An in vivo investigation of the structures responsible for corneal haze after photorefractive keratectomy and their effect on visual function

Corneal Scheimpflug Densitometry in Photorefractive Keratectomy Candidates

PURPOSE

To describe the normative data of corneal densitometry in photorefractive Keratectomy (PRK) candidates by using a Scheimpflug camera.

METHODS

Five hundred twenty-two eyes of 261 PRK candidates that underwent comprehensive optometric and ophthalmic examinations were enrolled in this study. Corneal densitometry values were obtained using a Scheimpflug camera (Pentacam HR; Oculus Optikgeräte GmbH, Wetzlar, Germany) and displayed in grayscale units (GSU).

RESULTS

Of 261 participants, 187 (71.6%) were women. The mean age of the participants was 28.79 ± 5.02 years (range: 21-40 years). The mean entire corneal densitometry was 14.86 ± 2.37 GSU. According to corneal depth, maximum and minimum densitometry values were seen in the anterior (30.28 ± 7.42 GSU) and posterior (9.86 ± 2.08 GSU) layers, respectively (P < 0.001). Based on the annulus over the cornea, the highest and lowest densitometry values were seen in the 10- to 12-mm (21.31 ± 4.26 GSU) and 2- to 6-mm zones (13.26 ± 2.34 GSU), respectively. However, this difference was not seen after 35 years of age. Individuals aged 30 to 35 years showed higher corneal densitometry values compared with subjects aged 20 to 25 years old (P < 0.001). In generalized linear models, the corneal thickness was associated with the densitometry value in the central and posterior corneal layers. There were no significant differences in sex and refractive errors among participants.

CONCLUSIONS

This study provided normal corneal densitometry data in PRK candidates. Corneal densitometry increased with age until 35 years of age and then decreased thereafter. This finding should be considered in patient selection for PRK and their postoperative examinations.

Evaluation of corneal structures in myopic eyes more than twenty-two years after photorefractive keratectomy

The aim of this study is to evaluate corneal epithelial thickness (CET), corneal densitometry (CD) in 84 myopic eyes (57 patients) more than 22 years after photorefractive keratectomy, using anterior segment-optical coherence tomography (AS-OCT) and Scheimpflug imaging system. The CET was significantly higher in all operated eyes than in unoperated eyes in central sector. A statistically significant increase in CD in corneal anterior layer of central sector was shown in groups of operated eyes with greater ablation depth respect to unoperated eyes. While there was no significant difference in CD between the operated eyes groups with lower ablation depth and unoperated eyes. A significant trend toward higher values in anterior CD with deeper ablations in central sector was found. These noninvasive imaging techniques allow to better understand the corneal remodeling process after photoablation and to monitor the patients over time.

Histopathologic Analysis of Explanted KAMRA Corneal Inlays Demonstrating Adherent Fibroconnective Tissue Scar Formation

Objective

To investigate the histologic composition of opaque membranes associated with corneal intrastromal inlays implanted for the surgical treatment of presbyopia.

Methods

This is an observational case series of KAMRA corneal inlays explanted due to the presence of adherent opaque membranes associated with peri-inlay corneal stromal haze and sent for histopathologic analysis. Routine histology was performed in addition to immunohistochemical staining with myofibroblast and keratocyte markers.

Results

Eleven explanted inlay specimens were received, of which, after histologic processing, four demonstrated suf-ficient cellular material for histopathologic analysis. The opaque membranes surrounding the explanted inlays were composed of fibroconnective tissue, and myofibroblasts (positive for smooth muscle actin immunostain) were the predominant cell type. Immunostaining for the keratocyte marker CD34 was negative, confirming that the membranes were the result of a reactive scar-tissue formation process and not simply normal corneal stroma adherent to the explant.

Conclusions

Corneal inlay implantation can lead to the formation of an adherent fibroconnective tissue membrane, suggesting keratocyte-to-myofibroblast transdifferentiation and reactive fibroconnective tissue scar formation that could potentially impact visual potential. Prospective patients should be counseled regarding the risk of this complication, as this may be associated with some risk of incomplete reversibility of the procedure.

Impact of topical anti-fibrotics on corneal nerve regeneration in vivo

Recent work in vitro has shown that fibroblasts and myofibroblasts have opposing effects on neurite outgrowth by peripheral sensory neurons. Here, we tested a prediction from this work that dampening the fibrotic response in the early phases of corneal wound healing in vivo could enhance reinnervation after a large, deep corneal injury such as that induced by photorefractive keratectomy (PRK). Since topical steroids and Mitomycin C (MMC) are often used clinically for mitigating corneal inflammation and scarring after PRK, they were ideal to test this prediction. Twenty adult cats underwent bilateral, myopic PRK over a 6 mm optical zone followed by either: (1) intraoperative MMC (n = 12 eyes), (2) intraoperative prednisolone acetate (PA) followed by twice daily topical application for 14 days (n = 12 eyes), or (3) no post-operative treatment (n = 16 eyes). Anti-fibrotic effects of MMC and PA were verified optically and histologically. First, optical coherence tomography (OCT) performed pre-operatively and 2, 4 and 12 weeks post-PRK was used to assess changes in corneal backscatter reflectivity. Post-mortem immunohistochemistry was then performed at 2, 4 and 12 weeks post-PRK, using antibodies against α-smooth muscle actin (α-SMA). Finally, immunohistochemistry with antibodies against βIII-tubulin (Tuj-1) was performed in the same corneas to quantify changes in nerve distribution relative to unoperated, control cat corneas. Two weeks after PRK, untreated corneas exhibited the greatest amount of staining for α-SMA, followed by PA-treated and MMC-treated eyes. This was matched by higher OCT-based stromal reflectivity values in untreated, than PA- and MMC-treated eyes. PA treatment appeared to slow epithelial healing and although normal epithelial thickness was restored by 12 weeks-post-PRK, intra-epithelial nerve length only reached ∼1/6 normal values in PA-treated eyes. Even peripheral cornea (outside the ablation zone) exhibited depressed intra-epithelial nerve densities after PA treatment. Stromal nerves were abundant under the α-SMA zone, but appeared to largely avoid it, creating an area of sub-epithelial stroma devoid of nerve trunks. In turn, this may have led to the lack of sub-basal and intra-epithelial nerves in the ablation zone of PA-treated eyes 4 weeks after PRK, and their continuing paucity 12 weeks after PRK. Intra-operative MMC, which sharply decreased α-SMA staining, was followed by rapid restoration of nerve densities in all corneal layers post-PRK compared to untreated corneas. Curiously, stromal nerves appeared unaffected by the development of large, stromal, acellular zones in MMC-treated corneas. Overall, it appears that post-PRK treatments that were most effective at reducing α-SMA-positive cells in the early post-operative period benefited nerve regeneration the most, resulting in more rapid restoration of nerve densities in all corneal layers of the ablation zone and of the corneal periphery.

