Progressive post-LASIK keratectasia: biomechanical instability or chronic disease process?

Posterior corneal surface stability after femtosecond laser-assisted in situ keratomileusis in patients with myopia and myopic astigmatism

PURPOSE

To evaluate the variation and stability of the posterior cornea surface parameters (posterior cornea curvature [PCC], posterior cornea astigmatism [PCA], and posterior cornea elevation [PCE]) after femtosecond laser-assisted in situ keratomileusis (LASIK) in patients with myopia and myopic astigmatism over a period of 6 months or longer.

METHODS

This retrospective study comprised 284 right eyes. Patients aged 18 years or older with myopia up to -12.00 D and/or astigmatism up to -6.00 DC and who underwent femtosecond LASIK were recruited. Patients were divided into three subgroups: low myopia (-0.50 to -3.00 D), moderate myopia (>-3.00 to ≤-6.00 D), and high myopia (>-6.00 D), according to their pre-LASIK spherical equivalent (SE). The variables included for analysis were PCC (central 0-3.0 mm, pericentral 3.0-6.0 mm, and peripheral region 6.0-9.0 mm), PCE, PCA, internal anterior chamber depth, intraocular pressure, and central cornea thickness at the pre- and post-LASIK stages.

RESULTS

The central PCC remained unchanged across all three myopia subgroups at 1 month when compared to the pre-LASIK stage and remained stable at 6 months. The pericentral regions became flatter across all myopia subgroups at 1 month postsurgery (P < 0.001) and remained unchanged at 6 months. This trend was not seen in the peripheral cornea regions, which remained unchanged at 1 and 6 months post-LASIK when compared to pre-LASIK mean readings. There were minimal changes in post-LASIK posterior cornea astigmatism throughout follow-up. There was no incidence of post-LASIK surgery ectasia in this study population.

CONCLUSION

Post-LASIK, the different cornea subregions behaved differently. Overall, the posterior cornea surface remained stable post-LASIK across all myopia subgroups throughout follow-up.

Corneal Collagen Crosslinking for Ectasia After Refractive Surgery: A Systematic Review and Meta-Analysis

Introduction

Corneal ectasia leads to progressive irregular corneal curvature and reduced visual acuity.

Objective

To assess the safety and effectiveness of corneal collagen cross-linking (CXL) for managing corneal ectasia resulting from refractive laser surgery (RSL).

Methods

A systematic review and meta-analysis were realized according to PRISMA guidelines. We searched PubMed, EMBASE, Cochrane, and Web of Science databases for studies on CXL in patients with ectasia after RLS. The outcomes of interest included visual acuity, refractive outcomes, topographic parameters (Kmax, index surface variance (ISV), index of Vertical Asymmetry (IVA), keratoconus index (KI), central keratoconus index (CKI), index of height asymmetry (IHA), index of height decentration (IHD) and Rmin (minimum sagittal curvature)), central corneal thickness, endothelial cell count, and possible adverse events. Statistical analysis was performed using the R software (version 4.2.3, R Foundation for Statistical Computing, Vienna, Austria).

Results

15 studies encompassing 421 patients (512 eyes) were included. The mean age was 32.03 ± 4.4 years. The pooled results showed a stable uncorrected visual acuity post-CXL, with a significant improvement in corrected distance visual acuity (SMD = 0.09; 95% CI: -0.07 to 0.26). The spherical equivalent decreased significantly (SMD = -0.09; 95% CI: -0.35, -0.02). The topographic parameter Kmax decreased significantly (SMD = 0.15; 95% CI:0.01 to 0.28); however, the other parameters, ISV, IVA, KI, CKI, IHA, IHD, and Rmin, did not change significantly. Central corneal thickness decreased significantly (SMD = 0.24; 95% CI:0.07 to 0.41), and the endothelial cell count remained stable The complications were rare.

Conclusion

CXL is a safe and effective technique for managing corneal ectasia after RLS.

Visual and Topographic Outcomes of Corneal Collagen Cross Linking for Post LASIK Ectasia

Purpose

To assess the topographic and visual outcomes of corneal cross-linking (CXL) for post-LASIK ectasia.

Methods

A retrospective case series of patients who had progressive post-LASIK ectasia, with at least 2 years of follow-up. They had epithelium-off CXL. Topographical and visual changes were recorded.

Results

The study included 21 eyes of 11 patients. At month 24, the final logMAR corrected distance visual acuity (CDVA) and spherical equivalent (SE) were stable compared to baseline (0.16, and −2.0 diopter (D), respectively). The final Kmean and Kmax were 42.5 and 47.4 D, respectively. Stability or improvement in CDVA, SE, and Kmax was seen in 17 eyes (81%). Significant corneal thinning was seen (438 vs 457 microns, p = 0.003). Thinning by 2% or more was seen in 12 eyes (57.1%). Failure of CXL was seen in 4 eyes (19%). No other ocular complications were seen.

Conclusion

CXL for post LASIK ectasia is a safe and effective modality. Despite corneal thinning, there was stability or improvement in topographic parameters and CDVA over the 2-year follow-up period.

Laser refractive surgery in pregnant or breastfeeding patients

This systematic review reported the outcomes of laser corneal refractive surgery in pregnant or breastfeeding patients. This study was performed by searching in PubMed, Web of Science, and Scopus databases, on June 15, 2020. Included were 128 eyes from a total of 64 patients, with the mean maximum follow-up was 39.2 ± 36.14 months. Time from surgery to complication ranged from 1 to 67 months, with a mean value of 23.42 ± 22.23 months. Photorefractive keratectomy and laser in situ keratomileusis surgery seem to be stable procedures that are not modified during pregnancy and safe to complete during breastfeeding. Nevertheless, the lack of weight prospective research avoids having a greater certainty on this matter, and because of transitory nature of pregnancy and breastfeeding, it could still be contemplated that surgery risk outweigh the benefits. Additional investigation will be necessary to clarify these issues.

