The Science of Fitting Soft Contact Lenses

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science of contact lens fitting

With so many different contact lens brands and materials on the market, it might be tempting to assume that these medical devices are one size fits all. But there is a very specific science to contact lenses fitting correctly on the eye, without being to loose (in medical terminology this is called flat) or too tight (or steep). A well fit contact lens will provide stable and clear vision, be centered across the cornea, fully cover the cornea, and adequately move with a blink (defined as 0.2 to 0.4 mm of vertical movement with a blink.) 

Well fit contact lens
A. A well fit contact lens showing good centration and full corneal coverage via. B. A poorly fitting contact lens showing inadequate corneal coverage and fluting of the edge via

Prior to disposable contact lenses entering the market in 1987 with the introduction of Johnson & Johnson Vision Care's Acuvue, soft contact lenses were custom designed for a patient's unique corneal shape, size, and prescription. When we average out corneal shape and size for most patients, we do find that the majority of patients fit into an "average" shape, which is why mass-produced soft contact lenses have become the predominant modality of soft contact lens wear. In fact, a 2017 mathematical analysis revealed that a 8.6 base curve and 14.2 mm diameter contact lens achieved a successful fitting relationship in 90% of patients. 


distribution of average cornea size
Most patients fall into an "average" range of eye size and shape that provides a successful contact lens fit from an average contact lens shape of 8.6 mm base curve and 14.2 mm diameter. via

To better understand the factors at play, we need to define the two measurements listed on your prescription contact lens box that dictate how a contact lens will fit. Base Curve is the curvature of the lens, and diameter is the overall size of the lens. 

Base Curve
Base Curve of the contact lens. In this example, base curve is 8.5 mm 
 
Diameter Contact Lens
Diameter of the contact lens. In this example, diameter is 14.3 mm


It is tempting to think that the base curve number will define how tight or loose (steep or flat) a contact lens will fit, but in actuality, how a contact lens fits is dictated by a relationship between the base curve and the diameter together, called the sagittal depth. Research shows that the base curve number written on a box of contact lenses is actually a poor predictor of how a contact lens will truly fit on the eye, and to complicate matters, the labelled base curve of a lens on the box many not even truly reflect the curvature of the lens as manufacturers often incorporate mathematical modifications to adjust how a contact lens fits in the periphery versus in the center. Base curve can also change as you wear a contact lens based on environmental factors like the lens drying out, temperature changes, and exposure to makeup or soaps.

As a doctor, selecting a contact lens for best fit is also challenging because the sagittal depth of any contact lens is not notated on the packaging. Additionally, research shows that base curves and diameters as they relate to sagittal depth is not universal across contact lens manufacturers. You would assume that two lenses made by two different companies but both with a 8.6 base curve and a 14.2 diameter would be nearly the same, but that is not always the case. Luckily research done in 2021 has shed some light as to the actual sagittal depth of popular daily disposable contact lenses on the market.

To use this data as a tool for improving contact lens fitting outcomes, there are a few key take-aways:

  • Smaller corneal diameters (smaller than 11.7 mm) are associated with lower sagittal depth
  • Larger corneal diameters (larger than 11.7 mm) are associated with higher sagittal depth
  • Troubleshooting poor vision
    • A lens that is moving excessively or rotating with an astigmatism lens will exhibit better stability with a higher sagittal depth
    • A lens that is too tight may be warping or vaulting as the patient blinks. In this case you would see minimal to normal lens movement, and a toric lens for astigmatism would look stable with no visible rotation. You can check for vaulting by using an autorefractor, topographer, or keratometer to assess the mire rings as the patient blinks. If the mires alter between blinks, the lens is vaulting and you need a lower sagittal depth lens.

Sagittal Depth of daily disposable contact lenses
via

The Take Home: Contact lenses are not one size fits all, and the numbers listed on the contact lens box are not truly indicative of how a contact lens will fit. New research is helping doctors understand the important fitting relationship between corneal size and shape and the available contact lens options on the market to provide the best fitting relationship and optimize both vision and comfort.

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