PRK laser refractive surgery

Photorefractive keratectomy is one of the surgical techniques that uses an excimer laser to treat refractive errors like myopia or astigmatism. 

Like the other techiques that use a laser, its aim is to correct refractive errors, modifying the corneal curvature, which is the cause of the majority of low degree myopias.

The cornea is the main lens of the eye and it is located in the surface. Like any lens, its power depends on the degree of its curvature.

The more curved a lens is, the greater its dioptric power, that is to say, the higher it is. We could make an analogy with a magnifying glass, the greater its curvature, the higher the magnification and, therefore, the better the near objects will be seen and the worse the distant objects will be seen, which is what happens in myopia.

If we decrease the curvature of the cornea, we also decrease its power and therefore reduce the dioptres of the eye.

The laser acts by reducing the central thickness of the cornea compared to the periphery, which causes an overall reduction in corneal curvature.

As the laser acts upon the surface of the tissue to be treated, we must first remove the epithelium (the fine skin that covers the cornea) to directly treat the stroma (rigid tissue that forms the large part of the corneal thickness).

Although the aim of the different laser techniques is to reduce the thickness of the corneal stroma, the differences between them involve the way we remove the epithelium. 

Thus, photokeratomileusis, better known as the LASIK technique, uses another type of laser (femtosecond laser) to make a cut in the anterior stroma below the epithelium, which we subsequently lift, to be able to apply the excimer laser (which reduces the corneal thickness and corrects the myopia) and which has a direct effect on the stroma. Subsequently, the stroma layer and epithelium that we had lifted are put in place and it is left to heal on its own. 

The photorefractive keratectomy, better known as the PRK technique, on the other hand, involves removing the epithelium, applying the excimer laser directly to the surface of the stroma, and then let it regenerate by itself, which normally happens in 2 or 3 days.

The main difference between the two techniques is that the PRK technique provides treatment to the surface of the corneal stroma while the LASIK technique does it so in a more profound way, which requires greater corneal thickness. For this reason, the PRK technique is usually used when the patient has a finer or less resistant cornea.

In general, laser surgery techniques are indicated to correct low myopia, normally below 7 or 8 dioptres, depending on the characteristics of the cornea (thickness, regularity, shape, resistance, etc) which will be determined after an exhaustive series of tests in the preoperative period. 

Laser treatment, in general, is used to correct myopia or astigmatism below 6-8 dioptres. An introcular lens implant is the technique of choice to correct myopia of a higher degree. 

The choice between the LASIK and PRK techniques depends on various factors. 

Firstly, the thickness and resistance of the cornea, reserving the PRK technique to thinner or more delicate corneas. 

Similarly, another indication of the PRK technique is, in an attempt to most effectively preserve the resistence of the cornea, to perform surgery on people who in their daily lives are more at risk of receiving a direct impact to the eye, such as those who practice risk or contact sports. 

So, why is PRK not practiced on all patients who want laser surgery? The answer is simple: because of the recovery time and the possible postoperative discomfort over the first few days. 

Removal of the epithelium requires a new re-epithelialisation or growth of a new skin, which will take three days or so. During this time, the patient may present discomfort and their eyesight may not be optimum, which is why relative rest is required during that period.

In addition, once the cornea is re-epithelialised, the period from an increase in visual acuity until reaching definitive vision may be 3 to 6 weeks. While with the LASIK technique, visual recovery is usually much shorter, 2 or 3 days. The visual result two months after the operation is similar in both techniques. 

Another problem that the LASIK technique may present is that is can affect the superficial sensitivity of the cornea, which translates as a higher probability of having problems with dry eye before the operation, for example, in people who have used contact lenses in an exhaustive way and for a prolonged period of time. 

Technically, PRK surgery is painless and simple, even more so than the LASIK technique. It involves removing the superficial epithelium with a sponge and another delicate instrument to then apply the excimer laser. In total, the technique does not usually last more than 3 or 4 minutes.

Subsequently, a protective contact lens, removed 3 or 4 days after the operation, is put in place to decrease the possible postoperative discomfort. A normal life can be resumed from then on. We remind you that although you can lead a normal life, visual recovery probably won't be complete until a good few weeks have passed. 

The result in terms of vision and reduction of dioptres is excellent, similar to other refractive techniques such as LASIK. 

However, we should remember that full visual recovery may take a few weeks. During this time the patient may go about their normal activities, without glasses now, while waiting for their vision to gradually increase until reaching the level of vision they had with glasses, but without them now. 

Corneal refractive surgery has a very high rate of effectiveness and level of safety. This means that the desired effect is obtained in the majority of cases with a very low level of risk. 

In any case, we must understand that, as it is a medical and surgical procedure that acts on the live tissue and not inert materials, the response may vary from one person to another. Therefore, a 100% successful result cannot be assured in all cases. Neither is it exemp from possible complications.

Like in any surgical procedure, we must take precaution to avoid possible infections, which is why strict compliance with postoperative treatment is necessary. 

As previously mentioned, visual recovery from this technique is slower than with the LASIK technique due to the fact that surface healing may cause mild opacification in the superficial stroma which will pass over a few weeks. This healing proceess and regularisation of the surface is usually longer when the number of dioptres is high.

In some cases, a feeling of dry eye may increase, particularly in cases of prolonged use of contact lenses before surgery. This means it is neccessary to use eye lubricant gels for a more or less prolonged time, depending on the symptomology and environmental conditions.