Three-Year Follow-Up of Laser In Situ Keratomileusis Treatments for Myopia: Multi-Center Cohort Study in Korean Population

This multi-center cohort study included 3401 myopic laser in situ keratomileusis (LASIK) procedures conducted in 1756 myopia patients between 2002 and 2005. Pre- and postoperative uncorrected visual acuity (UCVA), best corrected visual acuity (BCVA), and manifest refraction spherical equivalent (SE) were recorded. Factors predicting low postoperative efficacy (defined as a postoperative UCVA < 0.5) were identified using univariate and multivariate logistic regression analysis. Compared with 1 month postoperatively, logMAR UCVA at 3 months postoperatively was significantly decreased (p = 0.002) and that at 2 and 3 years was significantly increased (p < 0.001). LogMAR BCVA at 2 years postoperatively was significantly decreased compared with 1 month postoperatively (p = 0.008). Over the 3-year postoperative period, overall refractive predictability within ±1.00 D and ±0.50 D ranged from 69.0% to 86.2% and from 43.3% to 67.8%, respectively. This also decreased from 1 month to 6 months postoperatively (p < 0.005). Multivariate logistic regression analysis using generalized estimating equations, revealed that higher preoperative SE (odds ratio [OR], 2.58 and 7.23; p < 0.001) and lower preoperative BCVA (OR, 2.44; p = 0.003) were predictive of a low postoperative efficacy. In summary, myopic LASIK can be effective and safe with a high refractive predictability in a Korean population, but myopic regression occurs over time. Higher preoperative SE and lower preoperative BCVA are predictive of a low postoperative efficacy.

Crosslinking und Keratokonus

Ein Keratokonus führt zu einer progressiven Vorwölbung und Verdünnung der Hornhaut. Um dies aufzuhalten, kann ein Crosslinking durchgeführt werden. Dabei ist eine Behandlung nach dem „Dresdener Protokoll“ eine effektive und sichere Behandlungsmöglichkeit, aber auch zahlreiche neue Anwendungsprotokolle (akzeleriertes Crosslinking, transepitheliales Crosslinking) und sogar weitere Indikationen (refraktive Eingriffe, infektiöse Keratitis) wurden in den letzten Jahren veröffentlicht.

[Crosslinking and Keratoconus]

Keratoconus leads to a progressive protrusion and thinning of the cornea. In order to stop this, corneal crosslinking can be performed if the progression of the disease is proven. Crosslinking according to the "Dresden protocol" includes abrasion of the corneal epithelium, application of riboflavin eye drops and irradiation with UV-A light of an intensity of 3 mW/cm² for 30 minutes. The efficacy has been shown in several prospective randomized studies. One of the more recent developments is accelerated crosslinking, which allows a shorter irradiation time. On the other hand, the possibility of transepithelial crosslinking was presented, which does not require an abrasion of the cornea. This should reduce the occurrence of postoperative pain. The range of indications has also been expanded. Corneal crosslinking is used for post-LASIK keratectasia as well. It is also being considered for use in infectious keratitis. Topographically controlled crosslinking can likewise be used to try to positively influence the refractive power of the cornea. The risks of crosslinking include the occurrence of pain, haze or scarring, endothelial cell damage and, rarely, the occurrence of keratitis.

Evaluating the Ectasia Risk Score System in Cancelled Laser In Situ Keratomileusis Candidates

Purpose

To evaluate the ectasia risk score system in cancelled laser in situ keratomileusis (LASIK) candidates at an academic hospital.

Methods

LASIK candidates who had been cancelled by a surgeon considering the patient age, preoperative central corneal thickness, residual stromal bed thickness, or preoperative manifest refraction spherical equivalent were retrospectively reviewed, and their Randleman ectasia risk score system score was calculated.

Results

The mean ectasia score of 194 eyes (97 patients) was 4.5 ± 2.67; 40 (20.6%), 46 (23.7%), and 108 (55.7%) eyes were classified as low-, moderate-, and high-risk eyes, respectively. The topography was abnormal in 69% of the patients. The mean manifest refraction spherical equivalent, central corneal thickness, and estimated residual stromal bed thickness were 4 (+0.50 to –15.50) diopters, 520 (439 to 608) µm, and 312.38 (61.5 to 424.12) µm, respectively. The main cause of cancellation in low- and moderate-risk patients (86 eyes) was the presence of unstable refractive error in the past year.

Conclusion

Although promising, some other criteria, such as stable refraction, should be added to this scoring system to achieve greater practicality since a main cause of cancelling LASIK candidates in this study was the presence of unstable refraction.

Could the percent tissue altered (PTA) index be considered as a unique factor in ectasia risk assessment?

PURPOSE

To evaluate the efficacy, safety, predictability and stability of the percent tissue altered (PTA) formula in post-LASIK (laser-assisted in situ keratomileusis) ectasia risk assessment.

METHODS

One hundred and ninety-three eyes from 104 patients with low to moderate myopia that underwent femtosecond LASIK were included in this retrospective, observational and longitudinal study. Seventy-eight eyes were classified in the higher-PTA (> 40%) group and 115 eyes in the lower-PTA (< 40%) group. Spherical manifest refraction, cylinder manifest refraction, logMAR and Snellen previous corrected distance visual acuity (CDVA), uncorrected distance visual acuity (UDVA), anterior face Baiocchi Calossi Versaci index (BCVf) and posterior face (BCVb), central corneal thickness (CCT), estimated residual stromal bed (RSB), spherical aberration (SA) and root mean square (RMS) were reported. All patients were followed up for 4 years.

RESULTS

In the higher-PTA group, 97% of eyes reported UDVA 20/20 or better, and in the lower-PTA group, all eyes achieved 20/20 or better. No group reported decreased visual acuity. The higher-PTA group obtained 55% of eyes within ± 0.50 D and 90% within ± 1.00 D; and the lower-PTA group obtained 63% of eyes within ± 0.50 D and 90% within ± 1.00 D. Both groups showed a refraction correction of 0.50 D or more in 36% and 31% of eyes, in the higher-PTA and the lower-PTA groups, respectively.

CONCLUSION

Percent tissue altered index should not be considered as a unique variable in post-LASIK ectasia risk assessment. Ectasia susceptibility screening should integrate tomography and biomechanical variables in order to help us to decide any refractive treatment choice and increasing refractive surgery safety.

Evaluation of the percentage tissue altered as a risk factor for developing post-laser in situ keratomileusis ectasia

PURPOSE

To assess the currently recommended percentage tissue altered (PTA) metric for its ability to screen for ectasia after laser in situ keratomileusis (LASIK).

SETTING

Gavin Herbert Eye Institute, University of California, Irvine, California, USA, and Rothschild Foundation, Paris, France.

DESIGN

Retrospective case series.

METHODS

The study used a LASIK database created by 1 surgeon for LASIK cases with normal preoperative topography that had a minimum follow-up of 24 months with complete preoperative and intraoperative data to permit the calculation of PTA values to detect eyes at risk for developing ectasia.

