From the podium at this year’s Global Specialty Lens Symposium (Caroline et al, 2019), there was discussion regarding the sagittal depth (sag) of soft contact lenses and a call to major soft lens manufacturers and eyecare practitioners to adopt a fit by sag philosophy. After the lecture, a colleague and meeting attendee posed the question: “Is this solving a problem I don’t have?” Let’s discuss.
When It Isn’t Necessary
Currently, most commercial soft contact lenses are available in one, maybe two different base curve/diameter combinations. This certainly decreases the time required to determine the ideal contact lens fit for patients, and the soft contact lens industry did not get to this point of convenience by accident. Both on-eye performance (Roseman et al, 1993) and theoretical modeling (Young et al, 2017) have shown that a first-fit success rate of 90% or greater can be achieved with soft lens parameters that hover around the 8.6mm base curve/14.0mm diameter mark.
Suppose that human eye shape and size follow a normal distribution (Figure 1). From basic statistics, the hashed, black line that intersects the middle of the curve is the average eye, and more than 95% of the population falls within two standard deviations of the mean. The range for eye shape and size differs only in millimeter increments; thus, it is no surprise that such a large portion of the healthy cornea population can be successfully fit with similar soft lens parameters. Even when contact lens fitters carefully look for adequate corneal coverage and movement of a soft contact lens to deem it a “successful” fit, inadequate fits may be rare to none. By none I mean that if a non-ideal fit is observed with one soft contact lens brand, simply changing to another brand may yield success. Naturally, practitioners may rightly assume that their current soft lens fitting techniques are perfectly fine.
Figure 1. Average eye shape and size are assumed to follow a normal distribution pattern.
When It Makes a Difference
However, what if using a more detailed approach to soft contact lens fitting is solving a problem that you don’t know you have? While not ideal with regard to lens centration, Figure 2 shows a soft lens that has adequate movement and full corneal coverage (see video). Say that this patient has no complaints with comfort and vision. This lens is dispensed, and a “successful” fit is the outcome. But, in a head-to-head comparison of this lens with a lens that has a sag more appropriate for the sag of this eye, which lens would this patient report as more comfortable? Future studies are needed to shed light on this approach to soft lens fitting.
Figure 2. A right eye with keratometric values of 43.25/44.00 @ 090 fit with a soft spherical contact lens marked with an 8.6mm base curve and 14.0mm diameter.
While some may say that fitting soft lenses by sag should be reserved only for the those who fall at or beyond the extremes of the normal population, perhaps it is time to better serve all of the patients who fall under the curve in between. CLS
For references, please visit www.clspectrum.com/references and click on document #281.