A corneal topographer is your best friend when it comes to GP contact lens fitting. It provides useful visual information about the cornea (the color maps), but there are also many numerical values that make fitting GP lenses a precise science. Here are just a few of the most common values.
Keratometry Simulated K values provide a great starting point for GP fitting. They provide the amount of corneal astigmatism, which is useful for estimating residual astigmatism and choosing between spherical and toric base curves (see elevation notes below). As most corneal designs are meant to be fit with a close-to-alignment relationship, the K values are typically used to calculate an initial base curve to use.
Horizontal Visible Iris Diameter (HVID) HVID (sometimes called horizontal white-to-white or HWTW) is used to estimate the corneal diameter. Some instruments provide this value directly, while in other cases, eyecare practitioners use a ruler tool to take this measurement. HVID is useful for selecting a lens diameter for standard GPs, orthokeratology (ortho-k) lenses, scleral lenses, and even custom soft lenses.
Elevation Corneal elevation is arguably the most useful information of all when it comes to decision-making about corneal GP lens fitting. Deciding between spherical or toric base curves on a corneal lens is much more precise with elevation than with K values alone. Typically, a toric base curve is needed if the difference in elevation values between meridians is 60 to 80 microns or greater at an 8mm chord.
A similar rule is used to determine whether an ortho-k lens should have toric reverse and/or alignment curves, but here the rule is a difference of 30 microns or more. Elevation is also helpful in determining whether a corneal design will work on a keratoconic cornea. Here, the comparison is between the superior and the inferior cornea; success is more likely if the elevation difference is less than 350 microns (Zheng et al, 2015).
Aberrations Many topographers can give an estimate of anterior corneal aberration values. In lens fitting, I find this to be most useful when comparing pre- and post-ortho-k corneas. Ortho-k for myopia increases positive spherical aberration (PSA). A significant increase in PSA in adults can account for issues with decreased contrast noted by patients (and possible need to increase the size of the treatment zone). Conversely, an increase in PSA is beneficial for the myopia control effect, and smaller treatment zones can help to increase this change (Lau et al, 2020).
Topography and GP fitting were made for each other. Take some time to learn more about what the instruments have to offer the practice. CLS
- Zheng F, Caroline P, Kojima R, et al. Corneal elevation differences and the Initial selection of corneal and scleral contact lens. Poster presented at the Global Specialty Lens Symposium. Las Vegas, January 2015.
- Lau JK, Vincent SJ, Cheung SW, Cho P. Higher-order aberrations and axial elongation in myopic children treated with orthokeratology. Invest Ophthalmol Vis Sci. 2020 Feb;61:22.