Contact Lens Astigmatic Correction: An Update
Contact Lens Astigmatic Correction: An Update
More toric lens options equals more contact lens success for your astigmatic patients.
By Julie Ott Dekinder, OD
Dr. Dekinder is a clinical assistant professor at the University of Missouri-St. Louis College of Optometry.
In the world of contact lenses, providing astigmatic correction was once considered specialty lens fitting. In recent years, contact lens manufacturers have brought toric contact lenses to a new level, and practitioners are fitting nearly as many toric soft lenses as they are spherical soft lenses. Toric contact lenses are no longer just for correcting high amounts of cylinder, and they're more reproducible, with improved stability and better oneye comfort and vision than ever before. Both soft toric designs and GP lenses are available in state-of-the-art lens materials.
Utilizing a variety of the astigmatic lenses available will help you fit more patients quickly and efficiently. Knowing the available lens options can help make challenging astigmatic fits successful. Let's consider the different astigmatic options and discuss a few troubleshooting ideas.
Soft Toric Fitting
There are more than 25 soft toric lens manufacturers and nearly a hundred lens options. The majority of soft toric patients wear monthly and two-week replacement modalities, with more than a dozen options in the cylinder range from –0.75D to –2.25D. One daily disposable toric is currently available (CIBA Vision's Focus Dailies Toric). Recent expanded parameters have greatly increased the options for high ametropia and high cylinder correction. Most options offer axis selections in 5-degree or 10-degree increments. However, several made-to-order or around-the clock selections do exist. CooperVision's Preference toric is a quarterly replacement lens with astigmatic correction up to –9.95DC in its expanded range or XR design.
Soft toric candidates generally have –0.75DC or greater, with toric correction expected to add one-half to one line of improved acuity for patients who have low cylinder (Richdale et al, 2007). That being said, greater spherical refractive errors will generally tolerate small amount of uncorrected cylinder. The same physical and physiological standards should be met whether you fit a soft sphere or its soft toric counterpart. Wide apertures with normal lid tension and no redundant conjunctiva are ideal. Of course, prior to fitting you must rule out dry eyes, allergy, infection, meibomian gland dysfunction and corneal compromise.
As with most contact lens fits, treat with the least complex design. Soft torics are generally less complex than front-surface toric GP lenses. The back vertex power of a soft toric contact lens corrects the refractive error when the lens does not rotate and remains stable with eye movements. Soft toric contact lenses drape or conform to the eye without tear lens powers. Poor soft toric candidates are overly sensitive to axis mislocation or experience blur with less than five degrees of rotation. When the corneal meridians and the refractive axes are not aligned, the soft toric lens may align with the corneal meridians and cause oblique residual astigmatism (K readings 44.00/46.00 @090, MR –3.00 –2.00 x030). In this case, a soft toric correction may be unpredictable, especially with an unstable lens.
Lens stabilization is affected by lid shape, anatomy and tightness as well as by the lens design and prescription. Of course the eyelid and lens interaction is crucial as well as material, tear film, capillary attraction and lubricity. The peripheral corneal topography and ocular cyclo-rotation affect lens stability as well. Soft toric lenses are designed to minimize rotation with a thickness differential between the superior, middle and inferior portions of the lens. Prism ballasted lenses followed by double thin zone designs are most common for soft torics. Truncation, peri-ballasting and eccentric lenticulation are stabilization techniques on their own or in conjunction with prism ballasting or thin zones.
Soft Toric Troubleshooting
Once the diagnostic lenses have settled, measure the lens movement, stability, centration and coverage. You can assess lens rotation by measuring the angle of the slit lamp parallelepiped placed over the central or main toric marking. Guessing at lens rotation will generally underestimate rather than overestimate.
If there is no rotation and good visual acuity, comfort and fit or mild lens rotation and good visual acuity, comfort and fit then dispense the lens. When visual acuity is poor and the lens is rotating, then you need a new trial lens in a new power or in a different design or material. For stable, consistent misalignment you can compensate for rotation using LARS (Left-add, right-subtract as you face the patient). This method assumes that another lens with the same parameters (base curve, diameter and material) will have the same degree and direction (nasal or temporal) of rotation. The goal with LARS is to align the cylinder axis of the lens with the refractive cylinder of the axis.
The second option with poor acuity is to perform a spherocylinder over-refraction (SCOR). If the acuity doesn't improve with a SCOR or isn't consistent and stable with the SCOR, then you should consider an alternate design, brand or material. If the acuity does improve and is stable and consistent, you can calculate the new lens power by performing a cross cylinder calculation or with a cross cylinder calculator such as ToriTrack available from CooperVision.
Silicone Hydrogel Toric Lenses
In the United States, silicone hydrogel market share is growing at staggering levels. With the introduction of toric designs in silicone hydrogel materials, this category looks to see great market share increase and practitioner profitability over the next few years as well.
The two most widely available silicone hydrogel toric lenses are the PureVision Toric (Bausch & Lomb) and Acuvue Advance for Astigmatism (Vistakon). I find that both lenses are stable on the eye and provide good comfort and vision to most patients with the current lens parameters. The lenses are available in a range of both myopic and hyperopic powers, and both offer a maximum cylinder power of –2.25D.
The PureVision Toric is available as a 30-day daily wear or continuous wear lens and uses a prism ballast design for lens stabilization. The Acuvue Advance for Astigmatism is available in a two-week daily wear modality and incorporates what Vistakon calls an accelerated stabilization design that is similar to a thin zone type of design. The Acuvue Advance for Astigmatism also features Class 1 UV protection (>99 percent UVB and 95 percent UVA).
