Photorefractive keratectomy: implications of corneal wound healing

Management of Corneal Haze After Photorefractive Keratectomy

Photorefractive keratectomy (PRK) is a safe and popular corneal surgery performed worldwide. Nevertheless, there is potential risk of corneal haze development after surgery. Proper management of post PRK haze is important for good visual outcome. We performed a comprehensive review of the literature on the various risk factors and treatments for PRK haze, searching the PubMed, Google Scholar, SCOPUS, ScienceDirect, and Embase databases using relevant search terms. All articles in English from August 1989 through April 2023 were reviewed for this study, among which 102 articles were chosen to be included in the study. Depending on the characteristics of and examination findings on post PRK haze, different management options may be preferred. In the proposed framework, management of PRK haze should include a full workup that includes patient's subjective complaints and loss of vision as well as visual acuity, biomicroscopy, anterior segment optical coherence tomography, epithelial mapping, and Scheimpflug densitometry. Topical steroid treatment for haze should be stratified based on early- or late-onset haze. Mechanical debridement or superficial phototherapeutic keratectomy (PTK) may be used to treat superficial corneal haze. Deep PTK and/or PRK can be used to treat deep corneal haze. Mitomycin-C and topical steroids are prophylactic post-surgery agents to prevent recurrence of haze.

NBL1 Reduces Corneal Fibrosis and Scar Formation after Wounding

Corneal scarring is a leading cause of blindness. Currently, there is no treatment to prevent and/or reduce corneal scar formation under pathological conditions. Our previous data showed that the NBL1 protein, also termed the DAN Family BMP (Bone morphogenetic protein) Antagonist, was highly expressed in corneal stromal cells upon wounding. Here, we examined the function of NBL1 in corneal wound healing. Mouse corneas were mechanically wounded, followed by a 2-week treatment using NBL1. Wounded corneas treated with vehicle or an Fc tag served as controls. Compared with the controls, NBL1 treatment facilitated wound re-epithelialization, partially restored the stromal thickness, and significantly reduced corneal scar formation. NBL1 treatment did not decrease immune cell infiltration, indicating that the anti-scarring effect was not dependent on immune suppression. We further examined the anti-fibrotic effect of NBL1 on human corneas. Pairs of human corneas were induced to form myofibroblasts (a key player in fibrosis and scarring) upon wounding and incubation in a medium containing TGF-β1. The OS corneas were treated with Fc as a control, and the OD corneas were treated with NBL1. Compared with the control, human corneas treated with NBL1 had significantly fewer myofibroblasts, which was consistent with these mouse data. A further study revealed that NBL1 treatment inhibited BMP canonical (phospho-Smad1/5) and no-canonical (phospho-p38) pathways in human corneas. Data show that NBL1 reduced corneal fibrosis and scar formation in mice and cultured human corneas. The underlying molecular mechanism is not certain because both anti-fibrotic Smad1/5 and pro-fibrotic p38 pathways were inhibited upon NBL1 treatment. Whether the p38 pathway dominates the Smad1/5 pathway during corneal fibrosis, leading to the anti-fibrotic effect of NBL1, needs further investigation.

Poly(l-Histidine)-Mediated On-Demand Therapeutic Delivery of Roughened Ceria Nanocages for Treatment of Chemical Eye Injury

Development of topical bioactive formulations capable of overcoming the low bioavailability of conventional eye drops is critically important for efficient management of ocular chemical burns. Herein, a nanomedicine strategy is presented to harness the surface roughness-controlled ceria nanocages (SRCNs) and poly(l-histidine) surface coatings for triggering multiple bioactive roles of intrinsically therapeutic nanocarriers and promoting transport across corneal epithelial barriers as well as achieving on-demand release of dual drugs [acetylcholine chloride (ACh) and SB431542] at the lesion site. Specifically, the high surface roughness helps improve cellular uptake and therapeutic activity of SRCNs while exerting a negligible impact on good ocular biocompatibility of the nanomaterials. Moreover, the high poly(l-histidine) coating amount can endow the SRCNs with an ≈24-fold enhancement in corneal penetration and an effective smart release of ACh and SB431542 in response to endogenous pH changes caused by tissue injury/inflammation. In a rat model of alkali burn, topical single-dose nanoformulation can efficaciously reduce corneal wound areas (19-fold improvement as compared to a marketed eye drops), attenuate ≈93% abnormal blood vessels, and restore corneal transparency to almost normal at 4 days post-administration, suggesting great promise for designing multifunctional metallic nanotherapeutics for ocular pharmacology and tissue regenerative medicine.

A New Postoperative Regimen after CXL and PRK Using Topical NSAID and Steroids on the Open Ocular Surface

Corneal epithelium removal during photorefractive keratotomy (PRK), TransPRK, or corneal cross-linking (CXL) means that patients experience pain and inflammation after the procedure, which need to be carefully managed with topical drug regimens. One highly effective class of topical analgesics is non-steroidal anti-inflammatory drugs (NSAIDs), but these must be used carefully, as their use has been associated with delayed re-epithelialization and, in rare cases, corneal melting. However, our clinical experience has been that the concomitant use of topical corticosteroids obviates this risk. Here, we present a mechanistic explanation for our observations, our TransPRK and epithelium-off CXL protocols, and the postoperative medication regimens where topical NSAIDs are used in combination with topical steroid therapy during the first two postoperative days (where pain and inflammation levels are the highest). We detail the results of a single-center retrospective case analysis that examined eyes that underwent TransPRK (n = 301) or epithelium-off CXL (n = 576). Topical NSAID use in the first two postoperative days to control pain and inflammation after PRK/TransPRK or epithelium-off CXL, when used in combination with topical steroid therapy, does not appear to be associated with corneal melting or delayed epithelial healing. This approach may represent an improvement over current methods of handling post-surgical pain in procedures that require corneal epithelial debridement.

Quantification of biomechanical properties of human corneal scar using acoustic radiation force optical coherence elastography

Biomechanical properties of corneal scar are strongly correlated with many corneal diseases and some types of corneal surgery, however, there is no elasticity information available about corneal scar to date. Here, we proposed an acoustic radiation force optical coherence elastography system to evaluate corneal scar elasticity. Elasticity quantification was first conducted on ex vivo rabbit corneas, and the results validate the efficacy of our system. Then, experiments were performed on an ex vivo human scarred cornea, where the structural features, the elastic wave propagations, and the corresponding Young's modulus of both the scarred region and the normal region were achieved and based on this, 2D spatial distribution of Young's modulus of the scarred cornea was depicted. Up to our knowledge, we realized the first elasticity quantification of corneal scar, which may provide a potent tool to promote clinical research on the disorders and surgery of the cornea.

PresbyPRK vs presbyLASIK using the SUPRACOR algorithm and micromonovision in presbyopic hyperopic patients: visual and refractive results at 12 months

PURPOSE

To compare the visual outcome and refractive results between presbyopic photorefractive keratectomy (presbyPRK) and presbyopic laser in situ keratomileusis (presbyLASIK) in presbyopic hyperopic patients using micromonovision and the SUPRACOR algorithm.

SETTING

Percy Military Hospital and private Laser Victor Hugo Center, France.

DESIGN

Observational retrospective nonrandomized.

METHODS

Twenty-three patients who had undergone presbyPRK or presbyLASIK were included and followed up for 12-months. Far and near visual acuity without correction in monocular and binocular, measurement of distance visual acuity with correction in monocular, evaluation of spherical equivalent (SE), aberrometry, and patient satisfaction were measured and analyzed in this study.

RESULTS

The study included 26 eyes of 13 patients in the presbyLASIK group and 20 eyes of 10 patients in the presbyPRK group. The mean age was 55.43 ± 4.6 years. Uncorrected binocular distance visual acuity was 0.030 ± 0.05 logMAR in the presbyLASIK group and 0 logMAR in the presbyPRK group (P = .066). Binocular near visual acuity without correction was 0.21 ± 0.11 logMAR in the presbyLASIK group and 0.30 ± 0.15 logMAR in the presbyPRK group (P = .0398). For the dominant eyes, the SE was -0.08 ± 0.48 diopters (D) in the presbyLASIK group and 0.16 ± 0.82 D in the presbyPRK group (P = .3995). For nondominant eyes, the SE was -0.44 ± 0.50 D in the presbyLASIK group and 0.12 ± 0.65 D in the presbyPRK group (P = .0254).

