Corneal collagen cross-linking in patients with keratoconus from the Dresden protocol to customized solutions:theoretical basis

Keratoconus is an ectatic condition characterized by gradual corneal thinning,corneal protrusion,progressive irregular astigmatism,corneal fibrosis,and visual impairment.The therapeutic options regarding improvement of visual function include glasses or soft contact lenses correction for initial stages,gas-permeable rigid contact lenses,scleral lenses,implantation of intrastromal corneal ring or corneal transplants for most advanced stages.In keratoconus cases showing disease progression corneal collagen crosslinking(CXL)has been proven to be an effective,minimally invasive and safe procedure.CXL consists of a photochemical reaction of corneal collagen by riboflavin stimulation with ultraviolet A radiation,resulting in stromal crosslinks formation.The aim of this review is to carry out an examination of CXL methods based on theoretical basis and mathematical models,from the original Dresden protocol to the most recent developments in the technique,reporting the changes proposed in the last 15y and examining the advantages and disadvantages of the various treatment protocols.Finally,the limits of non-standardized methods and the perspectives offered by a customization of the treatment are highlighted.

A short-term study of corneal collagen cross-linking with hypo-osmolar riboflavin solution in keratoconic corneas

AIM: To report the 3mo outcomes of collagen crosslinking(CXL) with a hypo-osmolar riboflavin in thin corneas with the thinnest thickness less than 400 μm without epithelium.METHODS: Eight eyes in 6 patients with age 26.2±4.8y were included in the study. All patients underwent CXL using a hypo-osmolar riboflavin solution after its de-epithelization. Best corrected visual acuity, manifest refraction, the thinnest corneal thickness, and endothelial cell density were evaluated before and 3mo after the procedure.RESULTS: The mean thinnest thickness of the cornea was 408.5 ±29.0 μm before treatment and reduced to369.8 ±24.8 μm after the removal of epithelium. With the application of the hypo-osmolar riboflavin solution, the thickness increased to 445.0 ±26.5 μm before CXL and recover to 412.5 ±22.7 μm at 3mo after treatment, P =0.659). Before surgery, the mean K-value of the apex of the keratoconus corneas was 57.6 ±4.0 diopters, and slightly decreased(54.7±4.9 diopters) after surgery(P =0.085). Mean best-corrected visual acuity was 0.55 ±0.23 logarithm of the minimal angle of resolution, and increased to 0.53±0.26 logarithm after surgery(P =0.879).The endothelial cell density was 2706.4 ±201.6 cells/mm2 before treatment, and slightly decreased( 2641. 2 ±218.2 cells/mm2) at last fellow up(P =0.002).CONCLUSION: Corneal collagen cross-linking with a hypo-osmolar riboflavin in thin corneas seems to be a promising treatment. Further study should be done to evaluate the safety and efficiency of CXL in thin corneas for the long-term.

Corneal collagen cross-linking and liposomal amphotericin B combination therapy for fungal keratitis in rabbits

AIM： To observe the therapeutic effect of corneal collagen cross-linking（CXL） in combination with liposomal amphotericin B in fungal corneal ulcers.METHODS： New Zealand rabbits were induced fungal corneal ulcers by scratching and randomly divided into 3groups, i.e. control, treated with CXL, and combined therapy of CXL with 0.25% liposomal amphotericin B（n =5 each）. The corneal lesions were documented with slit-lamp and confocal microscopy on 3, 7, 14, 21 and 28 d after treatment. The corneas were examined with transmission electron microscopy（TEM） at 4wk.RESULTS： A rabbit corneal ulcer model of Fusarium was successfully established. The corneal epithelium defect areas in the two treatment groups were smaller than that in the control group on 3, 7, 14 and 21d（P 〈0.05）. The corneal epithelium defect areas of the combined group was smaller than that of the CXL group（P 〈0.05） on 7 and 14 d, but there were no statistical differences on 3, 21 and 28 d. The corneal epithelium defects of the two treatment groups have been healed by day 21. The corneal epithelium defects of the control group were healed on 28 d. The diameters of the corneal collagen fiber bundles（42.960 ±7.383 nm in the CXL group and 37.040±4.160 nm in the combined group） were thicker than that of the control group（24.900±1.868 nm）,but there was no difference between the two treatment groups. Some corneal collagen fiber bundles were distorted and with irregular arrangement, a large number of fibroblasts could be seen among them but no inflammatory cells in both treatment groups. CONCLUSION： CXL combined with liposomal amphotericin B have beneficial effects on fungal corneal ulcers. The combined therapy could alleviate corneal inflammattions, accelerate corneal repair, and shorten the course of disease.

