A recurring fitting concept that has been discussed at the podium at the Global Specialty Lens Symposium (GSLS) and among researchers is utilizing sagittal depth (sag) of the eye to determine the best-fitting soft contact lens. This ocular measurement can be captured at a specific chord length via instrumentation or estimated using corneal shape descriptors (Young, 1992). While a custom soft contact lens can be manufactured from these measurements, the same logic of fitting by sag can be applied when selecting a single soft diagnostic lens from multiple trial lens sets.

Research Inquiry

During the GSLS panel titled “What’s Special About Specialty Soft Contact Lenses,” Matt Lampa, OD, shed light on the importance of understanding the contact lens (CL) sag of the lenses used in everyday practice. This CL-sag value is not a metric found on the foil packaging or labeling for commercially available soft contact lenses; however, it is a value that can be measured with lab-based technology (Figures 1 and 2).

Methods & Results

Researchers measured the sag of 12 daily disposable and 15 reusable lenses, all labeled –3.00D, with varying base curves and diameters from four major manufacturers (van der Worp et al, 2021). The researchers looked at 17 toric lenses, all labeled 3.00 –0.75 x 180 (six daily disposable and 11 reusable). A spectral domain optical coherence tomography (OCT)-based instrument was used to measure the posterior CL-sag.

It was shown that there were variations in the CL-sag values among common off-the-shelf soft contact lenses. There were sag differences between lenses of the same marked base curve and, in some cases, very small differences between two base curves of the same lens brand/type. A similar evaluation was carried out by a different research group and a variation in the measured CL-sag of a sample of commercially available soft contact lenses was also reported (Norman et al, 2017).

Clinical Applications

In practice, central keratometric readings of the cornea (approximately 4mm worth of data) are often used to determine the initial soft diagnostic contact lens. When the same manufacturer offers two different base curves for the same lens type, it is tempting to fit the flatter lens option on flatter eyes and vice versa. However, the actual difference in CL-sag from these two theoretical lens options are minimal and, thus, may fit nearly identically on the eye (assuming they have the same diameters). With the CL-sag chart that was noted above, it is evident that there is a range in the CL-sag values of existing soft contact lens.

The Future

There is renewed interest in putting the science back into soft lens fitting. With the aid of technology, it is easy to see that there is quite a variation in off-the-shelf lenses and they are not one-size-fits-all. Further research will support this discussion and allow practitioners to look at their soft diagnostic CL trays with a fresh perspective. CLS

Special thanks to Ben Colderick, PhD, for acquiring these OCT images.

For references, please visit www.clspectrum.com/references and click on document #317.