CONCLUSIONS

PresbyPRK and presbyLASIK were comparable in efficacy, stability, predictability, and safety. PresbyPRK could be a safe and effective surgical alternative for the hyperopic presbyopic patient.

Clinical Applications of In Vivo Confocal Microscopy in Keratorefractive Surgery

PURPOSE

To review the contribution of in vivo confocal microscopy (IVCM) to the understanding of corneal wound healing following refractive surgery, and its role in the diagnosis and management of complications arising from keratorefractive procedures.

METHODS

Review of the basic science and clinical literature relating to the study of keratorefractive surgical procedures using IVCM.

RESULTS

Extensive research using IVCM has generated a comprehensive understanding of tissue responses after corneal refractive surgery. Epithelial thickness and stromal keratocyte density can be quantified postoperatively and studied longitudinally. Corneal nerve loss and subsequent reinnervation has been characterized and differs significantly between laser refractive techniques. IVCM has also been used to study complications arising from postoperative inflammation (diffuse lamellar keratitis, central toxic keratopathy, ring keratitis, and ectasia), infection (microbial keratitis), and neuropathy (dry eye and neuralgia). This imaging technique can have a critical role in the diagnosis of these complications and subsequent monitoring of treatment response. Manual processing of IVCM images is time-consuming and there may be significant interobserver and intraobserver variability with poor repeatability. However, increasing automation and the use of artificial intelligence is improving the speed and accuracy of image analysis.

CONCLUSIONS

IVCM has historically been confined to a research setting because image capture and subsequent processing was extremely labor intensive. However, advances in both hardware and software capabilities promise to allow the use of IVCM in routine clinical practice. Real-time evaluation of the cornea at a cellular level will be particularly useful in patients with inflammatory, infectious, or neuropathic complications of keratorefractive surgery. [J Refract Surg. 2021;37(7):493-503.].

Cell-Free Biological Approach for Corneal Stromal Wound Healing

Corneal opacification is the fourth most common cause of blindness globally behind cataracts, glaucoma, and age-related macular degeneration. The standard treatment of serious corneal scarring is corneal transplantation. Though it is effective for restoring vision, the treatment outcome is not optimal, due to limitations such as long-term graft survival, lifelong use of immunosuppressants, and a loss of corneal strength. Regulation of corneal stromal wound healing, along with inhibition or downregulation of corneal scarring is a promising approach to prevent corneal opacification. Pharmacological approaches have been suggested, however these are fraught with side effects. Tissue healing is an intricate process that involves cell death, proliferation, differentiation, and remodeling of the extracellular matrix. Current research on stromal wound healing is focused on corneal characteristics such as the immune response, angiogenesis, and cell signaling. Indeed, promising new technologies with the potential to modulate wound healing are under development. In this review, we provide an overview of cell-free strategies and some approaches under development that have the potential to control stromal fibrosis and scarring, especially in the context of early intervention.

Extracellular Matrix Deposition and Remodeling after Corneal Alkali Burn in Mice

Corneal transparency relies on the precise arrangement and orientation of collagen fibrils, made of mostly Type I and V collagen fibrils and proteoglycans (PGs). PGs are essential for correct collagen fibrillogenesis and maintaining corneal homeostasis. We investigated the spatial and temporal distribution of glycosaminoglycans (GAGs) and PGs after a chemical injury. The chemical composition of chondroitin sulfate (CS)/dermatan sulfate (DS) and heparan sulfate (HS) were characterized in mouse corneas 5 and 14 days after alkali burn (AB), and compared to uninjured corneas. The expression profile and corneal distribution of CS/DSPGs and keratan sulfate (KS) PGs were also analyzed. We found a significant overall increase in CS after AB, with an increase in sulfated forms of CS and a decrease in lesser sulfated forms of CS. Expression of the CSPGs biglycan and versican was increased after AB, while decorin expression was decreased. We also found an increase in KS expression 14 days after AB, with an increase in lumican and mimecan expression, and a decrease in keratocan expression. No significant changes in HS composition were noted after AB. Taken together, our study reveals significant changes in the composition of the extracellular matrix following a corneal chemical injury.

The Effect of SPARC on the Proliferation and Migration of Limbal Epithelial Stem Cells During the Corneal Epithelial Wound Healing

Secreted protein acidic and rich in cysteine (SPARC) shows a specific colocalization with limbal epithelial stem cells (LESCs) in vivo; however, the inherent relationship between SPARC and LESCs is still unclear. This study investigated the effects of SPARC on the maintenance of LESC stemness and corneal wound healing. To test the influence of different concentration of exogenous SPARC on the proliferation of LESCs, cell counting kit-8 assay and 5-ethynyl-2'-deoxyuridine staining were performed and the results indicated that 1 μg/mL SPARC was the optimum concentration for enhanced LESC proliferation. Compared with a control group, SPARC-treated group showed a higher expression of LESC-positive markers p63α, ABCG-2, and Bmi-1, and a lower level of differentiation marker cytokeratin-3 (CK3), thereby suggesting that SPARC could maintain LESC characteristic phenotype and suppress spontaneous epithelial differentiation in vitro. In vivo, exogenous SPARC accelerated the wound-healing process by both the enhancement of LESC proliferation and promoting the migration of the proliferating cells. However, the intact epithelium impaired this function of SPARC by contact inhibition.

The fibrinolytic system in the cornea: A key regulator of corneal wound healing and biological defense

The cornea is a relatively unique tissue in the body in that it possesses specific features such as a lack of blood vessels that contribute to its transparency. The cornea is supplied with soluble blood components such as albumin, globulin, and fibrinogen as well as with nutrients, oxygen, and bioactive substances by diffusion from aqueous humor and limbal vessels as well as a result of its exposure to tear fluid. The healthy cornea is largely devoid of cellular components of blood such as polymorphonuclear leukocytes, monocytes-macrophages, and platelets. The location of the cornea at the ocular surface renders it susceptible to external insults, and its avascular nature necessitates the operation of healing and defense mechanisms in a manner independent of a direct blood supply. The fibrinolytic system, which was first recognized for its role in the degradation of fibrin clots in the vasculature, has also been found to contribute to various biological processes outside of blood vessels. Fibrinolytic factors thus play an important role in biological defense of the cornea. In this review, we address the function of the fibrinolytic system in corneal defense including wound healing and the inflammatory response.

Acetylcholine decreases formation of myofibroblasts and excessive extracellular matrix production in an in vitro human corneal fibrosis model

Acetylcholine (ACh) has been reported to play various physiological roles, including wound healing in the cornea. Here, we study the role of ACh in the transition of corneal fibroblasts into myofibroblasts, and in consequence its role in the onset of fibrosis, in an in vitro human corneal fibrosis model. Primary human keratocytes were obtained from healthy corneas. Vitamin C (VitC) and transforming growth factor‐β1 (TGF‐β1) were used to induce fibrosis in corneal fibroblasts. qRT‐PCR and ELISA analyses showed that gene expression and production of collagen I, collagen III, collagen V, lumican, fibronectin (FN) and alpha‐smooth muscle actin (α‐SMA) were reduced by ACh in quiescent keratocytes. ACh treatment furthermore decreased gene expression and production of collagen I, collagen III, collagen V, lumican, FN and α‐SMA during the transition of corneal fibroblasts into myofibroblasts, after induction of fibrotic process. ACh inhibited corneal fibroblasts from developing contractile activity during the process of fibrosis, as assessed with collagen gel contraction assay. Moreover, the effect of ACh was dependent on activation of muscarinic ACh receptors. These results show that ACh has an anti‐fibrotic effect in an in vitro human corneal fibrosis model, as it negatively affects the transition of corneal fibroblasts into myofibroblasts. Therefore, ACh might play a role in the onset of fibrosis in the corneal stroma.

