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

[Optical quality after refractive corneal surgery]

Correction of myopia, hyperopia and astigmatism within its indicated margin by means of refractive corneal surgical procedures such as LASIK and surface ablation (e.g. PRK) is one of the standard procedures in ophthalmology. Now that advances in the fields of surgical techniques and the technical devices employed have further progressed in terms of safety and predictability, research also focuses on optical quality. "Optical quality" is not a clearly defined parameter, but can be captured indirectly by means of directly measured data. One has to start with the anatomical properties of the eye, which determine the optical images on the retinal level. The quality of the retinal image influences the eye's function, i.e. acuity and contrast perception. Finally, there is the subjective perception of the image we receive. "Optical quality" as such is reflected by the patient's evaluation of this image perception. Three phenomena are especially responsible for deterioration of the quality of the retinal image: diffraction, aberrations and dispersion. Some of the methods for measuring optical quality are subjective questionnaires, functional testing procedures for measuring visual acuity and contrast sensitivity, optical measuring procedures for the determination of optical quality, as well as biomicroscopy, aberrometry and corneal topography for assessing anatomical changes.

Wound healing following refractive surgery in hens

The wound-healing response is critical to the outcome of refractive surgery and studying wound healing contributes to an understanding of the pathophysiology of other corneal injuries. Animal models allow research to be conducted with sufficient samples and under controlled parameters. We studied the hen to determine the healing process from clinical, biophysical, and biological standpoints after photorefractive keratectomy (PRK). PRK (-6.0 diopters) was performed in hen eyes. At 3, 6, 12, 24, 48, and 72 h and 5, 7, 15, 30, and 60 days postoperatively, we studied the clinical follow-up, objective measurements of light transmission (direct transmittance), apoptosis by TUNEL assay, proliferation by immunocytochemical analysis of 5-bromo-2'-deoxyuridine, and expression of alpha smooth muscle actin (SMA) in myofibroblasts in the corneas. Hen corneas reepithelialize quickly. Haze developed from 5 to 60 days after surgery and was correlated with the appearance and finalization of the expression of SMA. The direct transmittance of light was low during the first 15 days and improved at 30 and 60 days. TUNEL-positive cells were observed 3 h after surgery and the numbers decreased thereafter. Epithelial proliferation began at 12 h and was greater at 48 h, while stromal cell proliferation began at 24 h and was greater at 72 h. The hen cornea is anatomically similar to the human cornea, and the manner in which it heals is a good model for studying different surgical techniques and pharmacologic assays.

Corneal keratocyte deficits after photorefractive keratectomy and laser in situ keratomileusis

PURPOSE

To measure changes in keratocyte density up to five 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 17 eyes of 11 patients received LASIK to correct a mean refractive error of -6.56 +/- 2.44 diopters. Corneas were examined by using confocal microscopy before and six months, one year, two years, three years, and five years after the procedures. Keratocyte densities were determined in five stromal layers in PRK patients and in six stromal layers in LASIK patients. Differences between preoperative and postoperative cell densities were compared by using paired t tests with Bonferroni correction for five comparisons.

RESULTS

After PRK, keratocyte density in the anterior stroma decreased by 40%, 42%, 45%, and 47% at six months, two years, three years, and five years, respectively (P < .001). At five years, keratocyte density decreased by 20% to 24% in the posterior stroma (P < .05). After LASIK, keratocyte density in the stromal flap decreased by 22% at six months (P < .02) and 37% at five years (P < .001). Keratocyte density in the anterior retroablation zone decreased by 18% (P < .001) at one year and 42% (P < .001) at five years. At five years, keratocyte density decreased by 19% to 22% (P < .05) in the posterior stroma.

CONCLUSIONS

Keratocyte density decreases for at least five years in the anterior stroma after PRK and in the stromal flap and the retroablation zone after LASIK.

A Standardized Drawing Scheme to Document Corneal Changes Following Refractive Corneal Surgery

PURPOSE

To present a systematic and standardized drawing scheme for unambiguous and reproducible documentation of corneal changes after incisional techniques, coagulative procedures, and lamellar and surface ablation excimer surgery.

METHODS

Standardized symbols in five colors representing specific corneal conditions after incisional surgery (eg, radial keratotomy, astigmatic keratotomy), coagulative procedures (eg, laser thermokeratoplasty, conductive keratoplasty), surface ablation (eg, photorefractive keratectomy, laser subepithelial keratomileusis, epi-LASIK), and LASIK are used to record corneal changes in frontal and sectional views.

RESULTS

Corneal changes following refractive corneal surgery were documented.

CONCLUSIONS

The drawing scheme permits specific features to be followed in the clinic in a clear and unambiguous manner.

Response of the Cornea for up to Four Years After Photorefractive Keratectomy for Myopia

PURPOSE

To analyze the long-term corneal topographic changes 4 years after myopic photorefractive keratectomy (PRK).

METHODS

This study comprised 15 patients (30 eyes) who had PRK surgery with a scanning-spot excimer laser (Chiron Technolas 217C; Bausch & Lomb, Dornach, Germany) and were followed up to 4 years after surgery. The eyes were subdivided into three groups according to the preoperative spherical equivalent refraction. Corneal topographic maps were obtained for all eyes with a Placido disc topographer. Preoperative and follow-up topographical data were imported into a custom software program, which computed the average composite corneal maps and difference maps for each study group to quantify the anterior corneal changes following laser ablation. The software delineated three concentric zones of the corneal surface to characterize the regional corneal remodeling following the surgery.

RESULTS

A significant central corneal steepening (approximately 0.25 D, P < .001) was calculated between the 1- and 4-year postoperative maps in all study groups. A significant steepening (P < .001) of the corneal periphery was also noted for the lower myopic ablations whereas a peripheral flattening (P < .001) was observed for the deeper ablations between 1 and 4 years after surgery.

CONCLUSIONS

The anterior corneal surface was observed to remodel for up to 4 years after surface ablation, steepening a mean of approximately 0.25 D.

How the cornea heals: cornea-specific repair mechanisms affecting surgical outcomes

In mammals, penetrating injuries typically heal by deposition of fibrotic "repair tissue" that fills and seals wounds but does not restore normal function. Excessive deposition of fibrotic repair tissue can lead to pathologies involving excessive scarring and contracture. In the cornea, fibrotic repair presents special challenges affecting both clarity and shape of the cornea. With the increasing popularity of surgical techniques that alter corneal refractive errors, understanding of cornea repair mechanisms has acquired new significance. The cornea has unique anatomic, cellular, molecular, and functional features that lead to important mechanistic differences in the process of repair in comparison with what occurs in skin and other organs. Moreover, corneal function calls for special outcomes. This review addresses these features from the viewpoint of the authors' research on factors of importance to understanding and improving surgical outcomes.

Analysis of dense-medium light scattering with applications to corneal tissue: experiments and Monte Carlo simulations

Dense-medium scattering is explored in the context of providing a quantitative measurement of turbidity, with specific application to corneal haze. A multiple-wavelength scattering technique is proposed to make use of two-color scattering response ratios, thereby providing a means for data normalization. A combination of measurements and simulations are reported to assess this technique, including light-scattering experiments for a range of polystyrene suspensions. Monte Carlo (MC) simulations were performed using a multiple-scattering algorithm based on full Mie scattering theory. The simulations were in excellent agreement with the polystyrene suspension experiments, thereby validating the MC model. The MC model was then used to simulate multiwavelength scattering in a corneal tissue model. Overall, the proposed multiwavelength scattering technique appears to be a feasible approach to quantify dense-medium scattering such as the manifestation of corneal haze, although more complex modeling of keratocyte scattering, and animal studies, are necessary.

Confocal Microscopy of Disposable and Nondisposable Heads for the Moria M2 Microkeratome

PURPOSE

To study the quality of the flap obtained with two different heads, disposable and nondisposable, and two different speeds, low and high, using the same Moria M2 microkeratome (Moria, Antony, France).

METHODS

In a prospective randomized study, 51 consecutive eyes (28 patients) underwent myopic LASIK (-4.39 +/- 2.15 diopters). The eyes were randomly assigned to four groups related to cutting head model (disposable and nondisposable) and cut speed (low and high). In all cases, the intended cut was 160 microm and the same M2 unit was used. During surgery, superficial measurements of the flap and the surgeon's opinion about the quality of the cut were recorded. One month after LASIK, the cut depth, interface particle density, and transparency of the cornea (confocal wound healing opacity index) were measured with a confocal microscope.

RESULTS

Flaps performed with a disposable cutting head and high speed were statistically significantly larger than the flaps created in the other groups (Kruskal-Wallis test; P = .041). No statistical differences were noted in cut depth, number of particles, and wound healing opacity index. The surgeon's opinion parameters were similar in all groups.

CONCLUSIONS

The Moria M2 microkeratome is safe and provided reproducible results, regardless of cutting head and speed. Disposable and nondisposable head models were similar in their outcome in the quality control model used in this study.

