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Biomedical subjects

G Simon

Publications and source records attributed to G Simon.

491 records · Page 28Linked to original sources
PubMedDate not supplied

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.

Artifacts↗
PubMedDate not supplied

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 instrument's 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.

Animals↗
PubMedDate not supplied

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 Bowman's 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.

Astigmatism↗
PubMedDate not supplied

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.

Cornea↗