Corneal collagen cross-linking: An introduction and literature review

Optometry (2012) 83, 33-42

Corneal collagen cross-linking: An introduction and 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.

KEYWORDS Collagen cross-linking; Keratoconus; Corneal ectasia; Riboflavin; Ultraviolet A

Abstract BACKGROUND: This literature review analyzes the scientific evidence available regarding corneal collagen cross-linking (CXL) as a treatment option for progressive keratectasia. METHODS: A literature search was performed using dates from 1990 to August 2010 regarding CXL Specific areas of focus for the literature review include safety and efficacy of the procedure as a standalone treatment or when used in conjunction with IntacsÒ corneal implants (Addition TechnologyÔ) or photorefractive keratectomy (PRK). RESULTS: A total of 50 clinical trials and studies were identified, 20 of which met the inclusion criteria. Results of the included literature support the conclusion that CXL is a safe and efficacious treatment for progressive keratectasia. The results of CXL alone have shown stabilization or improvement 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 of Intacs and has been proven successful when used in conjunction with PRK. CONCLUSION: CXL is an effective treatment for limiting the progression of keratectasia, thus reducing the need for penetrating keratoplasty. CXL has a similar side-effect profile and similar risk level as PRK. Optometry 2012;83:33-42

Progressive keratectasia is a result of progressive corneal disease or a sequela of refractive surgery. In the United States, there is no current treatment for keratectasia. Corneal collagen cross-linking (CXL) is a new treatment intended to halt the progression of keratectasia. The procedure, which is currently performed in other parts the world, uses ultraviolet (UV) light and riboflavin to strengthen the stromal collagen. Disclosure: The author has no financial or other vested interests in the products used in this literature review. * Corresponding author: Brandon J. Dahl, O.D., Weed Army Community Hospital, Optometry Services Box 105109, Fort Irwin, CA 92310 E-mail: [email protected]

The use of UV light long wave (UVA) irradiation and cross-linking as a means to strengthen a material is not a new concept. Dentists use UV light to induce cross-linking, which strengthens the material used for fillings.1,2 The polymer industry used UV light to harden adhesives.1 Crosslinking with glutaraldehyde is used to stabilize prosthetic heart valves.1,3 Cross-linking is also a natural phenomenon occurring within the cornea and crystalline lens with age. The corneal fibril diameter increases 4.5% throughout a person’s life because of an age-dependent glycosylation crosslinking.4-6 Within the lens, crystallins increase in molecular weight, and rigidity secondary to age-related cross-linking.6,7

1529-1839/$ - see front matter Ó 2012 American Optometric Association. All rights reserved. doi:10.1016/j.optm.2011.09.011

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CXL is the result of research commencing in the 1990s intended to identify biological glues that could strengthen corneal collagen.8 Researchers at the Technical University of Dresden in Germany noted diabetic patients rarely develop keratoconus because of a glycosylation-mediated cross-linking that strengthens the stromal tissue.8-12 Their research goal was to induce a similar cross-linking effect in nondiabetic corneas using sugars activated by ultraviolet light.10,13 The final result was a procedure using riboflavin and 370-nm UVA irradiation to induce cross-linking between collagen fibrils in the stroma.8,10,13 Riboflavin (the photosensitizing agent), when excited to a triplet state by UV exposure, releases free radicals or reactive oxygen species into the surrounding stroma.4 The free radicals cause hydrogen bond or cross-link formation between the amino acids on the collagen chains at the intraand interhelical levels as well as the intermicrofibrillar level.1,10 The intra- and interhelical cross-links cause an increase in collagen fiber diameter, and the intermicrofibrillar cross-links lead to an increase in spacing between collagen fibrils (see Figure 1).1,4,10 CXL is a possible treatment in cases of keratoconus, pellucid marginal degeneration, post-laser in situ keratomileusis (LASIK) ectasia, bullous keratopathy, infectious keratitis, and corneal melts. Patients previously considered poor candidates may be able to undergo laser refractive surgery if done in conjunction with CXL.

Methods A literature review using various health science databases from 1990 until August 2010 was performed. The search strategy included words pertaining to CXL, any indication for 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

        

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 studied the effects of CXL, either alone or in conjunction with IntacsÒ corneal implants (Addition TechnologyÔ, Des Plaines, Illinois) or photorefractive keratectomy (PRK), in vivo on human subjects. Sample populations needed to include at least 10 patients of any age range undergoing CXL who had a previous diagnosis of keratoconus, iatrogenic keratectasia, corneal ulcer, corneal melt, or bullous keratopathy.

Indications CXL is indicated in progressive keratectasia, such as keratoconus and the associated variants. Depending on the population studied, approximately 1 in 2,000 people in the United States suffer from keratoconus,14,16 with 20% of those patients eventually requiring a penetrating keratoplasty.1,14 Keratoconic corneas undergo changes in the collagen structure and organization as well as changes in the extracellular matrix.17-20 The alterations, along with keratocyte apoptosis and necrosis, lead to a 50% reduction in the biochemical resistance of the cornea.8 Individuals with early-to-moderate keratoconus showing signs of progression are better candidates for CXL than those with end-stage disease.8 In keratoconic individuals, CXL has also shown the ability to potentiate the effects of Intacs and is effective when used with PRK.10,21-24 CXL is indicated in patients who wish to undergo corneal laser corrective surgery but are considered poor candidates because of corneal thickness considerations.10 CXL is also indicated in patients suffering from keratectasia after corneal laser corrective surgery.10,25 In cases of keratectasia, Koller et al.26 recommend restricting inclusion criteria to patients with keratometry readings less than 58 diopters and patient age younger than 35 years. Other corneal conditions currently being studied as possible indications for CXL include corneal melting processes, infectious keratitis not responding to treatment, and bullous keratopathy.10,25,27-30

Contraindications Figure 1

A, Collagen fibril size and arrangement before undergoing CXL. B, Collagen fibril size and spacing after undergoing CXL. Note the increased size and spacing of the collagen fibrils post-CXL and the cross-links between the individual fibrils.

