When designing and prescribing custom soft contact lenses, the corneal feature most frequently relied upon is central corneal curvature, which is measured using a keratometry (K) value.
However, while the K value is useful in understanding how the cornea is shaped, it provides little information about the peripheral cornea, how the cornea changes from center to periphery (eccentricity), and overall corneal diameter. Clinical researchers have determined these other features actually have a greater impact on the overall sagittal depth of the eye than K value alone.1,2
Optometrists prescribing precision-fit custom soft toric and sphere contact lenses will find it useful to understand how the depth of the eye changes as all three of these anatomic features (central corneal curvature, corneal diameter, and eccentricity) change. This enables them to determine the appropriate base curve and diameter for custom soft contact lenses.
The corneal feature that has the greatest impact on sagittal height is the overall corneal diameter, or horizontal visible iris diameter (HVID). The larger the cornea is, the deeper the cornea will be. The corneal feature that has the next greatest impact on sagittal height is eccentricity, also known as corneal shape factor. As the normal human cornea transitions from center to periphery, it gradually begins to flatten. The rate at which it does so is referred to as “eccentricity.” The greater the amount of eccentricity, the faster the rate at which the cornea flattens. Lastly, the third corneal feature that affects sagittal height is corneal curvature, as measured by K value. As the central cornea steepens (K value increases), the depth of the eye increases.
Practice Pearl: Therefore, it can be said that when extremes are in combination — such as a large corneal diameter and steep central curvature, or a small corneal diameter and flat central curvature — the need for custom soft toric and sphere contact lenses is greater; so they should possibly be considered for patients with these specific anatomic features.
Understanding these corneal measurements and how they affect the depth of the eye will be very helpful in determining an accurate final lens prescription. To ensure the custom soft toric contact lenses you prescribe match the exact curvature and diameter of your patient’s eyes, use the SpecialEyes Arc Length Calculator. As you familiarize yourself with this tool, you’ll notice that the software incorporates both corneal diameter, otherwise known as horizontal visible iris diameter (HVID), and K values into the design of SpecialEyes custom soft contact lenses for a perfect fit and optimal comfort.
Matthew Lampa, OD, FAAO
Dr. Lampa is an Assistant Professor of Optometry at Pacific University and a consulting optometrist to SpecialEyes.
References:
1. Young, Graeme. “Ocular Sagittal Height and Soft Contact Lens Fit.” Journal of the British Contact Lens Association 15: 1 (1992): 45-49.
2. Young, Graeme; Holden, Brien; Cooke, Geoff. “Influence of Soft Contact Lenses Design on Clinical Performance.” Optometry and Vision Science 70:5 (May 1993): 393-403.