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All you need to know about refractive surgeries

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Laser Vision Correction

  • People who need to wear glasses to see clearly for distance are said to have refractive errors

  • Refractive errors need glasses or contact lenses for correction, otherwise the vision is blurry

  • Refractive errors result from a mismatch between the length of the eye and its focusing power. 

  • The mismatch can result in light focusing in front of, or behind, the retina of the eye, resulting in blurred vision

  • To correct refractive errors, the focusing power of the cornea of the eye can be changed using lasers

  • Lasers can be used to reshape the cornea, so that the eye now focuses light exactly on the retina, eliminating the need to wear glasses or contact lenses to see clearly

  • To correct myopia or near sightedness, you require to flatten the central cornea, relative to the periphery

  • To correct hyperopia or far sightedness, you require to steepen the central cornea, relative to the periphery

  • Typically, excimer lasers are used to remove tissue from the cornea, to change its shape.

  • If more tissue is removed from the centre, it flattens the central cornea

  • If more tissue is removed from the periphery of the cornea, it steepens the central cornea 

  • There are typically 3 ways to remove tissue from the cornea

  • These are Surface Ablation, LASIK and SMILE

Surface Ablation

  • In surface ablation, an excimer laser is used. 

  • An excimer laser can accurately remove tissue from the cornea, very thin layer after layer

  • As the name suggests, the excimer laser is scanned along the corneal surface, to selectively remove tissue from the cornea, to achieve the correction desired

  • The first layer of the cornea is called the epithelium. The epithelium has cells which can reproduce. If the epithelium is removed, it grows right back

  • To achieve permanent correction, therefore, the epithelium of the cornea is first debrided, exposing the layer of the cornea beneath. The excimer laser is then used on this(ese) exposed layer(s), thus achieving the required correction. The epithelium is then allowed to regrow.

  • This procedure is known as surface ablation

  • Surface ablation is safe and effective.

  • However, it does cause a lot of pain, and it also has a delayed visual recovery, till the epithelium regrows and becomes smooth

  • In the treatment of high refractive errors, it can lead to a heightened inflammatory reaction, causing the refractive error to partially come back, and also cause a fine ‘haze’ in the front layers of the cornea

  • To avoid these issues of surface ablation, most surgeons prefer the alternatives-like LASIK or SMILE

  • In some situations, however-a thin cornea, for example-, surface ablation may be the preferred choice

LASIK

  • LASIK also used the excimer laser to treat refractive errors. 

  • However, unlike in surface ablation, the epithelium of the cornea is not disturbed in LASIK

  • Instead, a femtosecond laser or a mechanical instrument is used to create a thin, tongue like, flap on the cornea, exposing its deeper layers directly, without disturbing the epithelium

  • This flap is reflected to one side, exposing the deeper layers of the cornea. Once exposed, the excimer laser is used to remove tissue from the cornea, to correct the refractive error. Once completed, the flap is replaced in its original position, and it soon sticks in place.

  • LASIK has rapid visual recovery and very little pain relative to surface ablation

  • LASIK also causes a much smaller inflammatory response in the cornea, and thus even high corrections are stable after LASIK relative to surface ablation

  • However, because a large incision is made to make the flap, a lot of nerves get cut during LASIK, and this leads to more dry eye after LASIK

  • Because the deeper layers of the cornea are being cut, LASIK also makes the cornea somewhat biomechanically weaker.

  • LASIK, with a mechanical instrument, is also a more tricky surgical procedure. However, when the LASIK flap is created with a femtosecond laser, the procedure can be very very safe.

Smile and Its Advantages

  • SMILE is a relatively new procedure for Laser Vision Correction

  • It uses a femtosecond laser to carve out a lenticule within the cornea of the eye. 

  • The shape of the lenticule is based on the amount of correction required

  • The lenticule is physically removed from the cornea from a small incision

  • SMILE requires a small incision, relative to LASIK

  • SMILE retains the advantages of LASIK over surface ablation-less pain, rapid visual recovery

  • SMILE promises less dry eye relative to LASIK

  • SMILE is also very predictable and effective, just as LASIK

  • SMILE has no flap, so fewer flap related complications

  • SMILE cannot be used to treat very high myopia, and cannot treat hyperopia (or plus numbers)

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