A Critical Overview of the Biological Effects of Mitomycin C Application on the Cornea Following Refractive Surgery

During the last 2 decades, modifying the shape of the cornea by means of laser photoablation has emerged as a successful and popular treatment option for refractive errors. Corneal surface ablation techniques such as photorefractive keratectomy (PRK) and laser-assisted subepithelial keratomileusis (LASEK) offer good refractive results while having a minimal impact on corneal biomechanical stability. Past limitations of these techniques included the long-term regression of refractive outcome and a vigorous healing response that reduced corneal clarity in some patients (giving rise to what is clinically described as “haze”). Mitomycin C (MMC) was introduced as a healing modulator and applied on the corneal surface after refractive surgery to address these drawbacks. This article critically reviews the available evidence on the biological effects, safety, and clinical benefits of the off-label use of MMC in corneal refractive surgery.

Preoperative refraction, age and optical zone as predictors of optical and visual quality after advanced surface ablation in patients with high myopia: a cross-sectional study

OBJECTIVE

To investigate the factors associated with optical and visual quality of advanced surface ablation in high myopia.

DESIGN

A cross-sectional study of high myopic eyes treated with laser epithelial keratomileusis (LASEK)/epipolis laser in situ keratomileusis (Epi-LASIK). SETTING6: Eye and ENT Hospital of Fudan University in Shanghai.

METHODS

One hundred and thirty-eight high myopic eyes (138 patients) (myopia -6 D or more) were examined more than 12 months after LASEK or Epi-LASIK with advanced surface ablation on the MEL 80 excimer laser (Zeiss AG, Jena, Germany). Refraction, higher order aberrations (HOAs) and contrast sensitivity before and after surgery were evaluated. Factors including preoperative refraction, age, gender, central corneal thickness, pupil size, optical diameter, ablation depth and flap creation method were analysed for association with postoperative high-order aberration, contrast and glare sensitivities, and different analytic diameters.

RESULTS

HOAs increased significantly postoperatively (p<0.05), with the most significant change found in Z(spherical aberration). At a 5 mm analysis diameter, increased coma was associated with age; increased spherical aberration difference was associated with age, optical zone diameter and method of epithelial flap creation. At a 3 mm analysis diameter, none of the factors contributed to changes in HOAs. Higher preoperative refractive error was associated with decreased contrast and glare sensitivity at each spatial frequency.

CONCLUSION

A larger optical zone diameter design is recommended to achieve better visual quality in advanced surface ablation for high myopia correction. Age and preoperative refraction may help predict postoperative visual quality.

Corneal Breakthrough Haze After Photorefractive Keratectomy With Mitomycin C: Incidence and Risk Factors

PURPOSE

To identify preoperative and intraoperative factors affecting breakthrough corneal haze incidence after photorefractive keratectomy (PRK) with mitomycin C (MMC).

METHODS

In this retrospective study of PRK performed at the Care Vision Refractive Laser Center, Tel Aviv, Israel, a total of 7535 eyes (n = 3854 patients; mean age ± SD, 26 ± 6 years; 55% men) underwent PRK with intraoperative MMC application. Patients with histories of corneal pathology or surgery were excluded. Incidence, time of onset, and corneal haze severity were documented on follow-up of 118 ± 110 days. Eyes were grouped by preoperative refraction [low (≤-3D), moderate (-3D to -6D), or high (>-6D) myopia; low (≤3D) or high (>3D) astigmatism; low or high hyperopia]; by intraoperative time (above or below 40 seconds); and by MMC application time (above or below 40 seconds). The main outcome measures were incidence, onset time, and severity of corneal haze.

RESULTS

The haze incidence was 2.1% in eyes with high myopia versus 1.1% in those with low to moderate myopia (P = 0.002), and 3.5 times higher in eyes with high than with low astigmatism (P < 0.05). The overall incidence was higher in eyes treated for hyperopia (10.8%) than for myopia (1.3%) (P = 0.0001). In eyes with moderate myopia, the haze incidence was lower in MMC application time ≥40 seconds (0%) than in <40 seconds (1.3%) (P = 0.03). After surgery, a mild early haze incidence peaked at 68.8 ± 6 days and severe late haze at 115 ± 17 days (P = 0.02).

CONCLUSIONS

Hyperopic and large myopic or astigmatic corrections carry higher risk of haze. Longer MMC application might have beneficial haze prevention.

Corneal densitometry after accelerated corneal collagen cross-linking in progressive keratoconus

PURPOSE

To analyze changes in corneal densitometry 3 months after accelerated corneal collagen cross-linking (CXL) measured with Scheimpflug tomography.

METHODS

In this study we reviewed charts and anterior segment data of patients who had undergone accelerated pulsed epithelium-off CXL (30 mW/cm² for 4 min, 8 min total radiation time) for treatment of progressive keratoconus in the Department of Ophthalmology, Goethe University, Frankfurt, Germany. Visual, topographic, pachymetric and densitometric data were extracted before surgery and at the 3-month follow-up. Corneal densitometry measurements from different corneal layers and zones obtained using Scheimpflug tomography (Pentacam HR, Oculus).

RESULTS

The study investigated 12 eyes of 12 patients. The anterior (120 μm) stromal layer within the 0.0 to 2.0 mm and 2.0 to 6.0 mm concentric zones showed a significant elevation of mean densitometry 3 months post-surgery (P = 0.045; P = 0.015) compared to baseline. A mean stromal demarcation line was apparent at a depth of 203.00 μm ± 13.53 (SD). After accelerated CXL, no change in mean corrected distance visual acuity (LogMAR) was observed but a thinning of the cornea measured by a significant reduction in central pachymetry (μm).

CONCLUSION

Accelerated CXL results in an increase in corneal densitometry, particularly in the anterior stromal layer within the two central concentric zones (0.0 to 2.0 mm and 2.0 to 6.0 mm) of the cornea at 3 months postoperatively. The changes in corneal densitometry of the anterior stromal layer did not correlate with postoperative visual acuity or central pachymetry.

Corneal Regeneration After Photorefractive Keratectomy: A Review

Photorefractive keratectomy (PRK) remodels corneal stroma to compensate refractive errors. The removal of epithelium and the ablation of stroma provoke the disruption of corneal nerves and a release of several peptides from tears, epithelium, stroma and nerves. A myriad of cytokines, growth factors, and matrix metalloproteases participate in the process of corneal wound healing. Their balance will determine if reepithelization and stromal remodeling are appropriate. The final aim is to achieve corneal transparency for restoring corneal function, and a proper visual quality. Therefore, wound-healing response is critical for a successful refractive surgery. Our goal is to provide an overview into how corneal wounding develops following PRK. We will also review the influence of intraoperative application of mitomycin C, bandage contact lenses, anti-inflammatory and other drugs in preventing corneal haze and post-PRK pain.

