Corneal collagen cross-linking

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Cross-linking procedure, UV light source[1]

Corneal collagen cross-linking[2] with riboflavin (vitamin B2) (also known as CXL, C3-R,[3] CCL[4] and KXL[5] ), better known as Cross-linking, is a parasurgical treatment for keratoconus.

It was first developed in Germany in 1998 and clinical trials have been in course since the same year; in Italy routine interventions have been successfully performed since 2005, while in the USA clinical trials commenced only in 2008. The procedure, with epithelium removed, is approved for use throughout Europe and Canada.

The procedure[edit]

Removed corneal epithelium during CCR operation on an eye with post-LASIK complication, from Kymionis et al., 2009[6]

The cross-linking involves a one-time application of riboflavin solution to the eye that is activated by illumination with UV-A light for approximately 30 or less minutes. The riboflavin causes new bonds to form across adjacent collagen strands in the stromal layer of the cornea, which recovers and preserves some of the cornea's mechanical strength. The corneal epithelial layer is generally removed to increase penetration of the riboflavin into the stroma.[7]

Patients that are considered for treatment must undergo an extensive clinical workup, including computerized corneal topography, endothelial microscopy, ultrasound pachymetry, b-scan sonography, keratometry and biomicroscopy.

Epithelium-Off (Dresden protocol)[edit]

The standard cross-linking technique, also called Dresden protocol[8] (CXL) requires the removal of central 9 nm of corneal epithelium layer (epi-off), followed by 30 minutes of riboflavin administration, subsequently, UVA light is applied for 30 minutes.

Transepithelial Cross-linking[edit]

In transepithelial or epithelium-on (epi-on) cross-linking technique, the corneal epithelium layer is left intact. Brian Boxer Wachler was the first to perform transepithelial crosslinking in 2004.[8] in this technique, because the epithelium is not removed, riboflavin loading requires more time than with epi-off techniques.

Accelerated Cross-linking[edit]

In accelerated cross-linking (also known as ACXL, KXL[9][10] ), UVA exposure time is reduced to less than 3 minutes, this is achieved by increasing the UVA power and reducing the exposure time, thereby maintaining the same energy on the eye as standard cross-linking while reducing cross-linking time.[11]

Combination with refractive eye surgeries[edit]

Cross-linking is not intended to correct vision, thus it is usually combined with refractive eye surgeries such as photorefractive keratectomy and intrastromal corneal ring segments. Cross-linking can also be used to avoid post-LASIK ectasia and to improve refractive outcomes.[12]

Cross-linking and LASIK[edit]

A Applying riboflavin for LASIK Xtra
Applying riboflavin on the stromal bed just after excimer laser ablation

Corneas normally become thinner after LASIK because the excimer laser used removes tissue. This effect is more marked when higher degrees of spectacle power or larger optical zones are treated.[13] In a small percentage of cases, this thinning causes the cornea to bulge forwards and become distorted in a condition called corneal ectasia.[14] In a similar manner to keratoconus, corneal collagen cross-linking has been found to benefit patients with post-LASIK corneal ectasia as well.[15] However, when cross-linking is performed only after the cornea becomes distorted, vision remains blurry even though the disease is stabilised. Simultaneously crosslinking corneal collagen with LASIK ('LASIK Xtra') aims to strengthen the cornea at the point of surgery and may be useful in cases where a very thin cornea is expected after the LASIK procedure. This would include cases of high spectacle power and patients with thin corneas before surgery. By strengthening the cornea and preventing its distortion, the patient avoids the risk of blurred vision which is experienced if ectasia is treated only after it happens. Definitive evidence that the procedure can reduce the risk of corneal ectasia will only become available a number of years later as corneal ectasia, if it happens, usually occurs in the late post-operative period. Some have also questioned the cost effectiveness of this procedure for preventing corneal ectasia, since corneal ectasia is a very rare complication and there may be many cases treated unnecessarily.[16] Recently, it has been found that LASIK Xtra adds stability to hyperopic treatments[17] and may also do the same for very high myopic treatments. As a result, some centers outside the USA are already performing LASIK Xtra routinely in such cases.[18]