The Effect of Topical Substance-P Plus Insulin-like Growth Factor-1 (IGF-1) on Epithelial Healing After Photorefractive Keratectomy in Rabbits

Purpose

To determine whether topical Substance-P (SP) plus insulin-like growth factor-1 (IGF-1) can improve corneal healing after photorefractive surface ablation in a rabbit.

Methods

After a 9.0-mm corneal de-epithelialization using a combination of chemical (18% alcohol) and mechanical debridement, excimer photorefractive surface ablation was performed bilaterally in eight rabbits (16 eyes) with an 8.0-mm ablation zone and 70-μm depth. The right eye was treated with SP (250 μg/mL) and IGF-1 (25 ng/mL) in hyaluronic acid, one drop twice a day, and the other eye treated with only hyaluronic acid. The epithelial healing process was documented photographically twice a day until healing was complete. Six rabbits were sacrificed 6 weeks after photorefractive keratectomy (PRK) and corneas examined histologically.

Results

Seven of eight rabbit eyes treated with SP/IGF-1 healed in a shorter time than the untreated eye. For rabbit #6, both eyes healed at the same time. The average healing time (total time until wound closure) for the treated eyes was 99 hours, while the average healing time for the untreated eyes was 170 hours (P = 0.0490). A persistent epithelial defect was found in two of the nontreated eyes but none in the treated eyes. Corneal pathology showed some degree of epithelial separation in the central corneal wound in three out of six nontreated eyes and in just the treated eye of rabbit #6.

Conclusion

Topical SP plus IGF-1 increases the epithelial healing rate after PRK. There may have been beneficial effects upon cell adhesion as well.

Translational Relevance

Better and faster healing.

Photorefractive keratectomy for patients with preoperative low Schirmer test value

Purpose

To compare dry eye signs and symptoms between patients with preoperative low and normal Schirmer test after Photorefractive keratectomy (PRK).

Methods

In this prospective, nonrandomized, comparative case series, 76 eyes of 76 patients were preoperatively categorized into two groups according to selected criteria for characterization of tear film status: the low Schirmer test value (STV) group and the normal STV group. For the tear function assessment, we performed a Schirmer test with and without anesthesia, tear break-up time (TBUT) test, and measurement dry eye symptoms using the Farsi translation of Ocular Surface Disease Index (OSDI) questionnaire pre- and 3 months post-operation.

Results

Postoperatively, the Schirmer and TBUT values were significantly lower in both groups than preoperatively (all p < 0.05). Deterioration in tear secretion was significantly greater in the low STV group (p = 0.012), but tear stability was more compromised in the normal STV group (p = 0.021). The changes in OSDI score were not significant between the two groups.

Conclusion

These results demonstrated that tear function deteriorates after PRK. Therefore, patients with low preoperative Schirmer test values should be thoroughly assessed for dry eye before proceeding with refractive surgery to eliminate postoperative complication.

Progress in corneal wound healing

Corneal wound healing is a complex process involving cell death, migration, proliferation, differentiation, and extracellular matrix remodeling. Many similarities are observed in the healing processes of corneal epithelial, stromal and endothelial cells, as well as cell-specific differences. Corneal epithelial healing largely depends on limbal stem cells and remodeling of the basement membrane. During stromal healing, keratocytes get transformed to motile and contractile myofibroblasts largely due to activation of transforming growth factor-β (TGF-β) system. Endothelial cells heal mostly by migration and spreading, with cell proliferation playing a secondary role. In the last decade, many aspects of wound healing process in different parts of the cornea have been elucidated, and some new therapeutic approaches have emerged. The concept of limbal stem cells received rigorous experimental corroboration, with new markers uncovered and new treatment options including gene and microRNA therapy tested in experimental systems. Transplantation of limbal stem cell-enriched cultures for efficient re-epithelialization in stem cell deficiency and corneal injuries has become reality in clinical setting. Mediators and course of events during stromal healing have been detailed, and new treatment regimens including gene (decorin) and stem cell therapy for excessive healing have been designed. This is a very important advance given the popularity of various refractive surgeries entailing stromal wound healing. Successful surgical ways of replacing the diseased endothelium have been clinically tested, and new approaches to accelerate endothelial healing and suppress endothelial-mesenchymal transformation have been proposed including Rho kinase (ROCK) inhibitor eye drops and gene therapy to activate TGF-β inhibitor SMAD7. Promising new technologies with potential for corneal wound healing manipulation including microRNA, induced pluripotent stem cells to generate corneal epithelium, and nanocarriers for corneal drug delivery are discussed. Attention is also paid to problems in wound healing understanding and treatment, such as lack of specific epithelial stem cell markers, reliable identification of stem cells, efficient prevention of haze and stromal scar formation, lack of data on wound regulating microRNAs in keratocytes and endothelial cells, as well as virtual lack of targeted systems for drug and gene delivery to select corneal cells.

Genomics of corneal wound healing: a review of the literature

Corneal wound healing is a complex process: its mechanisms and the underlying genetic control are not fully understood. It involves the integrated actions of multiple growth factors, cytokines and proteases produced by epithelial cells, stromal keratocytes, inflammatory cells and lacrimal gland cells. Following an epithelial insult, multiple cytokines are released triggering a cascade of events that leads to repair the epithelial defect and remodelling of the stroma to minimize the loss of transparency and function. In this review, we examine the literature surrounding the genomics of corneal wound healing with respect to the following topics: epithelial and stromal wound healing (including inhibition); corneal neovascularisation; the role of corneal nerves in wound healing; the endothelium; the role of aquaporins and aptamers. We also examine the effect of ectasia on corneal wound healing with regard to keratoconus and following corneal surgery. A better understanding of the cellular and molecular changes that occur during repair of corneal wounds will provide the opportunity to design treatments that selectively modulate key phases of the healing process resulting in scars that more closely resemble normal corneal architecture.

Corneal stromal cell plasticity: in vitro regulation of cell phenotype through cell-cell interactions in a three-dimensional model

In vivo, epithelial cells are connected both anatomically and functionally with stromal keratocytes. Co-culturing aims at recapturing this cellular anatomy and functionality by bringing together two or more cell types within the same culture environment. Corneal stromal cells were activated to their injury phenotype (fibroblasts) and expanded before being encapsulated in type I collagen hydrogels constructs. Three different epithelial-stromal co-culture methods were then examined: epithelial explant; transwell; and the use of conditioned media. The aim was to determine whether the native, inactivated keratocyte cell phenotype could be restored in vitro. Media supplementation with transforming growth factor beta-1 (TGF-β1) was then used to determine whether the inactivated stromal cells retained their plasticity in vitro and could be re-activated to the fibroblast phenotype. Finally, media supplementation with wortmannin was used to inhibit epithelial-stromal cell interactions. Two different nondestructive techniques, spherical indentation and optical coherence tomography, were used to reveal how epithelial-stromal co-culturing with TGF-β1, and wortmannin media supplementation, respectively, affect stromal cell behavior and differentiation in terms of construct contraction and elastic modulus measurement. Cell viability, phenotype, morphology, and protein expression were investigated to corroborate our mechanical findings. It was shown that activated stromal cells could be inactivated to a keratocyte phenotype via co-culturing and that they retained their plasticity in vitro. Activated corneal stromal cells that were fibroblastic in phenotype were successfully reverted to a nonactivated keratocyte cell lineage in terms of behavior and biological properties; and then back again via TGF-β1 media supplementation. It was then revealed that epithelial-stromal interactions can be blocked via the use of wortmannin inhibition. A greater understanding of stromal-epithelial interactions and what mediates them offers great pharmacological potential in the regulation of corneal wound healing, with the potential to treat corneal diseases and injury by which such interactions are vital.