CXL requires a minimum corneal thickness of 400 mm after removal of the epithelium. Individuals with corneas thinner than 400 mm should not undergo CXL because of possible endothelial cell damage.10,25,31,32 Patients with prior

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Literature Review

incisional refractive surgery (radial keratotomy or astigmatic keratotomy) should not be considered for CXL. Alterations within the corneal stroma post-CXL, particularly the contraction of the collagen lamellae, can cause the keratotomy incisions to rupture.10 Other contraindications include central corneal opacities and severe dry eye, which can hinder reepithelialization.10,25 Individuals who are pregnant, nursing, or have systemic collagen vascular disease should avoid undergoing CXL, as these populations have not been sufficiently investigated.10

Preoperative care A comprehensive eye examination before CXL, which includes refraction, keratometry, topography, and regional pachymetry is required.10,13,23,33 Some researchers recommend that patients discontinue contact lens use approximately 1 month before CXL to assure no corneal warpage is present.10,27,31,34 It should be noted, however, that this is not always possible for these patients. Many keratoconic patients are only able to achieve functional vision with rigid gas permeable contact lenses and would not be able to function without their contact lenses for 1 month. Assessment of endothelial cell count is recommended so post-CXL endothelial cell counts comparisons can be made.23,31,33 A broad-spectrum topical antibiotic should be initiated 4 times daily, 1 to 3 days before the procedure.10 Abad and Panesso10 suggest the use of gaba analogs (gabapentin or pregabalin) twice daily by mouth beginning 1 to 2 days before treatment to increase pain tolerance and comfort levels postoperatively. Prophylactic use of oral acyclovir should be considered, as an isolated case of herpes keratitis has been reported in a post-CXL individual with no prior history of keratitis.1,35

Procedural overview CXL is an outpatient procedure performed under topical anesthesia with the patient in the supine position. After application of the anesthetic, a lid speculum is applied to maintain a large aperture. Controversy still exists as to whether epithelial debridement is necessary, although, a 7- to 9-mm epithelial debridement is most often utilized.4,8,10,13,21,22,25,28,31,33-53 Riboflavin absorption is the biggest reason many researchers argue that epithelial debridement is necessary. The epithelium acts as a barrier to the riboflavin and prevents sufficient absorption leading to insufficient riboflavin shielding and possible endothelial cell damage.25,31,40,41,54-56 Wollensak and Iomdina55 showed that without epithelial debridement, the efficacy of CXL is decreased to one fifth the normal efficacy. Baiocchi et al.56 demonstrated in their study that the necessary stromal concentration of riboflavin is only achieved after epithelial debridement. Proponents of leaving the epithelium intact argue patients will experience significantly less postoperative

35 discomfort, are at less risk of serious infections and will have much improved healing times.23,24,57,58 These arguments, however, do not address the issue of riboflavin absorption and proper UV shielding. An alternative to complete debridement is performing an incomplete debridement in a crosshatch pattern, which can speed the reepithelialization process and reduce postoperative pain (see Figure 2).10 Kanellopoulos22 has suggested using a femtosecond laser to create a pocket 100 mm deep into the cornea. This allows for riboflavin delivery in two 0.1-mL doses into this pocket without significant epithelial compromise. The patient group only consisted of 10 patients; however, the procedure showed to be efficacious, and the patients experienced less postoperative pain.59 Regardless of whether epithelial debridement is performed, a 0.1% riboflavin solution in 20% dextran is subsequently applied every 2 to 5 minutes for 30 minutes before applying any UVA irradiation.4,8,10,13,21-25,28,31,36-55,60-63 Dextran is used to limit edema, maintain a controlled environment, and maximize the cross-linking effect.10 Additionally, before UVA exposure, riboflavin should been assessed on slit lamp biomicroscopy in the anterior chamber to assure the cornea is sufficiently saturated.10,24,25,34,40,41,54 The cornea is subsequently exposed to UVA irradiation for 30 minutes,4,8,13,21-25,28,31,36-54,61-63 during which time riboflavin is added every 2 to 5 minutes to ensure sufficient riboflavin shielding.4,10,13,23,24,28,33,38-41,43-54,61-63 Despite differences in procedural techniques, the UVA dosing remains consistent: wavelength 370 6 5 nm and (maximum absorption of riboflavin)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 UV source is used, but the average treatment diameter is 7-9 mm.4,8,10,23,24,31,33,36,39-41,47,51-54,61-63 This assures that only clear cornea is exposed to the UV irradiation, and the limbal 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 for 1 week,8,13,21,25,28,31,34,39,41,43,62 and a bandage contact

Figure 2

Cross-hatch epithelial debridement. This method has been suggested to decrease postoperative pain and speed up wound healing.

36 lens is left in place until reepithelialization is complete.8,10,13,21,25,28,31,33,34,38,40-43,54 Some controversy does exist surrounding the use of bandage soft contact lenses. Practitioners must weigh the benefit of increased patient comfort and improved wound healing compared with the increased risk of microbial keratitis and complications of steroid use with soft contact lenses.31,35,65,66 The individual should be followed up with daily until reepithelialization. The follow-up schedule is 1, 3, 6, and 12 months after reepithelialization is complete.8,27,31,39 Topography, bestcorrected visual acuity (BCVA), pachymetry, keratometry, and intraocular pressure measurements should be obtained at each follow-up.27,31,34,39 If a preoperative endothelial cell count was obtained, postoperative measurements should be repeated.23,33 If gaba analogs were initiated preoperatively, agents are continued 1 to 2 days postoperatively.10 Optional topical medications include steroids to decrease inflammation,4,8,21,23,25,28,41,62 or nonsteroidal anti-inflammatory medications (NSAIDs) for pain.10,39,41,42,51,61,67 When steroids or NSAIDs are incorporated, they should be discontinued after 2 to 3 days postoperatively to prevent impeding wound healing.10 Proparacaine 0.5% diluted to 0.125% in artificial tears may also be used every 5 minutes to break the pain cycle instead of an NSAID, then every 30 to 60 minutes thereafter.10,68 (An NSAID and the proparacaine mixture should not be used in conjunction with each other, as no studies were found showing they are a safe combination.)

