Article Date: 7/1/2004

MASKING ASTIGMATISM
Masking Cylinder with Aspheric Soft Lenses
This study found that an aspheric lens can improve the vision of low-cylinder patients.
By Nimesh Patel, OD, Linda L. Edmondson, AM, OD, FAAO, and William Edmondson II, MAT, OD, FAAO

Fitting individuals who have astigmatism is an art. When the refractive astigmatism equals that of the corneal toricity, we tend to lean toward prescribing GP lenses. However, this isn't always the best answer. When refractive cylinder is low to moderate (­0.25D to ­1.50D), you can successfully prescribe a soft contact lens.

Since the introduction of soft hydrogel lenses by Griffin Laboratories in the 1960s, claims have existed that soft lenses mask astigmatism. The company claimed that its soft spherical lens could mask anywhere from ­0.50D to ­2.00D of astigmatism. Research by Isen (1972), Gasson (1973) and Hodd (1980) supported this. Of note, many other studies show that similar soft lenses can mask no more than ­0.30D of astigmatism.

With the advent of wavefront technology, we know that a refraction entails more than just compensating for sphere and cylinder. A major source of decreased vision results from spherical aberrations. Theoretically, an aspheric lens should reduce this aberration and better focus light on the retina.

One lens brand that we use in our clinic is the Frequency 55 (CooperVision) lens. This lens is available in both spherical and aspheric designs, which enables us to see what differences the optics of a lens can make in visual outcome. Having a large database of patients available, we conducted a retrospective study including a large data pool. Because we sought to compare spherical lenses to aspheric lenses, we used only the two Frequency 55 lenses, as compared to other studies that have used more than two spherical lenses in their comparison.

Figure 1. The aspheric lenses masked significantly more astigmatism than the spherical lenses.

Performing the Study

We conducted a retrospective study that included all patients fit with the Frequency 55 and Frequency 55 aspheric at the Northeastern State University Oklahoma College of Optometry. Of the 133 eyes fit with these lenses, we excluded those that didn't have an astigmatic component or over-refraction.

We created a data set that included each patient's spectacle refraction, over-refraction, keratometer readings, visual acuity with the lenses and the amount of astigmatism masked. The keratometer readings reflected each patient's initial readings before the start of contact lens wear. We used the most recent spectacle refraction performed on each patient, and we used the over-refraction performed at each patient's last visit, after the patient had worn his lenses for more than three hours.

We measured visual acuities from a standard projector Snellen chart. We took keratometer readings using either an autokeratometer or manual keratometer. We performed non-cycloplegic refractions behind a standard phoroptor.

We divided the data into five different sets. The first set contained data for patients who were wearing spherical contact lenses. The second set contained all the patients who were wearing the aspheric lenses. The other sets included patients fit with the aspheric lenses, but we split these into with-the-rule (WTR), against-the-rule (ATR) and oblique astigmatism. We defined WTR astigmatism as 30 degrees on either side of 180 degrees, ATR as 30 degrees on either side of 90 degrees and oblique as everything in between.

We compared the amount of astigmatism masked within the different groups. We also compared the final visual acuity of all the eyes fit with the spherical vs. the aspheric lenses.

Reviewing the Results

Table 1 shows the profiles of the patients who participated in this study. Of the 133 eyes that were wearing the Frequency 55 spherical or aspheric lens, 71 used the aspheric lens and had an astigmatic correction, and 21 eyes used a spherical lens and had a measure of astigmatism. Forty-one eyes fit with either lens had no astigmatic correction or over-refraction. We included these 41 eyes in the comparison of visual acuity measures, but not in comparing the astigmatic data.

We found that 69 percent of the study patients fit with contact lenses had some measure of astigmatic refractive error, which is similar to the norm. The patients in this study had up to ­1.75D of astigmatism. The average refractive astigmatic error for patients fit with spherical contact lenses was ­0.4D, with a standard deviation of 0.15. Patients fit with the aspheric contact lens had an average refractive astigmatic compensation of 0.7D, with a standard deviation of 0.29.

The Frequency 55 spherical lenses masked on average 21 percent of the total refractive astigmatism. The Frequency 55 aspheric lenses masked 49 percent of the astigmatism on average (Figure 1). This difference in masking was statistically significant (p=0.0018).

