Astigmatism and Toric Soft Contact Lenses: Now Mainstream?

Posted by CooperVision on Friday, March 8, 2013

By Desmond Fonn

Introduction

Patients with refractive astigmatism who would rather wear contact lenses than spectacles should be fitted with toric soft lenses. This is a sweeping statement as some would argue that for visual reasons spherical rigid gas permeable (RGP) lenses will correct astigmatism resulting in visual acuity that would closely match their vision with spectacle lenses.

The argument is flawed if the refractive astigmatism is not all corneal, resulting in residual astigmatism induced by the RGP lens. Plus, there is always approximately 10% residual of the corneal astigmatism with a spherical RGP lens, but it is dependent on the refractive index of the RGP lens material. The range of astigmatism that I am referring to is 0.75 – 3.00D, and if vertexed to the corneal plane might be closer to 2.75D.

The reason for my bold statement is that the current inventory of toric soft contact lens designs would cater for that power range and manufacturing capability is so good that practitioners and patients can be assured of a predictable correction. There is little doubt as well about the vastly superior comfort of soft lenses [1]. In the low astigmatic range, masking of astigmatism with spherical soft lenses does not provide the level of vision of toric soft lenses [2-4], and the visual performance with aspheric lenses is also worse than soft toric lenses. [5]

Prevalence of Astigmatism

In an Australian study of 179 patients in the 1970s, Holden reported that 45% of the prospective contact lens patients exhibited 0.75 D or more of astigmatism and if the astigmatism of 1.00D or greater was corrected, 35% of the patients would require toric lenses but only 25% of the cohort exhibited 1.25 D or more.[6]

Young et al’s paper on the prevalence of astigmatism in relation to soft lens fitting included a summary table of 8 papers and showed that in a large age range, the prevalence ranged from 16 - 45% with astigmatism of 0.75 D or greater. [7] Young et al worked from a database of 11,624 patients (age range 8 to 70 years) and the prevalence of astigmatism of 0.75D group was 24.1% and 1.00 D or greater in both eyes was 15.0%.

The prevalence of astigmatism of 0.75 D or greater in myopes was almost twice that of hyperopes and the prevalence of with-the-rule (WTR) astigmatism was higher than against-the-rule (32.9% vs. 29.1%). Their summary showed approximately 33% of potential contact lens wearers require astigmatic correction based on the spectacle prescription from the database.

Cho et al’s paper on spherical and toric daily disposable lenses for low astigmatic corrections included a prevalence of astigmatism summary of 4 papers [9-12]. They wrote 46 to 63% of adults had astigmatism, but the percentage was dependent on the magnitude used as the threshold.

Percentage of the correctable population fitted with toric lenses

Quite appropriately the number and percentage of patients wearing toric lenses has grown quite substantially.

Here are some examples: In 1991, Pearson estimated that only 9% of all soft lens fits in the UK were soft torics [8]. Morgan et al reported that the proportion of toric soft lenses prescribed has grown from approximately 12% to 30% [13,14], and the trends in North America are very similar, perhaps as high as 38% [15,16]. Efron has stated that toric soft lenses now represent over 35% of all soft lenses prescribed, but this survey was restricted to seven countries with the exception of Japan where the percentage there is considerably lower.

Lens designs that stabilize the axis/orientation on the eye

The diameter and posterior surface shape or base curve of toric soft lenses are very similar to spherical lens dimensions, but the thickness profile will be substantially different in order to position the correcting cylinder in the correct meridian and to maintain it in that position. The methods include prism “ballast”, periballast, eccentric lenticulation, back-surface toricity, thin/thick zones, and various combinations of these designs.

Most of the recent design developments have incorporated the latter thickness differential designs, where the top and bottom lids govern the orientation of the lens and therefore the astigmatic correction and they seem to be working very well. In a study recently conducted at the CCLR (data on file), more than 60% of the silicone hydrogel toric lenses dispensed (three currently marketed toric silicone hydrogel lenses) had the same axis as the manifest spectacle correction and the axis of the remainder were within 10 degrees of the spectacle correction.

