Below, I have quoted a passage from an article that might be of some help regading toric lenses.
"Toric lenses, or lenses with cylinder power, typically are used to correct for astigmatism. For toric lenses, the cylinder power is along the cylinder radius, which is orthogonal to the cylinder axis. The cylinder power of the toric lens results in the eye focusing light rays at a common focal point. As indicated above, toricity is used to correct astigmatism so that it focuses light rays at a common point. However, the common focal point may be located at an incorrect position with respect to the retina. This condition is referred to as either "myopia" or "hyperopia". Myopia typically results from a steep cornea that causes light rays to focus at a focal point that is short of the retina. Conversely,
hyperopia is a case in which a
flat cornea causes light rays to focus at a focal point that is behind the retina. Sphere power is included in a lens to cause light to properly focus on the retina.
contact lenses may include either or both spherical correction or cylinder correction depending on the particular circumstances of the lens wearer's eyes. Lenses that have only spherical correction are spherically symmetric, and thus, any rotation of the lens on the eye has no effect and does not disturb the intended vision correction. Toric lenses, on the other hand, are typically designed to include areas of different thickness, resulting in a spherically asymmetric lens. As a result, toric contact lenses need to be rotationally stabilized in the eye to insure that the lens is properly oriented with respect to the eye in order to carry out its function of correcting the vision particular areas of the eye."
You can read this entire article at [url=http://www.patentstorm.us/patents/6135594/description.html]Toric contact lens with axis offset compensation and method and apparatus for manufacturing same - US Patent 6135594[/url]
In addition, here is a passage from an article regarding cornea curvature and fitting of lenses.
"Average corneas usually have an eccentricity of about 0.55 (e = 0.55). The SFs (e2) for the general population are as follows:
Normal 0.13 to 0.35
Borderline 0.02 to 0.12 and 0.36 to 0.46
Abnormal 1.0 to 0.01 and 0.47 to 1.0
Different corneas may have the same central radius of curvature (Sim K or keratometry) but display a different rate of peripheral flattening or eccentricity. Therefore, if you choose base curves based on the central keratometric reading, you should know that peripheral flattening determines the contact lens-to-cornea relationship. If an eye demonstrates rapid peripheral flattening (e = 0.8, for example), it would produce a steep lens-to-cornea fitting relationship because the sagittal depth of the spherical lens would surpass that of the aspheric eye. Contact lens modules take these differences into consideration when they preselect the best contact lens to fit the aspheric cornea. These contact lens nomograms calculate the overall fitting relationship, the diameter of the contact lens, the optic zone and the peripheral curve configuration to determine the best contact lens fit. They may help you preselect a contact lens prior to the actual diagnostic fitting."
This article can be read in its entirety at [url=http://www.clspectrum.com/article.aspx?article=12399]Contact Lens Spectrum[/url]
Hope this helps.
Welcome to the Contact Lenses Forum - Lens 101 forums. 
05-15-2008, 04:53 PM
Encore Toric Contacts and Flat Corneas
No Problem
Linear Mode
