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Scleral Lenses 101

Accreditation Number: ODO 002/001/02/2020

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This article is aimed at the novice scleral lens fitter.  It will address the steps to follow, as well as some basic concepts.  Before we start, we need to get a basic understanding of the scleral topography, as opposed to our traditional understanding of corneal topography.  One can imagine that when one designs dental implants (a stationary object), the interaction is different compared to the interaction between a scleral lens and a cornea.   It is therefore essential to look at what happens to the sclera once a scleral lens is inserted into the eye.

 By now, I think Optometry can claim that it has a proper understanding of the corneal shape. Optometry was the driving factor for RGP contact lenses and edge designs since the very beginning.  This resulted in the development of outstanding options for visual correction.  As scleral lenses gained popularity, researchers began to question how relevant Optometry’s knowledge of corneal shape is when designing a lens that lands beyond the cornea.

Scleral lenses from 1942 – Photo by Pieter Naudé

Firstly, remember that scanning of the sclera was not adequately done before. What was discovered is that it is not possible to make assumptions about the shape of the sclera and for that matter any part of the eye based on just the limited information we gathered from the cornea.  On the contrary, the anterior corneal surface is unique, especially the central 10 mm.  To optimise scleral lens design, one needs to integrate principles of scleral geometry as well.  Scleral lenses are not new.  The image below shows one of my own patient’s lenses.  She received them in South Africa in 1942 as hard contact lenses.  The problem back then was an oxygen supply.  This vital shortcoming was addressed by reducing the lens diameter.  Today we know them as RGP (rigid gas permeable) lenses.  It is only since gas permeable materials were developed (1970’s) and quite a while after that, it was decided to go back to these “bigger” lenses.  

Two exciting discoveries of the sclera

Well fitted scleral lens – Photo by Pieter Naudé

 At first, it was believed that the sclera just followed a spherical curve as the cornea did, but it was later discovered that the shape of the transition area between cornea and sclera is almost straight in most cases.  The anterior scleral shape is also tangential in most cases.  Secondly, the sclera is not rotationally symmetric outside of the corneal borders.  This had a significant impact on the latest scleral lens designs.  Since the discovery of this, lens designers opt to make lenses with quadrant specific parameter changes.  New topographers are seeing the light because we want to look at the sclera and not just the cornea alone, never mind the central 10 mm of the cornea. The same eye can have scleral shape inconsistencies from one meridian to the next.1

Anatomy of a scleral lens

The sagittal height (sag) of the anterior eye can be a helpful tool in the fitting of scleral lens designs.  Today, we believe that five anatomical features contribute to the sagittal height of the anterior eye (Figure 1):

  1. The central radius of curvature (approximately 3.0 mm) of the central cornea
  2. The mid-peripheral corneal eccentricity, which extends from the apex of the cornea to a chord of approximately 10.0 mm
  3. The overall corneal diameter
  4. The corneal angle that begins at the 10.0 mm chord and extends to the limbus.
  5. The scleral sag, which begins at the corneal diameter and extends out to any given chord (i.e., 13.0 mm to 20.0 mm).
Figure 1. The five anatomical features that contribute to the sagittal height of the anterior eye.

Most topographers can only measure accurately over a chord length of 10mm. So lets put the abovementioned theory to the test.  The majority of scleral lenses are designed over a chord length of 14 – 15 mm.  The sag height from a 15mm to 10mm chord length is consistent in most eyes, which = 2000 microns.  The most variance in sag height happens within the 10 mm chord length.  This is why we measure the central 10 mm. For example:  if the SAG value of the central 10 mm chord = 1900 microns, you add 2000 microns, which is equal to 3900 microns.  Now you just need to add 200 microns for a tear layer, and you have your first trial lens sag height that equals 4100 microns.

