Corneal collagen crosslinking (CXL) is a promising development in refractive surgery. CXL with riboflavin is a generally accepted treatment for corneal ectatic diseases; however, sucessful use of CXL was reported for bullous keratopathy, corneal melts, and infectious corneal ulcers. This article provides a brief overview of the use of CXL for each of these indications.
KERATOCONUS
Optimal corneal optics require a smooth, regular surface with a healthy tear film and epithelium. A regular arrangement of stromal cells and macromolecules is necessary for clear vision. The lattice arrangement of collagen fibrils acts as diffractive grating to reduce light scattering via destructive interference.1 In keratoconus, the arrangement of stromal fibrils is lost, the number of collagen lamellae decreases, and collagen bundles separate.2,3
Keratoconus is relatively frequent, with an incidence of one in 2,000. It often affects young patients, and there is no known effective treatment to stop progressive keratoconus. Eventually, approximately 21% of keratoconus patients require corneal transplantation.3 The aim of CXL is to correct the pathogenic causes of keratoconus by changing the intrinsic biomechanical properties of the corneal collagen. Additional chemical bonds are created within the stroma; photopolymerization occurs in the anterior stroma with minimized exposure to the surrounding structures of the eye.4
Wollensak performed the first clinical trial on CXL for the treatment of keratoconus,5 studying 22 patients treated with riboflavin and UV-A. CXL slightly reversed and flattened keratoconus (2.00 D) in 72% and improved BCVA in 68%. In 70% of eyes, regression was achieved; the maximal keratometry (K) readings and refractive error were reduced by 2.01 and 1.14 D, respectively. In a separate study, Caporossi et al6 showed a mean K reduction of 2.10 D.
Chan et al7 reported the first study combining the use of Intacs (Addition Technology, Inc., Des Plaines, Illinois) with CXL, leading to greater reductions in manifest refraction, steep K, and average K readings. Mean changes in UCVA, BCVA, sphere, and mean K values were 6.5 lines, 1 line, 0.12 D, and 1.34 D, respectively, after Intacs with CXL. With Intacs alone, the changes were 9.5 lines, 1 line, 0.25 D, and 0.21 D, respectively.
The efficacy of crosslinking with versus without epithelial removal is also controversial. Some believe that keeping the epithelium intact causes inadequate penetration of riboflavin, enhances UV-A penetration, and results in possible cell damage. However, Pinelli et al reported 6-month crosslinking results and found comparable outcomes with and without the epithelium in terms of changes in keratometry, vision, and endothelial cell count.8 Sharma and Boxer Wachler also reported similiar results after crosslinking without epithelial removal.9 Additionally, Podskochy et al10 showed increased keratocyte damage with UV-A light when the epithelium was removed. They speculated that epithelium may play a significant role in absorbing UV-A and thus protect the cornea and deeper structures from damage.
Clinical outcomes vary after CXL plus Intacs. This combination resulted in more regular topography with visual improvement.11 In advanced keratoconus, CXL may be considered after Intacs implantation to provide slight improvement in refractive and visual results with possible stabilization.
CORNEAL ULCERATION
Recently, CXL has been used for the treatment of corneal melting and ulcers. The development of corneal ulceration can be a disastrous sequela to bacterial, viral, or fungal infection. Even if bacterial toxins are not directly capable of collagenolytic activity, Pseudomonas species produce a protease capable of glycosaminoglycan destruction.12
Independent of the cause, stromal melting is usually preceded by a corneal epithelial defect; it is associated with an inappropriate inflammatory response. Ulceration is known to be secondary to the action of tissue collagenases, which perform the initial cleavage of stromal collagen fibrils, with further degredation of collagen and glycosaminoglycans involving proteases, peptidases, and cathepsins. The initiation of tissue collagen breakdown can potentially be controlled at the levels of collagenase concentration and activity by cellular and humoral activators and inhibitors.13
The cellular constituents responsible for ulceration and their interactions are subjects of extensive research interests. Although substantial evidence has implicated the actions and interactions of injured corneal epithelium and keratocytes, other pathologic and experimental studies have emphasized the role of acute inflammatory cells in the formation of ulcers. These inflammatuar cells contain more than a dozen lytic enzymes, such as collagenase, elastase, and cathepsin, in their lysosomes and are ubiquitous at the site of active ulcerations and in the tear film of melting coneas (Figure 1).14
Compared with controls, Spoerl et al15 found an impressive doubling in the digestion time following pepsin, trypsin, and collagenase digestion in corneas crosslinked with riboflavin and UV-A at 3 mW/cm². In this study, 60 enucleated porcine eyes were treated with riboflavin and UV-A irradiation (370 nm; irradiance of 1–3 mW/cm²) for 30 minutes and compared with 20 untreated control eyes. After CXL treatment, the corneal buttons were trephined and exposed to pepsin, trypsin, and collagenase solutions. Photochemical CXL of the cornea using riboflavin and UV-A resulted in a markedly increased resistance versus collagen digesting enzymes. This finding reflects the biochemical effect of the crosslinking treatment in addition to its already known biochemical effect.16 The stabilizing biochemical effect of crosslinking can be explained by the changes of the tertiary structure of the collagen fibrils, induced by crosslinking and preventing access of the proteolytic enzymes to their specific cleavage sites by steric hindrance.17
In summary, CXL in the cornea significantly increases resistance to collagenase, pepsin, and trypsin digestion—especially in the anterior half of the cornea. This is another important advantage of CXL in addition to the increase in biomechanical stability following this procedure.
