Section TWO Rigid gas-permeable lens fitting

The BOZR range is 7.00–8.60 mm in 0.05 mm steps over TDs of 9.60 and 10.20 mm. The BOZD is 5.00 mm for each diameter and the centre thickness at −3.00 D is 0.135 mm. The Astrocon is the basis of a front surface multifocal design.

MetrO2 (Cantor & Nissel)

A computer generated constant axial edge lift design with a back surface aspheric and front surface bicurve design. The incremental design results in a progression from a BOZR of 7.00 mm with a TD of 8.80 mm to a BOZR of 8.70 mm with a TD of 10.50 mm.

12.3 Principles of fitting

12.3.1 Fully aspheric lenses

Progressive eccentric aspheric lenses (e.g. Quasar) are generally fitted on  flattest ‘K’. The fluorescein pattern shows no obvious area of bearing over the central and mid-peripheral area while the lens edge gives a well-defined tears meniscus. The lens position should be central with 1.5–2 mm of movement on blinking. If the TD needs to be ordered larger (10.00 mm) or smaller (9.20 mm) than the 9.60 mm diagnostic lenses, there is no need to change either the BOZR or BVP.

With toric corneas, the central radius is chosen 0.10 mm steeper than flattest ‘K’. The lens should still give good centration but will show greater peripheral clearance along the vertical rather than horizontal meridian. Mid-peripheral bearing indicates the need for a flatter lens, whereas hard central touch or decentration requires a steeper fitting.

12.3.2 Mainly aspheric/part sphere

Persecon E

The Persecon E design is manufactured from CAB (see Section 7.1). It is  fitted slightly flatter than flattest ‘K’. The total diameter is chosen according to corneal size: 8.80 mm or 9.30 mm for corneas up to 11.0 mm, and 9.80 mm or 10.30 mm for those larger than 11.0 mm.

A correct fitting shows alignment or the suggestion of light central touch. In the case of decentration, a larger lens should be tried before steepening the curvature. The edge bevel is cut with a common tangent to the ellipse to give good comfort.

Aquila and Persecon 92E

Aquila and Persecon 92E are fitted at least 0.05 mm flatter than flattest ‘K’. The fluorescein fit is alignment to slightly flat with a small degree of edge clearance.

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As the elliptical base curve flattens towards the edge of the lens in the same way as the cornea, the TD can be altered without causing any change to the fluorescein pattern. However, if the base curve is altered, the BVP requires compensation in the usual way.

PRACTICAL ADVICE

The design of the Aquila and Persecon 92E differs from the earlier Persecon E so that the correct diagnostic set must be used to achieve an optimum fitting.

12.3.3 Mainly spherical/part asphere

These designs, with a spherical back optic and aspheric periphery (e.g. Menicon EX), require a relatively flat fitting. For a spherical cornea, the initial lens is 0.05 mm flatter than flattest ‘K’. As the corneal astigmatism increases, the first lens is selected along flattest ‘K’. If there is greater than 2.00 D of corneal astigmatism, the fitting should be 0.05 mm steeper than flattest ‘K’.

Quantum

Quantum (Bausch & Lomb) is a typical example of a design that has a small spherical cap with an aspheric periphery. It is therefore fitted differently. The spherical central radius is selected 0.10 mm steeper than flattest ‘K’ to allow the aspheric mid-peripheral portion to align with the cornea. The fluorescein pattern gives slight central pooling surrounded by an area of alignment with peripheral edge clearance. The lens design can give excessive edge clearance with steep corneas and too little clearance with flat corneas. Quantum does not ride as high as some of the flatter fitting designs and is useful where it is desirable to avoid lid attachment. The usual total diameter is 9.60 mm. To help centration, 10.20 mm lenses are available, but even 9.60 mm is too large for some small corneas. Only plus lenses are available in a 9.00 mm diameter.

Metro2 and Maxim Ultra

The initial trial lens is chosen on flattest ‘K’ or slightly steeper for up to 1.00 D of corneal astigmatism. This should maintain the appearance of alignment over the central area of the lens with gradually increasing clearance in the periphery. With greater than 1.00 D of astigmatism, the BOZR is selected 0.10–0.15 mm steeper than ‘K’. The lens should centre with 1–1.5 mm of movement on blinking.

Astrocon HDS

The first lens is chosen 0.10 mm steeper than ‘K’ for cylinders up to 0.75 D, on ‘K’ for up to 1.75 DC and 0.10 mm flatter than ‘K’ for up to 2.50 DC. Movement should be between 1.0 and 2.0 mm. The fluorescein picture should show alignment or minimal apical clearance. There should also be slight mid-peripheral

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bearing and 0.10–0.12 mm of edge clearance. If the initial fit appears flat, the 0.45 ‘e’ value option should be tried and, if steep, the 0.75.

12.3.4 General fitting considerations

Aspheric lenses

•Do not require such critical fitting because there are fewer parameters.

•Need to be fitted slightly flat to give adequate movement and sufficient edge clearance (except designs like Quantum).

•Flat fitting, however, tends to give decentration as a common problem.

•The variations in design mean that the manufacturer’s information should be read before fitting.

Spherical cornea

The ideal fit is alignment or slightly flatter than alignment.3 A slightly flat fit gives light central touch, but because of even pressure distribution any stress to the cornea is kept to a minimum.

A flat fit shows hard central touch surrounded by an excessive annulus of fluorescein. There is increased lid sensation and unstable vision.

A steep fit gives excessive central pooling with a sharp border, surrounded by a peripheral ring of hard touch and a very narrow meniscus of fluorescein at the periphery.

Toric cornea

The ideal fit is also alignment or slightly flatter than flattest ‘K’ to minimize lens flexure and maintain good acuity.

12.4 Fluorescein patterns compared with spherical lenses

•A much more gradual change in the fluorescein pattern from centre to periphery.

•True alignment can be achieved if the correct eccentricity has been assessed, as the p value of the lens and cornea can be chosen to be the same (e.g. using a videokeratoscope).

•The peripheral tear layer thickness increases more gradually towards the edge.

•Larger lenses have to be used to help centration.

PRACTICAL ADVICE

The axial edge lift of an aspheric lens is less than with the equivalent multicurve. It is sometimes possible to fit higher degrees of astigmatism because of the reduced edge stand-off in the steeper meridian.

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References

1.Bibby M. Computer assisted photokeratoscopy and contact lens design. Optician 1976;171(4423):37–43, 171(4424), 11–17, 171(4425), 15–17.

2.Meyler J, Ruston D. The development of a new aspheric RGP contact lens. Optician 1995;209(5487):30–8.

3.Morris J. RGP lenses Part 2 – Fitting procedures. Optician 2004;228(5976):28–35.

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