Laser Eye Treatment

The Munnerlyn Formula: The Mathematical Safety Net

If you decide to have LASIK or PRK, the calculations that determine your future eyesight will be based on a mathematical equation called the Munnerlyn formula. It ensures that the window of your eye has enough remaining corneal tissue underneath the newly sculpted lens to maintain the stability of the cornea. A crucial amount of the stroma, the main inner corneal layer, and the last two layers must remain intact. Developed by famous California physicist Dr. Charles Munnerlyn, the formula described in the medical equation, states:

The depth of the treatment zone (the thickness of the lens) = [the diameter of the lens (the treated zone) in millimeters squared x the number of diopters of correction] / 3

In effect, this formula means that the wider the diameter of your lens – and the greater your refractive error – the deeper your surgeon must laser into your cornea to correct your vision.

Put another way, the larger the diameter of your optical zone and the more nearsighted you are, the more corneal tissue the laser must remove. Ophthalmologists sometimes simply say that the depth of the laser treatment is proportional to the size of the optical zone.

This means that, depending on your correction, the wider your new lens, the thicker it must be. Of course, to treat nearsightedness, more laser light strikes the center of the cornea, thereby removing more tissue there than from the periphery. A small increase in the diameter of this curved optical zone causes a significant increase in the amount of the corneal tissue removed. Nevertheless, even a -6 diopter correction with a 6-millimeter optical zone only vaporizes 72 microns of tissue.

Your doctor will want you to look through a large optical zone that is about 6 millimeters in diameter. A wide, finely sculpted corneal lens is especially important for good night vision when the pupil dilates to let in more light. As the pupil widens beyond the lasered area for patients with a small optical zone, the light coming through the untreated edge of the cornea is unfocused. Severely nearsighted people often have large pupils. When the diameter of the uncorrected lens is too small for the size of the dilated pupil, the ring-shaped part of the cornea that remains “nearsighted” can cause halos around lights at night. Patients find this phenomenon extremely distracting. Physicians now have computer software that allows them to re-treat patients with this problem to make their optical zones larger.