RESULTS

Of the eyes, 593 eyes had complete data and met the inclusion criteria. Based on measured flap thickness, 126 eyes (21%) had a PTA value of 40% or more (mean 44) and a percentage of that flap thickness accounted for the PTA (mean 66.7%; range 34% to 92%). The mean attempted laser ablation was 79.8 μm ± 29.2 (SD), and the mean residual bed thickness was 304.4 ± 29.2 μm (range 212 to 369 μm). No eye developed ectasia over a mean follow-up of 30 months.

CONCLUSIONS

The current PTA calculation when applied to a LASIK population with normal preoperative topography and flap thickness measured with ultrasound did not predict the risk for ectasia. Differences between study populations and assumptions might have accounted for the different outcomes obtained in the initially published PTA study.

Intraocular Pressure-dependent Corneal Elasticity Measurement Using High-frequency Ultrasound

Measurement of corneal biomechanical properties can aid in predicting corneal responses to diseases and surgeries. For delineation of spatially resolved distribution of corneal elasticity, high-resolution elastography system is required. In this study, we demonstrate a high-resolution elastography system using high-frequency ultrasound for ex-vivo measurement of intraocular pressure (IOP)-dependent corneal wave speed. Tone bursts of 500 Hz vibrations were generated on the corneal surface using an electromagnetic shaker. A 35-MHz single-element transducer was used to track the resulting anti-symmetrical Lamb wave in the cornea. We acquired spatially resolved wave speed images of the cornea at IOPs of 7, 11, 15, 18, 22, and 29 mmHg. The IOP dependence of corneal wave speed is apparent from these images. Statistical analysis of measured wave speed as a function of IOP revealed a linear relation between wave speed and IOP c_s = 0.37 + 0.22 × IOP, with the coefficient of determination R² = 0.86. We also observed depth-dependent variations of wave speed in the cornea, decreasing from anterior toward posterior. This depth dependence is more pronounced at higher IOP values. This study demonstrates the potential of high-frequency ultrasound elastography in the characterization of spatially resolved corneal biomechanical properties.

Regional changes in corneal shape over a 6-month follow-up after femtosecond-assisted LASIK

PURPOSE

To assess the regional changes in corneal shape after femtosecond laser-assisted laser in situ keratomileusis (FS-LASIK) in patients with different myopia extents.

SETTING

Eye Hospital, Wenzhou Medical University, Wenzhou, China.

DESIGN

Retrospective case series.

METHODS

A retrospective study of myopic eyes treated with FS-LASIK was conducted to assess the shape changes within different corneal regions after surgery. Corneal curvature was measured in the central region (0 mm to 3.0 mm diameter), pericentral region (3.0 mm to 6.0 mm diameter) and peripheral region (6.0 mm to 9.0 mm diameter) preoperatively and from 1 week to 6 months postoperatively.

RESULTS

The study comprised 608 myopic eyes. During the 6-month follow-up, the anterior cornea became steeper in the central and pericentral regions, but flatter in the peripheral region (P < .01), representing a partial, gradual, yet significant reversal of the immediate change in corneal shape after laser ablation. In contrast, the posterior surface experienced significantly less change than the anterior surface, with the cornea becoming slightly flatter (P < .01) in the central region at 1 week postoperatively, and steeper elsewhere (P < .05), and then remaining stable during the rest of the follow-up. On the other hand, the anterior astigmatism had significant decreases in the central region (P < .01) and slight increases in the peripheral region (P < .01) 1 week postoperatively, and that remained stable over the follow-up period. In contrast, there were little or non-significant changes in the posterior astigmatism throughout the follow-up (P > .05).

CONCLUSIONS

Postoperative corneal shape changes were different in different regions. There were shape changes in individual corneal regions during the 6-month follow-up period that represented reverse trends; however, the changes were much smaller than the short-term changes observed 1 week after surgery.

Enhanced Tomographic Assessment to Detect Corneal Ectasia Based on Artificial Intelligence

PURPOSE

To improve the detection of corneal ectasia susceptibility using tomographic data.

DESIGN

Multicenter case-control study.

METHODS

Data from patients from 5 different clinics from South America, the United States, and Europe were evaluated. Artificial intelligence (AI) models were generated using Pentacam HR (Oculus, Wetzlar, Germany) parameters to discriminate the preoperative data of 3 groups: stable laser-assisted in situ keratomileusis (LASIK) cases (2980 patients with minimum follow-up of 7 years), ectasia susceptibility (71 eyes of 45 patients that developed post-LASIK ectasia [PLE]), and clinical keratoconus (KC; 182 patients). Model accuracy was independently tested in a different set of stable LASIK cases (298 patients with minimum follow-up of 4 years) and in 188 unoperated patients with very asymmetric ectasia (VAE); these patients presented normal topography (VAE-NT) in 1 eye and clinically diagnosed ectasia in the other (VAE-E). Accuracy was evaluated with ROC curves.

RESULTS

The random forest (RF) provided highest accuracy among AI models in this sample with 100% sensitivity for clinical ectasia (KC+VAE-E; cutoff 0.52), being named Pentacam Random Forest Index (PRFI). Considering all cases, the PRFI had an area under the curve (AUC) of 0.992 (94.2% sensitivity, 98.8% specificity; cutoff 0.216), being statistically higher than the Belin/Ambrósio deviation (BAD-D; AUC = 0.960, 87.3% sensitivity, 97.5% specificity; P = .006, DeLong's test). The optimized cutoff of 0.125 provided sensitivity of 85.2% for VAE-NT and 80% for PLE, with 96.6% specificity.

CONCLUSION

The PRFI enhances ectasia diagnosis. Further integrations with corneal biomechanical parameters and with the corneal impact from laser vision correction are needed for assessing ectasia risk.

Evaluation of corneal biomechanics in patients with keratectasia following LASIK using dynamic Scheimpflug analyzer

PURPOSE

To investigate the corneal biomechanics in eyes with keratectasia following LASIK using a dynamic Scheimpflug analyzer.

DESIGN

Case-Control study.

METHOD

The subjects in the study included 12 eyes with keratectasia after LASIK (KE), 24 eyes with keratoconus (KC), 17 eyes without keratectasia after LASIK (LASIK), and 34 eyes with normal corneas (Normal). Corneal biomechanics of the four groups were evaluated using a dynamic Scheimpflug analyzer.

