Toric rigid lenses 22 Chapter

As induced astigmatism is neutralized, the final powers are:

BVP along flatter meridian: = −2.00 D

BVP along steeper meridian: = −2.00+ (−2.33) = −4.33

Equivalent to:  −2.00/−2.33 along flatter meridian

22.6 Fitting bitorics

A bitoric is required when a back surface toric has created sufficient induced astigmatism (usually over 0.75 D) to warrant correction with a front surface cylinder.

•If corneal and ocular astigmatism are equal, the induced astigmatism created may require correction.

•If the ocular astigmatism is less than the corneal astigmatism, lenticular astigmatism is revealed by a rigid lens and requires neutralizing together with the induced astigmatism. In this case, a large front surface cylinder is needed.

= (−19.20) − (−18.00) = −1.20 D

This degree of induced astigmatism requires correction. The astigmatism induced by the back surface in air:

−480 − −480 = (−64.00) − (−60.00) = −4.00 D 7.50 8.00

Equivalent to −2.00/−2.80 along flatter meridian

22.7 Compromise back surface torics

If the ocular astigmatism is greater than the corneal astigmatism, a compromise fitting option is worth considering to avoid the use of a front surface cylinder. In these cases, some of the induced astigmatism will be neutralized by the residual astigmatism and the remaining induced astigmatism can be further eliminated by reducing the difference between the flatter and steeper radii.

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•Convert the spectacle prescription to an ocular prescription.

•Calculate the resultant induced astigmatism when the back surface toric lens is designed close to alignment.

•Calculate the residual astigmatism or use rule-of-thumb.

•Calculate the difference needed in BOZRs to create induced astigmatism that is equal or close to the residual astigmatism.

•Redesign the lens so that the flatter BOZR aligns with the flattest corneal meridian and the steeper BOZR satisfies the above calculation.

RULE OF THUMB

Choosing the lens radii to be approximately 75% of the corneal toric difference often avoids the need for a bitoric lens.

If the BOZRs were chosen to align r1 = 7.20 r2 = 8.00 For n = 1.336 n’ = 1.480

Induced astigmatism = −7.20144 − −8.00144

= (−20.00) − (−18.00) = −2.00 D

Front surface cylinder to compensate = (−2.00) − (−1.00) = −1.00 D However, by flattening the steeper BOZR to 7.40 mm

Induced astigmatism = −7.40144 − −8.00144

= (−19.45) − (−18.00) = −1.45 D Uncorrected astigmatism = (−1.45) − (−1.00) = −0.45 D

The patient would be expected to tolerate this small amount of astigmatism, therefore a bitoric is not needed with a back surface toric of 8.00 × 7.40 mm.

22.8 Fitting front surface torics

Method 1

•Use a large spherical diagnostic lens with power as near as possible to the final Rx and over-refract.

•Order a lens based on this power, usually with 1.5 .

•Always have the prism base marked so that the lens orientation can be measured on the eye.

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Toric rigid lenses 22 Chapter

•Over-refract and record the cylinder and axis.

•Note the orientation of the prism base.

•If the lens is unstable, consider a truncation of 0.50 mm, increasing the prism, or a larger TD.

•Order the front cylinder, taking into account any rotation of the lens.

•The cylinder can be worked on the initial lens if no other changes are needed.

Method 2

•Over-refract with a spherical diagnostic lens and order the final prescription lens directly with the front surface cylinder.

•This procedure is quicker, but an estimate of the final lens rotation is required (usually 5–15° nasal).

PRACTICAL ADVICE

•If only one eye requires a toric, there may be problems with tolerance.

•Use truncation if there is a problem with binocular vision because of prism in only one eye.

22.9Fitting toric peripheries

•Fitted when the cornea is more astigmatic in the periphery and a small lens design fails to work.

•Normally used when the corneal astigmatism is 2.00–3.00 D.

•With a spherical diagnostic set, the fitting should be assessed for central alignment and correct peripheral clearance in the flatter meridian.

•The radius for the steeper peripheral meridian can be determined either from the ‘K’ readings and looking up axial edge lift tables for each meridian, or from the fluorescein pattern. A minimum toric difference of 0.60 mm is usually required.

•The secondary curves are approximately 0.80–1.20 mm flatter.

•All toric periphery lenses with a spherical BOZR give an oval optic with the smallest diameter along the flattest meridian.

•If there are manufacturing problems with the theoretically designed periphery, an additional spherical edge curve can be used.

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•A toric periphery diagnostic set can be used for fitting.

•The prescription lens should give good acuity since the liquid lens neutralizes corneal astigmatism.

•A toric periphery is less expensive than a fully toroidal back surface design but represents a compromise.

22.10 Computers in toric lens fitting

Computer programmes are available which help in the design and ordering of bitoric rigid gas-permeable contact lenses. The required data are keratometry readings, spectacle Rx, BVD and, possibly, the selected BOZR, BOZD and TD. Most programmes provide the final lens specification along the flatter and steeper meridians as well as the ocular astigmatism, corneal astigmatism, residual astigmatism and induced astigmatism. From this information it becomes clear whether a front surface cylinder is necessary. Some programmes suggest the compromise fit required to avoid a front surface cylinder.

Most corneal topographers have a contact lens module which can create lens designs for back surface torics, bitorics or front surface torics. It is then possible for the practitioner to set the level of corneal toricity at which the fitting module will begin to suggest a toric lens. The programmes will simulate the expected fluorescein picture so changes to the BOZR can be made before lens ordering. However, the effect of lid tone, position and other ocular surface factors will affect the performance of the lens on the eye. The simulated fluorescein pattern is therefore only a starting point and assessment needs to be carried out in the usual way to determine the optimum fitting.

References

1.Douthwaite WA. Contact Lens Optics and Lens Design. 3rd ed. Oxford: Butterworth-Heinemann; 2006.

2.Christie C, Keirl A, editors. The correction of astigmatism with rigid gas permeable contact lenses. In: Clinical Optics and Refraction. Elsevier; 2007. p. 225–40.

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