The effect of the fourth-generation fluoroquinolones on corneal reepithelialization after penetrating keratoplasty

PURPOSE

To compare the effects of topical moxifloxacin and gatifloxacin on corneal reepithelialization after penetrating keratoplasty.

METHODS

In this single-center, open-label, prospective study, 46 consecutive patients requiring penetrating keratoplasty were alternately assigned to receive either postoperative moxifloxacin 0.5% ophthalmic solution (n = 23; Vigamox) or gatifloxacin 0.3% ophthalmic solution (n = 23; Zymar). The primary outcome measure was the area of the epithelial defect measured by slit-lamp evaluation during the postoperative period.

RESULTS

The mean area of the epithelial defect was similar in both treatment groups at baseline (P = 0.104), but it was significantly larger in the moxifloxacin group than in the gatifloxacin group at day 4 (23.9 mm2 versus 10.6 mm2, respectively; P < .001) and day 7 (3.8 mm2 versus 0.85 mm2, respectively; P = 0.03). At every follow-up visit after day 4, there were more eyes with complete reepithelialization of the corneal graft in the gatifloxacin group than in the moxifloxacin group (statistically significant at days 7 and 14).

CONCLUSIONS

The results of this preliminary study suggest that the postoperative use of gatifloxacin in corneal transplant patients may allow the early stages of corneal graft reepithelialization to proceed more quickly than does treatment with moxifloxacin. These findings suggest that a larger, masked, randomized clinical study in a more homogeneous patient population is warranted to further investigate the relative effects of moxifloxacin and gatifloxacin on postoperative corneal wound healing.

Quantitative assessment of ophthalmic viscosurgical device retention using in vivo confocal microscopy

PURPOSE

To develop and apply a new laboratory method for in vivo quantitative assessment of the retention of ophthalmic viscosurgical devices (OVDs) following phacoemulsification.

SETTING

Department of Ophthalmology, University of Texas Southwestern Medical Center, Dallas, Texas, USA.

METHODS

Studies of both eyes of New Zealand White rabbits were performed. Six OVDs were evaluated: Provisc and Healon (both sodium hyaluronate 1%), Healon5 (sodium hyaluronate 2.3%), Amvisc Plus (sodium hyaluronate 1.6%), Viscoat (sodium hyaluronate 3%-chondroitin sulfate 4%), and a new viscous-dispersive OVD, DisCoVisc (sodium hyaluronate 3%-chondroitin sulfate 4%). The OVD was injected to fill the anterior chamber and a phacoemulsification needle inserted with the tip positioned just anterior to the lens capsule. Simulated phacoemulsification was performed for 1 minute using flow rates of 20, 40, and 60 mL/min; a vacuum level of 300 mm Hg; and ultrasound power of 60% using a Legacy phacoemulsification unit. The needle was removed, and silicone oil (1000 centistokes) was injected into the anterior chamber. The distance between the corneal endothelium and the OVD-silicone oil interface was measured using in vivo confocal microscopy through-focusing (CMTF).

RESULTS

Significant differences in residual thickness were found between the OVDs tested. Specifically, the residual thickness of both DisCoVisc (mean 324.5 microm +/- 163.7 [SD]) and Viscoat (251.4 +/- 100.9 microm) was significantly greater than that of Provisc (9.5 +/- 16.7 microm), Healon (3.8 +/- 11.3 microm), Healon5 (0.6 +/- 2.4 microm), and Amvisc Plus (65.6 +/- 134.0 microm) (P < .05, Dunn test). Ophthalmic viscosurgical device retention was greatest with DisCoVisc; however, there was no statistically significant difference between DisCoVisc and Viscoat in residual thickness. The flow rate did not have a significant effect on the residual thickness (Friedman 2-way analysis of variance by ranks).

CONCLUSIONS

Residual OVD thickness following simulated phacoemulsification could be quantitatively measured using in vivo CMTF. The results are consistent with human surgical experience in that the dispersive OVD (Viscoat) was better retained than the cohesive compounds. DisCoVisc, a new viscous-dispersive OVD, also showed retention compared with Viscoat under the experimental conditions.

Atomic force microscopy analysis of normal and photoablated porcine corneas

We showed the capabilities and accuracy of atomic force microscopy (AFM) techniques for imaging and analyzing the corneal epithelium and the photoablated corneal stroma. Eight normal porcine corneas, half of which were ablated using a scanning-spot excimer laser, were examined. All the corneas were imaged in balanced salt solution after fixation in glutaraldehyde. In the normal untreated corneas we observed the epithelial surface showing the typical polygonal cells and presenting numerous microprojections. The superficial epithelial cells were classified in three types as a result of the anterior-surface roughness measurements. AFM images of the photoablated corneal specimens showed undulations and granule-like features on the ablated stromal surface, specific to 193-nm ArF laser irradiation. Nevertheless, the quantitative analysis confirmed the precision of excimer laser surgery in removing sub-micrometric amounts of tissue. AFM showed to be a high-resolved imaging tool for the scanning of both native as well as photoablated corneal specimens. Also, this technique permits precise topographic analysis of the corneal plane, in the nanometric scale, of which smoothness is an important physical characteristic and necessary to achieve an optimal optical quality of the eye.

Roughness of Excimer Laser Ablated Corneas With and Without Smoothing Measured With Atomic Force Microscopy

PURPOSE

To analyze the surface roughness of porcine corneas after excimer laser ablation with and without the smoothing procedure by means of atomic force microscopy.

METHODS

Excimer laser photorefractive keratectomy (PRK) was performed on eight porcine corneas. Immediately following the procedure, smoothing was performed on four corneas using a viscous solution of 0.25% sodium hyaluronate. The corneas were examined in balanced salt solution after fixation in 2.5% glutaraldehyde solution using atomic force microscopy. Quantitative analysis of the ablated stromal surface topography was performed using the section analysis module of the atomic force microscopy software. Repeated measurements were made over small areas (< or =50 microm2) near the center of each ablation, with a vertical resolution of <1 nm.

RESULTS

Images of the ablated stromal surface showed undulations and granule-like features on the ablated surface of the specimens. The specimens on which the smoothing procedure was performed (root-mean-square [RMS] rough: 0.152 +/- 0.014 microm) were more regular (P < .001) than those on which PRK alone was performed (RMS rough: 0.229 +/- 0.018 microm).

CONCLUSIONS

Atomic force microscopy analysis requires a simpler preparation of the specimens with respect to that necessary for scanning electron microscopy; for this reason, atomic force microscopy techniques are more reliable for the study of biological surfaces and prove to be a feasible method to establish the differences when comparing different laser techniques. Our investigations highlight that although the laser cut of scanning-spot excimer laser systems is precise in removing even the smallest amounts of tissue, the smoothing technique may still be useful to reduce post-ablation roughness.

Spatial Distribution of Keratan Sulfate in the Rabbit Cornea Following Photorefractive Keratectomy

PURPOSE

To examine the keratan sulfate content of the stroma and to assess its correlation with the healing process (epithelialization and keratocyte density) after photorefractive keratectomy (PRK).

METHODS

Using an Aesculap Meditec MEL 70(G-scan) excimer laser, -6.0 diopters (6.0-mm diameter, 82 microm photoablation depth), PRK was carried out on the right eye of 32 New Zealand pigmented rabbits. After enucleation (at days 1, 4, 7, 14, and 28 and months 2, 3, and 7; sub-groups of 4 animals), fluorescent immunohistochemistry was performed on sections from the central comea using monoclonal mouse anti-keratan-sulfate antibody, immunohistochemistry with proliferative cell nuclear antigen antibody, and hematoxylin-eosin histology. The left, untreated eyes served as controls. Cellular morphology and spatial distribution of keratan sulfate were recorded, stromal thickness measured, and keratocyte density calculated.

RESULTS

Keratan sulfate was found on the surface of migrating epithelial cells in the early stage (from days 1 to 7). In the stroma, three phases were noted. (Phase 1) Day 1 to 14, intense granular fluorescence appeared in the anterior stroma with hypocellularity. (Phase 2) Month 1 to 2, newly synthesized lamellar keratan sulfate restored the repopulating anterior stroma. Endothelial cells became keratan sulfate positive, while in the posterior stroma, lamellar-form keratan sulfate increased from week 1 and peaked at month 1 (100% increase). (Phase 3) Month 2 to 7, remodeling and deposition of keratan sulfate was noted, which was produced in phase 2.

CONCLUSIONS

Keratan sulfate was found in the epithelium, stroma, and endothelium. By controlling the interlamellar spacing, keratan sulfate plays a role in postoperative edema, remodeling of the corneal stroma, and simultaneous regulation of inflammation after PRK.

Corneal epithelial healing after photorefractive keratectomy: analytical study

PURPOSE

To characterize the velocity of epithelial migration after photorefractive keratectomy (PRK) with 3 different corneal ablation patterns.

