Toric GP Lenses – FAQ 2

How Do I Design and Fit a Bitoric Lens?

Management

Often the most imposing hurdle for designing a bitoric lens is simply determining the base curve radii and powers to order.

There are several calculators that will provide this information if refractive and keratometry values are inputted. Two calculators — the Mandell-Moore Guide and the GPLI Toric and Spherical Lens Calculator — are both available on this website (see “Useful References” below).

For example, the GPLI Toric and Spherical Lens Calculator recommends base curve radii equal to “On K” in the flat meridian and 0.75D flatter than the steep keratometry reading. Therefore, if the patient has keratometry values of 42.00 @ 180, 45.00 @ 090, and a refraction of -0.50 -3.00 x 180, the base curve radii determined by this calculator would be 42 (“On Flat K”) and 44.25 (0.75D flatter than “Steep K”). The powers calculated would be equal to simply calculating the tear lens power corrections in both meridians and using FAP (“flat add plus”).

In the flat (180°) meridian there is no tear lens correction, so the power calculated would be equivalent to the vertexed spherical refractive power or -0.50D.

The power in the steep meridian would be the vertexed refractive power in the steep meridian (equal to -3.50D) + tear lens correction or +0.75D, for a final value of -2.75D.

Laboratories can generate very good optics as well as peripheral designs. However, if there is interest in custom-designing all of the lens parameters, it is not difficult. The overall diameter would be similar to that of a spherical design. The center thickness would be identical to that of a spherical lens having a power equal to that of the most plus power meridian of the bitoric design (in this case, -0.50D).

Toric peripheral curves are recommended as well, especially if the astigmatism is limbus-to-limbus. They can be designed via reviewing the topography maps to determine how much toricity is present at approximately 4mm from center. Otherwise, a good starting point is to order toric secondary curves 1mm flatter than the base curve radius and peripheral curve radii 2mm flatter than the secondary curve radii.

However, today much success has been experienced with simply empirically designing these lenses using one of the aforementioned calculators and allowing the laboratory to recommend a specific lens material as well as the other lens parameters.

Useful References

GP Lens Institute

Publications

Acknowledgements

GP Lens Institute Advisory Committee members: Bruce Anderson OD, Marlane Brown OD, Carmen Castellano OD, Walter Choate OD, S. Barry Eiden OD, John Laurent OD, PhD, Derek Louie, OD, MS, Joe Shovlin OD, Frank Weinstock MD, Bruce Williams OD.