Article

CONTACT LENS DESIGN & MATERIALS

SOFT TORIC CONTACT LENSES FOR LOW ASTIGMATS

At a number of continuing education programs in several states over the past year, attendees were polled regarding their use of soft toric contact lenses. While not a scientifically valid study, the results are nonetheless informative.

The audiences were asked whether a patient who had 0.75D to 1.00D of cylinder after being vertexed to the cornea would be fit with a spherical or a toric soft contact lens in their practice. Approximately 20% to 25% in all instances responded that a spherical lens would be their choice. While lower than the 30% to 35% who responded similarly when queried about the topic five to 10 years ago, this still seems like a relatively significant number. When asked if they would fit that same patient in a spherical spectacle lens, there was unanimous agreement that they would not.

Why, then, is a toric correction seemingly not needed in soft contact lenses for low astigmats? Is the vision quality with soft toric lenses not really high enough? Is the effort required and the chair time needed to fit toric lenses too much to warrant their use in low astigmats?

Rotational Stability

The number of currently available soft contact lenses for astigmatism has never been greater, and the range of parameters available far exceeds what was possible only a few years ago. Modern design and manufacturing improvements have resulted in lenses with outstanding rotational stability. In the vast majority of cases, there will be little rotation observed. The lenses also remain very constant in their orientation with blinking and various eye movements.

Improved Vision

Therefore, combining outstanding optical quality with this rotational stability results in a very high likelihood that the vision will be noticeably improved in soft toric lenses compared to spherical corrections, even at relatively low levels of astigmatic correction.

In fact, the vision attained through some of the newest toric soft lenses may be enough improved over previous ones that power adjustments may be required when refitting patients. A good, current refraction is always the best starting point. However, if you are switching from a previous toric to a new one, do not neglect an over-refraction, as small refinements are not uncommon.

No Additional Chair Time

Due to their excellent stability and quick settling on the eye, chair time is not significantly increased when fitting low astigmatic corrections. The predictable nature of the rotation allows many first lens fits to be dispensed to evaluate the impact of adding the astigmatic correction. The prescription is then finalized with one return visit; the lenses will be torics if patients appreciated a value, or possibly back to spherical correction if they did not. In either case, the fitting is not complicated or time consuming.

New Designs Abound

Lenses will employ either prism, peri-ballast, or central thickness zones to attain stabilization. Prism-ballasted lenses will have base-down prism in the optic zone, while the other two methods will not. This is generally not of consequence, but if a toric is being fit in only one eye, it might be a consideration.

There have also been very rare instances in which yoked base-down prism has had an impact. For instance, a professional golfer fit with soft torics to correct low astigmatism reported sharp vision, but he also felt “thrown off.” The minor bilateral base-down prism created a fractional image shift that made the ball appear further from him, yet his swing “muscle memory” apparently conflicted with the location of where his brain said the ball was. When refit in toric lenses with no prism in the optic zones, he felt that the problem was solved.

Summary

There are many toric soft contact lenses to choose from in all wear modalities. It is neither complicated nor difficult to fit them, and the chair time is minimal to achieve this better vision correction. Why not offer lenses that give the very best acuity to all of your patients? CLS