SOFT LENSES FOR KERATOCONUS
Soft Contact Lenses for Keratoconus and Irregular Astigmatism
Specially designed custom soft lenses can provide great vision and comfort for irregular cornea patients.
By Joel A. Silbert, OD, FAAO
Practitioners have observed over the years that thicker soft lenses, or those manufactured in materials that are stiffer compared to HEMA-based hydrogels (like the CSI lens of the past, or the higher-modulus first-generation silicone hydrogels), have the ability to mask small amounts of corneal astigmatism. Unfortunately, such lenses did little for patients who manifested significant amounts of regular astigmatism, and they provided no help at all for patients who have irregular astigmatism and/or keratoconus. Practitioners have also tried toric soft lenses in managing keratoconus, with generally poor visual results except in the earliest stages of the condition.
The mainstay of keratoconus management has thus been with corneal GP contact lenses, which provide significant visual improvement by their outstanding ability to correct irregular astigmatism. However, patients often find that along with enhanced visual acuity comes physical discomfort. Discomfort with GP contact lenses may derive from chronic epithelial erosions and/or recurrent corneal abrasions; increased inferior edge lift and/or lens displacement; and back-surface lens deposits, lens adherence, and solution sensitivity issues (Vanderhoof and Mathe, 2012).
Attempts to deal with GP discomfort have included the use of “piggyback systems,” using either a frequent replacement soft lens or a custom soft lens with an anterior surface cut-out to “cradle” and position a properly fit habitual or GP keratoconus lens. Another alternative is to fit a hybrid lens, which is a GP lens with a soft skirt. Scleral GPs are yet another alternative, though their size can be intimidating and application and removal a challenge. While these options may be helpful in providing good vision and improved comfort, there remain patients who don’t succeed with them.
Wouldn’t it be ideal, then, to have a single lens that could provide improved comfort and still correct patients’ vision to a degree competitive with GP lenses?
Soft Lens Options for Keratoconus
Today, we do indeed have such options in the forms of specially designed, thick soft lenses that can be customized to correct both corneal and refractive astigmatism in patients who have keratoconus and other forms of irregular astigmatism (e.g., pellucid marginal degeneration [PMD], post-refractive surgery ectasia, and post-penetrating keratoplasty irregular astigmatism). This article will focus on soft lens options that are now available to help such patients. This underutilized modality now encompasses approximately a dozen different soft lens technologies available in both hydrogel and silicone hydrogel materials. For the sake of brevity, I will highlight four of these lens designs that I believe are representative of the scope of this new and growing technology. Table 1 lists other soft lens designs in this category.
|Bioperm Keratoconus (Orion Vision Group)|
|Concise K (ABB Optical Group)|
|Continental Keratoconus (Continental Soft Lens)|
|HydroKone and HydroKone Toric (Visionary Optics)|
|Keracon, Korb K1 & K2 (Gelflex)|
|Ocu-Flex K (ABB Optical Group)|
|UCL-55 (United Contact Lens)|
|YamaKone IC (GP Specialists)|
The Soft K Sphere and Toric designs were developed by Israel-based Soflex and are also available as the Soflex Eni-Eye Soft K and Soflex Eni-Eye Soft K Toric. U.S. distributors include Advanced Vision Technologies (AVT) and AccuLens. The Soft K is a silicone hydrogel lens designed for managing keratoconus, irregular astigmatism, and even post-surgical corneas. It is a daily wear lens that can be replaced on a quarterly schedule.
In the United States, the Soft K lens is manufactured in the Definitive material (Contamac, Inc), which has a 74% water content and a Dk of 60. The diameter is standardized at 14.2mm, and sphere powers correct +10.00D to –20.00D, with cylindrical correction up to –7.00D at any axis. Base curves range from 7.0mm to 8.2mm in 0.3mm steps. Using AVT’s five-lens fitting set, which includes all of these base curves, is necessary for observing lens performance, according to the company. This includes measuring whether the tear lens of the spherical diagnostic lens is correcting the irregularity of the cornea adequately, or whether a toric lens is required after suitable lens wearing time. An important element in fitting this lens is to be patient and allow adequate settling time prior to assessing lens movement, as initial tearing can make lenses look falsely loose. After selecting and applying the initial trial lens, a good 10 minutes is necessary before these measurements should be made.
