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Munnerlyn Formula
The most common used formula for calculating the depth the excimer laser will ablate during conventional Lasik, Bladeless Lasik, PRK, LASEK, or Epi-Lasik is called the Munnerlyn formula. Munnerlyn says the depth of the ablation
(in microns) per diopter of refractive change is equal to the square of the optical ablation zone measured in millimeters, divided by three.
Optical Ablation Zone
As an example, for a 6mm optical ablation zone, the depth of
ablation per diopter of correction would be 12 microns (6?)?=12.
Don't confuse the optical ablation zone with the transition zone. Adding the transition zone can require the
optical ablation zone to be a little deeper.
Transition Ablation Zone
The transition zone calculation can vary, but the general calculation
to calculate the optical ablation zone with a transition zone is
to add one-third of the transition zone to the calculation for the
optical ablation zone. An example: for a 6mm optical ablation zone
with a 1mm transition zone, the depth of ablation per diopter of
correction would be 13.34 microns ((6+(1?))?(6+(1?)))?=13.34. This
is for a standard, conventional excimer laser assisted refractive
surgery.
Wavefront Calculations Differs
Wavefront-guided ablations normally require significantly more
tissue removal than conventional ablations. This is due to the laser
attempting to limit the increase of higher order aberrations. The calculations for wavefront-guided
ablations are infinitely more complex, but a general rule of thumb
is to add about 30-40% of the total for a conventional ablation.
Fortunately, prior to surgery the wavefront-guided systems have
the ability to determine exactly how much tissue will be ablated
and where.
Maybe Less, Maybe More
These are only guidelines. There are many, many factors that
dictate changes in this formula. Atmospheric pressure and relative
humidity at the time of surgery will cause a change. Sex, age, and
race of the patient can require changes to this formula. If the
laser uses a gaussian profile flying spot rather than a broad-beam application, less tissue may required per diopter
of refractive change. The length of time during surgery will change
the hydration of the cornea and may require less laser energy for
the same correction. Different lasers of the same type require slightly
different formulas. Each doctor refines his or her formula based
upon real practical experience with a particular set of parameters.
Looking For Best Lasik Surgeon?
If you are ready to choose a doctor to be evaluated for conventional
or custom wavefront Lasik, Bladeless Lasik, PRK, or any refractive surgery procedure, we recommend you consider a doctor who has been evaluated and certified by the USAEyes nonprofit organization.
Locate a USAEyes Evaluated & Certified Lasik Doctor.
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