Excimer Laser Corneal Tissue Removal Calculations

The most common used formula for calculating the depth the excimer laser will ablate during conventional Lasik, All-Laser 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.

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.

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-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.

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 broadbeam 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.

If you are ready to choose a doctor to be evaluated for conventional or custom wavefront Lasik, All-Laser Lasik, PRK, LASEK, Epi-Lasik, NearVision CK, RLE, or any refractive surgery procedure, we highly recommend you consider a doctor who has been evaluated and certified by the USAEyes nonprofit organization. Locate a USAEyes Evaluated & Certified Lasik Laser Eye Surgery Doctor.

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

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Current Lasik Ablation Calculation Medical Journal News...

Theory concerning the ablation of corneal tissue with large-area, 193-nm excimer laser beams.

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Theory concerning the ablation of corneal tissue with large-area, 193-nm excimer laser beams.

J Biomed Opt. 2006 Nov-Dec;11(6):064032

Authors: Munnerlyn CR, Arnoldussen ME, Munnerlyn AL, Logan BA

Excimer laser beams (193 nm) of uniform fluence were studied to find out why they produce corneal ablations deeper at the edge than the center. Ablation depth profiles were taken of porcine corneas, including five dehydrated samples. Hydrated corneas and polymethyl methacrylate were ablated with and without central masks. Ablation plumes were photographed. Hydrated porcine corneas showed patterns of central underablation. As the incident beam increased, the crater exhibited increasingly shallower central ablation while maintaining nearly constant depth at the edges. Dehydrated corneas did not vary significantly. Masks did not alter the depth or shape of craters near ablation edges, but depth adjacent to the images of the masks was more than twice that with no mask. Depth adjacent to the mask image was nearly the same as at the edge of the zone. The rate of change in depth with position was nearly equal in both areas. Maximum plume density was centered over the entire ablation with and without the mask. Redeposition of plume particles is not the major cause of central underablation. Propagating transverse energy from the absorption of photons by peptide bonds increases pressure on excited components within the irradiated area, increasing recombination, which raises the ablation threshold.

PMID: 17212555 [PubMed - indexed for MEDLINE]