RESULTS

Compared with Normal (7.06 ± 0.54), the radius at the highest concavity (radius, mm) of LASIK (5.96 ± 0.76), KE (4.93 ± 0.61) and KC (5.39 ± 1.02) were significantly small. The Deflection Amplitude (HCDLA, mm) of Normal (0.94 ± 0.07) was significantly lower than those of KE (1.11 ± 0.10) and KC (1.06 ± 0.16), and was not significantly different from that of LASIK (0.98 ± 0.07). There were significant differences between LASIK and KE in radius and HCDLA (P < 0.05), whereas KE and KC had no differences in these parameters.

CONCLUSIONS

Corneal biomechanical features evaluated using the dynamic Scheimpflug analyzer suggest that biomechanical properties in eyes with keratectasia, keratoconus, and LASIK are different from those of normal eyes. Although the biomechanics in eyes with keratectasia differs from that in eyes with LASIK, it is similar to that in eyes with keratoconus.

Procollagen C-proteinase enhancer 1 (PCPE-1) functions as an anti-angiogenic factor and enhances epithelial recovery in injured cornea

Procollagen C-proteinase enhancer 1 (PCPE-1) has been characterized as a protein capable of enhancing the activity of bone morphogenetic protein 1/tolloid-like proteinases in the biosynthetic processing of C-propeptides from procollagens I-III. This processing step is thought necessary to the formation of collagen I-III monomers capable of forming fibrils. Thus, PCPE-1 is predicted to play an important role in scarring, as scar tissue is predominantly composed of fibrillar collagen. Corneal scarring is of great clinical importance, as it leads to loss of visual acuity and, in severe cases, blindness. Here, we investigate a possible role for PCPE-1 in corneal scarring. Although differences in corneal opacity associated with scarring following injury of Pcolce ^-/- and wild-type (WT) mice using full-thickness excision or alkali burn models of corneal injury were not grossly apparent, differences in procollagen I processing levels between Pcolce ^-/- and WT primary corneal keratocytes were consistent with a role for PCPE-1 in corneal collagen deposition. An unexpected finding was that neoangiogenesis, which follows alkali burn cornea injury, was strikingly increased in Pcolce ^-/- cornea, compared to WT. A series of aortic ring assays confirmed the anti-angiogenic effects of PCPE-1. Another unexpected finding was of abnormalities of epithelial basement membrane and of re-epithelialization following Pcolce ^-/- corneal injury. Thus, PCPE-1 appears to be of importance as an anti-angiogenic factor and in re-epithelialization following injury in cornea and perhaps in other tissues as well.

A review and meta-analysis of corneal cross-linking for post-laser vision correction ectasia

PURPOSE

The aim of this study was to review the safety and stability of cornea cross-linking (CXL) for the treatment of keratectasia after Excimer Laser Refractive Surgery.

METHODS

Eligible studies were identified by systematically searching PubMed, Embase, Web of Science and reference lists. Meta-analysis was performed using Stata 12.1 software. The primary outcome parameters included the changes of corrected distant visual acuity (CDVA), uncorrected visual acuity (UCVA), the maximum keratometry value (Kmax) and minimum keratometry value (Kmin), the surface regularity index (SRI), the surface asymmetry index (SAI), the keratoconus prediction index (KPI), corneal thickness, and endothelial cell count. Efficacy estimates were evaluated by weighted mean difference (WMD) and 95% confidence interval (CI) for absolute changes of the interested outcomes.

RESULTS

Seven studies involving 118 patients treated with CXL for progressive ectasia after laser-assisted in situ keratomileusis (LASIK) or photorefractive keratectomy (PRK) (140 eyes; the follow-up time range from 12 to 62 months) were included in the meta-analysis. The pooled results showed that there were no significant differences in Kmax and Kmin values after CXL (WMD = 0.584; 95% CI: -0.289 to 1.458; P = 0.19; WMD = 0.466; 95% CI: -0.625 to 1.556; P = 0.403, respectively). The CDVA improved significantly after CXL (WMD = 0.045; 95% CI: 0.010 to 0.079; P = 0.011), whereas UCVA did not differ statistically (WMD = 0.011; 95% CI: -0.055 to 0.077; P = 0.746). The changes were not statistically significant in SRI, SAI, and KPI (WMD = 0.116; 95% CI: -0.090 to 0.322; P = 0.269; WMD = 0.240; 95% CI: -0.200 to 0.681; P = 0.285; WMD = 0.045; 95% CI: -0.001 to 0.090; P = 0.056, respectively). Endothelial cell count and corneal thickness did not deteriorate (WMD = 12.634; 95% CI: -29.460 to 54.729; P = 0.556; WMD = 0.657; 95% CI: -9.402 to 10.717; P = 0.898, respectively).

CONCLUSION

The study showed that CXL is a promising treatment to stabilize the keratectasia after Excimer Laser Refractive Surgery. Further long-term follow-up studies are necessary to assess the persistence of the effect of the CXL.

Effects of ablation depth and repair time on the corneal elastic modulus after laser in situ keratomileusis

BACKGROUND

The biomechanical properties of the cornea should be taken into account in the refractive procedure in order to perform refractive surgery more accurately. The effects of the ablation depth and repair time on the elastic modulus of the rabbit cornea after laser in situ keratomileusis (LASIK) are still unclear.

METHODS

In this study, LASIK was performed on New Zealand rabbits with different ablation depth (only typical LASIK flaps were created; residual stroma bed was 50 or 30% of the whole cornea thickness respectively). The animals without any treatment were served as normal controls. The corneal thickness was measured by ultrasonic pachymetry before animals were humanly killed after 7 or 28 days post-operatively. The corneal elastic modulus was measured by uniaxial tensile testing. A mathematical procedure considering the actual geometrics of the cornea was created to analyze the corneal elastic modulus.

RESULTS

There were no obvious differences among all groups in the elastic modulus on after 7 days post-operatively. However, after 28th days post-operatively, there was a significant increase in the elastic modulus with 50 and 30% residual stroma bed; only the elastic modulus of the cornea with 30% residual stroma bed was significantly higher than that of 7 days.

CONCLUSIONS

Changes in elastic modulus after LASIK suggest that this biomechanical effect may correlate with the ablation depth and repair time.

Combined effects of interleukin-1β and cyclic stretching on metalloproteinase expression in corneal fibroblasts in vitro

Background

Corneal tensile strain increases if the cornea becomes thin or if intraocular pressure increases. However, the effects of mechanical stress on extracellular matrix (ECM) remodelling in the corneal repair process and the corneal anomalies are unknown.

