Toric soft lenses 23 Chapter

with minus cylinder of approximately twice the power (e.g. +0.50/−1.00) at a different axis, it is because of cylinder axis mislocation. This may be due to lens rotation or inaccurate manufacture and may be assessed from a suitable computer programme (see CD) or from Table 23.1 which gives cylinder powers commonly found with disposable or stock torics.

PRACTICAL ADVICE

•Any rotation of the lens should be routinely recorded in relation to the axis identification markings. This reveals any discrepancy between a trial lens and a prescription lens and builds up a history of how lenses locate on the cornea.

•Depending on the type of axis markings, the position at which a lens settles on the cornea can be recorded in any of the ways shown in Figure 23.6A–C.

23.5 Fitting examples

Front surface stock toric

Example:

Spectacle Rx: R.E.−3.00/−1.25 × 85 Keratometry: R.E. 7.85 mm (43.00 D) spherical

The astigmatism is entirely lenticular, so a front surface toric is preferred.

Fitting: CIBAVision, WCE Toric: FL (flat): 14.60 −3.00 (selecting the flatter of the two options)

Cylinder refraction:−1.25 × 85 (from spectacle Rx) Axis rotation: Assume 5° nasal (anticlockwise) rotation.

The nearest options available for this stock design are:

Cylinders: −1.00 D or −1.75 D. It is nearly always better to underrather than over-correct. Select −1.00 D.

Axes: 80° or 90°. Select 80° because of a more likely 5° nasal orientation. The cylinder axis would therefore rotate to the required 85°.

Lens specification: FL:14.60 −3.00/−1.00 × 80

Front surface toric with full range of parameters

Example:

Spectacle Rx: L.E. −2.50/−3.50 × 180 Keratometry: L.E. 8.23 mm along 180° (41.00 D)

7.72 mm along 90° (43.75 D)

The astigmatism is outside the range of most disposable or stock torics. It is mainly corneal but with 0.75 D lenticular. Select a front surface toric with a full range of parameters (e.g. CooperVision Z6 Toric).

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Section four Complex lenses

Likely best fitting for Z6 Toric: 9.00:14.00

Over-refraction: plano/−3.25 × 180 (using a thin daily disposable with power −2.50 DS)

Lens specification: allowing for 5° nasal rotation (clockwise for left eye): 9.00:14.00 − 2.50/−3.25 × 5

Suppose this prescription lens proved too mobile and also settled consistently with 10° temporal rotation (anticlockwise for left eye). Re-order:

Final lens specification: 8.70:14.00 −2.50/−3.25 × 170

Back surface disposable toric (e.g. Bausch & Lomb Purevision)

Spectacle Rx: R.E. −1.25 DS/−2.50 DC × 20 Keratometry: R.E. 7.96 mm along 20° (42.50 D)

7.50 mm along 110° (45.00 D)

The astigmatism is entirely corneal and within the range of a stock toric or disposable.

Single fitting: 8.60:14.00 −1.50

Example A:

Over-refraction: +0.25/−2.25 × 20 (using a spherical medium water content or silicone hydrogel daily disposable).

Disposable toric locates with 10° nasal rotation (anti-clockwise for right eye). Lens specification: 8.50:14.50 −1.25/–2.25 × 10

Example B:

Over-refraction: Plano/−2.75 × 25

Disposable toric locates with 20° temporal rotation (clockwise for right eye). With 0.50 D under-correction of the cylinder and a required axis theoretically at an oblique 45°, this example is unlikely to give a successful result. Discontinue and try another variety of toric to give better orientation on the cornea.

GENERAL ADVICE

•A successful fitting usually returns consistently to the same orientation within one or two blinks or after rotational displacement of the eye.

•Where a lens mislocates on the eye, rather than attempt to change its orientation (e.g. by tightening the fit or adjusting the truncation), assume that the various stabilizing factors (see Section 23.2.1) will always act on the lens in a similar way. Compensate in the axis of the correcting cylinder for any lens rotation on the eye.

•Fitting is less reliable with oblique cylinders.

•Take particular care when fitting monovision or essentially monocular patients. They are much more disturbed by any instability of vision caused by temporary lens rotation on blinking.

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•If no success is achieved after the second lens for a particular eye, try a different type of design.

•When inserting a lens, it helps to use the orientation markings to ensure more rapid and more comfortable settling.

References

1.Killpartrick MR. Apples, space-time and the watermelon seed. Ophthalmic Optician 1983;23:801–2.

2.Hickson-Curran S, Veys J, Dalton L. A new dual-thin zone disposable toric lens. Optician 2000;219:5736.

3.Lindsay RG. Toric contact lens fitting. In: Phillips AJ, Speedwell L, editors. Contact Lenses. 5th ed. Oxford: Butterworth-Heinemann; 2007. p. 255–71.

4.Holden BA, Frauenfelder G. Principles and practice of correcting astigmatism with soft contact lenses. Australian Journal of Optometry 1975;58:279–99.

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