THE CASE FOR LARGE - DIAMETER GPs
Large-Diameter Lenses: the Future of GPs?
A fresh assessment of large-diameter GP lenses and why they deserve new consideration.
By Langis Michaud, OD, MSc, FAAO(Dipl), FBCLA
|Dr. Michaud graduated from École d’optométrie de l’Université de Montréal where he now teaches as a full professor and chief of the contact lens department. Dr. Michaud is an extensively published author and clinical researcher in the contact lens field. He is the current president of the College of Optometrists of Quebec (Ordre des Optométristes du Québec). Dr. Michaud has been a consultant for Blanchard Laboratories and has received educational and/or research grants from Genzyme Canada, Allergan, Alcon/Ciba Vision, with honoraria from those companies as well as from B+L, CooperVision, and J&J Vision Care.|
Ask practitioners about the future of their contact lens practice, and you get mixed answers. A vast majority are still interested in building and/or maintaining a healthy contact lens practice but find it increasingly difficult. Some find patients less and less compliant, and believe that they ultimately end up purchasing contact lenses from Internet websites anyway. So they ask, why bother? For others, the contact lens market is still a challenging though less profitable area than before because of managed care organizations.
Finally, many practitioners are dismayed to find themselves spending a significant amount of chair time dealing with complications, still awaiting a significant breakthrough in technology to improve this frustrating situation. Perhaps they don’t realize that the new technology is already here, in the form of large-diameter GP lenses. This technology can help most practitioners to enhance and sustain a better future for their contact lens practice.
This article aims to explore how large-diameter GP lenses have evolved in recent years and why they should no longer be considered merely as a last resort for fitting patients. In fact, modern large-diameter GP lenses should now be considered a mainstream product that can be used to compensate for regular refractive errors, with comfort comparable to soft lenses. These lenses can revitalize a contact lens practice and may possibly become the future’s major growth sector in the contact lens market.
Large-Diameter GPs: Why Select Them?
Clinical indications for GP contact lenses have not changed over the years. We can use them to correct current refractive errors or to compensate for corneal irregularity.
Almost everyone experiences better visual acuity with GP lenses than with soft lenses, especially in low-contrast conditions and for the correction of higher refractive errors and astigmatism (Bailey et al, 2001). Research has shown that GP lenses, worn on a daily wear basis, are associated with fewer adverse events, infections, or infiltrative conditions (Cheng et al, 1999). However, their usage is limited due to initial and long-term discomfort. This is why small-diameter GP lenses are often chosen as a patient’s final option (Gill et al, 2010).
The major advantage of large-diameter GPs over the smaller lenses is certainly the initial comfort that they provide to patients (Segal et al, 2003). Because large-diameter GP lenses do not touch the cornea (Figure 1), they do not affect this very sensitive tissue. This is also a reason to not fit corneo-scleral lenses, a topic that will be discussed later in this article. Large-diameter GPs do not move on the eye, thus limiting the interaction between the lens and the lids (with smaller GP lenses, this interaction is the main cause of long-term discomfort). Because of a large-diameter GP’s fluid reservoir, the cornea and part of the ocular surface remain bathed in fluid during all wearing hours, thus alleviating ocular dryness (Alipour et al, 2012), end-of-day dryness, and problems related to lens dehydration with wear. Consequently, once fitted properly, large-diameter GP lenses are initially and over the long term as comfortable as soft lenses are (Michaud et al, 2012). In fact, when patients report discomfort, it is typically an indication that the large-diameter GP fit is not optimal.
Figure 1. An optical coherence tomography view of a scleral lens vaulting over the cornea.
Optically speaking, practitioners will agree that GP lenses provide better visual acuity compared to soft lenses, especially for higher refractive errors, any level of astigmatism, and, obviously, corneal irregularities or distortion (Bailey et al, 2001). Large-diameter GP lenses are very stable on the eye and rotate minimally. Consequently, visual acuity remains sharp and stable. For all of these reasons, large-diameter GP lenses are now becoming the lens of choice to address refractive conditions, especially for patients complaining about fluctuating vision with their current soft lenses.
There is no better way to compensate for refractive astigmatism. Also, for patients complaining about glare and halos, the optic zone of large-diameter GP lenses usually exceeds 8.5mm, which is far higher than any optic zone for soft toric lenses. For patients who have large pupils, and even for moderate myopes, large-diameter GPs become the lenses of choice in addressing their visual needs.
Imagine a young myopic and astigmatic patient (–3.00 –1.00 x 180) who has moderate-to-large pupils doing computer work eight hours a day. With regular soft lenses, he may complain about halos, glare, fluctuant and unstable vision (if masked for astigmatism or if toric lenses dehydrate over time), eye dryness, lens discomfort, etc. Once fitted with large-diameter GP lenses, this patient will likely gain better visual acuity that remains stable during all hours of wear (Michaud et al, 2012). End-of-day dryness is eliminated. Such a patient is the perfect candidate for a mini-scleral fit.