Control of scar tissue formation in the cornea: strategies in clinical and corneal tissue engineering

Corneal structure is highly organized and unified in architecture with structural and functional integration which mediates transparency and vision. Disease and injury are the second most common cause of blindness affecting over 10 million people worldwide. Ninety percent of blindness is permanent due to scarring and vascularization. Scarring caused via fibrotic cellular responses, heals the tissue, but fails to restore transparency. Controlling keratocyte activation and differentiation are key for the inhibition and prevention of fibrosis. Ophthalmic surgery techniques are continually developing to preserve and restore vision but corneal regression and scarring are often detrimental side effects and long term continuous follow up studies are lacking or discouraging. Appropriate corneal models may lead to a reduced need for corneal transplantation as presently there are insufficient numbers or suitable tissue to meet demand. Synthetic optical materials are under development for keratoprothesis although clinical use is limited due to implantation complications and high rejection rates. Tissue engineered corneas offer an alternative which more closely mimic the morphological, physiological and biomechanical properties of native corneas. However, replication of the native collagen fiber organization and retaining the phenotype of stromal cells which prevent scar-like tissue formation remains a challenge. Careful manipulation of culture environments are under investigation to determine a suitable environment that simulates native ECM organization and stimulates keratocyte migration and generation.

Structural and biochemical aspects of keratan sulphate in the cornea

Keratan sulphate (KS) is the predominant glycosaminoglycan (GAG) in the cornea of the eye, where it exists in proteoglycan (PG) form. KS-PGs have long been thought to play a pivotal role in the establishment and maintenance of the array of regularly-spaced and uniformly- thin collagen fibrils which make up the corneal stroma. This characteristic arrangement of fibrils allows light to pass through the cornea. Indeed, perturbations to the synthesis of KS-PG core proteins in genetically altered mice lead to structural matrix alterations and corneal opacification. Similarly, mutations in enzymes responsible for the sulphation of KS-GAG chains are causative for the inherited human disease, macular corneal dystrophy, which is manifested clinically by progressive corneal cloudiness starting in young adulthood.

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.

Nitric oxide levels of aqueous humor after photorefractive keratectomy

PURPOSE

To measure the nitric oxide (NO) levels of aqueous humor in rabbits after photorefractive keratectomy (PRK) and to evaluate the alterations of NO levels according to the PRK surgery steps, ablation depth, and time.

METHODS

Fifty eyes of 25 New Zealand white rabbits were included in the study. One eye was later randomly excluded from the study in order to equalize the number of eyes in groups. Eyes were divided into seven groups, each comprising seven eyes: unwounded control (Group 1), epithelial scrape (Group 2; aqueous humor samples taken at the 4th hour), superficial PRK (Group 3; samples taken at the 4th hour), deep PRK (Group 4; samples taken at the 4th hour), epithelial scrape (Group 5; samples taken at the 24th hour), superficial PRK (Group 6; samples taken at the 24th hour), and deep PRK (Group 7; samples taken at the 24th hour). The corneal epithelium was mechanically removed in surgical groups. The authors performed superficial corneal ablation (59 microm) in Groups 3 and 6 and deep corneal ablation (99 microm) in Groups 4 and 7. Aqueous humor samples were taken at the 4th hour (Groups 2-4) or 24th hour (Groups 5-7) after corneal surgeries. NO measurements were performed indirectly by using the Griess reaction with a spectrophotometer.

RESULTS

Aqueous humor NO levels 4 hours after corneal surgery were statistically significantly lower than the control group (p<0.05). However, there was no difference among the surgical groups at the 4th hour (p>0.05). At the 24th hour, the deep PRK group had significantly lower NO levels than both the control group and Groups 5 and 6 (p<0.05). NO levels were normalized at the 24th hour in epithelial scrape and superficial PRK groups (p>0.05) but remained stable at lower levels in deep PRK groups (p<0.05).

CONCLUSIONS

Corneal surgery caused low NO levels in aqueous humor 4 hours after surgery. However, 24 hours after surgery, NO levels normalized following epithelial scrape and superficial PRK and were stable at lower levels in the deep PRK group. Complications of deep PRK application are possibly induced by low NO existence in the aqueous humor.

Proteoglycan alterations in the rabbit corneal stroma after a lamellar incision

PURPOSE

To investigate proteoglycans under minimally damaged epithelium after a lamellar microkeratome incision.

SETTING

Collaborating university departments.

METHODS

Anterior lamellar caps were excised from rabbit corneas and then resutured in place. Healing tissue was examined by electron microscopy with proteoglycan staining.

RESULTS

In the weeks after surgery, regions of disorganized stromal matrix were populated by sulfated proteoglycan filaments that were much larger (up to 300 nm long) than those in quiescent stroma.

CONCLUSIONS

Large, sulfated proteoglycans existed in rabbit corneas healing from lamellar incisions. These molecules appear to be a normal feature of corneal wound healing; because of their water-binding capacity, they might aid tissue restructuring.

Corneal wound healing after photorefractive keratectomy: a 3-year confocal microscopy study

PURPOSE

To perform a sequential quantitative analysis of corneal wound healing after photorefractive keratectomy (PRK) by using confocal microscopy in vivo.

METHODS

In a prospective, nonrandomized, comparative trial performed in an institutional setting, 24 eyes of 14 patients received PRK to correct refractive errors between -1.25 and -5.75 D. Central corneas were examined preoperatively and at 1 day, 5 days, and 1, 3, 6, 12, 24, and 36 months after PRK by using confocal microscopy. A masked observer randomly examined 3 to 6 confocal scans per eye per visit to determine epithelial and stromal thickness, keratocyte density in 5 anterior-posterior stromal layers, corneal nerve density in the subbasal region and the stroma, and corneal light backscattering (corneal haze).

RESULTS

Epithelial thickness increased 21% (P < .001) by 12 months after PRK and thereafter remained unchanged to 36 months after PRK. There was no change in stromal thickness between 1 and 36 months after PRK (P = .35). The dense keratocyte population in the preoperative anterior 10% of the stroma (32,380 +/- 5,848 cells/mm3) that was partially or completely removed during photoablation was not reconstituted at 36 months in the anterior 10% of the post-PRK stroma (17,720 +/- 4,308 cells/mm3, P < .001). Subbasal nerve fiber bundle density was decreased 60% at 12 months after PRK (P < .001) before returning to densities at 24 and 36 months after PRK that were not significantly different from preoperative values (P = 1.0). Activated keratocytes and corneal haze peaked at 3 months after PRK.

CONCLUSIONS

Wounding of the cornea by PRK alters the normal structure, cellularity, and innervation of the cornea for up to 36 months.

Experimental Model of Corneal Haze in Chickens

PURPOSE

To develop an experimental animal model of corneal haze following photorefractive keratectomy (PRK).

METHODS

Fifteen Iber Braun hens underwent unilateral PRK for -9.00 D of myopia. The animals were sacrificed at 1, 3, and 6 months postoperatively, and light microscopy was performed.

RESULTS

Slit-lamp microscopy showed haze in the PRK-treated eyes. Histopathologic study disclosed epithelial hyperplasia, basement membrane abnormalities, and extensive anterior stromal disorganization.

CONCLUSIONS

An easy and inexpensive model of haze after PRK was developed in an animal with Bowman's layer. This new model could be useful to understand the pathophysiology and pharmacologic modulation of corneal haze.

Cellular proliferation and leukocyte infiltration in the rabbit cornea after photorefractive keratectomy

PURPOSE

To map the proliferative activity of corneal cells during wound healing following photorefractive keratectomy (PRK) and to compare two markers for proliferation.

METHODS

PRK, 5- mm in diameter with a -6 D setting, was performed in one eye of 28 New Zealand White Rabbits. The rabbits were sacrificed at time points between 12 hours and three months after surgery. The treated and fellow corneas were fixed in 10% formaldehyde, paraffin embedded, and immunohistochemically stained for proliferate cell nuclear antigen (PCNA) and at one time point, 1 week, also for Ki-67.

