Design, Materials, and Fitting of Toric Silicone Hydrogel Lenses
SIHY TORIC LENSES
Design, Materials, and Fitting of Toric Silicone Hydrogel Lenses
A look at what options are available in silicone hydrogel toric lenses as well as fitting tips for these newer designs.
By Tiffany M. Andrzejewski, OD, & Neil Pence, OD, FAAO
Toric hydrogel contact lenses have been in the marketplace for more than 30 years, while their silicone hydrogel (SiHy) counterparts have been around for less than a third of that. Compared to low-oxygen-permeable (low-Dk) hydrogel lenses, we now understand that SiHy lenses can substantially alleviate hypoxia-related complications such as limbal hyperemia and neovascularization (Covey, 2001). It's easy to understand today why SiHy lenses are rapidly becoming the contact lenses of choice for practitioners (Morgan, 2006).
We generally refit patients wearing traditional soft lens materials with Si-Hys when they sleep in their lenses, show clinical signs of hypoxia, or require thicker lens profiles. High myopes, high hyperopes, and especially astigmats can also benefit from silicone hydrogels. Eghbali et al (1996) calculated oxygen transmissibility (Dk/t) through the center and at points 3mm and 6mm superior and inferior to center for prism-ballasted toric soft lenses manufactured in low-Dk materials. They revealed that the inferior cornea receives only one-half to one-third of the amount of oxygen as compared to what the superior cornea receives, illustrating the need for increased oxygen permeability in toric lens designs.
SiHy Toric Availability
Power parameters available for toric SiHy lenses are quite vast and continue to be expanded on a regular basis, providing multiple options to allow patients to enjoy freedom from glasses. The options today are remarkably stable and predictable in their fitting characteristics. They also provide greater amounts of oxygen to the eye compared to hydrogel torics (Forister and Brennan, 2008). To date, six toric and four custom toric SiHy options are available in the United States with more to come. Bausch + Lomb (B+L) expects to launch the PureVision 2 HD for Astigmatism before the end of the summer, and this design will be widely available by the end of the year. The various SiHy lenses differ not only in material and water content, but also in optics and design. Tables 1 and 2 summarize the parameters of these options.
In fall 2010, Contamac received U.S. Food and Drug Administration (FDA) approval for Definitive, a 74-percent water content latheable SiHy material with a Dk of 60—making it suitable for custom lenses, in particular for torics. In the United States, Art Optical, Metro Optics, Unilens, and X-Cel Contacts are the four laboratories offering custom toric contact lenses in this new material.
To effectively neutralize astigmatic refractive error, a toric lens aligns its axis of cylinder correction to the axis of astigmatic error. Different stabilization designs keep the cylinder axis aligned with the eye. Some of these design features include prism ballast, thin zones (also known as double slab-off), posterior toric, chamfering, truncation, and combinations that incorporate different design features into a single lens (Edrington, 2011).
The Air Optix for Astigmatism (Ciba Vision [Ciba]), Biofinity Toric (CooperVision), Avaira Toric (CooperVision), and PureVision Toric (B+L) contact lenses are all stabilized with a variation of the prism-ballasted design. Prism-ballasted lens designs work by incorporating base-down prism by making the lower portion of the lens thicker to facilitate proper lens orientation.
The Air Optix for Astigmatism utilizes Ciba's “Precision Balance 8|4 Design,” which incorporates a wide optic zone with the thickest points of the lens located at 8 o'clock and 4 o'clock to assist with stabilization (Figure 1).
Figure 1. Air Optix for Astigmatism, Precision Balance 8 | 4 Design.
The Biofinity Toric and Avaira Toric incorporate CooperVision's “Optimized Ballast Design,” which has a junctionless, wide ballast area encircling the optic zone and a constant horizontal thickness to maximize stability and reduce rotation during blinking. In CooperVision's design, the toric's ballast is designed to remain constant across all powers to result in predictable and consistent performance despite the lens power.
The PureVision Toric uses a “Lo-Torque Design” that is similar to the company's SofLens 66 Toric. B+L has balanced the thickness of the midperiphery with a 360-degree chamfer to reduce the lens mass of the prism-ballasted design and to help improve patient comfort.
