corneal collagen cross-linking: an introduction and literature review

Optometry (2012) 83, 33-42

Corneal collagen cross-linking: An introductionand literature review

Brandon J. Dahl, O.D., Eric Spotts, O.D., and James Q. Truong, O.D.

Keller Army Community Hospital, West Point, New York; and State University of New York, College of Optometry,New York.

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KEYWORDSCollagen cross-linking;Keratoconus;Corneal ectasia;Riboflavin;Ultraviolet A

Disclosure: The author has no financ

ducts used in this literature review.

Corresponding author: Brandon J. Da

spital, Optometry Services Box 10510

E-mail: [email protected]

9-1839/$ - see front matter � 2012

:10.1016/j.optm.2011.09.011

AbstractBACKGROUND: This literature review analyzes the scientific evidence available regarding cornealcollagen cross-linking (CXL) as a treatment option for progressive keratectasia.METHODS: A literature search was performed using dates from 1990 to August 2010 regarding CXLSpecific areas of focus for the literature review include safety and efficacy of the procedure as a stand-alone treatment or when used in conjunction with Intacs� corneal implants (Addition Technology�) orphotorefractive keratectomy (PRK).RESULTS: A total of 50 clinical trials and studies were identified, 20 of which met the inclusioncriteria. Results of the included literature support the conclusion that CXL is a safe and efficacioustreatment for progressive keratectasia. The results of CXL alone have shown stabilization or improve-ment in the maximum keratometry readings, best-corrected visual acuity, uncorrected visual acuity,and spherical and cylinder refractive measurements. CXL has been shown to enhance the effects ofIntacs and has been proven successful when used in conjunction with PRK.CONCLUSION: CXL is an effective treatment for limiting the progression of keratectasia, thus reducingthe need for penetrating keratoplasty. CXL has a similar side-effect profile and similar risk level asPRK.Optometry 2012;83:33-42

Progressive keratectasia is a result of progressive cornealdisease or a sequela of refractive surgery. In the UnitedStates, there is no current treatment for keratectasia.Corneal collagen cross-linking (CXL) is a new treatmentintended to halt the progression of keratectasia. The proce-dure, which is currently performed in other parts the world,uses ultraviolet (UV) light and riboflavin to strengthen thestromal collagen.

ial or other vested interests in the

hl, O.D., Weed Army Community

9, Fort Irwin, CA 92310

American Optometric Association. All r

The use of UV light long wave (UVA) irradiation andcross-linking as a means to strengthen a material is not anew concept. Dentists use UV light to induce cross-linking,which strengthens the material used for fillings.1,2 The pol-ymer industry used UV light to harden adhesives.1 Cross-linking with glutaraldehyde is used to stabilize prostheticheart valves.1,3

Cross-linking is also a natural phenomenon occurringwithin the cornea and crystalline lens with age. Thecorneal fibril diameter increases 4.5% throughout a per-son’s life because of an age-dependent glycosylation cross-linking.4-6 Within the lens, crystallins increase inmolecular weight, and rigidity secondary to age-relatedcross-linking.6,7

ights reserved.

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http://dx.doi.org/10.1016/j.optm.2011.09.011

http://dx.doi.org/10.1016/j.optm.2011.09.011

34 Optometry, Vol 83, No 1, January 2012

CXL is the result of research commencing in the 1990sintended to identify biological glues that could strengthencorneal collagen.8 Researchers at the Technical Universityof Dresden in Germany noted diabetic patients rarely de-velop keratoconus because of a glycosylation-mediatedcross-linking that strengthens the stromal tissue.8-12 Theirresearch goal was to induce a similar cross-linking effectin nondiabetic corneas using sugars activated by ultravioletlight.10,13 The final result was a procedure using riboflavinand 370-nm UVA irradiation to induce cross-linkingbetween collagen fibrils in the stroma.8,10,13

Riboflavin (the photosensitizing agent), when excited toa triplet state by UV exposure, releases free radicals orreactive oxygen species into the surrounding stroma.4 Thefree radicals cause hydrogen bond or cross-link formationbetween the amino acids on the collagen chains at the intra-and interhelical levels as well as the intermicrofibrillarlevel.1,10 The intra- and interhelical cross-links cause an in-crease in collagen fiber diameter, and the intermicrofibrillarcross-links lead to an increase in spacing between collagenfibrils (see Figure 1).1,4,10 CXL is a possible treatment incases of keratoconus, pellucid marginal degeneration,post-laser in situ keratomileusis (LASIK) ectasia, bullouskeratopathy, infectious keratitis, and corneal melts. Patientspreviously considered poor candidates may be able toundergo laser refractive surgery if done in conjunctionwith CXL.

