IRREGULAR ASTIGMATISM 3
Irregular Astigmatism, Part 3
This third article of a three-part series discusses piggyback systems, hybrids, and specialty soft lenses.
By Susan Kovacich, OD
The first two installments of this three-part series (Parts 1 and 2 appeared in the April and May issues of Contact Lens Spectrum, respectively) reviewed the management of irregular astigmatism with small-/large-diameter corneal GP lenses and with scleral lenses. However, as previously mentioned, many patients are corneal GP contact lens-intolerant (Polse et al, 1999). Scleral lenses are more comfortable compared to other GP lens options, but these lenses have only recently experienced a resurgence. What options exist for corneal GP lens-intolerant patients who have irregular astigmatism, especially those who cannot or don’t want to wear or handle scleral lenses?
Before the advent of newer designs, such as hybrid and scleral lenses, there was a need for a system that leveraged the optics of a rigid GP lens and the comfort of a soft contact lens for patients who have irregular astigmatism. In the early 1970s, when soft lenses became available in the United States, Baldone (1973, 1985) suggested the use of a rigid contact lens (for vision) fit onto a soft contact lens (for centration) for irregular corneas.
This approach is termed “piggybacking” or use of a piggyback system (i.e., piggybacking a GP lens onto a soft carrier lens) (Figure 1). The soft and GP lenses in a piggyback system are fit independently. The soft lens should move and center well, and the GP lens should move on top of the soft lens to promote tear exchange.
Figure 1. Piggyback system with inferiorly decentered GP lens on a well-centered soft lens carrier.
The materials for both lenses should be highly oxygen permeable (i.e., a high-Dk silicone hydrogel soft lens and hyper-Dk GP lens). The diameter of the GP lens should match or be slightly smaller than the optical zone diameter of the soft lens. This two-lens system will be thick, so the GP lens should be made thin, and the patient should not be aware of the lens edge, which could result in intolerance of the system.
Rather than using two separate disinfection solutions, one for the soft lens and one for the GP lens, a disinfection system that can clean both types of lenses, such as a peroxide system, can be used. If two separate disinfection solutions are used, then patients should use a multipurpose solution indicated for soft lenses to rinse the GP lens before applying it onto the soft lens. One way to simplify the care and maintenance of piggybacking is to use daily disposable silicone hydrogel soft lenses (Bennett et al, 2013), but this will increase the cost of the system.
When a soft lens carrier is being fit under a well-centered GP lens to improve comfort, generally the power of the soft lens will be fairly low, usually –0.25D or +0.50D. The soft lens power will be neutralized by the tear lens that forms between the two lenses in the piggyback system and will not affect the power of the GP lens (Szczotka-Flynn et al, 2006).
This makes fitting a low-powered soft lens under a GP lens a fairly simple procedure that does not require much chair time. However, the soft lens carrier can accomplish more than just improved comfort; it can also help the GP lens center better, especially when fitting an off-axis challenging pathology such as an inferiorly located cone. In these cases, a plus-power soft lens (generally about +6.00D) is used to give the GP lens an elevated lenticular platform or button on which to center.
When fitting a keratoconus patient, the GP lens tends to be a high-minus-power lens; again, the power effect from the plus carrier lens will be much less than what you would predict by just adding the powers of the two lenses together—the result would be about 20% to 25% of the original soft lens power (Bennett et al, 2013).
Some piggyback systems actually are designed with a cut out or countersunk area in the soft lens carrier to give the GP lens an area into which the lens can settle, which allows it to center better. The countersunk portion is larger compared to the diameter of the GP lens to allow the GP lens to move. The thickness of the GP/countersunk area should not be thicker than the thickness of the outer area of the soft lens carrier, or the patient may feel the GP lens or the lens may dislocate with blinking.
Another problem with this system is that the GP lens can erode the edges of the countersunk portion of the soft lens carrier (Szczotka-Flynn et al, 2006). Also, the soft lens carrier of these systems is sometimes still made in a hydrogel material, which can result in hypoxia complications (Bennett et al, 2013).
