Custom Soft Contact Lenses: Prescribing and Practice
CUSTOM SOFT LENS ADVANCEMENTS
Custom Soft Contact Lenses: Prescribing and Practice
How to incorporate the latest advances in soft lens materials into your practice to solve patient needs.
By David Kading, OD, & Mile Brujic, OD
Dr. Kading is in practice in Seattle and is an adjunct faculty member at Pacific University. He has been a consultant/advisor to and has received research/education grants from Optometric Insights, Alcon Laboratories, Allergan, Art Optical, Bausch + Lomb, Contamac, CooperVision, Visionary Optics, Nicox, Paragon, TruForm Optics, SynergEyes, RPS Detectors, Unilens, Valeant, Valley Contax, and Vistakon.
Dr. Brujic is a partner of Premier Vision Group, a four-location optometric practice in northwest Ohio. He has received honoraria from Alcon Laboratories Inc., Aton Pharma (Valeant), B+L, Odyssey, Rapid Pathogen Screening, TelScreen, Transitions, Vistakon, and VMax Vision.
The future of contact lenses is becoming more and more exciting. Materials and designs have continued to improve over the past 30 years, and technological progress has brought us to a new age of both standard and custom lens materials and designs.
Custom lenses will always lag slightly behind standard lenses due to complexities in their utilization, but the advent of silicone hydrogel materials for custom lenses and advancements in custom soft lens design have resulted in an increased awareness of the custom soft lens world. Though custom soft lenses have been available in the United States for several years, the market has never been more replete with options. Contact lens designs to address keratoconus, post-surgery, high cylinder, and presbyopic needs are readily available from most custom soft lens laboratories.
Most of the custom soft lenses on the market tend to incorporate thickness to mask ocular irregularity caused by surgery, trauma, or disease. The increased thickness heightens the risk for hypoxia and peripheral neovascularization for these patients. Most of the irregular cornea lens designs incorporate a thinner edge profile, which has reduced the clinically perceived neovascularization. With the advent of custom latheable silicone hydrogel lenses, we anticipate that we will continue to see a reduction in hypoxia centrally.
The introduction of efrofilcon A (Definitive from Contamac) was perhaps the most exciting material advancement to date for custom soft contact lenses. Efrofilcon A has a Dk of 60, water content of 74 percent, and modulus of 0.35 MPa. When compared to current standard silicone hydrogel lenses on the market, it has a much higher water content and a much lower modulus (Figure 1).
Figure 1a. Water content versus oxygen permeability (Dk) of several popular silicone hydrogel lens materials.
Figure 1b. Lens material modulus (i.e., stiffness) versus oxygen permeability (Dk) of several popular silicone hydrogel lens materials.
Many of the current soft lens laboratories are starting to utilize the material for its obvious advantages in breathability.
Lagado Corporation, a subsidiary of Menicon America, has just received FDA clearance to use its new silicone hydrogel material in this category, Lagado LSH (mangofilcon A). The mid-range water content (49 percent) material can be utilized in a wide variety of designs. Lagado LSH will be plasma oxidized to produce a low wetting angle and improved wettability, promoting lens comfort, enhanced visual performance, a reduced potential for deposits, and ease of handling. The material will be available later this year to a select group of independent custom lens laboratories who must apply for and receive FDA clearance for contact lenses manufactured in Lagado LSH material.
As new advancements continue to be made in materials, ask your laboratory if the designs that you are utilizing are available in a silicone hydrogel material.
