Why PRK?
I came to a laser clinic for LASIK and they told me that I needed PRK:
What is the difference in the two procedures?
What are some of the reasons for doing PRK?
So why can I have PRK if I'm not a candidate for LASIK?
What are the advantages and disadvantages of the two procedures?
What is the difference in the two procedures?
There are basically two laser refractive procedures to correct myopia (nearsightedness), hyperopia (far-sightedness) and astigmatism.
The average corneal thickness in a Caucasian is 543 microns (1/2 mm), it is less in Blacks (520 microns) and even less in Asians (490-510 microns).
In order for the cornea to properly focus light it MUST maintain a certain structure and not be irregular. Removal of too much tissue can destroy it's BIOMECHANICS and render it structurally unsound. This leads to loss of vision that cannot be corrected by glasses and usually not by contact lenses.
In LASIK, a flap of approximately 150 microns is cut. This flap, which when put back adds very little to the structural integrity of the cornea (it's biomechanics). The laser is then applied which removes tissue (a prescription is ground off the cornea) and so one is left with a bed of tissue at the end of the LASIK procedure. This bed must be at least 250 microns thick so that the cornea is biomechanically sound and can focus light. We like to leave more to insure that there is enough tissue left so that a re-treatment, if necessary, can be done.
Example 1
Corneal thickness 500 microns
Flap thickness 150 microns
Laser tissue removal 50 microns
Corneal bed 500-150-50 = 300 microns
OK for LASIK with plenty of tissue left over for re-treatment.
Example 2
Corneal thickness 500 microns
Flap thickness 150 microns
Laser tissue removal 100 microns
Corneal bed 500-150-100 = 250 microns
OK for LASIK with no tissue left over for re-treatment - one time LASIK only
Example 3
Corneal thickness 500 microns
Flap thickness 150 microns
Laser tissue removal 150 microns
Corneal bed 500-150-150 = 200 microns
Can not do LASIK - Rule of 250 micron bed
So, if one doesn't have enough TISSUE for LASIK can one have laser surgery? Yes, they can have PRK (Advanced Surface Ablation, No-Touch TM - trademark names for PRK).
In PRK only the epithelium (thin surface layer) is removed (it has no structural integrity and grows back within days). Then the laser removes the prescription. In PRK we like to leave 350 microns at the end of the procedure.
Epithelium (the surface skin of the cornea) can be removed by many methods. There is a natural plane between the epithelium and the underlying tissue and it is easily removed with a laser, alcohol, scraping brushing it off etc. I have tried many different methods and presently find alcohol removal to be the easiest and most accurate although any method is probably as good depending on one's experience. It probably doesn't matter much which method one uses.
After the epithelium is removed, laser is applied and then a contact lens is placed on the cornea for 5-6 days to allow the epithelium to heal and keep the cornea comfortable (it bandages the eye).
One may or may not use a chemical agent to prevent hazing before the clear contact lens is put on. I personally use such an agent.
So why can I have PRK if I'm not a candidate for LASIK?
Look at Example 3
Corneal thickness 500 microns
Flap thickness 0 microns (we're doing PRK - no flap)
Laser tissue removal 150 microns
Corneal bed left over 500-150 = 350 microns
What are some of the reasons for doing PRK?
- Scar on the cornea that impairs vision - can be removed with PRK but will remain in the LASIK flap (the scar is not removed with LASIK).
- In corneas that are too thin for LASIK, PRK is an option.
- An area of epithelium that is poorly adherent to the underlying tissue and breaks down causing pain.
- The cornea has a problem (disease) for which removal of the tissue is indicated and PRK is the treatment for this.
- Certain occupations or hobbies, such as;
- Boxing,
- Martial Arts etc.
- Any sport, where there is contact with the eye. One doesn't want a flap that can be moved or injured.
What are the advantages and disadvantages of the two procedures?
LASIK Advantages:
- Quick healing (driving vision the next day in most cases)
- Little chance of haze
- Re-treatments are easier
Disadvantages:
- Uses more tissue
- Can have flap complications (rare occurrence in good hands)
PRK Advantages:
- Uses less tissue
- No flap created so no chance of flap complication
Disadvantages:
- Healing takes longer than LASIK (driving vision in 4 to 14 days)
- Slight risk of haze (little risk with modern laser and adjunctive chemical therapy)
- Delayed epithelial healing
We do approximately 70% of our patients with PRK and find that the end result of the two procedures to be excellent.