Side effects While some studies report no adverse events,8,13,33 the side-effect profile of CXL is similar to that of PRK.10 Individuals should expect blurry vision, lacrimation, and foreign body sensation for at least 24 to 48 hours.4 Blurry vision 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 thirds of the stroma and presents as a honeycomb or spongelike pattern8,69; this should resolve by 3 months postoperatively with concurrent improvement in vision.8,10 The proposed mechanism for the corneal edema is related to the complete rarefaction of keratocytes in the anterior stroma.8,61,67,69 Confocal microscopy found that the edema was only present in areas devoid of keratocytes.67 The riboflavin soak will cause the cornea to be stained yellow; the staining resolves within 3 to 4 days and will not cause a decrease in vision.18 Cases of more severe complications, including microbial keratitis,28,32,70-73 acanthamoeba keratitis,29 herpes keratitis,1,35 sterile infiltrates,74 corneal haze,8,13 and decreased visual acuity75 have all been reported in the literature. Additionally, a report by Gokhale and Vemuganti51 suggests a possible link between diclofenac and topical anesthetic use with corneal melt post-CXL. The case involved the use of prescription 0.5% proparacaine, which the

Optometry, Vol 83, No 1, January 2012 individual reported using frequently.51 Patients undergoing CXL are also at risk for endothelial and limbal cell damage caused by exposure to UV radiation or the free radicals liberated during cross-linking.10,64 As previously discussed, Koller et al.26 suggest keratometry readings greater than 58 diopters, older than 35 years, and BCVA 20/25 or better are risk factors for failure or complications.

Results The recommendations for exposure to wavelengths between 180 and 400 nm suggest radiant exposure should not exceed 1 J/cm.2,76 UVA irradiation of 370 nm wavelength at a dose of 3 mW/cm2 (5.4J/cm2) is used during CXL.4,8,10,13,21-25,28,31,33-52,54,62 With a corneal thickness of at least 400 mm, no endothelial damage has been reported8,25,31,33,41,47,67,77 Table 1 details the UV damage thresholds for ocular tissues and the correlation with actual exposure during CXL.25 Several clinical measurements have been shown to improve after CXL, including uncorrected visual acuity (UCVA), BCVA, maximum keratometry values, and spherical, spherical equivalent, and cylindrical refractive measurements. Tables 2 through 5 detail the measured improvements found in various clinical studies.27,31,33,38,39,41,47,62,67,78-82 In addition to the improvements observed above, eyes with keratoconus also experienced a significant reduction in 4 of the 7 keratoconus indices as detailed in Table 6.27 Double segment Intacs combined with transepithelial CXL showed more significant improvements in UCVA, BCVA, keratometry values, and sphere and cylinder refractive measurements than double segment Intacs alone (see Table 7).24 This study used Intacs with a 6.8-mm inner diameter, 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 with inferior segment Intacs compared with inferior-segment Intacs alone. Patients who underwent CXL with inferior Intacs showed a statistically significant decrease in cylinder refraction, steep keratometry values, and average Table 1 UV damage thresholds of ocular tissues compared with UV exposure during CXL7 Ocular tissue Epithelium* Endothelium Anterior lens surface Posterior lens surface Retina

Exposure during CXL 2

Damage threshold

5.4 J/cm 0.32 J/cm2 0.27 J/cm2

0.12–0.56 J/cm2 0.65 J/cm2 70 J/cm2

0.22 J/cm2

70 J/cm2

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.

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Change in maximum keratometry values after CXL

Study 31

Goldich et al. Wollensak et al.33 Arbelaez et al.38 Argrawal39 Vinciguerra et al.41 Wittig-Silva et al.47 Raiskup-Wolf et al.62 Raiskup-Wolf et al.62 Caporrossi et al.67 Jankov et al.78 Hoyer et al.79 Fourni
e et al.80 Coskunseven et al.81 Grewal et al.82

Measurement unit Δ*

Follow-up

D D D D D D D D D D D D D None

1 yr Variable 1 yr Variable 2 yr 12 mo 2 yr 3 yr 3 mo 6 mo 12–36 mo 3–18 mo 5–12 mo 12 mo

1.80 2.01 1.40 2.47 1.35 1.45 1.91 2.56 1.90 2.14 4.34 1.68 1.57 Stable

CXL 5 corneal collagen cross-linking; D 5 diopters. * Represents the decrease in maximum keratometry reading after CXL.

Table 4 linking

Change in corrected visual acuity after crossMeasurement unit Δ*

Study 31

Goldich et al. Wollensak et al.33 Arbelaez et al.38 Vinciguerra et al.41 Wittig-Silva et al.47 Raiskup-Wolf et al.62 Caporrossi et al.67

Snellen Snellen Snellen Snellen Snellen Snellen Snellen

lines lines lines lines lines lines lines

Follow-up

21.0 21.26 23.0 23.0† 21.0 21.0 21.66 †

1 1 1 2 1 2 3

yr yr yr yr yr yr 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.

improvement on topography; however, the effect regressed after 3 months.

Discussion keratometry values compared with those patients who only received Intacs implantation. No statistically significant difference in UCVA, BCVA, spherical refraction, and flat keratometry values were observed between the 2 groups (see Table 8).23 The only details given about the Intacs used in this study are that either .35 mm, .30 mm, or .25 mm Intacs segments were placed inferiorly.23 CXL and PRK used in combination is an effective treatment for keratoconus patients.21,22,83 Kymionis et al.83 performed PRK followed by same-day CXL. The eyes studied (N 5 14) showed improvements in spherical equivalent, defocus, UCVAs and BCVAs, and mean steep keratometry values (see Table 9).83 Kanellopoulos22 showed that simultaneous PRK and CXL (both procedures performed the same day) is more effective than subsequent PRK performed 6 months after CXL (see Table 10).22 Conductive keratoplasty and CXL was not shown to be an effective permutation for the treatment of keratoconus as demonstrated by Kymionis et al.46 Patients undergoing this series of procedures did experience significant

