Contact Lens Spectrum

CONTINUING EDUCATION - No Fee Required
Prescribing Soft Toric Contact Lenses
Prescribing soft torics is easier than you may think and is a skill that benefits many of your astigmatic patients.
BY PETER D. BERGENSKE, OD, FAAO

While spherical soft lenses satisfy a majority of patients, the use of toric soft lenses for astigmats continues to grow. Lens quality and precision in fitting have steadily improved for high and especially moderate astigmats. Estimates are that 75 percent of myopic astigmats have between 0.75D and 1.50D of astigmatism, and meeting their needs is a routine part of a successful contact lens practice.

Although GP lenses provide advantages for some astigmats, particularly those who have irregular corneal astigmatism, soft toric lenses are preferable for many patients who have low to moderate astigmatism in which a toric soft lens provides comparable acuity combined with greater comfort. With internal astigmatism, prism-ballasted GP lenses suffer from the vagaries of lens position and rotation, offering no real advantage over soft toric lenses.

Toric soft contact lens fitting is no longer a game of guesswork and finger crossing because sophisticated lens designs and systematic fitting systems have evolved into a clinical science that every practitioner can and should master.

Patients who have 0.50D or less of refractive cylinder are usually best fit with spherical soft lenses, while those who have 0.75D cylinder have yielded consistently better acuity and patient satisfaction with toric vs. spherical soft lenses. In addition, approximately one-third of patients have at least 1.00D of refractive astigmatism. Although it's possible to reasonably satisfy some of them with spherical correction, it doesn't provide optimal vision for many.

Soft spherical lens "masking" of cylinder is little more than myth. Although a thick soft contact lens may mask some small amounts of corneal surface irregularities, even lenses of substantially exaggerated center thickness have little effect on corneal cylinder. While aspheric surfaces have succeeded in improving acuity and satisfaction for some astigmats, no solid evidence of cylinder masking or correction exists. Front surface aspheric lenses apparently work by decreasing aberrations other than astigmatism. In eyes for which these aberrations are significant, the vision effect can be positive even though the residual astigmatism remains uncorrected. Perhaps this technology is a precursor to wavefront aberration-corrected lenses, which may eventually allow customized correction of a host of aberrations.

TABLE 1 Determination of Base Curve Radius Depending on Lens Diameter and "Effective K"
LENS DIAMETER (MM)	BASE CURVE FLATTER #009;THAN EFFECTIVE K BY
12.5	0.1mm
13.0	0.3mm
13.5	0.5mm
14.0	0.7mm
14.5	0.9mm
15.0	1.1mm
15.5	1.3mm
16.0	1.5mm

Soft toric lenses must fulfill the same good fitting criteria as spherical soft lenses to achieve good comfort, physiology, vision and precise correction of astigmatism. To ensure centration and minimize lens rotation, standard toric soft lenses are generally larger than spherical lenses, which increases sagittal depth and effectively steepens the fit. We often overlook the significance of lens diameter while paying undue attention to corneal and contact lens radius. Diameter affects fit more than radius, particularly with larger lenses.

Fortunately, the majority of corneas fall within a small range of diameters, and the average horizontal visible iris diameter (HVID) is about 11.8mm. The ideal overlap of a toric soft lens onto the conjunctiva is 1.0mm to 1.5mm on each side, and most toric soft lenses have diameters between 14.0mm and 15.0mm.

HVID becomes important when the corneal diameter is significantly greater or smaller than standard (approximately <11.3mm or >12.3mm). For smaller corneas, the standard lens diameter/base curve radius combination will have excessive sagittal depth and result in vaulting or a steep fit, impingement on conjunctival circulation, limbal epithelial hypertrophy and possible neovascularization. Conversely for the larger corneas, standard lens diameter/base curve radius combinations will likely fit unacceptably loose, causing decentration, rotation and lens awareness.

With small variances from normal HVID, adjusting the base curve radius is helpful, but some standard soft toric lenses are currently available in but one radius and diameter, which may necessitate custom fitting. In custom designing a lens for the abnormally large or small cornea, consider both the radius and diameter (the sagittal depth), and remember that corneal diameter alters the effect of the radius. This altered radius effect is called "effective K."

A rule of thumb (based on complex sagittal depth calculations) states that for each millimeter of diameter change, the effective K changes by 5.00D "K," or about 1.0mm in radius. Therefore, the effective K changes 1.00D for each 0.20mm change in diameter. For example, if 11.8mm is the "standard" HVID, then a 12.0mm cornea with 45.00 K reading has a sagittal depth equivalent to an 11.8mm/46.00D cornea, and an 11.6mm cornea with the same radius has sagittal depth equivalent to an 11.8mm/44.00D cornea. Use this thinking to determine the final lens radius.