Intraoperative and postoperative corneal thickness change after collagen crosslinking therapy

PURPOSE

To assess intraoperative and postoperative changes in corneal thickness subsequent to riboflavin-UVA (collagen crosslinking [CXL]) treatment.

METHODS

Forty-one eyes of 41 patients (mean age 27.97 ± 6.97 years) were treated with CXL technique. During treatment, isotonic riboflavin was instilled and corneal thickness measurements were obtained at the cornea apex, the thinnest point, and the pupil center at 15 and 30 minutes, then 3 days, 1 week, and 1, 3, 6, and 12 months after surgery using Pentacam HR and an ultrasound pachymeter.

RESULTS

A decrease in corneal thickness was detected 15 minutes intraoperatively with a value of 108.95 ± 48.6 µm, and 112.35 ± 47.3 µm at 30 minutes (p<0.001). Three days after the operation, no deviation was found from the initial values (p = 0.17). No further changes were detected during the follow-up period.

CONCLUSIONS

Isotonic riboflavin solution used during CXL treatment resulted in a significant decrease in corneal thickness, but its effect had disappeared by postoperative day 3.

Corneal refractive surgery: past to present

There have been many historical corneal refractive techniques and procedures developed over the years. From early techniques of radial keratotomy to modern excimer laser techniques, the field of refractive surgery is one of the most rapidly developing in ophthalmology. This review details the historical aspects of the many early techniques up to current techniques used on millions of eyes around the world.

Mechanical versus alcohol-assisted epithelial debridement during photorefractive keratectomy: a confocal microscopic clinical trial

PURPOSE

To compare the effects of mechanical versus alcohol-assisted epithelial debridement on corneal cellular elements after photorefractive keratectomy (PRK) using confocal microscopy.

METHODS

This randomized, clinical trial included 66 eyes from 33 patients with spherical equivalent refraction <-4.00 diopters (D). Mechanical versus alcohol-assisted epithelial debridement was performed during PRK. The right eye of each patient was randomly assigned to one group (mechanical group or alcohol-assisted group) and the fellow eye to the alternate group. Confocal examination was performed preoperatively and at 3 and 6 months postoperatively. Main outcome measures were keratocyte density and maximum anterior stromal light reflectivity.

RESULTS

Mean epithelial healing time was 3.2 ± 0.4 and 3.0 ± 0.3 days in the mechanical and alcohol-assisted groups, respectively (P=.001). Anterior retroablation stromal keratocyte density was 704.3 ± 119.9 cells/mm² and 734.3 ± 103.7 cells/mm² at 3 months (P=.05) and 643.8 ± 134.4 cells/mm² and 696.7 ± 129.6 cells/mm² at 6 months (P=.02) in the mechanical and alcohol-assisted groups, respectively. No significant difference was noted in midstromal and posterior keratocyte density between the two groups. Maximum reflectivity was 61.56 ± 12.64 international units (IU) and 56.93 ± 7.86 IU in the mechanical and alcohol-assisted groups, respectively, 3 months after surgery (P=.018). Corresponding values were 49.46 ± 4.97 IU and 48.98 ± 4.60 IU, respectively, 6 months after surgery (P=.628).

CONCLUSIONS

Due to more adverse effects of mechanical epithelial debridement on anterior keratocyte density and anterior stromal reflectivity, alcohol-assisted epithelial debridement is recommended as the procedure of choice for epithelial removal during PRK in patients with mild myopia.

Omega-3 fatty acids in dry eye and corneal nerve regeneration after refractive surgery

Dry eye (DE) is a multifactorial condition that affects the surface of the eye and induces an inflammatory response. Corneal nerves play an important role in the maintenance of a healthy ocular surface. Here we review corneal structure, nerve architecture, DE conditions, and nerve regeneration following corneal surgery and discuss how n-3 fatty acids affect the health of the cornea. Animal studies show that resolvins, compounds derived from eicosapentaenoic acid (EPA), increase tear volume and decrease inflammation induced by DE. After corneal surgery in rabbits, treatment with nerve growth factor (NGF) or pigment epithelial derived factor (PEDF) in conjunction with docosahexaenoic acid (DHA) increase nerve density and corneal epithelial cell proliferation. Increased synthesis of the novel docosanoid, neuroprotectin D1 (NPD1), was found in corneas after the animals were treated with PEDF and DHA. Topical application of these lipids derived from n-3 fatty acids could be useful in treating DE and prevent clinical complications such as cornea erosion and ulcerations.

Interface Haze Formation After Ultra Thin Flap Laser in Situ Keratomileusis

In this case series, five patients (10 eyes) underwent laser in siter keratomileusis (LASIK) for the correction of moderate myopia and astigmatism with the Schwind Carriazo Pendular microkeratome 90 mum head. Flap (superior hinged) thickness measured intraoperatively was assessed less than 70 mum in all eyes. On first month's postoperative examination, subepithelial mild corneal haze with consequent myopic regression was found in all patients. A significant improvement of haze formation and residual refractive error were observed during the following postoperative months. In conclusion, post-LASIK subepithelial corneal haze after thin flap creation is a temporarily potential complication that could affect patient's refractive error during the first postoperative month.

Trichostatin a inhibits corneal haze in vitro and in vivo

PURPOSE

Trichostatin A (TSA), a histone deacetylase inhibitor, has been shown to suppress TGF-beta-induced fibrogenesis in many nonocular tissues. The authors evaluated TSA cytotoxicity and its antifibrogenic activity on TGF-beta-driven fibrosis in the cornea with the use of in vitro and in vivo models.

METHODS

Human corneal fibroblasts (HSFs) were used for in vitro studies, and New Zealand White rabbits were used for in vivo studies. Haze in the rabbit cornea was produced with photorefractive keratectomy (PRK) using excimer laser. Trypan blue exclusion and MTT assays evaluated TSA cytotoxicity to the cornea. Density of haze in the rabbit eye was graded with slit lamp biomicroscopy. Real-time PCR, immunoblotting, or immunocytochemistry was used to measure alpha-smooth muscle actin (SMA), fibronectin, and collagen type IV mRNA or protein levels. TUNEL assay was used to detect cell death.

RESULTS

TSA concentrations of 250 nM or less were noncytotoxic and did not alter normal HSF morphology or proliferation. TGF-beta1 treatment of HSF significantly increased mRNA and protein levels of SMA (9-fold), fibronectin (2.5-fold), and collagen type IV (2-fold). TSA treatment showed 60% to 75% decreases in TGF-beta1-induced SMA and fibronectin mRNA levels and 1.5- to 3.0-fold decreases in protein levels but had no effect on collagen type IV mRNA or protein levels in vitro. Two-minute topical treatment of TSA on rabbit corneas subjected to -9 D PRK significantly decreased corneal haze in vivo.

CONCLUSIONS

TSA inhibits TGF-beta1-induced accumulation of extracellular matrix and myofibroblast formation in the human cornea in vitro and markedly decreases haze in rabbit cornea in vivo.

