The LASIK Envelope

Determining the Diameter of the Optical Zone, Ablation Depth, and Minimum Corneal Thickness Required for LASIK

The LASIK Envelope is comprised of three variables a surgeon takes into consideration when determining the treatment necessary, and in evaluating your suitability for the LASIK procedure.

      The three Ps (variables) impacting the LASIK Envelope are:
Pupil Size    =   The diameter of your pupil under low light conditions
Pachymetry    =   Corneal thickness
Power (RX)    =   Amount of correction necessary

The diameter of the optical (ablation) zone should be greater than the diameter of your pupil under normal low light conditions. The pupil naturally dilates at night to allow more light into the eye. Ablation (and centration) diameter is important because if the edge of the modified corneal lens overlaps the pupil or light axis then the patient may experience glare, light sensitivity or other symptoms.

The maximum depth of the ablation is restricted by the pachymetry (thickness) of the cornea. The thickness of your cornea must be measured to determine your suitability for the procedure. The absolute minimum of residual thickness is 250 microns. If the depth of the ablation leaves less than 250 microns of residual corneal tissue, the risk of the cornea shape (ectasia) changing, or blowing out is greatly increased.

Most surgeons like to leave
270 -275 microns of residual cornea in order to provide a safety margin to allow for enhancement (retreatment), should this be necessary.
A wider optical zone will require a deeper sculpting.

A higher refraction requires a more curved and deeper sculpting.

An individual's best combination can only be chosen by the surgeon after carefully weighing all relevant factors. In any situation where you have a high initial refraction, a thin initial corneal thickness or a large pupil, any compromise of the optical zone size can lead to a permanent problem with nighttime glare and halos.

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For more information contact:
Dr. Murray McFadden
(BSc, MD, FRCS(C), Diplomate of the
American Board of Ophthalmology)
© Copyright 1996-2005 Murray McFadden MD, Inc.

Telephone: (604) 530-3332
Fax: (604) 535-6258
SnailMail: 20434 64th Avenue, Unit #201,
Langley, BC Canada V2Y 1N4

This page last updated on September 17, 2001.
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