Generally, determine lens diameter by adding 3.0mm (allowing 1.5mm overlap) to the measured HVID. Calculate lens base curve radius by using another rule of thumb: make the radius of the lens flatter than the effective K radius by about 0.20mm for each millimeter of lens diameter greater than 11.8mm. For simplification, determine the diameter by choosing the desired overlap. Calculate effective K using the measured K and the HVID as described above. Convert effective K to millimeters, then adjust it based on lens diameter (Table 1).

For toric corneas, use the mean K as a starting point. Small errors in base curve radius have little effect on overall fit. This discussion about lens diameter and radius applies as well to custom-design spherical lens fitting, though custom design in toric lenses is more common and lens diameters tend to be larger.


Figure 1. Thickness profile of prism-ballasted lens.

Figure 2. Thickness profile of a double thin zone lens.

Astigmatic correction lenses have a toric curve within the optical zone of the front surface (front toric lenses) or the back surface (back toric lenses).

Toric lenses need to rotate and orient consistently, and you can usually achieve this by using a prism as "ballast" -- with or without inferior slab-off or eccentric lenticulation -- and several lens designs have no prism but use a double slab-off (double thin zone) geometry (Figures 1 and 2).

Toric soft lenses are generally categorized by both the cylinder location (front vs. back) and the method of stabilization (prism ballast vs. double thin zone), yielding four possible combinations (Table 2). There's little theoretical consensus in choosing one lens design over another, and practitioners most often employ trial and error clinically.

Past thinking was that back surface torics are more stable in toric corneas and front torics (with a spherical back curve) are more stable on spherical corneas. Neither has proven to make significant, consistent differences, nor do double thin zone designs have advantages over prism-ballasted lenses for against-the-rule cylinder. However, a front toric or back toric design may affect fitting in cases of high cylinder. This is because the toric optic zone is oval, which may affect optic zone size with back toric lenses and front surface interaction with the upper lid during the blink for front toric lenses. Often the diameter of the larger toric lens counteracts these effects, but they're worth considering when using trial lenses of low cylinder power to predict fitting characteristics of a high cylinder power lens.

The upper lid largely controls orientational stability during blinking, which tends to push away and rotate the thicker portions of the lens more than the thinner portions. This so-called "watermelon seed" effect explains why the double thin zone can prove effective even without prism.

The upper lid should properly orient an acceptably fitting toric lens on the eye. Sometimes the slight lateral vectors of upper and lower lid cause a limited clockwise or counterclockwise rotation of the lens. Limited lens rotation is normal, easily compensated for and not problematic if it's consistent and stable. Unstable or variable rotation is unacceptable as it varies cylinder effect. To determine stability and final rotational position, place a trial lens on the eye and allow it to equilibrate for up to 15 minutes. For accuracy, the trial lens should optimally have the identical base curve and diameter of the intended final lens, and ideally it should have the same cylinder power and axis. When no trial lens reasonably close to the predicted final lens is on hand, it's often better to order a lens with the required parameters that assumes no rotation and then use this for final determinations.

TABLE 2 The Four Basic Possible Designs for Soft Toric Contact Lenses
Prism ballast Front cylinder (Spherical base curve)	Thin zone Front cylinder (Spherical base curve)
Prism Ballast Back toric (Front sphere, toric base curve	Thin zone Back Toric (Front sphere, Toric base curve)

With prism-ballasted lenses, prismatic effect occurs throughout the lens. The amount of prism differs among toric lens designs, but it's generally 1.0 to 1.5 prism diopters. This may cause vertical binocularity problems with a non-prismatic lens on one eye and a prismatic lens on the other. Conversely, the prism may prove advantageous when you require base down prism of the appropriate amount for good binocularity over the same eye. You can create larger amounts of prism in custom lenses with or without cylinder correction, but prism direction other than base down is generally impractical.

The main challenge of toric soft lens fitting is to properly align and maintain the correcting soft lens cylinder with the refractive cylinder, and an acceptably stable fit helps. Non-customized "stock" lenses usually provide an adequate fit and rotational stability, and astigmatic correction for most patients is quite simple.

Corneal astigmatism is usually regular, and the "masking" and flexure of soft lenses negligibly affects the needed refractive power. Make vertex power compensation for both meridians when converting from spectacle plane to corneal plane, especially in higher lens powers, to avoid overcorrection of the cylinder for myopic astigmats and undercorrection for hyperopic astigmats.

