Defining Contrast Sensitivity

Diagram Explanations

The following diagram (Figure A) is the working diagram for all explanations within this page.

In Figure A, the eye's optical 'mechanics' have been broken down into the following xx sections:

• Cornea - The cornea is shown enlarged, and the various layers (Epithelium, Bowman's, Stroma, Descemet's Membrane and Endothelium) are color coded.
  
• Iris - The iris opens and closes to control the amount of light entering the inner eye. The aqueuous humor that separates the cornea and the iris is not shown.
  
• Lens - The lens is shown in Figure A in it's relaxed state. This is the assumed state for most figures in this page. It is assumed to be 'clear' and does not distort light.
  
• Retina - The retina is shown as a curved 'panel' that the image is displayed upon. This is highly simplified since the retina is extremely complex consisting of rods, cones, optic nerve network, etc., but will do for these illustrations.
  

PRK/LASIK Tissue Ablation Depths

Refractive Error Correction Attempted	Ablation Depth (6mm/7mm) Ablation Zone	Refractive Error Correction Attempted	Ablation Depth (6mm/7mm) Ablation Zone
-2.0	24/33 microns	>-3.0	36/49 microns
-4.0	48/66 microns	-5.0	60/82 microns
-6.0	72/98 microns	-7.0	84/115 microns
-8.0	96/131 microns	-9.0	108/147 microns
-10.0	120/162 microns	-11.0	132/178 microns
-12.0	144/193 microns	-13.0	156/209 microns
-14.0	168/225 microns	-15.0	180/240 microns

Observations:

• The amount of tissue removed increases greatly as you move up the refractive error curve. A LASIK ablation procedure begins approximately 160 microns (1/3) into the stroma (the cap is usually 160 microns or thicker). Thus a person having a 12D correction would actually have an 'effective' ablation depth of 304 microns (160 flap depth + 144 ablation depth) using a 6mm ablation zone. That's pretty deep, and you are getting close to the endothelium (within 200 microns) with the final laser pulses. This also presents the issue of cornea stability; i.e., just how much tissue removal can the cornea tolerate before ecstasia (instability, tissue breakdown) occurs?
  
• As the ablation depth increases, the transition to the unablated area must be steeper.
  
• Wider ablation zones (7mm) requires greater ablation depth.
  

Post-operative Contact Lens To Reduce Zone Transition Effects

Contact lens fitting may be a problem after refractive surgery since the wound profile does not 'fit' any standard contact design. Because the ablation wound tends to be a little bit off center, and the scarred wound edge rough, contacts tend to 'ride up' into the top of the eye, or pop out. They also tend to get 'bubble' pools of tears under them where the 'crater-shaped' wound is. In the case of RK, the worped cornea and elevated scars make contact lens wear nearly impossible. At least one contact lens vendor is now marketing contacts with 'flat' interiors to improve fit over post-operative corneas.

Patients who have had RK should not wear contacts; it is the worse thing they can do. They don't fit at all, they irritate, and they cause serious complications like ulcers, infections, and accelerated vascularization emanating around the scars and peripheral cornea where it merges into the sclera. What's more, they accelerate endothelial cell loss, which can lead to serious cornea edema and ulceration.

Even so, some post PRK patients and some RK patients do try to wear contacts after surgery. Current studies of contact lens wear fitting are short term, following patients who have been fitted for only a few weeks or months. Long term studies of contact lens wear and their success or failure will be documented in the CLAO journal.

Contacts tend to reduce irregular astimatism's effects, especially on night vision and reduce the effect of GASH. This is possible since the contact lens provides a 'new' cornea surface effectively eliminating the transition zone. But there are limitations. The light rays must still pass through the altered cornea tissue which can induce scattering and some residual irregular astigmatism.

Diagram(s) : Fitting Contact Lens after Surgery, how it causes cavitation bubbles or rides up and how it reduces scatter and multifocal effect.

Fitting Glasses After Surgery For Optimum Night Vision

Fitting glasses after refractive surgery is somewhat more difficult in many cases dependent on the patient's preoperative prescription and the results of surgery.

For people who have mild to moderate myopia, good refractive symmetry and small amounts of astigmatism, they may be surprised that the eyeglasses they require after surgery may have mild spherical corrections but unusual astigmatic cylinder corrections and greater difference in spherical diopters between the two eyes. Of course, people who had high myopia, severe astigmaatism, and signficant anisometropia prior to surgery may have 'more normal' prescriptions after surgery.

If you have any questions or comments, please contact eyeknowwhy@aol.com.