Gabriel Simon

Gel injection adjustable keratoplasty

Gel injection adjustable keratoplasty (GIAK) is a new refractive surgical procedure designed for the correction of myopia by injection of a gel substance into the peripheral corneal stroma. This paper describes the GIAK technique and reports the results obtained in 21 fresh cadaveric eyes using the procedure. After a deep interlamellar canal has been dissected with a helicoid spatula surrounding the visual axis, the gel is injected under keratometric control. In the 1st group of 14 eyes, the degree of correction varied from 2.2 to 12.8 D ; there was a direct relationship between the amount of gel injected and the keratometric change. In the 2nd group of 7 eyes, the adjustability of the procedure was demonstrated. Through partial extraction of the gel and subsequent modification of the corneal curvature, the previously induced keratometric changes could be reversed or altered to a specific extent. Following the initial injection of gel to a targeted flattest meridian power of 35 D, an average value of 35.8 ± 0.5 D was achieved in these eyes. We subsequently attempted to increase the flattest meridian to 40 D by partial removal of the gel and achieved a mean value of 40.2 ± 0.4 D. Average presurgical astigmatism of 1.497 ± 0.737 D was reduced to a postsurgical reading of 0.941 ± 0.590 D (P = 0.005, Students paired t-test), indicating an autocorrection by autodistribution of the gel inside the canaliculus (Laplaces law). GIAK is a simple, inexpensive procedure designed for the correction of myopia that has the added advantage of reducing preexisting astigmatism without encroaching on the visual axis.

Effect of corneal hydration on Goldmann applanation tonometry and corneal topography

BACKGROUND Measured ocular tension is reported to be positively related to corneal thickness. It has been speculated that an edematous cornea will cause low measurements. We studied the effects of corneal hydration on applanation tonometry and biomechanical behavior of the cornea. METHODS This study examines the effects of corneal hydration on applanation tonometry on four human cadaver eyes with the Goldmann Schmidt tonometer and on one cadaver eye with the Tono-Pen 2. Hydration was controlled osmotically and assessed by ultrasonic pachometry. True intraocular pressure (IOP) was controlled and was measured by a piezoelectric pressure transducer. The effect of hydration and IOP on anterior curvature was also assessed with computer topography. RESULTS The results of this study show an inverse relationship between corneal hydration and ocular tension measured with applanation tonometry. The Goldmann readings decreased by 9.2 +/- 0.7 mm Hg as the cornea is taken from minimum to maximum hydration. Anterior curvature was affected very little by hydration or increased IOP. CONCLUSIONS This study has shown, in vitro, that the tonometer readings are inversely related to its thickness when the cornea is edematous. The clinical implication is that when performing applanation tonometry, the hydration status of the cornea must be carefully examined to avoid possible diagnostic error.

Optics of the corneal epithelium

BACKGROUND The refractive effect of the corneal epithelium in relation to the cornea as a whole is relatively unknown, yet it may affect the efficacy of keratorefractive surgical procedures, most notably photorefractive keratectomy and epikeratoplasty. This article investigates the refractive effect of the epithelium on the cornea. METHODS We measured the corneal keratometry in 10 fresh human eye-bank eyes with and without epithelium. All readings were performed at 2.0- and 3.6-millimeter diameter zones with an automated keratometer. RESULTS When the epithelium was removed, there was an increase in corneal refractive power in every case. The mean change was +1.03 D (range, 0.55 to 1.85 D) at the central 2.0-millimeter diameter zone and +0.85 D (range, 0.29 to 1.60 D) at the 3.6-millimeter diameter zone. We also found a change in both the power and axis of astigmatism in most cases. CONCLUSIONS The corneal epithelium plays an active role in determining the final surface power of the cornea. It accounts for an average of 1.03 D of the power of the eye at the central 2-millimeter diameter zone. The difference in astigmatism suggests that the epithelium does not form a layer of uniform thickness over Bowmans layer. It appears that the epithelium attempts to minimize abrupt changes in surface contour. These findings suggest that in refractive procedures such as photorefractive keratectomy the refractive contribution of the epithelium must be taken into account to improve predictability.

Computer-assisted corneal topography: accuracy and reproducibility of the topographic modeling system.

