Section ONE Preliminaries

1.4 Manufacture of lenses

The majority of soft lenses are now disposable. Mass production manufacturing methods have therefore evolved for lenses to become cheaper and more  reproducible. With rigid lenses the emphasis is more on careful, stress-free production.

Regulation

One of the most important influences on contact lens manufacture during the 1990s was CE marking. Under the European Medical Device Directive (MDD), contact lenses are treated as medical devices and care products are treated as their accessories. Devices conforming to the directive should show the European standard CE marking and, from June 1998, it has been illegal to buy or sell a contact lens which does not have affixed the CE mark from a ‘Notified Body’ which would require the manufacturer to have some form of quality system. Manufacturers are required to have a formal quality control system such as ISO 9001/2000 and ISO 13485, the European equivalent.

1.4.1 Rigid lens manufacture

•Conventional lathes to cut the back and front lens surfaces from buttons.

•Computer numerically controlled (CNC) lathes. Four types are available with different types of automation, so that both spherical and aspheric surfaces can be cut.16

Polishing

The time and speed of polishing, together with the wetness and composition of the polish, are all very important. Frictional heating and over-polishing of the lens surface cause poor lens wettability.

1.4.2 Soft lens manufacture

•Lathing, as with rigid lenses, using buttons cut from rods. The finished lens is then hydrated.

•Spin casting, in which polymerization of the monomer and solvent takes place in open, spinning moulds (see Section 17.6).

•Cast moulding, which uses closed, disposable moulds with two components. Polymerization is by means of heat. The two methods are dry moulding, where the lens is moulded in the dry state and the edges finished by buffing; and wet moulding, in which the material is already hydrated (e.g. for disposable lenses).

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Background 1 Chapter

•Liquid edge moulding, in which lenses are cast in polypropylene moulds in the dry state. The contact lens edge is formed by accurate control of pressure on the mould and the volume of polymer employed, leaving the edge intact when the excess polymer (termed flash) has been squeezed out.17 There is no need to polish the edge with this process.

•Lightstream Technology, which eliminates the need for solvents and extraction of toxic residues, e.g. with CIBAVision’s PVA based nelfilcon A. Rigid quartz moulds are used but the front curve and base curve moulds never actually touch. A mechanical system holds them microns apart. A circular mask blocks the UV lightstream at the edge of the mould, preventing light interaction with the liquid material at the lens edge.   This liquid is washed away while the photo-lithographic process forms   the edge.

•Stabilized soft moulding (Vistakon) allows the lens to remain in the soft state throughout manufacture. The current process, called ‘Maximize’, has five stages. Steel moulds are used for injection moulding of the front and back curves for which the polypropylene primary packaging blisters are produced simultaneously. The material and diluent are added to the moulds which pass through a temperature controlled light activated polymerization tunnel. The lenses are then transported through a series of washing steps, the diluent being replaced with pure water. The lenses are transferred into primary packaging for automated inspection and the approved lenses are dosed with sterile saline and sealed under heat and pressure.

1.4.3 Toric lens manufacture

Soft lenses

•For conventional (non-disposable) torics, the method of choice is lathing using CNC lenticular back surface lathes and purposeful crimps to form the toric back surface under pressure. For the front surface, manual lenticular lathes with prism ballast capability are needed. The finished lens is polished before hydration.

•Disposable torics, which have a rather simpler design, are moulded.

•Recent developments are lathe waveform generators capable of producing complex, non-symmetrical geometries on surfaces that can be rotated off-axis.

Rigid gas-permeable lenses

•Conventional lathes in conjunction with crimping devices can be used to manufacture toric peripheries, back surface torics, bi-torics and front surface torics.18 The lens is lathed in the conventional manner, with a radius halfway between the required steeper and flatter meridians. It is then lathed a second time while held under a specified tension within the crimping device. The peripheral curves are then cut and the lens  

polished.

•Toric lathes that can generate a specified toric back surface.

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Section ONE Preliminaries

References

1.Jalbert I, Stapleton F. Effect of lens wear on corneal stroma: preliminary findings.

Australian and New Zealand Journal of Ophthalomology 1999;27:211–13.

2.Fatt I, Weissman BA. Physiology of the Eye. An Introduction to the Vegetative Functions. 2nd ed. Boston: Butterworth-Heinemann; 1992.