Table 3

When evaluating a new surgical procedure, safety is paramount. During CXL the ocular tissues are exposed to UVA irradiation, which, under normal circumstances, results in damage. The key to maintaining safety with CXL lies in the shielding effect of riboflavin. UVA absorption in combination with riboflavin is approximately 10 times less (.35 mW/cm2) than that of UVA alone (3.4 mW/cm2).10,63 Kolhaas et al.52 found that during CXL, 70% of the UVA irradiation is absorbed in the anterior 200 mm and 20% is absorbed in the next 200 mm of the cornea.52 Minimal UV irradiation penetrates to more posterior structures of the eye.25,31,52 These data are consistent with the expected depth of treatment of approximately 300 mm8,13,61 and supports the recommended minimal corneal thickness requirement of 400 mm to preserve endothelial integrity.1,22,25,33,37,40,61 Corneal transparency is essential for proper function; corneal fibril arrangement and spacing are essential in maintaining transparency. CXL causes an increase in collagen fiber diameter and fibril spacing (see Figure 2).4,36,84 To maintain clarity, the collagen fiber diameter cannot exceed

Change in UCVA after CXL

Study 31

Goldich et al. Arbelaez et al.38 Vinciguerra et al.41 Caporrossi et al.67

Measurement unit

Δ*

Snellen Snellen Snellen Snellen

12.0 25.0† 23.6 25.0†

lines lines lines lines

Follow-up †

1 1 3 1

yr yr mo yr

UCVA 5 uncorrected visual acuity; CXL 5 corneal collagen crosslinking. * (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.

Table 5 Change in sphere, spherical equivalent, and cylinder refraction measurement after CXL Study

Measurement unit 38

Arbelaez et al. Wollensak et al.33 Wollensak et al.33 Arbelaez et al.38 Argrawal39 Raiskup-Wolf et al.62

Δ (D) Follow-up

Sphere 1.26 Spherical equivalent 1.14 Cylinder 1.2 Cylinder 1.25 Cylinder 1.2 Cylinder 1.2

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

2 yr Variable Variable 1 yr Variable 2 yr

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Table 6

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

Name

Description

Preoperative

Postoperative*

Index of surface variance Index of vertical asymmetry Keratoconus index Center keratoconus index Index of height asymmetry Index of height decentration Aberration coefficient

Curvature variation from the mean curvature Curvature difference between superior and inferior Compares keratoconus sector with normal sector Compares central with peripheral curvature Height difference between superior and inferior Vertical decentration of height data Zernike coefficient of the anterior corneal surface

98 1.10 1.27 1.08 31.4 0.09 2.29

95† 1.09 1.25† 1.06† 24.7† 0.09 2.3

* Postoperative values measured at 12 months. † Statistically significant change.

one third the wavelength of visible light (150 nm).85 The 12.2% increase in collagen fibril diameter within the anterior stroma reported in CXL does not approach the threshold and thus does not affect corneal transparency.4,85 Despite the treatment being concentrated in the anterior stroma, the posterior collagen fibrils also increase in diameter after CXL. The anterior fibers increase 9.3% more than the posterior fibers; however, the posterior fibrils still increase by 4.6% after CXL.4 The increased fiber diameter after CXL may be secondary to an increase in molecular weight of type I stromal collagen after CXL as demonstrated on electrophoretic analysis.36 The increased fiber diameter after CXL may be responsible for the stromal demarcation line that is evident 2 weeks after CXL is performed.10,13 The demarcation line can be seen clinically on slit lamp biomicroscopy. The demarcation line appears as a faint change in corneal reflectivity extending approximately 60%, or 300 mm, into the cornea from the anterior surface.4,10,13 The change in reflectivity is not significant enough to result in any adverse effect on visual performance.13 No consensus has been obtained as to etiology of the demarcation line. Studies have suggested that the line is a clinical manifestation of the depth of treatment or a change in the refractive index within the stroma caused by the induced cross-links, or a result of an increase in collagen fiber diameter and fibril spacing.4,10,13

Table 7 Efficacy comparison of double-segment Intacs alone versus double-segment Intacs with CXL. Values represent improvement from preoperative measurements Measurement

Intacs only

Intacs/CXL

UCVA* BCVA* Spherical refraction Cylindrical refraction Mean K value Steepest K value

1.9 1.7 2.08 0.47 2.22 1.27

3.19 3.09 2.58 0.62 2.57 2.03

D D D D

D D D 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 a complete rarefaction of stromal keratocytes in the anterior 200 to 300 mm of the stroma as shown in Figure 3.1,8,25,61,84 Keratocytes in the untreated cornea are activated after the apoptotic event and begin repopulating the treated stroma around 3 months postoperatively. The repopulating keratocytes produce hyaluronan, which plays a key role in fibril spacing,86 increases the stromal resistance to UV damage, and contributes to an increase in extracellular matrix density.37,87 Keratocyte repopulation is complete at approximately 6 months postoperatively.5,8,13,25,37,45,87 Application of the apoptosis inhibitor zinc chloride is not beneficial, and it weakens the cross-linking effects of the procedure.61 Because keratocytes are responsible for stromal organization and maintenance, the lack of keratocytes at 6 months postoperatively may leave the cornea more prone to a melting process. The rarefaction of keratocytes and subsequent repopulation have been confirmed with confocal microscopy.1,8,88-90 CXL might play a role in preventing progressive keratectasia. Progressive keratectasia is accompanied by a decrease in corneal tensile strength. In vitro testing has found a significant increase in stromal rigidity after CXL.48,91 Porcine corneal tissue has shown a 70% increase in rigidity, whereas human tissue samples in vitro have shown to increase in rigidity by 328%48; however, rigidity increases are only evident in the anterior 200 mm of the

Table 8 Efficacy comparison of inferior segment Intacs alone versus inferior segment Intacs with CXL Measurement

Intacs with CXL

Intacs only

UCVA (Snellen)* BCVA (Snellen)* Spherical refraction Cylinder refraction Steep K Flat K Average K

6.5 1 0.12 2.73 1.94 1.05 1.34

9.5 1 0.25 1.48 0.89 0.64 0.21

D D D D D

D D D D D

CXL 5 corneal collagen cross-linking; D 5 diopters. Note. Values represent improvement from preoperative measurements. * Improvement measured by number of Snellen line.