1900  (The measured SAG height over 10mm Chord)
2000  (the standard SAG height to add to get from 10mm to 15mm Chord)
200    (An ideal tear layer height to achieve)
_____
4100    Total Sag of Trial Lens
______

Actual lens fitting:

The comfort of well-fit scleral lenses is generally outstanding. One can argue that comfort is a subjective measure, but with scleral lens wear, we don’t expect any pain or the slightest discomfort.  With an optimally fitted scleral lens, initial mild lens sensitivity or discomfort is occasionally experienced.  I always tell my patients, “we just inserted a huge foreign object onto your eye, surely you will feel something.”  There is more than one lens parameter we can change to improve comfort during scleral lens wear.  When fitting scleral lenses, pay attention to subjective complaints that will guide parameter changes to achieve comfortable scleral lens wear.  It is essential to note the severity of symptoms of lens discomfort immediately after lens application vs. the symptoms after hours of scleral lens wear.2

Scleral lens vs. RGP lens:

How do I decide between RGP or a scleral lens for a patient?  The first critical aspect is almost the worse driving factor to consider, but in South Africa, it remains a reality, and that is cost.  The cost of scleral lenses is still more than double the price of RGP lenses.  It would also need a lot more scans from a topographer and an OCT, which can also raise the cost. 

The second problem:  If we want to get ophthalmology to buy into the concept of scleral lenses as an alternative to corneal surgery, we need to lift our game.  We need to be accurate (microns), be able to monitor changes to the corneal shape and health.  We need to be able to respond fast when we see signs of inflammation, infections, and scarring.   This means that we need to acquire more equipment.  Although we can charge a fee for most procedures done, it would be unlikely that you will be able to fund all this just from scleral lens patients. Yes, you can start to fit scleral lenses without any fancy equipment and use your slitlamp.  But if you want to build a strong relationship with your network of professionals involved and build a successful scleral practice, then you will need to be able to measure in microns and be able to take digital photos of the eye.

The third hurdle to overcome is the multiple visits required with scleral lenses. To get the vision spot on, you don’t need more than three visits, but you would need to do a one-month follow-up and then a six-month follow-up.  I have a few patients who travel from as far as Zambia and Angola to get these lenses fitted.  This is when the turn-around of manufacturing becomes an issue.  With the local RGP lenses, I can get the patient in the final lens within three days (if you ask the lab nicely), compared to a quadrant specific scleral lens that will take fourteen days to receive the first trial.

Despite the abovementioned facts, I still prefer to fit sclerals over RGP lenses.   My approach these days, to help a patient better understand the difference, is to fit both options during the initial visit and give them a choice.  At least nine out of ten patients choose scleral lenses above RGP’s.  The last bit of motivation that I need is from the patient’s lifestyle.  Active people and outdoor lovers enjoy the benefits of scleral lenses.  Scleral lenses don’t fall out easily, don’t get dust particles stuck behind them, and the vision is very stable.

One last very important element of scleral lenses to remember is that for some patients, a scleral lens is the last and only option. A corneal graft may be an option, but the contra-indications of a graft totally outweighs the benefits. It is for these patients where scleral lenses truly change lives.  Patients can cry in your chair when they can see detail for the first time in ages.  They will forever, have the utmost respect for you as a practitioner.

What is available

Rather than focusing on product names, I think it is essential to understand the more general terms available.  Scleral lenses can be a semi-scleral or scleral lens.  Locally, in South Africa, they can manufacture both these options, but with limited parameters.  The Lathe cut technology is what is holding us back.  If more people fitted scleral lenses, it would make more sense for local labs to invest in this technology.  When we order more complex lenses, which is 90% of my scleral lens designs, then the only option is to use an imported product from the USA, Nieu-Zeeland, or the UK. When I refer to complex designs, I refer to lenses with toric peripheries or landing zones, quadrant specific SAG heights, toric base curves, different angles in the landing zones, and adjustable smart zones.

When we started to fit scleral lenses in South Africa, we would order a trial lens, and then ask the lab to make adjustments as per our requests.  This was acceptable for the lens you gave to the patient, but how do you repeat that process?  We learned that there is only one way to fit scleral lenses and that is to actually design a trial lens, then once you have seen how the lens interacts on the eye, you ask the lab to make changes to your design, but start from scratch on a new lens.  This improves the repeatability of the final product.  So, in short, we have access to all major brands of scleral lenses in South Africa. They take on average 14 working days to order in, and you would need about 2-3 attempts before the patient can use them.

The Consultation:

First Visit – My sequence of events during the consultations goes something like this.  I use the first visit to explain the theory behind the fitting and gather ocular information from the patient. Information needed is unaided and aided V/A’s, ocular history, I take slitlamp photos, we get topographies done, and then I use a trial set to determine my first custom lens for the patient.

Second Visit –  I want to look at the “fit” of the lens.  I cannot work on the refraction of the lens if the fit isn’t right.  I inform my patient on the first visit that the next lens we will receive is like a dental mould.  If my mould looks good, then we can work on the vision.  More than likely, I would get close with the fit of this lens and would look very critical at my base curve alignment and flexure of the lens (discussed later).  I need to pay careful attention to the over-refraction during this visit.