The aspheric lenses appeared to mask more astigmatism in ATR and oblique axis refractive corrections. The aspheric lenses masked on average 66 percent of the refractive astigmatism for ATR study individuals, 63 percent of the astigmatism for patients who have oblique astigmatic errors and 32 percent of the astigmatism for WTR patients (Figure 2). When we compared WTR to ATR and oblique axis, we noted statistical significance (WTR vs. ATR p=0.038, WTR vs. oblique p=0.017).

Comparing acuities, we found that the aspheric contact lenses provide much better acuity than the spherical contact lenses (p=0.018). Patients who were wearing aspheric lenses measured approximately half a line better for acuity than did patients who were wearing spherical lenses.

 

TABLE 1 Study Patient Profile

  SPHERICAL PATIENTS ASPHERIC PATIENTS
Number of patients 21  71
Mean Astigmatism 0.40D 0.69D
Standard Deviation 0.15D 0.29D
Median 0.50D 0.75D
Low 0.25D 0.25D
High 0.75D 1.75D

 

What Other Studies Have Found

Generally, when the corneal toricity is equal to the refractive astigmatism, we prefer to fit a GP lens. But in certain instances, a soft spherical or soft toric lens may be the lens of choice. Clinically, we've noticed that patients will tolerate a mild to moderate amount of residual astigmatism while wearing hydrogel and GP lenses. When patients who have low (less than ­1.00D) amounts of astigmatism present to the clinic, we often prescribe a soft lens, especially when we suspect that the astigmatism is internal. The greater the ratio of the spherical power to the cylindrical power, the better your chances of successfully fitting a spherical soft lens. Rakow (2000) states that an ideal ratio is three to one.

Soft contact lenses theoretically shouldn't mask astigmatism because they drape, matching the corneal curves. In practice, hydrophilic lenses flex variably on the cornea and make it difficult to predict residual astigmatism. Researchers most intently studied hydrogel lens masking when the lenses first appeared in the 1960s. Interest in this area has persisted over time as new lenses and technologies became available.

Snyder and Talley (1989), conclude that we shouldn't expect spherical soft contact lenses to predictably mask astigmatism. The researchers incorporated their data with that of Wechsler et al (1986), to include five different lens types. Both studies show that the lenses investigated unpredictably masked astigmatism. Not all of the subjects in these studies experienced lens wear, and patients wore the study lenses only in office.

In another study of the Bausch and Lomb SofLens, Sarver (1972) noted that the mean residual astigmatism was about 84 percent of the refractive astigmatism. In Sarver's data, the lens masked approximately 16 percent of astigmatism. Harris et al (1979) noted a statistically significant reduction in refractive astigmatism, but they didn't find any differences among the lenses they had used (B&L SofLens, Wesley-Jessen Hydrocurve II and AOSoft). In another study headed by Harris (1996) of CIBA Vision Clarity, Vistakon Acuvue and Vistakon Surevue lenses, the researchers showed that Surevue seemed to mask more astigmatism than the other two lenses, although the difference wasn't statistically significant.

All of the studies noted that some patients presented with a higher residual astigmatism on over-refraction. The paper by Snyder actually documented that the astigmatism increased in 16 percent of the cases. He linked this increase in the astigmatic correction to the difference between the refractive index of the cornea and the contact lens. Because, in theory, soft lenses drape across the cornea and respect the curves of the cornea, we expect this difference in the astigmatic over-refraction. Again, this isn't evident in most patients who wear soft lenses, which indicates that a perfect drape across the cornea doesn't occur.

Acuity is a major concern when lens wear results in uncorrected astigmatism. Dabkowski et al (1992) compared toric lenses vs. spherical lenses fit on low cylinder myopes. They found that the toric lenses provided a better Snellen acuity on two-week follow up, but no difference in the contrast sensitivity existed between patients fit with spherical and toric contact lenses.

Research by Cho and Woo (2001) showed that high- and low-contrast logMAR charts allow for better comparison of acuity. Their study examined whether thicker or thinner lenses provided better acuity. Using lenses that have center thicknesses of 0.06mm and 0.12mm, they noted that the thicker lenses provided better acuity, though this difference wasn't statistically significant. Cho also noted that in 70 percent of the cases in which patients wore the thicker lens, the astigmatic over-refraction was significantly reduced.

Gundel et al (1988) also used different contrast charts. They noted that spherical lenses reduced contrast sensitivity when fit on astigmatic eyes. They found the greatest reduction in the lower spatial frequencies that the standard Snellen chart doesn't measure. They weren't able to attribute the reduction in contrast to the hydrogel material or to the uncorrected astigmatism.