Methods

In this study, the combined effects of interleukin-1β (IL-1β) on matrix metalloproteinases (MMPs) in corneal fibroblasts under cyclic stretching were investigated in vitro. Cultured rabbit corneal fibroblasts were subjected to 5, 10 or 15 % cyclic equibiaxial stretching at 0.1 Hz for 36 h in the presence of IL-1β. Conditioned medium was harvested for the analysis of MMP2 and MMP9 protein production using the gelatin zymography and western blot techniques.

Results and conclusions

Cyclic equibiaxial stretching changed the cell morphology by increasing the contractility of F-actin fibres. IL-1β alone induced the expression of MMP9 and increased the production of MMP2, and 5 % stretching alone decreased the production of MMP2, which indicates that a low stretching magnitude can reduce ECM degradation. In the presence of IL-1β, 5 and 10 % stretching increased the production of MMP2, whereas 15 % stretching increased the production of MMP9. These results indicate that MMP expression is enhanced by cyclic mechanical stimulation in the presence of IL-1β, which is expected to contribute to corneal ECM degradation, leading to the development of post-refractive surgery keratectasia.

Acoustic Radiation Force Optical Coherence Elastography of Corneal Tissue

We report on a real-time acoustic radiation force optical coherence elastography (ARF-OCE) system to map the relative elasticity of corneal tissue. A modulated ARF is used as excitation to vibrate the cornea while OCE serves as detection of tissue response. To show feasibility of detecting mechanical contrast using this method, we performed tissue-equivalent agarose phantom studies with inclusions of a different stiffness. We obtained 3-D elastograms of a healthy cornea and a highly cross-linked cornea. Finally we induced a stiffness change on a small portion of a cornea and observed the differences in displacement.

Rate of corneal collagen crosslinking redo in private practice: risk factors and safety

Objective. To report the rate of progression of keratectasia after primary crosslinking (CXL) and evaluate the safety and efficiency of CXL redo. Materials and Methods. We conducted a retrospective analysis of the patients who underwent CXL between 2010 and 2013 at the Beirut Eye Specialist Hospital, Lebanon. Progression of keratectasia was based on the presence of an increase in maximum keratometry of 1.00 D, a change in the map difference between two consecutive topographies of 1.00 D, a deterioration of visual acuity, or any change in the refraction. Primary and redo CXL were done using the same protocol. Results. Among the 221 eyes of 130 patients who underwent CXL, 7 eyes (3.17%) of five patients met the criteria of progression. All patients reported a history of allergic conjunctivitis and eye rubbing and progressed within 9 to 48 months. No complications were noted and all patients were stable 1 year after CXL redo. Conclusion. Allergic conjunctivitis and eye rubbing were the only risk factors associated with keratoconus progression after CXL. A close followup is thus mandatory, even years after the procedure. CXL redo seems to be a safe and efficient technique to halt the progression after a primary CXL.

Platelet recruitment promotes keratocyte repopulation following corneal epithelial abrasion in the mouse

Corneal abrasion not only damages the epithelium but also induces stromal keratocyte death at the site of injury. While a coordinated cascade of inflammatory cell recruitment facilitates epithelial restoration, it is unclear if this cascade is necessary for keratocyte recovery. Since platelet and neutrophil (PMN) recruitment after corneal abrasion is beneficial to epithelial wound healing, we wanted to determine if these cells play a role in regulating keratocyte repopulation after epithelial abrasion. A 2 mm diameter central epithelial region was removed from the corneas of C57BL/6 wildtype (WT), P-selectin deficient (P-sel^-/-), and CD18 hypomorphic (CD18^hypo) mice using the Algerbrush II. Corneas were studied at 6h intervals out to 48h post-injury to evaluate platelet and PMN cell numbers; additional corneas were studied at 1, 4, 14, and 28 days post injury to evaluate keratocyte numbers. In WT mice, epithelial abrasion induced a loss of anterior central keratocytes and keratocyte recovery was rapid and incomplete, reaching ~70% of uninjured baseline values by 4 days post-injury but no further improvement at 28 days post-injury. Consistent with a beneficial role for platelets and PMNs in wound healing, keratocyte recovery was significantly depressed at 4 days post-injury (~30% of uninjured baseline) in P-sel^-/- mice, which are known to have impaired platelet and PMN recruitment after corneal abrasion. Passive transfer of platelets from WT, but not P-sel^-/-, into P-sel^-/- mice prior to injury restored anterior central keratocyte numbers at 4 days post-injury to P-sel^-/- uninjured baseline levels. While PMN infiltration in injured CD18^hypo mice was similar to injured WT mice, platelet recruitment was markedly decreased and anterior central keratocyte recovery was significantly reduced (~50% of baseline) at 4–28 days post-injury. Collectively, the data suggest platelets and platelet P-selectin are critical for efficient keratocyte recovery after corneal epithelial abrasion.

Laser in situ keratomileusis for -6.00 to -18.00 diopters of myopia and up to -5.00 diopters of astigmatism: 15-year follow-up

PURPOSE

To evaluate the long-term outcomes of laser in situ keratomileusis (LASIK) for high myopia with or without astigmatism.

SETTING

Vissum Instituto Oftalmologico de Alicante and Miguel Hernandez University, Alicante, Spain.

DESIGN

Retrospective-prospective case series.

METHODS

Laser in situ keratomileusis was performed using the Visx 20/20 excimer laser. The minimum follow-up was 15 years. The main outcome measures were uncorrected (UDVA) and corrected (CDVA) distance visual acuities, manifest refraction, and corneal topography.

RESULTS

This study included 40 patients (40 eyes) with a mean age of 51.08 years ± 6.67 (SD) (range 41 to 60 years) with high myopia (-6.00 to -18.00 diopters [D]). At 15 years, the safety index was 1.23 and the efficacy index, 0.95. During the follow-up, a significant increase in the dioptric power of all keratometric variables was detected (P≤.028, Friedman test), the most notable increase occurring between 3 months and 1 year (P≤.005). At 15 years, 46.15% of the eyes were within ±1.00 D of the attempted spherical equivalent and 64.10% were within ±2.00 D. The UDVA at 15 years was 20/25 or better in 43.59% of eyes and 20/40 or better in 64.10% of eyes. The postoperative CDVA was significantly better than preoperatively (P<.001). The postoperative complications were minor except in 1 eye in which ectasia occurred.

