Laser surgery (PRK and LASIK) for refractive errors

What is Excimer laser?

Excimer laser is a far Ultra Violet (UV) light energy of wavelength 193 microns. It is invisible to human eye. This laser breaks the chemical bonds within the molecules (photoablation) with minimal thermal damage to the surrounding tissues. Thus it is very precise and can remodel the cornea with an accuracy of more than a thousandth of a millimeter.

How does Excimer laser correct the refractive error?

The effect of excimer laser on the cornea is very similar to grinding of a glass lens to change its refractive power. In eyes with refractive errors, excimer laser because of its high level of precision, can change the shape of the cornea to change its refractive power to the desired state and thus correcting the refractive error.

In myopia, the central part of the cornea is made flatter, and thus decreasing the refractive power of the cornea and of the eye, and thus correcting the refractive error.

Similarly in hypermetropia, the laser removes a ring of tissue from the peripheral part of the cornea and thus makes the central part steeper and corrects the refractive error.

As we know, in astigmatism, the cornea is more curved in one direction. Excimer laser can correct astigmatism also by selectively ablating the cornea in the required direction.

What is PRK?

In PRK the surgeon removes the surface layer of the corneal epithelium over the area to be treated, and the excimer laser is applied to sculpt the corneal stromal front surface to a new shape. The surface epithelium of the eye normally heals by itself in 2-3 days for myopia treatment and 4-7 days for hypermetropia treatment.

PRK does not involve any incisions on the cornea and removes only small amount of tissue; therefore the globe is not weakened significantly. However, as the epithelium is scraped off, the nerve endings of the cornea get exposed and there might be moderate pain in the initial post-operative period. Also due to regrowth of cells and healing processes, there might be some haze of the cornea. The predictability of PRK is in the range of -1.0 to -8.0 diopters of myopia.

What is LASIK?

LASIK involves putting the PRK treatment not on the surface of the cornea, but under a protective corneal flap. A very thin (about 0.16 mm) and precise flap is raised by a special instrument known as microkeratome. The result is a corneal flap attached at one edge, the hinge. The surgeon folds the flap to expose the inner stromal layer of the cornea. The excimer laser treatment is applied on this stromal bed to remodel it. After this the flap is repositioned to its original position and it does not require any suture. Since the corneal epithelium has only been minimally disturbed, there is only mild discomfort after the procedure. LASIK is predictable and safe upto -15.0 diopters of myopia and 8.0 diopters of astigmatism.

Is LASIK better than PRK?

When comparing the benefits of the two procedures, LASIK outperforms PRK in many aspects. But as it is a surgical procedure, there are a few rare but serious potential complications and risks.

Advantages of LASIK over PRK are:

• • Less pain and discomfort with LASIK. The surface epithelium of the cornea is always removed while performing PRK but is left almost intact with LASIK.
  
  
• • Postoperative visual acuity is restored within hours or days with LASIK, rather than weeks or months with PRK for high myopia.
  
  
• • Less incidence of corneal haze (scarring) in the long term for LASIK than PRK.
  
  
• • There is a greater predictability of the correction with LASIK than PRK.
  
  
• • Less follow-up visits are required with LASIK than PRK.

What are the complications of LASIK?

The overall rate of significant complications in LASIK is of the order of 1-2%. These complications may be:

• • Flap complications: the flap may not be raised properly by the microkeratome, or it may be damaged during or after the operation.
• • Perforation of the eye if the flap is cut too deep.
• • Infection
• • Scarring of the cornea