Insight into Toric Contact Lenses
One practitioner explains why she prefers a particular soft toric contact lens.
By Ann Hoscheit, OD
Just like my toes have become a little misshapen from wearing high heels for so many years, and my wedding ring has created a permanent indentation on my finger, the eyes can conform to the shape of regularly worn contact lenses. In the case of contact lenses, the corneas adjust to the contour of the lenses.
The fit can vary for astigmatic patients, depending on the lens design they are wearing. Popular prism-ballasted toric contact lenses, for example, have an asymmetrical thickness that may be comfortable once the eye has adjusted to it. However, prism-ballasted lenses do not necessarily allow for optimal corneal response.
Stabilizing the Lens on the Cornea
Because astigmatic patients typically have toric corneas, which produces the distorted image, their contact lenses must compensate for the eyes' asymmetry. This makes rotational stability of a contact lens on the eye one of the most important features of a toric contact lens.
Prism-ballasted contact lenses maintain their position on the eye due to their design of varying profile thickness. Though this design limits rotation, it also makes the bottom of prism-ballasted lenses (at six o'clock) disproportionately thicker than the top, thereby influencing the natural topography of the eye.
The dual thin zone stabilization design of the Acuvue Toric (Vistakon) contact lens provides two opportunities for the eyelid to stabilize the lens with each blink, which effectively keeps the ultra-thin independent optic zone in place over the cornea. In addition, clinical studies involving both initial and follow-up visits demonstrated that more than 98 percent of Acuvue Toric contact lenses rotated 5 degrees or less with each blink. I have found that this minimal rotation, combined with triple scribe marks at three and nine o'clock, each 10 degrees apart, combine for increased precision when fitting.
TABLE 1 Soft Toric Lens Profiles
|CIBA Focus Toric||0.14mm|
|CIBA FreshLook Toric||0.11mm|
Reproducibility Is Key
As contact lens practitioners know, consistency is paramount for reproducing toric contact lenses. Deviation in orientation or incorrect power or axis can make a significant difference for the wearer's vision correction. Compared with competitors, including Bausch & Lomb's SofLens66 Toric and CIBA Vision's FreshLook Torics, Acuvue Toric contact lenses often vary less in sphere power, cylinder power and cylinder axis (based on standard tolerances determined by the International Organization for Standardization and the American National Standards Institute and on file at Vistakon).
Allowing the Eye to Breathe
Contact lenses restrict the cornea's essential oxygen intake, which could result in hypoxia-related conditions such as corneal stromal edema, microcysts and neovascularization. Over the past 30 years, manufacturers have made many advancements to increase oxygen transmissibility (Dk/t) to keep the eye healthy and comfortable. Manufacturers have worked to design better hydrogel lenses with increased oxygen transmissibility by decreasing thickness (t) and increasing oxygen permeability (Dk), as a function of the water content.
Most hydrogel contact lenses offer water content ranging from about 30 to 70 percent. However, etafilcon A (the polymer in Acuvue Toric contact lenses) delivers the highest Dk/t of any current toric soft contact lens brand at 30.6 (based on measurement of edge-corrected Dk).
Acuvue Toric contact lenses also have one of the thinnest profiles on the market, with a center thickness of 0.07mm (at 3.00D). Compare this with CIBA Vision's Focus Toric lens (0.14mm at 3.00D) and FreshLook Toric lens (0.11mm at 3.00D). The Acuvue Toric's thinner profile also leads to greater oxygen transmissibility (based on a Vistakon study). Both of these factors offer my patients improved corneal health with this lens.
Vision and Healthier Business
In addition to lens reproducibility, rotational stability and high Dk/t, the manufacturer recently added plus and high minus powers to the lens line to allow contact lens practitioners to fit more astigmatic patients. While practitioners have often chosen not to correct low astigmatic patients' vision with toric contact lenses due to increased chair time, inferior comfort (compared with spherical lenses) and negligible visual improvement, contemporary toric lens designs and expanded powers facilitate fitting for both astigmats new to contact lenses and those requiring residual astigmatism correction.
A study by Kurt J. Moody, OD, shows that the new expanded plus and high minus powers helped improve low astigmats' visual acuity and symptoms (such as ghosting, halos and glare). Study subjects preferred Acuvue Toric contact lenses to their habitual spherical soft contact lenses, and rated the quality of visual acuity the same as with their spectacles.
I believe that innovations in the fit and function of Acuvue Toric contact lenses provide an excellent option for prescribing comfortable, high-performance lenses to astigmatic patients. More importantly, eyes will reap the long-term benefits of well-designed, properly-fitted lenses.
Dr. Hoscheit served as a clinical investigator for Acuvue Toric contact lenses.
References are available upon request to the editors of Contact Lens Spectrum. To receive references via fax, call (800) 239-4684 and request document #90. (Have a fax number ready.)
Dr. Hoscheit practices in Gastonia, NC in a vertically integrated office with five optometrists and three ophthalmologists.