SETTING

Department of Ophthalmology, Catholic University of Rome, Rome, Italy.

METHODS

Fifteen patients (30 eyes) with mild to moderate myopia and with simple to compound myopic astigmatism were enrolled for this study. The surgical procedure consisted of standardized PRK with final smoothing performed using the Technolas Keracor 217C excimer laser. The reepithelialization process was evaluated at 0 hours, 20 hours, 40 hours, and 60 hours after surgery using a digital photo camera and custom software for measurement. Digital analysis of the images was performed. Corneal topographies were taken at 1 month, 3 months, 6 months, and 12 months after PRK.

RESULTS

The mean speed of radial migration in the 10 eyes (33%) in the low spherical ablation group was 0.087 mm/h +/- 0.008 (SD). This was significantly higher than that found in the 10 eyes (33%) in the high spherical ablation group (mean speed 0.078 +/- 0.007 mm/h; P<.001) and in the 10 eyes (33%) in the cross-cylinder ablation group (mean speed 0.055 +/- 0.014 mm/h; P<.001).

CONCLUSION

Analysis of the data shows that epithelial migration along the photoablated corneal surface depends on the ablation pattern. The epithelial sliding is highly influenced by local variations in the curvature of the stromal surface. The data demonstrate that faster epithelial wound healing after PRK is predictive of optimal visual performance.

Epithelial Innervation of Human Cornea

PURPOSE

Evaluation of a new method to visualize distribution and morphology of human corneal nerves (Adelta- and C-fibers) by means of fluorescence staining, confocal laser scanning microscopy, and 3-dimensional (3D) reconstruction.

METHODS

Trephinates of corneas with a diagnosis of Fuchs corneal dystrophy were sliced into layers of 200 microm thickness using a Draeger microkeratome (Storz, Germany). The anterior lamella was stained with the Life/Dead-Kit (Molecular Probes Inc.), examined by the confocal laser scanning microscope "Odyssey XL," step size between 0.5 and 1 microm, and optical sections were digitally 3D-reconstructed.

RESULTS

Immediate staining of explanted corneas by the Life/Dead-Kit gave a complete picture of the nerves in the central human cornea. Thin nerves running parallel to the Bowman layer in the subepithelial plexus perforate the Bowman layer orthogonally through tube-like structures. Passing the Bowman layer, Adelta- and C-fibers can be clearly distinguished by fiber diameter, and, while running in the basal epithelial plexus, by their spatial arrangement. Adelta-fibers run straight and parallel to the Bowman layer underneath the basal cell layer. C-fibers, after a short run parallel to the Bowman layer, send off multiple branches penetrating epithelial cell layers orthogonally, ending blindly in invaginations of the superficial cells. In contrast to C-fibers, Adelta-fibers show characteristic bulbous formations when kinking into the basal epithelial plexus.

CONCLUSIONS

Ex-vivo fluorescence staining of the cornea and 3D reconstructions of confocal scans provide a fast and easily reproducible tool to visualize nerves of the anterior living cornea at high resolution. This may help to clarify gross variations of nerve fiber patterns under various clinical and experimental conditions.

Correction of large amblyopiogenic refractive errors in children using the excimer laser

PURPOSE

We sought to determine whether laser subepithelial keratomileusis (LASEK) and photorefractive keratectomy (PRK) are effective methods for correcting amblyopiogenic refractive errors in children.

METHODS

Thirty-six eyes in 35 amblyopic children, who ranged in age from 4 to 16 years (mean, 8.4 years), received treatment for large magnitude ametropia. Seventy-two percent (25/35) of the children had a neurobehavioral disorder and/or were noncompliant with spectacle or contact lens wear. Myopia ranged from -3.25 to -24.25 D (mean, -11.48 D); one patient had hyperopia of +5.87 D. Correction was tailored to match the refractive error of the nonamblyopic eye. VISX Star S2/S3 excimer lasers were used in manual or auto-tracking modes, and corneal centration was achieved using brief, general anesthesia. Mean follow-up was 29.2 months (range, 4-42 months).

RESULTS

Myopia correction averaged -8.95 +/- 2.89 D (range, -3.25 to -15.50). Eighty-nine percent (31 children) were corrected to within +/- 1.00 D of goal refraction and the remaining 11% to within 2.0 D of the goal (most were undercorrected). Acuity improved postoperatively in 97%; by 1 optotype line in 37% and by 2 or more in 60%. No child lost acuity. Binocularity improved in 69% (24/35) and remained the same in 31%. Corneal haze measured grade 0-1 in 78%, grade 2 in 14%, and grade 3-4 in 8%. Myopic regression exceeding congruent with 1.0 D/year (0.08 D/month) occurred in 50% (18/36) of eyes treated. No substantial differences were observed in PRK- (n = 18) versus LASEK- (n = 17) treated children.

CONCLUSIONS

Laser refractive surgery is effective for correcting anisometropic myopia in amblyopic children. Recurrence of myopia is common. Further study is indicated to determine long-term stability and safety of the procedure in this population.

Correlation of the Appearance of the Keratoconic Cornea In Vivo by Confocal Microscopy and In Vitro by Light Microscopy

OBJECTIVE

To compare morphologic features of keratoconus as observed in vivo with a slit scanning confocal microscope and in vitro using light microscopy.

METHODS

Slit scanning confocal microscopy (CM) was used to evaluate the central cornea of 29 keratoconic subjects (mean age, 31 +/- 10 years; range, 16-49). Light microscopy (LM) examination was performed on 2 of the keratoconic corneas post-keratoplasty.

RESULTS

With CM, the epithelium appeared more abnormal with increasing severity of keratoconus. In severe disease, the epithelium displayed the following characteristics: superficial cells were elongated and spindle shaped, wing cell nuclei were larger and more irregularly spaced, and basal cells were flattened. These findings were confirmed by LM. Images obtained using CM revealed disruption to Bowman's layer and the occasional presence of epithelial cells and stromal keratocytes. This was shown with LM to be due to breaks in Bowman's layer. Stromal haze and hyperreflectivity observed with CM corresponded with apical scarring seen on slit-lamp biomicroscopy. Hyperreflective keratocyte nuclei observed with CM are thought to indicate the presence of fibroblastic cells seen with LM. Increasing levels of haze detected with CM were found with LM to be due to fibroblast accumulation and irregular collagen fibers. Descemet's membrane appeared normal with both CM and LM. Evidence of endothelial cell elongation was apparent in 1 subject with CM.

CONCLUSIONS

The current study confirms the application of CM for assessing morphologic alterations to the epithelium, Bowman's layer, and stroma in keratoconus. Many of the tissue changes observed with CM could be reconciled with observations made using LM. This work provides a framework against which tissue changes in keratoconus can be studied in a clinical context in vivo using CM.

Pathologic findings in postmortem corneas after successful laser in situ keratomileusis

PURPOSE

To examine the histologic and ultrastructural features of human corneas after successful laser in situ keratomileusis (LASIK).

METHODS

Corneas from 48 eyes of 25 postmortem patients were processed for histology and transmission electron microscopy (TEM). The 25 patients had LASIK between 3 months and 7 years prior to death. Evaluation of all 5 layers of the cornea and the LASIK flap interface region was done using routine histology, periodic acid-Schiff (PAS)-stained specimens, toluidine blue-stained thick sections, and TEM.

RESULTS

In patients for whom visual acuity was known, the first postoperative day uncorrected visual acuity was 20/15 to 20/30. In patients for whom clinical records were available, the postoperative corneal topography was normal and clinical examination showed a semicircular ring of haze at the wound margin of the LASIK flap. Histologically, the LASIK flap measured, on average, 142.7 microm (range, 100-200). A spectrum of abnormal histopathologic and ultrastructural findings was present in all corneas. Findings at the flap surface included elongated basal epithelial cells, epithelial hyperplasia, thickening and undulations of the epithelial basement membrane (EBM), and undulations of Bowman's layer. Findings in or adjacent to the wound included collagen lamellar disarray; activated keratocytes; quiescent keratocytes with small vacuoles; epithelial ingrowth; eosinophilic deposits; PAS-positive, electron-dense granular material interspersed with randomly ordered collagen fibrils; increased spacing between collagen fibrils; and widely spaced banded collagen. There was no observable correlation between postoperative intervals and the severity or type of pathologic change except for the accumulation the electron-dense granular material.

CONCLUSIONS

Permanent pathologic changes were present in all post-LASIK corneas. These changes were most prevalent in the lamellar interface wound. These changes along with other pathologic alterations in post-LASIK corneas may change the functionality of the cornea after LASIK.

Mitomycin C modulation of corneal wound healing after photorefractive keratectomy in highly myopic eyes

PURPOSE

To evaluate the role of topical mitomycin C in corneal wound healing (CWH) after photorefractive keratectomy (PRK) in highly myopic eyes.

DESIGN

Prospective, double-masked, randomized clinical trial.

PARTICIPANTS

Seventy-two eyes of 36 patients affected by high (>7 diopters) myopia.

