Keratoconus, without exception, has always been the number one indication for specialty lenses, particularly scleral lenses, for both pre and post-surgical intervention. Studies show that the prevalence of keratoconus in the general population was estimated to be 1:375. These values appear to be 5-fold to 10-fold higher than previously reported. Two reasons for this may be the ever-increasing use of corneal topographers in practices globally and increased awareness of the condition.
So, because keratoconus is here to stay – as is secondary ectasia (often resulting from refractive surgery) – this will remain a key consideration in every specialty lens practice. For severe irregularities in stage 3 and stage 4 keratoconus, scleral lenses come to the rescue – as they bridge over the entire cornea, resting on the scleral portion (the conjunctiva, really) on the ocular surface. This explains, at least in part, the vast success of scleral lenses over the last decade.
Cross-Linking
However, the landscape of the irregular cornea in keratoconus is set to change in a positive direction. First came the development of corneal cross-linking (CXL), which has now been around in Europe for 20 years (human studies of UV-induced CXL began in 2003 in Dresden, Germany). CXL does not cure or diminish keratoconus but can help stop further progression at a given stage. The primary purpose and literal definition of CXL is to halt the progression of ectasia, although this is not always entirely feasible. Typically, the earlier in the that the procedure is performed, the better the result. That said, the best candidates for CXL are individuals with progressing keratoconus or ectatic disease of the cornea, so ECPs must monitor for progression first before the procedure is considered. In other words, if the cornea is already irregular to at least some degree by then, and preventative CXL is not feasible.
So, while across Europe, and increasingly in other parts of the world, CXL is increasingly becoming the standard of care for young patients with progression, it is most likely we will see more keratoconus being diagnosed. However, with earlier monitoring it is to be expected that we will see corneal changes in the earlier stages of the condition. This may impact what correction methods we apply and as such glasses and soft (specialty) lenses may become more common as the first-line method of vision correction.
Lamellar Corneal Transplants
There have been major developments in corneal transplants for keratoconus and other diseases. Not too long ago, a penetrating keratoplasty – replacing the entire cornea in its full thickness with donor tissue – was the standard procedure. Now, an array of lamellar techniques in which only a portion of the cornea is replaced by donor tissue is increasingly used. This can provide major advantages, one being that the risk to the endothelium being far less, because the back part of the patients’ original cornea remains intact.
It was In the late nineties, that the technique of deep anterior lamellar keratoplasty (DALK) was developed. The thought behind this technique is to replace only the diseased part – that is, only the diseased anterior or front layers of the cornea are removed. The healthy part of the cornea is left in place (which is the important, healthy, innermost layer of the cornea called the endothelium, is retained) avoiding the major disadvantages of a full-thickness corneal transplantation.
Bowman Layer Transplant
Recently developed is another new surgical technique: Bowman layer transplantation. Just as with CXL, Bowman layer transplantation intends to prevent further deformation of the cornea and delay the need for a corneal transplant. This technique is indicated when the cornea is too thin for CXL. With a Bowman layer transplantation, a thin layer of donor cornea (limited to just a donor Bowman layer) is implanted between the connective tissue layers of the cornea, making the original cornea become firmer. Sometimes even two donor Bowman layers are placed on top of each other on the receiving eye for that purpose.
Corneal Allogenic Intrastromal Ring Segments And Keranatural
Visual rehabilitation in keratoconus is approached in a stepwise manner. Options include spectacles or contact lenses, advanced surface ablation combined with CXL, phakic IOL implantation, and intrastromal corneal ring segments (ICRSs). When these modalities are insufficient, DALK or PKP is required. Keratoconus patients are often young, and corneal transplantation carries the risks of rejection, long-term failure, weakened corneal wound from trauma, and ocular surface disease.