Reorientation or recovery refers to the toric lens’ ability to speedily relocate to the proper axis after the axis is misaligned, for example when physically misaligning the lens.

A number of terms to have been used to describe the lens recovery performance e.g., “reorientation speed”, or “rotational recovery”, Young et al. used “lens reorientation” to determine whether gravity affects the lens orientation or axis mis-location.

They found that the three prism-ballast design lenses rotated away from their baseline position “significantly more” than the accelerated stabilization lens design [18] and concluded that gravity does affect prism ballasted or peri-ballasted lenses rather than the thin/thick zone lenses when patients adopted a recumbent position. The four lens designs showed similar re-orientation speeds.

Fitting toric lenses

On the basis that inventory toric lenses will be used, the majority of practitioners’ inventory will be less than the 3000 lenses that Young has suggested will be needed for 90% of patients but this method of fitting and prescribing is the best. Even if the exact prescription isn’t within stock, a close approximation will with a high degree of certainty, confirm the lens that is required. The three elements that are necessary to fulfill the correct prescription, assuming that the lens fits symmetrically and centrally over the cornea without excessive movement, are:

  • 1. The refractive correction at the corneal plane
  • 2. The degree of rotation (if any) of the lens
  • 3. The rotational stability of the lens

If the refractive correction is known, the only function of the trial lens should be to ascertain if it fits correctly and to measure the amount of rotation of the lens and to compensate for the rotation when determining the final prescription. There are numerous texts and descriptive papers on when and how an over-refraction should be conducted, but that should not be necessary with the method that I have described.

Summary

As the average thickness of toric lenses is influenced by the by prism ballast or periballast and therefore increased thickness in parts of the lens, the oxygen transmissibility may be compromised and this will almost always be negated by the use of silicone hydrogel materials.

As silicone hydrogels,as a category, represent at least 60% of prescribed soft lenses there is every reason to believe that the same statistic applies to toric soft lenses. Fitting and prescribing should no longer be considered as specialty lenses except for those prescriptions that require custom designs.

As toric silicone hydrogel lenses are more expensive than spherical lenses practitioners should guard against non-compliance of exceeding the manufacturers recommended replacement frequency as this habit has been demonstrated in two studies [19,20].

References

1.      Fonn D, Gauthier CA, Pritchard N.Patient preferences and comparative ocular responses to rigid and soft contactlenses. Optom Vis Sci 1995; 72:857-863

2.      Snyder C, Talley DK: Masking of astigmatism withselected spherical soft contact lenses. J Am Optom Assoc 1989; 60:728-31.

3.       Richdale K,Bersten D, Mack K et al. Visual Acuity with Spherical and Toric Soft Contact Lenses in Low- to Moderate-Astigmatic Eyes. OptomVis Sci 2007; 84: 969-75.

4.       Cho P, Cheung SW, Charm J. Visualoutcome of Soflens Daily Disposable and Soflens Daily Disposable forAstigmatism in subjects with low astigmatism. Clin Exp Optom. 2012; 95: 43–47.

5.       Morgan PB,Efron SE, Efron N, Hill EA: Inefficacy of aspheric soft contact lenses for thecorrection of low levels of astigmatism. Optom Vis Sci 2005; 82: 823-8.

6.      Holden BA. The principles and practiceof correcting astigmatism with soft contact lenses.Aust J Optom 1975;58: 279–99.

7.      Young G, Sully A, Hunt C. Prevalenceof astigmatism in relation to soft contact lens fitting. Eye Contact Lens2011;37: 20 -25

8.      Pearson R. Contact lens trends in the UnitedKingdom in 1991. J Brit Contact Lens Assoc 1992; 15:17 - 23.9.      Anstice J. Astigmatism–its componentsand their changes with age. Am J Optom Arch Am Acad Optom 1971; 48: 1001–1006.

10.  Fledelius HC, Stubgaard M. Changes inrefraction and corneal curvature during growth and adult life. Across-sectional study. Acta Ophthalmol Copenh 1986; 64: 487–491.

