Section TWO Rigid gas-permeable lens fitting
•Fenestrations: number, position, size and finish.
•Tint.
•Prism ballast: increased edge thickness at the base.
•Truncation.
•Carrier design: assessed by edge measurement.
Table 13.1 Suggested tolerances
Parameter |
ISO suggested tolerances |
BOZR |
±0.05 mm |
|
|
BPZR |
±0.10 mm |
|
|
BOZD |
±0.20 mm |
|
|
TD |
±0.10 mm |
|
|
Edge and centre thickness |
±0.02 mm |
|
|
BVP |
±0.12 D up to ± 5.00 D |
|
±0.18 D from ± 5.00 D to ± 10.00 D |
|
±0.25 D from ± 10.00 D to ± 15.00 D |
|
±0.37 D over ± 15.00 D to ± 20.00 D |
|
±0.50 D over ± 20.00 D |
|
|
13.4 Tolerances
See Table 13.1.
PRACTICAL ADVICE
• The easiest methods for practitioner verification are:
BOZR |
Radiuscope |
Diameters |
Band magnifier |
Power |
Focimeter |
Thickness |
Thickness gauge |
Condition |
Slit lamp or band magnifier. |
•Rigid and PMMA lenses flatten on both front and back surfaces with hydration. Flattening relates to BVP (greater with high minus) and centre thickness.
•New lenses should be hydrated for 24 hours before a reliable measurement can be made.
•Very rapid changes in BOZR also occur on dehydration.
•Rigid gas-permeable diagnostic lenses should be kept hydrated. PMMA lenses in powers <±10.00 D are reliable if dry.
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Rigid lens specification and verification 13 Chapter 
References
1.Cagnolati W. Lens checking: soft and rigid. In: Phillips AJ, Speedwell L, editors. Contact Lenses. 5th ed. Oxford: Butterworth-Heinemann; 2007. p. 355–74.
2.Sarver MD, Kerr K. A radius of curvature measuring device for contact lenses.
American Journal of Optometry 1964;41:481–9.
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Section |
Rigid gas-permeable lens fitting |
TWO |
|
|
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Orthokeratology |
CHAPTER |
|
and reverse |
14 |
|
geometry lenses |
|
14.1 |
Historical |
169 |
|
|
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14.2 |
Current approach |
170 |
|
|
|
14.3 |
Reverse geometry lenses |
172 |
|
|
|
14.4 |
Clinical appearance of reverse geometry lenses |
178 |
|
|
|
14.5 |
Corneal topography |
179 |
|
|
|
14.6 |
Fitting routine |
180 |
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|
|
It has been observed for many years that conventionally fitted PMMA lenses appear to reduce the rate of increase of myopia.1,2 Modern gas-permeable lenses seem to produce a similar effect but to a lesser extent.3 A more active approach to myopia control is orthokeratology, defined as: the reduction, modification or elimination of a visual defect by the programmed application of contact lenses.4
Several other terms have also been applied to the same procedure, including corneal refractive therapy (CRT), overnight vision correction (OVC), reversible corneal therapy (RCT) and vision therapy.
There is now possible evidence that orthokeratology, apart from the temporary reduction in myopia, may have a significant long-term effect in reducing the progression of myopia in children and younger patients.5
In practical terms, orthokeratology applies to myopic eyes and aims to eliminate the refractive error or to reduce it to a sufficiently small degree that the patient can function without spectacles or contact lenses for most of the waking day. The result is achieved by flattening the cornea in a controlled fashion by wearing contact lenses with the BOZR significantly flatter than the corneal radius. When the desired result has been achieved, the new corneal shape is maintained by means of retainer lenses.
14.1 Historical
It is only recently that orthokeratology has begun to achieve any scientific acceptability. The procedure has been practised in the USA for over 30 years,6–8 but with controversial and mainly anecdotal results. It developed from the clinical observation that wearing PMMA lenses flattened the cornea and reduced
©2010 Elsevier Ltd, Inc, BV
DOI: 10.1016/B978-0-7506-7590-1.00011-X