TOPIC: Soft Torics Versus Spherical GPs for Correcting Astigmatism

Point: Soft Torics are the Lenses of First Choice for Astigmatism

By Carole Maldonado-Codina, PhD, MCOptom, FAAO, FBCLA

In general, astigmatism of more than 0.75D is considered “significant” and is the level at which most eyecare practitioners (ECPs) would consider correcting it with contact lenses (Young et al, 2002). Is it better to use spherical GP or soft toric (ST) lenses to correct astigmatism?

A recent study in the United Kingdom reports that the prevalence of patients who have astigmatism ≥ 0.75D in at least one eye is 47 percent (Young et al, 2011). Worldwide prescribing data show a significant decline in the number of GP lenses being fitted (Morgan et al, 2011). These data also show that the level of ST prescribing now closely matches the proportion of prospective lens wearers who have significant astigmatism in many markets. In effect, these data already tell us that ST lenses are now the lenses of first choice for astigmatism.

Defining Lens Wear Success

For patients, a contact lens needs to satisfy the following criteria:

1. Good Vision GP lenses have a reputation for providing crisp vision, but they are not immune from other factors that can affect vision such as deposition and poor wettability. Fitting GP lenses can be complicated by induced astigmatism where refractive astigmatism is not corneal in origin, and glare/flare problems are not uncommon in patients who have large pupils. ST lenses can overcome all of these difficulties. Improvements in ST designs mean that lenses stabilize quickly (Edrington, 2011) and are more reproducible, offering patients excellent vision. Ever-expanding parameters mean that more patients can be successfully fitted.
2. Comfort New GP wearers experience reduced comfort compared to new soft lens wearers (Morgan et al, 2003). Any period of discomfort in a new wearer is perilous in terms of their perseverance and potential discontinuation from lenses. Adaptation/re-adaptation problems with GP lenses can be a significant issue for part-time wearers.
3. Safety There is no doubt that GP lenses have a better track record in terms of inflammatory events (Morgan et al, 2005) and microbial keratitis (Dart et al, 2008; Stapleton et al, 2008). However, a recent U.K. survey indicates that although ECPs are aware of the ocular health benefits that GP lenses provide, most do not think that these benefits outweigh the perceived time-consuming nature of fitting these lenses and the belief that the lenses are uncomfortable (Gill et al, 2010). Adverse events in GP lenses are mainly of mechanical origin (Morgan et al, 2005) and can be particularly problematic if a spherical GP begins to compromise a more toric cornea. With the advent of silicone hydrogel materials, STs now rival the oxygen performance of GP lenses.
4. Convenience Manufacturing costs mean that GP lenses cannot be prescribed on a truly disposable basis. We now have ST lenses available on a daily disposable basis. Aside from convenience, there is evidence to suggest that more frequent contact lens replacement is advantageous in terms of vision, comfort, and ocular health.
5. Affordable Manufacturing technology is such that both contact lens types are now affordable—even daily disposable ST lenses. However, in many cases, loss of a single GP lens can significantly increase the running costs of these lenses.

A Healthy Future

Soft lens correction of astigmatism has advanced significantly in recent years. Enhanced parameters, better reproducibility, more sophisticated designs, better comfort, and improved materials mean patients no longer have to compromise. Prescribing trends reflect the increased confidence ECPs have in these lenses, which stands our profession in good stead for a healthy future. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #191.

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Dr. Maldonado-Codina is a lecturer in Optometry in the Faculty of Life Sciences at The University of Manchester and associate director of Eurolens Research. She has received research funding from AMO, Alcon, B+L, Ciba Vison, CooperVision, Johnson & Johnson, and Sauflon Pharmaceuticals.

Counterpoint: When Correcting Astigmatism, Vote Yes to GP Lenses

By Edward S. Bennett, OD, MSEd, FAAO

It is evident that soft toric lenses have numerous benefits and applications and, in recent years, have undergone many improvements. That said, GP lenses have numerous—and increasing—benefits that merit consideration for astigmatic patients.

Why You Should Try GP Lenses

Following are the benefits of GP lenses over soft torics.

1. Quality of Vision Vision quality continues to be an important benefit of GP lenses. In studies in which both soft and GP modalities were worn, patients preferred the vision of GP lenses (Fonn et al, 1995; Johnson and Schnider, 1991; Michaud et al, 2009). In a recent study, 20 highly astigmatic subjects (i.e., all >1.75D corneal cylinder; average refractive cylinder was –3.62D) wore soft toric lenses for one month and GP bitoric lenses for one month (Michaud et al, 2009). Although only two subjects were GP wearers entering the study (10 were wearing soft torics), 14 of 19 subjects completing the study preferred the vision of GPs.

Soft toric multifocal designs are limited in availability and compromised in optical quality due to the incorporation of both corrections. The ability of GP lenses to reduce higher-order aberrations is also a factor in their success in visual performance (Dorronsoro et al, 2003; Gemoules and Morris, 2007; Villa-Collar et al, 2009). In one study in which subjects were corrected with multiple modalities, correction with GPs resulted in significantly better optical quality compared to soft lenses or spectacle lenses (Hong et al, 2001).

The perceived differences in the complexity of the fitting process between soft toric and GP lenses are lessening as well. This is not only the result of improvements in manufacturing quality resulting in consistent, high-quality standard lens designs, but also the ability to fit spherical, toric, and multifocal GPs empirically with very good success such that patients often experience the hallmark benefit of good vision upon application of the initial lenses (Benoit, 2009; Pitts et al, 2001).

2. Ocular Health The low risk of ocular infection and inflammation due to the absence of limbal compression and the greater movement with the blink have always been benefits of GPs. In fact, GPs exhibit the least risk of infection (Stapleton et al, 2008). Although silicone hydrogels exhibit much greater oxygen permeability than do hydrogel materials, they have not reduced the risk of eye infection, especially if worn overnight (Stapleton et al, 2008; Schein et al, 2005).

3. Benefits to Young People Overnight orthokeratology (OOK) may reduce both myopia and also axial length progression and vitreous chamber depth by approximately 50 percent, compared to little to no effect for soft lens wearers (Walline, 2007) and spectacle wearers (Cho et al, 2005). When subjects wore both OOK and soft modalities in another study, more than two-thirds chose OOK at the conclusion of the study (Lipson et al, 2005).

What About Lens Awareness?

The initial wearing experience with GP lenses is better today due to such factors as the use of a topical anesthetic as well as newer, ultrathin designs with aspheric or pseudo-aspheric peripheries (Schnider, 1996; Bennett et al, 1998). However, the most important factor in providing initial GP comfort rivaling that of soft lenses is the increasing use of larger-diameter GP designs. In fact, smaller scleral designs are becoming more available to regular astigmatic patients and could easily represent a very viable alternative to soft toric lenses in the next few years (Jedlicka, 2010).

GP lenses should always be an option when fitting astigmatic patients. Soft toric wearers who are experiencing blurred vision due to uncorrected cylinder or lens rotation deserve the opportunity to experience better vision. CLS

For references, please visit www.clspectrum.com/references.asp and click on document #191.

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Dr. Bennett is assistant dean for Student Services and Alumni Relations at the University of Missouri-St. Louis College of Optometry and is executive director of the GP Lens Institute. You can reach him at ebennett@umsl.edu.