Clinical applications of corneal confocal microscopy

Corneal confocal microscopy is a novel clinical technique for the study of corneal cellular structure. It provides images which are comparable to in-vitro histochemical techniques delineating corneal epithelium, Bowman's layer, stroma, Descemet's membrane and the corneal endothelium. Because, corneal confocal microscopy is a non invasive technique for in vivo imaging of the living cornea it has huge clinical potential to investigate numerous corneal diseases. Thus far it has been used in the detection and management of pathologic and infectious conditions, corneal dystrophies and ecstasies, monitoring contact lens induced corneal changes and for pre and post surgical evaluation (PRK, LASIK and LASEK, flap evaluations and Radial Keratotomy), and penetrating keratoplasty. Most recently it has been used as a surrogate for peripheral nerve damage in a variety of peripheral neuropathies and may have potential in acting as a surrogate marker for endothelial abnormalities.

Effect of excimer laser repetition rate on outcomes after photorefractive keratectomy

PURPOSE

To compare the refractive outcomes after photorefractive keratectomy (PRK) for low to moderate myopic corrections using 2 excimer lasers with different repetition rates (200 Hz and 400 Hz).

SETTING

University refractive surgery center.

METHODS

This retrospective study included all consecutive patients who underwent PRK using the 200 Hz or the 400 Hz Allegretto laser platform (WaveLight Laser Technologie AG). Thirty-five patients (70 eyes) and 29 patients (58 eyes) had PRK with the 200 Hz platform and the 400 Hz platform, respectively, using the same surgical technique.

RESULTS

The mean follow-up was 13.22 months +/- 1.16 (SD) (range 11 to 15 months). No intraoperative or early postoperative (eg, late reepithelialization) complications were found in either group. At 1 year, 66 eyes (94.2%) in the 200 Hz group and 56 eyes (96.6%) in the 400 Hz group were within +/-1.00 diopter of the attempted correction. At 3 months, 20 eyes (29%) in the 200 Hz group and 27 eyes (46%) in the 400 Hz group had mild or moderate corneal haze (P = .03). These corneas showed progressive clearing over subsequent months without statistically significant differences in haze formation between the 2 groups. Twelve months after PRK, all corneas in both groups were clear.

CONCLUSION

Photorefractive keratectomy for the treatment of low to moderate refractive errors using a 200 Hz or 400 Hz excimer laser gave comparable results.

Contemporary in vivo confocal microscopy of the living human cornea using white light and laser scanning techniques: a major review

In vivo confocal imaging of the cornea has evolved exponentially over the last few decades and it has increasingly emerged from the laboratory to be used in the clinical setting in relation to inherited corneal diseases, corneal infections, contact lens wear and the effects of corneal surgery. This evolution has led to significant enhancement of our knowledge of the living cornea in both its physiological and pathological states. A number of in vivo confocal microscope devices using white, and more recently coherent, light sources have been developed to provide non-invasive assessment of the corneal microstructure at a lateral resolution of 1-2 microm. The fundamental principles of in vivo confocal microscopy and the key differences between these devices are highlighted in this review. By providing a systematic review of the extensive literature on the human cornea, this perspective paper aims to provide an overview of how in vivo confocal microscopy has contributed to our greater understanding of the human cornea in health, in disease, and following surgery, with a particular emphasis on quantitative data. The utility and limitations of available data are highlighted as are possibilities for the future development of this innovative technology.

Cellular effects of mitomycin-C on human corneas after photorefractive keratectomy

PURPOSE

To investigate the effects of mitomycin-C (MMC) on epithelial and keratocyte cell kinetics after photorefractive keratectomy (PRK) using an in vitro human cornea model.

SETTING

Department of Academic Ophthalmology, Rayne Institute, St. Thomas' Hospital, London, United Kingdom.

METHODS

Twenty-four human eye-bank corneas were placed in a specially designed acrylic corneal holder and cultured using the air-interface organ culture technique for up to 4 weeks. The corneas were divided into 3 groups. Group 1 consisted of 8 human corneas that had -9.00 diopter (D) myopic PRK without MMC application. Group 2 consisted of 8 corneas that had -9.00 D PRK with MMC (0.2 microg/mL) application for 1 minute on the stromal surface after ablation. Group 3 consisted of 8 corneas that had -9.00 D PRK with 2-minute exposure to MMC (0.2 microg/mL). Temporal events in epithelial and keratocyte cell kinetics were evaluated using digital imaging, confocal microscopy, and light microscopy.

RESULTS

Epithelial latency was significantly delayed with MMC application in Groups 2 and 3 (P<.001). Epithelial migration was delayed in Group 3 (2-minute exposure) compared to migration in Group 2 (P<.04), with a consequent delay in epithelial closure (P<.001). Group 3 corneas had poorly differentiated epithelium that was significantly thinner than in Groups 1 and 2 (P<.0001). A significant delay in keratocyte regeneration occurred after MMC application (P<.0005). At 4 weeks, the anterior stromal cell density was significantly lower in Group 3 than Group 2 (P<.001). There were no significant differences in the mid- and posterior stromal keratocyte density between the groups.

CONCLUSIONS

Results suggest that epithelial healing after MMC is characterized by prolonged latency and decreased migration rate dependent on exposure time. Mitomycin C application did not result in increased loss of keratocytes, but it significantly delayed keratocyte repopulation in the anterior stroma. The use of MMC 0.2 microg/mL for 1 minute resulted in optimum modulation of healing characterized by reduced keratocyte activation with normal epithelial differentiation.

Optical Quality Analysis After Surface Excimer Laser Ablation: The Relationship Between Wavefront Aberration and Subepithelial Haze

PURPOSE

To investigate the relationship between mild and moderate corneal haze and the distribution of higher order wavefront aberrations after photorefractive keratectomy (PRK).

METHODS

Thirty-six eyes from 18 patients who underwent PRK were divided into two groups: 10 eyes with corneal haze and 26 eyes without corneal haze (control). All eyes were evaluated up to 6 months after PRK. Wavefront aberrations were measured using a psychophysical wavefront sensor and the NIDEK OPD-Scan. Topography, point spread function, and modulation transfer function maps were obtained from the OPD-Scan.

RESULTS

The mean total higher order aberration was slightly higher in the corneal haze group than in the control group. This difference was not statistically significant. The mean third order coma aberrations were higher and mean fourth order spherical aberrations were lower in the haze group compared with the control group, although neither difference attained statistical significance. The t test values were 1.05, -0.38, -1.10, -0.08, and -0.23, when comparing the mean third, fourth, fifth, sixth, and seventh order aberrations, respectively. None of these differences attained statistical significance. In terms of Zernike coefficients, Z-1 and Z1 showed greater mean root-mean-square (RMS) in the haze group (0.33 and 0.35 microm, respectively) than those for the control group (0.26 and 0.23 microm, respectively) (t=0.71 and P=.49; t=0.84 and P=.43, respectively). However, ZO had lower RMS in the haze group (0.18 microm) than in the control group (0.28 microm). This difference also was not statistically significant.

