Problem Solving Soft Toric Contact Lenses
Understanding today’s soft toric designs and how they affect lens fit and stability will increase your success.
|Dr. Davis is a cofounder of EyeVis Eye and Research Institute and practices in Oak Lawn, Ill. He is an adjunct faculty member at Southern California College of Optometry, Illinois College of Optometry, Pennsylvania College of Optometry at Salus University, and University of Alabama in Birmingham. Dr. Davis is Diplomate in the Corneal, Contact Lens and Refractive Technology section of the American Academy of Optometry as well as an inductee in National Academy Practice in Optometry. He has received research funds from SynergEyes, CooperVision, Alcon, and ZeaVision and has a proprietary interest in SpecialEyes.|
|Dr. Eiden is president and medical director of North Suburban Vision Consultants as well as co-founder of EyeVis Eye and Vision Research Institute. He is an adjunct faculty member at The University of Illinois Medical Center as well as at the Indiana, Illinois, Salus, and PCO colleges of Optometry. Dr. Eiden is a consultant or advisor to CooperVision, SynergEyes, Alcon, and SpecialEyes, and he has received research funds from Vistakon, CooperVision, and B+L.|
By Robert L. Davis, OD, FAAO, & S. Barry Eiden, OD, FAAO
The universal key to successful contact lens fitting is achieving optimal vision, comfort, and ocular health response. One of the most effective ways to build patient loyalty while developing a contact lens specialty practice is to become proactive in delivering the best vision options for your patients. Precision management of astigmatism is one of the most effective ways to optimize your patients’ vision with contact lenses. A clear understanding of patients’ vision needs and expectations, considered in unison with their lifestyles and desires, will result in the highest levels of successful patient management.
Today’s array of soft toric lens options (including both frequent replacement stock lenses as well as custom lenses) can fit the majority of our patients who have astigmatism. How do you select the correct lens design for your patient? What are the advantages and disadvantages of each lens design? When do you select custom lenses over stock lenses? In this article we will describe the clinical relevance of managing patients who have astigmatism as well as proper lens selection and problem solving.
We Aren’t Prescribing Enough Toric Lenses
Practitioners would never disregard the astigmatic correction in a spectacle prescription, but to this day many practitioners try to fit soft contact lenses without correcting the existing cylinder component. Toric lens fittings increased from 22 percent to 28 percent from 2000 to 2009 (Efron et al, 2011). However, on average, 34.8 percent of the world’s population has more than 1.00D of astigmatism; therefore, toric lenses are under-utilized worldwide (Holden, 1975).
The barriers to practitioners fitting more soft toric lenses reside in their perceptions of increased chair time with soft toric fitting and of patient satisfaction with existing spherical modalities as well as in their concern about delivering the cheapest contact lens option, the difficulty in patient communication, and the impression that toric lenses provide a visual result no better than that of a spherical lens. Many practitioners do not offer a soft toric contact lens option to patients who have lower degrees of astigmatism and who are under the impression that their prescription cannot be properly corrected with soft lenses.
You can explain astigmatism to patients by demonstrating that the power along the horizontal meridian is different from that along the vertical meridian. Show the patient a “T” on the acuity chart and explain that the vertical leg of the “T” does not focus at the same location as the horizontal leg. It is then easier for patients to understand the difference between powers along the horizontal and vertical meridians and thus to understand the rotational effects of toric lens designs. Patients often say that they have been told that their eye is shaped like a football instead of like a baseball. Our experience indicates that this description just generates confusion.
Lens Thickness Implications
Stock frequent replacement soft toric lenses, due to their relative thinness, conform to the shape of the cornea like plastic wrap, thus allowing the astigmatic power of the cornea to manifest through the contact lens. The post-lens tear film is less than 10 microns of equal thickness throughout the optic zone in these lenses, which effectively eliminates any power contribution from the lacrimal lens (Holt, 1997). For soft torics to correct the total astigmatic error of the eye, the toric lens powers must be placed along the principal meridians of the eye.
As the thickness of a soft lens increases, a portion of the corneal astigmatism can be masked (Eiden and DeNaeyer, 2012). Clinically, we find that the materials of frequent replacement soft lenses can also mask up to 0.25D to 0.37D of the astigmatic power when they are stiffer or higher-modulus materials. Significantly increasing center thickness of a soft contact lens can even improve visual acuity in keratoconus patients in the early stages of the disease by masking their irregular corneal astigmatism. Conversely, Snyder and Talley (1989) concluded from their study that spherical soft lenses were not able to predictably mask astigmatism, although custom keratoconic soft lens designs such as KeraSoft (Bausch + Lomb [B+L]) or NovaKone (Alden Optical) have been shown to effectively mask larger amounts of astigmatism and corneal distortion depending on lens thickness (Roncone, 2011).
The advantage of masking the astigmatic power with a custom soft spherical lens design by manipulating center thickness is that lens rotation does not affect visual acuity. Rotation with thinner designs such as stock frequent replacement lenses effectively changes both the lens cylinder power and axis. Patients’ acuity will degrade until the lens shifts back to its appropriate rotational position. As the required cylinder power increases, so does the amount of visual acuity distortion caused by the lens rotation. Patients generally are more bothered by the “in and out” focus from unstable toric lens rotation than they are by the blur from under-corrected astigmatism.
