Potential Therapeutic Efficacy of a Virtual Pupil Made of Polarizing Plates for Correction of Aniridia

Abstract

Purpose.: To investigate whether diagonally-placed polarizing plates can mimic a functional pupil using an artificial eye.

Methods.: An artificial eye system was used to evaluate the visibility of the fundus and the quality of vision associated with polarizing plates, an artificial iris with a pupillary hole, or a black-diaphragm intraocular lens. The transparency of various wavelengths of light through each material was also evaluated.

Results.: The observational field was limited to the same extent through the artificial iris and when one polarizing plate with a pupillary hole and an intact polarizing plate were placed diagonally. The observational field could be widened as by a mydriatic pupil when the intact polarizing plate was removed, leaving the polarizing plate with the pupillary hole at the iris plane. The observational field limited through the black-diaphragm intraocular lens could not be changed. Visual quality was almost the same with the polarizing plates, artificial iris, or black-diaphragm intraocular lens. The strength of the light was slightly reduced when passed through one polarizing plate or two parallel polarizing plates. It was dramatically reduced when passed through two diagonal polarizing plates or a black-diaphragm.

Conclusions.: Diagonally placed polarizing plates can mimic a functional pupil using an artificial eye. If one polarizing plate with a pupillary hole is placed at the iris plane, a virtual pupil can be created by wearing polarizing glasses, and this system is controllable by wearing or removing the glasses. This system may be a suitable alternative treatment for aniridia.

Aniridia, or the lack of an iris, occurs both congenitally and traumatically. ^{1 –5} Congenital aniridia causes a significant reduction in visual acuity, because of the lack of the iris diaphragm and other associated anomalies. Cataracts may also exist or develop during the later stages of the condition, and secondary glaucoma may occur. Traumatic aniridia is often associated with corneal scars, bullous keratopathy, secondary glaucoma, and cataracts when the lens is intact. Several therapeutic methods such as eyelid surgery, and corneal tattooing had been applied for eyes with aniridia, ^6,7 however, implantation of a black-diaphragm intraocular (BDI) lens during cataract surgery, ^{2 –4} implantation of artificial iris prosthesis, ^8,9 and wearing colored contact lens ⁵ are the current standard therapeutic methods to reconstruct a functional pupil. The BDI lens and the artificial iris prosthesis provide a fixed pupil and do not require daily care, but limits fundus observation through the nonmydriatic fixed pupil. In contrast, colored contact lenses can be easily worn and removed, but require daily care and are not appropriate for patients who require filtering surgery against secondary glaucoma, because the contact lenses may result in bleb-associated complications. 

Polarizing glasses are widely used to protect eyes from strong sunlight and to reduce glare, not only in pseudophakic patients, but also in healthy people. Although normal light moves on many planes, polarizing plates allow only light in one plane to pass through the plates. Therefore, much of the glare can be eliminated. Although the light from the sun is not polarized, it can be separated into two polarized components. If two polarizing plates are set diagonally, the sun cannot transmit any light through the plates. That means that if one of two diagonally-placed polarizing plates has a small hole at the center, the light can pass through the hole, and if one plate is removed, the polarized light can pass through not only the hole but also the surface of the remaining polarizing plate. 

The authors thus presumed that if one polarizing plate with a hole were placed at the iris plane, a virtual pupil could be created by wearing polarizing glasses, which would be controllable by wearing or removing the glasses. This system may conquer the disadvantages of BDI lenses, artificial iris prostheses, and colored contact lenses, and the authors therefore evaluated the therapeutic potential of this system for aniridia using an artificial eye model. 

Methods

Iris Substitutes for Artificial Eye Experiment

An aluminum plate was purchased and a pupillary hole with a 3-mm diameter was created at the center, to be used for control experiments. Polarizing plates (KN3115820) were obtained from Tech-Jam Co. (Osaka, Japan). A 3-mm diameter hole was created in one plate. The light transparency of combinations with two polarizing plates is shown in Figures 1A and B. A BDI lens (model 67B), which has a 3-mm diameter pupil, was purchased from Morcher GmbH, Stuttgart, Germany (Fig. 1C). 

Figure 1.

 

Materials used for artificial eye experiments. (A) Light passes through two parallel polarizing plates. (B) Light is blocked by diagonally-placed polarizing plates. (C) Morcher's black-diaphragm intraocular lens (model 67B). (D) Schematic diagram of artificial eye.

Artificial Eye

An artificial eye, which was invented by Inoue et al., ¹⁰ was used (Fig. 1D). The artificial eye was constructed based on Gullstrand's model of the human eye. The body of the eye was made of metal. The cornea was made of polymethylmethacrylate. A 1951 United States Air Force (USAF) test target (Edmund Optics, Barrington, New Jersey) was glued to the posterior surface of the eye at the position of the retina. After either the aluminum plate with a hole, or the BDI lens, or the polarizing plate with a hole was placed at iris plane of the artificial eye, both the anterior chamber and the vitreous cavity were filled with pure water with care taken to remove all air bubbles and then United States Air Force chart at fundus was observed from the outside through a surgical microscope (model: SZX12, Olympus, Tokyo, Japan) and digitized by digital camera (E-410, Olympus). To simulate the retinal image in an eye, a charge-coupled device (CCD) camera (model CV-A1, Jai Co., Yokohama, Japan) was placed at retinal plane and then visual acuity chart was observed. ¹¹  

Transparency Test

To evaluate the transparency for different wavelengths of light through each material for the dense part of iris substrate, a spectrophotometer (U-4100; Hitachi Co., Tokyo, Japan) was used according to the protocol provided by the manufacturer. Each iris substrate was fixed on the test chamber between the light source and the detector in order that the light beam can pass through the dense part of iris substrate vertically. To evaluate the transparency of polarizing plate, single plate, two parallel, or two diagonal plates were tested. 