Table 1 illustrates the benefits of large-diameter versus smaller-diameter GP lenses.
Obviously, there are many arguments in favor of large-diameter GPs over smaller versions. In fact, because of the many benefits of large-diameter lenses, smaller ones will most likely be phased out over the next five years, except for very specific uses such as orthokeratology.
How Large-Diameter GP Lenses Work
The technique for fitting a large-diameter GP is simple. It involves vaulting the ocular surface by designing a lens that matches the patient’s ocular sagittal height. The lens is filled with fluid and applied to the eye. Any corneal irregularity is offset by the presence of this fluid layer. Consequently, the cornea and some of the ocular surface remain bathed in fluid during all waking hours. There is no dehydration of the ocular surface, despite some loss of fluid clearance during wear (van der Worp, 2010).
|Comparison of Large- Versus Small-Diameter Lenses|
|Ease of handling||X|
|Ease of fit (first 10 cases)||X||Ease of fit (long-term)||=||=|
|Stability (non-fluctuating VA)||X|
|Quality of vision (halos, glare, larger optic zone)||X|
|Cost: regular refractive error||X|
|Cost: irregular cornea||=||=|
|Treatment of ocular surface disease||X|
Clinical Indications for the Use of Large-Diameter GP Lenses
Many new designs have appeared on the market over the last several years, and clinical indications for the use of large-diameter GPs have evolved in response (DeNaeyer, 2012). Initially, large-diameter GP lenses were considered only to compensate for corneal irregularity or as part of the treatment for ocular surface disorders.
Modern large-diameter GP indications now include:
• Correction of current refractive errors including for pediatric populations (Figure 2).
• Managing soft contact lens patients dealing with unstable vision.
• Managing patients intolerant to their regular small-diameter GP lenses or who are fitted in a piggyback lens system.
• Managing patients complaining about ocular dryness or lens discomfort at the end of the day.
• Managing patients who have higher-order aberrations including symptoms of glare and halos at night.
• Managing presbyopic patients, especially those showing refractive astigmatism.
In a recent study conducted at four university clinics, a mini-scleral lens design (Onefit P+A, Blanchard) was compared to a well-accepted toric silicone hydrogel frequent replacement lens, Biofinity Toric (comfilcon A, CooperVision). All participants were current and asymptomatic soft lens wearers. At the end of the study, both lenses were found to provide similar comfort, but 75 percent of participants preferred the mini-scleral over the silicone hydrogel toric lens for visual acuity (Michaud et al, 2012). This was expected based upon the known properties of GP lenses over soft lenses. More surprisingly, 55 percent of the participants decided to remain fitted in mini-sclerals in the future, even while expressing no dissatisfaction with their regular soft lenses. The handling of the lenses, compared to soft lenses, was identified as a limiting factor for some who opted to remain in soft lenses. We can extrapolate that on a symptomatic population (fluctuant vision, end-of-day dryness, etc.), the switch from soft lenses to large-diameter GPs would have been even easier.
Figure 2. A mini-scleral lens fit on a pediatric patient.
Mini-Sclerals, Sclerals, and Corneo-Sclerals
This raises the question: which type of scleral lenses should practitioners select to address current needs in contact lens care? There are many options on the table (DeNaeyer, 2012), and it can be difficult to decide which product to choose in entering this market.
Any GP lens that exceeds the corneal diameter is considered a large-diameter GP. There are three different types of large-diameter GPs on the market, and it is important to understand their designs to appreciate their specific clinical use.
The first consideration is the design itself. Most large-diameter lenses are sclerals. This means that they are designed to vault the entire corneal surface, including the limbal area, and to land on the conjunctiva. A small number of other products are called corneo-scleral lenses, and are fitted to be supported partly by the cornea. Clinical experience suggests that this may not be appropriate for most patients, and in some cases (with keratoconus in particular), this corneal touch could generate a high level of mechanical stress on a limited area of the cornea, triggering a higher risk of scarring (Figure 3). This is why most experts in the field dislike and do not fit corneo-scleral lenses on fragile corneas, especially those subject to scarring (Caroline et al, 2013).
Figure 3. Corneal scarring visible under a corneo-scleral lens fitted on a keratoconus patient.
The second consideration is the size of the lens. Here we have to differentiate between mini-sclerals (14.5mm to 15.5mm in diameter) and true sclerals (over 16mm). While both can be considered to belong to the same category, their design and fit vary a great deal. They should be viewed as two different types of lenses, such as toric and spherical soft lenses.
