contact lenses
The Bitoric HGP Calculator
Fitting highly toric corneas doesn't have to be that hard.

Bitoric, hard GP (HGP) lenses often provide the best solution for highly toric corneas yet are overlooked by many clinicians due to perceived fitting difficulty. I recommend the divide and conquer strategy to simplify bitoric lens fits. I'll explain my approach here.

Step #1: The sum of its parts

You can break down any toric eye into two spherical components: one flat and one steep. Approach the complex bitoric fit as if fitting two spherical eyes. Combine the two spherical lenses to yield the final bitoric lens.

	Toric eye: Ref: -1.00D -6.00D X 180 K: 44.00@180/50.50@90 Final lens: BC: 43.75/49.75 Power: -0.75D/-5.75D
=
	Flat spherical eye: Ref: -1.00D Sph. K: 44.00@180/44.00 Sphere Final lens: BC: 43.75 Power: -0.75
+
	Steep spherical eye: Ref: -7.00D Sph. K: 50.50@180/50.50 Sphere Final lens: BC: 49.75 Power: -5.75D

Step #2:

Mimic a half diopter with-the-rule cornea. This creates a vertical rock, which is beneficial for tear exchange. To accomplish this, fit the vertical meridian an additional 0.50D flatter than you fit the horizontal meridian. View your toric patient's eye as one flat eye plus one steep eye that you will fit independently with two spherical lenses. Design the lens empirically or use a diagnostic lens; I'll explain both approaches.

Empirical fitting: I typically fit lenses 0.25D flatter than Keratometry (K). For the flat eye (see example below), I would prescribe a lens with a base curve of 43.75 and a power of 0.75D. Generally, an ideal fit is achieved with a 0.50D with-the-rule cornea. To emulate this, I fit the vertical meridian 0.50D flatter than the horizontal meridian for a total of 0.75D flatter than K. For the steep meridian, I would prescribe a lens with a base curve of 49.75 and a power of -5.75D (corrected for effectivity). Finally, combine the two for the final lens: base curve (BC): 43.75/49.75 and power: -0.75D/-5.75D (specify to the lab that these are drum readings).

Diagnostic lens fitting: Pick a trial lens that you would normally use to fit the flat eye. Do a spherocylindrical overrefraction. Fitting the flat eye is straightforward. Use the BC of the trial lens and calculate the power by adding the trial lens power plus the overrefraction along the flat meridian. Choose a BC for the steep eye 0.75D flatter than the steep K. Plug this base curve and the overrefraction along the steep meridian into this formula to calculate the lens power:

(BCtl - BCsl) + Ptl + OR = Psl

(tl=trial lens; sl=steep lens; P=power; OR=overrefraction)

Warpage of the trial lens may cause error, which can be observed by over K. Compensate for this.

The calculator

I programmed my computer to crunch the numbers to simplify bitoric lens fitting and refinement while preventing mathematical errors or omissions. Here's how it works:

I've plugged in the numbers from the case above. Just enter the refraction and keratometry readings. Under "Fit Adjustment," I've programmed the calculator to emulate one half diopter with-the-rule cornea. By default, the program chooses a horizontal base curve 0.25D flatter than K and the vertical 0.75D flatter than K. As the cylinder becomes more oblique, this becomes less accurate.

These numbers can be changed to accommodate individual patient need or doctor preference. The program converts the refraction to drum readings in order to calculate the final lens power. No power will appear in the final lens box if the refraction in that meridian is greater than ±3.75D. The lower right section displays the refraction for each meridian. If either number is greater than ±3.75D, look up the effective power and enter it into the box directly below. The final lens power will now appear. Combine the base curves and powers to order the final lens.

Diagnostic Lens Fitting

Here I trial fit the same patient using a spherical diagnostic lens with a base curve of 43.75D and power -3.00D. My overrefraction was +3.25D -1.00D X 90. Because the axis was within 10� of the steep meridian, I entered the refractive information in the lower portion of the Overrefraction section.

This time, no power was greater than 3.75D, so it was unnecessary to compensate for effectivity. Enter the flat and steep base curves manually, based on observance of the fit or empirically. Again, beware of warping of the trial lens.

Plug the bitoric lens parameters into the Diagnostic Fitting calculator to troubleshoot the lens when dispensed. The fluorescein pattern indicated the lens needed to be fit steeper in the flat meridian and flatter in the steep meridian.

To achieve this, I adjusted the BC to 44.25/49.25. The overrefraction was -0.50D -1.25D X 175. Enter this into the upper overrefraction boxes as the axis is within 10� of the flat meridian. The new bitoric lens to order is BC: 44.25/49.25 Power: -1.75D/ -7.00D.

The axis of the overrefraction moves away from that of the eye, creating a cross-cylinder proportional to the cylindrical component. At that point a spherical overrefraction may be more productive. Select a diameter as you would for any spherical patient. Order the lens in this form and you're done:

BCfl/BCst Power fl/Power st (specify drum readings)

(fl=flat; st=steep)

Happy fitting!

Dr. Kreda practices in a primary care setting in Lauderhill, Fla. He's a frequent lecturer and author. You can contact him at eyerx@comcast.net.