RESULTS

Following initial sliding of the epithelial cells, the proliferative activity in the wound area starts in the leading edge (24 hours) and is spread towards the periphery. The proliferative activity peaks after one week and subsides during the following two weeks. Early (24 hours) proliferative activity is also seen in the limbal epithelium which peaks after three days. The keratocytes express PCNA in the peripheral stroma 48 hours after injury. They then also migrate to repopulate the stroma under the wound area. The expression period lasts 1 week and subsides the following week. Leukocytes are found in the wound as early as 12 hours after injury. The cells disappear around the time of epithelial wound closure, i.e. after 3 days. The two proliferative markers PCNA and KI 67 show a similar distribution after surgery.

CONCLUSION

Epithelial proliferative activity starts earlier after injury, and is preceded by leukocyte presence in the wound. The PCNA expression starts later in the keratocytes but lasts somewhat longer (3 weeks). PCNA expression appears more efficient than Ki-67 to show proliferative activity of slow cycling cells in the cornea

Reduction in corneal haze and apoptosis by amniotic membrane matrix in excimer laser photoablation in rabbits

PURPOSE

To determine whether preserved human amniotic membrane can reduce corneal haze and keratocyte apoptosis induced by excimer laser photoablation in rabbit corneas.

METHODS

Excimer photoablation was performed bilaterally in 30 rabbits with a 6.0 mm ablation zone and 120 microm depth using the VISX Star laser with the phototherapeutic keratectomy (PTK) mode. One eye was randomly covered by preserved human amniotic membrane secured with 4 interrupted 10-0 nylon sutures, and the other eye served as the control. The amniotic membranes were removed at 1 week, and corneal haze was graded with slitlamp biomicroscopy by 3 masked corneal specialists biweekly for the ensuing 12 weeks until the rabbits were killed. Another 18 rabbits were divided into 4 subgroups and received PTK alone, PTK with membrane, PTK with sham sutures, or PTK with tarsorrhaphy. All eyes were studied histologically, and 3 eyes in each group were studied by in situ terminal deoxynucleotidyl transferase deoxy-UTP-nick end labeling assay at 1, 3, and 7 days and 12 weeks, respectively.

RESULTS

A consistent grading of differences in corneal haze scoring between the control corneas and the amniotic-membrane-covered corneas was noted among the 3 masked observers. Organized reticular post-PTK corneal haze peaked at 7 weeks in both groups, and the corneal haze score in the amniotic-membrane-covered group was significantly less than in the control group from 7 to 12 weeks (all P < .001). Compared to the control corneas, the amniotic-membrane-covered corneas had less inflammatory response at 1 and 3 days, less keratocyte apoptosis in the ablated anterior corneal stroma at 1, 3, and 7 days (P < .001), and less stromal fibroblast cellularity and epithelial hyperplasia at 12 weeks.

CONCLUSIONS

Amniotic membrane matrix introduced at an early stage of the corneal wound healing process effectively reduced corneal haze induced by excimer laser photoablation in rabbits. Studies linking suppression of apoptosis in the acute wound-healing process with reduction of subsequent corneal scarring may have useful clinical applications.

Corneal haze after photorefractive keratectomy using different epithelial removal techniques: mechanical debridement versus laser scrape

PURPOSE

To determine differences of corneal wound healing and haze after photorefractive keratectomy (PRK) using either mechanical epithelial debridement or laser-scrape epithelial removal in human subjects.

DESIGN

A 6-month randomized, masked, prospective, paired-eye clinical study.

PARTICIPANTS

Twenty eyes in 10 myopic patients treated between March 1999 and May 1999.

INTERVENTION

Photorefractive keratectomy treatments with two different epithelial removal techniques. Continuous z-scan of confocal image, termed confocal microscopy through focusing (CMTF), was performed before surgery and at 3 weeks, 6 weeks, 3 months, and 6 months after surgery.

MAIN OUTCOMES MEASURES

Epithelial and stromal thickness measurement, achieved stromal ablation depth, and objective assessment of corneal light-backscattering (corneal haze) were obtained from digital image analysis of the CMTF scans. Manifest refraction was also measured. Student's paired t test or two-way repeated-measures analysis of variance after rank transformation were performed to evaluate statistical differences between groups.

RESULTS

Comparison of the mean posttreatment spherical equivalent between the two techniques showed no statistically significant difference. In preoperative corneas, mean epithelial thickness was 50.08+/-3.70 microm in the mechanical debridement group and 50.49+/-4.01 microm in laser-scrape group (not significant). For both groups, the epithelium was significantly thinner at 3 weeks, but returned to preoperative values by 6 months, with no difference between groups. Planned stromal ablation depth by PRK was 59.38+/-11.48 microm (39-73 microm; n = 8) in the mechanical group and 57.75 +/- 7.21 microm (48-70 microm; n = 8) in the laser-scrape group. Achieved stromal ablation depth was not significantly different between the two groups. Most importantly, in both groups CMTF-measured corneal haze increased significantly after surgery, peaked at 3 months, and then decreased at 6 months, with no significant difference between groups. (Power = 0.96).

CONCLUSIONS

There is no significant difference in the corneal wound healing response between mechanical epithelial debridement versus laser-scrape technique in human myopic eyes undergoing PRK.

Topical vitamin E and hydrocortisone acetate treatment after photorefractive keratectomy

PURPOSE

To investigate the effects of topical vitamin E and hydrocortisone acetate treatments on corneal healing response after -10.0 D photorefractive keratectomy (PRK) in rabbits.

METHODS

Thirty-three New Zealand white rabbits were divided into four groups and -10 D PRK was performed under in vivo conditions. Following PRK, group 1 (n = 9) received no topical treatment and served as control. Group 2 (n = 8) received 0.1% hydrocortisone acetate ointment twice a day, group 3 (n = 8) received 1% vitamin E ointment and group 4 (n = 8) received both 0.1% hydrocortisone acetate and 1% vitamin E twice a day for a month. At the end of the third month, corneal haze was graded and the corneal hydroxyproline levels were measured, as a crude indicator of new collagen synthesis. Finally corneal samples were examined by transmission electron microscopy.

RESULTS

Non-homogeneously distributed strong haze was identified in group 1 which was greater than in the other groups; haze was least in groups 2 and 4. Corneal hydroxyproline levels were found to be significantly lower in groups 2, 3 and 4 compared with the control (Student's t-test, p < 0.05). Histopathologically, the most aggressive wound healing response was detected in group 1. The corneal wound healing response of group 2 was less than that of group 1 and equal to or more than that of group 4.

CONCLUSIONS

Deep corneal photoablation induces an aggressive healing response, and topical hydrocortisone acetate reduces this corneal wound healing effectively. The inhibitory effect of topical vitamin E on corneal wound healing seems to be less than that of hydrocortisone acetate, but combined treatment with these two drugs may have an additive effect in controlling corneal wound healing after PRK.

Confocal microscopy of a patient with irregular astigmatism after LASIK reoperations and relaxation incisions

PURPOSE

Laser-assisted in situ keratomileusis (LASIK) is widely used for correcting refractive errors. If the predicted refractive result is not achieved after the first operation, a re-operation can be performed by ablating more stromal tissue after reopening the flap. The goal of this study was to analyze, by using in vivo confocal microscopy, the morphologic changes associated with repeated LASIKs.

METHODS

Clinical examination, computed corneal topography, and real-time in vivo confocal microscopy were performed on both eyes of a 50-year-old patient with induced irregular astigmatism leading to decreased best-corrected vision in the left eye after LASIK. The left cornea had been operated on 5 times (LASIK with two reoperations followed by two relaxing incisions), and the right cornea twice (LASIK with one reoperation).

RESULTS

Microfolds at the level of the Bowman's layer and highly reflective particles at the flap interface were observed in both corneas. The subbasal nerve plexus was severed in the left eye. In addition, we identified epithelial material in the flap margin and inside one of the two relaxing incisions placed inferotemporally.

CONCLUSION

Repeated LASIKs may stretch the flap and result in microfolding at the Bowman's layer. This and deposition of particles in the flap interface may increase with the number of reoperations, challenging the healing response. Microfolding and occurrence of foreign material in the interface may add to the irregular astigmatism and poor visual outcome after LASIK. Clinical in vivo confocal microscopy offers new possibilities for the assessment of ultrastructural changes after corneal refractive surgery.