Third Visit – I now receive the second custom lens, and it needs to be the one that the patient can take home to go and practice handling.  They can only do this if they can see with the lens.  With a lot of practice, one starts to achieve this with your first lens, but don’t beat yourself up if it doesn’t happen straight away.   I spend a lot of time to look for small adjustments.  More detail to follow.  I look for B.C. alignment, limbal clearance, negative staining, and landing zone alignment.  With this lens, the over-refraction also needs to be carefully planned.  Rather ask the patient to return again to achieve some wearing time before making the last adjustments too quickly.  If you find an astigmatic over-refraction, make sure that lens rotation is very stable.  When you calculate your over-refraction, remember that it will change if you make any changes to the base curve off the lens. Apply the formula for changes to the base curve; for every 0.1 mm flatter Base Curve, you need to change the refraction with +0.50D and -0.50D if you go steeper.  Remember, my second lens should be close to perfect because big changes in the base curve were addressed during the evaluation of the first custom lens (second visit).

At the fourth visit – I now receive the third and final lens.  The patient usually comes in with the lens they took home after the previous visit, and they can give a good feedback about wearing time and cleaning pitfalls.  If necessary, we can even assess their handling techniques.  I have been pleasantly surprised about the creativity of some patients to try and get these lenses in their eyes. For example, a patient chopped off the bottom of the DMV lens holder and inserted it through the back of a polystyrene cup.  This allowed him to force both lids open with both hands instead of one.  He would then gently lower his face over the contact lens.  It worked for him.

If the patient managed to use the lens provided on the third visit, then you can spend adequate time examining the cornea.  Specifically look for staining, neovascularization, conjunctival prolapse, and limbal congestion.  With the lens on the eye, look for touch, excessive clearance, fluorescein leakage, build-up on the lens, and floaters trapped behind the lens, to name a few.

Scleral lens troubleshooting:

1. An air bubble or debris trapped underneath the lens – even a small one, can lead to increased lens awareness during the fitting process. With time, symptoms increase, and irritation becomes a reality. A tip to avoid application bubbles is to overfill the lens. Fill it so that the fluid creates a convex curve over the edge of the lens.  Ensure that the handling technique is correct.  Tilting the lens or pulling back at it can form these bubbles.

What to Do – It is critical to immediately remove and re-insert the lens without a bubble. I look for a bubble with the naked eye, but a Burton lamb can  also help.

Air Bubbles trapped underneath the lens – Photos by Pieter Naudé
Debris trapped underneath the lens

2. Edge Lift – Probably the most common reason for initial discomfort with scleral lenses is excessive edge lift or stand-off during the initial fitting process.  The area of discomfort is easily located, and the patient can almost pinpoint it.  A colleague in the USA likes to apply fluorescein and then sit back and see what happens.  One does not want to see a brightly illuminated green appearance under the edge.  Never does one want to see that all the fluorescein that you initially applied in the vault of the lens, drained out after 15 min.  OCT’s are handy to have, but one needs nothing more than a slit lamp, saline solution, and fluorescein to do this.  If in doubt, confirm edge lift with the addition of fluorescein after lens insertion.  Now, one can observe the tear flow from outside the lens margin to underneath the lens.  This tear flow can be spontaneous or forced by nudging the tear meniscus at the lens margin.4

What to do – Reducing the edge lift with a steeper peripheral curve will improve initial comfort.  Some labs indicate this in steps or degrees so make sure you understand the terminology for the product you are working with.

Edge Alignment – Photos by Pieter Naudé
Edge lift at landing zone

3. The edge profile is also important. I am not just talking about the alignment, but the shape of the very edge. Do you look with a magnifier at the edge of a lens when you receive it from a lab?  Probably not. We all apply blind faith and hope that the laboratory has done a good job. Visually examine the lens for any chips. Then look to see if the profile is shaped like a sharp or blunt knife. This causes discomfort, usually only after hours of wear.  Symptoms include eye ache, lens awareness, tightness, tiredness, soreness or the sensation of a bruise.

What to do – Ask the manufacturer for a more rounded edge.  When flattening peripheries, one can sometimes create sharper edges.