BACKGROUND Assessment of the efficacy and stability of novel refractive surgery techniques and comparative studies require accurate and reproducible corneal mapping technology, ideally to +/- 0.25 D. A vertically mounted Topographic Modeling System (TMS) system was evaluated. MATERIALS AND METHODS Six calibrated PMMA spheres (30.00, 35.00, 43.00, 50.00, 55.00, and 60.00 D) were used. Two examiners performed three measurements on each test sphere and compared appropriate data with a previously tested and calibrated digital auto-keratometer (SK-1, precision < 0.25 D). The accuracy and reproducibility of all 25 rings of the TMS were studied as a function of focus (250 to 500 microns), centering (100 to 300 microns), and integrity of the inner and outer reflected placido images to mimic tear breaks at the corneal apex and peripheral vignetting and shadows produced by eyelids and eyelashes. RESULTS The accuracy of the 25 rings decreased from the center to the periphery. The deviation scores (mean difference of three readings between measured and calibrated surface powers) of rings 3 through 25 were 100% within +/- 0.25 D only for the 43.00-diopter sphere. They were 56%, 68%, 43%, 13%, and 17% within +/- 0.25 D respectively for the 30.00-, 35.00-, 50.00-, 55.00-, and 60.00-diopter spheres. For 500 microns of defocusing, we observed a mean deviation of 0.35 D for the flattest sphere to 2.30 D for the 60.00-diopter sphere. For the same amount of defocusing, the error produced by the TMS increased progressively with the sphere dioptric power. Masking the first four and six rings, one by one, showed topographic maps to be valid only when all four central placido images were fully read by the computer. CONCLUSIONS The TMS clinical performance could be improved by 1) increasing focus depth of field, 2) blocking data processing when central rings are incomplete or missing, 3) increasing the accuracy over the 30.00- to 60.00-diopter range, and 4) providing the user with calibration spheres.

Ultraviolet Solid-state Laser (213-nm) Photorefractive Keratectomy

BACKGROUND The pulsed ultraviolet 213-nm solid-state laser has been demonstrated as an alternative to the gas argon-fluoride 193-nm excimer laser for photorefractive keratectomy (PRK). The authors studied the clinical course and histopathologic changes occurring in rabbit corneas after PRK with a 213-nm solid-state laser. METHODS The 213-nm output of neodymium:YAG frequency-quintupled laser was used to create 5-mm optical zone ablations in seven pigmented rabbit corneas. The radiant exposure was 250 mJ/cm2 delivered through a computer-controlled scanning delivery system with a spot size of 0.5 mm. The target ablation was 4.0 diopters with an estimated ablation depth of 40 microns. A clinical estimate of corneal epithelial healing and stromal haze was made at intervals over the 3-month study period. Animals were killed immediately after ablation, or at 10 days, 1 month, or 3 months after ablation. Corneal tissue was preserved for light microscopy and transmission electron microscopy at each study interval. RESULTS All corneas re-epithelialized within 10 days postoperatively. Anterior stromal haze was clinically visible at 3 days, increased until approximately 1 month, and then gradually decreased over the succeeding 2 months. Residual subepithelial haze was visible at 3 months. Results of histopathologic study documented normal epithelium healing over time; the basement membrane retained its regular thickness and hemidesmosomes were abundant at 3 months. The anterior stroma had an increased number of fibroblasts at 10 days, many of which remained until 1 month. A mild, transient, cellular reaction occurred throughout the thickness of the stroma and the endothelium. CONCLUSION Using the 213-nm ultraviolet solid-state laser with a scanning delivery system, PRK shows a similar clinical course and histopathologic findings to the 193-nm excimer PRK study in rabbits. It is a clinically viable procedure for refractive surgery and requires further human clinical trails to determine its efficacy.

Cataract surgery with a mid-infrared endo-laser system

With most current cataract surgery techniques, an intraocular lens (IOL) is implanted in the capsular bag to emetropize the eye for distance vision. Modern IOLs are made of flexible materials (e.g., silicone and acrylic elastomers) allowing the surgeon to fold and insert the IOL through a smaller limbal incision (4 mm), thus reducing the number of sutures. When using a scleral pocket technique, suturing of such small wounds might not be required. Recently, IOLs having 2 foci (multifocal IOLs) have been introduced. These implants give the patient a second focal plane at normal reading distance, but the double image reduces both visual acuity and contrast sensitivity. However, with all present surgical techniques, the patient loses the natural ability to accommodate. By directing laser energy into a flexible fiber, cataract removal might be performed endoscopically while minimizing trauma to healthy tissue. Bath successfully demonstrated the use of the 308 nm XeCl excimer laser for cataract removal. A significant drawback, however, lies in the fluorescence induced by the 308 nm laser pulses which may cause significant retinal damage. The use of UV radiation also raises serious concerns about carcinogenesis and cataractogenesis risk to both the patient and the surgeon.