CONCLUSIONS

In this study comparing the optical aberrations of eyes with and without corneal haze after PRK, corneal haze did not affect the magnitude and distribution of higher order aberrations in a predictable manner.

Clinical corneal confocal microscopy

Confocal microscopy allows non-invasive in vivo imaging of the ocular surface. Its unique physical properties enable microscopic examination of all layers of the cornea and have been used to investigate numerous corneal diseases: epithelial changes, numerous stromal degenerative or dystrophic diseases, endothelial pathologies, corneal deposits, infections, and traumatic lesions. It offers a new approach to study the physiological reactions of the cornea to different stimuli and the pathophysiologic events leading to corneal dysfunction in certain diseases. Confocal microscopy proves to be a powerful diagnostic tool and is especially of value in certain corneal diseases by allowing straightforward and non-invasive recognition of the pathologic conditions.

Effects of ablation diameter on long-term refractive stability and corneal transparency after photorefractive keratectomy

OBJECTIVE

To evaluate the effects of ablation diameter on long-term refractive stability and corneal transparency after photorefractive keratectomy (PRK).

DESIGN

Long-term, prospective, follow-up study.

PARTICIPANTS

One hundred twenty-three eyes treated between 1990 and 1993 at one institution as part of a number of ethical committee-approved clinical studies underwent long-term follow-up at 10 to 12 years after PRK.

INTERVENTION

Photorefractive keratectomy was performed using the Summit Technology UV 200 and OmniMed Excimer lasers with either 4.0-mm (n = 36), 5.0-mm (n = 47), or 6.0-mm (n = 40) optical zones and either -3.0-diopters (D) or -6.0-D myopic spherical corrections, based on the original Munnerlyn algorithms.

MAIN OUTCOME MEASURES

Refractive stability, refractive predictability, best spectacle-corrected visual acuity, and corneal haze.

RESULTS

Refractive outcome at 1 and 10 to 12 years of follow-up was better with 6.0-mm treatments, especially for -6.0-D corrections (P>0.001). The early hyperopic shift was significantly reduced with 6.0-mm zones, with less regression between 1 and 6 months, compared with 5.0- and 4.00-mm PRK, especially for -6.0-D corrections (P<0.001). The postoperative refraction remained stable between 1 and 10 to 12 years in all groups. Objective measurements of haze were less with 6.0-mm compared with 4.0- and 5.0-mm treatments (P<0.001). Night vision problems were significantly less with 6.0-mm PRK (P<0.01). There was no evidence of progressive hyperopic shift, corneal ectasia, or late onset of corneal haze in any of the eyes during the follow-up period.

CONCLUSIONS

Refractive stability was maintained between 1 year and 10 to 12 years after PRK with 4.0-, 5.0-, and 6.0-mm optical zones. A significant linear trend was observed in terms of refractive predictability, early hyperopic shift, regression, corneal transparency, and night haloes with better outcomes in PRK with a larger ablation zone. None of the eyes had sight-threatening complications such as ectasia or late-onset corneal haze during the follow-up.

Development of late-onset subepithelial corneal haze after laser-assisted subepithelial keratectomy with prophylactic intraoperative mitomycin-C

We present a case of dense, visually significant reticular haze that developed approximately 17 months after uneventful laser-assisted subepithelial keratectomy with mitomycin-C (MMC). The patient was successfully treated with manual debridement coupled with phototherapeutic keratectomy and intraoperative MMC.

Confocal Microscopy of Corneal Flap Microfolds After LASIK

PURPOSE

To describe the morphological characteristics of microfolds that appear at the corneal flap after LASIK, as seen under confocal microscopy.

METHODS

Twenty-one eyes that had undergone LASIK were examined, all within 3 weeks to 1 month after surgery. A central scan of the total corneal thickness was obtained by using confocal microscopy in vivo. Confocal images were captured and digitized. The longitudinal orientation (vertical, horizontal, and oblique) and morphological characteristics of the microfolds were described and recorded.

RESULTS

Six eyes had folds at the central corneal flap, visible as linear distortions in the confocal images: one fold had a vertical orientation, two were horizontal, and three were oblique. The folds were visible from the epithelial basal cell layer to the stromal portion of the flap and were deeper than Bowman's layer.

CONCLUSIONS

Confocal microscopy allowed visualization of microfolds after LASIK. With the appropriate software, it is possible to analyze the morphological characteristics of these folds. Flap microfolds after LASIK are deeper than Bowman's layer.

Recovery of corneal subbasal nerve density after PRK and LASIK

PURPOSE

To measure and compare the return of corneal innervation up to 5 years after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK).

DESIGN

Prospective, nonrandomized clinical trial.

METHODS

Eighteen eyes of 12 patients received PRK to correct a mean refractive error of -3.73 +/- 1.30 diopters, and 16 eyes of 11 patients received LASIK to correct a mean refractive error of -6.56 +/- 2.44 diopters. Corneas were examined by confocal microscopy before and at 1, 2, 3, and 5 years after the procedures. Subbasal nerve fiber bundles were measured to determine density (visible length of nerve/frame area) and expressed as micrometers per square millimeters. Differences were compared by Friedman's test and adjusted for multiple comparisons by the Student-Newman-Keuls procedure.

RESULTS

After PRK, mean subbasal nerve density was reduced by 59% at 1 year (2764 +/- 1321 microm/mm(2) [+/-SD]) when compared with preoperative (6786 +/- 1948 microm/mm(2); P < .001). By 2 years, subbasal nerve density (6242 +/- 1763 microm/mm(2)) was not significantly different from density before PRK and remained unchanged to 5 years (5903 +/- 3086 microm/mm(2)). After LASIK, subbasal nerve density was reduced by 51%, 35%, and 34% at 1, 2, and 3 years, respectively (P < .001). By 5 years, subbasal nerves had returned to densities (4441 +/- 2819 microm/mm(2)) that were not significantly different from densities before LASIK (5589 +/- 2436 microm/mm(2)).

CONCLUSION

Corneal subbasal nerve density does not recover to near preoperative densities until 5 years after LASIK, as compared with 2 years after PRK.

In vitro human corneal model to investigate stromal epithelial interactions following refractive surgery

PURPOSE

To develop an in vitro human corneal model to evaluate stromal epithelial interactions following corneal refractive surgical procedures.

SETTING

Department of Academic Ophthalmology, Rayne Institute, St. Thomas' Hospital, London, United Kingdom.

