Fitting single reverse geometry lenses.

Basil Bloom BSc(Hons) FCOptom

1997

The British Orthokeratology Society (BOKS)

BOKS was set up in 1996 by a group of Optometrists inspired by John Mountford’s lecture at the BCLA and subsequent workshops.

It is intended as a self-help group to encourage and promote Orthokeratology in this country.

Using his methods we can now predict the amount of myopia reduction and eliminate a large percentage of unsuccessful patients before we start fitting. He has developed a mathematical model, which means that the fit can be checked at each stage, and compared with the actual Rx and calculated Rx. This makes it a much less hit or miss approach and inspires confidence in both practitioner and patient.

Requirements

1. Reverse Geometry lenses

2. Corneal Topography Scanners

1. Reverse Geometry lenses

These have a secondary curve much steeper than the BOZR. Our present lens is 4 dioptres steeper. To calculate the secondary radius (BOZR2) you have to convert BOZR to dioptres, add 4 and recalculate the BOZR2.

The lens is fitted to give an apical clearance (TFL) of 10 microns (10u). This gives a positive pressure on the centre of the cornea to flatten it. The steep second curve, a tear reservoir (TR) in a ring around the cornea, produces a negative force, which tends to steepen the periphery, or at least enhance the flattening effect of the central positive force. This change in shape produces a reduction in Myopia and can also reduce ‘With the Rule’ astigmatism.

The lenses are fitted by calculating the sag of the cornea, adding the required TFL (10u) and finding the closest contact lens that matches this value. BOKS members have available a computer program, a spreadsheet program and a fitting manual to simplify this calculation.

We are using lenses from an American Company, Contex. They make a large number of Reverse Geometry lenses but the 704C is our present lens of choice.

It has a 7.00 Optic Diameter (BOZD)

It has a 4.00 D Tear Reservoir (TR)

Overall Diameter is 10.60 or 11.20 (TD)

The Edge is 1.00mm wide

The lens has an aspheric edge so the chord size is not the (TD - edge)

10.60 has 8.80 chord

11.20 has 9.40 chord

Fluorescein

The Fluorescein pattern is very strange.

There looks like a central touch (CT) area of 3 mm. There is a TFL of 10u here but as you can not see Fluorescein until it gets to 20u, it looks like flat touch but it is not and the lens must not touch.

There is a ring of deep Fluorescein which is the Tear Reservoir (TR)

Then there is a ring of peripheral touch (PT) which supports the lens.

Finally there is the band of Fluorescein under the Edge of the lens showing the Axial Edge Lift (AEL).

Fluorescein simulation of a Contex Lens

The lens must centre perfectly

CT should be about 3mm

TR should have sharply defined edges

TFL must not be less than 10u. A flat lens will cause central corneal abrasion and staining. When in doubt fit steep on your initial choice of lens.

Flat lenses ride high.

Steep lenses ride low.

Increase Diameter to improve centration but any change in parameter must be recalculated to give optimum TFL.

Changing the diameter will mean a change in BOZR.

Corneal Topography.

I use an Eyesys system, which gives a corneal plot and measurements.

The first plot will give Ro and e, which you need to calculate the first lens. It will also show you the overall shape of the cornea and this must be taken into consideration when selecting patients. Some types of astigmatism, even WTR do not respond as well as others.

The machine comes into its own when you have taken the second plot after a fitting and 6 hour trial. It will show both plots and a Difference plot, which will show the amount of corneal change and the expected change in Rx. You can compare this with your subjective Rx to confirm the results.

You can also monitor the type of corneal shape change. This is important because if the lens sits off centre it will cause peripheral steepening or compression. Neither of these is acceptable and can be present even with very good VA’s and comfort.

Patient selection

The corneal eccentricity gives an indication of the amount of myopia reduction possible. Each 0.20 of e gives 1.00D reduction so an e=0.60 will give a likely reduction of 3.00D of myopia. This must correlate with the patients Rx. If they have an Rx of -10.00, then reducing it by 3D will not help much. But if it is -2.50 it is worth going ahead with a fitting.

Astigmatism can also be reduced as long as it is ‘With the Rule’ (WTR). Minus Cyls within about 200 of 180 i.e. 200 to 1600 , and up to about -1.50 can be dealt with.

OrthoK can induce ATR astigmatism i.e. minus Cyls at 90, so these should not be selected.

The ability to predict the final reduction in myopia means that you can eliminate a large number of patients before the initial scan and then gradually eliminate the unsuccessful ones as the procedure continues. Your fitting results should correspond to the calculated reduction in myopia and the new shape of the cornea. Orthokeratology is no longer a random procedure.

Fitting Schedule

I like night therapy where the patient wears the lenses at night and removes them in the morning. The object is to achieve good vision all day without any correction. After the fitting, I give the patient a pair of trial lenses, and get them to sleep in them. They come back to the practice the next morning with the lenses in and I then look at the lenses, refract with and without lenses, photograph and do Eyesys scan.

With a -2.50 myopia you can expect about 1D reduction in Rx and a wonderful increase in vision. The UvA is much better than expected, probably 6/12 to 6/9.

When I am happy with the fit I order the final lenses for the patient.

n   Powered

Because of the lens design, there is a deep, positive liquid lens and this will eliminate a lot of the Rx. Get the Flattest K reading from the pre-fit scan and take it away from the Contact lens BOZR, convert this to dioptres and subtract the RX, this gives the final lens power. This is quite often close to Plano.

I then check the patient

n    Early morning after 3-4 days and PM that day.

n    Early Morning after 1 week

I would expect all day UVA to be 6/6 at this stage.

If all looks well I will check weekly for 2 weeks and then monthly.

But if the fit is not perfect you will have to recalculate the lens and reorder and start again.

This procedure is very time consuming and expensive in terms of materials and you should set your professional fees accordingly.

Membership

Membership of BOKS is open to all practitioners and I hope you will join us in practising one of the most exciting and demanding of contact lens specialities.

Membership Application Forms can be obtained from:

W1A 2FT

WEB SITE             www.boks.org.uk

EMAIL                  mail@boks.org.uk

Acknowledgement

Most of this information has been obtained from the work of John Mountford. I must thank him for his ongoing help to practitioners.