Practitioners should base their selection on the following elements:
• Type of patients in the practice
• Ease of fitting and troubleshooting GP contact lens fits
• Lens features
• Support from the manufacturers
Diameter Selection Versus Patients’ Needs: Scleral Versus Mini-Scleral
When confronted with an ocular disease such as keratoconjunctivitis sicca or a neurotrophic cornea, it is quite reasonable to want full coverage of the ocular surface. In that regard, relying on a larger-diameter lens (greater than 16mm to 18mm) makes perfect sense (Jacobs, 2008) (Figure 4).
Figure 4. Large scleral lenses are a better alternative to treat chronic dry eye conditions such as graft-versus-host disease, as in this case.
However, for the majority of patients, including those who have only corneal irregularity (keratoconus, post-graft, post-scar, post-LASIK) and/or regular refractive error, such large lenses are unnecessary. Mini-scleral lenses are certainly a better option, not only because they are far easier to fit, but also because they are easier to handle and are less intimidating for patients. Handling is a key issue and still represents the No. 1 reason for scleral lens fitting failure. Making handling easier for patients certainly would have a positive effect on this shortcoming and would position large-diameter GP lenses far better as mainstream products.
If we hope as professionals to develop a contact lens market with sustained growth, we will certainly want to incorporate large-diameter GP lenses. Consequently, we must simplify the learning process involved in fitting them and make them as easy for patients to wear as soft lenses are.
Understanding Conjunctival Anatomy
Based on the work of van der Worp (2010), we now know that the conjunctiva is toric in shape beyond 15mm of diameter, with marked differences between the nasal and the temporal quadrants. The angle between the cornea and the conjunctiva is also a key issue that varies a great deal from wearer to wearer and can influence the way large-diameter GPs settle on the ocular surface (Laflamme et al, 2013). Based on this knowledge, any lens with a diameter exceeding 15mm should be designed with toric peripheries, quadrant-specific, to align with the entire ocular surface without impacting the vasculature (clinically seen as blanching of the tissue) and/or the tear exchange that is vital to maintaining ocular health. Some authors will argue that a restriction in one quadrant is acceptable as long as patients are comfortable. However, comfort and ocular health are improved when back-toric, large-diameter GP lenses are prescribed (Visser et al, 2007).
Again, this point supports mini-scleral rather than larger lenses: with a reduced diameter, the lens can be produced with spherical peripheries, thus avoiding the complication of dealing with a toric conjunctival surface.
Clearance and Mechanical Support
Large-diameter GP lenses are supported by the conjunctiva but also by the fluid layer under its surface, acting as a water bed. Larger lenses are heavier and consequently require more support. This is why the clearance (height of the fluid layer under the lens, in microns) should be higher for larger sclerals compared to that needed with mini-scleral lenses.
The design of the lens also influences the level of clearance needed to obtain the best outcome. Some mini-scleral lenses are designed to create a wedge effect by limiting the clearance over the limbal area without bearing upon it, leaving a deeper reservoir centrally. This helps better support the lens while providing optimal oxygen delivery over the stem cells. This explains why these smaller lenses work so well without impacting the conjunctiva on which they land. With this wedge effect, they do not sink into the conjunctiva like the larger lenses do. In fact, when this configuration works, there is almost no impact on the conjunctiva during settling on the ocular surface. Consequently, these lenses never induce blanching, and vessel compression and lens seal-off are very rare occurrences. More importantly, this type of design allows for regular tear exchange, which is not always the case with larger scleral lenses.
Every scleral lens loses some fluid over the wearing time period. Larger lenses will lose more fluid and, because they are heavier, will sink deeper into the conjunctiva. This causes the loss of more than 50 percent of their initial clearance during the first hours of wear. More importantly, once stabilized, larger lenses do not allow for regular tear exchange.
Tear Exchange: Clouding the Issue
It is reported that up to 50 percent of patients fitted with bigger lenses can experience fogging of their vision after a few hours of wear (Visionary Optics Fitting Guide, 2013), because the fluid under the lens becomes milky and loaded with debris (Figure 5). Most of the time, the only remedy is to remove the lens, rinse it, and refill the bowl with a fresh, nonpreserved saline solution. Nobody really knows what causes this fogging, although a few factors can contribute to it.
Figure 5. Debris and mucus accumulation under an 18mm-diameter lens fitted with over 600μm of clearance.
According to one hypothesis, the friction between the lens and the conjunctiva stimulates the production of mucus, which remains trapped and not entirely dissolved under the lens. The larger the lens, the greater the friction with the conjunctiva. This rubbing effect can also stimulate inflammatory reactions and inflammatory mediators, and cells remain trapped under the lens surface. Finally, bacteria and by-products from the regular corneal metabolism also form part of this “toxic soup.”