METHODS

Fifty-six human donor corneas procured from the eye bank were placed in a specially designed acrylic corneal holder and were cultured using the air-interface organ culture technique for up to 4 weeks. Corneal refractive surgical procedures such as a simple epithelial defect, 4 diopter (D) and 9 D photorefractive keratectomy (PRK), 4 D and 9 D laser-assisted subepithelial keratectomy (LASEK), and 9 D laser in situ keratomileusis (LASIK) were performed on the model. Temporal events in epithelial and keratocyte cell kinetics were evaluated using digital imaging, confocal microscopy, and light microscopy. Two-way analysis of variance and Student t tests were used to assess statistical significance.

RESULTS

Epithelial healing following PRK was completed by 92 hours +/- 10 (SD) at a rate of 0.58 +/- 0.45 mm2/hour. In LASEK, the epithelial flap was replaced by regenerating peripheral epithelium that showed significant delay in epithelial closure (120 +/- 5 hours) with prolonged latency (24 +/- 4 hours, P<.0001) in comparison with PRK. The magnitude of keratocyte loss corresponded to ablation depth, and keratocyte regeneration was dependent on epithelial closure. In comparison, LASIK corneas showed a lesser percentage of keratocyte loss with poor recovery of keratocyte density in the stromal flap. Epithelial viability and keratocyte density were well preserved in the in vitro human model as observed in control corneas for up to 4 weeks.

CONCLUSIONS

The temporal events in stromal epithelial interactions in the in vitro human model closely mimicked in vivo observations. The human model further avoided species-specific variations and provided a suitable test bed for evaluating newer algorithms and therapeutic regimens following refractive surgery.

Matrix metalloproteinase distribution during early corneal wound healing

AIM

To compare matrix metalloproteinase (MMP) localisation in anterior keratectomy (AK) and lamellar keratectomy (LK) wounds.

METHODS

Wounds were produced in one eye of 24 rabbits. The AK wounds were made to approximately 120 microm in depth and then allowed to re-epithelialise. The LK wounds were of similar depth, but the anterior stroma and epithelium were replaced after a second deeper keratectomy had been performed. Immunohistochemistry was used to localise the MMP-1, -2, -3, and -9 at intervals from 4 h to 14 days following surgery. The contralateral eyes acted as controls.

RESULTS

After an AK wound MMP-1 was present at the leading edge of migrating epithelium after 18 h, while MMP-2 and -9 were localised behind the advancing epithelial edge. The presence of these enzymes rapidly fell to low levels after epithelial closure. There was only faint MMP-3 localisation between days 3 and 7. After an LK wound, MMP-1, -3, and -9 were not detected in the stromal interface, but MMP-2 was present at all time points.

CONCLUSIONS

This study suggests that after an AK wound, MMP-1 is a key mediator of epithelial migration, while MMP-2 and -9, and to a lesser extent MMP-3, may participate in the remodelling of corneal stroma and the reformation of epithelial basement membrane. In contrast, an LK wound results in a much lower stimulus for MMP activation. The action of MMP-2 in stromal repair is thus partly independent of epithelial injury.

Effect of Homoharringtonine on corneal haze after excimer laser photorefractive keratectomy in rabbits

To evaluate the inhibiting effect of Homoharringtonine (HHT) on the corneal haze after excimer laser photorefractive keratectomy (PRK) in rabbits. 18 healthy rabbits which underwent PRK were randomly divided into three groups (A, B and C). The refractive degree of ablation was - 10.0DS in each group. Group A was locally treated with a piece of filter paper soaked with 1 mg/mL HHT for 5 min, and then the entire cornea was repeatedly irrigated with balance solution; Group B was dropped with 0.1 mg/mL HHT after PRK for 3 months; Group C was the control group. Corneal haze, histopathology, response, ect. were investigated. The corneal haze was significantly less in group A, while the difference between group B and group C was insignificant. Keratocytes and fibrocytes in corneal stroma were more active up to 3 months in group B and group C. Intraoperative use of topical HHT can reduce corneal haze after PRK in rabbits.

Pirenoxine prevents oxidative effects of argon fluoride excimer laser irradiation in rabbit corneas: biochemical, histological and cytofluorimetric evaluations

The production of reactive oxygen species (ROS) associated with excimer laser irradiation is recognized as a possible cause of corneal haze following photorefractive keratectomy (PRK). Our work was aimed at investigating in vitro the oxidative effects induced by subablative laser fluences and at demonstrating the protective effectiveness of pirenoxine. Comparative trials of subablative fluence on rabbit eyes with or without 10(-5) M pirenoxine were carried out. Superoxide anion (O(2)(-)), conjugated diene (CD), and thiobarbituric acid reagent substance (TBARS) formation were analyzed. Cellular death was evaluated by flow cytometry. Histological examinations were also performed. No appraisable differences in O(2)(-),CD,andTBARS formation were detected soon after irradiation, whereas they all increased following incubation. Pirenoxine inhibited such increases. Cytofluorimetric and histological observations gave coherent results. The experimental data indicate that oxidative and toxic effects are ascribable to ROS avalanches triggered by laser irradiation-induced photodissociation and are inhibited by pirenoxine.

Grading of corneal transparency

AIM

To examine the academic literature on the grading of corneal transparency and to assess the potential use of objective image analysis.

METHOD

Reference databases of academic literature were searched and relevant manuscripts reviewed. Annunziato, Efron (Millennium Edition) and Vistakon-Synoptik corneal oedema grading scale images were analysed objectively for relative intensity, edges detected, variation in intensity and maximum intensity. In addition, corneal oedema was induced in one subject using a low oxygen transmissibility (Dk/t) hydrogel contact lens worn for 3h under a light eye patch. Recovery from oedema was monitored over time using ultrasound pachymetry, high and low contrast visual acuity measures, bulbar hyperaemia grading and transparency image analysis of the test and control eyes.

RESULTS

Several methods for assessing corneal transparency are described in the academic literature, but none have gained widespread use in clinical practice. The change in objective image analysis with printed scale grade was best described by quadratic parametric or sigmoid 3-parameter functions. 'Pupil image scales' (Annunziato and Vistakon-Synoptik) were best correlated to average intensity; however, the corneal section scale (Efron) was strongly correlated to variations in intensity. As expected, patching an eye wearing a low Dk/t hydrogel contact lens caused a significant (F = 119.2, p < 0.001) 14.3% increase in corneal thickness, which gradually recovered under open eye conditions. Corneal section image analysis was the most affected parameter and intensity variation across the slit width, in isolation, was the strongest correlate, accounting for 85.8% of the variance with time following patching, and 88.7% of the variance with corneal thickness.

CONCLUSION

Corneal oedema is best determined objectively by the intensity variation across the width of a corneal section. This can be easily measured using a slit-lamp camera connected to a computer. Oedema due to soft contact lens wear is not easily determined over the pupil area by sclerotic scatter illumination techniques.

Effect of exogenous keratinocyte growth factor on corneal epithelial migration after photorefractive keratectomy

PURPOSE

To investigate the effect of topical keratinocyte growth factor (KGF) on wound healing after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK).