The PureVision 2 HD for Astigmatism lens will feature a number of changes compared to the PureVision Toric design:
• HD Optics: Similar to the PureVision 2 HD spherical lens design, the toric lens uses HD optics to reduce spherical aberration, but will accomplish this differently than any other toric lens on the market in that it addresses spherical aberration reduction in both the spherical and cylinder meridians of the lens, which may reduce halos and glare to improve visual quality, especially in low light conditions.
• Auto-Align design: The PureVision 2 HD for Astigmatism lens has been redesigned completely with a hybrid-ballast design for stabilization (Figure 2). Additionally, the lens diameter has been increased to 14.5mm (with a large optic zone of 8.0mm) to also help improve lens stabilization.
Figure 2. Auto-Align Design of the PureVision 2 HD for Astigmatism lens.
• Thinner lens profile: The overall thickness profile of the lens has also been thinned significantly compared to the original PureVision Toric. This will help increase the oxygen transmission and, along with a thinner, rounded edge design, may result in improved comfort.
• ComfortMoist technology: PureVision 2 HD has a wetting agent (poloxamine) in the package to help improve wetting and comfort at application.
Unlike the other SiHy toric lens options, the Acuvue Oasys for Astigmatism (Vistakon) and Acuvue Advance for Astigmatism (Vistakon) designs both feature an “Accelerated Stabilization Design” rather than relying on traditional prism ballasting for orientation and stability. Vistakon incorporated four active zones of added thickness located in the midperiphery of the lens and a dual thin zone superiorly and inferiorly (Figure 3). The thin zones rest under the open eyelid, and thick zones rest outside the open eyelid. The lens is designed to be actively rotated into place upon blinking whenever it is misoriented and then held stable when the lens is correctly aligned. Consequently, it tends to quickly rotate into position and remain stable during wear.
Figure 3. Accelerated Stabilization Design of the Acuvue Oasys and Advance for Astigmatism lenses.
Lens Stability and Direction of Rotation
If the lens cylinder correction is misaligned, or aligned but swings off-axis during a blink, then a patient's vision will be compromised. It is this lack of consistency in rotational position and stability that is often the primary reason for patient success or failure with toric soft contact lenses (Goldsmith, 1991).
Both the upper and lower lids influence orientation of a toric soft lens. However, because the lids move in nearly perpendicular directions, they can influence orientation in different ways (Figure 4). In most cases, the downward force of the upper lid acts on asymmetric lens thickness profiles to force the lens into place (Young, 2005). Thus, variability has been found in the amount and direction of rotation among toric soft lens wearers due to factors such as eyelid anatomy, the thickness profile of the lens, and the fitting relationship between the lens and the eye (Hanks, 1989).
More recent studies have led to a better understanding of how patient and lens factors influence lens fit. The lid position, the upward or downward slope of the lids, and palpebral fissure size are all factors affecting lens orientation and stability (Young, 2002). Due to all of these extraneous factors, it's difficult to guarantee that all toric lenses will orient at the zero position; and contrary to the popular belief that the lenses will always rotate nasally (Hanks, 1989), a high proportion may rotate temporally, too. Most silicone hydrogel toric soft lenses will rotate within 5 degrees to 10 degrees of the zero position. However, because different lenses have different designs and interact with the lids in different ways, we cannot expect nor predict that each design will perform and orient to the same position on a given eye without the application of diagnostic lenses.
Figure 4. Conflicting actions of the upper and lower eyelid during blinking.
Clinicians at Indiana University have observed the rotational characteristics for several silicone hydrogel toric lens designs (Acuvue Oasys for Astigmatism, Acuvue Advance for Astigmatism, Biofinity Toric, Air Optix for Astigmatism, and PureVision Toric) to analyze their performance based on a records review of lenses ordered and dispensed in which some rotation was noted (Pence et al, 2006 to 2009). These studies showed that, in general, all five lenses demonstrated very little rotation in the majority of patients, exhibiting either no or a relatively small amount of rotation in all cases. Particularly, in more than 95 percent of eyes fit in any of the studied designs, the lenses rotated 10 degrees or less. More than 80 percent were within 5 degrees of zero. However, when rotation was present, Acuvue Advance for Astigmatism and Biofinity Toric exhibited a slight tendency to rotate nasally, while Air Optix for Astigmatism, PureVision Toric, and Acuvue Oasys for Astigmatism showed a slight tendency to rotate temporally. Understanding these tendencies and the fact that these designs overall are quite stable may help reduce chair time during the fitting process.