Clinical Observation of Transepithelial Corneal Collagen Cross-linking by Iontophoresis of Riboflavin in Treatment of Keratoconus

Purpose: To evaluate the efficacy and safety of transepithelial collagen cross-linking by iontophoretic delivery of riboflavin in treatment of progressive keratoconus.Methods:.Eleven patients(15 eyes) with progressive keratoconus were enrolled. After 0.1% riboflavin-distilled water solution was deliveried via transepithelial iontophpresis for 5 min with 1 m A current, and ultraviolet radiation(370 nm,.3 m W /cm2) was performed at a 1.5 cm distance for 30 min. The follow up were 6 months in all eyes. The uncorrected visual acuity, corrected visual acuity,endothelial cell counting, corneal thickness,.intraocular pressure, corneal curvature, corneal topography,.OCT and corneal opacity before and 6-month after surgery were analyzed.Results: At 6 month postoperatively, mean uncorrected visual acuity and corrected visual acuity changed from 0.36 to 0.30 and from 0.42 to 0.57 without statistical significance..The mean value of each index of corneal curvature declined without statistical significance.Kmax value dereased from 60.91 to59.91, and the astigmatism declined from 3.86 to 3.19. Central corneal thickness decreased from 460.93 μm to 455.40μm,.and thinnest corneal thickness declined from 450.87 μm to 440.60 μm with no statistical significance..Intraocular pressure was significantly elevated from 10.85 mm Hg to 12.62 mm Hg. Endothelial cell count did not change significantly. No corneal haze occurred. Mean depth of corneal demarcation line was 288.46 μm at 1 month postoperatively..Conclusion:.Transepithelial corneal collagen cross-linking by iontophoresis is effective and safe in the treatment of progressive keratoconus, and yields stable clinical outcomes during 6-month follow up..However,.long-term follow up is urgently required.

Comparison of Central Corneal Thickness using Ultrasound Pachymetry during Corneal Collagen Cross-linking

Purpose:To study variation in central corneal thickness (CCT) during corneal collagen cross-linking(CXL) using ultrasound pachymetry. Methods:Twenty patients (26 eyes) with progressing keratoconus undergoing riboflavin-UVA-induced CXL were involved in this study. Intraoperative CCT measurement using ultrasonic pachymetry was performed during the procedure.Measurements were obtained before operation, after epithelial removal, after riboflavin drop instillation, and after UVA irradiation. Results:Mean CCT was 495±56 and 450±52 μm before and after epithelial removal, respectively. Mean CCT was 443±42 and 411±39 μm after riboflavin drop instillation and after UVA irradiation, respectively. Statistically significant decreases in CCT occurred between the preoperative period and after epithelial removal, after riboflavin drop instillation and after UVA irradiation. Twenty-six eyes from 20 patients undergoing CXL were divided into 2 groups (I with CCT≥400 μm after UVA irradiation and II with CCT<400 μm after UVA irradiation). No statistically significant difference was noted between I and II in preoperative endothelial cell count, but a statistically greater postoperative endothelial cell count was noted in I compared to II. A statistically significant difference was evident between preoperative and postoperative endothelial cell counts in Group II (P<0.05). Conclusion:Performing CXL with the use of riboflavin and UVA irradiation resulted in a statistically significant decrease in CCT, to a level where the corneal endothelium may be damaged.(Eye Science 2013; 28:15-19)

Variation in Central Corneal Thickness during Open- or Closed-Eye Riboflavin Instillation for Corneal Collagen Cross-linking