For a keratoconic or prolate corneal topography (including PMD), start with the 7.6mm base curve. For post-surgical or oblate corneas, start with the 7.9mm base curve. Make base curve changes only after the appropriate settling period, going steeper if movement is excessive and flatter if there is inadequate or no movement. Once the best physical fit has been achieved, and after an additional 20 minutes has elapsed (for a total on-eye settling time of 30 minutes), then determine lens power.
Perform a spherical over-refraction (OR) initially. If good functional vision is found (20/40 or better), adjust the OR using vertex compensation if greater than ±4.00D and order the lens. Unless the vision is severely compromised, delay ordering a toric lens initially and ask the patient to wear the spherical Soft K lens for two weeks. During this period, there may be alterations in the corneal shape due to the controlled thickness of the central optical zone, which may reduce the eventual need for a toric. If, after two weeks, the cylindrical correction is still significant in magnitude and improves visual acuity, order the Soft K Toric lens with the compensated and vertexed OR factored into the diagnostic lens power. The toric lens should be ordered only if there is at least a two-line increase in acuity with its use.
The Soft K lens is double lenticulated to decrease lens mass and increase comfort, and the junctional area between the optic and the carrier is thus structurally stabilized (Figure 1). Located just outside of this structural ring at 3 o’clock and 9 o’clock are two pressure-balancing “vents” or fenestrations that are designed to enhance equilibration and can be used for measuring lens rotation (which then will need to be compensated for when ordering the final toric lens powers). The vents equalize the fluid pressure between the front and back surfaces and allow air bubbles to escape from behind the lens.
Figure 1. Design features of the Soft K lens.
COURTESY OF ADVANCED VISION TECHNOLOGIES
The NovaKone (Alden Optical) lens is a relatively new addition to this category that uses increased thickness as a lens parameter to compensate for corneal irregularity. The central zone of this soft lens can neutralize irregular corneal astigmatism much like a GP lens does due to its significantly increased thickness. It has an advantage, however, in that it can correct residual astigmatism by incorporating astigmatic correction on the front surface of the lens. Rotational stability for the toric application comes from Alden’s Dual Elliptical Stabilization design (Figure 2).
Figure 2. NovaKone lens design.
COURTESY OF ALDEN OPTICAL
The lens is manufactured in the 54% water hioxifilcon D material and comes in a standard 15.0mm diameter. It can be replaced quarterly. The 18-lens diagnostic set contains a wide range of base curves (from 6.6mm to 8.6mm in 0.4mm steps) and fitting curves (including flatter curves for the corneal regions outside of the cone). Table 2 lists the complete parameter range. These lenses have significant amounts of minus lens power and varying thicknesses.
|Material||Benz G4X 54%, hioxifilcon D|
|Diameter (mm)||15.0 as standard, others available in 0.1 steps|
|Base curve (central) (mm)||5.4, 5.8, 6.2, 6.6, 7.0, 7.4, 7.8, 8.2, 8.6 as standard, others available as 0.1mm steps|
|Fitting curve (paracentral) (mm)||8.2, 8.4, 8.6 as standard, others available in 0.1mm steps|
|Sphere power||+30.00D to –30.00D in 0.25D steps|
|Cylinder power||Up to –10.00D in 0.25D steps|
|Axis||1º to 180º in 1º steps|
|IT Factor* (increased thickness)||0 = Standard thickness|
|1, 2, 3, 4 incrementally thicker for higher levels of irregularity|
|* IT Factor is used to increase the lens thickness when irregularity is observed.|
Thickness control with the NovaKone lens is critical to its success. This variable is controlled as the Index of Thickness (IT) Factor, ranging from thinnest (level 0) to thickest (level 4) in incremental steps. Steeper curves for advanced cones with several IT Factors are provided in the diagnostic set for correcting irregular astigmatism. Its effectiveness is evident not only by the level of visual acuity obtained through sphero-cylindrical over-refraction performed over the spherical test lenses, but more importantly by assessing the quality of the front surface of the lens with an “over-K” measurement.
