Scleral Lenses for Healthy Eyes
A World of Opportunities
BY LANGIS MICHAUD, OD, MSC
At the end of a recent lecture, I was about to leave the room when one of my students came forward. “Sorry to keep you late, but you told us we have to consider scleral lenses as the future of the contact lens market. I’m struggling to see that happening anytime soon. I don’t believe that, at a given point, they will replace daily disposable lenses. Did I miss something? Are there that many patients with keratoconus?” The following is a summary of my answer.
BUSTING THE MYTH
In the past, scleral lenses were mostly fit to treat ocular surface disease and visual correction for irregular corneal astigmatism. However, with the recent evolution in designs and manufacturing processes, and considering their main characteristics, it would be a mistake to keep them as niche products, prescribed only by a few contact lens experts.1 Fortunately, the trend is headed in a different direction. The rebirth of the scleral lens modality is due in part to proactive practitioners who are thinking outside the box and looking to explore other patient conditions that can benefit from scleral lens technology. It is time to bust the myth that scleral lenses are specialty contact lenses used only with special corneas. They are, in fact, becoming mainstream.
SCLERAL LENSES: WHAT THEY CAN OFFER
This modality can offer many different positive outcomes compared with other ones, especially in fixing two major issues that often lead to dropout: vision and comfort.2
In general, scleral lenses have been known to help optimize visual acuity. They are made of rigid gas permeable (GP) materials known to be optically superior to any other type of material.3 Scleral lenses help to lower high-order aberrations through optical correction and better compensation of surface irregularities. Also, scleral lenses are designed with a larger optic zone that doesn’t interfere with a larger pupil diameter. Consequently, fewer halos and less glare are experienced due to lighting conditions or when driving.
Another important aspect is that scleral lenses move minimally once stabilized on the ocular surface. To provide good visual acuity, they also have to be well centered. This is easier to achieve with smaller diameter scleral lenses. Given the conjunctival anatomy and gravity, larger scleral lenses tend to decenter inferior (downward) and temporal, leading to a misalignment between the visual and optical axis, which contaminates visual perception.4 This element becomes crucial when correcting for presbyopia. Smaller diameter scleral lenses (15.5 mm or less) are usually preferred for normal corneas and healthy eyes.
Scleral lenses can also help improve the contact lens experience by enhancing short- and long-term comfort. First, scleral lenses don’t touch the cornea or limbus, which are two of the most sensitive parts of the body. When blinking, they also limit lens-to-lid interaction, which is the main cause of discomfort with small diameter GP lenses.
Higher lens coefficient of friction is thought to be associated with contact lens discomfort.5 Roflufocon A and E, and Hyfocon A materials are characterized by a very low wetting angle. This implies that the tear film can spread easily on the surface with minimal surface tension. Consequently, this lowers the amount of deposits on the lens surface, at least in healthy eyes without meibomian gland dysfunction and blepharitis. Lens moisture can also be improved with plasma treatment. Moreover, if hydrogen peroxide is used as a care regimen as directed without rubbing of the lens, plasma treatment will remain unaltered for several months. With an abrasive contact lens cleaner, plasma treatment may last 30 to 45 days. In my opinion, the new Hydra-Peg surfacing technology can also be considered as a good alternative to improve lens resistance to deposits.
Finally, scleral lenses never dehydrate, and the presence of the post-lens fluid reservoir helps to keep the ocular surface moist during all wearing hours, which enhances patient comfort.
To have a patient switch from soft contact lens wear, or to consider fitting scleral lenses on a naïve patient, we must identify a benefit or an unmet need that will be fulfilled with scleral lenses. The risk/benefit (R/B) ratio should be evaluated for every patient.
Benefits from scleral lens wear have been documented.6,7 For diseased eyes or irregular corneas, the R/B ratio is low, meaning that benefits outweigh the risks by a high proportion. Thus, in those situations, scleral lenses are considered a safe device, as long as they are not worn overnight. This ratio can be different for healthy eyes.