After making vertex power compensation, choose a trial lens of appropriate sphere and cylinder power. Assume that the initial trial lens will have no rotation, or that the 90-degree meridian of the lens (and the base-apex meridian of a prism-ballast lens) will coincide with the 90-degree meridian of the eye. Dispense the lens when rotation is zero, visual acuity is acceptable and other aspects of fit and comfort are good.

Note that even if lens rotation and axis orientation aren't perfect but acuity, comfort and overall fit are good, no further change is necessary. Conversely, if lens rotation isn't zero and visual acuity is unacceptable, then you must change the lens. The two common approaches are to simply compensate for the rotation or to perform a spherocylindrical over-refraction and calculate the crossed cylinder effect.


	Figure 3. Scribe markings for assessing toric lens rotation.

Compensating for rotation assumes that sub-standard acuity entirely results from lack of coincidence of the lens cylinder axis and the refractive cylinder. For stable and consistent misalignment, compensation is straightforward: Measure or estimate the amount of rotation and note the direction of rotation. Assume that other lenses with the same parameters will show the same direction and degree of rotation, so that if the first lens rotates 10 degrees counterclockwise, then a 10-degree clockwise rotation allows the cylinder axis to fall in the desired position.

Toric soft contact lenses include scribe markings to help in assessing lens rotation (Figure 3). Typically the marking indicates the 90-degree meridian at the bottom, or six o'clock position, though some mark the 180-degree meridian. It's common to include ancillary markings that are 10 degrees to 30 degrees to either side of the main marking. Estimate rotation by observing at the slit lamp, remembering that one clock hour corresponds to 30 degrees of rotation. Clinicians tend to underestimate rotation, especially with lenses that have a single marking at the six o'clock position.

You can more accurately measure rotation by rotating the slit lamp of the biomicroscope beam to correspond with the rotation of the scribe mark. You can then read the rotation directly off of a scale on the microscope or project it onto a protractor for this purpose (Figure 4). Accurate measurement of rotation is useful when applying the "left add, right subtract" (LARS) technique for rotation adjustment and when needed for calculating over-refraction information.

The goal is to align the cylinder axis of the lens with the refractive cylinder axis of the eye. A common error is to expect that compensation for rotation results in the lens markings to indicate no rotation, when in reality the rotation of the second lens must be the same as the rotation of the initial lens.

To compensate for rotation, apply LARS when you view the lens rotation facing the patient and in relation to the six o'clock position of the eye. For example, when projecting a vertical slit lamp beam onto the eye, you note that the scribe marking of the lens rotates 10 degrees to the left of the slit lamp beam and requires you to add 10 degrees to the refractive cylinder axis. If the refractive cylinder axis is 90, then you should order axis 100. Again, this assumes the new lens will rotate the same as the initial one. Although in many instances LARS provides adequate axis compensation, many errors can encroach and you may need sphero-cylindrical over-refraction.


Figure 4. Use slit rotation measurement to determine lens rotation on the eye. Place the light power straight ahead between the slit lamp objective.

Toric soft lenses are usually thick and may affect refraction, particularly in higher powers and with irregular astigmatism because of draping, masking and tear layer effects that render LARS an oversimplification. Over-refraction application varies, but the principle of cross-cylinder calculation remains constant.

Over-refraction and cross-cylinder calculation yield the same result when rotation is the only error factor, but over-refraction also accounts for errors that result from masking and tear layer effects that are otherwise not measurable. Two different approaches to over-refraction exist and each has application. The traditional approach considers trial lens power, rotation and over-refraction, while the ToriTrack (CooperVision) approach considers manifest refraction, trial lens power and over-refraction.

Over-refraction relies on the mathematics of combining crossed cylinders, and fortunately a number of available calculators perform the complex calculation. Online sources of cross-cylinder calculation programs include:

For the traditional approach, combine the sphere and cylinder of the trial contact lens with the sphero-cylindrical over-refraction. Note that you should take the rotation of the trial lens into account, both before calculation and before determining the axis of the resultant contact lens cylinder.

To apply the traditional approach, place a trial lens of known power on the eye and allow it to equilibrate. If visual acuity is less than expected, then perform a sphero-cylindrical over-refraction. Then calculate the effect of the crossed cylinders, taking into account the rotation of the lens on the eye to incorporate the true relationship of the trial lens and over-refraction cylinder axes. Once you determine that resultant, assume that the next lens will rotate as did the original trial lens and adjust the axis accordingly.