Role of the Corneal Epithelium in Refractive Changes Following Laser in situ Keratomileusis for High Myopia

PURPOSE

Epithelial hyperplasia is one of the factors that plays a role in myopic regression after photorefractive keratectomy (PRK). We evaluated the role of the epithelium following excimer laser in situ keratomileusis (LASIK) performed on highly myopic eyes.

METHODS

Fifty eyes of 32 patients (18 females and 14 males; mean age, 31.7+/-6.5 years) were treated with LASIK for myopia with an attempted correction ranging between -8.50 and -12.25 D (mean, -10.48+/-1.43 D). No sutures were placed. The thickness of the central epithelium was evaluated either with a 50-MHz ultrasonic pachymeter or an ultrasound biomicroscope. Postoperatively, topical corticosteroid drops were administered for at least 1 month.

RESULTS

Follow-up was at least 12 months. Epithelium measurements were thicker compared to preoperative measurements from the first week postoperatively (mean, +1.9 microm, +2.77%; P<.05). Epithelial thickness peaked at the third month after LASIK (mean, +6.5 microm, +9.5%; P<.05) and remained stable through 12 months. A negative correlation between epithelium thickness and manifest refraction from the first month postoperatively to the end of follow-up was present.

CONCLUSION

The epithelium seemed to have a role in the regression of refractive effect after LASIK to correct high myopia.

Corneal haze among blue eyes and brown eyes after photorefractive keratectomy

BACKGROUND

To compare the incidence and severity of corneal haze after photorefractive keratectomy (PRK) among white patients with blue eyes and Saudi patients with brown eyes.

DESIGN

Retrospective, nonrandomized, comparative trial.

PARTICIPANTS

A total of 150 patients (71 females and 79 males) were included in this study. Two hundred sixty-six eyes of 150 patients were subjected to PRK. One hundred blue eyes of 50 white patients and 166 brown eyes of 100 Saudi patients were included in this study.

METHODS

Two hundred sixty-six eyes of 150 patients were subjected to PRK with the Chiron Technolas Keracor 117C for the correction of myopia and astigmatism.

MAIN OUTCOME MEASURES

All patients had complete ophthalmologic examinations, visual acuity testing, intraocular pressure, pachymetry, corneal haze assessment (0-4+), and computerized corneal topography.

RESULTS

There were 266 eyes of 150 patients with 100 blue irides and 166 brown irides. The spherical equivalent was -0.50 diopter (D) to -8.75 D. The mean postoperative spherical equivalent at 6 months was -0.063 D (standard deviation [SD], +/-0.595) in blue eyes compared to -0.28 D (SD, +/-0.683) in brown eyes (P = 0.006). Ninety-five (95%) of 100 of the blue eyes achieved +/- 1 D of attempted correction compared to 148 (89.2%) of the 166 brown eyes. All patients with blue eyes had a visual acuity of 20/30 or better compared to 153 (92.2%) of the 166 brown eyes (P = 0.009). Forty-eight (18.04%) eyes had minimal to mild haze, 3 (1.12%) eyes had moderate haze, and 2 (0.75%) eyes had severe haze. The incidence of corneal haze among brown eyes was 48 (28.9%) of 166 eyes compared to 5 (5%) of 100 in blue eyes (P < 0.001). The difference remained significant after adjustment for age and gender with a P value of 0.0283. The relative risk for developing haze in brown eyes was found to be 7.72.

CONCLUSION

The incidence of corneal haze after PRK was significantly higher among Saudi patients with brown irides than among white patients with blue irides. This suggests that racial factors may play a role in the development of corneal haze.

Photorefractive keratectomy for hyperopia: long-term nonlinear and vector analysis of refractive outcome

PURPOSE

To characterize the refractive changes after excimer laser photorefractive keratectomy for the correction of hyperopia over a follow-up up to 3 years and to assess refractive stability and changes in astigmatism.

DESIGN

Noncomparative, nonrandomized, retrospective, interventional case series.

PARTICIPANTS

Thirty-eight hyperopic eyes of 28 patients (age range, 33-62 years) with refraction in the range +1.00 to +8.00 diopters (D). Mean attempted correction was +3.33+/-0.98 D (range, +1.00 to +4.00 D). Data were compared to those from 216 eyes treated for myopia in the range -1.00 to -12.70 D.

INTERVENTION

The hyperopic correction was made using an erodible mask inserted in the laser optical pathway to produce a circular ablation measuring 6.5 mm in diameter. An axicon was then used to create a blend transition zone from 6.5 mm up to 9.4 mm in diameter. Eyes were evaluated 3 to 11 times (5.5+/-2.4) over a 3- to 34-month follow-up (16.8+/-8.4 months).

MAIN OUTCOME MEASURES

Vector analysis of refractive error, applying a nonlinear statistical model fitting the spherical equivalent, and the sphere component data. The fit parameters were the long-term error at stabilization (epsilon(infinity)), the amount of regression (epsilon0), being the difference of refractive errors immediately after surgery and at stabilization, and the time constant (T1/2) giving the temporal scale length by which the overcorrection halves (regression half-life). Cylinder was analyzed by a linear regression.

RESULTS

The initial overcorrection was much larger after hyperopic treatments than myopic ones (epsilon0 = -3.26+/-0.35 D vs. +1.43+/-0.35 D), and it takes typically four times longer to regress (T1/2 = 3.30+/-0.91 months). Refractive stabilization is reached after more than 1 year, with a satisfactory refractive result. The hyperopic treatment induces a mean astigmatism of 1.00 D, which remains constant throughout the follow-up, and tends to be aligned along the with-the-rule meridian.

CONCLUSIONS

The advantages of a reasonably well-designed algorithm to correct hyperopia (epsilon(infinity) = +0.20+/-0.23 D) are counterbalanced by the long time to refractive stabilization and by the induced astigmatism.

Comparison of wound healing after photorefractive keratectomy and laser in situ keratomileusis in rabbits

PURPOSE

To evaluate and compare the corneal wound-healing process after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK).

SETTING

Kangnam St. Mary's Hospital, Seoul, Korea.

METHODS

Two surgical procedures, PRK with the VISX Star excimer laser and LASIK with a MicroTech microkeratome, were performed in 24 rabbit eyes. In the PRK group (n = 12 eyes), the rabbit cornea was treated with a 20 microns ablation. In the LASIK group (n = 12 eyes), a 20 microns laser ablation was performed after a 150 microns thick hinged corneal flap had been made. During both procedures, dichlorotriazinyl aminofluorescien (DTAF) dye was applied to the ablated stromal bed; in the LASIK group, the stromal side of the corneal flap was also stained with DTAF to differentiate regenerated collagen from normal stromal tissue. Corneal wound healing was evaluated postoperatively at 1, 4, 8, and 12 weeks using light, electron, and fluorescence microscopy. The amount of regenerated stromal tissue and the number of keratocytes were analyzed by an image-analysis system.

RESULTS

In the PRK group, epithelial migration and regeneration were observed in the ablated area without any stromal regeneration 1 week postoperatively. However, newly regenerated, irregularly arranged stromal collagen, with epithelial hyperplasia in the ablated area, was observed 4 to 12 weeks postoperatively by light and fluorescence microscopy. The number of keratocytes in the surgical area was also increased. In ultrastructural observation using an electron microscope, the shape of keratocytes in the ablated area was changed, and the number of rough and smooth endoplasmic reticuli, ribosomes, mitochondria, and electron-dense vesicles in the cytoplasm were increased, suggesting that the cells were activated. In the LASIK group, there was no observed regenerated collagen between the corneal flap and the ablated stromal bed except in the wound margin. Lamellated, parallel collagen fibers in the cornealstroma were not disturbed. However, in the wound margin, corneal epithelial ingrowth between the flap and the stromal bed was observed, as was some regenerated stromal tissue. The amount of regenerated stromal tissue and the number of keratocytes in the wound area were statistically smaller than those in the PRK group (P < .05). Observation by electron microscopy showed no activated keratocytes, unlike in the PRK group. The collagen fibers in the wound area were parallel.