As for the risks, theoretical calculations suggest that scleral lenses are linked to the presence of central chronic corneal hypoxia (2-4%),8-10 if the lenses are too thick (>250 μm) and if the central clearance is too high (>200 μm).11 With a clearance of 400 μm, there is 33% less oxygen delivery to the cornea than with a clearance of 200 μm.12
At this point, studies have not yet predicted the clinical outcome of such chronic hypoxic stress on normal and healthy tissue. However, a few case reports mention the negative impact of chronic edema when the cornea is compromised after surgery or with a reduced endothelial cell count of less than 1,000 cells/mm.13,14
Some authors compare an induced level of corneal edema to physiological edema overnight.15 This is misleading, because overnight physiological edema is eliminated within the first hour after awakening, and the cornea quickly restores itself. Corneal edema induced by scleral lens wear continues throughout the wearing hours with no chance for recovery. In addition, if lenses are used very soon after waking, the cornea never has a chance to recover.
When it comes time to fit young, normal, healthy corneas, these elements raise the risk/benefit ratio. Fortunately, there are ways to alleviate negative outcomes, such as providing less corneal clearance and using thinner lenses.
WHO ARE THE HEALTHY CORNEA CANDIDATES?
First, any patient wearing soft or small diameter GP lenses who complains of discomfort or experiences visual disturbances is a prime candidate to be fit with scleral lenses. This symptomatic population represents up to 40% of patients in an optometric practice.16 The key is to proactively identify these patients, which means asking the right questions.17
Although there have been many advances in contact lens technology, the contact lens dropout rate remains high, around 16%.17-23 As such, practitioners should dig deeper and proactively look for signs and clues to predict which patients have the potential for contact lens dropout. In my opinion, three major factors play a role: vision, comfort, and handling difficulties. Specific questions should be asked to ensure that lenses are comfortable and vision remains sharp and stable during all wearing hours, and that the lenses are easy to insert and remove.
Perhaps, ask this question: “If you could improve one thing about your contact lens experience, what would it be?” Unmet needs can be an opportunity to consider scleral lenses.
FIRST OPPORTUNITY: HIGH REFRACTIVE ERROR PATIENTS
Patients with high myopia, astigmatism, and hyperopia are challenging. There aren’t many contact lens options that can correct vision properly while maintaining optimal ocular health. In some cases, scleral lenses can help.
For highly myopic patients, scleral lenses can improve visual acuity, because a larger optic zone can be used and, thus, won’t interfere with the patient’s pupil diameter. With soft lenses, the spherical optic zone is limited to 6 to 7 mm, or even smaller if combined with a toric design. It is not rare to see moderate to highly myopic patients display a pupil of the same size as the optic zone. Under low illumination, the pupil dilates and expands outside of the optic zone, generating haloes and glare. With a larger optic zone of 8 to 10 mm, scleral lenses are a better alternative to improve visual acuity. The lens optic zone is large enough that it won’t interfere with the patient’s pupil.
It is possible to further improve the vision of highly myopic patients (>-6.00D) by relying on an oblate scleral lens design. A Q value is equal to 0 because the radius is constant. There is no flattening of the curve from the center to the periphery. A prolate cornea, which is a parabola, generates a negative Q value of -0.25, because its periphery is flatter than the central curvature. In young patients, the crystalline lens also induces -0.25 Q value. A total negative Q value is linked to higher levels of spherical aberrations, which impact the image quality perceived by patients. The higher the refractive error, the more negative the impact.
An oblate surface generates a positive Q value. In theory, this can counterbalance the natural negative Q value of the eye, especially in younger patients. Therefore, an oblate lens fit on a prolate cornea is expected to reduce spherical aberration and improve the optical outcome. Thus, the higher the refractive error, the greater the benefit.
An oblate scleral lens also has a flatter base curve compared with a corresponding prolate lens. Similar to a smaller diameter GP lens, the power of the scleral lens is determined by its radius or base curve and thickness. In smaller diameter GPs, contact lenses can be fit slightly flatter than the cornea without risking warpage of the ocular surface. With a scleral lens, a lens with the same sagittal height but with a flatter base curve can be designed because the lens doesn’t touch the cornea. Depending on the design or the manufacturer, the gain could be up to 5 diopters. Consequently, a patient needing a correction of -10.00D can be prescribed an oblate scleral lens of -6.00D and still see better.