Endothelial Damage by the Corneal Hessburg-Barron Vacuum Trephine

BACKGROUND Tissue damage in corneal transplantation procedures was previously assessed in comparative pilot studies using vacuum-assisted metal blade trephines and a noncontact hydrogen fluoride laser trephine system. With the laser, endothelial damage was less than 20 x 30 microns at the edge of the posterior corneal perforation. METHODS In this study, a second generation vacuum-assisted Hessburg-Barron trephine was evaluated in full-thickness penetrating trephination performed in 16 rabbit eyes and eight fresh human donor eyes. RESULTS We found a relatively large, 150-micrometer wide, annular zone of endothelial damage on the recipient cornea located 1.25 mm away from the trephined opening. Our study demonstrates that endothelial damage is linked to the instruments vacuum fixation ring. CONCLUSION With this trephine, endothelial damage may be avoided by using partial-depth trephination, filling the anterior chamber with viscoelastic and completing the procedure using a metal blade or diamond knife.

Modification, calibration, and comparative testing of an automated surgical keratometer.

A new digital surgical keratometer (SK-1, Canon Inc) designed to fit existing operating microscopes and provide quantitative measurements at a nominal 2.0- and 3.6-mm diameter optical zone of the central cornea was modified for single foot-switch operation by the surgeon. The instrument was tested against three calibrated steel spheres, an astigmatic test-jig, and compared with a standard clinical keratometer (CLC, A.O. Co) in two studies involving six eye bank eyes and five healthy human corneas. Assessed with the steel spheres, the SK-1 calibration error was -0.042 diopters at the 3.6-mm diameter and +0.207 D at the 2.0-mm chord, while it was -0.120 D for the CLC. With cylindrical power greater than 0.50 D, the astigmatic angle was accurate to +/- 1 degree. After correction for calibration error, the CLC readings on eye bank eyes were 1.00 D higher in average (range +0.45 to +1.90) than those obtained with the SK-1 which gave lower keratometric values at the 2-mm chord diameter (-0.254 D, range: +0.05 to -0.7). The human subject corrected readings at the 3.6-mm chord were within 0.137 D (range -0.09 to +0.41) from the CLC values. A mean corrected difference of +0.068 D (range -0.27 to +0.30) was found between the two chords. The SK-1 instrument was easier to use than the CLC. A more accurate calibration and the addition of automated averaging and video display of readings as well as of a motorized Placido disc would further augment its clinical usefulness.

Laser Scleral Buckling for Retinal Reattachment

The holmium:yttrium aluminum garnet (Ho:YAG) laser produces infrared light that is absorbed inside scleral tissues, which increases local temperature, thereby shrinking collagen and inducing a buckle effect. Scleral indentation was induced in ten cadaver eyes by shrinkage of scleral collagen fibers by using a pulsed solid-state Ho:YAG (2.1-microns) laser with fiberoptic delivery. The amount of laser-induced buckling effect is controlled by selecting laser treatment settings such as beam spot, radiant exposure, and number of pulses. With treatment using 11.3 +/- 1.2 J/cm2 of laser radiant exposure and five pulses, laser-induced scleral shrinkage affected only the external two thirds of the scleral tissue. No thermal damage or disruption was observed in subjacent retinal pigment epithelium, choroid, or retina. The coupling of two appropriately selected lasers may allow laser-induced scleral buckling and transscleral retinal photocoagulation by using the same laser probe for retinal reattachment.

Artificial orbit system for experimental surgery with enucleated globes

A portable system for holding, accurately positioning, and restoring the physiologic integrity of enucleated globes during experimental anterior segment procedures and pars plana vitrectomy is described. A 0 to 500-mm Hg vacuum fixation ring and positioning apparatus mechanically locks globes into known spatial position while they are supported in a 2 to 6-mm Hg pressurized, pliable socket designed to match the size and pressure of a natural orbit. A gravity infusion system and an implantable piezoelectric pressure sensor in conjunction with a syringe and stopcock assembly allow for controlled inflation of globes to intraocular pressure (IOP) levels from 0 to 70 mm Hg, and to over 300 mm Hg. The fixation vacuum level, and intraocular and extraocular pressures are simultaneously monitored on 3-digit LED displays, and permanent records of each can be generated by routing data to storage devices. The anterior segment remains accessible at all times for surgical procedures and diagnostic measurements. The entire system is portable, allowing globes to be transported to multiple experimental stations, while all pressure settings and the position of the globe are maintained. To demonstrate applications of the artificial orbit system, we describe experiments using it to calibrate applanation tonometers, and to study the relationship between IOP and corneal curvature as well as the effect of trephination procedures and limbal vacuum fixation on IOP.