For example, if a trial lens has a power of 1.75 2.00 x 80, then 10 degrees clockwise rotation occurs (the marker at six o'clock moves left). On the eye (and relative to the over-refraction) this cylinder has axis 70 degrees. If the over-refraction to best acuity is +0.50 2.00 x 100, then combine that with the adjusted lens power of 1.75 2.00 x 70. Then the calculated resultant is 1.37 3.75 x 85. Then assume that a lens with this power will also rotate the same 10 degrees clockwise as the original lens did. Apply LARS and add back the 10-degree shift so the final lens would be 1.37 3.75 x 95. Most modern cross-cylinder calculators take the rotation of the trial lens into account, so you need only input the trial lens power and rotation along with the over-refraction.

ToriTrack provides a clever and useful variation of the over-refraction calculation. The software uses the over-refraction data differently and includes taking the manifest refraction into account based on the assumption that it yields the best-corrected visual acuity. While this is generally true for patients who have regular astigmatism, it's not likely true for patients who have irregular corneas.


	Figure 5. Effect of varying degrees of rotation. (Induced cylinder error as a function of cylinder power of the rotated lens.)

ToriTrack requires you to input the manifest refraction, the trial lens power and the over-refraction. Assuming the over-refraction combined with the trial contact lens yields the best acuity, ToriTrack deduces the amount of rotation that must have occurred with the trial lens. Therefore, it doesn't require clinical measurement of rotation or its input into the calculation. In essence, ToriTrack anchors its calculation around the manifest refraction and uses this to filter out the effects of crossed cylinders, rotation and the effects that result from the presence of the lens on the eye.

ToriTrack also vertex corrects the refraction and calculates effective K if you also input the K readings and HVID. The lens selection yields a list of CooperVision products expected to fit and available in the determined power range. However, the calculations aren't unique to these products and you can use them for any brand of toric soft contact lens.

While ToriTrack is useful, easy and applicable in the majority of toric soft lens fittings, for most cases of regular astigmatism, the traditional method yields the same result. ToriTrack isn't effective with irregular astigmatism, as the manifest refraction isn't likely to represent the optimal correction, so in these cases the traditional approach is likely better.

While sphero-cylindrical over-refraction is a useful tool, you must apply it appropriately (whenever acuity is less than expected or desired). If the acuity improves with the over-refraction, then use it to determine a final lens. Remember that if the subsequent lens rotates differently than the lens on which you performed the over-refraction, then the result will be less than optimal.

It's common with toric soft contact lenses to bias the correction toward the least amount of cylinder that provides acceptable acuity because the impact of unstable axis position or rotation is directly related to the cylinder power (Figure 5). You may try a cylinder power less than the vertexed manifest correction and compensate with a small adjustment of spherical power (spherical equivalent specific to the reduced cylinder power).

To test a patient's acceptance of the lower cylinder power, adjust the cylinder in the phoropter to the lesser amount and adjust the sphere to the best acuity. A study found that the soft lens with reduced cylinder provided as good or better acuity as this adjusted refraction in more than 90 percent of subjects. Lower cylinder power may not only improve stability of vision, it may also make the difference between using stock lenses and costlier extended range or custom lenses. Current availability of daily disposable toric soft lenses (Focus Dailies Toric, CIBA Vision) is limited to 0.75D cylinder only. Surprisingly, many patients who show refractive cylinder up to 2.00D do quite well with this lens.


Figure 6. Inferior corneal steepening associated with toric soft lens wear.

Combining soft torics and multifocals has been challenging. Some companies have made advances so that success is achievable for some patients. A number of approaches to multifocal design exist, and currently all soft multifocal torics incorporate the multifocal effect on one surface and the toric curve on the other, so that a back surface toric has the multifocal correction on the front surface.

All soft multifocal toric designs are based on their spherical counterparts. Fit the spherical multifocal trial lenses to determine performance and then perform a cylindrical over-refraction in a trial frame. Order the toric version of the multifocal based on this. Assume that negligible rotational or other factors that thicker and often larger soft toric multifocals induce will occur. Consider the first toric multifocal that you order as another level of trial lens -- it's common to use two or more trial lenses for each eye to achieve success. Prepare yourself and patients for a relatively long and possibly expensive process. Nevertheless, you can succeed with persistence and proper application of toric and multifocal fitting strategies.

With the upcoming release of the first silicone hydrogel toric lenses, we expect many of the physiological problems associated with soft toric lenses to abate. On the other hand, the stiffer modulus of spherical silicone hydrogels may cause significant, although transient, cor-neal distortion. How toric lenses made of these materials may affect topography remains to be seen.

