Section TWO Rigid gas-permeable lens fitting
The standard design consists of five curves with a TD of 10.60 mm and edge lift of 165 m.
Example of empirically fitted lens using Dreamlite software from No. 7:
7.85orthoK 10.50 RC 7.10 AC 7.35 OZ 6.00 E = 0.6 + 0.75 Where:
7.85= BOZR
10.50= TD
7.10= Reverse curve
7.35= Alignment curve
6.00= BOZD
0.6= Eccentricity of the final peripheral curve
+0.75 = BVP
Practical advice
•The apical radius is different from flattest ‘K’ (see Section 2.4).
•Corneas with the same radius but different e values require different fittings.
•The two eyes of the same patient frequently require slightly different fittings.
•Higher degrees of myopia require smaller treatment zones.
•The minimum treatment zone is 3.2 mm to avoid flare.
14.4 Clinical appearance of reverse geometry lenses
14.4.1 Fluorescein pattern
The suitability of the lens fitting is determined with fluorescein. The appearance is quite unlike that found with a conventional rigid lens and similar in some respects to that seen with silicone elastomer lenses (see Section 32.9). The pattern consists of central touch over approximately the central 4 mm surrounded by an annulus of fluorescein representing the steeper tear reservoir (reverse, relief or return zone). The TR is in turn surrounded by the alignment curve, an area of mid-peripheral touch which further supports the lens and leads into the band of edge clearance (Figure 14.3).
14.4.2 Other fitting points
•The lens must give good centration.
•The area of central touch should be 3.5–4.5 mm wide.
•The TR should have sharply defined edges.
•TLT must not be less than about 5 m. A lens which is too flat may cause central abrasion and staining. When in doubt, fit steeper.
• Flat lenses ride high.
• Steep lenses ride low.
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Figure 14.3 Fluorescein pattern of reverse geometry lens
•Steep lenses often trap small bubbles in the TR.
•Increase TD to improve centration but any change in parameters must be recalculated to give optimum TLT.
•A change in TD or BOZR gives a smaller change to lens sag than a change in alignment curve.
Practical advice
•Fluorescein only becomes visible when the TLT approaches 20 µm. The central area has been calculated for a TLT of 10 µm or less and only gives the appearance of touch.
•If apical staining is seen, the lens is too flat.
14.5 Corneal topography
Modern instrumentation enables the mapping of corneal topography over most of its surface (see Section 2.3) and this has become an integral part of the fitting and aftercare procedures. Those corneal plots most useful in orthokeratology are the tangential, subtractive and numeric. The measurement of corneal topography is essential because:
•It provides measurements for the apical radius (R0) and eccentricity (e).
•It can help predict potential failures prior to fitting.
•It provides a permanent record of corneal shape and power at all stages of the procedure.
•The subtractive plot gives a record of progress.
•It demonstrates graphically any unacceptable degree of corneal distortion either before or during fitting.
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•The type of distortion gives indications as to whether the fit is flat, steep or decentred.
•Some topographers can be used to measure the HVID.
14.6 Fitting routine
14.6.1 Fitting
•Full ocular and slit lamp examination.
•Assessment of pupil diameter.
•Refraction including unaided vision before commencing orthokeratology.
•Corneal topography.
•Numeric plots to establish apical radius and e value.
•Calculation of the first fitting lens.
•A wearing period of 6–10 hours, either daily or overnight.
A lengthy trial is necessary to provide the following information:
•The new unaided vision.
•Degree of refractive change.
•Extent of initial shape change.
•Movement and centration of the lens after settling.
•Corneal response in terms of staining.
•Whether the patient wishes to continue with the procedure.
A satisfactory response usually gives:
•A significant improvement in unaided vision.
•A reduction in myopia of at least 1.00 D.
•No corneal staining or other adverse signs.
•Maintained lens centration and mobility.
•Minimal, if any, corneal distortion.
In some cases, it is necessary to repeat the trial with a different lens because of an unsatisfactory result at the end of several hours. This may have caused:
•Borderline or minimal improvement in unaided vision.
•An insignificant reduction in myopia.
•Unacceptable lens decentration.
•Corneal distortion.
•Corneal staining even with a well-fitting lens.
•Excessive dimpling
•Poor patient tolerance.
14.6.2Overnight wear
Lenses for orthokeratology were originally worn during the day, but current designs are worn overnight to give a more rapid response. Overnight wear has
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several additional advantages. There is no discomfort from dust, foreign bodies, dry eyes or other environmental factors. Lenses are less likely to be lost or damaged and, gradually, the waking day becomes increasingly free of both contact lenses and spectacles.
Overnight use is not the same as extended wear because the orthokeratology lenses are removed during the day. Nevertheless, materials must have a high Dk of at least 90 and a Dk/t greater than 40.
Patients should be warned of the possibility of lens adhesion on waking and should be advised how to free the lens with careful massage to avoid any risk of corneal abrasion. Fenestration can also be used to facilitate tears flow but may cause dimpling or change the fitting characteristics. The first check-up is carried out before the lenses are removed.
14.6.3 Aftercare
Aftercare following the first overnight and at subsequent visits is directed at assessing the factors mentioned in 14.6.1.
•The lens fit is checked on the eye with fluorescein looking for signs of decentration, adhesion or excessive dimpling.
•The acuities are recorded with lenses.
•Lenses are removed and over-refraction carried out, recording both aided and unaided vision. The quality of the retinoscopy reflex gives an indication of corneal distortion.
•Slit lamp examination carefully checks the corneas for any signs of staining or other physiological problems.
•‘K’ readings are taken to give a rapid indication of corneal flattening and astigmatism.