Methods

A literature review using various health science databasesfrom 1990 until August 2010 was performed. The searchstrategy included words pertaining to CXL, any indicationfor this procedure, and for frequently published authors.The search terms include the following:

� CXL� Collagen cross-linking� Progressive keratectasia or progressive corneal ectasia� Keratectasia or corneal ectasia treatment� Keratoconus and keratoconus treatment

Figure 1 A, Collagen fibril size and arrangement before undergoing

CXL. B, Collagen fibril size and spacing after undergoing CXL. Note the in-

creased size and spacing of the collagen fibrils post-CXL and the cross-links

between the individual fibrils.

� Riboflavin and UVA irradiation� Riboflavin and UVA cross-linking� Corneal collagen glycosylation reaction� Wollensak, Gregory� Seiler, Theo� Spoerl, Ebergard� Mazzotta, Cosimo� Kanellopoulos, Anastasios John� Kymionis, George

The inclusion criteria were original articles that studiedthe effects of CXL, either alone or in conjunction withIntacs� corneal implants (Addition Technology�, DesPlaines, Illinois) or photorefractive keratectomy (PRK),in vivo on human subjects. Sample populations needed toinclude at least 10 patients of any age range undergoingCXL who had a previous diagnosis of keratoconus, iatro-genic keratectasia, corneal ulcer, corneal melt, or bullouskeratopathy.

Indications

CXL is indicated in progressive keratectasia, such askeratoconus and the associated variants. Depending onthe population studied, approximately 1 in 2,000 people inthe United States suffer from keratoconus,14,16 with 20% ofthose patients eventually requiring a penetrating kerato-plasty.1,14 Keratoconic corneas undergo changes in thecollagen structure and organization as well as changes inthe extracellular matrix.17-20 The alterations, along withkeratocyte apoptosis and necrosis, lead to a 50% reductionin the biochemical resistance of the cornea.8 Individualswith early-to-moderate keratoconus showing signs ofprogression are better candidates for CXL than those withend-stage disease.8 In keratoconic individuals, CXL hasalso shown the ability to potentiate the effects of Intacsand is effective when used with PRK.10,21-24

CXL is indicated in patients who wish to undergocorneal laser corrective surgery but are considered poorcandidates because of corneal thickness considerations.10

CXL is also indicated in patients suffering from keratecta-sia after corneal laser corrective surgery.10,25 In cases ofkeratectasia, Koller et al.26 recommend restricting inclusioncriteria to patients with keratometry readings less than58 diopters and patient age younger than 35 years. Othercorneal conditions currently being studied as possible indi-cations for CXL include corneal melting processes, infec-tious keratitis not responding to treatment, and bullouskeratopathy.10,25,27-30

Contraindications

CXL requires a minimum corneal thickness of 400 mm afterremoval of the epithelium. Individuals with corneas thinnerthan 400 mm should not undergo CXL because of possibleendothelial cell damage.10,25,31,32 Patients with prior

Dahl et al Literature Review 35

incisional refractive surgery (radial keratotomy or astig-matic keratotomy) should not be considered for CXL. Al-terations within the corneal stroma post-CXL, particularlythe contraction of the collagen lamellae, can cause the ker-atotomy incisions to rupture.10 Other contraindications in-clude central corneal opacities and severe dry eye, whichcan hinder reepithelialization.10,25 Individuals who arepregnant, nursing, or have systemic collagen vascular dis-ease should avoid undergoing CXL, as these populationshave not been sufficiently investigated.10

