The British OrthoKeratology Society
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The British OrthoKeratology Society
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CORNEAL REFLECTIVE THERAPYBasil H. Bloom BSc(Hons) FCOptom. In the 2002 Paragon introduced in the USA their new advanced Orthokeratology system which they call Corneal Reflective Therapy or CRT. The Paragon CRT Lens consists of a series of curves and angles designed to optimise the controlled redistribution of corneal tissue during overnight lens wear. Recent studies by Swarbrick in 1998(Ref. 1) and by Alharbi in 2001 (Ref 2) have shown that the topographical changes taking place beneath corneal reshaping lenses are predominately epithelial. Their results can be explained by the fact that the human epithelium is about 50 microns thick and that half the thickness is occupied by the basal cells which remain immobile and firmly attached to Bowman's layer by a series of anchors (hemidesmosomes) . This indicates that between 20 to 25 microns of mobile wing and surface cells are available for redistribution. (Ref 3) The controlled redistribution of epithelium can be indirectly visualised by viewing the difference display map on a topographer. This display allows the practitioner to compare the pre-fitting topography with the post-fitting topography while a third map shows the difference, clearly illustrating the flattening of the central cornea and subsequent steepening of the mid-peripheral cornea and showing any flaws in the fit such as a flat or decentered lens. (Fig 10)
The Paragon CRT lens is the only lens that is FDA approved for overnight corneal refractive therapy. A total of 205 patients from 11 sites participated in the FDA clinical evaluation on the safety and efficacy of Paragon CRT for overnight therapy. Subjects were fitted with Paragon CRT lenses in both Paragon HDS and Paragon HDS 100 materials and monitored for nine months. The pre-treatment manifest refraction sphere powers were as follows: -0.75 to -2.00 (37%) -2.00 to -4.00 (51 %) Above -4.00 (12%). Figure 1 shows a scattergram of achieved correction (at the end of the day) vs. attempted correction. The middle bar represents the target prescription of Plano, the top bar represents an over correction of 1.00D and the lower bar an under correction of 1.00D. At the conclusion of the study, 67.4 percent of the subjects had visual acuity of 20/20 or better, 93.3 percent had 20/32 or better, and 94.4 percent had 20/40 or better. Additionally, no adverse events were reported during the study.(Ref 4)
The Paragon CRT lens consists of three primary zones. (FIG 2)
1.
THE
TREATMENT ZONE.
2. THE RETURN ZONE (RZD) (Fig 3)
2.
THE LANDING ZONE (Fig 4) Although the differences between the lenses in the fitting set appear to be very large compared to the other lenses we are used to, in practice they work very well. For example The Ultravision BE lens has an 0.008mm sag difference and a 0.25 degree angle change. This angle change results in a 0.008mm change in sag, which is a 0.035mm change in sag/degree, which is close to the value of 0.025mm change in sag/ degree for the CRT system. PARAGONS CRT lenses, manufactured in Paragon HDS 100, are best fitted from an inventory system which allows lenses to be dispensed to the patient at the initial visit. The inventory significantly reduces chair time and allows the patient to begin their refractive therapy the same day. However, in the UK, we need to take the appropriate sterilising measures as laid down for reusable contact lenses. (Ref5) The lenses are supplied in groups of 9 lenses, all with the same base curve with three different sag heights and 3 different tangent angles. (Fig 5)
A lens calculator slide rule, the Paragon CRT Initial Lens Selector, is provided.
The actual lens fitting is a simple three-step process. Step 1 Base Curve Selection Keratometric readings are obtained. The flat K and the spherical component of the refractive error (vertexed to the plane of the cornea) are cross-referenced with the Paragon CRT Initial Lens Selector. This gives the initial diagnostic lens.
Step 2
Evaluate Sag (RZD) (Figure 7) The initial diagnostic lens is placed on the eye and evaluated with fluorescein. The fit is acceptable if the lens provides centration over the pupil, a 4mm area of central treatment and adequate touch in the peripheral cornea.
If the initial diagnostic lens
decenters superiorly, the overall sagittal height of the lens is inadequate, and
the lens is resting too heavily on the apex of the cornea. Apply a lens with the
next greater RZD and evaluate for centration. A flat fitting lens will decenter
superiorly.(Fig 8) If the initial diagnostic lens exhibits excessive apical clearance, inadequate treatment will result. In this case the RZD can be decreased bringing the base curve into closer apposition to the cornea. These lenses follow my FAST fitting rule; ‘If the lens is Flat, Add to the sag, if the lens is Steep, Take away from the sag.’
