Stromal wound healing explains refractive instability and haze development after photorefractive keratectomy: a 1-year confocal microscopic study

Effect of Acute Biomechanical Changes on Corneal Curvature After Photokeratectomy

PURPOSE

Unintended hyperopic shift is a common yet poorly understood complication of phototherapeutic keratectomy (PTK) that raises fundamental questions about the etiology of corneal curvature change in PRK and LASIK. We investigated the relative contributions of ablation profile and peripheral stromal thickening to intraoperative PTK-induced central flattening, and propose a biomechanical model of the acute corneal response to central ablation.

METHODS

Fourteen de-epithelialized eye bank globes from seven donors underwent either broadbeam ablation (approximately 100-microm depth, no programmed dioptric change) or sham photoablation in paired-control fashion. Peripheral stromal thickness changes and the pattern of thickness loss across each ablation zone were evaluated by optical section image analysis as predictors of acute corneal flattening.

RESULTS

Relative to sham ablation, keratectomy caused significant anterior corneal flattening (-6.3+/-3.2 D, P = .002). Concomitant peripheral stromal thickening (+57+/-43 microm, P = .01) was a significant predictor of acute hyperopic shift (r = 0.68, P = .047). Ablation pattern bias did not consistently favor hyperopia and was a poor lone predictor of hyperopic shift.

CONCLUSIONS

Unintended keratectomy-induced hyperopic shift is replicable in a human donor model and is associated with significant thickening of the unablated peripheral stroma. This biomechanical response may have a considerable impact on early refractive outcomes in PTK, PRK, and LASIK.

Extracellular matrix and wound healing

Over the years, most researchers have approached corneal epithelial and stromal wound healing as separate events. However, it is becoming increasingly clear that even the simplest epithelial debridement wound results in keratocyte death and a subsequent stromal response to regenerate the affected area. Thus, the interaction between stromal and epithelial healing must be considered to fully understand corneal wound healing. Although wound healing has been an active area of research for many years, the advent of refractive surgery has stimulated research into the regulation of wound repair and provided important insights into the molecular components involved in repair. Epithelial and stromal wound healing are influenced by extracellular matrix components. The purpose of the current article is to review progress in the year 2000 toward understanding mechanisms involved in corneal wound healing and how extracellular matrix affects the healing processes.

Laser in situ keratomileusis enhancement after photorefractive keratectomy

OBJECTIVE

To describe the safety, effectiveness, and predictability of laser in situ keratomileusis (LASIK) for correcting residual myopia after primary photorefractive keratectomy (PRK).

DESIGN

A retrospective, noncomparative case series.

PARTICIPANTS AND INTERVENTION

Thirty-six consecutive eyes of 30 patients underwent LASIK after primary PRK. A Multiscan Schwind excimer laser was used for LASIK enhancement.

MAIN OUTCOME MEASURES

Uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity, refraction, videokeratography, and complications were determined before and after LASIK retreatment. Follow-up was at least 12 months.

RESULTS

Before LASIK, 11.11% of eyes showed a UCVA of 20/40 or better. This increased to 94.44% 12 months after LASIK. A UCVA of 20/25 or better was achieved in 0% before and in 72.22% after retreatment. Refraction +/-0.5 diopters (spherical equivalent) represented 0% of eyes before and 77.78% of eyes after enhancement. Before LASIK, two eyes had significant haze. Haze remained in these two eyes and appeared in another eye.

CONCLUSIONS

Laser in situ keratomileusis proved to be safe and effective for treating residual myopia after PRK. Care must be taken when considering LASIK retreatment in patients with significant haze after primary PRK.

Measurement of corneal sublayer thickness and transparency in transgenic mice with altered corneal clarity using in vivo confocal microscopy

Measurement of sublayer thickness and transparency at cellular level in the living animal are critical to understanding the role of specific transgenes and transgene products in controlling corneal development and maintenance of transparency. Using two different transgenic mouse strains having altered corneal clarity, we have evaluated the ability of in vivo confocal microscopy to measure corneal haze and localize light scattering structures. Projection of 2-D and 3-D image information identified the nature and location of light scattering within the cornea and allowed correlation of unique structural differences to transgene expression. Our findings suggest that in vivo confocal microscopy can be used to identify the effects of transgene expression on mouse corneal transparency.

Pathology of ocular irritation with bleaching agents in the rabbit low-volume eye test

Despite differences in the processes leading to tissue damage, the ocular irritation response to various surfactants, two concentrations of an acid and an alkali, and an acetone, alcohol, aromatic amine, and aldehyde has been shown to depend on the extent of initial injury. The purpose of this study was to assess the extent to which this fundamental relationship exists for bleaching agents in the rabbit low-volume eye test. Ten microl of sodium perborate monohydrate (NaBO3), sodium hypochlorite (NaOCl), 10% hydrogen peroxide (H2O2), and 15% H2O2 was applied directly to the cornea of the right eye of each rabbit. Macroscopic assessments for irritation were made 3 hours after dosing and periodically until 35 days. Light microscopic examinations were conducted on tissues obtained at 3 hr and on 1, 3, and 35 days. In vivo confocal microscopy (CM) and measurements of dead corneal epithelial cells and keratocytes at 3 hours and 1 day were used to characterize quantitatively initial corneal injury, while in vivo CM performed at 3 hours and 1, 3, 7, 14, and 35 days was used to characterize quantitatively the corneal changes over time. The changes with NaBO3 and NaOCl were consistent with mild irritancy. For both, corneal injury was limited to the epithelium and superficial stroma. The changes with 10% H202 and 15% H2O2 were consistent with severe irritation. Both concentrations affected the epithelium and deep stroma, with 15% H2O2 also at times affecting the endothelium. However, unlike other irritants previously studied, with 10% H2O2 and 15% H2O2 there was an incongruity between the extent of epithelial and stromal injury, with stromal injury being more extensive than epithelial injury. A similar, although less dramatic, effect was observed with NaBO3. Additionally, there was still significant keratocyte loss at 35 days with 10% H2O2 and 15% H2O2 even though the eyes at times were considered to be macroscopically normal. These observations highlight the need to include both epithelial and stromal components in an ex vivo or in vitro alternative assay. In conclusion, these results continue to support and extend our hypothesis that ocular irritation is principally defined by the extent of initial injury despite clear differences in the means by which irritants cause tissue damage. Importantly, we have identified unique differences in the ocular injury and responses occurring with bleaching agents that are important to consider in the development and validation of alternative ocular irritation tests to characterize a broad range of materials differing in type and irritancy.