Purpose:To assess the effect of having an open or closed eye on the variation in central corneal thickness during riboflavin instillation for corneal collagen cross-linking (CXL).Methods:.Thirty eyes of 15 New Zealand White rabbits underwent an in vivo anterior segment optical coherence tomography (OCT) examination at 0,10,20,and 30 min after riboflavin instillation on the de-epithelialized corneal surface.Each eye of every rabbit was randomly placed into one of two different treatment groups(open-eye or closed-eye)during the instillation;.the examinations were performed one after the other.After instillation for 30 min,the changes in the corneal stroma and anterior chamber were observed by slit lamp.Results:.A significant decrease in the central corneal thickness(CCT).was demonstrated during riboflavin instillation;.the variations were smaller in the measurements performed with the eye closed than with the eye open.(81.36±15.13μm and129.20±12.05μm respectively)..Both methods turned the corneal stroma and anterior chamber yellow.Conclusion:Keeping the eye closed during riboflavin instillation reduced the decrease in the CCT..The same yellow change in the corneal stroma and anterior chamber occurred,but the exposure time of the ocular surface was shorter.Therefore,.keeping the eye closed was a more effective and safer method than keeping the eye open.

Corneal collagen cross-linking epithelium-on vs. epithelium-off: a systematic review and meta-analysis

Background The purpose of the study was to determine the advantages and disadvantages of epi-on corneal cross-linking(CXL)techniques compared with standard epi-off CXL.Methods We searched MEDLINE and EMBASE for randomized controlled trials(RCTs)and non-randomized studies of interventions(NRSIs)and we evaluated the selected papers according to the Cochrane risk of bias tool.We considered,as primary outcomes,average Kmax flattening,changes in uncorrected and corrected distance visual acuity(UDVA and CDVA);as secondary outcomes,we considered changes in pachymetry values and endothelial cell density(ECD).We also investigated adverse events related to the treatments and treatment failure.Meta-analysis was conducted with a fixed or random-effects model using weighted mean difference(MD)with 95%confidence interval(CI)as the effect size.Results A total of 15 studies were included and among these 15 trials,9 were RCTs and 6 were NRSIs,but only 4 studies showed no high risk of bias and were included in this meta-analysis.Our analysis revealed significant postoperative differences in CDVA(MD=0.07;95%CI 0.04 to 0.10;P<0.001),and no significative differences in UDVA,Kmax,central corneal thickness(CCT)and ECD(P>0.05).Epi-on CXL protocol was found to be significantly less prompt to have risks of delay in epithelial healing(P=0.035)and persistent stromal haze(P=0.026).Conclusion Epi-on CXL is as effective as epi-off CXL.Except for a higher significant improvement in CDVA with current epi-on protocols,our meta-analysis demonstrates that epi-on and epi-off CXL have comparable effects on visual,topographic,pachymetric,and endothelial parameters.Epi-on CXL has clinical advantages in terms of comfort and avoidance of complications as it reduces the risk of developing delay in epithelial healing and persistent stromal haze.

Early changes in corneal densitometry after FS-LASIK combined with accelerated corneal cross-linking for correction of high myopia

AIM:To observe the effects of femtosecond laserassisted excimer laser in situ keratomileusis combined with accelerated corneal cross-linking(FS-LASIK Xtra)on corneal densitometry after correcting for high myopia.METHODS:In this prospectively study,130 patients underwent FS-LASIK or FS-LASIK Xtra for high myopia.Their right eyes were selected for inclusion in the study,of which 65 cases of 65 eyes in the FS-LASIK group,65 patients with 65 eyes in the FS-LASIK Xtra group.Patients were evaluated for corneal densitometry at 1,3,and 6mo postoperatively using Pentacam Scheimpflug imaging.RESULTS:Preoperative differences in corneal densitometry between the FS-LASIK and FS-LASIK Xtra groups in different ranges were not statistically significant(P>0.05).Layer-by-layer analysis revealed statistically significant differences in the anterior(120μm),central,and total layer corneal densitometry between the FS-LASIK and FS-LASIK Xtra groups at 1 and 3mo postoperatively(all P<0.05),the FS-LASIK Xtra group is higher than that of the FS-LASIK group.Analysis of different diameter ranges showed statistically significant differences between the FS-LASIK group and the FS-LASIK Xtra group at 1mo postoperatively in the ranges of 0–2,2–6,and 6–10 mm(both P<0.05);At 3mo postoperatively,the FS-LASIK Xtra group is higher than that of the FS-LASIK group in the ranges of 0–2 and 2–6 mm(P<0.05).At 6mo postoperatively,there were no statistically significant differences in corneal densitometry between the FS-LASIK group and the FS-LASIK Xtra group in different diameter ranges(all P>0.05).CONCLUSION:There is an increase in internal corneal densitometry during the early postoperative period after FS-LASIK Xtra for correction of high myopia.However,the densitometry values decreased to the level of conventional FS-LASIK at 6mo after surgery,with the most significant changes observed in the superficial central zone.

