Lens Design
Criterion with "Canted Zone Technology" is indicated for those very irregular corneas where excessive edge lift is an issue. Patients who exhibit severe ectasia from Pullucid Marginal Degeneration or corneal distortion with excessive inferior corneal steepness are excellent candidates for Criterion. "Canted Zone Technology" allows for different individual base curves in four quadrants of the posterior surface eliminating excessive edge lift.

Pre-Fitting Examination
Assuming there are no contraindications, begin by taking the Keratometer readings. Determine the flatter K reading ("Flat K") and amount of corneal astigmatism. If corneal topography is done take note to the steepest area on the map and the most symmetrical plain of the cornea. In addition you want to observe the limbal size of the cornea.

Selecting Lens Size
The first parameter you should select is lens size. Since most patient's corneas are 11.5mm in diameter an 11.0 mm lens will be selected. The overall diameter should be .5 mm smaller that the limbal area of the eye.

Choosing Base Curve
The most difficult parameter to determine on a distorted corneal is the base curve. An accurate "K" reading is many times difficult to obtain. Your goal is to determine the flat "K" and how steep the cornea is in its steepest area. Using a topographer or peripheral "K's" will allow you to obtain the steepest area of the cornea. Take the steepest area and subtract it from your flat "K" to determine the total corneal cylinder. A good start point for your initial trial lens will be a base curve that is steeper than the flat "K" by adding 1/4 of the total corneal cylinder to that flat "K". If topography is available the base curve in the zero, ninety and one hundred and eighty quadrants should be the curve that is 4mm off center in the most symmetrical plane of the topography. The Two hundred and seventy quadrant usually will be 3 to 5 diopters steeper than the other three quadrants.

Calculating Lens Power
Lens power is best determined by over-refraction with a trial lens in place. The spherical value of the over-refraction is simply added to the trial lens power to determine the power prescribed.

Center Thickness
Center thickness is a function of lens design and should be calculated by the laboratory.

Ideal Fit
The lens should ideally move freely with the blink. However, there will be less movement with a Criterion than a conventional lens design. A minimum of 1 mm of movement is required for good tear exchange. Depending on the severity of the distortion the fluorescein pattern may be irregular and lens position may de-center.

Fluorescein Pattern
Lens selection may be aided by an examination of the fluorescein pattern. There is no ideal pattern when dealing with a distorted cornea. There will be areas of pooling and bearing. However, the peripheral curve area should show a definite green band. Caution: A lens that seals off and does not move will be apt to cause a physiological problem. Make sure the lens fluorescein pattern does not seal off at the intermediate or peripheral areas. If there is excessive pooling with edge lift in the inferior position steepen the 270 degree quadrant.