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Table 9 Comparison of pre- and postoperative measurements in patients undergoing PRK followed by sameday CXL Measurement Preoperative

Postoperative

Spherical 23.03 D 21.25 D equivalent Defocus 4.67 D 3.04 D UCVA 0.99 logMAR 0.16 logMAR BCVA 0.21 logMAR 0.11 logMAR Mean steep K 48.20 D 45.13 D

D 1.75 D 1.63 0.83 0.10 3.07

D logMAR logMAR D

PRK 5 photorefractive keratectomy; CXL 5 corneal collagen cross-linking; UCVA 5 uncorrected visual acuity; BCVA 5 bestcorrected visual acuity; D 5 diopters.

cornea.52 The increase in corneal rigidity after CXL could decrease the risk of progressive keratectasia. Corneas having undergone CXL showed a greater resistance to chemical degradation, which is critical in halting the progression of keratectasia.8,53 The demonstrated increase in anticollagenase activity is particularly important in individuals with keratoconus or corneal ulcerative processes. Keratoconic corneas are more susceptible to collagenase,8,92 and collagenolytic activity is one of the main mechanisms causing corneal ulceration.92,93 An in vitro chemical degradation study53 compared the vulnerability of untreated corneas and CXL-treated corneas with pepsin, collagenase, and trypsin. Corneas treated by CXL degraded by day 13 with pepsin, day 14 with collagenase, and day 5 with trypsin. Untreated corneas showed a much greater vulnerability, degrading by day 6 with pepsin and collagenase, and day 2 with trypsin. The benefit of the increased resistance to chemical degradation after CXL is evident in various case reports28,29,93-95 of patients with infectious keratitis and bullous keratopathy. The subjects showed subjective and clinical improvements in visual acuity and pain levels as well as decreases in opacification and cicatricial corneal tissue after treatment with CXL.28,29,93-95 In cases of keratomycosis, CXL may improve amphotericin B effectiveness against Candida albicans, Fusarium sp, and Aspergillus fumigatus.96

Table 10 Comparison of sequential versus simultaneous CXL and PRK. Values represent the reduction in measurements after both procedures Measurement

Sequential

Simultaneous

UCVA BCVA Spherical equivalent Mean K reading

0.41 0.25 2.50 2.75

0.76 0.28 3.20 3.50

logMAR logMAR D D

logMAR logMAR D D

CXL 5 corneal collagen cross-linking; PRK 5 photorefractive keratectomy; D 5 diopters. Note. Values represent the reduction in measurements after both procedures.

Figure 3 Representation of keratocyte apoptosis after CXL with eventual repopulation beginning at 3 months postoperatively and completing approximately 6 months postoperatively.

Researchers are investigating other ways to allow for CXL treatment in patients with less than 400 mm corneal thickness. Kymionis et al.40 have suggested performing pachymetry-guided epithelial debridement instead of complete debridement. All areas thinner than 400 mm before epithelial debridement on regional pachymetry are left intact. The procedure would allow individuals with corneal thickness bordering 400 mm to undergo CXL. Kymionis et al.40 reported a case of pachymetry-guided epithelial debridement. After 9 months, no adverse events were observed. Increased follow-up periods and an increased study cohort are needed to verify the safety and efficacy of pachymetry-guided epithelial debridement.40 Researchers have also investigated the use of hypoosmolar riboflavin in patients with corneal thickness less than 400 mm.10,54,64 Instead of the riboflavin application described above, 2 drops of hypotonic 0.1% riboflavin solution are added every 10 seconds to swell the cornea until the corneal thickness is greater than 400 mm.10 It is, however, unclear if the cross-linking effect on an overhydrated cornea will be the same as on a cornea with normal hydration.54 Areas for future study include the indication and efficacy of retreatment. Collagen turnover in the cornea ranges from 2 to 7 years, which may indicate the need for retreatment if the renewed collagen does not exhibit the same increased strength and resistance to degradation.10,33 Further research into the need for an adjusted nomogram when using PRK and CXL together is required to reduce the likelihood of overcorrections and necessary retreatment.21 Also, the regularization of the corneal surface could take up to 1 year27; therefore, the refractive error could be variable during that period of regularization. To satisfy the patient’s visual demands, trial contact lenses or frequent spectacle lens replacement may be necessary.

40

Conclusion CXL has shown to be effective at limiting the progression of keratectasia. To date, the longest reported follow-up has been 6 years. The patient population is significantly reduced in the 6-year follow-up group; however, individuals have shown good stability.62 Practitioners should consider enrolling keratoconus patients in ongoing clinical trials, especially those who show signs of progression as defined by changes in topography, keratometry readings, and new onset of anterior segment findings such hydrops.8,14,15 Caution should be taken with patients whose keratometry readings are above 58 diopters, are older than 35 years, or have BCVA better then 20/25 because they are at higher risk for failure and decreased vision after CXL.26 Patients with microbial keratitis not responding to treatment and bullous keratopathy may also benefit from enrolling in clinical trials. These patients may not have a large improvement in vision, but CXL can significantly improve their comfort level. A list of the ongoing clinical trials investigating CXL can be found on the U.S. National Institutes of Health Web site.97 No timetable is available for when and if CXL will be approved for use in the United States.