Fitting Soft Lenses
In addition to their increased power capabilities, the latest soft lenses stand apart from off-the-rack lenses in other design features—specifically, their increased prism and thickness profiles as well as base curve and diameter availabilities. Of greatest
significance in soft lens designs is sagittal depth. Often overlooked in the importance of base curve, corneal diameter plays a critical role in lens fit. One study looked at corneal diameter of 200 consecutive right eyes. The researchers found that the average corneal diameter was 11.8mm (Caroline and André, 2002) (Figure 2a). In a more recent evaluation of 100 optometry students, the average horizontal visible iris diameter (HVID) was 11.8mm as well (Figure 2b). Patients who have corneal diameters that are much smaller or larger than this may need a custom lens. Together, base curve and diameter are the key elements that affect soft lens stability on the ocular surface. By ensuring the proper sagittal depth through diameter evaluation, the lenses are better able to stabilize on the eye, making them more comfortable. Patients’ vision is subsequently enhanced, whether they have 1.00D of cylinder or keratoconus. It is critical to work with your local lab regarding custom lens fitting, as their design features can alter the fit significantly from one lens to another.
|Measuring Corneal Diameter|
Corneal diameter is typically reported to laboratories as HVID (horizontal visible iris diameter). Some slit lamps come with ocular reticules that allow for on-sight measurements. Another method is to utilize measuring calipers from your topographer to measure white-to-white (or limbus-to-limbus) measurements. If this is not possible due to lid interaction or the topography image being cut off, then measure DVID (diagonal visible iris diameter) by going from white-to-white in a diagonal manner, being careful to pass through the center of the corneal rings (Figure 3). Another, less precise way to measure HVID is to utilize a PD ruler held in close proximity to the eye (Figure 4). If all else fails, you can estimate corneal diameter—place a lens of known diameter upon the eye, manually center it, and estimate the amount of overlap nasally and temporally. For example, a 14.0mm soft lens will overlap a 12.0mm cornea by 1mm on each side. If the lens appears to overlap more or less, then estimate the size of the cornea and provide this information to your laboratory upon lens ordering.
With improvements in latheability, soft toric lenses have continued to improve over time (Figure 5). Throughout the years, we have had patients tell us how other practitioners said that they could not wear contact lenses due to the amount of their astigmatism. Although contact lens options for astigmatic patients have been available for years, the availability of custom toric lenses now makes this statement an anachronism. Your local soft lens laboratory can make custom lens designs in any axis, cylinder, or spherical amount. Many companies can manufacture powers down to the tenth of a diopter.
A significant advantage of customized toric lenses is that their axis can be narrowed down to the degree that is measured during the spectacle lens refraction or through lens over-refraction. This data can be given to the laboratory, allowing for the manufacture of a much more precise lens. Such lenses should not be confined only to patients who have high amounts of toricity, but should also be considered for patients who are sensitive to lens rotation.
Figure 3. Measuring diagonal visible iris diameter via a corneal topographer.
A simple way to evaluate patients’ axis stability and rotational sensitivity is to perform the Becherer Twist Test. With a patient’s best-corrected spectacle lens correction in the phoropter, occlude one eye and begin to slowly rotate the axis of the patient’s cylinder. Ask the patient to report to you when he first notices blurring of the letters. Take note of the amount of rotation and repeat the test by spinning the axis in the other direction. Repeat the test with the other eye and evaluate the amount of axis rotation that is acceptable for each patient. It may surprise you how much rotation is acceptable to some patients and how little rotation is suitable for others. A patient who has a cylinder amount of 1.50D and an axis of 175 degrees who is sensitive to a 4-degree Becherer Twist Test may struggle with a standard off-the-rack lens with axis of 180 degrees. Consider custom toric lenses for these patients to aid them in their sensitivity through a custom axis (Quinn, 2009).
Figure 4. Measuring horizontal visible iris diameter via the traditional approach (i.e., PD ruler).
It is estimated that there are 122 million Americans who have presbyopia (U.S. Department of Commerce, Bureau of the Census. Intercensal Estimates of the Resident Population by Sex and Age for the United States: April 1, 2000 to July 1, 2010). Accounting for nearly one out of every three persons in the population, they represent a significant percentage of our patients. Having reduced or lost their amplitude of accommodation, every one of them needs visual correction to aid in their visual needs, and contact lenses offer exceptional options for these patients. Through standard, GP, and custom soft lenses, this population base has many options available to them.
