prescribing for astigmatism
A Recipe for SPE
BY PETER D. BERGENSKE, OD, FAAO
The Spherical Power Effect (SPE) lens design is useful in cases where a toric base improves the lens-cornea bearing relationship, but a spherical lens would provide neutralization of the refractive astigmatism. The SPE is a bitoric lens that behaves optically as a spherical lens on the eye.
The SPE design is elegantly simple: the cylinder power of the lens in air is equal to the back surface toricity expressed in diopters K. A lens requiring a base curve of 43.00 x 46.00 (3.0 toricity), must have a 3.00 cylinder, such as -1.00 @ 43.00 and -4.00 @ 46.00, for the lens to have spherical power effect on the eye (Figure 1). The principle advantage of this design is that lens rotation does not affect vision.
Figure 1. The "classic" SPE design. Note the cylinder power is equal to the toricity expressed in diopters K.
1. Determine the flat base curve. The goal is to achieve a horizontal meridian that shows central alignment and light mid-peri-pheral bearing. This typically requires a curve 0.25D to 0.50D flatter than flat K. If you have a topographer, use the curvature 3.0 mm to 4.0 mm temporal on the axial map. (We are assuming a with-the-rule cornea, the most common for this lens design).
2. Calculate the power in the flat meridian. Think of this exactly like a spherical base curve. Adjust the sphere component of the spectacle power by the amount flatter than K. Alternatively you can place a spherical diagnostic lens on the eye and over-refract.
3. Choose the steep base curve. Select a curve that is steep enough to stabilize the lens, but flat enough to allow vertical lens movement. Make the steep curve flatter than the steep K by about 1.00D. This simulates the fit of a spherical lens on a cornea that has a low degree of toricity.
4. Calculate the base curve toricity. This is the difference between the flat base and the steep base. Let's call this delta K.
5. Calculate the power in the steep meridian. We want the cylinder component of the lens power equal to delta K, so the power in the steep meridian (Fs) is just the power in the flat meridian (Ff) less delta K. Or, Fs = Ff delta K.
6. Choose diameter, optic zone and periphery. Determine all these as you would if fitting a spherical lens with a base curve the same as the flat base curve of this lens.
7. Choose a lens material. This design can be made in any RGP material. If you are unsure, talk with your lab to see what they prefer to use when making a toric lens. They know what works best for them in producing a stable lens with good optics.
That's it! Consider SPE the next time you need a toric base lens, if for no other reason than simplicity. Another advantage is that the power can be determined by calculation without K readings or refractive error. You can obtain the only necessary information from trial lens fitting and over-refraction. This is particularly useful when fitting distorted corneas with spurious keratometric and refraction findings.
Figure 2. Right eye refraction: 5.25 2.75 x 178. Perfect candidate for the SPE design.
Note the match-up of refractive cylinder and toricity.
Dr. Bergenske, a Past Chair of the American Academy of Optometry's Section on Cornea and Contact Lenses, has practiced for over 20 years in Wisconsin and now is on the faculty at Pacific University College of Optometry. E-mail him at firstname.lastname@example.org.