TransPRK - Transepithelial PhotoRefractive Keratectomy


Corneal collagen cross linking with Riboflavin (Vitamin B2), or CXL, is a procedure to strengthen a weakened cornea. The commonest cause of a weakened cornea is called Keratoconus - "kerato" means cornea and "conus" meaning conical i.e. a conical cornea.

Jerry Tan Eye Surgery has been offering cross-linking since 2008.

Keratoconus - Corneal collagen cross linking with Riboflavin

The cause of keratoconus is unknown but there is strong evidence that this disease has a genetic basis. Other conditions resulting from a weakened cornea are Pellucid Marginal Degeneration and more recently Corneal Ectasia secondary to LASIK or PRK(photorefractive keratectomy).

CXL was devised by IROC co-founder Prof. Theo Seiler together with Prof. Eberhard Spoerl from the University of Dresden, Germany. CXL minimises the need for invasive corneal transplantation for people suffering from weakened corneas like Keratoconus and Corneal Ectasia.

It works by increasing the stiffness and rigidity of the cornea, thus stabilising the ectasia. In other words, the more cross-linking there is, the stronger the cornea becomes.

It is a known fact that in Singapore, and the world over, the number of available donor corneas is significantly lower than the corresponding demand. Corneal transplantation has also been associated with a number of complications, like rejection with resultant clouding and failure of the corneal transplant. CXL is a much less invasive procedure that can delay the need, and if done early can obviate, the need for a corneal transplant.

How does CXL work?

Your cornea is mostly made up of collagen fibres that are arranged in bundles, and the strength and rigidity of the cornea is determined by how strongly these fibres are linked together. Thus, when CXL is done on the cornea, the collagen fibres become linked together more strongly hence the term cross-linking.

Picture showing advantages of Corneal collagen crosslinking

This effect is more pronounced nearer the corneal surface. The biomechanical strength of the cornea itself is improved by a factor of 4. Over the course of a lifetime, your cornea becomes progressively stiffer due to natural cross-linking between the fibres.

CXL makes use of Riboflavin (Vitamin B2) and UV light, for instance through photo-polymerisation. Riboflavin is a naturally occurring compound which strongly absorbs Ultra Violet (UV) light. When Riboflavin is applied to the cornea and exposed to UV light at the same time, this enhances the effect of the cross-linking procedure as well as absorbs the UV light to protect the inner layers of the cornea and intraocular structures from the potentially damaging effects of the light rays.

Benefits of CXL

As CXL is effective in stabilising ectasia, this means there will be a reduction in the myopic or astigmatic conditions suffered by people with ectasia. Before the discovery of CXL, it was always thought that Excimer laser of myopia or astigmatism was not possible when ectasia is present. This was because it was thought that by removing corneal tissue with the laser, the cornea would become less stable and the ectasia made worse.

However, once the cornea is stabilised by CXL, it is possible to perform some laser ablation while maintaining the structural stability of the cornea and any remaining optical defect could then be corrected by spectacles, soft lenses or Implantable Collamer Lens.

Alternatively we offer another method using CXL to correct keratoconus and corneal ectasia. A topography-guided PRK is done to 'regularise' the cornea and reduce the distortion after which a CXL is performed to strengthen the cornea and 'fix' the cornea in a less distorted condition. This will both strengthen the cornea and give the patient a chance of an increase in the clarity of vision. It may also make the fitting of spectacles and contact lenses easier.

Concerns about CXL

Presently, it is not known whether the stabilising effect of CXL is permanent, but the treatment could potentially be repeated if necessary. Animal studies, however, show the effect lasts between 10 to 20 years.

As UV light is known to be damaging to cells, it has been noted that keratocytes in the outer layers of the treated cornea die. However, these cells are replaced by the ones which migrate from other parts of the cornea.

If the patient's corneal thickness is less than 400 microns, CXL treatment can still be performed using a specially prepared Riboflavin. UV light can damage the innermost layer of endothelial cells of the cornea but no cases of permanent corneal injury after CXL have been reported worldwide.

Further to that, concerns that UV could damage the retina and lens are unfounded due to the fact that Riboflavin blocks the UV to an extent that no measurable damage will occur.

What happens when you undergo CXL treatment?

The treatment begins with topical anaesthesia. The epithelial cells on the surface of the cornea are removed from the central region (7mm), and the Riboflavin drops are applied. Once these drops have sufficiently penetrated the eye, the UV light is focused on the central area of the cornea. Finally, a bandage soft contact lens is applied to be worn for 3-4 days until the surface epithelial cells grow back.

During the first few days the eye will be sore and teary. The patient will experience slight haziness of vision for several months after CXL.

Fortunately, vision stabilises within a month and the haziness will eventually clear.

A 2nd generation system, the Avedro KXL system is now available for strengthening the cornea by cross-linking its collagen as well as provides shorter treatment time and better precision.

The Avedro KXL system