Lens Rotation Implications
Success in fitting toric soft contact lenses requires that the cylinder power and axis remain in constant alignment with the meridians of the manifest refraction. The ability of a lens to return to its proper orientation or to a misaligned but stable position following rotation is a major consideration when choosing toric lenses (Bernstein, 1991). Rotational recovery refers to how quickly a lens returns to the desired orientation following disruption, such as from eye rubbing (Cairns, 2010). Lens rotation is a critical element that can contribute to reduced or poor visual quality, which could in turn lead to patient dissatisfaction and lens discontinuation. Therefore, it is important to ask your patients if they are experiencing symptoms consistent with rotational issues (e.g., fluctuating vision or blur after gaze changes) so that corrective measures can be initiated.
To maximize rotational stability and recovery, soft toric contact lenses utilize several stabilization designs. Two time-tested stabilization methods are prism ballasting and dynamic stabilization/double thin zone. In the prism-ballasted design, the bottom of the lens is thicker compared to the top. Using a concept known as the “watermelon seed principle,” this design incorporates lid pressure to keep the lens in place. Gravity has been shown not to have an effect on lens positioning (Kilpatrick, 1983). By thickening the lower portion of the contact lens, the upper eyelid can slide over the thin superior portion during the blink, thus forcing the thicker inferior portion down into the correct position. Thin zone dynamic stabilization or double slab-off design involves removing contact lens material from the superior and inferior portions of the lens, leveraging the thicker region of the lens within the interpalpebral space (Kovacich, 2008).
Today’s manufacturers of frequent replacement silicone hydrogel toric contact lenses utilize modified and proprietary stabilization designs. PureVision Toric (B+L) lenses feature a modified prism-ballasted design with a bevel along the entire circumference of the lens that the company says reduces its weight and allows the contact lens to tuck under the eyelids during the blink. B+L’s PureVision 2 for Astigmatism lenses incorporate Auto Align Design, which combines hybrid ballasting (peri and prism design), a large diameter (14.5mm), and large optic zone (8mm) to improve stability and comfort, according to the company. Air Optix for Astigmatism (Alcon) lenses have a modified prism-ballasted design in which the 6 o’clock position is not the thickest portion of the contact lens. Rather the 4 o’clock and 8 o’clock positions are thicker. The company says that the thinner lens profile at 6 o’clock minimizes interaction with the lower eyelid and improves stability. Acuvue Oasys for Astigmatism and Acuvue Advance for Astigmatism (Vistakon) lenses utilize an accelerated stabilization design, which features four thicker stability zones that rest within the palpebral aperture above and below the 3 o’clock and 9 o’clock positions. The design is not considered ballasted as its stability is independent of weight (gravity) (Kovacich, 2008). The accelerated stabilization design utilizes a thin edge and thicker middle to stabilize the lens, which becomes less reliant upon lid-to-lens interaction, according to the company. For its Biofinity Toric and Avaira Toric lenses, CooperVision utilizes a junctionless, wide ballast area encircling the optic zone and a constant horizontal thickness to maximize stability and reduce rotation during blinking. The toric’s ballast is designed to remain constant across all powers to result in predictable and consistent performance despite the lens power (Andrzejewski and Pence, 2011).
Solving Challenges to Soft Toric Performance
Lens-to-eyelid interactions are the primary causes of soft toric lens rotation. The resistance of a contact lens to the force of the blink is a critical factor in achieving success with soft toric lenses. Thinner designs with steeper peripheral and midperipheral front surface areas reduce the eyelid’s ability to grasp and rotate a contact lens (White, 1997). A successful toric design allows the lid to glide over the lens surface. Lens deposits and dehydration resulting from increased osmolarity and reduced blink rate can reduce comfort after hours of wear (McMonnies, 2007). This causes variable vision especially during computer use or while driving a car (Parker, 2011).
Many soft toric lens problems originate from the shape of the palpebral aperture. If the outer canthus is higher compared to the inner canthus, the lens will tend to rotate temporally. Conversely, when the outer canthus is lower compared to the inner canthus, the lens will tend to rotate nasally. Therefore, examining your patients’ anatomy may help solve lens rotation instability.
One method of counteracting lens misalignment (a lens that rotates to a consistent location) is known as “LARS” (lens rotation left add; lens rotation right subtract). Reticules available on most slit lamps can help measure the exact amount of misalignment, as can keeping in mind that one clock hour of rotation equals approximately 30 degrees. The most accurate method to compensate for lens rotation (along with other challenges to visual performance with toric soft lenses such as lens draping and tear layer effects) is to perform an accurate sphero-cylindrical over-refraction (SCOR). Perform an over-refraction only after you have confirmed rotational stability. If the endpoint visual acuity and vision quality is excellent following the SCOR, then calculating the combined cross cylinders should result in a lens that provides excellent vision. If the SCOR outcome does not produce a good visual outcome, then poor physical fit (i.e., base curve radius, thickness, rotational instability, etc.) may be at fault. Alternately, it could also indicate defective optics in the contact lens.
