Introduction
In 1945, Norman Bier and Joseph Dallos separately described how fenestrating a sealed scleral lens allowed for a tear exchange that provided oxygen to the cornea, with the result that the lenses could be worn for extended periods of time.
The introduction of high-oxygen-permeable gas permeable materials has revolutionized the scleral lens and once again provided patients with the very best contact lens for many ocular conditions.
All current scleral designs are sealed lenses. Oxygen is supplied to the cornea through the lens and, as Irving Fatt has suggested, with tear exchange from under the scleral skirt of the lens.
Benefits
For many years of fitting scleral lenses, in glass, PMMA, and gas permeable materials, I have fitted only fenestrated scleral lenses. The benefits of a fenestration are a constant flow of fresh tears over the cornea and the clearance of debris from behind the lens. All-day comfortable wear is normally achieved and is maintained for many years.
The fitting process is undertaken using a series of “Fenestrated Lenses for Optical Measurement” (FLOM), initially described by Norman Bier. This is a series of lenses with varying base curves and diameters to provide different sags and lens clearance. Fenestrations will result in an air bubble. The bubble should be located at the limbus and must circulate around the limbus with normal eye movement. The bubble must not be stationary and must not infringe on the pupil area. Once the lens relationship to the cornea and bubble are achieved, by applying digital pressure on the lens, the final correct lens fitting and size of bubble can be accurately estimated.
Bubble
Unlike sealed lenses, fitting a keratoconic cornea with a standard Bier FLOM lens will result in a large bubble likely infringing onto the pupil area. The fitting of such a cornea requires the lens to include a central spherical back surface with the peripheral central curves following the contour of the cone. I always encourage practitioners to learn how to modify lenses (corneal and scleral) to fine-tune their fit. It is not difficult and does not take much time to do, and such modification is undertaken while the patient is in the practice.
A series of FLOM lenses has been developed for keratoconic corneas that includes reverse geometry back surface designs. This allows for a lens fitting that requires minimal modifications.
Lens Settling
It is said that the problem with fenestrated lenses is that they settle on the eye, requiring lens modification. Lenses do settle, but it is not difficult at the initial fitting evaluation to estimate how much the lens will settle and to allow for this when ordering the lens. With the advent of OCT instrumentation, lens design and estimation of lens settling is much more precise. Not needing to fill the lens makes insertion easy, especially for pediatric patients.
Tear Flushing
A scleral lens that allows for the circulation and the continual flushing of tears is important for the health of the cornea. I expect that in the future, fenestrations or lens vents will be the norm for scleral lenses.
Advances in technology and fitting methods will provide more precise lenses that will be clinically better designed. With the advent of 3D printing, I can visualize a more precise fitting of scleral lenses with the necessary features allowing for the constant circulation of tears to and from the corneal surface.
Closing Remarks
The resulting 3D fenestrated or ventilated gas permeable scleral lenses designed from the data of advanced instruments will incorporate the necessary lens clearance to allow for lens settling. I believe that the resultant lenses will relate to the cornea and sclera precisely as required so that when applied to the eye, they will provide optimum optical vision and comfort, ensuring long-term ocular health.