METHODS

In each patient, one eye was randomly assigned to PRK with intraoperative topical 0.02% mitomycin C application, and the fellow eye was treated with a placebo. Postoperatively, mitomycin C-treated eyes received artificial tears (3 times daily, tapered in 3 months), whereas the fellow eye was treated with fluorometholone sodium 2% and artificial tears (3 times daily, tapered in 3 months).

MAIN OUTCOME MEASURES

Uncorrected visual acuity (UCVA) and best-corrected visual acuity (BCVA), contrast sensitivity, manifest refraction, and biomicroscopy. Contrast sensitivity was determined using the Pelli-Robson chart. Corneal confocal microscopy documented CWH.

RESULTS

Mean follow-up was 18 months (range, 12-36). No side effects or toxic effects were documented. At 12-month follow-up examination, UCVAs (logarithm of the minimum angle of resolution) were 0.4+/-0.48 and 0.5+/-0.53 (P = .03) in mitomycin C-treated eyes and corticosteroid-treated eyes, respectively. At 1 year, corneal haze developed in 20% of corticosteroid-treated eyes, versus 0% of mitomycin C-treated eyes. At 12, 24, and 36 months, corneal confocal microscopy showed activated keratocytes and extracellular matrix significantly more evident in untreated eyes (Ps = 0.004, 0.024, and 0.046, respectively).

CONCLUSION

Topical intraoperative application of 0.02% mitomycin C can reduce haze formation in highly myopic eyes undergoing PRK.

Corneal power, thickness, and stiffness: Results of a prospective randomized controlled trial of PRK and LASIK for myopia

PURPOSE

To compare the short-, medium-, and long-term changes in corneal optical power and corneal aberrations, central corneal thickness, and corneal "stiffness" assessed by pneumotonometry readings in patients having laser in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) for myopia.

SETTING

Department of Ophthalmology, Arhus University Hospital, Arhus, Denmark.

METHODS

One eye of each of 45 patients with myopia ranging from -6.00 to -8.00 diopters (D) (spherical equivalent spectacle refraction [SER]) was randomized to LASIK (n=25; mean SER -7.12 D +/- 0.57 [SD]) or PRK (n=20; mean SER -6.91 +/- 0.57 D). Data were collected prospectively before and 1, 3, 6, 12, and 36 months after surgery. Measurements included corneal topography (TMS-1, Tomey), corneal thickness (ultrasound pachymetry), and apparent intraocular pressure (IOP) (pneumotonometry). Retreatments were not performed during the first year, and retreated eyes were excluded from the 3-year follow-up. Changes in corneal power and aberrations, thickness, and apparent IOP were calculated in a pair-wise manner for 3 time periods: short term (preoperative to 1 month after surgery), medium term (1 to 12 months after surgery), and long term (1 to 3 years after surgery).

RESULTS

In the short term, corneal power decreased equally in LASIK and PRK eyes. Spherical aberrations and coma-like aberrations increased equally, while corneal thickness decreased significantly less in LASIK eyes than in PRK eyes. The apparent IOP decreased more in LASIK eyes than in PRK eyes. In the medium term, corneal power increased significantly in both groups. Spherical aberrations decreased significantly in PRK eyes but not in LASIK eyes. From 1 to 12 months, corneal thickness increased more in PRK eyes than in LASIK eyes. During this period, the apparent IOP increased significantly in LASIK eyes. In the long term, corneal power and corneal aberrations did not change significantly in either group. Corneal thickness increased slightly but significantly in both groups. The apparent IOP increased significantly more in PRK eyes.

CONCLUSIONS

Differences between LASIK and PRK related to time-dependent events affecting corneal shape and structural integrity were present. Peripheral changes in flap hydration in LASIK eyes and epithelial and/or stromal thickening in PRK eyes appeared to be the most important factors in optical power changes in the first year after treatment. The changes in apparent IOP suggest that some interlamellar healing occurred during the first year after LASIK. After LASIK and PRK, corneal bending stiffness seemed permanently decreased, although some restiffening may occur in PRK eyes in the long term.

Differences in nasal and temporal responses of the cornea after photorefractive keratectomy

PURPOSE

To examine the differences in the biomechanical response of the peripheral regions of the cornea after photorefractive keratectomy (PRK).

SETTING

Department of Ophthalmology, Catholic University of Rome, Rome, Italy.

METHODS

Preoperative and 1-, 3-, 6-, and 12-month postoperative corneal topographies of 70 eyes that had PRK with the Technolas 217C excimer laser (Bausch & Lomb) were obtained. The eyes were divided into 4 groups according to the preoperative spherical equivalent refraction. Preoperative and follow-up topographic data were imported into custom software that computed the average composite corneal map and difference maps in each group to scientifically evaluate the corneal response to the surgery. The software was also used to analyze regional corneal changes after the laser ablation. Corneal peripheries up to 9.0 mm were evaluated.

RESULTS

The preoperative corneas had a flatter nasal periphery than temporal periphery. The corneal surfaces in the right eyes and left eyes showed a mirror symmetry. Significant differences in the regional response of the cornea were observed (P<.05), with a greater increase in the curvature of the nasal periphery than in the temporal periphery.

CONCLUSIONS

To refine modeling of the cornea, the different regional anatomic features and biomechanical responses must be considered. Modifying existing ablation algorithms to compensate for the differences between nasal and temporal corneal flattening of the preoperative corneal surface and between the nasal and temporal responses may improve the postoperative corneal shape and quality of peripheral optics.

Prediction of Corneal Haze Using an Ablation Depth/Corneal Thickness Ratio After Laser Epithelial Keratomileusis

PURPOSE

To investigate the usefulness of ablation depth/corneal thickness (AD/CT) ratio to predict corneal haze after laser epithelial keratomileusis (LASEK) using a retrospective, comparative, interventional case series.

METHODS

Fifty patients (90 eyes; mean age 40.9 years) with myopia, hyperopia, and/or astigmatism underwent bilateral or unilateral LASEK for correction of refractive error. After epithelial flaps were created using an 18% alcohol solution, bilateral or unilateral LASEK was performed using the Alcon Autonomous LADARVision 4000 excimer laser. Visual acuity (best spectacle-corrected and uncorrected) and refractive error were measured before and after LASEK. Corneas were assessed by two independent evaluators under a slit-lamp biomicroscope with broad tangential illumination. The relative haze scale was quantitated: 0 (clear), 0.5+ (trace), 1+ (mild), 2+ (moderate), 3+ (marked), and 4+ (severe).

RESULTS

Mean preoperative spherical equivalent refraction was -5.46 +/- 3.74 D (range -12.375 to +5.00 D), mean ablation depth was 93.04 +/- 45.03 microm (range 21.2 to 207.2 microm), and mean AD/CT ratio was 0.18 +/- 0.09 (range 0.04 to 0.41). Of 90 eyes, 40 eyes had a higher ablation depth (AD/CT ratio > 0.18) and 50 eyes had a lower ablation depth (AD/CT ratio < 0.18); 92.5% of eyes in the higher ratio group developed clinically significant haze (1+ or greater). In the lower ratio group, 94% of eyes developed no more than 1+ corneal haze, if any.

CONCLUSION

The ablation depth/corneal thickness ratio is useful for predicting corneal haze after LASEK. An AD/CT ratio of 0.18 or more suggests that patients have a high risk of developing clinically significant haze (1+ or more) after LASEK.

Corneal wound healing

PURPOSE

To examine the latest theories in understanding wound healing, primarily as it pertains to refractive surgery but also, briefly, incisional surgery and nonsurgical corneal conditions (iatrogenic injury and noninfectious corneal ulcers).

RECENT FINDINGS

Corneal wound healing involves transformation of fibroblasts; intercellular signaling-for example between epithelial and stromal cells-from cytokines, neuropeptides, growth factors, and chemokines; action of matrix metalloproteinases; and protection of tissue from free radical damage. There may be a common wound healing pathway after different types of surgery and injury to the cornea-for example, the wound healing cascades after laser in situ keratomileusis and photorefractive keratectomy may be similar, and yet the effects on the cornea differ because of the extent of disruption to the basement membrane-but the outcomes may depend on small differences in the cascade.

SUMMARY

Interest in the study of corneal wound healing has increased with the proliferation of keratorefractive surgery. Important contributions come from intercellular signaling, fibroblast transformation, remodeling of the extracellular matrix, and free radical scavengers. Despite these advances, a formalized, unified vision of the wound healing cascade remains elusive.

Quality assessment model of 3 different microkeratomes through confocal microscopy

PURPOSE

To study the quality of the cut created by 3 microkeratomes from 2 different generations using corneal confocal microscopy.

SETTING

Department of Refractive Surgery, Instituto Oftalmológico de Alicante, Alicante, Spain.