As long as the quality of the over-K mires is good with a test lens (regardless of the amount of astigmatism measured), this implies that irregular astigmatism has been corrected by the test lens (Figure 3). Thus, good visual acuity can usually be achieved via sphero-cylindrical over-refraction. This astigmatism correction is then incorporated into the rotationally stabilized toric lens design (compensate for toric rotation as you normally would do with any toric lens, assuming good lens movement and centration are observed). Two laser marks on the lens horizontal axis aid in toric rotational measurement. You may have to repeat this process with more than one IT Factor to determine the best lens. It is not uncommon to find that higher IT Factors can achieve better acuity with less additional toric refractive correction required. Having said that, however, you should still attempt to use the lowest IT Factor thickness that produces a satisfactory visual outcome. This is to provide the most comfortable lens for the patient as well as to facilitate oxygen transmission to the eye.
Figure 3. Over-K mires over a custom keratoconus soft lens design appear regular, indicating that over-refraction is likely to provide good visual acuity.
COURTESY OF MARK ANDRÉ, FAAO, & PATRICK CAROLINE, FAAO
The fitting technique for the NovaKone lens involves a four-step process, which works from the inside out:
1. Determine the central base curve. Use the supplied initial lens selection table that recommends the base curve/fitting curve combination based on average Ks (or Sim Ks if using topography) for the central 3mm to 4mm zone. This test lens is applied, filled with high-molecular-weight fluorescein, and evaluated to achieve an acceptable lens that demonstrates neither excessive bearing over the cone, nor excessive apical clearance (Figure 4).
Figure 4. Fitting evaluation of NovaKone using high-molecular-weight sodium fluorescein.
COURTESY OF ALDEN OPTICAL
2. Determine lens thickness (IT Factor). Use the lowest IT Factor that produces regular mires verified by over-keratometry or topography. Cones that are more centrally located will typically do well with low IT Factors. If significant cone decentration occurs, then switch to a higher IT Factor.
3. Determine the total lens power needed. Compensate for vertex distance as well as for rotation when adding the sphero-cylindrical over-refraction to your spherical test lens. Note: An effective over-refraction can be determined using an auto-refractor. Any changes made in the IT Factor or base curve will necessitate an additional over-refraction, as the optics will be different.
4. Determine the paracentral fitting curve. Typical soft lens fitting characteristics should be observed. Excessive lens movement or edge lift requires use of a steeper fitting curve. Little or no movement or edge impingement requires a change to a flatter fitting curve. Figure 5 shows an optimal fit.
Figure 5. Appearance of optimally fitting NovaKone soft lens for keratoconus.
Photo courtesy of Alden Optical
The KeraSoft IC lens was developed in the United Kingdom by UltraVision CLPL and is distributed in the United States through Bausch + Lomb (B+L) and several authorized manufacturing labs (ABB Optical Group, Art Optical, and Metro Optics). As with the NovaKone lens, this lens can be employed for managing keratoconus (all stages), PMD, post-corneal surgery including post-LASIK ectasia, and other irregular astigmatic conditions. (Note: practitioners wishing to fit either the NovaKone lens or the KeraSoft IC lens must be certified through an online certification process.)
KeraSoft IC has an aspheric design and is manufactured in the Definitive material. It is designed for quarterly replacement. Fitting involves an eight-lens diagnostic set with the standard 14.5mm diameter (it can also be ordered in 14.0mm, 15.0mm, and 15.5mm), plano power, and base curves ranging from 7.80mm to 8.60mm (the full base curve range is 7.40mm to 9.40mm in 0.2mm steps). Most lenses in the set have a standard periphery (Figure 6) (also available in peripheries ranging from Steep 1 to Steep 4 and from Flat 1 to Flat 4), but the flatter base curves also include lenses with flat and steep peripheries for unusual fitting circumstances. It is a small set, but one that I find very useful in its intelligent design. Sphere powers can be ordered from +20.00D to –20.00D, with cylinder powers from –0.50D to –12.00D in 0.25D increments in axes from 1º to 180º in 1º increments.