CONCLUSION

Stromal wound healing in the LASIK group was minimal compared with that in the PRK group, except in the wound margin. These results may support the clinical findings of less corneal haze in the human cornea after LASIK.

Regression and epithelial hyperplasia after myopic photorefractive keratectomy in a human cornea

We present the histology of a cornea of a woman who had excimer laser photorefractive keratectomy (PRK) for myopia 6 months before she died in an accident. Preoperative spherical refraction was -6.00 diopters (D) with an astigmatism of -0.50 D. Six months postoperatively, refraction was -4.50 D. Slight corneal haze was noted at 1 and 3 months. The corneal histology showed marked epithelial hyperplasia in the center of the ablation zone without subepithelial deposition of newly synthesized collagen, proteoglycans, or both. The epithelial thickness was 38 microns in the untreated area and 93 microns in the center of the ablation. In conclusion, postoperative epithelial hyperplasia was responsible for regression after PRK in this eye.

Keratocyte density in vivo after photorefractive keratectomy in humans

PURPOSE

To determine changes in keratocyte density in central human corneas in vivo after photorefractive keratectomy (PRK).

METHODS

Fifteen patients (25 eyes) received excimer PRK (VISX Star) with epithelial removal by laser-scrape (43 microns ablation followed by manual scrape) to correct myopia between -1.5 D and -7.25 D. Corneas were examined by using confocal microscopy in vivo before PRK and at 1 day, 5 days, 1 month, and 3 months after PRK. A custom automated image-processing algorithm identified bright objects (keratocytes) against a dark background and estimated keratocyte density by using the number and size of the objects. Cell density was quantified in anteroposterior stromal regions after PRK and compared to cell density in corresponding pre-PRK regions.

RESULTS

One day after PRK, keratocyte density increased 9% in the anterior third of the stroma (pi = .003), was unchanged in the middle third of the stroma (pi = .481), and decreased 6% in the posterior third of the stroma (pi = .038). Keratocyte density remained elevated in the anterior stroma to 3 months after PRK; at this time, there was a 13% increase in keratocyte density throughout the full-thickness stroma (pi < .001).

CONCLUSIONS

Keratocyte density was increased in the anterior stroma immediately after PRK in humans. Three months later, keratocyte density was increased in all anteroposterior stromal regions, suggesting that PRK affects keratocytes throughout the entire central cornea.

Corneal Wound Healing After Laser in situ Keratomileusis in Rabbits

BACKGROUND

The aim of this study was to characterize the cell biology of wound healing in rabbit corneas subjected to laser in situ keratomileusis (LASIK).

METHODS

Rabbit corneas underwent LASIK with various multizone photoablations or only a lamellar keratotomy followed by repositioning of the flap. We looked for indications for an active wound healing process. Immunohistochemistry for the extradomain A cellular fibronectin (EDA-cFn) or tenascin (Tn) and routine histology were examined.

RESULTS

Four days after LASIK or lamellar keratotomy followed by repositioning of the flap, epithelial plugs and prominent keratocytes as well as Tn and EDA-cFn immunoreactions-indicative of a wound-healing process-appeared in the wound margins. Epithelial plugs were less conspicous, and prominent, presumably activated, keratocytes were no longer identified at the wound margin at 2.5 and 5 months after wounding. However, EDA-cFn and Tn immunoreactivities could still be observed. Only the stromal cells located in the periphery of the flap and in relatively close contact with the epithelium were surrounded by scar tissue expressing immunoreactivity for EDA-cFn or Tn. The central corneal stroma was devoid of scar tissue.

CONCLUSION

Results indicate that the wound healing reaction after LASIK takes place only at the periphery of the microkeratome wound, leaving the central optical zone clear.

The Effect of Topical Corticosteroids on Refractive Outcome and Corneal Haze after Photorefractive Keratectomy

BACKGROUND

The effect of topical corticosteroids after excimer laser photorefractive keratectomy (PRK) remains a matter of some controversy. Refractive effects may be different according to the amount of myopia and timing of instillation.

METHODS

Two groups of patients were studied: Study A consisted of 215 eyes (128 patients) with PRK (mean baseline myopia, -6.53 +/- 2.22 D) that received no corticosteroids (No Corticosteroid Group) unless significant regression or corneal haze appeared (Delayed Corticosteroid Group), and in Study B, we randomly assigned eyes to the Initial Corticosteroid Group (mean baseline myopia, -6.39 +/- 1.84 D) or the No/delayed Corticosteroid Group (mean baseline myopia -5.78 +/- 2.02 D). Clinical results after PRK for low-to-moderate and high myopia were compared.

RESULTS

In the first group, 70.9% (73 eyes) of moderately myopic eyes (mean, -4.56 +/- 1.10 D) belonged to the No Corticosteroid Group that had a mean refraction of -5.39 +/- 1.77 D. Delayed Corticosteroid Group eyes were more myopic (mean, -7.52 +/- 2.10 D), and showed more severe haze than those in the No Corticosteroid Group. In study B, only in high myopes with more than -6.00 D (mean, -7.76 +/- 1.15 D) did refraction and corneal haze outcomes show significant difference between the Initial Corticosteroid Group and the No/delayed Corticosteroid Group.

CONCLUSIONS

The effects of topical corticosteroids after PRK were less in moderate myopes compared to high myopes. Delayed instillation of corticosteroids did not reverse the regression or haze whereas initial instillation showed a beneficial effect on high myopes but not on moderate myopes.

Short-term effects of flurbiprofen and diclofenac on refractive outcome and corneal haze after photorefractive keratectomy

PURPOSE

To evaluate the short-term effects of topical nonsteroidal anti-inflammatory drugs (NSAIDs) on refractive outcome and corneal haze after excimer laser photorefractive keratectomy (PRK) according to the degree of myopia and to compare the results with those of topical steroids.

SETTING

Seoul National University Hospital, Seoul, Korea.

METHODS

Patients were divided into two groups: low to moderate myopia (-6.00 diopters [D] or less) and high myopia (greater than 6.00 D). Then, each patient was randomly assigned to one of three drug subgroups for initial management (4 months post-PRK): corticosteroids (fluorometholone 0.1%); flurbiprofen sodium 0.03% (Ocufen); diclofenac sodium 0.1% (Decrol). Follow-up was 6 months.

RESULTS

In eyes with low to moderate myopia, the steroid and diclofenac subgroups had significantly different refractions 2 and 4 months postoperatively but no difference at 6 months; subjective haze grading was consistently lower in the steroid subgroup than in the NSAID subgroups (flurbiprofen, diclofenac) after 2 months. In eyes with high myopia, the steroid subgroup had significantly less myopic regression after 3 weeks and lower subjective haze after 2 months than the NSAID subgroups. The steroid subgroup had severe myopic regression or corneal haze less frequently than the NSAID subgroups.

CONCLUSION

Topical NSAIDs were less effective than topical steroids in reducing myopic regression and haze after PRK, especially in highly myopic eyes.

Factors affecting epithelial hyperplasia after photorefractive keratectomy

PURPOSE

To determine the effect of patient age, postoperative time, ablation zone diameter and depth, attempted correction, and corneal topography on postoperative corneal epithelial thickness after photorefractive keratectomy (PRK).

SETTING

Private clinic and university hospital, Stockholm, Sweden.

METHODS

This retrospective, unmasked study comprised 136 myopic patients treated unilaterally with PRK. Seventy eyes had been treated with the Summit excimer laser 27 months +/- 7 (SD) earlier using ablation zone diameters of 4.1 to 5.0 mm. Sixty-six eyes had been treated with the VISX excimer laser 6 +/- 3 months earlier using a 6.0 mm zone diameter. The untreated fellow eyes served as controls. Epithelial thickness was measured at a standardized central corneal area with a modified optical pachymeter, and corneal topography was determined using computerized videokeratoscopy.

