In contrast to incisional keratotomy, corneas that have undergone photorefractive keratectomy may be difficult to detect by inspection with slitlamp biomicroscopy alone. Eye bank corneas that have undergone high myopic refractive surgical correction could potentially result in substantial postoperative hyperopic correction if used as donor tissue for corneal transplantation. Surface irregularities or displacement of the treated optical zone within the graft in relation to the entrance pupil of the recipient could result in significant induced astigmatism and distortion. This study examines computerized videokeratographic screening of eye bank globes as a strategy for detecting myopic photorefractive keratectomy.
Preoperative and postoperative corneal topographic maps of freshly enucleated human and rabbit eyes that have undergone myopic photorefractive keratectomy with an excimer laser were placed in a globe-fixating device and analyzed using a vertically oriented videokeratoscope. The same system was applied in an actual eye bank setting, and potentially transplantable globes from donors without a history of corneal surgery were analyzed.
Computerized videokeratography using a vertically mounted system reliably detected photorefractive keratectomy in 12 of 12 human eye bank corneas treated by excimer photorefractive keratectomy in a range between −1.5 to −6.0 diopters. This method also detected similar changes on lased rabbit corneas enucleated 6 weeks after excimer surgery. Data processed with the tangential mode yielded a "bull's-eye" topography pattern reflecting central corneal flattening that was more sensitive in detecting myopic corrections than the conventional axial formula–based color maps. False-positive results were not detected in 96 cadaver globes sequentially screened in the eye bank.
Computerized videokeratography represents a feasible method to screen donor globes for myopic photorefractive keratectomy as shown by the in vitro and rabbit models. However, only whole globes and not corneoscleral sections are amenable to processing with this technique. Tangential maps provided greater sensitivity in detecting low myopic corrections than the axial formula–based color maps.
A, Components of the dismantled globe-fixating device. A indicates aluminum block stand; B, donut-shaped plastic cap; C, spring platform; and D, cone-shaped metal casing with adjustable screw. The spring platform is placed at the bottom of the metal casing with the flat piece adjacent to the tip of the screw. B, View from the top showing the human eye bank globe mounted in the globe-fixating device. Centration is determined by measuring the distance from the limbus to the inner edge of the plastic cap with a caliper at all quadrants. C, EyeSys Corneal Analysis System (EyeSys Technologies, Houston, Tex). The modified vertical-oriented videokeratoscope is seen to the right. D, side view of globe-fixating device.
A, Composite axial formula–based color maps of a patient who had −1.5-diopter photorefractive keratectomy (PRK) corrections. Top left, Preoperative map; top right, 3 days postoperatively, the central flattening effect (dark blue) of PRK is readily discerned; bottom left and right, 3 and 6 months postoperatively, respectively. Central flattening is no longer discernible. B, Composite maps of the same patient in part A processed with the tangential mode. Top left, preoperatively; top right, 3 days postoperatively showing "bull's-eye" pattern denoting central corneal flattening; bottom left, 3 months postoperatively (central flattening is visible); and bottom right, 6 months postoperatively.
A, Tangential map generated by computerized videokeratography of a human eye bank globe prior to the performance of myopic photorefractive keratectomy (PRK) with the Summit OmniMed excimer laser (Summit Technology, Waltham, Mass) depicting normal corneal asphericity. B, Corneal topographic maps of the same human eye bank globe in part A after being treated with 1.5 diopters of myopic PRK correction. Left, Tangential map showing central corneal flattening (blue). Right, Profile map showing central inflection corresponding to the central flattening effect of myopic PRK.
A, Videokeratographic analysis of the contralateral control eye of the same rabbit in part B, enucleated at the same time as the other eye. This control eye was not treated with excimer photorefractive keratectomy (PRK). The cornea is aspheric and is steeper centrally than peripherally. B, Videokeratographic analysis of an enucleated rabbit eye 6 weeks after −2.00-diopter PRK. Note inferiorly decentered ablation zone (blue) seen on the tangential map to the left. Profile map to the right reflects the findings seen on the tangential map.
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