Rigid Gas Permeable Corneal and Corneoscleral Lens Fitting

Silicone-acrylate (or siloxanyl-acrylate)

Although highly permeable to oxygen, pure silicone polymers are soft and hydrophobic. They are therefore combined with PMMA to enhance the hardness and stability of the material. To ensure adequate wettability, hydrophilic monomers (e.g. methacrylic acid) are also incorporated into the polymer so that surface treatment or special solutions are required to maintain adequate wettability. This allows subsequent laboratory and practitioner modification or repolishing of the lens. Different additives such as dimethacrylate may be used to enhance tensile strength and resistance to deformation.

The ratio of PMMA to silicone is generally 65% PMMA to 35% silicone. The greater the proportion of silicone, the higher the oxygen permeability, but the poorer the wettability, stability and strength. Similarly, the greater the proportion of wetting agent, the greater the surface wettability, but the poorer the dimensional stability and the greater the affinity for surface deposition due to increased surface reactivity.

The range of Dk values of the materials commonly available is limited (generally 9–30 units *

* Oxygen permeability units or Dk units are expressed as units of 10 ^–11 (cm mL O ₂ )/(cm ² s mm Hg). See also Chapter 2 .

), although by incorporating proprietary compounds, some laboratories have achieved higher values without sacrificing other desirable properties.

As with all RGP materials, care is needed during manufacture, although there is less susceptibility to distortion than with earlier materials such as CAB. Warpage often indicates excessive overheating during the button blocking or lathing procedures (see Chapter 29 ). Ammoniated polishes are best avoided, and alcohols, esters, ketones and aromatic hydrocarbons will damage the material.

Fluorosilicone-acrylate (fluoropolymers)

Fluorocarbon gases have been widely used since the early 1900s as a form of refrigerant (e.g. freon) and a gaseous propellant for aerosol sprays. Fluoroplastics include high-temperature cable insulation, heat-resistant plastics for piping and gaskets, and low-friction and anti-stick applications for nonlubricated bearings and coatings for cooking utensils (e.g. Teflon). More recently, researchers discovered that liquid fluorochemicals are highly efficient carriers of oxygen and carbon dioxide. These properties led to the use of fluorochemicals in physiological salt solution as a form of artificial blood ( ). This ‘blood’ supplies oxygen to the body tissues while carrying away waste products such as carbon dioxide.

Early reference to fluoroplastics for contact lenses appeared in patents by the DuPont Corporation in 1970 and Gaylord in 1974. These early contact lens materials never reached the commercial stage of development, partly due to the poor wetting characteristics, softness, high specific gravity (1.60) and low index of refraction (1.388) ( ).

Polymer Technology were the first to attempt to avoid these problems in the ‘Boston Equalens’ by using a polymer incorporating a fluorinated monomer and combining it with a silicone-acrylate moiety. The fluorinated component enhances the material's capability of resisting mucus adhesion and deposit formation while promoting its affinity for tear mucin and soluble proteins for superior wettability in vivo.

Many materials now incorporate an ultraviolet (UV) absorber within the polymer matrix in response to the growing concern over the potential cataractogenic and retinal toxic effects of UV light.

Fluorosilicone-acrylates (and a small number of silicone-acrylates) now make up the majority of RGP lenses being fitted.

Siloxanylstyrene-fluoromethacrylate

This material represents a further modification to the fluorosilicone-acrylates. Currently only available as the Menicon Z lens, this polymer is composed of siloxanylstyrene, fluoromethacrylate and benzotriazole UV absorber. The surface is modified to improve wettability (see below). The result is a material with good structural integrity and hyperoxygen transmissibility.

Modified surface materials

Some lenses have their surfaces modified to optimise the surface wettability. These lenses claim to have the physical characteristics (durability, flexure resistance and oxygen transmissibility) of RGP lenses but the wettability and easier adaptability of hydrophilic lenses. This is achieved by either surface plasma treatment or incorporation of hydrophilic monomers into the lens material.

In plasma treatment the lens is placed in a specialised vacuum chamber and bombarded with oxygen ions through the use of a radio frequency generator. This not only removes any remaining residuals left over from the manufacturing process but also significantly improves the wetting angle of the lens.

The Novalens (Ocutec Ltd) is composed of rosilfocon A, a hydrophilic styryl-silicone material. The lens consists of a rigid core and a surface polymer that has hydrophilic properties.

Aquasil (Aquaperm) is a silicone-acrylate copolymer which, when immersed in a weak acidic solution, produces a thin layer of pHEMA on the surface 15–20 µm thick. The hydroxyl groups of pHEMA that are permanently bonded to the polymer are claimed to be nonreactive. Solution uptake is less than 2%, making solution sensitivity unlikely, provided that the correct cleaning regime is used. The hydrophilised surface can be retreated if it is worn off or removed by repolishing or modification.

