There are just three possible reasons for fitting a toric rigid lens:

• Correct residual astigmatism
• Fit a toric cornea
• Both of these

Case One

Let's start with a rare one — a front toric, sphere base rigid lens. Lots of folks say there is no need for this any more, but here is an example where it is just the right thing!

Patient JR:

• S/P RK OU
• OD +0.50- 2.00 x 38 20/40
• Spherical trial lens: 7.90/-3.00
• Over-Ref: -1.00 -1.25 x 85 20/25
• Spherical lens fits and neutralizes the irregular astigmatism
• Residual cyl is "normal" against-the-rule astigmatism
• "Ideal" front toric candidate!

Power for this lens is simply:

• -3.00 (trial lens power)
• -1.00-1.25 x 90 (over-refraction)
• -4.00 -1.25 x ?
• What about the axis?

Blink of upper lid is bi-directional:

• Nasal
• Downward
• Lower gives a slight nasal "kick"
• This usually causes slight base-nasal rotation of the lens

Most accurate:

• Order trial lens with prism, but just spherical power
• Mark base of prism
• Put on eye, observe and measure rotation, over-refract for cyl power and axis
• Order final lens based on findings, using LARS as you would with a soft toric

SWAG method:

• With a rigid lens, most often there is 10-15 degrees nasal rotation for each eye (right on the right eye, left on the left)
• May order empirically using LARS and this assumption

Final Lens for this patient:
7.90 BCR/ 9.4 OAD/ 7.8 OZD/-4.00-1.25 x 75 c 1.25 (BD)

Case Two

Another relatively "exotic" lens is the back surface toric, with spherical front surface. It can be the lens of choice when there is adequate corneal cylinder for a toric base AND the refractive cylinder exceeds the corneal cylinder. These are almost always cases of high against-the-rule cylinder.

Patient EK:

• K: 46.50@10/43.50 OD
• Rx -0.50-4.25x100

Note:

• Soft lens requires 4.50 diopter cyl
• Rigid lens requires 1.25 diopter cyl
• Base curve cylinder will be helpful in fit

Here's the topography:

We need to simulate fit of spherical lens on a cornea with low with-the-rule toricity. Also, we will need to get enough base curve toricity to induce the extra cyl power we want.

• Need "bite" along horizontal: steep curve will be close to 46.00
• Need to allow for vertical movement, so make the flat meridian 43.00
• Note toric base lens usually needs to be flatter than "K" in both meridians.
• Optics refresher: remember the 1-2-3 rule o' thumb. To get 1 diopter of cyl on the eye you need 2 diopters "K" toricity on the lens, and you will measure 3 diopters of cylinder in air.
• This is the case for back cylinder, front sphere lenses. It is an approximation and varies with index of refraction of the lens material.
• If we have 3.0 diopters "K" toricity, we should get close to the 1.25 cylinder effect on the eye that we want.
• This lens will be tight and not likely to rotate.
• Power will be relative to the flat meridian. As we are 0.50 D flatter than K, power specified should be Plano in the flat meridian. On the lensometer we will expect to measure powers of Plano and slightly less than -4.50.

Case Three

The last case illustrated why the back toric is usually not the right choice: it always over-corrects the corneal cylinder. More commonly, there is close match of the corneal and refractive cylinders, i.e. a non-flexing spherical lens would correct the cylinder. When this is the case, but a spherical lens doesn't fit, we need to use a toric base. If we just use a toric base, it will over-correct the cylinder, so to compensate we put a plus cylinder on the front that neutralizes the induced cylinder.

Patient PC:

• -2.75 -5.75 x 12
• K 42.00/47.50

Note the close match of corneal and refractive cylinder.

Here's the topo:

And here is what a spherical lens looks like. This was a 44.00 D.K. sphere. Over-refraction indicates power of -4.50 corrects refractive error.

Obviously this fit is unacceptable, though it works optically. Let's make it fit:

We know that 44.00/-4.50 is optically acceptable. If we go flatter in the flat meridian and steeper in the steep, we can adjust the powers to maintain the same optics.

Make the flat meridian 41.75; this is 2.25 flatter than 44.00. Adjusting power by this amount, this meridian should be -2.25.

Make the steep meridian about 1.00 flatter than steep "K", say 46.50. This is 2.50 steeper than 44.00. Adjusting power by this amount, this meridian should be -7.00.

So, final answer: BC 41.75/46.50 Power -2.25/-7.00

Note well: the base curve cylinder in diopters "K" is 4.75, and the lens shows -4.75 cylinder in air. This is the essence of spherical power effect (SPE) design.

If we had this base in a back surface toric, it would measure closer to 7.00 cylinder in air, and over-correct the cylinder on the eye by about 2 diopters! The lab puts a front (plus) cylinder on the to get the power we need — this is a bitoric lens.