Step 3
Evaluate LZA (Fig 9) Once the appropriate return zone depth has been established, the LZA can be easily evaluated with fluorescein. If the peripheral fluorescein picture is too thin, then the LZA needs to be reduced by one degree, this will also have the effect of the lowering the lens and therefore reducing the sag height by 0.025. Conversely if the periphery is to wide, the LZA needs to be increased which will also increase the sag height by 0.025mm. You can think of the LZA working like flaps on the edge of the lens. Increasing the LZA will bring them closer to the cornea, lifting up the lens and increasing the sag. Therefore, after a change in LZA, the fit of the lens will need to be re-evaluated. (The LZA is measured from the horizontal so you change it in the way that you see it; if it is too wide you increase it and if it is too narrow you reduce it.) The mnemonic here is WIND; ‘If the edge lift is Wide, then Increase the angle, if the edge lift is Narrow then Decrease the angle.’ After the initial diagnostic fitting, the patient is instructed in the appropriate application/ removal and lens care techniques and is rebooked for a morning appointment following overnight wear. The patient removes the lenses at the practice, then applies them, along with rewetting drops, 10 minutes prior to bed the evening before the appointment. If the patient has no previous experience of RGP lenses, they are advised to wear the lenses for one hour the first evening and to sleep in the lenses in the next evening. The patient returns for the next appointment while wearing the lenses. I perform an over-refraction to verify the accuracy of the treatment base curve. I instil fluorescein and examine the lenses for overall lens movement and centration. . Following lens removal, I perform a slit lamp examination, followed by topography, the most accurate tool in identifying where the lenses have positioned in the closed eye. The optimum map shows a well centred treatment zone (5mm to 6mm in diameter) over the pupil.(Fig 10).
The upper left plot is the pre-fit map, the lower left plot is the over night wear map and the larger map on the right is the difference map which shows the actual change in corneal shape. I then check vision and do a full refraction. I would expect to see a reduction in myopia of about 2 dioptres at this stage. The higher prescriptions will take two to three days to be corrected completely although the change will only last a few hours at this early stage. The longer the lenses are worn, the longer the change will last up to a maximum of about 18 hours. If topography shows an incorrectly fitting lens then either the RZD or the LZA will need to be altered. (Fig 11).
This shows a low riding lens so the first change would be to decrease the LZA. This has the effect of increasing the edge lift and lowering the lens, i.e. reducing the sag. Paragon supplies flowcharts to assist in these decisions. I would advise that only one parameter is changed at a time. The corrected lens is then dispensed from the fitting set and another overnight trial undertaken. This continues until the correct fit is obtained. If the fit is correct but the prescription change is incorrect, then the optic radius is altered to compensate for this. If the patient is under corrected you flatten the base curve by 0.10 for each 0.50 of myopia, conversely steepen the base curve for over correction. This mnemonic is a bit tortuous, SMILE, ‘Steepen the BCOR if there is too Much change, Increase (flatten) the radius if there is too Little Effect.’ The final lenses are then dispensed from the fitting set and new lenses ordered to replace them. If the lens is not available from the fitting set it will need to be specially ordered from Paragon. I check my patients one-week, one month, three months and six months after a successful fitting. This procedure is very intensive in terms of chair time and your fees need to reflect this.
An example of fitting a patient with CRT lenses. I will describe the fitting procedure for the left lens only. The patient is a 24 year-old Male studying hotel management in Switzerland. He has previously worn disposable soft contact lenses, both daily and monthly. He wanted to try orthokeratology because he found some discomfort in the air conditioned hotels. Left Eye Data Vision <6/60 Rx –2.50 / -0.75 x 20 VA 6/5 Topography was unexceptional with 0.50D of astigmatism. Keratometry readings (from Topographer) 7.78 (43.38D) @ 99 7.87 (42.88D) @ 9 Ro=7.81 e=0.51 HVID=12.6 Power change required –3.00D Using the lens calculator the first diagnostic lens suggested was 8.60 525 32. This was placed on the patient’s eye and allowed to settle for 30 minutes. Fluorescein was instilled and the lens fit evaluated with a slit lamp. (Fig. 12).
Position and movement was satisfactory and the lens showed the classic doughnut fluorescein appearance. The lens was dispensed to the patient to wear for one night.
First overnight Trial The patient returned for a morning appointment wearing the lenses. (Fig 13).
Vision 6/9 Rx –0.50DS VA 6/5.
Corneal topography showed that the lens was riding high indicating that the sag of the lens was insufficient. (FIG 14)
We tend to describe this incorrectly as a flat fitting lens, with traditional lenses the BCOR dictates the fit but with orthokeratology lenses the sag controls the fit and the BCOR controls the prescription change. Slit lamp examination did not show any corneal staining. If the sag of the lens is too small then there will be no apical clearance and one will see central corneal staining. This follows Blooms law, which states that a correctly fitting orthokeratology lens has apical clearance at all stages of wearing. (Ref 6) However corneal staining does not always indicate that the sag of the lens is insufficient. With certain lens designs fenestrations are needed to avoid staining. (Ref 7) The choice here is either to change the LZA if you consider that this is incorrect or to increase the sag. I felt that the LZA was satisfactory so I increased the sag by 0.025 giving 8.60 .550 32 and dispensed the lens for another overnight trial. Second overnight trialThe patient returned for a morning appointment wearing the lenses. (Fig 15).