In any piggyback system, the soft lens carrier also provides a protective barrier between the GP lens and the cornea. This can be helpful when there are elevated areas of the cornea. For instance, keratoconus can progress and cause the GP lens to abrade the top of the cone, resulting in irritation or pain, contact lens intolerance, and the possibility of scarring. A piggyback system can be used temporarily while a new GP lens is ordered, or it can be permanent (Szczotka-Flynn et al, 2006).
However, due to the drawbacks of piggybacking, including the cost of two separate contact lens types, different contact lens solutions, and dissimilar contact lens replacement schedules, the piggyback system has been replaced in many instances by hybrid or scleral lenses. As the GP and soft lenses are sold separately, there is no data on how often this system is used, but the piggyback system still has a place as a problem-solver in the management of irregular astigmatism.
As manufacturing techniques improved, the next step was to try to combine the piggyback system into one lens with a center portion of rigid lens material/optics surrounded by a soft lens skirt; this is commonly known as a hybrid lens. There are several benefits to a one-lens hybrid system: the lower cost of not having to purchase both GP and soft lenses, and the simplified maintenance of not having to care for two different lenses. Another important feature is that the built-in GP portion of a hybrid lens is stabilized by the soft lens skirt and remains centered over the visual axis (Figure 2); this is unlike the GP lens of a piggyback system, which can become decentered (Bennett et al, 2013; Szczotka-Flynn et al, 2006) as previously mentioned.
Figure 2. Hybrid lens with centered GP portion of lens.
However, the original manufacturing of lenses that transitioned from a rigid center to a soft skirt was difficult; early generation lenses often tore at the rigid/soft lens interface. With these manufacturing difficulties, lens orders took a long time, and manufacturing consistency was a problem. Also, early hybrid lenses were low in Dk in both the GP and soft lens portions. The lenses tended to not move much, and adherence and hypoxia complications were common. An early example of this lens type was the Softperm lens by Ciba Vision (Bennett et al, 2013).
Manufacturing techniques of these lenses have improved over time. Current hybrid lenses, especially the SynergEyes family of lenses, are more durable and less likely to split at the rigid lens/soft lens junction. Also, current lenses are made with higher-Dk materials in both the rigid and soft lens regions, making them much more compatible with the cornea and reducing hypoxia complications.
These lenses should be monitored for tightening over time. Newer designs for irregular corneas are fit by lens vault similar to scleral lenses (Bennett et al, 2013). Hybrid lenses come in spherical, multifocal, and specialty designs for irregular astigmatism. In 2015 in the United States, approximately 2% of the total contact lens share was hybrids (Nichols, 2016), but only a portion of that percentage were used to correct irregular astigmatism. This modality is expected to continue to grow (Nichols, 2016).
Specialty Soft Contact Lenses
While GP lenses or lenses having a center GP portion (such as hybrids) are considered to have the best optics to correct irregular astigmatism, some patients have a strong bias against these lenses. Some patients have had a bad experience with GP lenses in the past and greatly desire a soft contact lens to manage their vision problems.
Conventional soft lenses are not generally used to manage significant irregular astigmatism as they drape over the cornea without being able to form the smooth optical surface and use the tear lens like GP lenses do. Early specialty soft contact lenses designed to correct irregular astigmatism were very thick to try to mask the astigmatism; this combined with the then-available low-Dk materials made hypoxia complications common.
Newer custom lathe-cut lenses are available, with some available in higher-Dk silicone hydrogel material. However, they still tend to be thick, which reduces the oxygen transmission.