3.Rivera RK, Polse KA. Effects of hypoxia and hypercapnia on contact   lens-induced corneal acidosis. Optometry and Vision Science 1996;73(3):178–83.

4.Mandell R, Fatt I. Thinning of the human cornea on awakening. Nature 1965;208:292.

5.Madigan MC, Holden BA. Reduced epithelial adhesion after extended contact lens wear correlates with reduced hemidesmosome density in cat cornea. Investigative Ophthalmolology and Visual Science 1992;33:314–23.

6.Millodot M, O’Leary DJ. Effect of oxygen deprivation on corneal sensitivity. Acta Ophthalmologica 1980;58:434.

7.Fatt I, St Helen R. Oxygen tension under an oxygen permeable contact lens.

American Journal of Optometry 1971;48:545.

8.Morris JA. An overview of the hard gas permeable oxygen race. Optometry Today 1985;(March):168–72.

9.Hough DA. A Guide to Contact Lens Standards. London: British Contact Lens Association; 2000.

10.Brennan NA. Average thickness of a hydrogel lens for gas transmissiblity calculations. American Journal of Optometry 1984;61:627.

11.Brennan N, Efron N, Holden BA. Further developments in the RGP Dk controversy.

International Eyecare 1986;2:508–9.

12.Fatt I, Rasson JE, Melpolder JB. Measuring oxygen permeability of gas permeable hard and hydrogel lenses and flat samples in air. Internatiional Contact Lens Clinic 1987;14:389–91.

13.Holden BA, Newton-Howes J, Winterton L, Fatt I, Hamano H, La Hood D. The Dk project: an interlaboratory comparison of Dk/L measurements. Optometry and Vision Science 1990;67:476–81.

14.Holden BA, Mertz GW. Critical oxygen levels to avoid corneal oedema for daily and extended wear contact lenses. Investigative Ophthalmology and Visual Science

1984;25:1161–7.

15.Pearson RM. Rigid gas permeable wettability and maintenance. Contax 1987;(Sept):8–16.

16.Hough A. Rigid lens manufacture in the 1990s. Optician 1997;214:24–8.

17.Hough A. Soft lens manufacture in the1990s: managing unit costs to compete effectively. Optician 1997;213:35–41.

18.Meyler J, Ruston D. Toric RGP contact lenses made easy. Optician 1995;209:30–5.

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Section

Preliminaries ONE

Instrumentation CHAPTER2

2.1 Slit lamp

The slit lamp provides the best method of observing ocular tissue in section under high or low magnification.

2.1.1 Instrument controls and focus

Instrument controls allow for variation in height, lateral movement and focusing. The illumination and observation systems are focused at a common point unless they are uncoupled to allow independent movement.

The optical system contains an objective, typically with ×3 to ×3.5 magnification, and an eyepiece with variable or interchangeable power. The normal range of total magnification gives ×6, ×10, ×16, ×25 and ×40, but with optional eyepieces up to ×70 is possible. Zoom optics give a smoother change of magnification.

The illumination system provides a slit width and height of up to 14 mm and contains a variety of filters depending on the slit lamp model. The range available includes:

•White light.

•Neutral density or heat absorption filters – to reduce the light intensity.

•Cobalt-blue filter – for use with fluorescein.

•Yellow filter (Wratten 12) – a barrier filter placed in front of the viewing system. This is used in conjunction with the cobalt-blue filter to enhance the contrast of fluorescein. It allows the transmission of green fluorescent

©2010 Elsevier Ltd, Inc, BV

DOI: 10.1016/B978-0-7506-7590-1.00011-X

Section ONE Preliminaries

‘Against’

C

‘With’

Figure 2.1  Focusing the slit beam (C, focus point)

light while at the same time blocking the blue light reflected from the corneal surface.

•Green ‘red-free’ filter – to enhance contrast when looking for corneal vascularization. This also increases the visibility of rose bengal staining.

•Diffusion filter (usually external).

•Polarizing filter – to reduce unwanted specular reflections and enhance the visibility of subtle changes.

•Red filter (Wratten 25) – enhances lissamine green stain when placed in front of the viewing system. Used in conjunction with white light. The filter blocks the green light from the stained area making it appear darker and more visible.

