Jesús Merayo-Lloves

Validation of the Estimation of Corneal Aberrations From Videokeratography in Keratoconus

PURPOSE To validate the estimation of corneal aberrations from videokeratography against a laser ray tracing technique that measured total eye aberrations, in eyes without keratoconus (ie, cornea-dominated wave aberrations). METHODS We measured total and corneal wave aberrations of three eyes diagnosed with keratoconus by slit-lamp microscopy and corneal topography: two eyes from one patient with early keratoconus and one eye with more advanced keratoconus. Total aberrations were measured with laser ray tracing. Corneal aberrations were obtained from corneal elevation data measured with a corneal videokeratoscope using custom software that performs virtual ray tracing on the measured front corneal surface. RESULTS The keratoconus eyes showed a dramatic increase in aberrations (both corneal and total) particularly coma-like terms, which were 3.74 times higher on average than normal. Anterior corneal surface aberrations and total aberrations Were similar in keratoconus. This similarity was greater for the early keratoconus patient, suggesting a possible implication of the posterior corneal surface in advanced keratoconus. CONCLUSIONS The similarity found between corneal and total aberration patterns in keratoconus provided a cross-validation of both types of measurements (corneal topography and aberrometry). Both techniques were useful in diagnosing and quantifying optical degradation imposed by keratoconus.

Corneal Biomechanical Changes after Collagen Cross-Linking from Porcine Eye Inflation Experiments

PURPOSE Understanding corneal biomechanics is important to refractive or therapeutic corneal treatments. The authors studied the corneal response to variable intraocular pressure (IOP) in porcines eyes after UV collagen cross-linking (CXL), in comparison with untreated eyes. METHODS Twenty-three enucleated eyes were treated with standard CXL conditions (365 nm, 3 mW, 30 minutes), and 15 contralateral eyes served as control. Eyes (within a humidity- and temperature-monitored wet chamber) were measured by Scheimpflug corneal three-dimensional topographer. Images were obtained automatically while IOP either remained constant (14 eyes) or increased (24 eyes) by 40 mm Hg and then decreased (4-mm Hg steps). Measurements were performed immediately after treatment and 24 hours later. Corneal geometry was analyzed as a function of IOP, and whole globe stress-strain curves were calculated. RESULTS Instillation of riboflavin-dextran solution reduced corneal thickness (by 281 +/- 5 microm). Cross-linking produced a 1.54x reduction in corneal thinning and 2.8x reduction in corneal apical rise with increased IOP. Anterior and posterior cornea flattened with increased IOP (less flattening in CXL eyes) and became steeper with decreased IOP. The horizontal meridian flattened significantly (P < 0.01) more than the vertical meridian. Youngs modulus was higher in cross-linked eyes (1.096 +/- 0.30 kN/m(2)) than in non-cross-linked eyes (0.692 +/- 0.30 kN/m(2)). Hysteresis in nontreated eyes was also larger than in cross-linked eyes. CONCLUSIONS Cross-linking stiffened porcine corneas significantly. Both experimental data and stress-strain analysis are valuable for finite element models to improve understanding of CXL and its predictability. Although differences are expected between human corneas in vivo and porcine corneas ex vivo, the results are consistent with clinical data found in patients. The apparent biomechanical anisotropy of pig corneas must be confirmed in humans.

Corneal and total optical aberrations in a unilateral aphakic patient

Purpose: To measure corneal and total optical aberrations in the normal and treated eye of a unilateral aphakic patient to (1) cross‐validate techniques in an eye in which corneal and total aberrations should be almost identical (aphakic eye) and (2) compare the interactions of corneal and internal aberrations in the normal eye with those in the aphakic eye. Setting: Instituto de Óptica, Consejo Superior de Investigaciones Científicas, Madrid, Spain. Methods: Aberrations in both eyes of a unilateral aphakic patient were measured using laser ray tracing. Corneal aberrations were obtained from corneal elevation data measured with a corneal videokeratoscope (Humphrey Instruments) using custom software that performs virtual ray tracing on the measured front corneal surface. Results: There was a 98.4% correspondence between the total and corneal aberration pattern in the aphakic eye (6.5 mm pupil). In the normal eye, the total spherical aberration was much lower than the corneal spherical aberration; this did not occur in the aphakic eye. Conclusions: The posterior corneal surface contributed slightly to the aberrations in the normal cornea (2% at most). The crystalline lens appears to play a compensatory role in the total spherical aberration in normal eyes.