Corneal Collagen Cross-linking may also be combined with other treatments to improve corneal strength or optical refraction. Successful treatment methods include Mini Asymmetric Radial Keratotomy (MARK),[19] corneal ring segment inserts (Intacs or Ferrara rings), topography-guided laser, or Keraflex.[20][21][22] Corrective lenses are normally required after these treatments, but with smaller, more normalized prescriptions. Increased corneal symmetry allows for more comfortable contact lens wear, often of daily disposable lenses. These newer methods have an important role in limiting deterioration of vision, increasing unaided and uncorrected vision, and strongly reducing the case for corneal transplantation.[23]

References[edit]

  1. ^ Renesto Ada, C; Sartori, M; Campos, M (Jan–Feb 2011). "[Cross-linking and intrastromal corneal ring segment].". Arquivos brasileiros de oftalmologia 74 (1): 67–74. doi:10.1590/s0004-27492011000100017. PMID 21670914. 
  2. ^ "Corneal Collagen Cross-Linking". American Academy of Ophthalmology. 
  3. ^ "Holcomb C3-R®- Proprietary, Non-Surgical Corneal Crosslinking". Brian Boxer Wachler. 
  4. ^ Romppainen, T.; Bachmann, L. M.; Kaufmann, C.; Kniestedt, C.; Mrochen, M.; Thiel, M. A. (1 December 2007). "Effect of Riboflavin-UVA Induced Collagen Cross-linking on Intraocular Pressure Measurement". Investigative Ophthalmology & Visual Science. pp. 5494–5498. doi:10.1167/iovs.06-1479. 
  5. ^ "KXL, Accelerated Crosslinking". Avedro. 
  6. ^ Kymionis GD, Diakonis VF, Coskunseven E, Jankov M, Yoo SH, Pallikaris IG (2009). "Customized pachymetric guided epithelial debridement for corneal collagen cross linking". BMC Ophthalmology 9: 10. doi:10.1186/1471-2415-9-10. PMC 2744909. PMID 19715585. 
  7. ^ Spoerl E, Wollensak G, Dittert DD, Seiler T (2004). "Thermomechanical behavior of collagen-cross-linked porcine cornea". Ophthalmologica 218 (2): 136–40. doi:10.1159/000076150. PMID 15004504. 
  8. ^ a b Kathryn M. Hatch, William B. Trattler. (16 May 2012). "Corneal Crosslinking: Epi-on or Epi-off?". Medscape. 
  9. ^ "Accelerated Cross-linking". Avedro. 
  10. ^ "Corneal Collagen Cross-linking for Keratoconus". 
  11. ^ Bethke, Walter. "A Cross Section of Cross-Linking". Review of Ophthalmology. 
  12. ^ Stephenson, Michelle. "LASIK Xtra: Is It for Everyone?". Review of Ophthalmology. 
  13. ^ Munnerlyn, CR, Koons, SJ, Marshall, J. "Photorefractive keratectomy: a technique for laser refractive surgery," J. Refract. Surg., 1988, 14, 46-52
  14. ^ Pallikaris IG,Kymionis GD,Astyrakakis NI. Corneal ectasia induced by laser in situ keratomileusis. J Cataract Refract Surg2001; 27:1796–1802
  15. ^ Vincigeurra P,Camesasca FI,Albe E,Trazza S.Corneal collagen cross-linking for ectasia after excimer laser refractive surgery:1-year results. J Refract Surg 2009; 22:1–12.
  16. ^ http://avedro.com/WP/wp-content/uploads/2012/05/Straits-Times-Article.pdf
  17. ^ http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3775081
  18. ^ http://www.revophth.com/content/c/46548/
  19. ^ "CRSTodayEurope.com > August 2009 > Refractive Changes Following CXL". Bmctoday.net. Retrieved 2012-12-15. 
  20. ^ Chan CC, Sharma M, Wachler BS (January 2007). "Effect of inferior-segment Intacs with and without C3-R on keratoconus". Journal of Cataract and Refractive Surgery 33 (1): 75–80. doi:10.1016/j.jcrs.2006.09.012. PMID 17189797. 
  21. ^ Straub, Laura (March 2010). "Thermo-Biomechanics for Treating Keratoconus and Refractive Errors". Cataract and Refractive Surgery Today. Retrieved 15 December 2012. 
  22. ^ By: (2010-05-07). "Thermo-biomechanical system gets CE mark | OphthalmologyTimes". Modernmedicine.com. Retrieved 2013-04-02. 
  23. ^ Hersch, Peter S. (1 June 2010). "Keratoconus approach is promising". Ophthalmology Times Europe. Retrieved 2 April 2013. 

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