CONCLUSIONS

Laser in situ keratomileusis for high myopia was safe over the long term. However, significant myopic regression with time was detected. Low preoperative pachymetry and low residual stromal bed were predictors of keratometric regression.

FINANCIAL DISCLOSURE

No author has a financial or proprietary interest in any material or method mentioned.

Noncontact depth-resolved micro-scale optical coherence elastography of the cornea

High-resolution elastographic assessment of the cornea can greatly assist clinical diagnosis and treatment of various ocular diseases. Here, we report on the first noncontact depth-resolved micro-scale optical coherence elastography of the cornea achieved using shear wave imaging optical coherence tomography (SWI-OCT) combined with the spectral analysis of the corneal Lamb wave propagation. This imaging method relies on a focused air-puff device to load the cornea with highly-localized low-pressure short-duration air stream and applies phase-resolved OCT detection to capture the low-amplitude deformation with nano-scale sensitivity. The SWI-OCT system is used here to image the corneal Lamb wave propagation with the frame rate the same as the OCT A-line acquisition speed. Based on the spectral analysis of the corneal temporal deformation profiles, the phase velocity of the Lamb wave is obtained at different depths for the major frequency components, which shows the depthwise distribution of the corneal stiffness related to its structural features. Our pilot experiments on ex vivo rabbit eyes demonstrate the feasibility of this method in depth-resolved micro-scale elastography of the cornea. The assessment of the Lamb wave dispersion is also presented, suggesting the potential for the quantitative measurement of corneal viscoelasticity.

New method of microwave thermokeratoplasty to correct myopia in 33 eyes: one-year results

PURPOSE

To assess the safety, predictability, and stability of a new microwave thermokeratoplasty procedure to correct myopia.

SETTING

Cornea and refractive surgery subspecialty.

DESIGN

Prospective clinical trial.

METHOD

Thermokeratoplasty was performed in myopic eyes at a single center in Turkey from June 2009 to June 2010. The attempted corrections ranged from -1.25 to -5.75 diopters (D). The main outcome measures were changes in logMAR uncorrected distance visual acuity (UDVA) and in keratometry (K) values.

RESULTS

The procedure was performed in 33 eyes (patients aged 20 to 45 years). The mean preoperative logMAR UDVA (0.76 ± 0.24 [SD]) significantly improved to 0.19 ± 0.20 at 1 month, postoperatively. By 3 months, the mean UDVA had markedly regressed to 0.59 ± 0.29; however, the residual improvement remained statistically significant. At 12 months, the mean logMAR UDVA was 0.72 ± 0.26. The mean K values were 43.9 ± 1.36 D preoperatively, 41.25 ± 2.63 D at 1 month, 43.4 ± 1.69 D at 3 months, and 44.1 ± 1.09 D at 12 months. The mean endothelial cell density was 2836 ± 342 cells/mm(2) preoperatively and were statistically unchanged 12 months postoperatively (2732 ± 353 cell/mm(2)). No patient lost lines of corrected distance visual acuity by 12 months postoperatively.

CONCLUSIONS

The new thermokeratoplasty procedure produced the desired reduction in myopia and improvement in postoperative UDVA 1 month postoperatively without significant side effects. However, early and complete regression shows the need for further development of this technique.

FINANCIAL DISCLOSURE

Drs. Yilmaz and Marshall are paid consultants to Avedro, Inc., and Dr. Muller is president and CEO of Avedro, Inc. No other author has a financial or proprietary interest in any material or method mentioned.

Unilateral ectasia after LASIK in a patient with abnormal topography but normal tomography

PURPOSE

To report a case of unilateral ectasia developing after LASIK in a patient with abnormal topography but normal tomographic indices.

METHODS

Case report and literature review.

RESULTS

A patient was treated bilaterally for myopia using LASIK. Five years and 5 months postoperatively, unilateral ectasia in the right eye was diagnosed. Preoperatively, anterior curvature (Placido) map of the right cornea showed an asymmetry with 1.8 diopters of steepening when evaluated from upper left to lower right meridians, and a skewing of the steepest radial axes. The calculated KISA% index was 128.4 in the right eye and 5.6 in the left eye. Conversely, posterior elevation map and pachymetry map of the right eye showed no frank abnormalities.

CONCLUSIONS

This case is interesting because it shows that for this patient, the anterior curvature (Placido) map was more sensitive to detect cornea at risk of post-LASIK ectasia than the tomographic features

Corneal ectasia after myopic laser in situ keratomileusis: a long-term study

Background

The purpose of this study was to evaluate the long-term postoperative incidence of and key factors in the genesis of corneal ectasia after myopic laser-assisted in situ keratomileusis (LASIK) in a large number of cases.

Methods

A retrospective review of one surgeon’s myopic LASIK database was performed. Patients were stratified into two groups based on date of surgery, ie, group 1 (1313 eyes) from 1999 to 2001 and group 2 (2714 eyes) from 2001 to 2003. Visual acuity, refraction, pachymetry, and corneal topography data were available for each patient from examinations performed both before and after the refractive procedures.

Results

Of the 4027 surgically treated eyes, 23 (0.57%) developed keratectasia during the follow-up period, which was a minimum seven years; nine eyes (0.69%) were from group 1 and 14 eyes (0.51%) were from group 2. The onset of corneal ectasia was at 2.57 ± 1.04 (range 1–4) years and 2.64 ± 1.29 (range 0.5–5) years, respectively, for groups 1 and 2. The most important preoperative risk factors using the Randleman Ectasia Risk Score System were manifest refractive spherical error in group 1 and a thin residual stromal bed in group 2. Each of the cases that developed corneal ectasia had risk factors that were identified.

Conclusion

Ectasia was an uncommon outcome after an otherwise uncomplicated laser in situ keratomileusis procedure. The variables present in eyes developing postoperative LASIK ectasia can be better understood using the Randleman Ectasia Risk Score System.

In vivo estimation of elastic wave parameters using phase-stabilized swept source optical coherence elastography

We report a highly sensitive method based on phase-stabilized swept source optical coherence elastography (PhS-SSOCE) to measure elastic wave propagation in soft tissues in vivo. The waves were introduced using a mechanical stimulus and were assessed using the phase response of the swept source optical coherence tomography signal. The technique was utilized to measure age-related changes in elastic flexural wave velocity and attenuation in mice cornea in vivo. Results demonstrate that the wave velocity increases with animal age, supporting previous observations that stiffness of mice cornea gradually increases with age. Our studies suggest that the PhS-SSOCE technique could potentially be used to obtain biomechanical properties of ocular tissues in vivo.