METHODS

Two different studies were conducted: Study 1 and Study 2. Study 1 was a prospective analysis using confocal microscopy examination data from 2 reference groups: Group A (control) with 20 nonoperated eyes of 20 healthy volunteers, and Group B with 50 eyes of 30 patients operated on with the Bausch & Lomb Automated Corneal Shaper (ACS) microkeratome. Study 2 was a prospective randomized double-masked study in which 40 eyes of 20 patients underwent myopic laser in situ keratomileusis by 1 surgeon. The right and left eyes of each patient were randomly and alternatively assigned to Group C, flap made with the Hansatome microkeratome, or Group D, flap made with the Moria M2. Immediately after surgery, the microscopic appearance of the cut was subjectively evaluated by the surgeon. One month postoperatively, flap thickness, particle density, and the subclinical confocal wound healing opacity (WHO) index were evaluated with the corneal confocal microscope.

RESULTS

Surgeon handling comfort was nearly the same with the Hansatome as with the M2 (P =.540). However, the apparent quality of the resulting flap was better with the M2 microkeratome (P =.041). The depth of the cuts made by the 3 microkeratomes were significantly different (P<.001), with the ACS flaps being thinner than the flaps made with the Hansatome or the M2. Particle density at the interface was significantly poorer in the eyes operated on with the ACS, but in these cases the WHO index was significantly greater (P<.001 in both cases).

CONCLUSIONS

Confocal microscopy is a very useful tool to evaluate the quality of the cut made by different microkeratomes. Overall, the predictability in flap thickness and the apparent quality of the cut made by the Moria M2 microkeratome are better than those obtained with the Hansatome or the ACS. The thin flap made by the ACS microkeratome produced a significantly greater WHO index than the thicker flaps created with the other 2 microkeratomes.

Late-onset corneal haze and myopic regression after photorefractive keratectomy (PRK)

OBJECTIVE

To report the incidence and clinical course of a series of patients who developed both delayed-onset, clinically significant progressive haze and myopic regression after photorefractive keratectomy (PRK).

METHODS

In this retrospective case series, the charts of 542 consecutive patients who had undergone PRK with the VISX Star Excimer or Nidek EC-5000 laser between July 1996 and October 1998 and who had a minimum of 6 months of follow-up were reviewed. Ten eyes of 8 patients developed progressive haze to greater than 1+ and myopic regression equal to or more than -1 D 3 months or more after PRK. The historical and clinical features were reviewed.

RESULTS

The incidence of combined progressive haze and myopic regression was 1.8%. The average age was 40.5 years. Three of the 8 patients were female. The median spherical equivalent (SE) attempted correction was -6.69 D (range -4.00 to -12.25 D). Five patients who underwent bilateral PRK had unilateral involvement. The mean SE regression was -2.01 +/- 0.79 D (range -1.00 to -3.00 D). Regression plateaued at a mean of 9.8 months. Haze ranging up to 4+ peaked at a mean of 7.4 months. Topical steroid treatment and/or epithelial scraping was attempted in 3 eyes but was ineffective.

CONCLUSIONS

Combined delayed-onset progressive haze and myopic regression can occur after PRK. In such cases, the amount of haze appears to correlate with the magnitude of attempted initial correction (r = 0.639, P = 0.046) although not with the magnitude of subsequent regression. Patients may need at least 10 months of follow-up to achieve a stable refraction and level of haze. These observations suggest a need for improved understanding of corneal wound healing following PRK and of biologic factors that may contribute to variability in outcomes.

Keratocyte reflectivity and corneal haze

Corneal transparency is a remarkable characteristic that is essential for vision. Biophysical models of corneal transparency have entirely focused on the stromal extracellular matrix and disruption of the regular array of collagen fibres as the main reason for corneal haziness. Therefore, disorder of corneal transparency has traditionally been explained by a combination of three main factors: (1) abnormal water content (i.e. swelling or edema); (2) abnormal collagen fibre diameter, spacing, and orientation (i.e. scar tissue or fibrosis); and (3) abnormal accumulation of macromolecules (proteins, glycosaminoglycans, lipids, etc.) as in many corneal dystrophies. Here, clinical and experimental data are provided to support the concept that corneal keratocytes, which are normally invisible and transmit light, may show intense light scattering in injured corneas. Thus, the existence of a fourth group of corneal transparency disorders is proposed that predominantly are associated with abnormal cellular-based reflections from multiple layers of stromal keratocytes. In this group of patients, the light scattering structures (keratocyte nuclei, cell-body, and cell-processes) cannot be discriminated using standard slit-lamp biomicroscopy but requires a confocal microscopic examination. Despite their importance, almost nothing is known about the physical basis for the invisibility and haziness of the keratocytes. A more comprehensive model to understand corneal transparency is needed and should include the interaction of visible light with the physical structure of the keratocyte and its subcellular constituents.

Mitomycin C Alters Corneal Stromal Wound Healing and Corneal Haze in Rabbits After Argon-Fluoride Excimer Laser Photorefractive Keratectomy

PURPOSE

To investigate the effects of mitomycin C (MMC) on rabbit cornea wound healing after photorefractive keratectomy (PRK).

MATERIALS AND METHODS

Rabbit corneas were stained with dichlorotriazinyl aminofluorescein immediately after PRK. MMC was applied to the right eye and phosphate-buffered salt solution (PBS) to the left. Corneal epithelial wound healing rate and corneal haze were examined. Ultrasound pachymetry was performed. Stromal collagen regeneration was evaluated by fluorescent microscopy. We used terminal deoxyribonucleotidyl transferase-mediated D-uridine 5'-triphosphated-digoxigenin nick-end labeling (TUNEL) assay and transmission electron microscopy (TEM) to evaluate keratocyte apoptosis.

RESULTS

In eyes treated with MMC, there was no delay to the healing rate of corneal epithelial wound, and less haze 4 weeks after PRK. Ultrasound pachymetry showed thinner corneal thickness in MMC-treated eyes at week 4. Corneal stromal thickness regression was less in MMC-treated eyes observed by fluorescent microscope at week 4. Keratocyte apoptosis was noted in both MMC- and PBS-treated eyes by TUNEL assay and TEM observation. This study discovered the phenomenon that MMC prolongs keratocyte apoptosis.

CONCLUSIONS

Applying MMC after PRK is an effective method to decrease haze formation and corneal stromal thickness regression in rabbit corneas. The effect may be related to MMC prolonging keratocyte apoptosis.

Laser Subepithelial Keratomileusis for Myopia of -6 to -10 Diopters With Astigmatism With the MEL60 Laser

PURPOSE

To evaluate the efficacy, predictability, and safety of laser subepithelial keratomileusis (LASEK) for treatment of high myopia with astigmatism.

METHODS

LASEK was performed in 61 eyes of 36 consecutive patients with myopic spherical equivalent refraction of -6.00 to -10.00 D using the Aesculap-Meditec MEL60 excimer laser. Data were collected prospectively with a follow-up of 6 to 17 months. Main outcome measures recorded were UCVA, BSCVA, residual refractive error, corneal haze, and complications.

RESULTS

Ninety-six percent of eyes achieved 20/40 or better UCVA at 1 month. At 12 months, 64% of eyes achieved 20/20 and 92% achieved 20/40 or better UCVA. Two eyes lost 2 lines of BSCVA at 6 or 12 months. Accuracy of correction was +/- 0.50 D from emmetropia in 82% of eyes, and +/- 1.00 D in 90% at 12 months. No eye showed more than grade 1 haze. Grade 1 haze was observed in three eyes at 12 months. One patient had mild postoperative keratitis, which was successfully treated.

CONCLUSIONS

LASEK with the Aesculap-Meditec MEL60 excimer laser appeared to be safe, effective, and highly predictable in treating high myopia.

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.

Five-Year Follow-Up of Photorefractive Keratectomy for Myopia

PURPOSE

To analyze long-term results of photorefractive keratectomy (PRK) for myopia and myopic astigmatism.

METHODS

This retrospective study included 15 eyes of 8 patients who were examined annually for 5 years after PRK. The Nidek EC-5000 laser with an ablation zone of 5.0 mm was used. Evaluations included spherical equivalent manifest refraction, uncorrected visual acuity (UCVA), best spectacle-corrected visual acuity (BSCVA), corneal haze, and corneal astigmatism calculated by Fourier analysis of videokeratography data.

RESULTS

A tendency toward myopic regression was most evident within the first postoperative year, with manifest refraction changing from +0.80 +/- 1.62 D at 1 week to -0.45 +/- 0.70 D at 1 year postoperatively (P = .007). Regression continued after the second postoperative year. There was a statistically significant difference between manifest refraction at 2 years (-0.36 +/- 0.75 D) and 5 years (-1.11 +/- 1.12 D) (P = .002). Postoperative UCVA stabilized from 3 months up to 3 years, but slightly deteriorated at 4 years and thereafter due to the myopic refractive shift. BSCVA remained stable throughout the 5-year follow-up period. Several eyes developed mild corneal haze after surgery, but haze was minimal in the majority of patients by 1 year and continued to fade over time. The asymmetry component of the cornea significantly increased after surgery, with all postoperative values significantly higher than before PRK (P < .05). Higher order irregularity increased after surgery, with a statistically significant difference between preoperative and 1 year postoperative (P < .05), but values after 2 years were not different from preoperative baseline.

