Scleral lenses are ideal for the correction of irregularly shaped cornea, whether that irregularity has been caused by keratoconus, traumatic injury, laser surgery or a corneal graft.
The scleral lens provides a smooth refracting surface, and the irregularities of the cornea are cancelled out by the liquid lens between the back of the lens and the front surface of the cornea.
In other types of contact lens fitting, the aim is to match the curvature of the lens as close as possible to the shape of the cornea. With a scleral lens however, the aim is not to touch the cornea at all, but to ‘vault’ over the corneal surface and land beyond the limbus, on the scleral conjunctiva. One needs therefore not to think in terms of radii of curvature, but in terms of sagittal heights.
Manufacturers will offer guidance on the choice of initial lens, depending on HVID and type of irregularity. The lens is then inserted with non-preserved saline and fluorescein in the bowl. Once the lens is inserted, and no air bubbles are present, an initial observation may be made.
Viewing the lens with an oblique narrow beam and white light provides a cross-sectional view that allows the practitioner to assess the height of the lens above the apex of the cornea, or any cone or protuberance that might be present. At this early stage a clearance of about 300 microns is required, (which equates to the thickness of the lens for comparison).
The edges should be examined to ensure that they provide an adequate seal and are parallel with the scleral, and the limbus is examined to ensure that fluorescein is present with no contact with the limbus and the lens. An initial evaluation of vision at this time can give an indication of what visual acuity is likely to be achieved. This can motivate the patient significantly to persist with what can be a long fitting procedure.
After this initial examination and any change of lens has been made, the patient needs to wear the lenses for a minimum of two hours – longer, if possible. This is to allow the lens to settle into the sclera. This process takes time and cannot be rushed. The lens will settle by up to 150 microns, and that is why adequate clearance must be allowed for at the initial examination.
At the second visit, the fluorescein should still be visible under the lens. Its absence can indicate a poor seal around the edge. The height and clearance of the lens can be assessed, and a final judgement made.
Scleral lenses are not difficult to fit, but need a different skill set than is needed for regular lens fitting. The benefits to the irregular cornea patient in terms of vision quality, and the improvement in comfort for the dry eye patient are immeasurable.
The novice practitioner needs to attend a workshop and receive hands-on training from a skilled practitioner or technician rather than rely upon his existing skills.
While a slit lamp can be used to assess the fitting of a lens, the accuracy and clarity provided by OCT imaging makes for a much more accurate procedure, and a better result for the patient.