CONTACT LENS CASE REPORTS
WHERE DID WE GO WRONG WITH SOFT LENS PARAMETERS?
PATRICK J. CAROLINE, FAAO, & MARK P. ANDRÉ, FAAO
We believe that the contact lens industry is in general agreement with the following criteria related to soft lens fitting: 1) that optimum overall lens diameter (OAD) should be approximately 2.0mm to 2.50mm larger than the horizontal visible iris diameter (Figure 1); and 2) as a general rule, smaller-diameter corneas require lenses with less sagittal height compared to larger-diameter corneas (Figure 2).
Figure 1. Optimum soft lens diameter is most commonly between 2.0mm and 2.5mm larger than the horizontal visible iris diameter.
Figure 2. Smaller-diameter corneas require lenses with less sagittal height; larger corneas require lenses with greater sagittal height.
Changing Soft Contact Lens Sagittal Height
Given those two fundamental fitting parameters, soft lens manufacturers in the early 1970s had two choices for creating lenses with varying sagittal heights. Option #1 was to keep the OAD the same and create different sagittal heights by changing the base curve radius (BCR). In Figure 3, all four lenses have a uniform 14.5mm diameter, and changes in BCR (8.9mm, 8.6mm, 8.3mm, and 8.0mm) result in sag heights from 3,652 to 4,501 microns. In this case, flattening the BCR from 8.3mm to 8.6mm results in a 276-micron decrease in lens sagittal height.
Figure 3. Option #1 in creating soft lenses of varying sagittal heights is to keep the overall lens diameter constant and change the base curve radius.
In Option #2 (Figure 4), the BCR of the lenses remains constant at 8.6mm, and the sagittal height is created by changing OAD (13.5mm, 14.0mm, 14.5mm, and 15.0mm). This results in lens sag heights from 3,191 to 4,290 microns. In this case, increasing the OAD from 14.0mm to 14.5mm increases the sag height by 364 microns.
Figure 4. Sagittal depth change can also be made by keeping the base curve constant and changing the overall lens diameter.
It’s Time to Choose Wisely
Today, clinicians are in agreement that soft contact lens fitting is an exercise of raising and lowering the sagittal height of the lens onto (or off of) the ocular surface. We now know that this can be effectively accomplished by changing the BCR or OAD. We believe that 45 years ago, our industry chose poorly…it chose to change lens sagittal height by making one diameter (i.e., 14.0mm) to fit every size cornea. It is clear today that because of the strong association between corneal diameter and sagittal height, a much more logical approach would have been to vary the OAD.
Is it too late to change this rather significant oversight? We believe that is a question that contact lens manufacturers will need to address and justify sooner than later. CLS
Patrick Caroline is an associate professor of optometry at Pacific University. He is also a consultant to Contamac.
Mark André is an associate professor of optometry at Pacific University. He is also a consultant to CooperVision