Article

PRESCRIBING FOR ASTIGMATISM

CONTACT LENS ANATOMY: THE PRISM BALLAST

Incorporating cylinder power into a contact lens, whether soft or GP, has always been a challenge for contact lens manufacturers. Unlike a pair of glasses, a contact lens on the eye is subject to rotational forces generated by the eyelids during a blink. Because of this, creating a lens with rotational stability is crucial to prevent visual symptoms such as dizziness and asthenopia. One of the oldest but still common methods to achieve this stability is prism ballasting, which is used across almost all types of contact lenses.

Prism Ballasting in Corneal GP Contact Lenses

Prism ballasting in corneal GP lenses is commonly used to achieve rotational stability for patients who have a visually significant amount of residual astigmatism. Historically, in the manufacturing process, the prism was generated by offsetting the plastic button when it was cut. This results in a lens edge that is thicker on one side (base) than on the other (apex). Modern CNC lathes are able to cut prism into corneal GP lenses while maintaining uniform edge thickness, reducing discomfort that may be induced by excessive lower eyelid interaction.

Problems can arise in unilateral wear of a prism-ballasted lens because the vertical base-down prism is incorporated into the optic zone and can exacerbate a pre-existing vertical deviation. To minimize this, many manufacturers instead utilize a peri-ballast design in which a high-minus-power lenticular carrier is used in conjunction with thinning of the superior lens edge (Mandell, 1988).

Prism Ballasting in Scleral GP Contact Lenses

Residual astigmatism may also be a barrier to achieving optimal vision for scleral contact lenses, often more so than with corneal GPs due to the positive coma aberration that can be induced by lens decentration. For patients in whom a spherical scleral lens results in an optimal fit, prism ballasting can be used to stabilize a front-toric power by the same mechanism as with a corneal GP contact lens.

Unfortunately, this may result in additional lens decentration and visual aberrations. The shift toward using toric haptic curves for scleral alignment has reduced the need for, and usage of, prism ballasting in scleral lenses.

Prism Ballasting Pearls

When ordering GP contact lenses with a prism ballast, it is important to request a laser-inked dot at the base of the prism to help in evaluating the fit.

If the prism base does not position at 6 o’clock, but the lens is otherwise rotationally stable, use LARS (left add, right subtract) or Snyder’s Rule (Snyder, 1989) to correct the residual astigmatism.

Occasionally, a patient’s lid forces and anatomy may result in rotational instability even with an adequate amount of prism ballasting. In these cases, consider prescribing a truncated (corneal GP lens) or dual-thin-zone (scleral GP lens) design. A bitoric base curve in corneal GP lenses or a toric haptic in scleral lenses can also help with achieving rotational stabilization.

The most visually stable solution for correcting residual astigmatism is a pair of overlay spectacles, as it eliminates the need for rotational stability. This is often the best option for presbyopes, who can choose to wear progressives or bifocals over their contact lenses for optimal correction at all distances.

Do not forget that prism ballasting a spherical-powered corneal GP contact lens can be useful in centering an otherwise superiorly decentered fit. On the other hand, scleral lenses tend to decenter inferiorly—hence, prism is seldom used to improve the overall fit. CLS

Special thanks to Daren Nygren of Custom Craft Lens Service Inc. for his help in writing this article.

For references, please visit www.clspectrum.com/references and click on document #269.