The Business of Contact Lenses

The Basics of Residual Astigmatism and GP Lenses

By Clarke D. Newman, OD, FAAO

In keeping with the theme of returning to the basics to improve outcomes and control costs, we will visit the issue of residual astigmatism (RA). Essentially, RA is work undone on our part.

Clinically, small amounts of RA, say, 0.50D or less, usually are not noticed by patients—unless they notice. Chasing amounts this small is not a good idea unless a patient is really complaining about it because you might induce more astigmatism than you solve.

RA may either be induced by an optical prosthetic or be physiological. Physiological RA (PRA) is the amount of astigmatism neither induced nor corrected by an optical prosthetic. It can be predicted by comparing the refractive error, the anterior corneal curvatures, and the optics of the ocular prosthetic.

I spoke about hydrogel contact lens correction in my August column, so we will talk about GP lenses here.

Correction Options

For GP lenses, correcting astigmatism involves a couple of things. First, you must prevent lens flexure, which means you have to know how to identify it. If a lens designed with a spherical surface demonstrates a toric over-keratometry reading, but the lens reads spherical on the radiuscope, the lens is flexing on the eye. If the radiuscope also demonstrates toricity but the lens reads spherical on the lensometer, the lens is warped and needs to be replaced.

Flexure can be controlled by increasing the center and junctional thicknesses of the lens and by flattening the lens profile to reduce the sagittal depth. Achieving lid attachment can reduce lens flexure as well. Also, toric peripheral curves when combined with spherical base curves can control flexure.

Inducing lens flexure to correct RA is an older technique that we do not use much anymore.

That leaves the GP toric options. The front toric option works when the RA is all or almost all inside the eye. These lenses should be bigger in diameter, and you should use a periballast instead of a true prism ballast.

The base, or back-surface, toric is used when the geometric alignment of the anterior ocular surface and the posterior lens surface demands a toric lens surface, and the RA is equal to the induced cylinder that results from the index difference between the lens and the cornea, which is rare.

A low-index material induces half of the cylinder of the higher-index materials. So, if you change materials, you change the RA amount induced by the back surface. Low-index materials are best for bitoric lenses because they have the thinnest junctional thicknesses by inducing less RA for which to compensate.

A bitoric lens is needed when the geometric alignment demands a toric back surface and there is also a significant amount of PRA left to correct. The good news here is that the more astigmatism you have on the front of the cornea, the more likely that the spherical power effect (SPE) lens will work. This lens corrects the induced cylinder from the back only and behaves, optically, like a spherical lens.

The Mandell-Moore Guide is helpful if the patient has with- the-rule astigmatism, but the best way to think of proper alignment is in terms of vertical and horizontal instead of steep and flat.

Looking Ahead

In my next column, I will detail how toric alignment should take place and how I modified the Mandell-Moore Guide to make that happen no matter what the corneal cylinder orientation is. CLS