11.  Satterfield DS. Prevalence andvariation of astigmatism in a military population. J Am Optom Assoc 1989;60: 14–18


12.  Read SA, Collins MJ, Carney LG. Areview of astigmatism and its possible genesis. Clin Exp Optom 2007; 90:5–19.


13.  Morgan PB, Efron N. Prescribing softcontact lenses for astigmatism. Cont Lens Ant Eye 2009;32: 97–98.


14.  Morgan P. Trends in UK contact lensprescribing 2010. Optician 2010; 239: 34–35.


15.  Barr JT. Contact lenses 2001. ContLens Spectrum 2002; 17:22–28.


16.  Nichols JJ. Contact lenses 2009. ContLens Spectrum 2010; 25:20–27.


17.  Efron N, Morgan P, Helland M et al.Soft toric contact lens prescribing in different countries. Contact Lens & Anterior Eye. 2011; 34: 36–38


18.   Young G, McIlraith R, Hunt C. Clinical Evaluation of Factors Affecting SoftToric Lens Orientation Optom Vis Sci 2009;86: E1259- E1266


19.  Richter D, Dumbleton K, Guthrie S, Woods C, JonesL, Fonn D: Patient and practitioner compliance with silicone hydrogel and dailydisposable lens replacement in Canada. Can J Optom 2010;72: 10-19


20.  Dumbleton K, Woods C, Jones L, Fonn D: Comfort andvision with silicone hydrogel lenses: Effects of compliance. Optom VisSci 2010; 

Fitting Toric Contact Lenses

Posted by CooperVision on Wednesday, January 23, 2013

As an eye care professional, you know that not all patients are the same with the same needs. Some patients may need a little extra time when it comes to fitting them with the right contact lenses. These include patients who have astigmatism that may need to be fit with toric contact lenses. As a world leader of toric contact lenses, CooperVision has the products and the expertise to help you successfully fit toric lenses.

Here are some helpful tips that you can use when you want to fit toric contact lenses:

Ask yourself the right questions:

  • Is this patient a candidate for toric soft contact lenses?
  • What material best meets the patient's vision and lifestyle needs?
  • Which lens is available for the parameters that are necessary to fit the patient?

Make sure that you know what tools are available to you.

CooperVision offers a variety of support when it comes to fitting toric contact lenses. In fact, we have an entire section devoted to you on our practitioner website that gives you easy access to a variety of helpful toric fitting information, including procedures for diagnostic and empirical fitting, helpful case studies, and much more.

Use the fitting guides offered by contact lens manufacturers.

It is essential to fit your patient correctly with a toric contact lens if you want a successful practice. So why not use the tools offered to you by contact lens manufacturers that can help you do that? CooperVision offers a great tool called the ToriTrak Calculator that is the most accurate method of fitting our toric contact lenses and is designed to eliminate errors in estimating lens rotation.

No matter what your fitting needs may be, CooperVision is able to provide you with the tools to fit your toric patients.

Prescribing For Astigmatism

Posted by CooperVision on Wednesday, April 4, 2012

For eye care professionals, fitting contact lenses for patients with astigmatism can be challenging. However, the challenges can be worth the effort for some patients. Select astigmatic patients may benefit from contact lenses that can offer better vision than eyeglasses. Contact lenses may provide clearer vision and a wider field of view than eye glasses.

Soft contact lenses conform to the shape of the eye; which means that standard soft lenses may not seem effective in correcting astigmatism. But special toric soft contact lenses are available to provide a correction for many types of astigmatism. In fact, even patients who have low astigmatism (0.75DC to 1.00DC) benefit visually when fitted with toric soft lenses instead of with spherical soft lenses. A recent study reported that up to 30% of astigmatic patients reported that they had not tried toric soft lenses because their eye care practitioner did not recommend them. One reason why eye care practitioners shy away from prescribing toric lenses for low astigmatic patients is because they believe that patients can tolerate a constant slight blur from a spherical lens better than a sporadic blur from a rotationally unstable toric lens. So how do eye care professionals ensure the best fit for toric lenses on their astigmatic patients?