CONCLUSION

Fifteen eyes with a baseline refraction of -3.00 to -9.00 D had PRK with the Nidek EC-5000 laser and a 5-mm-diameter ablation zone. Myopic regression occurred in the first year, with continued mild regression of approximately -0.75 D between 2 and 5 years. Nevertheless, the results show the procedure was relatively safe and effective in this group.

Transparency, swelling and scarring in the corneal stroma

PURPOSE

This paper briefly reviews current explanations for corneal transparency and uses a well-developed model to try to explain the increased light scattering either accompanying corneal swelling or following phototherapeutic keratectomy (PTK).

METHODS

The direct summation of fields (DSF) method was used to compute light transmission as a function of wavelength. The method requires input of a number of structural parameters. Some of these were obtained from electron micrographs and others were calculated from X-ray diffraction data.

RESULTS

By swelling sections of stroma cut from different depths in the tissue, we have shown that fluid entering the cornea causes more swelling in the posterior lamellae than in the anterior lamellae. Furthermore, posterior lamellae can reach a higher final hydration than anterior lamellae. Collagen-free regions ('lakes') exist in corneas swollen in vitro and in Fuch's dystrophy corneas, many of which may be caused by the death of cells. The DSF method shows that local fibril disordering, increased refractive index mismatch, and increased corneal thickness together can account for a 20% increase in light scattering in a Fuch's dystrophy cornea at H=5.8 compared to the normal cornea. Additional scattering is probably caused by 'lakes'. The DSF method applied to PTK rabbit stroma with high levels of haze suggests that the newly deposited collagen is not the cause of the increased light scattering.

CONCLUSIONS

Fluid is not uniformly distributed within the corneal stroma when the cornea swells. Increased hydration of posterior lamellae may be because of known differences in the glycosaminoglycans between the anterior and posterior stroma. Lamellar interweave in the anterior stroma probably limits the extent to which the constituent lamellae can swell. The DSF method can be used to account for increased light scattering in oedematous corneas but cannot account for haze following PTK.

Confocal microscopy

OBJECTIVE

To review the available evidence for the use of confocal microscopy in diagnosing infectious keratitis and for other applications for ophthalmic practice.

METHODS

A MEDLINE search of the peer-reviewed literature for the years 1990 to 2001 yielded 94 citations. The search was limited to studies of human subjects published in English with abstracts. The Ophthalmic Technology Assessment Committee Cornea Panel evaluated these 94 articles for possible clinical relevance and selected 51 (54%) for content review by the panel members. Of these 51 articles, 24 were selected for the panel methodologist to review and rate according to the strength of evidence.

RESULTS

Of the 24 articles, 21 (87.5%) were classified as case reports or case series and were rated as level III evidence. Three articles were classified as independent, masked, or objective comparisons performed in a narrow spectrum of patients or in a nonconsecutive series of patients and were rated as level II evidence. No studies were rated as level I evidence, defined as an independent masked comparison of an appropriate spectrum of consecutive patients.

CONCLUSION

Confocal microscopy is a new technology with clinical applications in ophthalmology. Although confocal microscopy has been used in other fields of medicine, the optical transparency of the cornea and other structures of the eye provides a unique opportunity to apply this technology. The targeted literature review of 24 articles found no level I studies to support the use of confocal microscopy in the management of eye disorders. Three level II studies pertained to promising clinical applications of the confocal microscope and provided evidence that supports the use of confocal microscopy as an adjunctive modality for diagnosing Acanthamoeba keratitis. The remaining 21 articles, rated as level III evidence, focus on the use of confocal microscopy to facilitate the diagnosis of infectious keratitis, including amoebic and fungal, but currently there are no definitive studies of its role in the differential diagnosis of this condition. There are also level III studies that support the use of the confocal microscope in refractive surgery. Facilitating the diagnosis of infectious keratitis and applying the confocal microscope to refractive surgery may hold the greatest promise of this new technology.

Comparison of Four Corneal Pachymetry Techniques in Corneal Refractive Surgery

PURPOSE

To assess the effectiveness of optical and ultrasonic pachymetry when examining corneas of patients before and after laser in situ keratomileusis (LASIK).

METHODS

We conducted a prospective study of 25 patients (50 eyes) who had LASIK for myopia or myopic astigmatism (mean spherical equivalent refraction -4.80 +/- 3.60 D; range -1.75 to -12.00 D). Corneal thickness was measured using 1) conventional ultrasonic pachymetry (20 MHz probe) in the central cornea, 2) ASL confocal microscopy (CMTF analysis), 3) Orbscan II Analyzer, and 4) ultrasound biomicroscopy (UBM) (50 MHz probe) preoperatively and 3 months postoperatively.

RESULTS

Preoperative pachymetry values measured with the Orbscan II Analyzer (551.3 +/- 27.3 microm) were almost identical to those measured with the 20 MHz ultrasonic probe (553.7 +/- 25.7 microm; t-test P = .652). The mean preoperative pachymetry for UBM was 566.7 +/- 38.8 microm and for CMTF was 553.2 +/- 15.5 microm. The optic and ultrasonic system measurements differed in operated eyes; 20 MHz ultrasonic pachymetry yielded an average 478.7 +/- 23.5 microm and the UBM, 482.4 +/- 40.3 microm. These measurements differed from the Orbscan II (Tukey test; P = .023) measurement of 461.1 +/- 37.5 microm but not from the confocal microscopy measurement of 475.2 +/- 41.8 microm. UBM registered greater values that were more variable than those obtained with ultrasonic pachymetry.

CONCLUSIONS

In non-operated eyes, corneal thickness was similar using conventional ultrasound, Orbscan II, or confocal techniques, with ultrasound biomicroscopy giving thicker readings.

Stromal haze after laser in situ keratomileusis

PURPOSE

To report clinical and confocal microscopy characteristics of haze-like opacities in corneas after laser in situ keratomileusis (LASIK).

SETTING

Department of Ophthalmology, Johann Wolfgang Goethe-University, Frankfurt am Main, Germany.

METHODS

Eighteen eyes of 11 patients with clinically apparent corneal clouding were examined by slitlamp and confocal microscopy (Confoscan P4, Tomey) 1 to 9 months after primary LASIK or LASIK retreatment.

RESULTS

Postoperative slitlamp examination showed faint, white, snowflake-like clouding at the interface level in all patients. One patient had folds and rather diffuse haze-like opacities. Confocal microscopy revealed highly reflective structures in the flap stroma and at the interface level in all patients, probably due to numerous activated keratocytes and their processes. The confocal microscopy appearance was similar to that of photorefractive keratectomy haze.

CONCLUSION

Focal wound-healing reactions in the central flap stroma and interface resulting in significant keratocyte activation could be observed after LASIK.

Biomechanical and optical characteristics of a corneal stromal equivalent

Cell matrix interactions are important in understanding the healing characteristics of the cornea after refractive surgery or transplantation. The purpose of this study was to characterize in more detail the evolution of biomechanical and optical properties of a stromal equivalent (stromal fibroblasts cultured in a collagen matrix). Human corneal stromal fibroblasts were cultured in a collagen matrix. Compaction and modulus were determined for the stromal equivalent as a function of time in culture and matrix composition. The corneal stromal fibroblasts were stained for alpha-smooth muscle actin expression as an indicator of myofibroblast phenotype. The nominal modulus of the collagen matrix was 364 +/- 41 Pa initial and decreased initially with time in culture and then slowly increased to 177 +/- 75 Pa after 21 days. The addition of chondroitin sulfate decreased the contraction of the matrix and enhanced its transparency. Cell phenotype studies showed dynamic changes in the expression of alpha-smooth muscle actin with time in culture. These results indicate that the contractile behavior of corneal stromal cells can be influenced by both matrix composition and time in culture. Changes in contractile phenotype after completion of the contraction process also indicate that significant cellular changes persist beyond the initial matrix-remodeling phase.

Corneal nerves: structure, contents and function

This review provides a comprehensive analysis of the structure, neurochemical content, and functions of corneal nerves, with special emphasis on human corneal nerves. A revised interpretation of human corneal nerve architecture is presented based on recent observations obtained by in vivo confocal microscopy (IVCM), immunohistochemistry, and ultrastructural analyses of serial-sectioned human corneas. Current data on the neurotransmitter and neuropeptide contents of corneal nerves are discussed, as are the mechanisms by which corneal neurochemicals and associated neurotrophins modulate corneal physiology, homeostasis and wound healing. The results of recent clinical studies of topically applied neuropeptides and neurotrophins to treat neurotrophic keratitis are reviewed. Recommendations for using IVCM to evaluate corneal nerves in health and disease are presented.