Preoperative care

A comprehensive eye examination before CXL, whichincludes refraction, keratometry, topography, and regionalpachymetry is required.10,13,23,33 Some researchers recom-mend that patients discontinue contact lens use approxi-mately 1 month before CXL to assure no corneal warpageis present.10,27,31,34 It should be noted, however, that this isnot always possible for these patients. Many keratoconicpatients are only able to achieve functional vision with rigidgas permeable contact lenses and would not be able to func-tion without their contact lenses for 1 month. Assessment ofendothelial cell count is recommended so post-CXL endo-thelial cell counts comparisons can be made.23,31,33 Abroad-spectrum topical antibiotic should be initiated 4 timesdaily, 1 to 3 days before the procedure.10

Abad and Panesso10 suggest the use of gaba analogs(gabapentin or pregabalin) twice daily by mouth beginning1 to 2 days before treatment to increase pain tolerance andcomfort levels postoperatively. Prophylactic use of oralacyclovir should be considered, as an isolated case ofherpes keratitis has been reported in a post-CXL individualwith no prior history of keratitis.1,35

Figure 2 Cross-hatch epithelial debridement. This method has been

suggested to decrease postoperative pain and speed up wound healing.

Procedural overview

CXL is an outpatient procedure performed under topicalanesthesia with the patient in the supine position. Afterapplication of the anesthetic, a lid speculum is applied tomaintain a large aperture. Controversy still exists asto whether epithelial debridement is necessary, although,a 7- to 9-mm epithelial debridement is most oftenutilized.4,8,10,13,21,22,25,28,31,33-53 Riboflavin absorption isthe biggest reason many researchers argue that epithelialdebridement is necessary. The epithelium acts as a barrierto the riboflavin and prevents sufficient absorption leadingto insufficient riboflavin shielding and possible endothelialcell damage.25,31,40,41,54-56 Wollensak and Iomdina55

showed that without epithelial debridement, the efficacyof CXL is decreased to one fifth the normal efficacy. Baioc-chi et al.56 demonstrated in their study that the necessarystromal concentration of riboflavin is only achieved afterepithelial debridement.

Proponents of leaving the epithelium intact arguepatients will experience significantly less postoperative

discomfort, are at less risk of serious infections and willhave much improved healing times.23,24,57,58 Thesearguments, however, do not address the issue of riboflavinabsorption and proper UV shielding. An alternative tocomplete debridement is performing an incompletedebridement in a crosshatch pattern, which can speed thereepithelialization process and reduce postoperative pain(see Figure 2).10 Kanellopoulos22 has suggested using afemtosecond laser to create a pocket 100 mm deep intothe cornea. This allows for riboflavin delivery in two0.1-mL doses into this pocket without significant epithelialcompromise. The patient group only consisted of 10patients; however, the procedure showed to be efficacious,and the patients experienced less postoperative pain.59

Regardless ofwhether epithelial debridement is performed,a 0.1% riboflavin solution in 20% dextran is subsequentlyapplied every 2 to 5 minutes for 30 minutes before applyingany UVA irradiation.4,8,10,13,21-25,28,31,36-55,60-63 Dextran isused to limit edema, maintain a controlled environment, andmaximize the cross-linking effect.10 Additionally, beforeUVAexposure, riboflavin shouldbeen assessedon slit lampbi-omicroscopy in the anterior chamber to assure the cornea issufficiently saturated.10,24,25,34,40,41,54 The cornea issubsequently exposed to UVA irradiation for 30 min-utes,4,8,13,21-25,28,31,36-54,61-63 during which time riboflavin isadded every 2 to 5 minutes to ensure sufficient riboflavinshielding.4,10,13,23,24,28,33,38-41,43-54,61-63 Despite differencesin procedural techniques, the UVA dosing remains consistent:wavelength 3706 5 nm and (maximum absorption of ribofla-vin)62 power 3 mW/cm2 (5 J/cm2).4,8,10,13,21-25,28,31,33-52,54,62

Treatment zone diameter varies depending on which UVsource is used, but the average treatment diameter is7-9 mm.4,8,10,23,24,31,33,36,39-41,47,51-54,61-63 This assures thatonly clear cornea is exposed to theUVirradiation, and the lim-bal cells are protected.10,64

Postoperative care

Postoperative care for CXL is very similar to that of PRK.Topical antibiotics are applied 4 times daily for1 week,8,13,21,25,28,31,34,39,41,43,62 and a bandage contact

Table 1 UV damage thresholds of ocular tissues comparedwith UV exposure during CXL7

Ocular tissue Exposure during CXL Damage threshold

Epithelium* 5.4 J/cm2 0.12–0.56 J/cm2

Endothelium 0.32 J/cm2 0.65 J/cm2

Anterior lenssurface

0.27 J/cm2 70 J/cm2

Posterior lenssurface

0.22 J/cm2 70 J/cm2

Retina 0.22 J/cm2 7.7 J/cm2

UV 5 ultraviolet; CXL 5 corneal collagen cross-linking.