The key is that eye care practitioners are consistent with their fits. CooperVision offers eye care practitioner tools such as a ToriTrak Calculator , which can help eye care practitioners fit their astigmatic patients. It is the most accurate method of fitting our toric contact lenses and is designed to eliminate errors in estimating lens rotation. Make sure to register for other fitting tools here and to follow us on Twitter @CooperVision for more information.

What Is Astigmatism?

Posted by CooperVision on Friday, February 17, 2012

While most people know about refractive errors of the eye such as nearsightedness or farsightedness, some eye care patients may ask, “What is astigmatism?” after their eye exam with an eye care practitioner. Astigmatism is a common refractive error that causes vision to be out of focus because the cornea is abnormally curved. While it is not known what causes astigmatism, it is typically present from birth and accompanied with other refractive errors such as nearsightedness or farsightedness. It can even occur after specific types of eye surgery such as cataract surgery.

Astigmatism is commonly diagnosed early in life, so it is important to make sure to be aware of the symptoms and schedule eye exams for younger children. Symptoms that can occur with astigmatism are blurred vision, headaches or eye strain after prolonged tasks such as reading. Astigmatism can be detected during a simple eye exam. The test for astigmatism that an eye practitioner uses is known as a retinoscopy. However, this test is slowly being replaced with an automated refraction.

Astigmatism can be treated in three ways: traditional glasses, surgery and contact lenses. While astigmatism had to be treated with hard contact lenses in the past, it can now be treated with soft toric lenses such as CooperVision’s Biofinity Toric line. CooperVision has created this toric lens for patients with astigmatism with two major benefits. It is made from silicon hydrogel which allows for more oxygen to pass through the eye. It is also our most stable toric design. When a patient with astigmatism wears a toric lens, it is essential that the lens is stable so that when the patient blinks, the lens doesn’t move too much. A toric lens needs to stay in place because there are two different powers in the lens versus a traditional spherical lens which is all one power. Toric lenses must remain in position on the patient’s eye for optimal visual acuity.

Remember, astigmatism is common and simple to detect. So if you want to learn more about astigmatism, ask your eye care professional. For more information on CooperVision’s Biofinity Toric lenses, visit our product page here .

Exceptional Stability, Vision and Comfort All in One Lens

Posted by CooperVision on Thursday, February 24, 2011

Avaira Toric contact lenses are the latest addition to our 2-week silicone hydrogel family of lenses. So if you have an astigmatism, you now have the option to wear a lens that has an exceptional ability to stay moist and transmit high levels of oxygen through the lens material as a result of our exclusive AQUAFORM® Comfort Science.  The unique material binds water within the lens without the need for additional surface treatments or wetting agents – giving you a comfortable lens day after day.

Avaira Toric at a glance:
• Moist and comfortable from the inside out
• Lets plenty of oxygen flow through to your eye
• Visual performance you can count on
• Softer, more flexible lens material incorporating a UV blocker*

As a world leader in toric contact lenses, CooperVision’s Avaira Toric lenses are designed for consistent fit and visual performance – helping you see clearly every time you blink. Click here for information on rebates for Avaira Toric contact lenses.

*Warning: UV-absorbing contact lenses are not substitutes for protective UV-absorbing eyewear, such as UV-absorbing goggles or sunglasses, as they do not completely cover the eye and surrounding area. Patients should continue to use UV-absorbing eyewear as directed.

About On Eye

On Eye is the contact lens blog from CooperVision. On this site, you will find insights about fitting, technology, and the business of contact lenses. The On Eye blog is designed to meet the needs of both Eye Care Practitioners and consumers. ECP and medical professional-specific portions of the blog will be password protected in order to protect and reserve the privacy of the profession. To read more about our terms of use, please see the Legal tab.

Calendar

<<  April 2014  >>
MoTuWeThFrSaSu
31123456
78910111213
14151617181920
21222324252627
2829301234
567891011

View posts in large calendar