Some of these lenses are made specifically to fit different types of irregular astigmatism caused by distinct pathologies, such as keratoconus or pellucid marginal degeneration. Other lenses have features—such as front-aspheric or aspheric prism-ballasted designs, balanced thickness, and spherical aberration control—to make them better equipped to correct irregular astigmatism compared to earlier soft lens designs. Some new lens designs also incorporate peripheral curves that can be steepened or flattened and individual sectors that can also be adjusted to improve fit and comfort as with GP lens designs. The high amounts of cylinder in these specialty toric lenses make rotational stability extremely important for a good visual result (Bennett et al, 2013).
According to my personal experience, it is helpful when fitting these specialty soft lenses to perform an automated over-refraction to make sure enough cylinder is being incorporated into the lenses. That being said, these specialty soft lenses often do not correct the vision quite as sharply as GP optics do; if a patient is switching from a GP modality, a line or two of acuity may be lost. For patients who have irregular astigmatism and cannot wear another type of lens comfortably, this may be an acceptable compromise.
Putting It All Together
Many factors are important when selecting a lens for managing irregular astigmatism. The amount of irregular astigmatism, the location and size of the area of the cornea affected, the size of eyelid fissures, and patients’ ability to manage the care and handling of lenses are all significant. Most patients who have previously been diagnosed with irregular astigmatism know that they are challenging to fit with contact lenses; newly diagnosed patients need to be educated on their condition and how complicated contact lens management may be.
Managing expectations is especially difficult in cases in which the visual acuity is not corrected to the degree that the patient anticipates. All patients can benefit from knowing about the different lens modalities that are indicated for their individual treatment plan and the limitations of these lens types. Table 1 lists a summary of the lens indications and limitations.
|Type of Lens||Indications||Limitations|
|Small-diameter GP keratoconic corneal lenses (< 9.8mm)||Small, centrally located “nipple cones”||Lenses can decenter; only works for a subset of irregular corneas located near the visual axis|
|Large-diameter GP corneal lenses (10.0mm to 12.5mm)||Larger areas of irregular astigmatism off the visual axis, such as inferiorly located cones||Cornea may be too irregular (i.e., more than 350 microns in elevation difference)|
|Scleral lenses (15.0mm to 25.0mm)||Very irregular corneas with large amounts of corneal eccentricity||Can be difficult to apply and remove; too large for small fissures; expensive|
|Piggyback lens systems (GP lens fit over a soft lens carrier)||GP intolerance; soft lens carrier helps the GP lens center better and can protect the cornea from the GP lens||A two-lens system is expensive with complicated care/replacement schedule; GP lens may decenter|
|Hybrid lenses (GP center with soft lens skirt)||GP intolerance; GP portion of lens is stabilized over visual axis by soft lens skirt||May tighten over time|
|Specialty soft contact lenses||GP intolerance; patients who do not want GP lenses or cannot be managed with scleral lenses||May not correct all astigmatism, resulting in reduced visual acuity; hypoxia complications|
As we have seen throughout this three-part series, the evolution of contact lenses used to manage irregular astigmatism has progressed with advances in materials, design, and manufacturing. Changes in the surgical management of irregular astigmatism, including corneal collagen cross-linking and partial thickness or lamellar corneal transplantation procedures, are also changing the way irregular corneas are being treated and impacting contact lens management.
Figure 3. Specialty soft contact lens for irregular astigmatism.
We are fortunate today to have many good options to offer to our patients, who otherwise might be seriously hampered by substandard visual acuity that adversely affects their quality of life. Keeping up to date with these changes, and knowing what options currently exist and in which cases these options are best utilized, can help eyecare practitioners make the best choices in meeting the visual needs of these challenging patients. CLS
For references, please visit www.clspectrum.com/references and click on document #247.
Dr. Kovacich graduated from Indiana University (IU) School of Optometry and completed a hospital-based residency at the St. Louis VAMC. After practicing for 10 years, she returned to IU and is an associate clinical professor in the Cornea and Contact Lens Service. She was a consultant to Alcon and has received travel funding from Alcon, Bausch + Lomb, Johnson & Johnson Vision Care, Inc., and CooperVision.