Figure 6. KeraSoft IC lens with STD Periphery.
COURTESY OF BAUSCH + LOMB
The KeraSoft IC fitting process follows a rigid protocol known as the MoRoCCo VA system. Initial lens design for keratoconus involves first looking at a patient’s corneal shape, which can vary from steep central cones to low decentered cones that require different base curves also depending on the severity of the cone. PMD corneas often require flatter base curves and may require additional peripheral modification of the flat superior zone and steep inferior zone. Post-surgical corneas will often need steeper peripheries. Using the Corneal Shape Recognition and Corneal Profile Charts supplied by the manufacturer are thus the first step needed to help select an initial lens for evaluation.
The MoRoCCo VA system is then employed for the fitting assessment as follows:
• Mo (Movement) Up to 2mm of movement is acceptable as long as the patient is comfortable. Change the base curve radius if movement is greater than this, if movement is less than 1mm, or if the patient is uncomfortable. Note: Be prepared to see much more movement with all of these soft keratoconus lenses than you see with traditional soft lenses. Increased patient comfort comes with adaptation as well, and patient education here is important.
• Ro (Rotation) To assess rotation, use the vertical laser mark, which should be at 6 o’clock and stable, with up to 10º of acceptable rotation. Be sure to compensation for rotation. A slightly flatter base curve may improve the stability, but unstable rotation results from a flat fit. Stable but greater than 10º of rotation implies a tight fit, in which case the base curve radius should be flattened.
• C (Centration) Centration should be good, but a minimal amount of decentration is acceptable if acuity is stable and clear. If vision improves immediately after the blink, then the fit is tight. Lateral decentration with a drop on up gaze indicates a flat fit.
• Co (Comfort) Comfort should be good; if there is persistent edge awareness, try a steeper base curve radius. If there is discomfort and the lens is stationary, then the lens is likely tight.
• VA (Visual Acuity) Visual acuity should be stable, with no fluctuation between blinks. Vision that is worse after the blink is likely due to a flat fit. If clearer after the blink, then a steep fit is likely. If good acuity cannot be achieved after the above adjustments, this may be due to a very flat peripheral cornea. Peripheral modification may be needed in such cases, or the lens design discontinued in favor of other lens options.
Note: The MoRoCCo VA lens fitting procedure is recorded on the supplied KeraSoft IC Dynamic Assessment Form.
The periphery of the KeraSoft IC lens can be steepened or flattened independently of the base curve. This customizable process is known as Sector Management Control (SMC). Up to two sectors in the periphery can be modified when necessary, such as with post-graft corneas that require flatter base curves but need peripheral steepening. Also, low cones and PMD corneas may require SMC steepening of the inferior sector and possibly flattening of the superior sector. Lens Sector Angles (quadrants) are designated and specified as either STD (Standard), STP (Steep) 1 through 4, or FLT (Flat) 1 through 4 (up to four steps of change are available) (Figure 7). A1 and A2 define the beginning and end of the first sector, A3 and A4 define the beginning and end of the second sector. Blend areas are automatically set once sector angles are defined.
Figure 7. Kerasoft IC Sector Management Control (SMC). For more irregular corneas, up to two sectors of the periphery can be modified independently of the base curve and customized to practitioner specifications (indicated in less than 10% of KeraSoft IC fits).
COURTESY OF BAUSCH + LOMB
Flexlens Tri-Curve Keratoconus
A hydrogel lens available for several years that features significantly increased thickness is the Flexlens Tri-Curve Keratoconus (X-Cel Contacts) lens. It is available in both 49% or 59% hioxifilcon, in 55% methafilcon, and more recently in the Definitive silicone hydrogel material, which is the material of choice to offset the reduced oxygen transmission associated with thick hydrogels. The center thickness ranges from 0.40mm to 0.65mm, and the lens incorporates two peripheral curves (Figure 8). A flat secondary curve of 1.2mm to 1.8mm and a peripheral curve of 2.2mm to 2.8mm is used to aid lens alignment.