* Photokeratitis is not a sequelae when epithelial debridement is

performed.


lens is left in place until reepithelialization is com-plete.8,10,13,21,25,28,31,33,34,38,40-43,54 Some controversy doesexist surrounding the use of bandage soft contact lenses.Practitioners must weigh the benefit of increased patientcomfort and improved wound healing compared with theincreased risk of microbial keratitis and complications ofsteroid use with soft contact lenses.31,35,65,66 The individualshould be followed up with daily until reepithelialization.The follow-up schedule is 1, 3, 6, and 12 months afterreepithelialization is complete.8,27,31,39 Topography, best-corrected visual acuity (BCVA), pachymetry, keratometry,and intraocular pressure measurements should be obtainedat each follow-up.27,31,34,39 If a preoperative endothelialcell count was obtained, postoperative measurementsshould be repeated.23,33

If gaba analogs were initiated preoperatively, agents arecontinued 1 to 2 days postoperatively.10 Optional topicalmedications include steroids to decrease inflamma-tion,4,8,21,23,25,28,41,62 or nonsteroidal anti-inflammatorymedications (NSAIDs) for pain.10,39,41,42,51,61,67 Whensteroids or NSAIDs are incorporated, they should bediscontinued after 2 to 3 days postoperatively to preventimpeding wound healing.10 Proparacaine 0.5% diluted to0.125% in artificial tears may also be used every 5 minutesto break the pain cycle instead of an NSAID, then every 30to 60 minutes thereafter.10,68 (An NSAID and the propara-caine mixture should not be used in conjunction with eachother, as no studies were found showing they are a safecombination.)

Side effects

While some studies report no adverse events,8,13,33 theside-effect profile of CXL is similar to that of PRK.10

Individuals should expect blurry vision, lacrimation, andforeign body sensation for at least 24 to 48 hours.4 Blurryvision is transient but can last up to 1 month or longer,depending on the amount of corneal edema present.10

Stromal edema is concentrated in the anterior two thirdsof the stroma and presents as a honeycomb or sponge-like pattern8,69; this should resolve by 3 months post-operatively with concurrent improvement in vision.8,10

The proposed mechanism for the corneal edema is relatedto the complete rarefaction of keratocytes in the anteriorstroma.8,61,67,69 Confocal microscopy found that the edemawas only present in areas devoid of keratocytes.67 The ribo-flavin soak will cause the cornea to be stained yellow; thestaining resolves within 3 to 4 days and will not cause a de-crease in vision.18

Cases of more severe complications, including microbialkeratitis,28,32,70-73 acanthamoeba keratitis,29 herpes kerati-tis,1,35 sterile infiltrates,74 corneal haze,8,13 and decreasedvisual acuity75 have all been reported in the literature.Additionally, a report by Gokhale and Vemuganti51

suggests a possible link between diclofenac and topical an-esthetic use with corneal melt post-CXL. The case involvedthe use of prescription 0.5% proparacaine, which the

individual reported using frequently.51 Patients undergoingCXL are also at risk for endothelial and limbal cell damagecaused by exposure to UV radiation or the free radicalsliberated during cross-linking.10,64 As previously discussed,Koller et al.26 suggest keratometry readings greater than 58diopters, older than 35 years, and BCVA 20/25 or better arerisk factors for failure or complications.