The O2Optix Toric (CIBA Vision) has been available to a limited market with limited parameters. CIBA has reported a full nationwide launch as Air Optix for Astigmatism in 2008. This lens is currently FDA-approved as a two-week daily wear or six-night extended wear lens. (Table 1)
One disadvantage of the silicone hydrogel toric options is the relatively limited power availability as compared to the hydrogel options. However, some patients who previously suffered from neovascularization, myopic creep and corneal distortion from hypoxia have found that they can wear lenses again because of the physiologic improvements due to increased oxygen. (See Figure 1)
Frequent Replacement Hydrogel Toric Lenses
Disposable hydrogel toric lenses have been the mainstay in soft toric lenses for several years. The brands are numerous, but a few stand out and deserve mention. SofLens Toric (B&L) is one of the most well known and prescribed toric lenses. It has good stability with a back-surface toric design and is available in toric powers up to –2.75D in 10-degree steps.
Figure 1. Chart showing Dk values for some toric lenses now available.
Two more work horse, reliable back-surface design torics are the Frequency 55 Toric and the Proclear Toric (CooperVision). These two designs are available in astigmatic powers up to –2.25D in 10-degree steps and offer the benefits of extended parameters in the Frequency 55 Toric XR and the newly launched Proclear Toric XR with up to –5.75D of astigmatic correction.
CooperVision has the most expansive line of frequent replacement toric lenses with the Vertex Toric and XR, Biomedics Toric EW, Proclear Toric DW and XR, Frequency 55 Toric FW and XR and the Preference Toric DW and XR.
In addition to its silicone hydrogel and daily disposable options, CIBA Vision offers the monthly replacement Focus Toric in a prism ballast design and the Freshlook ColorBlends Toric, which is a thin zone design.
Daily disposable toric lenses haven't expanded in recent years. It's possible that lens cost and limited parameters make them not a viable choice for many patients. However, it's not an option that practitioners should overlook. The Focus Dailies Toric is available in both plus and minus sphere powers and in two cylinder powers, –0.75D and –1.50D, at axes of 180 degrees and 90 degrees.
As previously stated, low cylinder patients will benefit from toric correction especially if they have low sphere amounts as well. Hydrogel Vision introduced its Extreme H2O 54% Toric LC designed as an affordable toric option for low cylinder patients (–0.50DC to –1.00DC). The lens is available in sphere powers of plano to –6.00D with a cylinder power of –0.65D and an axis range of 30 degrees to 180 degrees in 30-degree steps.
The new multifocal toric soft lens designs are a vast improvement compared to their predecessors. I've had success with both the Ultravue 2000T Multifocal Toric (CooperVision) and the CIBASoft Progressive Toric (CIBA Vision). Both lenses are annual replacement. The manufacturers suggest that you provide all patient information to a consultant who helps choose the initial lens. The information required includes keratometry values, refraction with add power and dominant eye. The newest multifocal toric lens on the market is the Proclear Multifocal Toric (CooperVision). It has a more desirable monthly replacement schedule and features a wide variety of parameters. The Proclear omafilcon material with its PC Technology also boasts improved comfort for dry eye patients. The Proclear Multifocal Toric XR launched in 2007 with power availability of +20.00D to –20.00D in 0.50D steps above ±6.50D, cylinder powers up to –5.75D and add powers up to +4.00D. Metro Optics has recently launched the lathe-cut SaturEyes Multifocal Toric with add powers up to +2.50D.
Figure 2. High with-the-rule astigmatism [(a) right eye and (b) left eye] are often best corrected with a bitoric GP lens.
CooperVision's Proclear Tailor Made Toric and Vistakon's Acuvue Toric will both be discontinued for 2008.
Dry Eye Patients
The Proclear Toric and Acuvue Advance for Astigmatism are both toric lenses marketed for dry eye patients that work well in many who have low tear breakup times.
If your patient has dry eye symptoms as well as an associated lid disease, you might prefer a more frequently replaced lens.
GP Bitoric Lenses
The quality of vision achieved with GP contact lenses always makes them a viable modality for correcting astigmatism. Patients exhibiting 2.50D or greater of corneal cylinder are often excellent candidates for GP bitoric lenses (Figure 2). I find bitoric GP lenses easy to fit and design.
As with all other soft and GP lens designs, fit GP bitoric lenses either empirically or with a bitoric diagnostic fitting set. For in-office fitting, a spherical power effect (SPE) fitting set is ideal. You can contact your laboratory for a set. Bitoric designs are fit similar to spherical designs with the base curve radii slightly flatter than the keratometry reading in each meridian. As with spherical lenses, an alignment lens-to-cornea fitting relationship is desired. Obtain the final lens powers by simply adding the over-refraction to — in this case — both diagnostic lens powers.
If you don't have a diagnostic fitting set, several easy-to-use empirical fitting methods are available. The Mandell-Moore Bitoric Fitting Guide (available at www.gpli.info) is a simple fill-in-the-blank form in which you input the keratometry values and refraction. This information, in combination with the base curve radii selection "fit factor," results in the calculation of both powers and base curve radii.
A new dynamic empirical method is the "Quinn Toric Calculator," which will launch on www.gpli.info in early 2008. This program, developed by Tom Quinn, OD, MS, FAAO, allows you to input the refractive information and then shows the resultant calculations as optical crosses on the eye. This represents not only a visually dynamic method of obtaining base curve radii, powers and other GP lens parameters, but will also represent a great teaching tool. All of the empirical methods essentially perform both tear lens calculations (vertical and horizontal meridians) for you.
More and More Options
The options for astigmatic patients are almost limitless if you're willing to try the different lenses available. The toric lenses of today work well and I enjoy utilizing all of them as frequently as possible. I know that we've come a long way in the field of toric contact lenses, and with all of the recent developments I know we're headed in the right direction to continue advancing the toric contact lens correction of astigmatic patients. CLS
Contact Lens Spectrum, Issue: December 2007