Another advantage of lowering the amount of minus power is that minification will be reduced. The image size perceived by the patient is 7-10% bigger with a -6.00D vs -10.00D correction.24 The higher the initial refractive initial error, the greater the gain. This undoubtedly contributes to enhancing visual acuity and is also useful when correcting presbyopia. It is preferable to rely on a -6.00D, +2.25, compared with a -10.00D with the same add power.
Enhancing vision with oblate designs makes sense, albeit fitting scleral lenses in Asian eyes can be challenging due to small interpalpebral fissures. Smaller diameter scleral lenses must certainly be considered. In addition, the ocular surface of Asian eyes is unlike those of Caucasian eyes.25 In Asian eyes, the cornea is smaller, generating less sagittal height for the same corneal curvature, the mid-periphery is more prolate, but the limbal zone and the transition with the conjunctiva is kept the same. Overall, traditionally designed scleral lenses do not fit adequately on Asian eyes and tend to significantly decenter temporally. Thus, Asian eyes require a customized design, which is available at this time in prolate and oblate designs, as well as to optimize post-lasik corneas or enhance high myopia correction.
Figure 1. A scleral lens on a normal cornea.
COURTESY OF MELISSA BARNETT, OD
Correction of astigmatism in general, and high astigmatism in particular, is very sensitive to the position, movement, and rotation of the contact lens on the eye. The lens must remain stable, without rotating upon blinking or in different directions of gaze. Scleral lenses are the best lenses to fulfill these requirements and they often outperform other type of contact lenses, with the exception of hybrid lenses. However, scleral lenses can compensate up to 3.5D of regular corneal astigmatism due to the post-lens fluid reservoir, which is not the case for hybrid lenses. This includes 98% of all patients with astigmatism, without physiological limitations, such as 3:00 and 9:00 staining regularly encountered with small diameter GPs.
Clinical studies have demonstrated that most astigmatic patients will perceive the difference between spherical scleral lenses and toric soft lenses.26,27 Vision is crisper and more stable with scleral lenses. The same is true for patients wearing soft toric lenses in challenging environments, including low humidity, wind, and air conditioning. However, in some cases, residual astigmatism will be present. It is important to identify the source of the residual or induced astigmatism. Contrary to general belief, a portion of residual astigmatism may come from a non-uniform tear layer under the scleral lens28 and the presence of lenticular astigmatism.
Larger diameter scleral lenses tend to decenter more, and, consequently, the tear reservoir adopts a prismatic shape, which is thinner on top and thicker on the bottom. Lenticular astigmatism can become apparent when corneal astigmatism is fully compensated by scleral lenses. Finally, if the lens is not supported in all quadrants, lens distortion may be induced. This happens when a larger scleral lens (16mm or more) is fit with spherical peripheral curves on a toric conjunctiva.
It is believed that induced astigmatism can come from scleral lens flexure as demonstrated by performing a keratometry/topography reading over the scleral lenses once worn.29 Flexure is well documented in corneal gas permeable lenses. When misalignment of the lens to corneal surface occurs, the rigid lens may bend. This bend will induce astigmatism and produce a change in lens surface keratometry. It is speculated that the blink expresses the post-lens tear fluid and the lack of fluid cannot resist against the bend in the lens. While this is reported in corneal gas permeable lenses with a variety of materials, the phenomenon has been observed with scleral lenses but has not been thoroughly investigated.
In my clinical experience, increasing the center thickness of the lens as with corneal gas permeable lenses does not reduce this occurrence but increases the risk of reduced oxygen delivery to the cornea. Just as the cornea demonstrates toricity, the scleral has asymmetry and toricity as well. Similarly, adding back surface toric curves to the scleral landing zone improves overall scleral alignment and has been observed to reduce presumed lens flexure and induced astigmatism.
There are three options to alleviate induced flexure. The first option is to make the tear layer more uniform in shape. The second option is to design toric peripheries instead of spherical ones when the conjunctiva displays 1.00D or more toricity. Finally, if residual astigmatism is still present or if it is lenticular in origin, a front-toric scleral lens design is indicated.
Scleral lenses are not the most appropriate way to compensate for high hyperopia, unless there is a significant amount of corneal and refractive astigmatism involved. In fact, the associated risk of generating corneal edema is significantly higher when very thick lenses are fit. This becomes crucial when dealing with young post-surgical aphakic patients. Small diameter GPs may be better suited in these circumstances. Regardless, careful monitoring of the corneal condition is required.