Presence of mitomycin-C in the anterior chamber after photorefractive keratectomy

PURPOSE: To assess the presence of mitomycin‐C (MMC) in hen aqueous humor after photorefractive keratectomy (PRK). SETTING: Instituto Universitario de Oftalmobiología Aplicada, Faculty of Medicine, University of Valladolid, and Department of Analytical Chemistry, Faculty of Sciences, University of Valladolid, Valladolid, Spain. METHODS: Mitomycin‐C 0.02% was applied topically for 2 minutes to a right hens eye after PRK (Group A) and to the left eye with intact epithelium (Group B). At different time points (10, 30, 60, 360, and 720 minutes), aqueous humor was extracted and high‐performance liquid chromatography was performed to detect and quantify MMC levels. RESULTS: The mean maximum drug concentration of MMC measured in the aqueous humor was 187.250 μg/L ± 4.349 (SD) in Group A and 93.000 ± 4.899 μg/L in Group B, both detected 10 minutes after topical application. Statistically significant differences were found between Groups A and B at 10, 30, and 60 minutes, with decreasing MMC levels in both groups but a higher concentration in Group A. After 360 minutes, MMC levels were undetectable in Group B and after 720 minutes in Group A. CONCLUSIONS: Mitomycin‐C was detectable in the aqueous humor of the hen eye after topical application in PRK‐treated eyes and in eyes with intact epithelium. The presence of MMC is of concern as it may lead to ocular toxicity in the long term.

What Causes Eye Pain

Eye pain is an unpleasant sensory and emotional experience including sensory-discriminative, emotional, cognitive, and behavioral components and supported by distinct, interconnected peripheral and central nervous system elements. Normal or physiological pain results of the stimulation by noxious stimuli of sensory axons of trigeminal ganglion (TG) neurons innervating the eye. These are functionally heterogeneous. Mechano-nociceptors are only excited by noxious mechanical forces. Polymodal nociceptors also respond to heat, exogenous irritants, and endogenous inflammatory mediators, whereas cold thermoreceptors detect moderate temperature changes. Their distinct sensitivity to stimulating forces is determined by the expression of specific classes of ion channels: Piezo2 for mechanical forces, TRPV1 and TRPA1 for heat and chemical agents, and TRPM8 for cold. Pricking pain is evoked by mechano-nociceptors, while polymodal nociceptors are responsible of burning and stinging eye pain; sensations of dryness appear to be mainly evoked by cold thermoreceptors. Mediators released by local inflammation, increase the excitability of eye polymodal nociceptors causing their sensitization and the augmented pain sensations. During chronic inflammation, additional, long-lasting changes in the expression and function of stimulus-transducing and voltage-sensitive ion channels develop, thereby altering polymodal terminal’s excitability and evoking chronic inflammatory pain. When trauma, infections, or metabolic processes directly damage eye nerve terminals, these display aberrant impulse firing due to an abnormal expression of transducing and excitability-modulating ion channels. This malfunction evokes ‘neuropathic pain’ which may also result from abnormal function of higher brain structures where ocular TG neurons project. Eye diseases or ocular surface surgery cause different levels of inflammation and/or nerve injury, which in turn activate sensory fibers of the eye in a variable degree. When inflammation dominates (allergic or actinic kerato-conjunctivitis), polymodal nociceptors are primarily stimulated and sensitized, causing pain. In uncomplicated photorefractive surgery and moderate dry eye, cold thermoreceptors appear to be mainly affected, evoking predominant sensations of unpleasant dryness.

Minor Influence of Myopic Laser In Situ Keratomileusis on the Posterior Corneal Surface

PURPOSE To check whether myopic LASIK induces changes on the posterior corneal surface. METHODS A Scheimpflug system (Pentacam; Oculus, GmbH, Wetzlar, Germany) was used to measure preoperative and postoperative posterior corneal topography in 27 eyes (of 14 subjects) that had undergone standard myopic LASIK surgery (attempted corrections between -1.25 and -8.50 D) and on 18 nonoperated eyes (9 subjects). A hybrid porcine-plastic eye model was developed to validate the measurement technique. Longitudinal displacement of the posterior corneal apex and changes of the apical radius of curvature and asphericity were computed. RESULTS Measurements on a hybrid model eye of known posterior corneal geometry showed that the measured posterior corneal radius of curvature was minimally affected by the geometry of the anterior surface. The measurements on patients showed that, on average, the only relevant (though clinically unimportant) change in radius of curvature and asphericity occurred the first day after surgery (DeltaR = -28 +/- 34 microm and DeltaQ = -0.06 +/- 0.06). No statistically significant change was observed afterward. The change in radius was more pronounced in the vertical direction than in the horizontal direction. On average, there was no significant displacement of the posterior corneal apex. Individual changes over time did not show a systematic trend across patients, and control subjects experienced changes of the same order of magnitude. CONCLUSIONS The Pentacam Scheimpflug system can be used reliably to assess changes in the posterior corneal radius of curvature after LASIK. There is no evidence of surgically induced changes in the corneal posterior surface beyond 1 week after surgery.