Traditional ICRSs are made of PMMA and implanted in the midperipheral cornea to induce flattening and regularize corneal curvature. This produces a decrease in astigmatism and an improvement in patients’ visual acuity. Known complications of the technique are melt of the overlying cornea and extrusion. As described by Soosan Jacob, MS, FRCS, DNB, corneal allogenic intrastromal ring segments (CAIRS) are a modification of the technique that uses allogenic human donor corneal tissue. The tissue is deepithelialized, deendothelialized, and cut into the desired arc shape with a double-bladed trephine. The donor tissue is then implanted in channels in the midperipheral cornea cut with a femtosecond laser. Because of the reduced risk of device extrusion, the channels may be shallower than for ICRSs and thus have a greater capacity to induce flattening. An all-laser version of CAIRS has been reported in which the donor rings are fashioned with a femtosecond laser. CAIRS has been shown to induce regularization of the cornea, centralize the cone, and reduce regular and irregular astigmatism.
An issue with CAIRS is that it requires the use of transplant-grade corneal donor tissue, thus adding to demand. KeraNatural products (Lions VisionGift) are acellular rings and arcs derived from human donor corneas that are deemed to be unsuitable for transplantation owing to low endothelial cell counts. The rings and arcs are e-beam irradiated and sterile and have a shelf-life of 2 years. KeraNatural products have been used in CAIRS, reducing demand for donor tissue.
Soft Lenses to the Rescue
All these techniques may lead to a significant reduction of severe irregular corneas. This may also mean that scleral lenses will not always be necessary. Eye care practitioners around the globe increasingly consider a soft lens as an excellent first option after glasses fail to provide adequate visual acuity. The advantages of this is obvious: scleral lenses are large and handling sometimes can be an issue, wearing time is not always optimal, and limitations such as mid-day fogging, redness and potential intraocular pressure increase and other frustrating problems are eliminated if keratoconus is not left to progress to the point of needing more complex lenses. Especially the ease of use, comfort and cost are regularly mentioned as a plus for soft lenses.
This doesn’t always work with standard (off-the-shelf) soft lenses. First, the overall sagittal height of the ocular surface regularly goes beyond 4,000 microns over a 15mm chord in these corneas. Because of this, standard commercially available lenses on the extreme right-hand side of the Pacific SAG charts are recommended³. If this is not feasible, ‘out-of-standard soft lenses with higher SAG values can be considered. These have virtually no limit in SAG value height. Customized soft lenses based on corneal topography – with tangential peripheral shapes, almost like a scleral soft lens – are on the market as well (also with monthly replacement in silicone hydrogel materials). The question is: Does it still correct or at least mask the corneal irregularities enough for acceptable vision?
'Masking'
If the answer to the latter question is negative, there is a newer category of soft specialty lenses specifically for keratoconus. These lenses generally have increased central thicknesses, typically in the 300-450 micron range (a standard soft disposable lens may be 80 microns thick) to (partially) mask the irregularity of the cornea to achieve acceptable vision. Although these lenses can be made in silicone hydrogel materials, the Dk/t is lower because of the ‘t’ (thickness) of the lens, and patients wearing these lenses should be monitored for hypoxia. Some lenses can be ordered in 100-micron steps to increase or decrease thickness. Some lenses also have the option for ‘sector management,’ in which one quadrant can be ordered flatter or steeper (like quadrant-specific corneal or scleral lenses) to align the lens better with the keratoconic eye.
Illustrative of the latest surge in interest in soft specialty lenses for the irregular cornea is that recently some new soft keratoconus lens designs have entered the market. Some of these also try to improve vision further by using aspheric front optics. Trial sets in some of these lenses are also marked in sagittal height increments.
In Summary
So, while on the one hand, corneas don’t get nearly as irregular as they used to for a variety of reasons, on the other hand, the lens material, design and optics of soft specialty lenses are improving. The landscape of the keratoconic/irregular cornea, therefore, seems to be changing. And with that, the landscape for specialty lenses in that indication is also transforming, with a potentially more prominent place for soft lenses in that arena. It may be time to adhere to that – in clinical practice and from an industry perspective.
Educational Series & the Future
According to the annual Eurolens survey published in Contact Lens Spectrum (Jan 2023), at least 86% of ALL lenses fitted for ALL patients today are soft. But we can’t fit all KC patients with standard lenses. Therefore future looks bright for specialty lenses, especially when it comes to soft specialty lenses. We are just beginning to 'scratch the surface'. Stay tuned!
