I came to your 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 (the biomechanics) of the cornea. 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.
"Over the past few years, it has become apparent to me and to many other laser refractive surgeons that the gold standard as far as the amount of tissue that should be left remaining in the corneal bed after surgery should be more than 250 microns in order to be 100% safe. The reason for this, is that with the older criterion of 250 microns in the bed, there has been an occasional "LASIK surprise" - a condition called ectasia, where there is a disturbance in the biomechanical strength of the cornea. This is a weak spot in the cornea that can cause it to bulge and become irregular. This is a rare occurrence, but when one is performing a large volume of LASIK procedures, one can see an occasional case or two in their practice. These occur as a complete surprise, as all the parameters pre-operatively were normal. We do know statistically that the thinner the cornea is originally, thus resulting in a thinner corneal bed post-surgically, the greater the chance of ectasia occuring. With this in mind, I have changed the criterion for LASIK here at Coal Harbour Eye Centre, and will not leave less than 300 microns in the bed and will not cut a flap on a cornea that is less than 520 microns. There is no good science for this, yet I have implemented this change as an added safety margin to safeguard your procedure." August, 2008.
Steven L. Kirzner, MD
Example 1
Corneal thickness 525 microns
Flap thickness 150 microns
Laser tissue removal 50 microns
Corneal bed 525-150-50 = 325 microns
OK for LASIK with tissue left over for re-treatment.
Example 2
Corneal thickness 525 microns
Flap thickness 150 microns
Laser tissue removal 75 microns
Corneal bed 525-150-75 = 300 microns
OK for LASIK with no tissue left over for re-treatment - one time LASIK only as Dr. Kirzner will not leave a corneal bed with less than 300 microns.
Example 3
Corneal thickness 525 microns
Flap thickness 150 microns
Laser tissue removal 150 microns
Corneal bed 525-150-150 = 225 microns
Can not do LASIK - Rule of 300 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 (no additional microns are needed for the creation of a flap, as in LASIK).
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, or 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 the surgeon's experience.
After the epithelium is removed, the 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 525 microns
Flap thickness 0 microns (we're doing PRK - no flap)
Laser tissue removal 150 microns
Corneal bed left over 525-150 = 375 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:
- Fire Fighting, Police Officer, RCMP, Military, etc.
- Boxing, Martial Arts, any sport where there is contact with the eye (one doesn't want a flap that can be moved or injured), etc.
• More discussion of PRK vs LASIK to follow.
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, as at Coal Harbour Eye Centre)
* Delayed epithelial healing
