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Excimer Laser Corneal Tissue Removal Calculations

For Lasik, Bladeless Lasik, PRK, LASEK, and Epi-Lasik.

Image of a desktop calculator.  
Precise calculations may be made to determine safety.  

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.

Personalized Answers

If this article did not fully answer your questions, use our free Ask Lasik Expert patient forum.

Recent Munnerlyn Formula Medical Journal Articles...

Related Articles

Theoretical Analysis of the Effect of Pupil Size, Initial Myopic Level, and Optical Zone on Quality of Vision After Corneal Refractive Surgery.

J Refract Surg. 2012 Nov 12;:1-5

Authors: Alarcón A, Rubiño M, Pérez-Ocón F, Jiménez JR

PURPOSE: To analyze the theoretical effect that pupil size, optical zone, and initial myopic level have on the final retinal image after corneal refractive surgery.
METHODS: A schematic myopic eye model corrected by the Munnerlyn formula was used to analyze the optical quality of the final retinal image. Root-mean-square radius spot and modulation transfer function were calculated by ray tracing to evaluate retinal image quality.
RESULTS: Pupil size had a negative effect on the retinal image only when it was greater than the diameter of the optical zone. In addition, the greater the initial myopic level, the more the pupil size affected image quality. Thus, a clear dependence exists between the initial myopic level and effect that the pupil size can have on the retinal image after laser refractive surgery.
CONCLUSIONS: Pupil size may be a risk factor for night vision disturbances, but only when it is larger than the theoretical optical zones utilized in this study. Its effect depends not only on the optical zone size, but also on the initial myopic level. Therefore, this relationship should be taken into account during patient selection for refractive surgery.

PMID: 24601483 [PubMed - as supplied by publisher]


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