Long-term comparison of laser in situ keratomileusis versus laser surface ablation in corneas thinner than 470 μm

PURPOSE

To compare long-term refractive and visual outcomes of laser in situ keratomileusis (LASIK) and laser surface ablation in eyes with corneas thinner than 470 μm.

SETTING

Private clinics, Spain.

DESIGN

Comparative case series.

METHODS

The study comprised eyes with myopic error and corneas thinner than 470 μm that had at least 2.9 years of postoperative follow-up between September 2001 and June 2007. The main outcome measures were safety, efficacy, predictability, and complications.

RESULTS

The mean central corneal thickness was 462.0 μm (range 440 to 469 μm) in the LASIK group (n = 40) and 458.1 μm (range 420 to 469 μm) in the laser surface ablation group (n = 88). All eyes had normal preoperative topography. In the LASIK group after a mean follow-up of 5.1 years ± 1.5 (SD), the safety index was 1.07, efficacy was 0.99, and predictability (± 1.00 diopter [D]) was 0.93. In the laser surface ablation group after a mean follow-up of 4.8 ± 1.3 years, the safety index was 1.01, efficacy was 0.93, and predictability (± 1.00 D) was 0.92. The mean residual corneal bed thickness in all eyes was 345 ± 25 μm (range 270 to 399 μm). No major complications occurred. The safety index was better in the LASIK group than in the laser surface ablation group.

CONCLUSION

Both techniques were effective, safe, and predictable in eyes with corneas thinner than 470 μm, normal preoperative topography, and a residual corneal bed thickness greater than 250 μm.

Noncontact all-optical measurement of corneal elasticity

We report on a noninvasive and noncontact all-optical method to measure the elasticity of the cornea. We use a pulsed laser to excite surface acoustic waves (SAW) that propagate on the corneal surface, then use a phase-sensitive optical coherence tomography system to remotely record the SAWs from which the corneal elasticity is estimated. In addition, the system is able to provide real-time tomographic images of the cornea being examined, an important consideration for clinical studies. While precisely maintaining a range of intraocular pressures (IOP), a series of measurements is performed on ex vivo intact primate eyes. The measurement results not only demonstrate the feasibility of the proposed system to remotely measure the corneal elasticity, but also suggest a strong correlation between the corneal stiffness and the true IOP.

Corneal collagen crosslinking for corneal ectasia of post-LASIK: one-year results

AIM

To evaluate the efficacy and safety of corneal collagen crosslinking (CXL) to prevent the progression of post-laser in situ keratomileusis (LASIK) corneal ectasia.

METHODS

In a prospective, nonrandomized, single-centre study, CXL was performed in 20 eyes of 11 patients who had LASIK for myopic astigmatism and subsequently developed keratectasia.The procedure included instillation of 0.1% riboflavin-20% dextrane solution 30 minutes before UVA irradiation and every 5 minutes for an additional 30 minutes during irradiation. The eyes were evaluated preoperatively and at 1-, 3-, 6-, and 12-month intervals. The complete ophthalmologic examination comprised uncorrected visual acuity, best spectacle-corrected visual acuity, endothelial cell count, ultrasound pachymetry, corneal topography, and in vivo confocal microscopy.

RESULTS

CXL appeared to stabilise or partially reverse the progression of post-LASIK corneal ectasia without apparent complication in our cohort. UCVA and BCVA improvements were statistically significant(P<0.05) beyond 12 months after surgery (improvement of 0.07 and 0.13 logMAR at 1 year, respectively). Mean baseline flattest meridian keratometry and mean steepest meridian keratometry reduction (improvement of 2.00 and 1.50 diopters(D), respectively) were statistically significant (P<0.05) at 12 months postoperatively. At 1 year after CXL, mean endothelial cell count did not deteriorate. Mean thinnest cornea pachymetry increased significantly.

CONCLUSION

The results of the study showed a long-term stability of post-LASIK corneal ectasia after crosslinking without relevant side effects. It seems to be a safe and promising procedure to stop the progression of post-LASIK keratectasia, thereby avoiding or delaying keratoplasty.

Optical quality after myopic photorefractive keratectomy and laser in situ keratomileusis: comparison using a double-pass system

PURPOSE

To use a double-pass system to compare the optical quality after photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) for mild to moderate myopia.

SETTING

Universitat Politècnica de Catalunya, Terrassa, Barcelona Institute of Ocular Microsurgery, Barcelona, Spain.

DESIGN

Comparative case series.

METHODS

Optical quality was assessed with a clinical double-pass system preoperatively and 3 months after PRK or LASIK. The modulation transfer function (MTF), retinal image quality parameters (MTF cutoff frequency, Strehl ratio), and intraocular scattering (objective scatter index [OSI]) were calculated.

RESULTS

This study evaluated 34 eyes that had PRK and 55 eyes that had LASIK. Both PRK and LASIK had a statistically significant impact on retinal image quality, although no significant differences between the techniques were observed. The MTF at 30 cycles per degree decreased by a factor of 1.50 in the PRK group and by a factor of 1.32 in the LASIK group. The MTF cutoff frequency decreased by a factor of 1.04 in the PRK group and by a factor of 1.06 in the LASIK group. The Strehl ratio decreased by a factor of 1.10 and 1.07, respectively. Photorefractive keratectomy and LASIK increased the objective scatter index by factors of 1.48 and 1.57, respectively. Significant correlations between the preoperative refraction and the OSI were found.

CONCLUSIONS

Retinal image quality was similarly reduced with PRK and LASIK, with no significant differences between the 2 methods. Some PRK patients had a residual refractive error that might have been related to corneal-wound healing still present 3 months postoperatively.

Keratectasia following LASIK in a patient with uncomplicated PRK in the fellow eye

We present a case of unilateral keratectasia in a laser refractive surgery patient. Laser in situ keratomileusis (LASIK) was performed in the first eye, but because of difficulty lifting the femtosecond-created cap in the second eye, photorefractive keratectomy was performed in that eye. Neither eye had risk factors for keratectasia; both had identical low scores on the Randleman risk factor score. Although femtosecond laser caps were created in both eyes, ectasia developed in only the LASIK eye, in which the cap was lifted. We believe this is the first case of this complication reported in the literature. It highlights our incomplete knowledge of the risk factors for keratectasia following LASIK and suggests that unlifted flaps do not undergo the same biomechanical weakening as flaps that are lifted.