Experimental study on the treatment of rabbit corneal melting after alkali burn with Collagen cross-linking

AIM: To evaluate the effect of Collagen cross-linking on the prevention of melting in rabbit corneas after alkali burn. METHODS: Twenty New Zealand white rabbits were randomly divided into model control group and collagen cross-linking treatment group. The second group of rabbits received collagen cross linked treatment. Both groups were applied with antibiotic eye drops to prevent infection. The corneas were evaluated for melting, opacity, pathological and immunohistochemistry, record the changes when 28 days after the animals were killed. RESULTS: In the control group, 6 out of 8 rabbits showed corneal melting after injury (14 +/- 4) days, while two corneal perforated. In collagen cross-linking treatment group, one rabbit showed corneal melting after injury 23 days, without corneal perforation; corneal dissolution rate between the two groups was significantly different (P <0.05). Pathological examination suggested that in the treatment group, mild corneal edema, mild damage to collagen fibers, inflammatory cell infiltration was significantly less than the control group. Immunohistochemistry showed that corneal collagen fibers arranged in neat rows in the control group. CONCLUSION: Collagen cross-linking treatment not only can prevent and delay the corneal melting after alkali burn, but also can reduce the destruction of corneal collagen fibers and infiltration of inflammatory cells in the corneal tissue.

Different accelerated corneal collagen cross-linking treatment modalities in progressive keratoconus

Background:To compare the outcomes of two different protocols of accelerated corneal crosslinking(CXL)on visual,corneal high order aberrations(HOA)and topographic parameters in patients with progressive keratoconus.Methods:In this prospective comparative study,sixty-six eyes of 66 patients with progressive keratoconus were divided into two groups;37 eyes in Group 1 received 18 mW/cm^(2) for five minutes,and 29 eyes in Group 2 were treated with 9 mW/cm^(2) for 10 min.The uncorrected distant visual acuity(UCVA),best-corrected distant visual acuity(BCVA),corneal HOAs and topography parameters were measured preoperatively and postoperatively at the end of 12 months.The data for the two groups were compared statistically.Results:The mean UCVA and BCVA were significantly improved at the postoperative 12 months compared with the preoperative values in both groups(P<0.05 for all).A significant improvement in corneal HOAs was observed in both groups(P<0.05 for all).The change in corneal coma value was significantly higher in Group 2(P<0.05).The change in keratometric values K1,K2,AvgK and maximum keratometry(AKf)were significantly higher in Group 2(P<0.05 for all).The regression model showed that the most important factor predicting the change in AKf was the type of CXL(β=−0.482,P=0.005).Conclusions:Accelerated CXL using 10 min of UVA irradiance at 9 mW/cm^(2) showed better topographic improvements and coma values than five minutes of UVA irradiance at 18 mW/cm^(2) independent of keratoconus severity.

Scheimpflug imaged corneal changes on anterior and posterior surfaces after collagen cross-linking

AIM:To compare the anterior and posterior corneal parameters before and after collagen cross-linking therapy for keratoconus.METHODS:Collagen cross-linking was performed in31 eyes of 31 keratoconus patients(mean age 30.6±8.9y).Prior to treatment and an average 7mo after therapy,Scheimpflug analysis was performed using Pentacam HR.In addition to corneal thickness assessments,corneal radius,elevation,and aberrometric measurements were performed both on anterior and posterior corneal surfaces.Data obtained before and after surgery were statistically analyzed.RESULTS:In terms of horizontal and vertical corneal radius,and central corneal thickness no deviations were observed an average 7mo after operation.Corneal higher order aberration showed no difference neither on anterior nor on posterior corneal surfaces.During follow-up period,no significant deviation was detected regarding elevation values obtained by measurement in mm units between the 3.0-8.0 mm-zones.CONCLUSION:Corneal stabilization could be observed in terms of anterior and posterior corneal surfaces,elevation and higher order aberration values 7mo after collagen cross-linking therapy for keratoconus.