Results

The recommendations for exposure to wavelengthsbetween 180 and 400 nm suggest radiant exposure shouldnot exceed 1 J/cm.2,76 UVA irradiation of 370 nm wave-length at a dose of 3 mW/cm2 (5.4J/cm2) is used duringCXL.4,8,10,13,21-25,28,31,33-52,54,62 With a corneal thicknessof at least 400 mm, no endothelial damage has beenreported8,25,31,33,41,47,67,77 Table 1 details the UV damagethresholds for ocular tissues and the correlation with actualexposure during CXL.25

Several clinical measurements have been shownto improve after CXL, including uncorrected visualacuity (UCVA), BCVA, maximum keratometry values,and spherical, spherical equivalent, and cylindricalrefractive measurements. Tables 2 through 5 detail themeasured improvements found in various clinicalstudies.27,31,33,38,39,41,47,62,67,78-82 In addition to theimprovements observed above, eyes with keratoconusalso experienced a significant reduction in 4 of the 7keratoconus indices as detailed in Table 6.27

Double segment Intacs combined with transepithelialCXL showed more significant improvements in UCVA,BCVA, keratometry values, and sphere and cylinder refrac-tive measurements than double segment Intacs alone(see Table 7).24 This study used Intacs with a 6.8-mm innerdiameter, an 8.1-mm outer diameter, and a 150� curvature,which were placed inferior and superiorly in the cornea.Chan et al.23 showed that CXL had additive effects withinferior segment Intacs compared with inferior-segmentIntacs alone. Patients who underwent CXL with inferiorIntacs showed a statistically significant decrease in cylinderrefraction, steep keratometry values, and average

Table 2 Change in maximum keratometry values after CXL

Study Measurement unit Δ* Follow-up

Goldich et al.31 D 1.80 1 yrWollensak et al.33 D 2.01 VariableArbelaez et al.38 D 1.40 1 yrArgrawal39 D 2.47 VariableVinciguerra et al.41 D 1.35 2 yrWittig-Silva et al.47 D 1.45 12 moRaiskup-Wolf et al.62 D 1.91 2 yrRaiskup-Wolf et al.62 D 2.56 3 yrCaporrossi et al.67 D 1.90 3 moJankov et al.78 D 2.14 6 moHoyer et al.79 D 4.34 12–36 moFourni�e et al.80 D 1.68 3–18 moCoskunseven et al.81 D 1.57 5–12 moGrewal et al.82 None Stable 12 mo

CXL 5 corneal collagen cross-linking; D 5 diopters.

* Represents the decrease in maximum keratometry reading after

CXL.

Table 4 Change in corrected visual acuity after cross-linking


Goldich et al.31 Snellen lines 21.0† 1 yrWollensak et al.33 Snellen lines 21.26 1 yrArbelaez et al.38 Snellen lines 23.0 1 yrVinciguerra et al.41 Snellen lines 23.0† 2 yrWittig-Silva et al.47 Snellen lines 21.0 1 yrRaiskup-Wolf et al.62 Snellen lines 21.0 2 yrCaporrossi et al.67 Snellen lines 21.66 3 mo

* (2) Represents improvement on acuity, (1) represents loss of

acuity.

† Approximate value, original data reported in logMAR converted

to Snellen and then Snellen line improvement for data consistency.


keratometry values compared with those patients who onlyreceived Intacs implantation. No statistically significant dif-ference in UCVA, BCVA, spherical refraction, and flat kera-tometry values were observed between the 2 groups (seeTable 8).23 The only details given about the Intacs used inthis study are that either .35 mm, .30 mm, or .25 mm Intacssegments were placed inferiorly.23

CXL and PRK used in combination is an effectivetreatment for keratoconus patients.21,22,83 Kymionis et al.83

performed PRK followed by same-day CXL. The eyesstudied (N 5 14) showed improvements in spherical equiv-alent, defocus, UCVAs and BCVAs, and mean steepkeratometry values (see Table 9).83 Kanellopoulos22

showed that simultaneous PRK and CXL (both proceduresperformed the same day) is more effective than subsequentPRK performed 6 months after CXL (see Table 10).22

Conductive keratoplasty and CXL was not shown tobe an effective permutation for the treatment of keratoco-nus as demonstrated by Kymionis et al.46 Patients undergo-ing this series of procedures did experience significant

Table 3 Change in UCVA after CXL


Goldich et al.31 Snellen lines 12.0† 1 yrArbelaez et al.38 Snellen lines 25.0† 1 yrVinciguerra et al.41 Snellen lines 23.6 3 moCaporrossi et al.67 Snellen lines 25.0† 1 yr

UCVA 5 uncorrected visual acuity; CXL 5 corneal collagen cross-

linking.

