During Lasik treatment the laser eye surgeon uses a device (either a femtosecond laser or a mechanical device known as a keratome) to make a very thin flap on the surface of the cornea which has a hinge so that the flap remains attached to the rest of the cornea. The hinge is usually on the uppermost portion of the cornea although it can sometimes be on the nasal side. The surgeon will lift the flap exposing the portion of the cornea underneath it. For your interest, the cornea is approximately 0.5mm thick. Modern flaps are approximately 0.1mm thick (that’s very thin!) and this leaves about 0.4mm of cornea tissue underneath (also very thin!). You will be pleased to learn that thin or not, the cornea is an extremely robust tissue and very difficult to rip or tear. It is therefore perfectly possible to lift up such a thin flap and allow the excimer laser access to the cornea underneath. The excimer laser emits short wavelength ultraviolet light. This type of ultraviolet light has the remarkable property of interacting with the collagen of the cornea and vapourising it without causing heating. This is where the term ‘cold laser’ comes from. Since the excimer laser energy ablates or removes corneal tissue, the energy can be programmed to be applied in such a way as to remove precisely quantified amounts of cornea. The cornea can therefore be flattened or steepened according to the patient’s needs. A steeper cornea has more focusing power and is therefore used to treat hyperopia (long sightedness). A flattened cornea has less focusing power and is therefore used to treat myopia (short sightedness). Astigmatism is an irregular shape to the cornea – here the excimer laser is used to smooth the contour of the cornea so it becomes regular. Only minute quantities of tissue are removed – approximately 12 microns of cornea tissue has to be taken away to treat every unit (dioptre) of myopia or astigmatism. Minute or not, the patient enjoys a dramatic change in the quality of vision and quality of life as a result!
This article has so far described Lasik – but what of Lasek and Epi-Lasik? In these technologies, no flap is created. Instead a surface treatment approach is used with either the laser or a chemical being applied to remove the renewable surface lining of the cornea (the cornea epithelium). In the opinion of Eyesite surgeon Dr Simon Levy, using the laser to remove the cornea epithelium is a greatly superior technology to using a chemical. The Alcon Wavelight Refractive Suite EX500 excimer laser has an extremely sophisticated module that removes the cornea epithelium (which is only 50 microns thick, thats 1/20 of a mm!) in such a way that the distant edge of the ablation has a curved shape. This makes regrowth of the epithelium much faster (in fact it often occurs within only one day) and greatly reduces the discomfort associated with the surface treatment. After removal of the cornea epithelium the cornea underneath is lasered directly, without the creation of a flap.
You may ask why Lasek or Epi-Lasek might be preferable to Lasik, given that these surface eye laser treatment are inevitably associated with some discomfort or even pain for a day or two afterwards and that vision recovery is quite a bit slower than after Lasik. The answer is that some patients prefer the ‘less surgical’ approach inherent in not making a flap. Also, for patients who have thin corneas, surface treatments such as Epi-Lasik and Lasek may be preferable because less of the patient’s thin cornea is disturbed than would be the case if a flap were made. Why? – although the total amount of cornea removed by the excimer laser is the same with both techniques, with Lasik the thickness of the flap (generally about 100microns) must be added to the total amount of cornea ablated by the laser to calculate the total amount of cornea tissue that has been sacrificed to perform the successful laser eye surgery procedure. Therefore Lasik will always thin the cornea significantly more than Lasek and Epi-Lasek.
To enjoy Eyesite’s reliable and sophisticated laser eye surgery service to fix your vision and focusing problems – please contact Eyesite either by phone or email. Eyesite’s optical surgeon has the experience and credentials to greatly enhance your eyesight and change your life for the better for ever!