Experimental evaluation of optimized ablation patterns for laser refractive surgery

A new experimental model based on plastic (Filofocon A) artificial eyes was used to study the ablation profiles and the outcomes of three state-of-the-art refractive surgery excimer lasers provided with narrow-beam flying spot and optimized algorithms (Ladarvision 4000, Alcon; Technolas 217 Z100, Bausch and Lomb; Allegretto wave Eye-Q, Wavelight). The 3-D ablation patterns produced by myopic laser corrections (-9, -6 and -3 D) on flat and spherical surfaces of Filofocon A were measured using high resolution optical profilometry. We found significant differences across lasers in the shape and depth of the ablation patterns. A comparison of the ablation patterns on flat and on spherical surfaces provided a measurement of the laser efficiency losses from the center to the periphery at each point of the spherical plastic corneas. This effect also varied across lasers, depending on their fluence (120-400 mJ/cm(2)). Estimates of the post-operative corneal shapes were obtained from the measurement on Filofocon A and plastic-corneal tissue correction factors. The predicted post-operative corneal ablation shape, ablated volume, asphericity and spherical aberration varied across lasers, as well as the relative contribution of ablation pattern designs and efficiency losses to the increased asphericity. Although the results show that the algorithms have been optimized to reduce the induction of spherical aberration, they would still benefit from the application of correction factors for efficiency effects derived from a systematic approach using experimental plastic models. These models have proved useful (1) to assess the outcomes of different lasers or ablation algorithms, (2) for precise calibration and testing of the lasers, and (3) to calculate experimental correction factors for efficiency effects.

Intrastromal corneal ring segments: visual outcomes from a large case series

Background:  To evaluate the clinical safety and efficacy of implanted Ferrara intrastromal corneal ring segments in a large sample of patients with ectatic corneal disease.

Intrastromal Corneal Ring Segments Implantation in Patients With Keratoconus: 10-Year Follow-Up

PURPOSE To evaluate the long-term safety and efficacy of Ferrara intrastromal corneal ring segments (ICRS) (Ferrara Ring; AJL, Boecillo, Spain) in patients with keratoconus. METHODS The chart records of 36 eyes of 30 patients with keratoconus implanted with ICRS, operated on between July 1996 and January 2002, were retrospectively reviewed. The following parameters were studied: uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), keratometry (K), and central corneal thickness. The outcomes were evaluated at 5 and 10 years after ICRS implantation. RESULTS The mean UDVA (logMAR) improved from 1.01 ± 0.28 (20/200 Snellen) to 0.71 ± 0.38 (20/100 Snellen) at 5 years (P < .05) and 0.67 ± 0.25 (20/90 Snellen) at 10 years (P = .735). The mean CDVA (logMAR) improved from 0.45 ± 0.45 (20/55 Snellen) to 0.24 ± 0.19 (20/35 Snellen) at 5 years (P < .05) and 0.29 ± 0.09 (20/38 Snellen) at 10 years (P = .292). The mean maximum K value decreased from 54.99 ± 6.33 to 50.58 ± 5.11 D at 5 years (P < .05) and 50.65 ± 5.17 D at 10 years (P = .854). The mean minimum K value decreased from 48.85 ± 5.70 to 46.90 ± 5.08 D at 5 years (P < .05) and 47.12 ± 4.22 D at 10 years (P = .945). The central corneal thickness decreased from 457.42 ± 58.21 to 421.34 ± 74.12 μm at 5 years (P = .039) and 434.32 ± 77.65 μm at 10 years (P = .427). CONCLUSIONS Intrastromal corneal ring segments can effectively improve UDVA and CDVA 10 years after implantation in patients with keratoconus.

Inferior Intrastromal Corneal Ring Segments in Paracentral Keratoconus With No Coincident Topographic and Coma Axis

PURPOSE To assess the visual and refractive outcomes of implantation of intrastromal corneal ring segments (ICRS) in keratoconic eyes with no coincident topographic and comatic axes. METHODS Forty-one keratoconic eyes of 39 patients with no coincident topographic and comatic axes were implanted inferiorly with a Ferrara-type ICRS (Keraring SI6; Mediphacos Inc., Belo Horizonte, Brazil) of 150° of arc with a thickness of 150, 200, and 250 μm. Uncorrected distance visual acuity (UDVA), corrected distance visual acuity (CDVA), corneal coma-like root mean square, and residual refractive errors analyzed using vector analysis were recorded before and 6 months after the ICRS implantation. RESULTS Mean UDVA was 0.76 ± 0.41 logMAR before and 0.53 ± 0.46 logMAR after surgery (P = .0006). CDVA was 0.13 ± 0.14 logMAR before and 0.07 ± 0.09 logMAR after surgery (P = .0007). Two eyes (4.9%) lost two lines or more of CDVA, 3 eyes (7.32%) lost one line, 16 eyes (39.02%) had no change in CDVA, 9 eyes (21.95%) gained one line, and 11 eyes (26.83%) gained two lines or more of CDVA. The safety index was 1.10. Spherical equivalent was significantly reduced after ICRS implantation (P < .001). Corneal coma-like root mean square changed from 0.80 ± 0.53 μm before surgery to 0.61 ± 0.59 μm after surgery (P = .02) for 4.5 mm of pupil size. CONCLUSIONS One Ferrara-type ICRS of 150° of arc with a thickness of 150, 200, or 250 μm implanted inferiorly may reduce both astigmatism and corneal coma-like aberrations in keratoconic eyes with no coincident topographic and comatic axes, providing an improvement of UDVA and CDVA values.