A 33-year-old male patient who was diagnosed with bilateral keratoconus at puberty came to the Instituto de Olhos Dr. Saul Bastos in 2008. He had a previous history of failure in achieving an ideal GP lens fit and had undergone intrastromal ring implants OS, with subsequent extrusion of the superior segment and infection. We prescribed aspheric intralimbal GPs that he wore until 2011, when we observed significant ectasia progression in both eyes. We then fit 17.5mm scleral lenses, which provided excellent results.
In 2013, the patient underwent penetrating keratoplasty OD and intracorneal ring implants with femtosecond laser. Despite the better anterior corneal profile obtained with the procedures, visual acuity was still poor, and the corneal topography still presented irregular astigmatism, especially OS. Figure 1 shows the patient’s current corneal topography; figure 2 shows Scheimpflug views of the corneal profiles in different contrast settings.
Refining the Fit
Fitting scleral lenses in this case helped us to overcome difficulties normally found in post-surgical and irregular corneas. We have improved the design of his scleral lenses over the last few years. Our objective is to always refine the lens to achieve the best anterior eye and corneal alignment.
We used a 19.4-mm diameter full scleral lens in a design that provides a larger landing zone to help alleviate the pressure over the conjunctiva. Figure 3 shows the cross-section view of both lenses. This scleral lens design has a multi-aspheric posterior curve that allows a customized vault, with a spline haptic developed to achieve the best landing zone. It also features ventilating invisible channels to allow tears or preservative-free lubricant to enter and slowly renew the tear reservoir.
The posterior haptic is quadrant-specific and is calculated by observation. We do not have an instrument to measure scleral topography, but by evaluating specific trial lenses, we are able to achieve the best combination of splines. We have found that if the differences at the quadrants are small, and if there are no scleral obstacles, the lens can be applied in any position and will rotate to the proper orientation in a matter of minutes, especially when the lens has most of the liquid lens reservoir.
Although most of the scleral lens designs that we have seen use a minimum center thickness of 0.25mm, we use less than 0.20mm in powers greater than –6.50D and have experienced no problems with flexure that would induce residual cylinder or with lens adesion from negative pressure.
One interesting fact in this particular case was that we were able to fit the same lens design despite the different corneal topography and treatment. The final scleral lens parameters were OD 42.50D base curve, –5.00D power, overall diameter (OAD) 19.4 mm, optic zone (OZ) 14.0 mm, sag 6.144 mm and OS 42.50D base curve, –4.25D power, OAD 19.4 mm, OZ 14.0 mm, sag 6.144 mm.
Scleral lens fitting has the benefit of being forgiving for different elevations and areas of the cornea. In some cases, there will inevitably be some areas in which the posterior surface of the lens is better aligned to the corneal surface compared to others. We always look for possible fit improvements when a patient comes for a refit, even if the fitting was adequate, the patient is satisfied, and visual acuity is good.
The best time to evaluate a scleral lens fitting is always after 3 to 4 hours of lens wear, as by that time the lens has already settled and will not change significantly. We also prefer to instill fluorescein after the lens is settled to observe whether the channels are working. Ten to 15 minutes is usually enough to observe the fluorescein entering the liquid lens reservoir; in 1 hour, the overall fluid in the vault will be mixed with the contrast.
An important area to view when evaluating a scleral lens fit is the transition over the limbus; the goal is to prevent any touch and to ensure a smooth landing on the sclera. An inadequate fit at this critical area may induce problems such as conjunctival prolapse and conjunctival debris that will cause fogging. We have been successfully dealing with fogging by designing a smoother transition over the limbus and onto the sclera.
Customization in scleral lens fitting is a healthy direction to pursue. It is possible to adequately define the necessary lens modifications by observation and by using a good trial lens set from a manufacturer that will work with you on the specific changes required to achieve an optimal fit. While not essential, it is important in scleral lens fitting to always try to extend the limits of what you can achieve. CLS
For references, please visit www.clspectrum.com/references and click on document #SE2017.