Key words: intraocular lenses, flexible IOL's, lens testing
More and more ophthalmic surgeons are choosing to implant flexible intraocular lenses (IOLs), because these lenses permit smaller incisions which heal with less scarring and subsequent optical distortion of the eye. However, little information is available about the long-term stability of the new implant materials used in these lenses. Concern about this biocompatibility issue has led to extended follow-up periods for approvals of IOLs. The use of flexible lens materials has led to a number of unexpected post-approval consequences. Discoloration, haziness, and glistening have all been reported. For example, earlier this year, Alcon voluntarily recalled all distributed units of its ACRYSOF flexible IOL model for just this type of unwanted instability.
Although problems with lens materials are now well known, their effects on vision have been poorly studied. Little has been done beyond visual acuity testing. There has been no clear guidance from the ophthalmic community about the expected consequence of lens material problems. In response to this need, OST scientists have developed a bench test method that provides predictions about the visual deficit to be expected from changes in the lens optical material. The OST method begins by conducting tests of resolution, using the suspect lens in a model eye. Changes that would affect visual acuity and contrast sensitivity become evident. Resolution testing is followed by a new test for scattered light. This test uses a pinhole light source imaged through the test IOL onto the diode array of a CCD camera. This test measures both the amount of light undergoing near forward scatter, as well as the total amount of light either absorbed or scattered by the lens. Scattered light creates a veiling glare that can be disabling in situations where target illumination is limited and an intense light source is nearby. An example of this situation is driving at night with oncoming automobile headlights.
Microscopic bubbles were induced in IOLs by an incubation process. The bubble-forming incubation process was performed on three sets of test lenses. One set, identified as non-incubated, had no bubbles and served as a negative control. These IOLs were then tested for scattered light. A sample result for scattered light is shown below. As indicated by the figure significant amounts of scattered light were measurable by this technique. FDA intends to use this technique to screen new or unusual lens materials as a test of suitability. [PostME, ProA, Enf]
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Key words: lens testing, intraocular lenses
All intraocular lenses are tested by the manufacturer before they are released for distribution. The test performed by most firms is a simple resolution test performed with the lens in air. More sophisticated testing is possible in theory; however valid concerns about sterility dictate that lens handling be held to a minimum. Thus, air testing has become virtually an industry standard.
Silicone intraocular lenses pose special problems with this kind of testing. The index of refraction of silicone is lower than that of acrylic. To produce the same optical power in the eye, the silicone lens must have greater surface curvature. This presents no problem in situ ; but during manufacture, the testing of these more highly curved surfaces is affected by spherical aberration. Images formed in air with silicone lenses are not as clear as those formed with acrylic IOLs of the same in situ power. Thus, silicone IOLs are released with lower quality standards for resolving power in air. This reduced resolving power has the potential of masking unwanted optical affects that may disturb vision after the IOL is placed in the eye.
OST scientists have been studying this dilemma and have designed a special lens that could be used during silicone lens testing to eliminate the effects of spherical aberration. This special lens, known as a "null lens," would undo the effects of spherical aberration while leaving the other optical properties under test the same. Thus, in principle, silicone lenses could be tested against the more demanding criteria applied to conventional acrylic lenses. The null lens was computer designed using ray-trace software. The null lens consists of two optical elements working together to induce a negative amount of spherical curvature that is then canceled by the IOL under test. For a perfectly made silicone IOL, the resulting resolution should be limited only by the effects of diffraction.
Bench testing of this first design is currently underway. This testing will determine if the tolerances of the computer design are practical. The effects of positioning and manufacturing errors during the alignment and testing procedures must be evaluated. Complete cancellation of spherical aberration requires a perfect match of the null lens to the test IOL. However, the amount of spherical aberration induced by a lens surface depends upon the curvature of the surface. Thus, different IOL powers produce different amounts of spherical aberration. Since a null lens has usefulness over a limited range of powers, research is underway to examine the useful range of powers provided by a single design. [ProA, Stds, PostMS]
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In previous years, OST scientists collaborated with ophthalmologists in two prospective studies on the use of operating microscopes during cataract surgery. The first study evaluated the incidence of retinal photic injury; the second evaluated means for reducing retinal photic injuries. This work established a clear dose-response relationship between the incidence of retinal photic injuries and retinal dose during cataract surgery. OST scientists and their collaborators also established that the incidence of retinal photic injury from operating microscopes might be reduced by using instruments having reduced output in the ultraviolet and visible spectral regions. The use of certain techniques, such as oblique illumination during parts of the procedure that do not require direct illumination of the interior of the eye, can also contribute to a reduction in injuries.
OST scientists also conducted studies in FY 95 to evaluate the risks of retinal photic injury from intraocular illuminators used during vitreo-retinal and other intraocular surgeries. In this work, optical radiation emission data were obtained from several common intraocular illuminators being marketed in the United States. The preliminary results of this study indicate that the risk of retinal photic injury from these devices is similar to that from operating microscopes.
The work on operating microscopes culminated in a draft FDA Health Advisory
on the risks of retinal photic injury from operating microscopes. Additional
activities included active participation in the development of ANSI and
ISO performance standards for the quality of the optical radiation emissions
from operating microscopes. [PostME, ProA, Stds]
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