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

References 1. Dhaliwal J, Kaufman S. Corneal collagen cross-linking: a confocal, electron, and light microscopy study of eye bank corneas. Cornea 2009;28:62-7. 2. Ruyter I. Compositesdcharacterization of composite filling materials: reactor response. Adv Dent Res 1988;2:22. 3. Golomb F, Schoen J, Smith S, et al. The role of glutaraldehyde-induced cross-links in calcification of bovine pericardium used in cardiac valve bioprostheses. Am J Pathol 1987;127:122-30. 4. Wollensak G, Wilsch M, Spoerl E, et al. Collagen fiber diameter in the rabbit cornea after collagen crosslinking by riboflavin/UVA. Cornea 2004;23:503-7. 5. Doxer A, Misof K, Grabner B, et al. Collagen fibrils in the human corneal stroma: structure and aging. Invest Ophthalmol Vis Sci 1998; 39:644-8. 6. Bailey AJ. Structure, function and ageing of the collagens of the eye. Eye 1987;1:175-83. 7. Goosey JD, Zingler JS, Kinoshita JH. Cross-linking of the lens crystallins in a photodynamic system: a process mediated by singlet oxygen. Science 1980;208:1278-80. 8. Mazzotta C, Balestrazzi A, Traversi C, et al. Treatment of progressive keratoconus by riboflavin-UVA-induced cross-linking of corneal collagen: ultrastructural analysis by Heidelberg Retinal Tomograph II in vivo confocal microscopy in humans. Cornea 2007;26:390-7. 9. Sady C, Hosrof K, Nagaraj RH. Advanced Maillard reaction and crosslinking of corneal collagen in diabetes. Biochem Biophys Res Commun 1995;214:793-7.

Optometry, Vol 83, No 1, January 2012 10. Abad J, Panesso J. Corneal collagen cross-linking induced by UVA and riboflavin (CXL). Techniques Ophthalmol 2008;6:8-12. 11. Seiler T, Huhle S, Spoerl E, et al. Manifest diabetes and keratoconus: a retrospective case-control study. Graefe’s Arch Clin Exp Ophthalmol 2000;238:822-5. 12. Kuo IC, Broman A, Pirouzmanesh A, et al. Is there an association between diabetes and keratoconus? Ophthalmology 2006;113:184-90. 13. Seiler T, Hafezi F. Corneal cross-linking-induced stromal demarcation line. Cornea 2006;25:1057-9. 14. Rabinowitz YS. Keratoconus. Surv Ophthalmol 1998;42:297-319. 15. Maguire LJ, Lowry JC. Identifying progression of subclinical keratoconus by serial topography analysis. Am J Ophthalmol 1991;112:41-5. 16. National Eye Institute. Facts about the cornea and corneal disease. Keratoconus. Available at: http://www.nei.nih.gov/health/cornealdisease/#h. Accessed February 12, 2006. 17. Tuori AJ, Virtanen I, Aine E, et al. The immunohistochemical composition of corneal basement membrane in keratoconus. Curr Eye Res 1997;16:792-801. 18. Cheng EL, Maruyama I, SundarRaj N, et al. Expression of type XII collagen and hemidesmosome-associated proteins in keratoconus corneas. Curr Eye Res 2001;22:333-40. 19. Radner W, Zehemayer M, Skorpik Ch, et al. Altered organization of collagen in apex of keratoconus corneas. Ophthalmic Res 1998;30: 327-32. 20. Kenney MC, Nesburn AB, Burgeson RE, et al. Abnormalities of the extracellular matrix in keratoconus corneas. Cornea 1997;16:345-51. 21. Kanellopoulos JA, Binder PS. Collagen cross-linking (CCL) with sequential topography-guided PRK. Cornea 2007;26:891-5. 22. Kanellopoulos AJ. Comparison of sequential vs same-day simultaneous collagen cross-linking and topography guided PRK for treatment of keratoconus. J Refract Surg 2009;25:S812-8. 23. Chan C, Sharma M, Boxer Wachler B. Effect of inferior-segment Intacs with and without C3-R on keratoconus. J Cataract Refract Surg 2007;23:75-80. 24. Ertan A, Karacal H, Kamburoglu G. Refractive and topographic results of transepithelial cross-linking treatment in eyes with Intacs. Cornea 2009;28:719-23. 25. Spoerl E, Mrochen M, Sliney D, et al. Safety of UVA-riboflavin crosslinking of the cornea. Cornea 2007;26:385-9. 26. Koller T, Mrochen M, Seiler T. Complication and failure rates after corneal crosslinking. J Cataract Refract Surg 2009;35:1358-62. 27. Koller T, Iseli HP, Hafezi F, et al. Scheimpflug imaging of corneas after collagen cross-linking. Cornea 2009;28:510-5. 28. Kozobolis V, Labiris G, Gkika M, et al. UV-A collagen cross-linking treatment of bullous keratopathy combined with corneal ulcer. Cornea 2010;29:235-8. 29. Moren H, Malmsj€o M, Mortensen J, et al. Riboflavin and ultraviolet A collagen crosslinking of the cornea for the treatment of keratitis. Cornea 2010;29:102-4. 30. Ghanem RC, Santhiago MR, Berti TB, et al. Collagen crosslinking with riboflavin and ultraviolet-a in eyes with pseudophakic bullous keratopathy. J Cataract Refract Surg 2010;36:1444. 31. Goldich Y, Marcovich A, Barkana Y, et al. Safety of corneal collagen cross-linking with UV-A and riboflavin in progressive keratoconus. Cornea 2010;29:409-11. 32. Samaras K, Lake D. Corneal collagen cross linking (CXL): 1 review. Int Opthalmol Clin 2010;50:89-100. 33. Wollensak G, Spoerl E, Seiler T. Riboflavin/ultraviolet-A-induced crosslinking for the treatment of keratoconus. Am J Ophthalmol 2003;135:620-7. 34. Goldich Y, Barkana Y, Morad Y, Hartstein M, Avni I, Zadok D. Can we measure corneal biochemical changes after collagen cross-linking in eyes with keratoconus-a pilot study. Cornea 2009;28:498-502. 35. Rama P, Di Matteao F, Matuska S, et al. Acanthamoeba keratitis with perforation after corneal crosslinking and bandage contact lens use. J Cataract Refract Surg 2009;35:788-91. 36. Wollensak G, Redl B. Gel electrophoretic analysis of corneal collagen after photodynamic cross-linking treatment. Cornea 2008;27:353-6.