* (2) Represents improvement on acuity, (1) represents loss of

acuity.

† Approximate value, original data reported in logMAR converted

to Snellen and then Snellen line improvement for data consistency.

improvement on topography; however, the effect regressedafter 3 months.

Discussion

When evaluating a new surgical procedure, safety isparamount. During CXL the ocular tissues are exposed toUVA irradiation, which, under normal circumstances,results in damage. The key to maintaining safety withCXL lies in the shielding effect of riboflavin. UVAabsorption in combination with riboflavin is approximately10 times less (.35 mW/cm2) than that of UVA alone(3.4 mW/cm2).10,63 Kolhaas et al.52 found that duringCXL, 70% of the UVA irradiation is absorbed in the ante-rior 200 mm and 20% is absorbed in the next 200 mm of thecornea.52 Minimal UV irradiation penetrates to more poste-rior structures of the eye.25,31,52 These data are consistentwith the expected depth of treatment of approximately300 mm8,13,61 and supports the recommended minimalcorneal thickness requirement of 400 mm to preserveendothelial integrity.1,22,25,33,37,40,61

Corneal transparency is essential for proper function;corneal fibril arrangement and spacing are essential inmaintaining transparency. CXL causes an increase in colla-gen fiber diameter and fibril spacing (see Figure 2).4,36,84 Tomaintain clarity, the collagen fiber diameter cannot exceed

Table 5 Change in sphere, spherical equivalent, andcylinder refraction measurement after CXL

Study Measurement unit Δ (D) Follow-up

Arbelaez et al.38 Sphere 1.26 2 yrWollensak et al.33 Spherical equivalent 1.14 VariableWollensak et al.33 Cylinder 1.2 VariableArbelaez et al.38 Cylinder 1.25 1 yrArgrawal39 Cylinder 1.2 VariableRaiskup-Wolf et al.62 Cylinder 1.2 2 yr

CXL 5 corneal collagen cross-linking; D 5 diopters.

Table 6 Keratoconus indices and changes in each index after undergoing corneal collagen cross-linking27

Name Description Preoperative Postoperative*

Index of surface variance Curvature variation from the mean curvature 98 95†

Index of vertical asymmetry Curvature difference between superior and inferior 1.10 1.09Keratoconus index Compares keratoconus sector with normal sector 1.27 1.25†

Center keratoconus index Compares central with peripheral curvature 1.08 1.06†

Index of height asymmetry Height difference between superior and inferior 31.4 24.7†

Index of height decentration Vertical decentration of height data 0.09 0.09Aberration coefficient Zernike coefficient of the anterior corneal surface 2.29 2.3

* Postoperative values measured at 12 months.

† Statistically significant change.


one third the wavelength of visible light (150 nm).85 The12.2% increase in collagen fibril diameter within the anteriorstroma reported in CXL does not approach the threshold andthus does not affect corneal transparency.4,85 Despite thetreatment being concentrated in the anterior stroma, the pos-terior collagen fibrils also increase in diameter after CXL.The anterior fibers increase 9.3% more than the posteriorfibers; however, the posterior fibrils still increase by 4.6%after CXL.4 The increased fiber diameter after CXL maybe secondary to an increase in molecular weight of typeI stromal collagen after CXL as demonstrated on electropho-retic analysis.36

The increased fiber diameter after CXL may be respon-sible for the stromal demarcation line that is evident2 weeks after CXL is performed.10,13 The demarcationline can be seen clinically on slit lamp biomicroscopy.The demarcation line appears as a faint change in cornealreflectivity extending approximately 60%, or 300 mm,into the cornea from the anterior surface.4,10,13 The changein reflectivity is not significant enough to result in anyadverse effect on visual performance.13 No consensus hasbeen obtained as to etiology of the demarcation line. Stud-ies have suggested that the line is a clinical manifestation ofthe depth of treatment or a change in the refractive indexwithin the stroma caused by the induced cross-links, or aresult of an increase in collagen fiber diameter and fibrilspacing.4,10,13

Table 7 Efficacy comparison of double-segment Intacsalone versus double-segment Intacs with CXL. Valuesrepresent improvement from preoperative measurements

Measurement Intacs only Intacs/CXL

UCVA* 1.9 3.19BCVA* 1.7 3.09Spherical refraction 2.08 D 2.58 DCylindrical refraction 0.47 D 0.62 DMean K value 2.22 D 2.57 DSteepest K value 1.27 D 2.03 D

CXL 5 corneal collagen cross-linking; D 5 diopters. Note.