Lens Rotation in Real-World Vision In a typical assessment of toric soft lenses, the patient is sitting upright at a slit lamp and is looking in primary gaze. However, we know that this doesn't accurately typify the range of eye movements and postural positions that patients assume during a given day or activity. Few studies have been done that observe how these lenses perform when a patient is lying down horizontally or viewing in extreme gazes. The studies performed to investigate the orientation of SiHy toric lenses in dynamic situations have noted that not only is there a change in orientation, but also in visual acuity. A study by Young et al (2009) determined that both prism-ballasted and accelerated stabilization design (ASD) lenses show similar re-orientation speeds when rotated 45 degrees off center. However, the prism-ballasted lenses tended to rotate more from their normal settled position compared to that of the ASD lenses when a patient was in the recumbent position, thus impacting visual acuity. This information is important to remember, as our patients lead active lives. Although our assessment of lens performance in the examination room may be adequate, in real world situations patients may notice disruptive fluctuations in their vision; therefore, it is our job to ensure that we not only mention this possibility to our patients, but remedy the situation when necessary.
When to Correct Astigmatism
“Marissa” is a 25-year-old graduate student who has a spectacle prescription of –2.25 –0.75 x 175 OD and –2.75 –1.00 x 168 OS with best-corrected visual acuity (OD, OS, and OU) of 20/20. Her visual acuity with her current spherical contact lenses was 20/25-OD and 20/30+2 OS. She presented with the complaint of reduced vision with her contact lenses and an increasing frequency of headaches at the end of the day. She was wearing silicone hydrogel lenses with powers of –2.50D OD and –3.25D OS. After refitting her in toric SiHy lenses, her visual acuity with contact lens wear improved to 20/20 OD and OS. She returned for her follow-up visit ecstatic with her vision and noticed relief from her headaches.
“Marissa” is a good example of how patients, even those who have a low amount of astigmatism, can benefit from having their astigmatism corrected through their contact lenses. For “Marissa” this was especially true because she has compelling visual demands as a graduate student.
The majority of toric fitting sets are available with 0.75D to 2.25D of cylinder in 0.50D steps (Table 1). However, there remains some disagreement as to when toric lenses are indicated. The recently published results of a 10-year survey pertaining to soft lens toric prescribing habits from seven countries indicated that refractive cylinder of 0.75D or less is not routinely being corrected with toric lenses (Efron, 2011). Historically, a significant number of practitioners have ignored low levels of astigmatism, making the decision to fit patients with spherical or aspheric lenses in an attempt to fit simpler designs, increase lens comfort or stability of vision, or limit patient cost. Despite the fact that it has been shown that “masking of astigmatism” with spherical lenses is minimal (Snyder, 1989) and that aspheric lenses are ineffective at correcting astigmatism as compared to soft toric designs (Morgan, 2005), the number of patients fit with toric lenses still remains lower than it should be.
We all have different cutoffs for how much astigmatism to leave uncorrected. Naturally, this cutoff varies among patients depending on their refraction as well as their visual demands. One factor we should consider when selecting our own cutoff is that several studies have confirmed that low-to-moderate astigmats (0.75DC to 1.25DC) have significantly better visual acuity with toric lenses than with spherical equivalent lenses (Richdale et al, 2007; Dabkoski et al, 1992).
Fitting Tips for Toric Lenses
Following are some tips that may help you successfully fit toric soft contact lenses:
1. Start with an up-to-date refraction. If the power in any meridian is >–4.00D, then vertex convert the powers for both meridians.
2. Choose the closest power to the spectacle refraction. If the exact prescription is not available:
• Increase minus sphere power on pre-presbyopic myopes if the cylinder power is under-corrected.
• Do not over-correct the cylinder.
• Use the exact or the closest axis possible.
3. Allow the lens to settle, then check rotation and note the range.
• If necessary, compensate for the axis rotation by applying LARS (left-add, right-subtract) to the refractive cylinder axis.
• Keep in mind that the new contact lens should rotate the same amount as the original lens.
4. To assess stability, check orientation with blinking and version movements.
• Ask patients to look up and blink, look down and blink, and look in horizontal gaze.