Because of the considerable thickness of most soft toric lenses, physiological compromise can occur to a greater degree than what occurs with spherical soft lenses. Hypoxic complications common with overnight wear of medium- and low-Dk lenses, such as corneal neovascularization, myopic creep and corneal distortion, are also common with daily wear of toric soft lenses.

The neovascularization from chronic corneal hypoxia common with overnight wear of low-Dk lenses typically is most pronounced near the superior limbus on the part of the cornea that's normally covered by the upper eyelid. With prism-ballasted toric soft lenses, however, the neovascularization is common at the inferior cornea, which the thickest part of the lens covers. Neovascularization is unacceptable in cosmetic lens wear and requires a strategy to halt and hopefully reverse it by increasing oxygen availability. With toric soft lens wear, better oxygenation of the cornea could result from higher-Dk lenses, thinner lens design (double thin zone), fitting with non-prismatic spherical or aspheric lenses or refitting with GPs.

Myopic creep is also associated with lens-induced chronic corneal hypoxia. Toric soft lenses, especially those of low-Dk or substantial thickness, may cause this. The differential thickness of toric soft lenses may induce uneven effects and cause cylinder correction shift and other optical aberrations.

Corneal distortion occurs in many toric soft contact lens wearers and is a great concern. Although typically reversible upon cessation of lens wear, such distortion can go unrecognized and result in errors in refitting, spectacle lens prescribing and, most significantly, in refractive surgery considerations. In relation to refractive surgery, long-term wear of prismatic toric soft contact lenses can cause corneal topographical changes that mimic forme fruste keratoconus (Figure 6), and the potential refractive surgery candidate may then be unnecessarily declined for consideration of a refractive procedure. On the other hand, if such distortion remains undetected, then the patient may undergo a corneal refractive procedure with a less-than-optimal post-surgical result.

A toric soft lens wearer who is seeking refractive surgery should cease lens wear, and you should follow him serially until refraction and topography are stable, much as you would do with GP lens wearers. The amount of time for reaching stability varies widely, and we can't accurately predict it for given individuals. You can also best distinguish contact lens distortion from subclinical keratoconus by cessation of lens wear followed by serial refraction and topography.

The success with and ease of prescribing toric soft contact lenses have improved dramatically over the years to the point where they are a routine part of contact lens practice. Up-to-date knowledge of fitting, lens designs, cylinder power and alignment, rotation compensation, over-refraction, multifocal torics and physiologic factors contribute to success.

Dr. Bergenske has practiced for more than 20 years in Wisconsin and now is on the faculty at Pacific University College of Optometry.

This article is offered in conjunction with the New England College of Optometry

This course is supported by an educational grant from VISTAKON^®, Division of Johnson & Johnson Vision Care, Inc.

To obtain credit for this course, you must: 1. Be a licensed optometrist.; 2. Read and study the course presented in Spectrum; 3. Complete the test within nine months of the publication date of this issue; 4. Obtain a grade of 70 percent or higher on the exam.

There is no tuition or processing fee to you. The CE section is subsidized by Vistakon to support your professional enhancement. NEWENCO will correct the optometry tests, notify examinees of their scores, and issue certificates of credit to those who have passed the test.

Please allow 12 weeks for processing, which will be done in the order that they are received. You are encouraged to respond early.

To receive COPE-approved credit for this continuing education article, please print and complete the requested information on the official test form which is available as a PDF download.

You will need Adobe Acrobat Reader to view a PDF file. You can download it here if you need.

Please print out the form, blacken the most appropriate answers, enclose it in an envelope and mail it no later than November 30, 2005 to:

The New England College of Optometry
Department of Continuing Education
424 Beacon Street
Boston, MA 02215

1. The estimated percentage of myopic astigmats who have 0.75D to 1.50D of astigmatism is:

2. In general, you should fit patients with which of these amounts of refractive cylinder with toric soft lenses?

9. A spectacle plane refraction of 5.00 = 4.00 x 180 is what at the corneal plane?

11. On the eye, lens scribe marks have rotated 10 degrees to the left of the slit lamp beam and the patient's refractive cylinder axis is 90. You should order axis:

14. A study found that soft toric lenses with reduced cylinder power provided good acuity in what percent of patients?

15. Current availability of daily disposable toric soft lenses have what cylinders?

17. The most recent development in toric soft lenses is the introduction of lenses made from:

18. Hypoxic complications common with daily wear of prism-ballasted toric soft lenses are:

19. With prism-ballasted soft torics, where on the cornea is neovascularization more common?