Other studies have examined whether lens thickness helps mask astigmatism. Harris et al (1996) used contact lenses with center thicknesses between 0.04mm and 0.22mm lenses and found no statistically significant difference in the astigmatism masked. Bernstein et al (1991) also supported the notion that lens thickness has no influence on the amount of astigmatism masked. Both studies noted that the amount of astigmatism masked was variable and unpredictable. Most other research on this topic has reached this conclusion.

 

TABLE 2 Frequency 55 Aspheric and Spherical Lens Parameters

LENS  MATERIAL CENTER THICKNESS DK BC (MM) DIAMETER (MM)
Frequency 55 Methafilcon A 0.07mm 18.8 8.4, 8.7, 9.0 14.2
Spheric (55% water)                 
Frequency 55  Methafilcon A 0.09mm 18.8 8.4, 8.7 14.4 
Aspheric  (55% water)             

 

A Look at Our Data

Our retrospective study found a definite difference in the astigmatism masked by the spherical vs. the aspheric lenses. As we stated previously, the lenses that we used were both made of the same material and by the same company. The aspheric lens is slightly larger (by 0.2mm) and thicker (by 0.02mm) (Table 2) than the spherical lens. We know of no research on soft lenses which would indicate whether these differences are significant enough to cause the results that we see in our data.

We feel that the aspheric design played a large part in the amount of astigmatism masked. Theoretically, the aspheric lens should reduce a number of aberrations and also reduce the circle of least confusion. This would explain why low astigmats probably see better with these lenses. Clinically, using the Shack-Hartmann wavefront analyzer, it's hard to realize a difference in the aberrations between spherical and aspheric lenses (Ross et al, 2002).

Our data also shows that there's a significant difference between ATR, WTR and oblique axis masking. We noticed that the lenses masked ATR and oblique axis refractive errors more than they masked WTR refractive errors. The previous research data hasn't noted this. More research into this area is required to prove or disprove this hypothesis.

Figure 2. The aspheric lenses masked differing amounts of astigmatism depending on the type of astigmatism.

What are the Clinical Implications?

Our data has clear clinical implications. Correcting small amounts of astigmatism is becoming an important part of practice as we see more patients who are interested in Corneal Refractive Therapy (CRT) and LASIK. Often we find residual astigmatism after either of these procedures that is a problem for some patients.

Many LASIK patients present with low amounts of astigmatism that they find bothersome. In rare instances, the procedure results in residual central islands, which cause irregular astigmatism. In many cases in which the surgeon needs to reposition a flap, irregular astigmatism results. Our best option in such cases would be to fit these patients with a reverse geometry GP lens. However, for occasional wear, GP lenses are probably not the lens of choice, and you should consider a soft aspheric lens.

Chou and Wachler (2001) presented a case report in which certain lenses with certain center thicknesses helped reduce the amount of residual astigmatism and irregular astigmatism. In their case study, it wasn't predictable which lens would provide the better acuity and greater masking. For some time we have known that thick contact lenses help keratoconus patients in the early stages of the disease. Seeing the effects of aspheric lenses in our study, we would expect aspheric lenses to help reduce irregular astigmatism just as thick lenses do, which increases the number of lenses we can choose from when fitting these patients.

CRT patients complain that they occasionally experience slight distortion with some reduction in vision. Topographies often show a slightly decentered lens on that particular day. Refraction on those days may be a spherical equivalent of plano. Prescribing an aspheric lens for occasional wear has, in our practical experience, proven to help many of these patients. Daily disposable lenses have also proven beneficial (and more convenient). An aspheric daily design (currently not available) would probably be the ideal lens of choice for these patients.

 

Dr. Patel is an assistant professor at the Northeastern State University Oklahoma College of Optometry.

Dr. Linda Edmondson serves as a contact lens clinical faculty member and as chief of the Low Vision Services at the Northeastern State University Oklahoma College of Optometry.
Dr. William Edmondson serves as chief of the Contact Lens Clinic at Northeastern State University College of Optometry and as a clinical instructor in the Acute Eye Care Clinic at W.W. Hastings IHS Hospital.

The authors have no financial interest in any of the products mentioned in this article.

To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #107.

 


Contact Lens Spectrum, Issue: July 2004