Corneal collagen cross-linking(CXL)in thin corneas

Corneal collagen cross-linking(CXL)is a therapeutic procedure aiming at increasing the corneal stiffness in the keratoconus eyes by induction of cross-links within the extracellular matrix.It is achieved by ultraviolet-A(370 nm)irradiation of the cornea after saturation with the photosensitizer riboflavin.In the conventional CXL protocol,a minimum de-epithelialized corneal thickness of 400μm is recommended to avoid potential irradiation damage to the corneal endothelium.In advanced keratoconus,however,stromal thickness is often lower than 400μm,which limits the application of CXL in that category.Efforts have been undertaken to modify the conventional CXL procedure to be applicable in thin corneas.The current review discusses different techniques employed to achieve this end and their results.The overall safety and efficacy of the modified CXL protocols are good,as most of them managed to halt the progression of keratectasia without postoperative complications.However,the evidence of safety and efficacy in the use of modified CXL protocols is still limited to few studies with few patients involved.Controlled studies with long-term follow-up are required to confirm the safety and efficacy of the modified protocols.

Topography versus non-topography-guided photorefractive keratectomy with corneal cross-linking variations in keratoconus

AIM:To compare the visual results of non-topographyguided and topography-guided photorefractive keratectomy(PRK)applying sequential and simultaneous corneal crosslinking(CXL)treatment for keratoconus.METHODS:Interventional and comparative prospective study.Sixty-nine eyes(36 patients)suffering from keratoconus(stages 1 Amsler-Krumeich classification)were divided into four groups:sequential topography-guided photorefractive keratectomy with CXL,simultaneous topography-guided photorefractive keratectomy with CXL,simultaneous nontopography guided photorefractive keratectomy with CXL,and sequential non-topography guided photorefractive keratectomy with CXL.The main outcome measures were pre-and postoperative uncorrected distance visual acuity(UDVA),best corrected distance visual acuity(CDVA),manifest refraction,contrast sensitivity,and keratometry.RESULTS:All analyzed visual,contrast sensitivity,and refractive parameters showed a significant improvement in the four groups(all P<0.05).A noticeable improvement was seen in keratometry in all the groups,and a remarkable difference was observed between topography-guided groups in comparison to non-topography-guided groups(P<0.05).Interestingly,the improvement in all parameters showed a degree of stability to the end of the follow-up.CONCLUSION:The treatment priorities in all four groups are safety,efficacy,and predictability in the correction of the sphero-cylindrical errors in mild and moderate keratoconus.No significant differences among groups in the recorded objective outcomes were found.

Comparison of corneal biological parameters between transepithelial and epithelium-off corneal cross-linking in keratoconus

AIM:To evaluate the differences in corneal biological parameters between transepithelial and epithelium-off corneal cross-linking in keratoconus.METHODS:In our prospective clinical trial,40 patients(60 eyes)with progressive keratoconus were randomized to undergo corneal cross-linking with transepithelial(TE group,n=30)or epithelium-off(EO group,n=30)keratoconus.Examinations comprised topography,corneal biomechanical analysis and specular microscopy at 6 mo postoperatively.RESULTS:The keratometer values were not significantly different between the TE and EO corneal cross-linked groups in different periods(each P>0.05).The corneal thickness of the EO group was greater than that of the TE group at 1 wk after the operation(each P<0.05).Regarding corneal biomechanical responses,the EO group showed a longer second applanation length than TE group(P=0.003).Regarding the corneal endothelial function,standard deviation of the endothelial cell size,and coefficient of variation in the cell area,the values of EO group were larger than those of TE group at 1 wk(P=0.011,0.026),and the percentage of hexagonal cells in EO group was lower than that in TE group at 1 and 6 mo(P=0.018,0.019).CONCLUSION:Epithelium-off corneal cross-linking may strengthen corneal biomechanics better than TE procedure can.However,the TE procedure with a lower ultraviolet-A irradiation intensity would be safer for corneal endothelial function.