Dahl et al

Literature Review

37. Wollensak G, Iomdina E, Dittert D, et al. Wound healing in the rabbit cornea after corneal collagen cross-linking with riboflavin and UVA. Cornea 2007;26:600-5. 38. Arbelaez MC, Bernardita S, Vidal C, et al. Collagen cross-linking with riboflavin and ultraviolet-A light in keratoconus: one year results. Oman J Ophthalmol 2009;2:33-8. 39. Argrawal V. Corneal collagen cross-linking with riboflavin and ultraviolet-A light for keratoconus: results in Indian eyes. Indian J Ophthalmol 2009;57:111-4. 40. Kymionis G, Diakonis V, Coskunseven E, et al. Customized pachymetric guided epithelial debridement for corneal collagen cross linking. BMC Ophthalmology 2009;9:10. 41. Vinciguerra P, Albe E, Trazza S, et al. Intraoperative and postoperative effects of corneal collagen cross-linking on progressive keratoconus. Arch Ophthalmol 2009;127:1258-65. 42. Zamora K, Males J. Polymicrobial keratitis after a collagen crosslinking procedure with postoperative use of a contact lens: a case report. Cornea 2009;28:474-6. 43. Rabinowitz Y. Collagen cross-linking: what you should know about this potential new treatment. Ophthalmology Management 2009;13: 44-6,48,63. 44. Kahn Y, Behrens A. Corneal collagen cross-linking for the treatment of diseases of the eye. Contemporary Ophthalmology 2009;8:1-8. 45. Hung M. Corneal collagen cross-linking: new hope for patients with corneal ectasia. Contemporary Optometry 2010;8:1-5. 46. Kymionis G, Kontadakis G, Naoumidi T, et al. Conductive keratoplasty followed by collagen cross-linking with riboflavin-UVA-A in patients with keratoconus. Cornea 2010;29:239-43. 47. Wittig-Silva C, Whiting M, Lamoureax E, et al. A randomized controlled trial of corneal collagen cross-linking in progressive keratoconus: preliminary results. J Refract Surg 2008;24:S720-5. 48. Wollensak G, Spoerl E, Seiler T. Stress-strain measurements of human and porcine corneas after riboflavin-ultraviolet-A-induced cross-linking. J Cataract Refract Surg 2003;29:1780-5. 49. Kymionis GD, Portaliou DM, Bouzoukis DI, et al. Herpetic keratitis with iritis after corneal crosslinking with riboflavin and ultraviolet A for keratoconus. J Cataract Refract Surg 2007;33:1982-4. 50. Hayes S, O’Brart DP, Lamdin LS, et al. Effect of complete epithelial debridement before riboflavin-ultraviolet-A corneal collagen cross-linking therapy. J Cataract Refract Surg 2008;34:657-61. 51. Gokhale N, Vemuganti G. Diclofenac-induced acute corneal melt after collagen cross-linking for keratoconus. Cornea 2010;29:117-9. 52. Kolhaas M, Spoerl E, Schilde T, et al. Biomechanical evidence of the distribution of cross-links in corneas treated with riboflavin and ultraviolet A light. J cataract Refract Surg 2006;32:279-83. 53. Spoerl S, Wollensak G, Seiler T. Increased resistance of crosslinked corneal against enzymatic digestion. Curr Eye Res 2004;29:35-40. 54. Hafezi F, Kanellopoulos J, Wiltfang R, et al. Corneal collagen crosslinking with riboflavin and ultraviolet A to treat induced keratectasia after laser in situ keratomileusis. J Cataract Refract Surg 2007;33:2035-40. 55. Wollensak G, Iomdina E. Biomechanical and histological changes after corneal crosslinking with and without epithelial debridement. J Cataract Refract Surg 2009;35:540-6. 56. Baiocchi S, Mazzotta C, Cerretani D, et al. Corneal crosslinking: riboflavin concentration in corneal stroma exposed with and without epithelium. J Cataract Refract Surg 2010;36:186-8. 57. Pinelli R. The refractive surgery society results using C3R. Paper presented at: The Second International Congress on Crosslinking; December 8-9, 2006: Zurich, Switzerland. 58. Hafezi F, Mrochen M, Iseli HP, et al. Collagen crosslinking with ultraviolet-A and hypoosmolar riboflavin solution in thin corneas. J Cataract Refract Surg 2009;35:621-4. 59. Kanellopoulos AJ. Collagen cross-linking in early keratoconus with riboflavin in a femtosecond laser-created pocket: initial clinical results. J Refract Surg 2009;25:1034-7. 60. Sharma N, Boxer-Wachler BS. Corneal collagen CXL with riboflavin for corneal stabilization. Paper presented at: The American Academy

41

61.

62.

63. 64. 65. 66.

67.

68.

69. 70.

71.

72.

73.

74.

75. 76.

77.

78.

79.

80.

81.

82.

