendency to flake off unevenly. Applying a drop of water to the surface will magnify and clarify the relationship between the lines and spaces (Figure 2). This is one of the few advantages of the acrylic materials hydrophobic quality. Working with the blade to trim the lines through the water will give a better sense of the final appearance.
When I have achieved the desired result, I begin to paint the stroma, limbus, and sclera. I use dry pigment for my painting, and build up many layers of glazing to obtain a more realistic result (Figure 3). On the sclera, I use red cotton fibers to simulate the individual vascular pattern.
When this is completed, I use a painting shell to compare the prosthesis with the seeing eye (Figure 4). If satisfied with the results so far, I move on to packing the clear acrylic. After the prosthesis has cured and been removed from the flask, I grind, pumice, and polish the prosthesis. Now it is ready to be inserted into the patient's socket (Figure 5).
Figures 6 and 7 show a finished prosthesis (thin shell) that demonstrates how the pupil can appear to have changed in size to reflect changing light situations. This is much more noticeable in lighter eyes, but can be observed in all eye colors and depends on light intensity. Figures 8A-8D show this progression.
We are all familiar with the process in which brighter light causes lenses to become darker. If a material could be found that would act in a opposite manner (i.e. bright light would cause a dark area to appear lighter), then this material could be used within the portion of the collarette around the pupil. This would create the appearance of a smaller pupil in bright light and a larger pupil in lower light situations. Until a material with these properties is discovered, however, we must rely on techniques such as I have described to give a more natural appearance to the prostheses.
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