Confocal microscopy of Thygeson's superficial punctate keratopathy

PURPOSE

To describe the changes seen with the confocal microscope in Thygeson's superficial punctate keratopathy (TSPK).

METHODS

Confocal microscopy was performed on six patients with TSPK presenting to Manchester Royal Eye Hospital from October 1999 to June 2001. Both eyes were examined including the normal fellow eyes of two patients with unilateral TSPK. Images were compared with age-matched controls.

RESULTS

Irregular nerve fibers, masked by considerable haze, were seen in the subepithelial nerve plexus immediately above Bowman's membrane in TSPK. Generalized haze was also present in the anterior stroma along with areas of high reflectivity, microdots, and reflective bodies. Keratocytes had highly reflective nuclei and cell bodies of irregular size, orientation, and shape. The changes were related to the duration of TSPK, were not seen in normal fellow eyes, and were also present in areas without lesions.

CONCLUSION

Confocal imaging of TSPK demonstrated considerable abnormalities in the subepithelial nerve plexus, Bowman's membrane, and the anterior stroma that were related to disease duration. Similar changes have been reported in a variety of corneal diseases and following laser corneal surgery. Recent evidence has supported interactions between the epithelium and Bowman's membrane and the anterior stroma in corneal disease and after laser corneal surgery. We speculate that such interactions may have a role in TSPK.

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.

Changes in Central Corneal Thickness After Laser in situ Keratomileusis and Photorefractive Keratectomy

PURPOSE

To evaluate changes in corneal thickness after laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) in eyes with the same preoperative refraction, correlate these changes to postoperative refractive outcomes, and compare corneal healing process in a standardized subset of patients.

METHODS

Central corneal thickness was measured by contact ultrasound pachymetry in 14 eyes of 8 patients with preoperative myopia of -6.00 D who had LASIK, and in 14 eyes of 8 patients with the same preoperative refractive error who had PRK. Measurements were taken preoperatively, and 1 week, 3, and 6 months after surgery. Data were evaluated and compared using the paired Student t-test and Pearson correlation coefficient.

RESULTS

Mean preoperative central corneal thickness in the LASIK group was 549.14 +/- 37.4 microm, and in the PRK group, 552.64 +/- 34.9 microm. At 1-week postoperatively, mean central corneal thickness in the LASIK eyes was 467.28 +/- 29 microm and in the PRK eyes, 473.85 +/- 39.2 microm; at 6 months, central corneal thickness had increased in both groups compared to the 1-week values; LASIK eyes had a mean central corneal thickness of 481.42 +/- 23.0 microm and PRK, 481.50 +/- 35.3 microm. Mean postoperative refraction after 6 months was -0.48 +/- 0.30 D in the LASIK group and -0.67 +/- 0.35 D in the PRK group.

CONCLUSION

Increase in central corneal thickness between 1 week and 6 months postoperatively occurred in both LASIK and PRK eyes, but differences were not statistically significant. No statistically significant differences were found in myopic regression between the two patient groups.

Corneal forward shift after excimer laser keratorefractive surgery

The excimer laser keratorefractive surgery inevitably compromises structural integrity of the cornea by the surgical tissue subtraction and loss of integrity of Bowman's membrane. Forward shift of the cornea is commonly seen after both photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK). Antero-posterior movement of the cornea is evaluated by measuring the posterior corneal elevation with the scanning-slit corneal topography (Orbscan). Eyes with thinner cornea, higher intraocular pressure, and higher myopia requiring greater laser ablation were more predisposed to forward shift of the cornea. After PRK, there was a trend toward progressive forward shift of the cornea, but the progression stabilized 6 months after surgery. Because progressive thinning and expansion of the cornea were not observed during the one-year observation period after PRK, the forward shift of the cornea does not represent true ectasia. Forward shift of both corneal surfaces would add to the tendency toward myopic regression after excimer laser surgery.

In vivo confocal microscopy of the human cornea

AIMS

To describe the optics of in vivo confocal microscopy, its advantages over previous methods, and to summarise the literature that arose from its use for the observation of the human cornea. A critical review of the clinical usefulness of this new technology for the corneal examination is undertaken.

METHODS

Confocal microscopes obtain increased resolution by limiting the illumination and observation systems to a single point. Rapid scanning is used to reconstruct a full field of view and allows for "real time" viewing.

RESULTS

Coronal sections of the in situ epithelium, Bowman's membrane, stroma, and endothelium can be visualised at a resolution of 1-2 micro m. A backscattered light intensity curve allows objective measurements of sublayer thickness and corneal haze to be taken. In vivo confocal microscopy is therefore particularly useful in the areas of infective keratitis, corneal dystrophies, refractive surgery, and contact lens wear, where it aids in differential diagnosis and detection of subtle short and long term changes. Real time endothelial cell assessment can also be performed.

CONCLUSION

Because of their ability to visualise living tissue at cellular levels, confocal microscopes have proved useful additions to the current clinical tools.

In vivo microstructural analysis of the cornea in Scheie's syndrome

PURPOSE

To analyze clinical and in vivo microstructural characteristics of both corneas of a 13-year-old male subject with Scheie's syndrome and compare the observations with the pathologic reports in the literature.

METHODS

Standard clinical examination and real-time, slit-scanning in vivo confocal microscopy were performed and repeated after 1 year.

RESULTS

In vivo confocal microscopy images at all cellular layers demonstrated brighter intercellular spaces than those of normal corneas. Cicatrization of the anterior stroma was identified, and the keratocytes of the middle and posterior stroma exhibited markedly altered morphology, often round or elliptical in shape, and with clearly demarcated, hyporeflective centers. The nerve fibers of the subbasal plexus were somewhat more irregular and difficult to distinguish, possibly due to underlying fibrosis.

CONCLUSIONS

The potential of in vivo confocal microscopy to highlight microstructural alterations of the intact human cornea and evaluate such changes over time might reduce reliance on histopathologic investigations in such conditions and contribute to the ophthalmic management of the mucopolysaccharidoses in the future.

Quantification of simulated corneal haze by measuring back-scattered light

The Scheimpflug anterior eye segment analysis system EAS-1000 was modified to specifically quantitate the degree of corneal haze. In order to confirm the basic measurement performance of the new system, TSPC-3 hazemeter, in vitro experiments were conducted. Using white latex microsphere solutions, tests were performed for the linearity of the measurements, the influence of slit lamp illuminance and the influence of the length of the light path through the latex solution. The validity of Beer's law was confirmed, indicating that a sufficiently wide range of latex concentrations (haze intensities) can be covered by the hazemeter. The scattering intensity measured with the hazemeter showed a significant linear correlation with the latex concentration. The obtained scattering intensity was directly proportional to the illuminance of the slit lamp. The ratio of integrated scattering values was proportional to that of the light path length. It was shown that the TSPC-3 hazemeter possesses sufficient capability for a quantitative evaluation of corneal haze.

Validation of confocal microscopy through focusing for corneal sublayer pachymetry

PURPOSE

To evaluate the accuracy and precision of confocal microscopy through focusing (CMTF) for corneal sublayer pachymetry.

METHODS

A tandem scanning confocal microscope equipped with a nonapplanating contact objective was used to perform CMTF. The accuracy of CMTF measurements was evaluated using nine custom-made calibration contact lenses (PMMA) with varying thickness (300-600 microm) and radius of curvature (7.0-9.0 mm). The influence of immersion fluid stabilization and the consequence of prolonged corneal examination were assessed by performing CMTF in rabbits. Additionally, factors related to the instrumental setup and to sedation of experimental animals were examined.

RESULTS

For all calibration contact lenses, the thickness measured by CMTF was within +/-1.0 microm of the certified value. Varying the target thickness or radius of curvature had no consistent impact on the high accuracy of CMTF. When performing CMTF in vivo, z-axis motion was readily identified by sampling and comparing both in- and out-scans. Apart from involuntary eye movements, z-axis motion was due to initial thinning of the immersion fluid with stabilization obtained after approximately 1.5 minutes. Continued confocal examination led to slight but significant swelling of both the stroma (0.5 microm/min) and epithelium (0.1 microm/min).

CONCLUSIONS

CMTF is an accurate and precise technique for corneal sublayer pachymetry with concurrent display of cellular morphology. The precision of CMTF can be improved by allowing time for methylcellulose stabilization and by performing repeated two-way (in and out) scans to account for z-axis motion.

Corneal optical coherence tomography before and after phototherapeutic keratectomy for recurrent epithelial erosions(2)

PURPOSE

To study the representation of corneal structures with optical coherence tomography (OCT) before and after excimer laser phototherapeutic keratectomy (PTK) for recurrent epithelial erosions.

SETTING

Departments of Ophthalmology, Vivantes Klinikum Neukölln, Berlin, and Medizinische Universität, Lübeck, Germany.