In addition to the above stated lens fitting characteristics to correct for an ideal fit, custom lenses offer a larger parameter profile compared to standard lenses. This includes increased sphere powers, cylinder powers, and multifocal add powers. Through low add powers, patients who are emerging into their presbyopic years and spend a considerable amount of time doing near work may benefit from +0.25D to +0.75D add powers. On the other end of the spectrum, patients who have advanced presbyopia may benefit from add powers over +3.00D.
Manufacturing capabilities extend to add power ranges that are limited only by practitioner imagination. This sort of technology may be widely utilized in the future for viewing virtual and augmented reality images on a projected spectacle plane for entertainment and other intriguing uses. An example is available at: http://innovega-inc.com/benefits.php.
Zone Sizes One of the most significant features that custom multifocal contact lenses can offer is variability in zone size. Being able to customize the size of the center-near or center-distance portion of multifocal lenses allows practitioners to enhance the way that some patients see through their standard multifocal lenses. For those who have substandard pupil sizes or dynamic pupil size changes, being able to alter the asphericity or zone size may allow patients who have previously struggled with their multifocal contact lenses to have a much more stable wearing experience.
Figure 5. Lathe-cutting a custom soft lens.
Soft Colored/Tinted Lenses
Soft colored lenses make up a significant percentage of the current contact lens market. We estimate that over 1 million patients in the United States wear cosmetically colored lenses.
It should not surprise us that colored lenses can be custom made to therapeutically improve our patients’ lives by either shifting the color spectrum (as in red/green color deficiencies) or by correcting for ocular damage, trauma, or congenital corneal or iris abnormalities. Many of the custom laboratories (and some that deal with colored lenses exclusively) are able to tint lenses to shift the color perceptions that patients perceive. For patients who have color deficiencies, this can be simulated beforehand with a loose red filter trial lens. Other filters are available that can demonstrate the color shifts that are possible. In addition to color deficiencies, such tinted lenses may be helpful when worn during certain sports or hobbies.
|Fine-Tuning Custom Soft Lenses|
Joan, a 34-year-old female, was referred to our office for contact lens fitting due to instability with her soft toric lenses. A review of her ocular health revealed unremarkable findings anteriorly and posteriorly. Her refractive status was OD –3.25 –4.25 X 002 and OS –3.00 –3.75 X 172. Upon evaluating her topography, we noted that her HVID was smaller (10.5mm) compared the standard corneal HVID of 11.8mm (Figure 6). After vertexing, lenses were ordered for her with a 13.5mm diameter rather than the standard 14.5mm lenses. The patient reported stability of vision and comfort with the new lenses compared to her prior lenses.
Figure 6. The relatively small (10.5mm) horizontal visible iris diameter in Case 1.
Beyond color shifting, custom colored lenses can be utilized to reduce glare and photophobia for patients who have damaged their iris or cornea in some way (See “An Eye Trauma Solution” on p. 29). Additionally, they can be used to occlude the eye in cases of sudden onset diplopia or in patients who have neurologically uncorrectable diplopia (such as sudden onset nystagmus). The zone sizes and colors can be altered to help reduce patient symptoms and/or to help balance the cosmetic differences in patients with different colored eyes.
Figure 7. The Softchrome In-Office Tinting System.
|An Eye Trauma Solution|
Frank, a 62-year-old male, presented to the office with a history of a metallic penetrating foreign body 20 years ago. His best-corrected vision was 20/70 with spectacle lenses. He stated that with increasing cataract in his fellow eye, he was getting nervous about losing his vision and was curious whether a custom contact lens could be made for him. Utilizing a soft toric lens and the Softchrome In-Office Tinting System (Figure 7), we offset the center of the pupil of the lens to better match the traumatically altered pupil that resulted from the injury (Figure 8a). He is able to achieve 20/25- vision through the custom tinted lens (Figure 8b).
