Cadaver Studies Aid Phakic IOL Research

By Cheryl Guttman

Charleston - Studies of cadaver eyes can help identify potential design issues of phakic IOLs before the lenses go to the clinical trial stage, reports Liliana Werner, MD, PhD, assistant professor of ophthalmology, Storm Eye Institute, Medical University of South Carolina, Charleston.

Dr. Werner conducts research at the Storm Eye Institute’s Center for Research on Ocular Therapeutics and Biodevices and the Magill Research Center for Vision Correction. Collaborating with David J. Apple, MD, Center Director, and other researchers, Dr. Werner is examining IOL-anatomic relationships in cadaver eyes implanted in the laboratory with different phakic IOL models.

In addition, pathological post-mortem studies of human eyes that had been implanted with different lenses are underway to derive information on IOL-ocular tissue interactions. These studies can facilitate the design of lenses with reduced potential to cause such complications as cataracts, corneal endothelial cell loss, and pupil ovalization (Figure 1), she notes:

<"There has been renewed interest in phakic IOLs as an alternative to corneal refractive surgery modalities because of their potential to correct any refractive error.> However, no particular model has yet emerged as the design of choice. By allowing characterization of the intraocular behaviour of different types of IOLs and identifying problems that may arise in association with different design features or material types, our studies can guide the design process."

Lessons about Sizing
Evaluation of the different types of phakic IOLs is possible using the various techniques for cadaver eye preparation that have been developed at the research center (Figure 2). Using the Miyake-Apple posterior view and a “side-view” technique proposed by Assia and Apple, the investigators have been analyzing the fixation of posterior chamber models. Fresh human cadaver eyes are being used to study the effects of inaccurate sizing of angle-fixated anterior chamber IOLs. In those particular investigations, the white-to-white distance is measured and then under- or oversized lenses are implanted purposely.

"The choice of lens overall diameter is critical for success with implantation of this type of IOL. To date, however, there is no perfect system to determine the internal diameter of the anterior chamber angle and we are dependent on the estimation of the external white-to-white distance, which may be inaccurate and often misleading. Pre-clinical determination of the behavior of new lens designs in situations of under- or oversizing is important, as some may eventually be more susceptible than others to complications related to inappropriate sizing," said Dr. Werner.

Closed system studies of phakic anterior chamber lenses can be performed with non-fixated human cadaver eyes only when the quality and transparency of the cornea is adequate. Otherwise, an open-sky system is used in which the eyes are prepared before IOL implantation with the “corneal button-hole” technique (Figure 3). After injecting Karnovsky’s solution to preserve the anterior chamber geometry and dimensions, a 6.5-mm disc of central cornea is excised to allow direct observation of the subsequently implanted IOL and anterior chamber angle. Specimen preparation with the corneal button-hole technique has also facilitated the evaluation of foot plate positioning for angle-fixated anterior chamber IOLs, Dr. Werner noted.

With interest increasing in the development of foldable phakic IOLs, human cadaver eyes are being used as well to evaluate the incision size necessary for insertion/injection of these devices. In addition, studies are being done to characterize the unfolding behaviour, fixation, and sizing of foldable phakic IOLs.

Other research at the Center is focusing on defining the basic dimensions of the anterior chamber and seeking correlations between corneal diameter and both the angle-to-angle and sulcus-to-sulcus distance (Figure 4). For these investigations, the white-to-white distance is measured in cadaver eyes at the horizontal and vertical meridians. Then, the eye is fixated with a special technique and cut sagitally at the center of the cornea horizontally (3-9 o’clock meridian) or vertically (6-12 o’clock meridian). Using the same surgical calipres used for determining the white-to-white distance, the specimens are then used for direct measurement of the diameters of the anterior chamber and sulcus.

"Results to date indicate an absence of correlation between the white-to-white and the anterior chamber and sulcus diameters. Thus, development of new methods to estimate these diameters are necessary for more successful use of phakic IOLs,” noted Dr. Werner.

Findings from those anatomical studies will be published soon in a peer-reviewed journal.
In addition to Dr. Apple, Dr. Werner’s collaborators at the Center include Drs. Andrea M. Izak, Suresh K. Pandey, Rupal H. Trivedi, and Josef M. Schmidbauer.