Efficacy of iontophoresis-assisted epithelium-on corneal cross-linking for keratoconus

Corneal cross-linking（CXL） is a noninvasive therapeutic procedure for keratoconus that is aimed at improving corneal biomechanical properties by induction of covalent cross-links between stromal proteins. It is accomplished by ultraviolet A（UVA） radiation of the cornea, which is first saturated with photosensitizing riboflavin. It has been shown that standard epithelium-off CXL（S-CXL） is efficacious, and it has been recommended as the standard of care procedure for keratoconus. However, epithelial removal leads to pain, transient vision loss, and a higher risk of corneal infection. To avoid these disadvantages, transepithelial CXL was developed. Recently, iontophoresis has been adopted to increase riboflavin penetration through the epithelium. Several clinical observations have demonstrated the safety and efficacy of iontophoresisassisted epithelium-on CXL（I-CXL） for keratoconus. This review aimed to provide a comprehensive summary of the published studies regarding I-CXL and a comparison between I-CXL and S-CXL. All articles used in this review were mainly retrieved from the Pub Med database. Original articles and reviews were selected if they were related to the I-CXL technique or related to the comparison between I-CXL and S-CXL.

Analysis of the effective dose of ultraviolet light in corneal cross-linking

AIM: To analyze the efficacy of ultraviolet (UV) light initiating corneal cross-linking (CXL). METHODS: The time-dependent absorption of UV light due to the depletion of the initiator (riboflavin) was calculated. The effective dose of CXL with corneal surface covered by a thin layer of riboflavin was derived analytically. The cross linking time was calculated by the depletion level of the riboflavin concentration. A comprehensive method was used to derive analytic formulas. RESULTS: The effective dose of CXL was reduced by a factor (R) which was proportional to the thickness (d) and concentrations (C-0) of the riboflavin surface layer. Our calculations showed that the conventional dose of 5.4 J/cm(2) had a reduced effective dose of 4.3 and 3.45 J/cm(2), for d was 100 and 200 pm, respectively, and C-0=0.1%. The surface cross linking time was calculated to be T*=10.75s, for a depletion level of 0.135 and UV initial intensity of 30 mW/cm(2). The volume T* was exponentially increasing and proportional to exp (bdC(0)), with b being the steady state absorption coefficient. CONCLUSION: The effective dose of CXL is reduced by a factor proportional to the thickness and concentrations of the riboflavin surface layer. The wasted dose should be avoided by washing out the extra riboflavin surface layer prior to the UV light exposure.

Trans-epithelial accelerated corneal cross-linking for keratoconus in children

The aim of the study is to evaluate the safety and effi cacy of trans-epithelial accelerated corneal cross-linking（TE-ACXL） in children with progressive keratoconus.Retrospective,case-series of 23 eyes of 14 children who underwent TE-ACXL.Evaluations were performed at baseline and 1,3,6,12 and 18 mo postoperatively.Mean follow-up time of 23.82±3.15 mo and mean age was 13.7±1.4 y（range 11 to 16 y）.Mean preoperative uncorrected distance visual acuity changed from 0.92±0.45 log MAR（20/160） to 0.71±0.40 log MAR（20/100）（P=0.001）.Mean keratometry（Km） changed from 53.87± 6.03 to 53.00±5.81（P=0.001）.Pachymetry did not have significant changes at last follow-up（P=0.30）.The mean preoperative sphere was-5.58±2.48 and-4.89±4.66 D（P=0.11） at last follow-up;refractive cylinder from-5.58±2.48 to-5.02±2.23（P=0.046）.In conclusion,tomographic and refractive stability are shown in over 91% of eyes with pediatric progressive keratoconus who underwent TE-ACXL.

A novel analysis of Scheimpflug total corneal refractive power following corneal cross-linking in mild to moderate keratoconus

