Soft Lens Fitting Nomogram

Just like the RGP fiting nomgram their are rules of thumb that are used for the fitting of Soft Lenses. Here is a common fitting philosophy to help you fit patients more easily and accurately in soft lenses. Since most lenses come in three base curves:

If Flattest K is
> 45.00D	Fit the steeper Base Curve
41.00D to 45.00D	Fit the median Base Curve
< 41.00D	Fit the flattest Base Curve

The HVID (Horizontal Visible Iris Diameter) is going to play a role in the fit as well. The average HVID is anywhere from 11.6mm to 12.0mm in size. To get the diameter of the soft lens just add 3mm or 1.5mm to each side of the lens to allow for good coverage. Now if the patient has a smaller or even larger diameter HVID that would mean that the diameter of the lens is going to be smaller or larger respectively. Also the lenses sagittal height is going to mean that the fit is going to change. Bellow is a compensation table to apply to the flattest base before chosing the Base Curve.

Flat K Compensation
HVID	Compensation
10.0mm	-8.00D
10.2mm	-7.00D
10.4mm	-6.00D
10.6mm	-5.00D
10.8mm	-4.00D
11.0mm	-3.00D
11.2mm	-2.00D
11.4mm	-1.00D
11.6mm	0.00D
11.8mm	0.00D
12.0mm	0.00D
12.2mm	+1.00D
12.4mm	+2.00D
12.6mm	+3.00D
12.8mm	+4.00D
13.0mm	+5.00D

Now take the adjusted K’s and convert it to mm with the following equation:

Base Curve = 337.5 / Adjusted K’s

+0.3mm to the Base Curve for the final lens

These compensations are performed due to the elliptical nature of the cornea. A good way to remember these compensations is with the following rules.

For every o.2mm difference from the average HVID (11.6mm to 12.0mm) compensate by adding or subtracting 1.00D respectvely to the flattest K’s, then convert the K’s over to mm radius and add 0.3mm to that measure. Also add 3.0mm to the HVID to come up with the final soft lens diameter.

For power Vertex the spectacle Rx to the corneal plane.