Figure 8. Flexlens Tri-Curve Keratoconus lens.
COURTESY OF X-CEL GROUP
Other standard parameters include diameters from 8.0mm to 16.0mm in 0.1mm steps, base curves from 5.0mm to 11.0mm in 0.1mm steps, and sphere powers from +50.00D to –50.00D. The diagnostic set parameters include base curves of 6.0mm to 8.7mm in 0.3mm steps, diameters of 14.0mm and 14.5mm, power of –3.00D, and center thickness of 0.45mm.
Toric powers, unfortunately, have not yet been offered with this lens design. Keratoconus patients who have moderate amounts of irregular astigmatism may do well with this design, but those who have significant residual astigmatism would need the Flexlens Piggyback design instead, which is a soft lens with a cut-out to hold a GP lens.
Clinical Observations and Conclusion
These four soft lens designs represent a category of excellent technologies for correcting keratoconus and irregular astigmatism when adequate patient comfort cannot be achieved with GP keratoconus lenses. Although experienced fitters have been “biased” over the years to think in terms of corneal GP lenses for keratoconus and other unusual corneas, we are now blessed with many options beyond the conventional interpalpebral keratoconus lens. Intralimbal GPs, semi-scleral and scleral GP lenses, piggybacks, and hybrid lenses have all been good attempts to provide the desired visual acuity of a rigid lens with features to enhance on-eye comfort. Customized soft lenses, especially those made in silicone hydrogel materials, can provide good functional vision and excellent physiological responses when these other options are unsuccessful or undesirable.
While some of the silicone hydrogel lens designs discussed are somewhat complex, and NovaKone and KeraSoft IC require practitioner certification, they may be able to correct high degrees of corneal irregularity. Others, such as the Soft K Sphere and Toric and the Flexlens Tri-Curve Keratoconus, are relatively easy to fit and also provide excellent outcomes for keratoconic and post-surgical eyes.
The gains in patient comfort with customized soft lenses are well worth the additional chair time spent in fitting these lenses. Good lens movement and the use of silicone hydrogel materials can help provide irregular corneas with improved metabolism and reduce the risks of vascularization. Because of the increased thickness of these materials, be prepared to see more movement than with other types of soft contact lenses. About 0.75mm to 1mm of movement is desirable. Greater amounts may result in increased lens awareness; however, this does improve with adaptation.
Both practitioners and patients need to have realistic expectations, and both need to recognize that fitting these lenses requires additional time for on-eye assessments, with the likelihood that multiple visits will be needed to achieve the final optimal outcome. You can compensate for the increased office time by billing medical and/or optical insurance for “medically necessary” contact lenses and fitting services as well as billing medical insurance for follow-up office visits. (Use Code 92072 for keratoconus fitting and the appropriate office visit code for each follow-up visit.)
Materials may be covered under “medical necessity” provisions or may have to be paid for out-of-pocket by patients when this coverage is not present. Customized soft lenses, as with most keratoconus lenses, are more costly. Most can now be offered for quarterly replacement. Thus, these points need to be fully researched and discussed with patients prior to lens fitting, so that all parties are aware of and agree to the financial considerations.
Best of all in fitting these contact lenses is the satisfaction that comes from having another good management option for pleasing previously dissatisfied patients. CLS
For references, please visit www.clspectrum.com/references and click on document #229.
Dr. Silbert is professor of Optometric Medicine at Pennsylvania College of Optometry at Salus University. He is director of the Cornea and Specialty Contact Lens Service at The Eye Institute and director of Contact Lens Programs. Dr. Silbert is a Diplomate of the Cornea, Contact Lenses and Refractive Technologies Section of the American Academy of Optometry and former chair of the Association of Optometric Contact Lens Educators.