• Look for lens lag without rotation.
• Sometimes it is valuable to use the lid to rotate the lens off-axis, then observe how quickly it returns to its original position with normal blinking.
5. Check the patient's acuity. Vision, at minimum, should equal spectacle lens visual acuity. Misorientation can explain most cases of poor visual acuity with toric lenses and can become more significant as the cylinder increases. This is why it's sometimes more efficient to check the fit and/or visual acuity before proceeding to performing an over-refraction. If the lens is stable and well-oriented yet vision is still reduced, perform a sphero-cylindrical over-refraction.
6. Dispense or order diagnostic lenses. If vision is adequate, dispense diagnostic lenses to the patient from your diagnostic kit or order diagnostic lenses based on your over-refraction. When determining lenses to order based on your over-refraction, several internet sites let you enter the soft toric diagnostic lens over-refraction data to obtain a suggested sphere, cylinder, and axis correction using resultant cross-cylinder calculations. Coopervision.com and eyedock.com are two such sites that provide this useful tool.
Evaluating the Fit
Evaluating the fit of a toric soft lens is no more difficult than assessing a soft lens:
1. Start with a static assessment of the lens; ensure that the lens is centered and the cornea is fully covered to prevent desiccation.
2. Assess lens rotation. Evaluate lens orientation in respect to the lens markings and measure the amount of mislocation from the zero position. Most slit lamps allow you to rotate the beam to align with the axis of the lens markings and then read the amount of rotation directly off the slit lamp. If not available on your slit lamp, try visualizing a clock dial on the lens, with each clock hour being equivalent to 30 degrees of deviation. Toric soft lenses rarely adopt a fixed position and, when observed for a few minutes, may vary by 5 degrees to 10 degrees. Therefore, it's better to note a range of orientation positions (i.e. zero degrees to 5 degrees temporal) rather than a single value. This also assists you in deciding which cylinder axis to select when lens parameters have limited axes available.
3. Perform a dynamic assessment. The lens should move 0.5mm to 1mm with the blink. When the patient looks upward and in lateral gaze, the cornea should still remain covered with minimal lens rotation. Perform a “push up test” if minimal movement is observed.
4. Check for lens stability. Since soft toric contact lenses were introduced, a number of techniques have been developed to assess rotational stability. In addition to a simple orientation assessment of a well-settled lens on an eye looking straight ahead in primary gaze, evaluating the rate or degree of orientation recovery is another essential method to assess soft toric contact lenses. The ability of a lens to return to its original position following misorientation, for whatever reason, can have a profound effect on the vision correction that the lens provides.
Better lens reproducibility, more frequent replacement schedules, availability of expanded parameters, more oxygen-permeable and better wetting materials, and improved lens designs have all contributed to very high success rates when prescribing SiHy torics. The health benefits, particularly the increased oxygen permeability, of SiHy lenses are highly documented throughout the literature. Sufficient material oxygen permeability often results in a healthy corneal environment, with minimal-to-no effects of hypoxia and an ability to combat common lens-related complications (Fonn et al, 2006). Newer toric lens designs tend to reduce lens rotation and improve rotational stability for clear and stable vision.
With the advent of high-Dk SiHy lenses, not only have improvements in clinical signs of hypoxia been documented, but several studies have also noted improvement in overall comfort and in patient symptoms of end-of-day dryness (Dumbleton et al, 2006; Riley, 2006). Thus, SiHy torics are not only easy to fit and comfortable for our patients, but they are a healthy option. We believe that they should become the toric lenses of choice for both new contact lens wearers and for existing patients requiring astigmatism correction. CLS
To obtain references for this article, please visit http://www.clspectrum.com/references.asp and click on document #190.
|Dr. Andrzejewski is a 2010 graduate of the Illinois College of Optometry and has recently completed the Cornea and Contact Lens Residency at Indiana University. Following her residency she is looking to join a practice where she can utilize her skills fitting specialty contact lenses.
|Dr. Pence is the associate dean for Clinical and Patient Care Services, Indiana University School of Optometry in Bloomington, Indiana. He is a consultant or advisor to B+L, Ciba Vision, and Vistakon, and has received research funding from AMO. You can reach him at email@example.com.|
Contact Lens Spectrum, Issue: September 2011