Patrick and Tracy Caroline on a helicopter trip.

Patrick Caroline's subject was "On eye videography of our successes and failures with OK." Pat gave another of his superb clinical demonstrations. He said that he came late to OK but his students have a great interest in it. His elective, one of eight available, had a 50% take up amongst students. He has coined Caroline’s Rule - "Corneal power can only be redistributed not changed." In other words, central flattening of the cornea has to be associated with steepening elsewhere. He speculated on what actually moved with OK. The choices are the epithelium, Bowman's membrane or the stroma. The stroma has 225 lamellae stretching from limbus to limbus and it would be difficult to move just a portion of it; Bowman's is too stiff to move so it must be the epithelium, as Helen Swarbrick postulates.

Refractive surgery uses Munilyn's formula to work out how much tissue to remove for a given prescription. It is (Treatment Zone² x Rx)/8 (n-1) . This shows that the tissue removed is proportional to the diameter of the Treatment Zone Diameter (TzD); for a –3.00D myope, a 5 mm TzD needs a 25µ m ablation depth but a 7mm. TzD needs a 49µ m depth.

Pat stated that OK is also limited by the TzD; the smaller the zone the larger the change and this explains the few anomalous results where large changes of Rx are reported. These do occur but the TzD is so small as to be useless. The relationship between TzD and Power is:

The epithelium is 50µ m deep out of a total corneal thickness of 540µ m and assuming a maximum thinning of 60% this will give about a 3.00D change with a 5 –6 mm pupil, which is what BOKS members find in practice. Mr Caroline thought that this thinning does not damage the epithelium as long as it is healthy and this is confirmed by the fact that OK patients revert to their original prescription in about 3 days. Long term PMMA wearers took weeks to recover and this is possibly due to reduced epithelial metabolism caused by anoxia.

He felt we are dealing with a semi closed fluid system with a differential pressure distribution and to prevent leakage, we ought to fit larger lenses to seal around any central toric portion. As it is difficult to assess the fluorescein fit in the centre of the lens, (there is usually none visible), he advised that one ought to look at the appearance of the fluorescein outside the lens (where there is a 10µ m thick film) to compare it with the centre.

Figure 5 FARGO4 Schematic

The change produces a variable area of sphericalisation with a change in refractive error. To increase the change in Rx we must reduce the area of change by manipulating the design of the DRGL. His new BE Lens is another DRGL and he provides a computer program for initial fitting calculations. Once the pre-fit readings are entered, it gives the calculated change in Rx and the TzD. If more change in Rx is required, this is entered and the program calculates the resultant TzD. If this is acceptable it will calculate the lens design needed to achieve the change. (This is exactly the approach that Pat Caroline suggested in his lecture).

A system has also been developed to measure the elasticity of the cornea. A probe is placed on the cornea to indent it and the time is taken until the indentation disappears.

The initial tests were started in 1994 and they were able to produce up to a 9.00D change in rabbits, who have no Bowman's membrane. Last year clinical tests were started on human volunteers in Mexico. The first generation lenses took an average of 78 days to achieve 6/6 vision; the second generation lenses took an average of 24 days to achieve 6/6 vision and the third generation lenses took an average of 7 days to achieve 6/6 vision, but these were used with the stabilising drops.

All corneas were clear and the change in Rx has been shown to be stable for up to 1 year. They have found that as the central cornea flattens, the para-central area steepens. This is what we assume happens with traditional OK.

The present protocol uses a 4 curve, DRGL with an OZ of 4 mm and the treatment protocol is as follows:

Their results seem to indicate that the effect is purely stromal with no epithelial involvement. If there were changes due to epithelial movement we should expect to see a regression effect in about 24 hours, which does not happen.

There were various research presentations throughout the meeting. Roger Tabb presented details of the Pacific University College of Optometry "Night Wear End Result OK study" (NERO). This is being undertaken to gain FDA approval for this mode of fitting. It consists of 31 subjects of whom 26 completed the course. The objective was to sustain a minimum of 6/12 vision all day.

Results were:

All eyes reached 6/9, 96% reached 6/7.5, 88% reached 6/6, 58% reached 6/4.5.

At 1 week the best VA was held for 10 hours; at 3 weeks the best VA was held for 15 hours and at 6 weeks the best VA was held for 16.69 hours.

Ed Bennett, of the Ohio State University School of Optometry spoke on " Benefits of RGP Lenses and Children." This study is in progress in Singapore to find out if RGP lens wear slows down myopia progression and retards the eyes' axial elongation. 300 children aged 9 to 11 years are taking part, with half in RGP and half in spectacles and they are being followed for 4 years. The readings taken are A scan ultrasound, topography, objective and subjective refraction, prescription and VA. The questions they hope to have answered are: do the RGP wearers develop clinically significantly less myopia; is the change axial or a change in curvature of the cornea and can they adapt to RGP lenses and keep wearing them?

In an ongoing study in Ohio concerning CL and myopia regression, 110 8 – 11 year olds, with up to –4.00D and VA of 6/6 are being studied over 3 years. They were adapted to wear RGPs and then some were randomly assigned to soft lenses. 86% adapted to RGP lenses. Preliminary results of both these studies will be available by the end of the year.

The conference finished off with a panel discussion on "OK around the World" where John Mountford and I explained and defended our position. Whilst most of the OK lens designers were taking into consideration the eccentricity of the cornea, very few of the practitioners were aware of its importance in calculating the best initial fitting lens. Most do not use fitting sets or topographers and order the lenses using K readings, relying on the manufactures to do the calculations. Those who do use fitting sets use a flatter than K method that usually results in too steep a lens which is ineffective. This explains why their procedures are more laborious. Most of them are using the lens as a daily wear lens, giving improved vision when it is removed in the evening. Although this opens up the procedure to the higher myope who can benefit form this improved functional vision, it does not seem to utilise the true benefits of overnight wear: good vision without contact lenses or spectacles which lasts all day. Those practitioners I spoke too were eager to obtain the sag-fitting computer programs used by BOKS. Many were using OK lenses on children in the hope that it will prevent or slow down the onset of myopia. Asian families in particular are concerned with the actual number of their child’s prescription. Anything that reduces this, even temporally is welcomed.

Although some of the lectures were not as scientifically rigorous as I am used to hearing at other meetings such as the BCLA, this was an important conference. All of the DRGL seem similar in design, with small variations in the periphery and the graphs which analyse the applied force on the cornea are also very similar. Although the designers differ in the explanation of how their lenses work they are all agreed that as the area of the treatment zone decreases the potential change in the Rx increases. So we can now design individual lenses to achieve a specific reduction in Rx. With the conventional RGL lens this was not possible. New lens designs to correct hyperopia are being developed and with-the-rule astigmatism can be reduced by 50%. I feel that OK has come of age and I have returned invigorated and eagerly awaiting the delivery of my DRGL fitting set.

Acknowledgements

My thanks are due to BOKS for paying the conference fees and to NERF for reducing the said fee to their membership level. Figures 4,5,8,9 and 10 are by courtesy of Jim Day.

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