SECOND OPPORTUNITY: PRESBYOPIC CORRECTION
The most dramatic dropout rate is seen when presbyopia sets in, shortly after age 40.30 Prior to age 40, 20% of contact lenses wearers are decreased to less than 10% a few years later.30 Patients often quit wearing contact lenses due to poor vision and are unaware that multifocal options exist.
Another factor is the paucity of soft toric contact lens multifocals able to properly correct astigmatic presbyopes. Full correction of refractive astigmatism is mandatory to achieve a successful presbyopic contact lens fit. Masking astigmatism is not an option. Small diameter GPs or hybrid multifocal lenses can be considered; however, scleral lenses are another good option because they also address the dry eye disease that often emerges concomitantly with presbyopia. In the last 2 years, many new multifocal scleral lens designs have been launched,31 which reveals the potential market growth of this segment.
To be successful, scleral lenses should be precisely centered on the cornea. This may be easier to achieve with smaller diameter scleral lenses (15 mm or less). However, if a larger lens is needed, toric peripheral curves may aid lens centration. If lens misalignment is present, the optics of the lens can be decentered to match the visual axis, enhancing the visual outcome. This process is more complicated and requires increased chair time and laboratory consultation.
THIRD OPPORTUNITY: ALLERGY CONTROL
Almost 60% of people with allergic rhinitis, and 40% in the general population, present with ocular symptoms.32,33 Controlling allergen exposure is crucial when managing allergic conditions. Pollens and airborne particles are difficult to control. Oral medications have the potential to dry the ocular surface. Many patients need topical medications to control their ocular symptoms.
Contact lens wear, in general, positively reduces allergen exposure to the ocular surface. Scleral lenses can be more likely to further reduce allergen exposure because they cover the cornea and part of the conjunctiva. Once lens stabilization occurs, tear exchange is minimal. Consequently, chemical mediators released on the ocular surface during allergic reactions have less access to the area under the scleral lens. Thus, scleral lenses should be considered as an option for patients with moderate to severe allergic ocular symptoms.
FOURTH OPPORTUNITY: SPORTS ACTIVITIES
More and more people are active and involved in sporting activities. For practical reasons, many of them prefer wearing contact lenses when playing sports. Contact lenses also offer UV protection when playing outside. However, dust and wind can complicate lens wear. An astigmatic patient wearing soft toric lenses may experience lens rotation or visual fluctuation and discomfort when exposed to such conditions.
Scleral lenses shield the ocular surface from challenging outdoor conditions. Compared with soft lenses, they don’t dehydrate. Scleral lenses also maintain their parameters. Compared with small diameter GPs, they don’t become uncomfortable if dust and particles invade the space between the cornea and the back surface. Finally, some scleral lenses also offer UV protection. Scleral lenses are now used more frequently to fit athletes, even at a younger age. In these cases, the risk/benefit ratio for scleral lens wear should be carefully evaluated.
Scleral lenses also have a protective aspect. Case reports have demonstrated that scleral lenses exposed to ocular trauma can protect the cornea and ocular surface,34,35 but scleral lens wear for ocular protection is off-label. Practitioners should not recommend their usage when ANSI protective glasses are required.
A VALUABLE OPTION
I believe that scleral lenses are amazing devices that have proven their outstanding clinical value for the treatment of ocular disease and for visual correction of irregular corneal astigmatism.
The same benefits can be applied for patients with healthy corneas, especially to correct high refractive errors and/or presbyopia, to relieve ocular allegies, or to improve sports performance. Practitioners should also consider scleral lenses as an option to retain patients in contact lenses who are prone to dropping out due to vision or comfort issues.
Scleral lenses help to preserve ocular health by optimizing oxygen delivery to the cornea. Smaller diameter scleral lenses should be designed as thin as possible (<250 μm), with appropriate clearance (<200 μm) whenever possible. Handling may be easier with the use of smaller diameter scleral lenses.
Scleral lenses are an excellent option for patients with healthy eyes. They have the opportunity to significantly improve the contact lens wearing experience and the patient’s quality of life with customized lens designs. I believe they truly represent a unique opportunity. •
For references, please visit www.clspectrum.com/references and click on document #SCLERAL2016.