Pathology of ocular irritation with acetone, cyclohexanol, parafluoroaniline, and formaldehyde in the rabbit low-volume eye test

The ocular irritation responses to 11 different surfactants and two concentrations of acetic acid and sodium hydroxide have been shown to depend on the extent of initial injury, despite marked differences in the processes leading to tissue damage. The purpose of these studies was to determine the extent to which this fundamental relationship applies to other nonsurfactants. Ten microl of acetone (ACT). cyclohexanol (CY), parafluoroaniline (PF), or 37% formaldehyde (FA) was directly applied to the cornea of the right eye of each rabbit. Eyes and eyelids were macroscopically scored for signs of irritation beginning 3 hours after dosing and periodically until recovery or 35 days. Tissues were obtained for light microscopic examination after 3 hours and on days 1, 3, and 35. Initial corneal injury was characterized quantitatively at 3 hours and I day using in vivo confocal microscopy (CM) and by postmortem quantitation of dead corneal epithelial cells and keratocytes using a Live Dead Assay (L/D, Molecular Probes) and scanning laser CM. Corneal changes over time were characterized quantitatively using in vivo CM performed at 3 hours and 1, 3, 7, 14, and 35 days. The changes with ACT were consistent with mild irritation. Corneal injury was limited to the epithelium and superficial stroma, with the mean normalized depth of injury (NDI) being less than 10% with the majority of regions showing no stromal injury. Changes with CY and PF were consistent with moderate to severe irritation, and FA caused severe irritation. Specifically, corneal injury by CY and PF tended to involve the epithelium and anterior stroma, with the mean NDI being 10.4% to 23.8%, while injury with FA involved the epithelium, deep stroma, and at times the endothelium. Interestingly, with FA significantly less injury was observed at 3 hours with a dramatic increase in injury observed at 1 day and thereafter. In conclusion, these results continue to support and extend our hypothesis that ocular irritation is principally defined by the extent of initial injury despite clear differences in the means by which irritants cause tissue damage. We believe this approach can be applied to developing alternative assays based on injury to ex vivo eyes or injury to an in vitro corneal equivalent system.

Quantitative characterization of acid- and alkali-induced corneal injury in the low-volume eye test

Defining the extent of initial injury has proven to be a useful basis for differentiating the ocular irritation potential of surfactants; however, the applicability of this method to other types of irritants has not been demonstrated. In the following studies we characterized the extent of corneal injury following exposure to different concentrations of acetic acid and sodium hydroxide (NaOH) in the rabbit low-volume eye test. Groups of rabbits received 3% acetic acid, 10% acetic acid, 2% NaOH, or 8% NaOH and were evaluated in vivo by macroscopic and in vivo confocal microscopic examination and postmortem using a live/dead staining kit and scanning laser confocal microscopic examination. Quantitative assessment of macroscopic scores, corneal surface epithelial cell size, corneal epithelial thickness, corneal thickness, depth of stromal injury, corneal light scattering (confocal microscopy through focusing, CMTF), and number of dead cells was conducted at various times, including the following: at 3 hours and at 1, 3, 7, 14, and 35 days. Based on macroscopic scores, the order of ocular irritancy potential was 3% acetic acid < 2% NaOH < 10% acetic acid < 8% NaOH. Evaluation of the quantitative in vivo and postmortem microscopic live/dead data revealed a slight decrease in epithelial thickness and an increase in dead epithelial cell numbers with 3% acetic acid. With 2% NaOH, significant focal changes in epithelial cell size, epithelial thickness, corneal thickness, and number of dead surface epithelial cells occurred at 3 hours and at 1 day, with injury to only a very small number of corneal stromal keratocytes, despite the presence of epithelial denudation. Changes with 10% acetic acid were similar to those noted with 2% NaOH at 3 hours and 1 day, but these changes were more diffuse and included stromal injury to a depth of 7.2 +/- 9.3% of the corneal thickness, with significant numbers of dead keratocytes. Eight percent NaOH, on the other hand, caused focally extensive injury that averaged 26.3 +/- 18.4% of the corneal thickness at 1 day, with significant light scattering from the cornea, which did not return to normal by 35 days postinjury. Overall, these data indicate that ocular irritation as a result of acetic acid and NaOH was associated with changes similar to those observed with surfactants (ie, slight irritants damage the corneal epithelium, mild and moderate irritants damage the corneal epithelium and anterior stromal cells, and severe irritants damage the corneal epithelium and deep stroma). To our knowledge, this is the first time that the ocular irritation potential for different types of materials (acid/alkali, surfactants) has been shown to be primarily dependent on the initial area and depth of injury.