Empirical Lens Fitting Gets a Makeover

It’s interesting how technology changes so fast. My very first scleral lens fit was in 2008. Back then, we were limited by the modifications we could make to a lens. I even remember being encouraged to take a Dremel tool to the edge of a scleral lens to adjust for pingueculas and blebs.

Fast forward to today. Contact lens manufacturers have made significant investments in their technology. Computerized lathe cut machines make it possible to design lenses with submicron precision. We can now manipulate edges and curves in ways that weren’t possible even a few years ago. But how useful is this technology if we can’t provide our labs with the necessary information about our patients? This ability to customize lenses puts the onus on us to provide more precise measurements.

Several instrumentation companies have answered the call to provide tools that help facilitate a more advanced fitting process. With these tools, practitioners are finding themselves moving away from diagnostic fitting to a more empirical process.

HISTORY OF EMPIRICAL FITTING

Historically, empirical contact lens fitting hasn’t been seen as a particularly successful practice.¹ Previously, we were only able to provide a spectacle prescription, along with horizontal visible iris diameter, pupil size, and keratometry readings. Although these findings are sufficient on a symmetrical cornea, any shape that was out of the ordinary made for a less-than-optimal result. Poor outcomes had everything to do with the limitations of the data we were providing. Keratometry, for example, only measures two points in a 3-mm to 4-mm zone, which is a very small region of the cornea that does not provide any information about the areas central or peripheral to these points. It also assumes that the principal meridians are 90˚ apart and are unable to measure irregular corneas.²

EMPIRICAL FITTING TODAY

With the instrumentation at our disposal today, however, we can approach empirical fitting with much more confidence. For example, corneal topographers supply tens of thousands of data points, and whether we use Placido disc, Scheimpflug, or scanning-slit topography, they each provide information that streamlines the contact lens fitting process.

Fitting software provided by these companies can take the entire cornea into consideration, allowing us to order lenses empirically (Figure). This technology is transforming how practitioners are fitting soft contact lenses, gas permeable lenses (including orthokeratology), hybrid lenses, and scleral lenses.

Empirical scleral lens fitting can be a bit challenging with traditional corneal topographers because these devices do not reach far enough out onto the sclera. Studies show that the human sclera is rotationally asymmetric, and its shape varies considerably between patients.³ Three scleral mapping instruments will measure the topography of the eye beyond the limbus: the sMap3D (Visionary Optics), the Pentacam Cornea Scleral Profile (CSP) report (Oculus), and the Eaglet-Eye Eye Surface Profiler (ESP) (Eaglet Eye).

These instruments obtain many more data points than your traditional topographer. The sMap3D collects 1,000,000 data points on the ocular surface,⁴ the Pentacam CSP Report takes 250 images for more than 100,000 data points,⁵ and the Eaglet-Eye ESP provides 350,000 data points.⁶ With all of that information, these companies can seamlessly help you decide on the first diagnostic lens or, in some instances, design a completely customized lens.^7,8

ADDING UP THE BENEFITS

Empirical contact lens fitting has a number of advantages. Patients are reported to have a greater initial satisfaction with their visit due to the reduced chair time and higher first fit success rate.^9,10 Because we are not reusing diagnostic lenses, empirical fitting is also viewed as a safer approach compared to diagnostic fitting.¹¹ This view is taken because we are potentially reducing the risk of infection by decreasing the number of people who handle the lenses.

As with many technologies in our industry, fitting empirically has come a long way. Not only is the process of receiving data easier, but manufacturers have streamlined the system of ordering and receiving initial lenses that have a high probability of being the final lens. Although I do not see diagnostic fitting becoming obsolete in the near future, I do see it becoming a more challenging process when compared to the precision and accuracy of empirical fitting software.