No matter the nature of the elements involved, the main issue relates to tear exchange, another unknown factor with scleral lenses. How much tear exchange occurs during a full day of wear? No one knows the answer. If this exchange is insufficient, there is no doubt that mucus, by-products, and inflammatory cells will remain trapped under the lens and will not be washed away by regular tear flow. Due to the mechanics of scleral lenses, a larger lens will sink deeper into the conjunctival tissue, and consequently will reduce the tear exchange further. This is confirmed by the fact that mini-scleral lenses are not generally associated with this fogging effect. They sink less on the conjunctiva and consequently allow more tear exchange and have less impact on the tissue, reducing the mucus and inflammatory reaction.
Clinical Impacts of a Higher Clearance
When the clearance is too high, visual acuity could be reduced (Rathi et al, 2012) and the lens tends to become less stable on the ocular surface. Contrary to a soft lens, a scleral lens that moves with blinking is usually fitted with too much clearance. This implies that its sagittal value is too high relative to that of the fitted eye, leading to excessive clearance over its surface.
A higher clearance can also lead to a loss of oxygen transmissibility (Michaud et al, 2012). The appropriate amount of clearance that we can allow under a lens to alleviate corneal hypoxia is currently a topic of considerable debate. In theory, with the most permeable material on the market, clearance should not exceed 150 to 200 microns based on Harvitt-Bonanno criteria (Michaud et al, 2012). With a higher clearance, we limit oxygen transmissibility and induce hypoxia. On the other hand, some authors who propose fitting lenses initially with more than 400 microns of clearance report almost no edema or corneal neovascularization over time, except in cases in which the cornea, especially the endothelial layer, was already compromised (post-graft, guttata, etc.). This apparent contradiction can be explained by the fact that, starting with 400 microns, larger lenses very rapidly (within a few hours) end up with less than 200 microns of clearance. This means that they can initiate some corneal edema when first worn but, after a few hours, oxygen begins to be available again, thus reducing the clinical consequences. What is created is a transitory edema, as yet with no known long-term consequences.
We then come to the comfort zone of contact lens fitters: Do you prefer to fit a lens that never induces edema or one that induces a transitory edematous response on a day-to-day basis?
Once again, this factor can argue in favor of mini-scleral lenses because, from the beginning to the end of wearing time, the clearance needed to support the lens never exceeds the level that could generate hypoxia. Consequently, in most cases in which bigger lenses are not needed, it is safer to fit patients with mini-scleral lenses.
Where to Begin?
You may now be convinced to fit scleral lenses, probably mini-sclerals rather than corneo-sclerals at the outset, because you want to address the current refractive errors of your symptomatic wearers. To ease the process and reduce the learning curve’s steepness and length, we have to select lenses that are:
1. Smaller (most problems related to sclerals happen when lenses are fitted with diameters exceeding 15mm).
2. Available in a design that is easy to understand.
• To help you determine the best vault to adapt on a given ocular surface.
• Featuring parameters that are easy to evaluate and modify.
3. Labeled in a language that you can understand.
• Base curves are more intuitive compared to sagittal values. The steeper the base curve, the higher the vault over the cornea. Usually, steeper base curves are also associated with steeper peripheral curves, which helps when troubleshooting edge issues. For the sagittal value, a normal eye is characterized by a 4.3 sagittal height on average. Obviously, a keratoconic eye will require a higher sagittal height, while an oblate cornea will be best fitted with a sagittal height lower than 4.3 and/or with a reverse geometry design.
4. Known for having a proven track record of clinical success.
5. Distributed by a manufacturer offering a competent consultation team.
If you bear all of this in mind, you will be ready to build the future of your contact lens practice. Large-diameter GP contact lenses address the two most common reasons for contact lens dropouts: unstable vision and induced discomfort. They are reliable and provide a very good clinical experience to everyone who tries them.
Multifocal designs are available, and new designs are in the pipeline. This will help address the unmet needs of presbyopic patients who also show some level of astigmatism and who have a more unstable tear film. Very few soft lens products are available to meet their needs. With reliable large-diameter GP multi-focal lenses, these patients will certainly be a prime target demographic of the future contact lens practice.
Large-diameter GPs are already changing the nature of the contact lens market. The future challenge will be to continue to make these products mainstream, which does demand some additional commitment from eyecare practitioners, but also innovation in new designs from manufacturers, particularly in multifocal lens designs.
Once you understand fitting principles, the differences between lenses, and their specific clinical use, it becomes as easy to use large-diameter GP lenses as it is to use soft lenses to correct current refractive errors or irregular corneas.
If you have never considered large-diameter GP lenses before, now is the time to explore their potential to revitalize your contact lens practice and as a powerful tool to change the lives of patients. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #215.