Figure 8a. Individual in Case 2 prior to application of a custom tinted lens.
Figure 8b. Individual in Case 2 after application of a custom tinted lens.
Keratoconus has traditionally been thought of as a condition requiring GP contact lenses. Although the majority of our patients are able to succeed in both vision and comfort with this modality, some struggle with comfort and are forced to look for alternatives. Historically this has resulted in a corneal transplant, but now with increased availability of hybrid, scleral, and custom soft contact lenses (Figure 9), our patients have new options.
Figure 9. A profile view of a custom soft lens for keratoconus.
The primary feature of soft keratoconus lenses is that they tend to be thicker centrally compared to in the periphery, with a central thickness of 0.30mm to 0.40mm, which is about four times thicker than standard soft lenses. The thicker optical zone, which usually has a steeper base curve, allows the lens to mask most of the irregular astigmatism resulting from keratoconus. Most of the current designs have adjustable standard peripheral and midperipheral curves that resemble lenses used every day. The adjustment of these curvatures allows for fitting alterations that are independent of the central base curve (Figure 10). These adjustments may be required for better centration, movement, and comfort (Eiden and DeNaeyer, 2012; Kinoshita et al, 2008).
Figure 10. An on-eye view of a custom soft lens for keratoconus exhibiting the different zones.
Recently, Bausch + Lomb introduced in the United States the Kerasoft IC lens design, which had been utilized for several prior years in Europe. The Kerasoft IC lens is slightly different compared to other custom keratoconus designs in that it uses an aspheric front surface to manage vision and a two-sectored modifiable periphery for fit. Additionally, it is currently the only keratoconus lens design available in a silicone hydrogel material (Figure 11).
Figure 11. The KeraSoft IC custom soft lens for keratoconus from Bausch + Lomb.
As the fitting of these types of contact lenses is so customized, fitting sets are preferred over empirical fitting. This will help to improve visual acuity outcomes with proper over-refractions and base curve alterations, ultimately reducing overall chair time.
One of the major reasons why patients seek out and undergo a corneal transplant is because of contact lens intolerance. With the introduction of newer technology, we are seeing such patients stay in contact lenses longer. However, for patients who have undergone some form of a corneal transplant, many still require a contact lens fitting. Although patients who have corneal transplants can present with any shape of cornea, many of them have an oblate-shaped (flat central curvature relative to normal “steep” periphery) eye. The oblate-shaped cornea is also typical for patients who have undergone myopic refractive procedures such as Laser-Assisted in situ Keratomileusis (LASIK) or radial keratotomy (RK) correction. Although this pertains to only a small percentage of LASIK patients, many RK patients eventually require vision correction that can’t be achieved with spectacles or standard contact lenses.
In cases of refractive surgery or transplant surgery, most patients require a reverse geometry contact lens. With new lathing capabilities and improved materials, patients can now benefit from custom soft lens technology. These lenses utilize at least two custom zones in which the central base curve is flatter compared to the peripheral zone. Like the traditional keratoconus design, the peripheral zone tends to resemble a standard traditional soft lens base curve, as the peripheral cornea is minimally altered by surgery. The flatter central base curve may also have a thicker profile to mask any irregularity that may be present following the surgery (Kinoshita et al, 2008).
The Future of Custom Lenses
Significant improvements have occurred with custom soft lenses in the last three years. This is largely due to the advent of latheable silicone hydrogel materials, which have spurred GP laboratories to invest in the technology. Newer lenses are continually being brought to market in toric, multifocal, and custom irregular cornea designs. We are anxious for new materials to come to market, as well as improved designs that can be even further customized for our patients. As customization becomes more standardized, we can expect that technology will drive innovation in producing contact lenses for advanced aberration control, high-definition lenses, superior customized multifocals, and computerized contact lenses. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #213.
Contact Lens Spectrum, Volume: 28 , Issue: August 2013, page(s): 24 - 29 55