contact lens case reports
Using HVID and Central K Readings To Design a Custom Lens
BY PATRICK J. CAROLINE, FCLSA, FAAO, & MARK P. ANDRÉ, FCLSA, FAAO
In our last two case reports, we discussed how horizontal visible iris diameter (HVID) contributes to the overall sagittal height of the cornea. We demonstrated the need to adjust central keratometric readings when fitting corneas that are larger or smaller than normal.
This month we will describe how to design a custom soft lens based on central keratometric readings and overall corneal diameter.
Figure 1. A slit lamp eyepiece with a built-in measuring reticule.
Measure the patient's HVID and add 2.0mm for the appropriate amount of scleral drape. For example, if the HVID is 12.5mm, the suggested lens diameter would be 14.5mm. HVID is best measured using a slit lamp eyepiece with a built-in measuring reticule (Figure 1), which allows for an accuracy of approximately 0.2mm.
Next, we determine the "effective K" value. This is an estimation of the overall sagittal height of the cornea based on central K readings and the HVID. Therefore, if the HVID differs from normal (11.8mm), add 1.00D to the horizontal K reading for every 0.2mm larger or subtract 1.00D for every 0.2mm smaller. For example, if the flat K is 42.00 diopters, and the HVID is 12.5mm (0.7mm larger than 11.8mm), we add 3.50D to the flat K for an effective K of 45.50 diopters or 7.42mm.
TABLE 1: The Soft Lens Fitting Nomogram
|SOFT LENS DIAMETER||FIT FACTOR|
Once you have determined the effective K and lens diameter, refer to Table 1, the Soft Lens Fitting Nomogram, to select the appropriate fit factor. Determine the base curve radius of the contact lens by adding the effective K in mm's to the fit factor.
In our example, the suggested lens is equal to the effective K of 7.42mm (+) the fit factor of 0.90mm, for a base curve of 8.32mm, with a suggested overall diameter of 14.5mm
Determine the power of the contact lens by vertexing the spectacle prescription to the corneal plane. We recommend a diagnostic lens fitting with an over-refraction in cases where irregular astigmatism may be present.
Patrick Caroline is an associate professor of optometry at Pacific University and an assistant professor of ophthalmology at the Oregon Health Sciences University.
Mark André is director of contact lens services at the Oregon Health Sciences University.