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:
+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.
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on Monday, February 4th, 2008 at 11:08 pm and is filed under Uncategorized, Ophthalmic.
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