SCLERAL CXL FOR GLAUCOMA, PATHOLOGIC MYOPIA
With a controversial pathogenesis, progressive myopia remains an unsolved problem in ophthalmology. One important feature of severe myopia is a pathologic change in the biomechanically weakened sclera with progressive thinning, probably due to a disturbed feedback mechanism of emmetropization after visual deprivation.18
At the wavelength of 465 nm, 35% of light is absorbed by the sclera, 57% is reflected, and transmission of the nonreflected light is approximately 8%, and therefore is sufficient for scleral CXL.19 Wollensak et al20 studied scleral CXL in living rabbit eyes using riboflavin and a wavelength of 365 nm at 4.2 mW/cm² to induce CXL. They measured an increased ultimate stress up to 228% and also found retinal damage with retinal pigment epithelium and photoreceptor and outer nuclear layer death when exposed to the riboflavin.
Iseli et al21 treated six rabbits with topical riboflavin (0.5%) and blue light (465 nm) on the equatorial sclera using a light emitting diode source with an exposure area of 9 mm. Four weeks after the treatment, there was a threefold increase in stiffening at the stress-strain curve.
The stress-strain parameter values and the Young's modulus of sclera in myopic human eyes were significantly lower than those of the age norm.22 Therefore, several methods of surgical scleroplasty for scleral strengthening have been developed, including injections of a polymeric composition that form a foamed gel under Tenon's capsule and induces scar tissue. Scleral reinforcement operations have also been described in which donor sclera, facia lata, or synthetic bands are placed around the back of the globe and sutured to the sclera. These reduce the progression of axial elongation;23,24 however, they do not regenerate the internal structure and crosslinking properties of the weakened sclera.
The myopic optic nerve head is more susceptible to the several likely forms of intraocular pressure (IOP)-related insult at all levels of IOP. The myopic scleral canal may be unusually large, abnormally shaped, and/or tilted, leading to elevated levels of IOP-related stress for a given level of IOP.25-27 The myopic peripapillary sclera and lamina may be thin, leading to higher IOP-related laminar and scleral wall stress and deformation.28 Additionally, extracellular matrix of myopic eye may be abnormally weak, causing larger laminar deformations for a given level of IOP. The size of the axially myopic eye should increase IOP-related scleral stress for any given level.25,28,29
The presence of several clinical relationships would support the notion that an axially myopic eye is more susceptible to both normal and elevated levels of IOP. These may be related to peripapillary sclera and lamina cribrosas that are, in fact, thinner in myopic eyes than in well-matched normal eyes.
We know that impaired CXL is an important factor in the weakening process of the myopic sclera.30 Wollensak et al31 demonstrated an impressive stiffening effect of CXL in vivo in rabbit sclera after glyceraldehyde treatment for 14 days. Biomechanical tests showed that the proposed method of scleral reinforcement induced a highly significant improvement in the biomechanical parameters of the sclera. An entirely new scleral-based approach for the prevention and treatment of progressive myopia may become possible. Using sub-Tenon's injections with a large injection volume, it should be possible to achieve a treatment effect on the entire sclera because of the easy spread of the injected fluid in the sub-Tenon's space around the globe.32,33 However, the UV-A technique allows CXL in a clearly defined scleral target area, which may be an advantage in disorders such as myopic scleral staphyloma. Also, with riboflavin/UV-A irradiation, the extent of the treatment area is easly controllable because of the visible UV-A—light-induced fluorescence on the scleral surface.31
In the future, it may be possible to treat patients with progressive myopia using scleral CXL with riboflavin and blue light at 465 nm.
BULLOUS KERATOPATHY
Without sufficient endothelial pump function, the cornea swells and fluid accumulates in the extracellular spaces between the collagen fibers and lamellae.34 In corneal edema, the interfiber collagen spacing has been increased due to fluid accumulation. The concept of stromal compaction, with enhanced resistance to osmotic and hydrostatic fluid accumulation, introduces another attractive application for CXL technique. Krueger et al35 showed that staged UV-A CXL (15mW/cm²) with femtosecond—laser-facilitated intrastromal 0.1% riboflavin administration may be a safe and effective method for managing bullous keratopathy. The investigators also introduced the concept of staging the intrastromal riboflavin administration through a second, more anterior stromal pocket after the deeper CXL is achieved. In this method, the superficial layers may also undergo stromal compaction and reduce fluid accumulation anteriorly.
The ability to permanently strengthen the inherently weakened cornea promises to be a major advancement in ophthalmology. CXL is a simple, safe, and effective procedure for the management of ectatic, inflammatory diseases of the cornea as well as corneal melting and bullous keratopathy. In the near future, CXL treatments may become a standard of care.
Aylin Kiliç Ertan, MD, is the Chief Doctor and Head of the Department of Cataract and Refractive Surgery, Kudret Eye Hospital, Ankara, Turkey. Dr. Ertan states that she has no financial interest in the products or companies mentioned. She may be reached at tel: +90 312 4466464; fax: +90 312 4464771; e-mail: aylinclzy@gmail.com.