SETTING

Department of Ophthalmology, Rayne Institute, St. Thomas' Hospital, London, United Kingdom, St. Erick's Eye Hospital, Stockholm, Sweden, and the University of Regensberg, Regensberg, Germany.

METHODS

In a placebo-controlled trial, 24 New Zealand white female rabbits were divided into 3 equal groups. Group 1 (n=8) had myopic PRK (6.0 diopters [D]) using the Technolas 217z laser (Bausch & Lomb). Group 2 and Group 3 had myopic LASIK (6.0 D) with a flap depth of 140 microm and 180 microm, respectively. Topical KGF (20 microg/mL) was administered to half the treated eyes in each group intraoperatively and postoperatively; the other half received placebo eyedrops. Epithelial closure, corneal haze, and keratocyte activation in the rabbit eyes were analyzed and compared with those in placebo-controlled eyes for 5 weeks postoperatively.

RESULTS

In Group 1, the mean reepithelialization after PRK was 0.10 mm2/h +/- 0.02 (SD) in the KGF group and 0.33 +/- 0.05 mm2/h in the control group (P=.001). There was no significant difference in the mean backscatter between the KGF eyes (154 +/- 45.95) and the control eyes (141 +/- 38.45) after PRK (P=.42). Histology revealed reduced epithelial cell layers in the KGF group and comparable keratocyte density as in the control group. In Groups 2 and 3, there was no significant difference in backscatter, epithelial layers, and keratocyte density between KGF and control eyes after LASIK.

CONCLUSIONS

Topical KGF (20 microg/mL) delayed reepithelialization after PRK. It had no effect on stromal wound healing in LASIK eyes with an intact epithelial barrier.

Automated superficial lamellar keratectomy augmented by excimer laser masked PTK in the management of severe superficial corneal opacities

AIM

To assess superficial lamellar keratectomy augmented by excimer laser smoothening with sodium hyaluronate 0.25%, for the management of superficial corneal opacities.

METHODS

Consecutive procedure performed in 14 eyes (13 patients) with an automated microkeratome and excimer laser phototherapeutic keratectomy (PTK) smoothening using sodium hyaluronate 0.25%.

MAIN OUTCOME MEASURES

UCVA, BCVA, pachymetry, degree of haze, ray tracing analysis, and complications. Mean follow up was 12 (SD 1.6) months.

RESULTS

Mean preoperative haze from previous corneal refractive surgeries was 3.5 (SD 0.5) (11/14 cases). In one case, opacity was caused by ocular trauma and in two by infectious keratitis. The mean preoperative UCVA was 0.7 logMAR (0.2 (SD 0.13) decimal value). BCVA was 0.4 logMAR (0.4 (SD 0.17) decimal value). Mean preoperative corneal pachymetry was 508 (SD 62.5) micro m and mean opacity depth measured by corneal confocal microscopy was 115.2 (SD 49.4) micro m. At 6 months, 71.4% of the eyes with previous corneal refractive surgery showed grade I haze or less. Mean postoperative corneal pachymetry at 6 months was 352.36 (SD 49.05) micro m.

CONCLUSIONS

Automated superficial lamellar keratectomy combined with excimer laser PTK smoothening assisted by sodium hyaluronate 0.25% induces a significant improvement of corneal transparency and visual acuity in cases of corneal opacity caused by previous refractive surgery, ocular trauma, and keratitis.

Decreased keratocyte death after laser-assisted subepithelial keratectomy and photorefractive keratectomy in rabbits

PURPOSE

To compare keratocyte loss in the corneal stroma after laser-assisted subepithelial keratectomy (LASEK) and photorefractive keratectomy (PRK) in rabbits.

SETTING

Department of Ophthalmology, University of Essen, Essen, and the Institute of Anatomy, University of Bochum, Bochum, Germany.

METHODS

Laser-assisted subepithelial keratectomy and PRK were performed in rabbits and studied 1, 3, 10, and 20 days after surgery. Excimer photoablation was done unilaterally with a 6.0 mm ablation zone and an 80 microm depth, equivalent to -6.0 diopters. Keratocyte death was analyzed using DNA fragmentation-detecting terminal deoxynucleotidyl transferase deoxy-UTR-nick end labeling (TUNEL) assay and transmission electron microscopy.

RESULTS

Numerous TUNEL-positive keratocytes occurred 1 day after PRK; the number decreased significantly after 3 days. After LASEK, significantly fewer TUNEL-positive keratocytes were noted at the early time points (P<.001 at 1 day; P< or =.05 at 3 days). At 10 days, the number of TUNEL-positive keratocytes decreased in both groups but remained significantly higher after PRK than after LASEK (P<.001). Twenty days after both procedures, no significant signs of keratocyte death were found in the corneal stroma. Transmission electron microscopy revealed few apoptotic keratocytes after LASEK. After PRK, apoptotic keratocytes, characterized by chromatin condensation, apoptotic bodies, and cell shrinkage, were scattered in the stroma. The ultrastructural findings confirmed the results obtained with the TUNEL assay.

CONCLUSIONS

Laser-assisted subepithelial keratectomy induced significantly less apoptotic keratocyte death than PRK and promoted wound healing in the acute phase after photoablation. This procedure may offer the possibility of treating higher myopia with a decreased risk for developing wound healing-related complications known to occur after PRK.

Using the Freiburg Acuity and Contrast Test to measure visual performance in USAF personnel after PRK

PURPOSE

Photorefractive keratectomy (PRK) may be an alternative to spectacle and contact lens wear for United States Air Force (USAF) aircrew and may offer some distinct advantages in operational situations. However, any residual corneal haze or scar formation from PRK could exacerbate the disabling effects of a bright glare source on a complex visual task. The USAF recently completed a longitudinal clinical evaluation of the long-term effects of PRK on visual performance, including the experiment described herein.

METHODS

After baseline data were collected, 20 nonflying active duty USAF personnel underwent PRK. Visual performance was then measured at 6, 12, and 24 months after PRK. Visual acuity (VA) and contrast sensitivity (CS) data were collected by using the Freiburg Acuity and Contrast Test (FrACT), with the subject viewing half of the runs through a polycarbonate windscreen. Experimental runs were completed under 3 glare conditions: no glare source and with either a broadband or a green laser (532-nm) glare annulus (luminance approximately 6090 cd/m) surrounding the Landolt C stimulus.

RESULTS

Systematic effects of PRK on VA relative to baseline were not identified. However, VA was almost 2 full Snellen lines worse with the laser glare source in place versus the broadband glare source. A significant drop-off was observed in CS performance after PRK under conditions of no glare and broadband glare; this was the case both with and without the windscreen. As with VA, laser glare disrupted CS performance significantly and more than broadband glare did.

CONCLUSIONS

PRK does not appear to have affected VA, but the changes in CS might represent a true decline in visual performance. The greater disruptive effects from laser versus broadband glare may be a result of increased masking from coherent spatial noise (speckle) surrounding the laser stimulus.