Results

Transparency of Each Material

A spectrophotometer was used to evaluate the transparency for different wavelengths of light through each material. The aluminum plate completely blocked any light (data not shown). One polarizing plate reduced the transmission of light according to the wavelength of light (Fig. 2A). Two parallel polarizing plates showed a similar pattern, but with slightly more reduction than one plate alone (Fig. 2B). Two diagonal polarizing plates completely blocked wavelengths under 780 nm, whereas the black-diaphragm of the BDI lens blocked wavelengths under 720 nm (Figs. 2C, 2D). The black-diaphragm of the BDI lens transmitted long wavelength red light. 

Figure 2.

 

Transparency observed for different materials using an artificial eye. (A) One polarizing plate. (B) Two parallel polarizing plates. (C) Two diagonally-placed polarizing plates. (D) Black-diaphragm of Mocher's intraocular lens (model 67B).

Observational Field

Fundus observation was performed. The observational field was the same between the fixed pupil of the aluminum plate, the BDI lens, and the two diagonal polarizing plates (one at iris plane and 1 at spectacle plane; Figs. 3A, 3B, 3C). However, the brightness was slightly reduced through polarizing plates (Fig. 3C). The observational field associated with different angles between the two polarizing plates was also evaluated. Using only one polarizing plate at iris plane, the observational field was as wide as that associated with a mydriatic pupil (Fig. 3D). When the polarizing plate at spectacle-plane was rotated at 45°, the outer field was dark but transparent (Fig. 3E). 

Figure 3.

 

Observational field observed for different materials using an artificial eye. The observational field was narrow through an aluminum plate with a 3-mm hole (A), a black-diaphragm intraocular lens with a 3-mm pupil (model 67B) (B), and two diagonal polarizing plates with a 3-mm pupil (C). The observational field was wide through one polarizing plate with a 3-mm pupil (D). The center of the observational field was bright and the outside was dark through two polarizing plates placed at 45° (E).

Visual Quality

A visual acuity chart was observed from inside the artificial eye. There was no obvious difference between the three materials (Fig. 4). 

Figure 4.

 

Visual quality observed for different materials using an artificial eye. Aluminum plate with a 3-mm hole (A, B). Black-diaphragm intraocular lens with a 3-mm pupil (model 67B) (C, D). Two diagonal polarizing plates with a 3-mm pupil (E, F).

Discussion

The BDI lens was invented in 1991 ² and has given therapeutic opportunities to patients with aniridia. There has been some improvement in its design but all the previously invented BDI lenses and iris prostheses have a fixed pupil. ^{2 –4,8,9} Becaise aniridic eyes are often associated with other conditions, not only in congenital cases but also in traumatic cases, a mydriatic pupil is required to observe retinal problems. In this study, we used a BDI lens (model 67B), which has a 3-mm diameter pupil because the size is closest to the natural pupillary size under day light. Currently, surgeons prefer to use BDI lenses which have a much larger pupil to guarantee the window to observe the peripheral retina although the size may be too large to keep better vision for the patients. Our system may solve this dilemma and the surgeons would prefer to use a polarizing iris plate with a smaller pupil. 

Another therapeutic option for patients with aniridia is wearing colored contact lenses. ⁵ Because contact lenses can be easily removed, the eye can be observed immediately. However, contact lenses require daily care and are not appropriate for patients requiring filtering surgery against secondary glaucoma, because they may result in bleb-associated complications. 

To overcome the problems associated with current therapeutic methods, the authors examined the efficacy of a combination of two polarizing plates. This study showed, in an artificial eye, that two diagonal polarizing plates blocked the transmission of light when one was placed at the iris plane and another was placed separately in front of the eye. These plates thus create a virtual functional pupil that can be dilated quickly by removing the plate in front of the eye, which would presumably be provided as glasses. The polarizing plate at the iris plane may be provided on the surface of a normal intraocular lens or independently. As shown in Figure 3, if the lens of the glasses is rotatable, the intensity of the light can be adjusted. Two polarizing plates can be set in any combination of directions. Polarizing glasses, used for ordinary leisure activities such as sailing and driving, polarize light in the vertical meridian because this reduces glare from reflected light polarized horizontally. It would therefore seem logical to use horizontally polarized material in the intraocular lens because this would be at 90° to most standard polarizing sunglasses ¹² and a special purchase would not be needed. However, the effect of the use of polarizing lenses in everyday life is not known, the orientation for polarization in this system should be optimized. 

Another potential application of this system may be for occlusive lens. In rare cases patients with intractable diplopia request an occlusive lens, which is essentially a black intraocular lens. ¹³  

It is possible to take infrared photographs through this lens but all visible light is blocked. The polarizing system might be helpful in these cases as well. 

A disadvantage of this system is that the patient has to wear polarizing glasses to obtain a miotic pupil, which is often used by pseudophakic patients. As in Figure 3C, the brightness may be reduced through polarizing glasses. The polarizing plates used in this study were made for industrial purposes, and therefore, the long-term stability and in vivo function and safety are as yet unclear. Further in vivo studies are thus required; however, this study showed a new potential therapeutic method for the correction of aniridia. 

Footnotes

 Presented at the 64th Annual Congress of Japan Clinical Ophthalmology, Kobe, Japan, November 2010.

Footnotes

 Disclosure: M. Akimoto, Hoya Co (F); K. Mitomo, Hoya Co (E)

The authors thank Makoto Inoue, Toru Noda, and Kazuhiko Ohnuma, who kindly provided their artificial eye model.