METHODS

This prospective study comprised 15 eyes of 14 patients with recurrent epithelial erosions. The central corneal and epithelial thickness as well as the wound-healing response in the anterior corneal stroma were assessed with slitlamp-adapted OCT before and after PTK.

RESULTS

After PTK, the symptoms improved in all patients without loss of best corrected, glare, or low-contrast visual acuity. The mean central corneal OCT thickness was 540 microm +/-28 (SD) preoperatively, 492 +/- 36 microm immediately after epithelial debridement and PTK, and 519 +/- 25 microm after 7 weeks (P <.01). The mean central epithelial OCT thickness changed from 70 +/- 13 microm preoperatively to 60 +/- 7 microm after 7 weeks (P >.01). Changes in the light-scattering properties in the anterior subepithelial stroma revealed a hyperreflective area with a mean thickness of 46 +/- 13 microm after 7 weeks.

CONCLUSIONS

Using noncontact corneal OCT, corneal and epithelial thickness changes and the wound-healing response in the anterior corneal stroma could be evaluated after PTK in patients with recurrent epithelial erosions.

Extent of initial corneal injury as the mechanistic basis for ocular irritation: key findings and recommendations for the development of alternative assays

Currently, there are no recognized alternative tests to eliminate the use of animals in ocular irritation testing. A major reason no replacement alternatives have been developed is that the current in vivo data set provides no perspective regarding the critical cellular and molecular changes involved in initial ocular injury, subsequent responses, and repair processes in standard in vivo tests. Without this perspective, mechanistically based replacement tests cannot be developed and validated. We have proposed that the level of ocular irritation is related to the extent of initial injury, and that regardless of the processes leading to tissue damage, the extent of initial injury is the principal factor determining the outcome of ocular irritation. This article summarizes the results from our studies of various surfactants and nonsurfactants of differing irritancy that support our hypothesis. Our findings indicate that a mechanistically based alternative to in vivo ocular irritation tests would be the microscopic or biochemical measurement of initial injury using either ex vivo or in vitro corneal equivalent systems composed of corneal epithelial, stromal keratocyte, and corneal endothelial cell layers. This work also provides a well-characterized panel of materials of varying types and irritation for use in developing and validating alternative tests.

Effect of myopic laser in situ keratomileusis on epithelial and stromal thickness: a confocal microscopy study

PURPOSE

To determine changes in central epithelial and stromal thickness in human corneas in vivo after laser in situ keratomileusis (LASIK).

DESIGN

Prospective, nonrandomized, comparative trial.

PARTICIPANTS

Eighteen eyes of 12 patients received LASIK (performed using the VISX Star laser [VISX, Santa Ana, CA]) with a planned 180- micro m flap (created using an automated Hansatome microkeratome [Bausch & Lomb, Irvine, CA]) to correct refractive errors between -2.0 diopters (D) and -11.0 D.

METHODS

Corneas were examined by using confocal microscopy in vivo before LASIK and at 1 week and 1, 3, 6, and 12 months after LASIK. Epithelial thickness was the distance between images of the surface epithelium and subbasal nerve plexus or, when nerves were not visible, the subbasal peak (if present in the light intensity profile). Total flap thickness was the distance between images of the surface epithelium and interface debris (or peak), and total stromal thickness was the distance between images of the most anterior keratocytes and endothelium.

MAIN OUTCOME MEASURES

Corneal epithelial and stromal thickness.

RESULTS

Epithelial thickness before LASIK was 46 +/- 5 micro m (mean +/- standard deviation) and increased 22% by 1 month after LASIK (56 +/- 5 micro m; P = 0.01). Thereafter, epithelial thickness did not change, but remained thicker at 12 months after LASIK (54 +/- 8 micro m) than before LASIK (P = 0.02). Total flap thickness at 1 month after LASIK was 160 +/-28 micro m and did not change thereafter. Changes in total stromal thickness between 1 and 12 months after LASIK were not significant.

CONCLUSIONS

The central corneal epithelium was thicker in the first year after LASIK than before LASIK. There was no change in central stromal thickness between 1 month and 12 months after LASIK.

Laser-assisted subepithelial keratectomy for low to high myopia and astigmatism

PURPOSE

To evaluate the safety and efficacy of laser-assisted subepithelial keratectomy (LASEK) for the treatment of low to high myopia and astigmatism.

SETTING

Solo private practice, Mountain View, California, USA.

METHODS

Laser-assisted subepithelial keratectomy was performed in 146 eyes of 83 consecutive patients with myopia or myopic astigmatism using a VISX Star S2 excimer laser (72 eyes) or a Nidek EC-5000 excimer laser (74 eyes). The mean preoperative myopic spherical equivalent was -5.32 diopters (D) (range -1.25 to -14.38 D). Data were collected prospectively with a follow-up of 1 to 12 months. Outcome measurements included uncorrected visual acuity (UCVA), manifest refraction, best spectacle-corrected visual acuity (BSCVA), corneal haze, and complications.

RESULTS

After 6 and 12 months, no eye lost 2 or more lines of BSCVA. After 6 months, the UCVA was 20/20 in 57% of eyes and 20/40 or better in 96%. After 12 months, it was 20/20 in 56% of eyes and 20/40 or better in 96%. No eye developed corneal haze that affected visual acuity. There were no serious or vision-threatening complications.

CONCLUSIONS

Laser-assisted subepithelial keratectomy was safe and effective in treating a wide range of myopia and astigmatism. The potential advantages of LASEK over laser in situ keratomileusis (LASIK) include the elimination of stromal flap complications and greater choice in patient selection. The disadvantages include varying degrees of pain for 2 days and blurry vision for several days postoperatively.

A prototype two-detector confocal microscope for in vivo corneal imaging

Confocal microscopy through-focusing (CMTF) of the cornea produces a three-dimensional (3-D) display of corneal structure and intensity profiles that allow objective measurements of corneal sublayer thickness and relative assessment backscattering of light. In this study, a prototype confocal instrument was evaluated in which a photon counting photomultiplier tube (PMT) detector was added to provide faster and more quantitative measurements, while still maintaining the imaging capability of the microscope. To acquire images and measure backscattered light simultaneously, an uncoated pellicle beam splitter was incorporated into the light path of the confocal microscope. This beam splitter reflects 8% of the confocal signal to the PMT. The CMTF scans were performed on four rabbits using the prototype instrument. Corneal images and 3-D reconstructions acquired with and without the beam splitter in the light path appeared identical. Both the camera and PMT CMTF curves had easily identifiable peaks corresponding to the epithelium, basal lamina, and endothelium. No significant differences were found between PMT and camera CMTF measurements of epithelial, stromal, or corneal thickness (n = 12 scans). Furthermore, a high correlation was found between camera and PMT measurements (linear regression analysis, y = 0.999 x -0.4, r = 0.99, p < 0.001). The data suggest that by adding a pellicle beam splitter, CMTF intensity data can be acquired using a PMT. The PMT has a faster sampling rate and greater dynamic range than the camera and provides a count of the photons detected. Thus, the instrument has the potential for improving corneal pachymetry and back-scattering measurements while still providing high-resolution corneal images.

In vivo confocal microscopy through-focusing to measure corneal flap thickness after laser in situ keratomileusis

PURPOSE

To measure flap thickness in laser in situ keratomileusis (LASIK) patients using in vivo confocal microscopy through-focusing (CMTF) and compare measured versus intended flap thickness achieved by 2 microkeratomes, the Automated Corneal Shaper(R) (ACS) (Chiron Bausch & Lomb) and the Hansatome (Bausch & Lomb).

SETTING

Department of Ophthalmology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas, USA.

METHODS

Twenty-seven eyes of 27 patients were examined by in vivo CMTF 3 to 12 months after LASIK was performed with the ACS (12 patients) or Hansatome (15 patients) microkeratome. The central cornea was scanned, and the epithelium, flap, stroma, and total corneal thickness were measured. Normalized flap thickness (NFT) was also calculated to account for changes in epithelial thickness that may have occurred postoperatively.

RESULTS

The mean posterior stromal thickness was 341.1 microm +/- 53.9 (SD) (range 233 to 431 microm) in the ACS group and 320.3 +/- 42.3 microm (range 258 to 382 microm) in the Hansatome group. The mean nonnormalized flap thickness was 132.7 +/- 12.5 microm (range 11 to 151 microm) in the ACS group and 167.4 +/- 21.4 microm (range 141 to 209 microm) in the Hansatome group. The NFT was 129.6 +/- 9.5 microm and 158.4 +/- 22.1 microm, respectively. Both microkeratomes cut significantly less than intended (P <.05); however, the ACS cut a thinner-than-intended thickness in all cases, and the Hansatome cut thicker than intended in 13% of cases. The Hansatome also showed significantly greater variability in flap thickness than the ACS (P <.05).

CONCLUSIONS

A significant difference in precision was noted between the 2 microkeratomes. The findings emphasize the importance of performing thickness measurements and the usefulness of in vivo CMTF in making these determinations to ensure the safety and effectiveness of LASIK.