Corneal GPs for Keratoconus: Proven Designs With High Success
TOPIC: Corneal Versus Scleral for Keratoconus
Corneal GPs for Keratoconus: Proven Designs With High Success
By Bruce Anderson, OD, FAAO
One of the greatest pleasures in fitting keratoconus patients with contact lenses is achieving excellent vision with comfortable fitting contact lenses. Because of their high level of success, corneal lenses are my initial consideration when fitting keratoconus patients.
New Keratoconus Fits
Whenever fitting keratoconus patients who are new to lens wear, you want to ensure that each patient will be as successful as possible. I define success as being able to wear lenses comfortably all day with good and stable vision.
To provide this level of comfort and success with contact lenses, think “large.” As my fitting philosophy and arsenal of available lenses have evolved over the years, I have migrated toward larger-diameter corneal lenses for most of these new cone fits. My lens design of first choice is now usually in the diameter range of 9.5mm to 11.5mm, with the majority at the 11.2mm size. These lenses settle very quickly and exhibit less movement, which allows a rapid adaptation and stable vision. In more advanced cones, this large diameter holds the lens very securely and has the ability to vault the cone to decrease any apex irritation.
Current Keratoconus Wearers
Most long-term keratoconus patients are currently fit into corneal GP lenses. The lens size and style depends on how advanced the keratoconus may be and for how long the patient has been wearing GP lenses. In years past, the designs have been smaller in diameter, ranging from 8.5mm to 9.0mm. More recent design and fit trends have been toward somewhat larger lens diameters. Current larger-diameter corneal cone lens designs may start around 9.2mm, with the larger intralimbal designs up to 14.5mm.
If a patient is very successful with his current lens designs, I am slow to make major changes. I will provide newer and more permeable materials. I will also frequently increase the diameter of the lenses slightly to provide more pupil and corneal coverage. This will provide a more stable and comfortable fit, as well as improved night vision. However, on most of these patients, I will keep them in similar design corneal lenses.
So, the argument for long-term corneal lens patients is to keep the success that they may already be experiencing with their current lenses, but also improve on what is working well for them.
Other Corneal Lens Advantages
Keratoconus lens wearers may occasionally have residual astigmatism. This is typically determined when the best-corrected vision is less than expected, and uncorrected astigmatism is measured in the over-refraction. For these situations, a front-surface toric lens can be designed to provide enhanced vision. For the adventurous and brave GP lens fitters, bifocal lenses can also be designed for keratoconus patients. I have several patients who have moderate cones wearing translating segmented bifocal lenses very successfully.
The Best First Choice
According to an informal survey of several leading GP manufacturing labs, the primary lenses ordered for fitting keratoconus are corneal lenses. It has been estimated from these discussions that 80 to 95 percent of the lenses manufactured for keratoconus are corneal. Also, the diameters of the lenses used for keratoconus have been increasing as new designs are introduced.
In summary, corneal lenses have a very high fitting success rate. A tremendous number of designs are available through different manufacturers. These designs can be highly customized to provide excellent comfort with the best acuity possible. For these reasons, I consider fitting a corneal GP lens for keratoconus as a first choice. CLS
Dr. Anderson is the director of the Contact Lens and Low Vision Clinics at the University of South Florida, College of Medicine, Department of Ophthalmology in Tampa. He is a Diplomate in the Cornea and Contact Lenses section of the American Academy of Optometry. He specializes in keratoconus and other difficult lens fits, and he is a clinical investigator for several contact lens companies.
Sclerals Can Offer Better Comfort and Stability for Keratoconus
By Gregory W. DeNaeyer, OD, FAAO
Scleral contact lenses have become an important part of modern specialty lens fitting. In many cases, they offer numerous advantages over corneal GP lenses for fitting irregular corneas. This is especially true in the case of keratoconus, which in its moderate to advanced stages can challenge even the most experienced lens fitters.
Lens Fit and Stability
Achieving lens centration and stability is often an issue when fitting corneal GP contact lenses on keratoconus patients. By their nature, corneal GP contact lenses center over the steep area of the cornea, which unfortunately for many keratoconus patients is inferior to the central visual axis. Best potential vision can certainly be affected in such cases if the patient has to look through the peripheral optics of a decentered contact lens, and comfort is diminished with blinking over the lens edge. For steeper eyes, a corneal GP contact lens can often decenter off the cornea or eject from the eye.
The large size and semi-sealing fit of a scleral contact lens solves these centration and stability issues. A keratoconus patient with a properly fit scleral contact lens will always be looking through the central optic zone with a lens that won't move, decenter, or spontaneously fall out.
In addition, trying to successfully balance a small corneal GP lens on the apex of a keratoconic cornea can take multiple lens remakes. Scleral lenses are far more forgiving compared to corneal GP lenses, and so the fitting is usually faster and requires fewer adjustments. Ultimately, this speeds up the process and reduces chair time.
Corneal Health and Comfort
Fitting a corneal GP lens on a patient who has advancing keratoconus often involves compromise. More often than not, the fitter is trying to balance between contact lens touch and vault. Severe keratoconus is associated with more touch and sometimes bearing to achieve a successful fit. Increased lens bearing on the corneal surface can cause secondary epitheliopathy leading to irritation and lens intolerance. Piggybacking can help, but then the patient has to wear two lenses.
A scleral contact lens is able to avoid any negative lens-to-cornea interaction by completely vaulting over the corneal surface.
Scleral contact lenses are able to lock in on the eye using a prism ballast or toric/quadrant-specific back surfaces. They are further stabilized by their semi-sealing fit. This leads to the future possibility of adding higher-order aberration correction to the lens prescription beyond the front-surface asphericity that is offered in some corneal GP keratoconus lens designs.
Seeing is Believing
Keratoconus patients who have moderate to advanced progression but clear corneas are often referred to our practice for corneal transplant evaluation because they have become contact lens intolerant. Typically these patients have been to several practitioners and have tried multiple corneal GP lens designs without success.
By simply putting a diagnostic scleral lens on their eye(s), we are able to demonstrate in a matter of minutes that many of these patients can achieve comfortable and stable lens wear with a lens that provides excellent vision. I often wonder if those same patients would have gone on to have a corneal transplant had this option not been demonstrated.
Corneal GP lenses are still an important modality for managing keratoconus, and I utilize them for select patients. However, with all of the benefits that scleral lenses offer, don't we owe it to our keratoconus patients to fit them in a lens design that often is more comfortable and stable— and may keep them out of the operating room? CLS
Dr. DeNaeyer is the clinical director for Arena Eye Surgeons in Columbus, Ohio. His primary interests include specialty contact lenses. He is also a consultant or advisor to MedLens Innovations, Inc. Contact him at email@example.com.
Contact Lens Spectrum, Issue: December 2010