of Ophthalmology Annual Meeting; October 15-18, 2005: Chicago, Illinois. Wollensak G, Spoerl E, Wilsch M, et al. Keratocyte apoptosis after corneal collagen cross-linking using riboflavin/UVA treatment. Cornea 2004;23:43-9. Raiskup-Wolf F, Hoyer A, Eberhard S, et al. Collagen cross-linking with riboflavin and ultraviolet-A light in keratoconus: long term results. J Cataract Refract Surg 2008;34:796-801. Wollensak G, Spoerl E, Reber F, et al. Corneal endothelial cytotoxicity of riboflavin/UVA treatment in vitro. Ophthalmic Res 2003;35:324-8. Snibson G. Collagen cross-linking: a new treatment paradigm in corneal diseaseda review. Clin Ex Opthalmol 2010;38:141-53. Thoft RA, Mobilia EF. Complications with therapeutic extended wear soft contact lenses. Int Ophthalmol Clin 1987;21:197. Aquavella JV, del Cerro M, Musco PS, et al. The effect of a collagen bandage lens on corneal wound healing: a preliminary report. Ophthalmic Surg 1987;18:570. Caporossi A, Balocchi S, Mazzotta C, et al. Parasurgical therapy for keratoconus by riboflavin–ultraviolet type A rays induced cross-linking of corneal collagen: preliminary refractive results in an Italian study. J Cataract Refract Surg 2006;32:837-45. Verma S, Corbett MC, Marshall J. A prospective, randomized, double-masked trial to evaluate the role of topical anesthetics in controlling pain after photorefractive keratectomy. Ophthalmology 1995;102:1918-24. Wollensak G, Herbst H. Significance of the lacunar hydration pattern after corneal cross linking. Cornea 2010;29:899-903. Pollhammer M, Cursiefen C. Bacterial keratitis early after corneal crosslinking with riboflavin and ultraviolet-A. J Cataract Refract Surg 2009;35:588-9. Perez-Santonja JJ, Artola A, Javaloy J, et al. Microbial keratitis after corneal collagen crosslinking. J Cataract Refract Surg 2009;35: 1138-40. Koppen C, Vyryghem JC, Gobin L, et al. Keratitis and corneal scarring after UVA/riboflavin cross-linking for keratoconus. J Refract Surg 2009;25:S819-23. Sharma N, Maharan P, Signh G, et al. Pseudomonas keratitis after collagen crosslinking for keratoconus: case report and review of literature. J Cataract Refract Surg 2010;36:517-20. Angunawela RI, Arnalich-Montiel F, Allan BD. Peripheral sterile corneal infiltrates and melting after collagen crosslinking for keratoconus. J Cataract Refract Surg 2009;35:606-7. Koller T, Mrochen M, Seiler T. Complication and failure rates after corneal crosslinking. J Cataract Refract Surg 2009;35:1358-62. Matthes R. Guidelines on limits of exposure to ultraviolet radiation of wavelengths between 180 and 480 nm (incoherent optical radiation). Health Phys 2004;87:171-86. Henriquez MA, Izquierdo L Jr, Bernilla C, et al. Riboflavin/ultraviolet a corneal collagen cross-linking for the treatment of keratoconus: visual outcomes and Scheimpflung analysis. Cornea 2011; 30:281-6. Jankov MR 2nd, Hafezi F, Beko M, et al. [Corneal Cross-linking for the treatment of keratoconus: preliminary results]. Arq Bras Oftalmol 2008;71:813-8. Hoyer A, Raiskup-Wolf F, Spoerl E, et al. [Collagen cross-linking with riboflavin and UVA light in keratoconusdresults from Dresden]. Ophthalmologe 2009;106:133-40. Fourni
e P, Galiacy S, Arne JL, et al. [Corneal collagen cross-linking with ultraviolet-A light and riboflavin for the treatment of progressive keratoconus]. J Fr Ophtalmol 2009;32:1-7. Coskunseven E, Jankov MR 2nd, Hafezi F. Contralateral eye study of corneal collagen cross-linking with riboflavin and UVA irradiation in patients with keratoconus. J Refract Surg 2009;25:371-6. Grewal DS, Brar GS, Jain R, et al. Corneal collagen crosslinking using riboflavin and ultraviolet-A light for keratoconus: one-year analysis using Scheimpflug imaging. J Cataract Refract Surg 2009; 35:425-32.

42 83. Kymionis G, Kontadakis G, Kounis G, et al. Simultaneous topography-guided PRK followed by corneal collagen cross-linking for keratoconus. J Refract Surg 2009;25:S807-11. 84. Mencucci R, Marini M, Paladini I, et al. Effects of riboflavin/UVA cross-linking on keratocytes and collagen fibers in human corneas. Clin Ex Opthalmol 2010;38:49-56. 85. Goldman JN, Benedek GB, Dohlman CH, et al. Structural alterations affecting transparency in swollen human corneas. Invest Ophthal Vis Sci 1968;7:501-19. 86. Podskochy A, Fagerholm P. Cellular response and reactive hyaluronan production in UV-exposed rabbit corneas. Cornea 1998;17:640-5. 87. Podskochy A, Fagerholm R. Repeated UVR exposures cause keratocyte resistance to apoptosis and hyaluronan accumulation in the rabbit cornea. Acta Ophthalmol Scand 2001;79:603-8. 88. Kymionis GD, Diakionis VF, Kalyvianaki M, et al. One-year follow-up of corneal confocal microscopy after corneal cross-linking in patients with post laser in situ keratomileusis ectasia and keratoconus. Am J Ophthalmol 2009;147:774-8. 89. Croxatto JO, Tytiun AE, Argento CJ. Sequential in vivo confocal microscopy study of corneal wound healing after cross-linking in patients with keratoconus. J Refract Surg 2009:1-8. 90. Mazzotta C, Traversi C, Baiocchi S, et al. Corneal healing after riboflavin ultraviolet-A collagen cross-linking determined by confocal

Optometry, Vol 83, No 1, January 2012

91.

92.

93. 94.

95.

96.

97.

laser scanning microscopy in vivo: early and late modifications. Am J Ophthalmol 2008;146:527-33. He X, Spoerl E, Tang J, et al. Measurement of corneal changes after collagen crosslinking using a noninvasive ultrasound system. J Cataract Refract Surg 2010;36:1207-12. Kao WW, Vergnes JP, Ebert J, et al. Increased collagenase and gelatinase activities in keratoconus. Biochem Biophys Res Commun 1982; 107:929-36. Kenyon K. Inflammatory mechanisms in corneal ulceration. Trans Am Ophthalmol Soc 1985;83:610-63. Krueger RR, Ramos-Esteban JC, Kanellopoulos AJ. Staged intrastromal delivery of riboflavin with UVA cross-linking in advanced bullous keratopathy: laboratory investigation and first clinical case. J Refract Surg 2008;24:S730-6. Iseli H, Thiel M, Hafezi F, et al. Ultraviolet A/Riboflavin corneal cross-linking for infectious keratitis associated with corneal melts. Cornea 2008;27:590-4. Sauer A, Letscher-Bru V, Speeg-Schatz C, et al. In vitro efficacy on antifungal treatment using riboflavin/UV-A (365 nm) combination and amphotericin B. Invest Opthalmol Vis Sci 2010;51:3950-3. Clinicaltrials.gov. A service of the U.S. National Institutes of Health. Available at: http://www.clinicaltrials.gov/ct2/results?term5corneal1 collagen1crosslinking.