•It is essential at this stage to use topography to record the change in corneal shape with the difference maps and confirm the absence of corneal distortion (see 14.6.4 below). The desired result is a Bull’s eye plot (Figure 14.4).
Practical advice
After orthokeratology, autorefractometers do not give a reliable assessment of the refractive error and therefore of the anticipated unaided acuity. This is important in some vocations where these instruments are used during a medical examination.
14.6.4 Problem solving
Poor acuity
Poor acuity is generally due to:
•A decentred lens.
•An otherwise poor fitting.
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•Uncorrected astigmatism.
•Corneal distortion or induced astigmatism.
•Ghosting at night because of lens decentration or a small treatment area.
Figure 14.4 Difference map showing bull’s eye topographical plot
Under-responders
Sometimes it is not possible to achieve good acuity because of an insufficient reduction in myopia. These patients are termed under or slow responders. The courses of action are:
•Allow more time if both fitting and topography appear satisfactory.
•Refit with a different design of reverse geometry lens.
Where the effects of orthokeratology last for an insufficient time, it may be necessary to use a flatter fitting to achieve a greater reduction in myopia.
A high riding lens
A high riding lens occurs if the alignment curve is too flat or with an excessively tight upper lid. It can be corrected by:
•Increasing the overall sag of the lens.
•Steepening the alignment curve with a comparable adjustment to the reverse curve.
•Using a larger TD.
•Increasing the lens mass with a greater centre thickness; incorporating prism; or using a material of higher specific gravity.
•Steepening the radius of the front surface lenticulation to reduce the effect of the upper lid or adjusting edge shape and thickness.
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A low riding lens
Low riding occurs if the alignment curve is too steep, or with lid pressure or gravity. It can be corrected by:
•Decreasing the overall sag of the lens.
•Flattening the alignment fitting curve with a comparable adjustment to the reverse curve.
•Decreasing the lens mass with reduced centre thickness or using a material of lower specific gravity.
•Adjusting edge shape, thickness or lenticulation to increase the upwards effect of the lid.
Lateral decentration
Lateral decentration may be caused by a flat alignment curve, against-the-rule astigmatism or a decentred corneal apex. It may be corrected by:
•Increasing the overall sag of the lens.
•A steeper alignment curve.
•A larger TD.
The assessment of lens position is always judged in conjunction with corneal topography maps. The desired result is a centred bull’s eye pattern (Figure 14.5) which indicates good centration and accurate corneal topography. Unsatisfactory results give either ‘central islands’ or a ‘smiley face’. These patterns are caused by an incorrect sag because of an inappropriate alignment curve, lens decentration or inaccurate topography.
Figure 14.5 Difference map showing central islands with a steep fitting
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Figure 14.6 Difference map showing ‘a smiley face’ with a flat fitting on a toric cornea
Central islands
Central islands are areas of incomplete treatment (Figure 14.6) caused by a steep fitting or resistant areas of the cornea. They give rise to reduced acuity or distorted vision. Central islands suggest corneal topography that underestimates the corneal sag indicating a steeper fitting than necessary. Sometimes they resolve with time (another week or so), otherwise correction is by:
•Reducing the overall sag of the lens.
•A flatter fitting to increase the area of central touch.
•Improving centration.
Smiley faces
Smiley faces are usually found inferior to the central treatment zone. They represent areas of localized corneal steepening caused by a flat fitting with insufficient sag which, in turn, allows superior lens decentration. Smiley faces suggest corneal topography that overestimates the corneal sag indicating a flatter fitting than necessary. Correction is by:
•Increasing the overall sag of the lens.
•A steeper fitting.
•A larger TD.
Lens adhesion
A fairly high percentage of patients experience lens adhesion on waking. This need not be a problem if there are no subjective symptoms and lens mobility
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returns fairly rapidly. Patients should be instructed in how to remove lenses without risk of damage to the corneal epithelium.
A lens which adheres may be improved by:
•Using a smaller TD.
•Altering the alignment fitting curve in relation to any lens decentration.
•Flattening the peripheral curve to increase edge lift.
•Fenestration but this may also change the fitting characteristics.
Central corneal staining
Central corneal staining is almost invariably associated with excessive apical touch with a flat fitting so that a steeper lens is required.
Dimpling
Dimpling may indicate too deep a tear reservoir with a steep fitting which needs adjustment. Sometimes dimpling occurs with a perfectly satisfactory fitting where fenestrations allow the introduction of air bubbles. These may be ignored if there is no interference with vision.
14.6.5 Subsequent aftercare and follow-up
Patients should be checked at the end of the first overnight and after 1 or 2 weeks. Subsequently, they should be monitored on a regular basis. It is important that examinations take place at the same time of day for each visit, to maintain a consistent record of refraction and unaided vision.
Careful attention should be paid to lens condition and the patient’s cleaning regimen. Since orthokeratology works entirely on the basis of lens pressure at the corneal apex with a carefully controlled TLT, even a very small amount of deposit on the inside lens surface is sufficient to cause a central abrasion.
Similarly, a lens which has distorted through use or poor maintenance is unlikely to continue giving a good result. Orthokeratology lenses should therefore be replaced at least once a year and some systems recommend new lenses every 6 months.
14.6.6 Retainer lenses
Orthokeratology is a reversible procedure. When the optimum result has been achieved, a final pair of retainer lenses is used to maintain the new corneal shape and stabilize the improvement in visual acuity. The lenses are usually worn overnight but possibly for limited periods during the day. The required time depends on the individual patient. Mountford12 suggests that 60% of patients ultimately require lens wear only every second night and 10% as little as twice a week.
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