The lamp housing can be rotated so that that the slit beam may be used in a range of meridians. This is a useful technique for measurement (e.g. tear prism height) or axis location if associated with a graticule (e.g. for soft toric markings).

Focus is achieved by rotating the slit beam about its fulcrum1 (Figure 2.1).

•If the illuminated area moves with the direction of the arm, the projected slit is in front of the focus position.

•If the illuminated area moves against the direction of the arm, the projected slit is beyond the focus position.

•If the illuminated area remains stationary as the arm moves, the slit is exactly in focus.

2.1.2Methods of illumination

Direct methods

The slit beam and microscope are focused at the same point to give:

•Diffuse illumination.

•Direct focal illumination.

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Instrumentation 2 Chapter

•Indirect illumination.

•Specular reflection.

•Sclerotic scatter.

Indirect methods

The beam and microscope are uncoupled so that they are no longer focused at the same point to give:

•Indirect (proper) illumination.

•Sclerotic scatter.

•Retro-illumination.

2.1.3 Recommended slit lamp routine

•The instrument height is set for a halfway point in its travel range. The eyepieces are adjusted for the observer’s prescription and pupillary distance (PD).

•The patient is made comfortable in relation to the height of the headrest and the instrument table.

•The patient closes the eyes and a slit beam is focused onto the eyelids. The beam is moved to the outer canthus without moving the instrument out of its range of focus, and the patient asked to open the eyes.

•Diffuse illumination is used for a general look at the ocular tissues and lids under low magnification.

•In a darkened room, the cornea is examined with sclerotic scatter using either the microscope or the unaided eye for signs of opacities or oedema (Figure 2.2).

Figure 2.2  Sclerotic scatter by indirect method

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Section ONE Preliminaries

Figure 2.3  Direct focal illumination

•Starting at the temporal limbus, the corneal tissue is scanned using direct focal illumination and a parallelepiped at least 2 mm wide. The slit beam is

set between 40° and 60° to the temporal side of the centrally placed microscope. Magnification should be about ×20. From the corneal apex to the nasal limbus, the illumination should be swung to the nasal side of the microscope (Figure 2.3).

•The beam is reduced to an optic section and the cornea examined in the same manner, localizing the depth of any abnormality discovered with the wide beam.

•The patient looks up and down, to take in the superior and inferior limbal areas.

•The direct slit beam is oscillated as the cornea is traversed. Any abnormality changes the scattering of light in the tissues and aids identification. The whole field of view contained by the beam and its surrounds is continuously observed with a combination of direct and indirect illumination.

•As the direct beam is moved across the cornea, specular reflection of the tear film occurs. Examination of this bright area, looking for any debris or

oiliness, gives a qualitative assessment of the tear film. Changing focus to the rear of the beam section brings an area of endothelium into view (Figure 2.4). A general appreciation of endothelial regularity is obtained, rather than specific cell details. Each zone appears twice – nasally and temporally – as the reflected light using this method enters only one eyepiece at a time.

N.B. The endothelium looks like a patch of beaten gold to the side of the much brighter specular zone.

• The slit lamp is uncoupled to examine any corneal abnormality by means of retro-illumination. Direct, marginal and indirect retro-illumination

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Instrumentation 2 Chapter

Figure 2.4  Specular reflection showing the endothelium

Figure 2.5  Retro-illumination

ascertain whether any abnormality is more or less dense than the surrounding tissue. This is revealed by the way the light converges or diverges around the abnormality, and aids in its identification (Figure 2.5).2

•A small drop of fluorescein is instilled into each eye, just enough to fill the tear prism but not too much as it will mask any take-up of the stain.

•On fluorescein instillation ask the patient not to blink while observing whether the stain immediately fluoresces. This is more easily seen by looking at the conjunctiva or the tear meniscus. If the fluorescein has a dull orange hue, it is not mixing easily into the tears and suggests poor tear quality. After a few blinks normal fluorescence should occur.

•Both the conjunctiva and corneal epithelium are then examined for evidence of staining using a broad scanning beam and the cobalt blue filter. A yellow barrier filter (Kodak Wratten 12) in the observation system gives enhanced contrast and assists examination of any staining which may be present. Other stains used are lissamine green, rose bengal and, less commonly, alcian blue.

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