Rounded edge [5] – Photos by Pieter Naudé
Sharp edge [5]
4. Limbal Clearance – Optic zone base curve plays a role with limbal clearance and lens centration. If the base curve is too flat, expect a high clearance over the limbal area. When the base curve is too steep, you can expect very little clearance or even touch.  Both can cause discomfort or decentration. It is also likely for the discomfort to be associated with the initial fit.  In extreme cases, the patient may report a foreign body sensation and excessive tearing after lens removal.  The patient might also complain that the lens adheres to the eye and struggles to remove it.  Verify adequate central clearance (150-250 microns) and then judge the limbal clearance (fluorescein and slit lamp) aiming for close alignment of 50-100 microns.

What to do – to eliminate limbal bearing (touch), increased central clearance, or flattening of the base curve of the optic zone can alleviate the issue. Depending on your manufacturer, it may be possible to increase the optic zone diameter. The opposite is applicable to reduce limbal clearance.

Limbal bearing – Photos by Pieter Naudé
Good limbal clearance

5. Landing zone misalignment (lens flexure)– This is in my view one of the most critical issues that can improve comfort. It was once assumed that, like the cornea, the sclera features a curved shape.  As researchers looked more closely at maps and moulds of the eye, they discovered that from the peripheral cornea onward, the sclera often continues in a straight line, only 25% of cases showed a curved anterior sclera.  In the majority of cases, the sclera is tangential.3 When there is misalignment on the scleral landing zone, it can create bubbles entering from the lens margin, which end up underneath the lens in the post lens fluid reservoir. Note that these bubbles were not present directly after the insertion of the lens and will cause eventual discomfort.  Look for conjunctival staining along the flat meridian of the sclera after three hours (think 3 and 9 o’clock staining). This can identify lens bearing in these areas. There are two techniques to look for excessive edge lift off; negative staining or the shadow test.

Poor landing zone alignment can also be a cause for protein build-up.

What to do – In order to resolve this, one needs to order a toric peripheral curve on the lens — in other words, steepening the peripheral curve in the meridian where it is standing off the most. Some labs can design a toric sagittal value for the lens, and this is the way I prefer to get better alignment in the periphery.  It is also possible to order quadrant specific landing zones if needed.

Alignment in flat meridian – Photos by Pieter Naudé
Too steep in steep meridian

Insertion and cleaning

This can be overwhelming for some patients, but I do my own instructions with all my patients.  I fitted lenses on myself and can now relate better to the patient experience.

I always start to train the patients with a DMV lens holder as a starting point. The DMV lens holder works well for insertion and removal, but sometimes the standard RGP lens sucker works better for removal.  Once they have mastered the insertion with a DMV, then you can consider the teach them the tri-pod method, where they balance the lens on the thumb, index, and middle finger.

Put the mirror flat on the table and get them to fixate on the black centre spot of the DMV lens holder. This helps to align the lens properly over the Iris. Another important aspect, in the beginning, is to show them how to fill the lens with enough fluid correctly.  More than often, they would use too little, the fluid need to bulge over the edge of the lens.  Once they get a handle on the technique, it is actually easier than inserting a soft lens.

With the removal, I ask them to store the lens in AOSept solution or any peroxide system.  This helps to remove all protein from the surface of the lens and helps to prevent build-up the following day. A clean lens is a better lens. One can introduce additional alcohol-based cleaners to clean the lens even further, but I evaluate this on my one-month follow-up exam.

Conclusion

Don’t be intimidated by scleral lens fits.  Remember that when done correctly, one does not touch the cornea at all. Suction forces behind the lens should be minimal.  My advice is to ask an experienced colleague to guide you through your first fit.  You can contact the Contact Lens Society to ask for contact lens mentors in your region to assist you and be a “phone a friend” helpline. It is also important to assess the lens fit over increasing wearing times, e.g., 30 min, three hours, one day, and then again one month and one year.

References

[1]Van der Worp E, Graf T, Caroline P. Exploring beyond the corneal borders. Contact Lens Spectrum 2010;6:26-32.

[2] Melissa Barnett, OD, FAAO, FSLS, FBCLA, enhancing scleral lens comfort, 2018;04

[3] Van der Worp E, Graf T, Caroline P. Exploring beyond the corneal borders. Contact Lens Spectrum 2010;6:26-32.

[4] Van der Worp E, Graf T, Caroline P. Exploring beyond the corneal borders. Contact Lens Spectrum 2010;6:26-32.

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