Vision 6/9 Rx –0.75DS VA 6/5. Topography showed a well fitting lens but the over refraction of -0.75 needed to be reduced.(Fig 16)
Using the SMILE mnemonic, the BCOR was increased by 0.10 to 8.70 550 32 and the lenses were again dispensed for an overnight trial
Third overnight trial. The patient returned for a morning appointment wearing the lenses. (Fig 17).
Vision 6/6 Rx Plano/-0.25x180 VA 6/5. Topography showed a well fitting lens and the power change was –2.50D, which was satisfactory. (Fig18).
These lenses were dispensed and I expect to see the patient in a month’s time. The power change will probably increase slightly which will make him slightly hyperoptic in the morning, which will extend the effectiveness of the lenses. The Orthok effect will only last a few hours at the beginning of lens wear but after two to three weeks it should last between 18 and 24 hours.
Fig. 19 is an illustration of the effect of changing the BCOR.
The lower left difference map shows the change of –1.22D with the 8.6 base curve lens compared to the prefit eye and the lower right difference map the additional change of –0.47D induced by the 8.7 base curve. The topography difference maps invariably show a lower change than the refraction of the patient. One possibility is in the compression of the cornea produced by orthokeratology may change the refractive index slightly. If you look at the photographs of the three lenses Figs 13,15,17 after overnight wear, it is very difficult to see many differences between them. Fig 13 is possible riding slightly high but careful evaluation of the movement is needed to establish this. However the topography maps are clear and enable you evaluate the fit. You do need to have a corneal topographer to fit orthokeratology lenses safely and satisfactorily. The Paragon CRT system will be available in the UK in the summer although Paragon has made available in number of fitting sets to practitioners in this country. As a member of the British Orthokeratology Society committee, I was given one to evaluate. I found few problems with the lenses; I had great difficulty in removing the lenses and a number of my patients also have this problem. Paragon supply a rubber sucker with the patient care kits so obviously other people also have difficulty. A single fenestration in the return zone solves this problem but it reduces the effectiveness of the lenses by 0.50 dioptres. (Figure 20).
Paragon are unwilling to supply fenestrated lenses so these have to be done at another laboratory. In the cases where I have supplied unfenestrated lenses and the patients have had problems with the removal, most have reported that the lenses get easier to remove the longer they wear them. I do not know whether this is due to their improved handling ability or the change in the corneal shape. The ability to fit from stock is extremely useful and to have control over the three parameters of the lens gives one confidence in problem solving. I suppose an analogy would be between custom-made and shop bought shoes. The previous orthokeratology lenses are custom-made but the success of the CRT system suggests that the accuracy we were previously assuming was necessary in the calculation of the lenses was too high. I am confident that the CRT lenses are effective for low and medium degrees of myopia but I am not yet convinced that they are as efficient for the higher powers. The CRT literature says they are suitable for up to –6.00D prescriptions but the associated data shows that the range is +/-1.00D for these higher powers. This brings it closer to the value of –4.50D we can easily achieve with the other lens designs. The power change is directly proportional the treatment zone (TZ), and over –4.50 gives a sub 3mm TZ diameter which is too small to be of practical use.(ref 8) Paragon have designed an efficient and innovative fitting system, which simplifies a very complicated procedure and also speeds up what used to be a very lengthy process. I can recommend the Paragon CRT system to anyone who wishes to start fitting Orthokeratology lenses.
Basil H. Bloom BSc (Hons) FCOptom 2 Byeways Twickenham Middx TW2 5JN 020-8894-7149
Independent practitioner. Founder member and treasurer of The British Orthokeratology Society
References
1. Swarbrick,H.A. Wong,G. O`Leary,D.J. ‘Corneal Response To Orthokeratology’.Optom Vis Sci 1998/11/00//;75(11):791-799
2. Swarbrick HA, Alharbi A (2001): Overnight Ortho-Keratology induces central corneal thinning. Invest Ophthamol Vis Sci 42 (4): S597
3. P. Caroline. ‘What makes CRT different?’ Contact Lens Spectrum, Sept 2002.:10-13
4.FDA:Paragon CRT™ (paflufocon B), Paragon CRT™ 100 (paflufocon D), Paragon Quadra RG™ (paflufocon B), and Paragon Quadra RG™ 100 (paflufocon D) Rigid Gas Permeable Contact Lenses for Corneal Refractive Therapy - P870024/S043. http://www.fda.gov/cdrh/pdf/P870024S043.html
5. The College Of Optometrists,The Association Of British Dispensing Opticians,’Guidance On The Re-Use Of Contact Lenses And Ophthalmic Devices’. http://www.college-optometrists.org/professional/cjd.pdf
6. Bloom B H ‘ Fitting The BE orthokeratology lens’ CD-ROM. http://www.boks.org.uk
7. Bloom B H ‘When Nothing Solves The Problem’. BCLA Show And Tell. Royal Society Of Medicine January 2003.
8. Mountford J. ‘Advanced Orthokeratology, Patient Selection and Trial Fitting’, Optician Sept6,2002,V224,5867,26-37
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Copyright © 2005
The British Orthokeratology Society
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