Refractive need and lens design can also play a role in soft toric lens success. Lenses for patients requiring with-the-rule minus cylinders have increased vertical lens thickness and steeper curvature vertically (if back-surface toric); against-the-rule minus cylinders increase horizontal lens thickness and have steeper curvature horizontally. With higher astigmatic powers, these effects are exacerbated. In patients who have against-the-rule astigmatism, the thicker, steeper meridians are similar on both sides horizontally and are parallel to the movement of the upper lid, often resulting in no lens rotation or slightly nasal rotation. The lens can move up and down but will not fall off horizontally. In with-the-rule astigmatism, the thicker, steeper meridians are similar on both sides vertically and are perpendicular to the upper lid, so movement and lens rotation may be nasal or temporal. With rotational problems the lens is not balanced and falls off of the vertically steeper meridian. Oblique toric soft lenses present the greatest challenge to rotational stability (White, 1997). That being said, with improvements in today’s soft toric lens stabilization methods, issues such as direction and magnitude of astigmatic power have relatively little influence on lens fitting outcomes—yet they should be considered when problem solving.
Most soft torics are manufactured with back-surface toricity. This improves stabilization, especially with higher corneal toricity, as the back of the lens conforms to the toric cornea. The toric curves of many back toric lenses are confined to the central optic zone. This reduces differential edge thickness and blink-related lens rotation. During the blink, the upper eyelid moves downward and the lower eyelid moves medially to force tears toward the puncta (Hom and Bruce, 2006). Therefore, the eyelid’s directional movement during the blink is another force causing the lens to move off of the cylinder axis.
Lens diameter is another important parameter that can influence toric lens stabilization (Jackson, 2012). Typically, stability as it relates to lens diameter is related to palpebral aperture size. As pressure from the eyelid blink deforms and relaxes the contact lens, the tear film is placed under pressure and presses through the contact lens onto the cornea. Young (2003) found more stable lens orientation for subjects who had smaller palpebral fissures because of less movement after the blink, although realignment is slower to reposition. Increasing the lens diameter can help achieve stable lens orientation by increasing the surface area contact between the lids and the lens and reducing lid interaction with the lens edge. It is the reduced contact area with smaller contact lenses that increases the friction between the contact lens and the lid and causes inconsistent rotational effects. The pressure of the lid-lens interaction is greater compared to the lens-cornea forces that cause the lens to move. By increasing the lens diameter, the lid forces cannot generate enough torsional pull to rotate the lens. The principle of the “watermelon seed” effect comes into play and allows better stabilization.
One primary problem of fitting contact lenses in the examination room is that it is not reflective of real-life situations. Chamberlain (2011) demonstrates that clinicians’ assessment of toric lens stability in the chair is not well correlated to lens stability once patients leave the examination room. A patient’s visual world cannot be duplicated in our examination room due to humidity changes, lighting conditions, head orientations, and visual demands. Eye movements and changes in head position create stress on toric lens orientation that is specific to patients’ body orientations when engaged in visual tasks. Depending on the lens design, eyelid vectors and blink forces can cause unpredictable rotation. Ballasted lenses are particularly prone to rotate with head tilt. Studies have shown that they can rotate as much as 30 degrees with a 90-degree head tilt, while lenses with more stability points designed to reduce the effect of gravity rotate just 11 degrees (McIlraith et al, 2010). The everyday life situation of looking into a rearview mirror and then back at the road is one example of forces acting on a lens that can cause patient dissatisfaction with soft toric lenses (Zikos et al, 2007).
The Place for Custom Soft Toric Lenses
Custom design soft toric lenses offer an opportunity to manage astigmatic refractive errors in a highly precise way. Extended parameter ranges in lens powers, axis increments, diameters, base curves, peripheral curves, lens thickness profiles, and stabilization methods can dramatically increase the percentage of astigmatic patients whom we can successfully fit in soft toric lenses. These changes can also be made in a variety of materials to increase lens oxygen permeability as well as water content and material wettability. Computer-generated lathes can manufacture spherical powers of ±30.00D with a cylinder power up to 15.00D. Optic zone modifications can solve problems related to flare and glare from large pupils. Megalocorneas and microcorneas can now be fitted with toric lenses. Some post-surgical cases, keratoconus, and pellucid marginal degeneration can now be managed with custom soft toric lenses (White, 2010). Lower lid contour issues, upper lid influences on the lens, unique palpebral apertures, conjunctiva irregularities from pinguecula and pterygium, and oblique astigmatism can now be resolved with customizable toric lenses.
The early days of soft toric lens fabrication created lenses that were like snowflakes (no two were alike). Today, not only can they be reproduced with consistency, they can address our astigmatic patients’ vision needs in a highly precise and successful manner. Understanding contemporary soft toric lens designs and how to manipulate parameters to problem solve will increase your success with both stock frequent replacement toric lenses and customized toric lenses. Developing your skills with soft toric lenses will allow you to move on to the next stage of complexity, that of fitting soft toric multifocal contact lenses. We look forward to addressing those challenges with you in a future article. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #207.