Update on laser subepithelial keratectomy (LASEK)

PURPOSE OF REVIEW

This study reviews current concepts in laser subepithelial keratectomy (LASEK), variations in LASEK techniques, the role of pharmacology in LASEK, and optimizing outcomes in LASEK.

RECENT FINDINGS

Recent studies continue to support the use of LASEK over that of LASIK in the correction of refractive error. In addition, the advent of pharmacological/biologic intervention, improved algorithms, and wavefront technology have expanded the armamentarium available to ophthalmologists in the maximization of LASEK outcomes.

SUMMARY

LASEK offers an excellent profile in terms of both final outcome (uncorrected visual acuity) and safety (best corrected visual acuity). Untoward effects of LASEK are readily prevented/treated with a variety of agents. Postoperative pain can be ameliorated using topical and oral analgesia. Infection can be most effectively addressed with the fourth generation of fluoroquinolones. Haze may be treated or prevented using numerous remedies namely autologous serum, steroids, ascorbic acid, mitomycin-c, and NSAIDS. Wavefront combined with LASEK rather than with LASIK may offer the best refractive outcome.

Treatment of dense subepithelial corneal haze after laser-assisted subepithelial keratectomy

We report a case of dense and visually significant corneal haze after laser-assisted subepithelial keratectomy (LASEK). Visually significant corneal haze after LASEK can be successfully treated with manual debridement and intraoperative mitomycin-C.

Interaction between injured corneal epithelial cells and stromal cells

PURPOSE

To review the effects of injured corneal epithelial cells on myofibroblastic cell formation in corneal stroma after excimer laser surgery.

METHODS

Denudation of epithelium alone, photorefractive keratectomy (PRK), laser in situ keratomileusis (LASIK), and LASIK with denudation of epithelium were performed in rabbit eyes. Postoperative anterior stromal haze was assessed using a standard scale. Immunohistochemical methods were used to detect alpha-smooth muscle actin (alpha-SMA), a marker of myofibroblastic cells, and type III collagen in subepithelial corneal tissue. Rabbit corneal fibroblasts were cultured on collagen gels with or without cocultured corneal epithelial cells, or with partially scraped epithelial cells, on a companion plate separated by a permeable membrane. Gel thickness was measured daily to evaluate fibroblast-induced gel contraction. The total number of fibroblasts per gel was determined. Myofibroblasts were counted using immunocytochemical identification with alpha-SMA. Transforming growth factor (TGF)-beta was assayed in media on days 3 and 6; these procedures were also carried out in the presence of anti-TGF-beta antibody.

RESULTS

Three weeks after surgery, the presence of alpha-SMA-positive long-extended and spindle-shaped stromal cells as well as synthesis of type III collagen were observed in the subepithelial stromal layer, corresponding to corneal haze, in eyes that underwent PRK and LASIK with denudation of epithelium, but not in those that underwent denudation of epithelium alone or LASIK. Gel contraction, number of alpha-SMA-positive cells, and total cell number were significantly greater on gels with injured epithelial cells than on those without epithelial cells or with uninjured epithelial cells, as was TGF-beta concentration in media. Anti-TGF-beta antibody eliminated these differences.

CONCLUSIONS

The intact corneal epithelium might play an important role in curbing differentiation of myofibroblasts in corneal wound healing. Injured epithelial cells stimulate fibroblast myodifferentiation through one or more soluble factors, including TGF-beta.

Laser in situ keratomileusis in human corneas

PURPOSE

To establish an in vitro model of laser in situ keratomileusis (LASIK) in human donor eyes and to test its validity in comparison with animal models.

SETTING

Department of Anatomy, Friedrich-Alexander Unviersity, Erlangen, Germany.

METHODS

Laser in situ keratomileusis was performed on 20 organ-cultured human corneal buttons. The excimer laser ablations ranged from 0 to 12.0 diopters. The corneas were maintained in culture for up to 6 months and then evaluated with light microscopy and transmission electron microscopy. In addition, corneal sections were immunohistochemically stained for collagen type III, laminin, and fibronectin. The main outcome measures were the ultrastructural and immunohistochemical features of the stromal incision interface.

RESULTS

Ultrastructural investigations in the peripheral cornea revealed a disarrangement of collagen fibers, indicating scar formation. These findings were not observed in the central area. Immunohistochemical staining for fibronectin and collagen type III was detected over the entire stromal incision interface, whereas laminin staining was related to the ingrowth of epithelial cells.

CONCLUSIONS

The morphological changes after LASIK in an organ culture model can simulate the in vivo situation. Therefore, this model appears appropriate to use in further study of corneal wound-healing changes after LASIK.

Aberrant Corneal Nerve Regeneration after PRK

PURPOSE

To report a case of aberrant corneal nerve regeneration after myopic photorefractive keratectomy (PRK).

METHODS

One patient underwent bilateral PRK to correct a refractive error of -5.50 D in each eye. Thirteen months after the original PRK, the left eye underwent an uncomplicated PRK reoperation to correct a regression of -1.00 D. The central corneas were examined by confocal microscopy preoperatively in both eyes, at 1 and 2 years after the original PRK in the right eye, and before and 1 and 2 years after the PRK reoperation in the left eye.

RESULTS

Aberrant anterior stromal nerves with a coiled course and irregular branching pattern were identified 22 micro m deep to the most anterior keratocyte layer at 1 year after the PRK reoperation in the left eye and remained unchanged 2 years after reoperation. No abnormal stromal nerves were identified in the left eye before the reoperation or at any time in the right eye.

CONCLUSION

Aberrant regeneration of corneal stromal nerves may occur after myopic PRK reoperation.

Persistent Haze and Disorganization of Anterior Stromal Collagen Appear Unrelated Following Phototherapeutic Keratectomy

PURPOSE

The theoretical effects on corneal transparency induced by changes in collagen fibril packing following phototherapeutic keratectomy were compared to changes in objective measurements of haze.

METHODS

Phototherapeutic keratectomy was performed on the right eyes of four young rabbits; left eyes were used as controls. Postoperative slit-lamp measurements of haze were taken at regular intervals up to 19 months. Wounded stromas were studied by synchrotron x-ray diffraction to calculate the average interfibrillar spacing of the collagen fibrils. These data were combined with transmission electron microscope measurements, and the summation of scattered fields method was used to predict the transmission of visible light.

RESULTS

Objective measurements of haze were higher than the baseline control throughout the study. Electron micrographs of anterior stroma in 8-month-old wounds displayed irregularly spaced and poorly organized fibrils and x-ray diffraction indicated larger mean interfibrillar spacing compared to the controls. However, the predicted transmission of visible light through the anterior stromal scar tissue was not significantly different than normal.

CONCLUSIONS

Following phototherapeutic keratectomy, anterior corneal collagen fibrils were more widely spaced and unevenly organized than in the normal rabbit cornea. However, this did not cause a significant loss of transparency and was therefore unlikely to contribute to haze.