Section TWO Rigid gas-permeable lens fitting
8.5 Lid attachment lenses
Lid attachment (hitch-up) utilizes the edge contour and shape of the anterior peripheral surface of the lens to increase lid–lens adhesion.10,11
•Lid attachment occurs when the peripheral lens contour remains in constant contact with the upper lid margin after blinking or eye closure.
•The lens therefore moves with the upper lid and returns to a superior position on the cornea after blinking.
•Minus lenses give a natural lid attachment on most eyes because of their edge shape, but larger diameter lenses are often necessary (Figure 8.8).
•When the upper lid is in its normal position, its upward retention effect on the lens is greater than the downwards pull of gravity or the centration forces of the tears meniscus.
•Plus lenses give the reverse effect and tend to escape from lid retention because of their edge shape.
•The correct anterior lenticular construction is essential.
A B
Figure 8.8 Korb edge contour (A), compared with a standard edge design (B)
Advantages
•More comfortable.
•Helps maintain normal blinking.
•Counteracts low-riding lenses.
•Helps tears exchange on blinking.
•Lenses can be made thinner with high powers.
•Less 3 and 9 o’clock staining.
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Disadvantages
•Flare from lower edge of pupil.
•Peripheral curves may need to be individually designed.
•Front surface may require complex construction.
8.6 Interpalpebral lenses
This technique aims to give good centration using very thin lenses with a total diameter smaller than the vertical palpebral aperture. It has mainly been applied to PMMA to improve the physiological performance and where thinner lenses can be more easily manufactured.
Fitting
TD: |
At least 2.00 mm larger than maximum pupil diameter, i.e. usually |
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7.50–8.50 mm. |
BOZR: |
Up to 0.15 mm steeper than flattest ‘K’ to give the appearance of an |
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alignment fit. |
BOZD: |
5.00–7.00 mm. |
Example 1: 7.80:6.50/8.60:7.50/10.40:8.50 tc = 0.10
Example 2: 7.80:7.00/10.50:8.00 tc = 0.08
Advantages
•Better for narrow lid apertures.
•Less sensation with sensitive lids.
•Better for corneas with irregular periphery.
•Often successful with moderate or even highly toric corneas where a small lens may permit a spherical design.
•Less disturbance of corneal metabolism.
•Lens positions away from the limbus and may help with 3 and 9 o’clock staining or limbal disturbance.
Disadvantages
•More difficult to manufacture because they must be made thinner.
•Difficult to handle.
•Difficult to remove.
•Fragile edges.
•Flare.
•Disincentive to blinking and may give increased 3 and 9 o’clock staining.
References
1.Hayashi TT, Fatt I. Forces retaining a contact lens on the eye between blinks.
American Journal of Optometry and Physiological Optics 1980;57:485–507.
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2.Morris J. RGP lenses Part 1– Materials, manufacturing and design. Optician 2004;228(5971):28–35.
3.La Hood D. Edge shape and comfort of rigid lenses. American Journal of Optometry and Physiological Optics 1988;65:613–18.
4.Bennett AG. Aspherical contact lens surfaces. Ophthalmic Optician 1968;8:1037– 40, 1297–1300, 1311, 9:222–230.
5.Stone J. Corneal lenses with constant axial edge lift. Ophthalmic Optician 1975;15:818–24.
6.Hodd FAB. A design study of the back surface of corneal contact lenses. Ophthalmic Optician 1966;6:1175–8, 1187–1190, 1203, 1229–1232, 1235–1238, 7:14–16, 19–21, 39.
7.Douthwaite WA. Contact Lens Optics and Lens Design. 2nd ed. Oxford: ButterworthHeinemann; 1995.
8.Rabbetts RB. Spreadsheet programs for contact lens back surface geometry. Journal of the British Contact Lens Association 1993;16:129–33.
9.Atkinson TCO. The development of the back surface design of rigid lenses. Contax 1987;November:5–18.
10. Korb DR, Korb JE. A new concept in contact lens design. Journal of the American Optometric Association 1970;41:1023.
11. Mackie I. Design compensation in corneal lens fitting. In Symposium on Contact Lenses: Transactions of the New Orleans Academy of Ophthalmology. Mosby: St Louis; 1973.
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Rigid gas-permeable lens fitting |
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Development of |
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rigid lens design |
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9.1 |
Introduction |
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9.2 |
Current bicurve, tricurve and multicurve designs |
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9.3 |
Current aspheric lenses |
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9.4 |
Reverse geometry lenses |
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9.1 Introduction
The PMMA corneal lens was designed in 1947.1 It consisted of a single curve 0.30 mm flatter than ‘K’ with a total diameter of about 11.00 mm. By the early 1950s, the cornea was recognized as more complicated than a simple sphere so that a bicurve construction was introduced. Further improvements were made by adding a flatter, third curve to assist tear circulation. The complex elliptical shape of the cornea was more fully understood by the late 1960s and multicurve lenses evolved together with the first aspheric constructions.2
9.1.1 Early lens designs
Early lens designs amongst others included:
•The Bier contour technique (1957)3 – a spherical bicurve design, fitted
with apical alignment and peripheral clearance. Example: 7.80:6.80/8.60:9.65 −3.00.
•Modified contour technique – a tricurve, modified version of the Bier contour technique, fitted with central alignment, a first peripheral curve
also giving some degree of alignment and a small, flat peripheral curve. Example: 7.80:6.50/8.30:9.10/12.25:9.50 −3.00.
•Bayshore technique (1962)4 – a small tricurve, fitted to give central
clearance and peripheral alignment with a central fenestration. Example: 7.60:6.00/8.80:7.00/17.00:7.60 −3.00. Single central fenestration, 0.20 mm.
•Conoid lens (1967)5 – fitted to give apical clearance with a spherical BOZR and a conical periphery commonly tangential to the BOZR with a fenestration 0.20 mm in from the lens edge, just within the optic zone.
©2010 Elsevier Ltd, Inc, BV
DOI: 10.1016/B978-0-7506-7590-1.00011-X