RESULTS

In the Summit group, the epithelial layer in the PRK eyes was 12.0 microns (21%) thicker than in the control eyes (P < .001; 95% confidence interval [CI] 9.35 to 14.3 microns). This thickness differential correlated significantly with increased ablation depth and attempted correction. In the VISX group, the epithelium in the treated eyes was 7.0 microns (7%) thinner (P = .0009; 95% CI -1.9 to -6.7 microns) and thickness did not correlate with ablation depth or attempted correction. There was no correlation between epithelial hyperplasia and patient age or postoperative follow-up. With the laser groups combined, epithelial hyperplasia was greater with smaller zone sizes and a greater rate of change in power at the edge of the ablation zone.

CONCLUSION

The factors associated with an increase in epithelial thickness were small ablation zones, greater attempted corrections, and deeper ablations. Larger, smoother ablation profiles may result in less epithelial hyperplasia.

Effect of beam variables on corneal sensitivity after excimer laser photorefractive keratectomy

AIM

To investigate changes in corneal touch sensitivity following excimer laser photorefractive keratectomy (PRK) using different beam configurations.

METHODS

20 subjects were given a unilateral -3.00 D correction with either a 5 mm (26 micrograms, n = 10) or 6 mm (42 micrograms, n = 10) beam diameter. Thirty subjects underwent a unilateral -6.00 D correction with 5 mm (62 micrograms, n = 10), 6 mm (78 micrograms, n = 10), or multizone (62 micrograms, n = 10) treatments. The multizone treatment was 6 mm in diameter with the depth of the 5 mm treatment. Corneal sensitivity was measured using a slit-lamp mounted Cochet-Bonnet aesthesiometer before and at 1, 3, 6, and 12 months after PRK. Stimulus locations included points lying within the ablated zone (central) and outside (peripheral). These were compared with the equivalent locations in control (untreated) eyes.

RESULTS

There was a significant reduction in corneal sensitivity within the central (ablated) zone in all treatment groups after PRK. In most groups a return to full sensitivity was achieved by 6 months with the exception of the multizone treatment group which showed significant corneal hypoaesthesia at 12 months. Peripheral corneal sensitivity was also reduced in this group up to 3 months after the procedure. A comparison between the -3.00 D and -6.00 D treatment groups showed no significant difference. However, combining data from all treatment groups, a significant correlation was found between the interocular difference in central corneal sensitivity and postoperative haze at 3 and 6 months.

CONCLUSIONS

For corrections up to -6.00 D ablation depth and treatment zone diameter do not appear to be clinically important determinants of corneal hypoaesthesia. In contrast, postoperative corneal haze appears to correlate with sensitivity loss.

Real-time confocal microscopic observations on human corneal nerves and wound healing after excimer laser photorefractive keratectomy

PURPOSE

Corneal wound healing after excimer laser photorefractive keratectomy (PRK) passes through a series of characteristic stages which have earlier been defined by means of histological, histochemical, and biochemical approaches. We investigated the potential of confocal microscopy to verify morphological changes in human corneas in vivo after PRK.

METHODS

Ten corneas of eight patients that had earlier undergone PRK were examined at different postoperative time points (7 days-34 months). One of the PRK patients was examined sequentially three times. Three additional corneas, which had earlier undergone corneal grafting surgery and then were subjected to excimer laser photoastigmatic keratectomy (PARK), were studied as well. Seven healthy untreated corneas served as controls to define the normal morphology of human cornea. A tandem scanning confocal microscope (TSCM) was used to generate real-time images of the corneas on an S-VHS videotape. The images were either digitized and further processed or the individual video frames were produced with a video printer.

RESULTS

Seven days post-PRK in vivo confocal microscopy revealed the presence of morphologically immature surface epithelial cells. Delicate nerves, activated keratocytes and deposition of extracellular light-reflecting scar tissue were perceived. The epithelium appeared normal one month post-PRK. Ongoing activation of the anterior stromal keratocytes along with extracellular scar tissue were detected. We also observed increasing numbers of regenerating subepithelial nerve leashes with somewhat twisted pattern. Highly reflective, presumably activated keratocytes were no longer detected 6-7 months post-PRK. Hypercellularity with scar tissue could still be found up to 30 months post-PRK. Only one cornea examined 34 months post-PRK showed normal keratocyte morphology and recovery of the anterior stroma. However, the morphology of subepithelial nerves was still somewhat abnormal. The two corneal grafts examined 11 or 32 months post-PARK exhibited a normal-appearing epithelium but considerable stromal hypercellularity and extracellular scar deposition. The subepithelial nerves were poorly regenerated in one eye and fairly well detectable in the other. The third graft examined 15 months post-PARK revealed the presence of enlarged surface epithelial cells and dense stromal scarring but no nerves.

CONCLUSION

TSCM clinically confirms the earlier histological data on healing of excimer laser wounds. It offers a distinct improvement in the assessment of excimer laser-treated corneas, as it enables cellular details and nerves to be perceived in vivo. In addition the thickness of the stromal scar can be be measured for e.g. planning of phototherapeutic keratectomy.

Comparison of Corneal Epithelial Wound Healing after Photorefractive Keratectomy in the Rabbit with Two Types of Excimer Lasers

PURPOSE

To evaluate the differences in epithelial wound healing following photorefractive keratectomy when performed with the Summit UV 200 LA and the VISX 20/20 excimer lasers.

METHODS

Sixteen New Zealand rabbits were divided into two groups. One group was treated with the Summit laser and the other group treated with the VISX 20/20 laser. The treatment consisted of a -6.00 diopter photorefractive keratectomy with a 5-mm diameter treatment zone. Epithelial wound healing was followed by photography at 4 hour intervals for 64 hours. The length of the wound edge and the size, shape, and closure time of the wound were measured.

RESULTS

The median wound edge length at 4 hours was 18.3 mm for the Summit laser and 16.7 mm for the VISX laser. The median wound size at 4 hours was 22.0 mm2 for the Summit and 21.2 mm2 for the VISX. The median wound closure time was 53.4 hours for the Summit laser and 54.0 hours for the VISX laser.

CONCLUSION

There was no statistically significant difference in the epithelial healing of rabbit corneal wounds created by photorefractive keratectomy when performed with two current ophthalmic excimer lasers, the Summit UV 200 LA and the VISX 20/20.

Quantification of stromal thinning, epithelial thickness, and corneal haze after photorefractive keratectomy using in vivo confocal microscopy

PURPOSE

The authors establish, for the first time, observer-independent quantification of stromal thinning, epithelial thickness, and corneal haze after excimer laser photorefractive keratectomy (PRK) using a unique, new form of in vivo confocal microscopy.

METHODS

Rapid, continuous z-scans of high-resolution confocal images, termed confocal microscopy through focusing (CMTF), were performed in the central corneal area of 17 patients before and 1 month after PRK for low- to moderate-grade myopia (-2.88-9.13 diopters [D]). Corneal, epithelial, and stromal thickness measurements and an objective haze estimate were obtained from each CMTF scan by digital image analysis.

RESULTS

Epithelial thickness averaged 51 +/- 4 microns before and 45 +/- 10 microns 1 month post-PRK (P < 0.005), whereas stromal thinning ranged from 20 to 154 microns, representing a direct estimate of the actual photoablation depth. Corneal thickness averaged 560 +/- 36 microns before PRK and 462 +/- 52 microns at 1 month. The change in corneal thickness correlated closely with the change in spherical equivalent refraction (r = 0.94, P < 0.0001); linear regression analysis revealed a value of 14.3 microns corneal thinning per diopter of correction. A significant correlation was found between the objective CMTF haze estimate and a clinical haze grading obtained by slit-lamp examination (r = 0.73, P < 0.001).

CONCLUSIONS

Confocal microscopy through focusing is a new, powerful in vivo tool that enables quantitative, unbiased evaluation of PRK procedures over time by providing epithelial and stromal thickness analysis, photoablation depth assessment, and unbiased haze measurement. The method is uniquely valuable in the pre- and postoperative assessment of PRK patients and for determining the optimal treatment strategy, especially in assessing refractive and visual outcomes in individual cases.