Values represent improvement from preoperative measurements.

* Improvement measured by number of Snellen line.

Histologic analysis has found that CXL causes acomplete rarefaction of stromal keratocytes in the anterior200 to 300 mm of the stroma as shown in Figure 3.1,8,25,61,84

Keratocytes in the untreated cornea are activated after theapoptotic event and begin repopulating the treated stromaaround 3 months postoperatively. The repopulating kerato-cytes produce hyaluronan, which plays a key role in fibrilspacing,86 increases the stromal resistance to UV damage,and contributes to an increase in extracellular matrixdensity.37,87 Keratocyte repopulation is complete atapproximately 6 months postoperatively.5,8,13,25,37,45,87

Application of the apoptosis inhibitor zinc chloride is notbeneficial, and it weakens the cross-linking effects of theprocedure.61 Because keratocytes are responsible forstromal organization and maintenance, the lack of kerato-cytes at 6 months postoperatively may leave the corneamore prone to a melting process. The rarefaction of kerato-cytes and subsequent repopulation have been confirmedwith confocal microscopy.1,8,88-90

CXL might play a role in preventing progressive kera-tectasia. Progressive keratectasia is accompanied by adecrease in corneal tensile strength. In vitro testing hasfound a significant increase in stromal rigidity afterCXL.48,91 Porcine corneal tissue has shown a 70% increasein rigidity, whereas human tissue samples in vitro haveshown to increase in rigidity by 328%48; however, rigidityincreases are only evident in the anterior 200 mm of the

Table 8 Efficacy comparison of inferior segment Intacsalone versus inferior segment Intacs with CXL

Measurement Intacs with CXL Intacs only

UCVA (Snellen)* 6.5 9.5BCVA (Snellen)* 1 1Spherical refraction 0.12 D 0.25 DCylinder refraction 2.73 D 1.48 DSteep K 1.94 D 0.89 DFlat K 1.05 D 0.64 DAverage K 1.34 D 0.21 D

CXL 5 corneal collagen cross-linking; D 5 diopters. Note.

Values represent improvement from preoperative measurements.

* Improvement measured by number of Snellen line.

Figure 3 Representation of keratocyte apoptosis after CXL with even-

tual repopulation beginning at 3 months postoperatively and completing

approximately 6 months postoperatively.

Table 9 Comparison of pre- and postoperativemeasurements in patients undergoing PRK followed by same-day CXL

Measurement Preoperative Postoperative D

Sphericalequivalent

23.03 D 21.25 D 1.75 D

Defocus 4.67 D 3.04 D 1.63 DUCVA 0.99 logMAR 0.16 logMAR 0.83 logMARBCVA 0.21 logMAR 0.11 logMAR 0.10 logMARMean steep K 48.20 D 45.13 D 3.07 D

PRK 5 photorefractive keratectomy; CXL 5 corneal collagen

cross-linking; UCVA 5 uncorrected visual acuity; BCVA 5 best-

corrected visual acuity; D 5 diopters.


cornea.52 The increase in corneal rigidity after CXL coulddecrease the risk of progressive keratectasia.