AIM:To detect an earlier improvement in mild to moderate keratoconus following corneal cross-linking(CXL)with total corneal refractive power(TCRP)using ray tracing method.METHODS:A total of 40 eyes of 30 consecutive patients who underwent CXL for progressive keratoconus were retrospectively enrolled.The following keratometric parameters provided by Pentacam HR,including maximum keratometry(Kmax),steepest keratometry(Ksteep),3 mm zonal TCRP centered over corneal apex(TCRPapex,zone 3 mm),zonal mean keratometry and TCRP centered over corneal cone(Kmcone,zone and TCRPcone,zone 1,2,3 mm)were evaluated preoperatively and 1,3,6,and 12 mo postoperatively.Groups 1 and 2 were defined based on Kmax at postoperative 1 mo as improved(the initial improvement group)or worsen(the initial deterioration group)compared to the preoperative level.RESULTS:In the overall group,only keratometric parameters based on ray tracing method displayed significant improvement early at 3 mo postoperatively,in which TCRPcone,zone 1 mm and 2 mm exhibited the largest flattening(0.57 D and 0.53 D,respectively).In Group 1,only Kmax,Kmcone,zone 2 mm and TCRPcone,zone 2 mm showed significant improvement initially at 1 mo postoperatively,in which Kmax exhibited the largest improvement(1.05 D),followed by TCRPcone,zone 2 mm(0.82 D).In Group 2,only keratometric parameters based on ray tracing method and Kmcone,zone 3 mm showed slight but not significant improvement early at 3 mo,in which TCRPcone,zone 3 mm displayed the most improvement(0.19 D),followed by TCRPcone,zone 2 mm(0.15 D).CONCLUSION:The findings indicate that a 2 mm zonal TCRP centered over Kmax could earlier detect keratometric improvement by CXL compared to other commonly used parameters in mild to moderate keratoconic eyes.

Changes of matrix metalloproteinases in the stroma after corneal cross-linking in rabbits

AIM:To observe changes in the content of matrix metalloproteinases(MMPs)in the corneal stroma after corneal cross-linking(CXL)in rabbits,and further explore the corneal pathophysiological process after CXL.METHODS:Forty-two rabbits(42 eyes)were randomly divided into seven groups.One group served as the control group,while the other six groups were treated with CXL.The concentrations of MMPs in corneal stroma were evaluated through parallel reaction monitoring at baseline and 3,7,15,30,90,and 180 d after treatment.RESULTS:The levels of MMP-2 in the corneal stroma of rabbits were 0.76±0.07,2.78±1.39,4.12±0.69,2.00±0.29,2.00±0.30,1.22±0.18,and 1.35±0.18(10^(-9)mol/g)at baseline and 3,7,15,30,90,and 180 d after treatment,respectively.The contents of tissue inhibitor of metalloproteinase-1(TIMP-1)were 1.83±0.26,7.94±0.58,6.95±2.64,3.81±0.48,3.07±0.92,1.72±0.19,and 1.69±0.74(10^(-9)mol/g),respectively.The ratios of MMP-2/TIMP-1 were 0.42±0.33,0.36±0.20,0.62±0.10,0.54±0.15,0.68±0.13,0.71±0.10,and 0.68±0.09,respectively.After CXL,the expression of MMP-2 and TIMP-1 in the rabbit corneal stroma was initially increased and subsequently decreased.The levels of MMP-2 remained higher than those recorded at baseline 180 d after treatment,but it was not statistically significant.The levels of TIMP-1 returned to baseline levels at 90 d after treatment.The ratio of MMP-2/TIMP-1 started to rise from 7 d after CXL.It was significantly higher than that calculated at baseline 30-180 d after CXL.The results for MMP-1,-3,-7,-9,-13,and TIMP-2 were negative.CONCLUSION:CXL can lead to changes in the content of MMP-2 and TIMP-1 in the rabbit corneal stroma.The ratio of MMP-2/TIMP-1 remains higher versus baseline,indicating that MMP-2 is involved in the corneal pathophysiological process after CXL.

Evaluation of corneal endothelium after UVA/riboflavin cross-linking in thin keratoconic corneas

Dear Sir,UVA/riboflavin cross-linking （CXL） has been used clinically applied for the treatment of keratoconus and corneal edema via enhancement of corneal stiffness The safety of the corneal endothelium is of prime importance during CXL treatment. In clinical practice, a corneal thickness （CT） of 400um has traditionally been regarded as the minimum treatable thickness, thereby avoiding damage to the corneal endothelium Although CXL has been applied to thinner corneas, using a hypoosmotic solution onto cornea and inducing edema . CXL safety still needs further evaluation because of lower relative concentration of collagen in the hydrated stroma . This study aims to evaluate the changes of corneal endothelial density （ECD） in cases where the CT is 〈400 um before iatrogenic corneal swelling and CXL treatment.