High-resolution quantitative imaging of cornea elasticity using supersonic shear imaging

The noninvasive estimation of in vivo mechanical properties of cornea is envisioned to find several applications in ophthalmology. Such high-resolution measurements of local cornea stiffness could lead to a better anticipation and understanding of corneal pathologies such as Keratoconus. It could also provide a quantitative evaluation of corneal biomechanical response after corneal refractive surgeries and a tool for evaluating the efficacy of new cornea treatments such as cornea transplant using femtosecond laser or therapy based on Riboflavin/UltraViolet-A Corneal Cross Linking (UVA CXL). In the very important issue of glaucoma diagnosis and management, the fine tuning corneal elasticity measurement could also succeed to strongly correlate the applanation tonometry with the "true" intra-ocular pressure (IOP). This initial investigation evaluates the ability of ultrafast and high-resolution ultrasonic systems to provide a real-time and quantitative mapping of corneal viscoelasticity. Quantitative elasticity maps were acquired ex vivo on porcine cornea using the supersonic shear imaging (SSI) technique. A conventional 15 MHz linear probe was used to perform conventional ultrasonic imaging of the cornea. A dedicated ultrasonic sequence combines the generation of a remote palpation in the cornea and ultrafast (20,000 frames/s) ultrasonic imaging of the resulting corneal displacements that evolve into a shear wave propagation whose local speed was directly linked to local elasticity. A quantitative high-resolution map (150 microm resolution) of local corneal elasticity can be provided by this dedicated sequence of ultrasonic insonifications. Quantitative maps of corneal elasticity were obtained on ex vivo freshly enucleated porcine corneas. In the cornea, a quite homogenous stiffness map was found with a 190 kPa +/- 32 kPa mean elasticity. The influence of photodynamic Riboflavin/UVA induced CXL was measured. A significant Young's modulus increase was obtained with a mean 890 kPa +/- 250 kPa posttreatment Young's modulus (460% increase), located in the anterior part of the cornea. Simulations based on 3-D time domain finite differences simulation were also performed and found to be in good agreement with ex vivo experiments. The SSI technique can perform real-time, noninvasive, high-resolution, and quantitative maps of the whole corneal elasticity. This technique could be real time and straightforward adapted for a very wide field of in vivo investigations.

Healing Process at the Flap Edge in Its Influence in the Development of Corneal Ectasia After LASIK

PURPOSE

Post-laser in situ keratomileusis (LASIK) corneal ectasia is a progressive deformation of the gross corneal anatomy that occurs after surgery. However, this is a rare event even after deep lamellar keratoplasty. We hypothesize that the strength of the lamellar keratoplasty wound is derived from the sutures that enhance the wound edge healing response. This study compares, in a rabbit model, the stability of previously sutured and unsutured microkeratome flaps.

METHODS

Unilateral 160-micro m-thick LASIK flaps using a mechanical microkeratome was performed in 20 rabbit eyes. Animals were then divided in two groups: In group A, the flap was left without sutures. In group B, the flap was sutured with 12 interrupted 10/0 nylon stitches that were removed after 3 weeks under general anesthesia. Six weeks after surgery, all rabbits had corneal topographies performed at their baseline intraocular pressure (IOP) (14 mmHg) and at two artificially increased pressures (25 and 45 mmHg) using an anterior chamber maintainer implanted in the inferior limbal area. The animals were humanely euthanized, and immunohistological analysis of the corneas was performed.

RESULTS

A delta K1 value, which indicates the difference in the simulated keratometric value at baseline and the one measured at 25 mmHg, was calculated for all eyes. It showed a mean steepening effect of 2.74 D +/- 0.38 D in group A compared with 1.08 D +/- 0.27 D in group B (p < 0.05). Similarly, a delta K2 value, which indicates the difference in the simulated keratometric value at baseline and the one obtained at 45 mmHg, was registered. It showed a mean steepening effect of 3.02 D +/- 0.87 D in group A compared with 0.75 D +/- 0.44 D in group B (p < 0.05). Six weeks after surgery, the peripheral flap interface in group B consisted of 14.3% +/- 4.15% of positive monoclonal mouse anti-alpha smooth muscle actin (alpha-SMA) cells compared with 4.18 +/- 3.76% in group A (p < 0.05).

CONCLUSIONS

The addition of sutures in the corneal flap after LASIK appears to reduce the amount of corneal steepening when the IOP is artificially increased up to 25 mmHg in this rabbit model. Our results suggest that an increase in the amount of myofibroblastas induced by the sutures may be responsible for this behavior. Corneal ectasia may be related to the clinically observed lack of corneal wound-healing at the edge of the flap that allows the cornea to bulge. By stimulating a stronger wound-healing response at the edge of the flap, the cornea may better resist steepening under increased IOP conditions and improve the long-term stability of LASIK surgery in borderline thin corneas.

Keratectasia after laser in situ keratomileusis: a histopathologic and immunohistochemical study

OBJECTIVE

To examine histopathologic and immunohistochemical features of human corneal buttons from patients who developed keratectasia after laser in situ keratomileusis (LASIK).

METHODS

Five corneal buttons were obtained during penetrating keratoplasty from patients who developed keratectasia after LASIK. Histologic features were examined by hematoxylin-eosin staining using paraffin-embedded sections and by transmission electron microscopy. Immunostaining for alpha(1)-proteinase inhibitor, Sp1, and matrix metalloproteinases 1, 2, and 3 was performed with 2 healthy corneas and 2 corneas with keratoconus as controls.

RESULTS

Central stromal thinning was observed after hematoxylin-eosin staining in all corneas with keratectasia. No histologic features specific to keratoconus, including Bowman layer disruption, were identified in the corneas with keratectasia. By transmission electron microscopy, collagen fibril thinning and decreased interfibril distance were observed in the stromal bed. Immunostaining intensity and/or pattern for alpha(1)-proteinase inhibitor and Sp1 in the corneas with keratectasia was comparable to that of healthy corneas and differed from that in the corneas with keratoconus. No significant staining with anti-matrix metalloproteinases 1, 2, and 3 antibodies was observed in either the corneas with keratectasia or the healthy corneas.

CONCLUSIONS

Histologic findings suggest that post-LASIK keratectasia results in collagen fibril thinning and decreased interfibril distance within the residual stromal bed. Discrepant results between keratectasia and keratoconus suggest that the pathogenesis of the 2 conditions differ.