Corneas having undergone CXL showed a greaterresistance to chemical degradation, which is critical inhalting the progression of keratectasia.8,53 The demon-strated increase in anticollagenase activity is particularlyimportant in individuals with keratoconus or corneal ulcer-ative processes. Keratoconic corneas are more susceptibleto collagenase,8,92 and collagenolytic activity is one ofthe main mechanisms causing corneal ulceration.92,93

An in vitro chemical degradation study53 compared thevulnerability of untreated corneas and CXL-treated corneaswith pepsin, collagenase, and trypsin. Corneas treated byCXL degraded by day 13 with pepsin, day 14 with colla-genase, and day 5 with trypsin. Untreated corneas showeda much greater vulnerability, degrading by day 6 withpepsin and collagenase, and day 2 with trypsin. The benefitof the increased resistance to chemical degradation afterCXL is evident in various case reports28,29,93-95 of patientswith infectious keratitis and bullous keratopathy. Thesubjects showed subjective and clinical improvements invisual acuity and pain levels as well as decreases in opaci-fication and cicatricial corneal tissue after treatment withCXL.28,29,93-95 In cases of keratomycosis, CXL mayimprove amphotericin B effectiveness against Candidaalbicans, Fusarium sp, and Aspergillus fumigatus.96

Table 10 Comparison of sequential versus simultaneous CXLand PRK. Values represent the reduction in measurementsafter both procedures

Measurement Sequential Simultaneous

UCVA 0.41 logMAR 0.76 logMARBCVA 0.25 logMAR 0.28 logMARSpherical equivalent 2.50 D 3.20 DMean K reading 2.75 D 3.50 D

CXL 5 corneal collagen cross-linking; PRK 5 photorefractive

keratectomy; D 5 diopters. Note. Values represent the reduction

in measurements after both procedures.

Researchers are investigating other ways to allow forCXL treatment in patients with less than 400 mm cornealthickness. Kymionis et al.40 have suggested performingpachymetry-guided epithelial debridement instead of com-plete debridement. All areas thinner than 400 mm beforeepithelial debridement on regional pachymetry are leftintact. The procedure would allow individuals with cornealthickness bordering 400 mm to undergo CXL. Kymioniset al.40 reported a case of pachymetry-guided epithelialdebridement. After 9 months, no adverse events were ob-served. Increased follow-up periods and an increased studycohort are needed to verify the safety and efficacy ofpachymetry-guided epithelial debridement.40

Researchers have also investigated the use of hypo-osmolar riboflavin in patients with corneal thickness lessthan 400 mm.10,54,64 Instead of the riboflavin applicationdescribed above, 2 drops of hypotonic 0.1% riboflavinsolution are added every 10 seconds to swell the corneauntil the corneal thickness is greater than 400 mm.10 It is,however, unclear if the cross-linking effect on an over-hydrated cornea will be the same as on a cornea withnormal hydration.54

Areas for future study include the indication and efficacyof retreatment. Collagen turnover in the cornea ranges from2 to 7 years, which may indicate the need for retreatment ifthe renewed collagen does not exhibit the same increasedstrength and resistance to degradation.10,33 Further researchinto the need for an adjusted nomogram when using PRKand CXL together is required to reduce the likelihood ofovercorrections and necessary retreatment.21 Also, theregularization of the corneal surface could take up to1 year27; therefore, the refractive error could be variableduring that period of regularization. To satisfy the patient’svisual demands, trial contact lenses or frequent spectaclelens replacement may be necessary.


Conclusion

CXL has shown to be effective at limiting the progression ofkeratectasia. To date, the longest reported follow-up hasbeen 6 years. The patient population is significantly reducedin the 6-year follow-up group; however, individuals haveshown good stability.62 Practitioners should consider enroll-ing keratoconus patients in ongoing clinical trials, especiallythose who show signs of progression as defined by changesin topography, keratometry readings, and new onset of ante-rior segment findings such hydrops.8,14,15 Caution should betaken with patients whose keratometry readings are above58 diopters, are older than 35 years, or have BCVA betterthen 20/25 because they are at higher risk for failure and de-creased vision after CXL.26

Patients with microbial keratitis not responding totreatment and bullous keratopathy may also benefit fromenrolling in clinical trials. These patients may not have alarge improvement in vision, but CXL can significantlyimprove their comfort level. A list of the ongoing clinicaltrials investigating CXL can be found on the U.S. NationalInstitutes of Health Web site.97 No timetable is available forwhen and if CXL will be approved for use in the UnitedStates.

Acknowledgements

The authors thank the staff at the West Point optometryclinic, especially Eric Spotts, O.D., and James Q. Truong,O.D. The authors also thank Mary Davidian, M.D.

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corneal collagen cross-linking: an introduction and literature review

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