A different technique is used in Menicon Super-EX , which is a surface-modified fluorosilicone-acrylate material. In this case the very high Dk value has been gained at the expense of surface wettability, hence the surface treatment. Lens modifications are not possible with this design, and the lens is contraindicated for patients with a Dry eye problem. The lens is mainly indicated for those patients with a high physiological demand or for extended wear.

The SEED S-1 (Seed Co., Japan) uses polyethylene glycol, a hydrophilic monomer, grafted onto the surface of a fluorosilicone-acrylate RGP material by plasma treatment and polymerisation. Improved hydrophilicity and the ability to resist contamination and spoiling are claimed.

The Hybrid FS Plus (Contamac) , Tyro-97 (Lagado Corporation) incorporate small amounts of hydrophilic monomer into a fluorosilicone-acrylate RGP material during the polymerisation process. Surface treatment is therefore unnecessary, and the material can be machined, processed and modified as with any other RGP material. Water uptake is less than 0.85% in the Hybrid FS Plus so that the material displays great stability. Conventional RGP solutions can be used.

The Comfort O ₂ lens (David Thomas, UK) is a rigid silicone hydrogel polymer claimed to hydrate on the surface like a soft silicone hydrogel while remaining rigid in the lens interior. It has a Dk of 56 and can be made to any prescription.

Opinions vary as to the value of these modified materials. found no difference in initial comfort or wettability between the Novalens and two other RGP materials. Of 20 patients fitted by the author with an Aquasil material lens in one eye and a conventional FSA material in the other, only one could detect any improvement in comfort or wettability. On the other hand, reported improvements using the Aquasil material in patients over 35 years of age, in patients showing GPC or dry eye and in PMMA refit cases. Similar good results were reported by and .

Material and design classification

The International Organization for Standardization (ISO) has produced a method of classification of both soft and rigid lenses which differs from the US Food and Drug Administration (FDA) ‘Approved Names’ system (USAN). The objective is to identify each material more accurately ( Table 9.1 and Table 10.4 ).

Material names, therefore, indicate the polymer (whether it is a first or subsequent development of that polymer), the constituents and the Dk. Nowadays, nearly all new materials will be Group III lenses ( Table 9.2 ).

For example: Paragon HDS is called Paflufocon B III 3:

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  Paflu – name of polymer mix

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  focon – rigid lens material

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  B – second generation of this polymer

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  III – fluorosilicone-acrylate

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  3 – ISO Dk is between 31 and 60 units.

Terms Relating to Corneal Lenses

Detailed terminology relating to corneal lenses is summarised in the Glossary of terms at the beginning of the book. For ease of reference, the major terms are summarised in Fig. 9.2 (see also Chapter 30 ).

Capillary attraction/Post-lens tear layer forces

The force of attraction between the lens and the cornea varies inversely with the distance between the two surfaces ( ); i.e. the more closely a lens surface matches the corneal contour, the greater the force of attraction. Because the cornea is not spherical, the posterior lens surface is made either of multiple flattening curves or aspherical curves. In practice, however, although it is desirable to achieve a reasonable area of corneal ‘alignment’ to aid capillary attraction and prevent corneal insult, a lens that conforms exactly to the corneal contour over the whole of its surface would not be comfortably tolerated. The capillary attraction would be so great that there would be minimal lens movement or tear circulation beneath the lens.

Tear fluid squeeze pressure (TFSP)

Since the lens does not exactly match the shape of the eye, capillary attraction is not a major force holding the lens in place – it is the TFSP. This is the pressure that develops behind the optic zone in the Post-lens tear film. It centres the lens by opposing the gravity force (see below) that acts to decentre the lens inferiorly and the eyelid force (ELF) that acts to decentre the lens superiorly at equilibrium. During blinking, the TFSP is the main recentration force, as its dynamic action creates a symmetrical force on the contact lens ( ). This force is proportional to the irregularity of the Post-lens tear layer in that region and directly proportional to the tear layer thickness (TLT): The greater the TLT, the greater the TFSP. Manipulation of the fitting affects the TLT in conventional RGP fitting (see below), and this pressure is utilised in orthokeratology (see Chapter 19 ).

showed that if a lens with zero apical tear thickness is placed on the cornea, the lack of TFSP allows the lid to manipulate the lens position until a tear layer develops. This occurs as the lens decentres, usually superiorly, and continues until the surface tension forces around the lens edge balance the squeeze force so that a quasistatic state re-emerges.

Gravity

The effects of gravity on the lens are best envisaged using the concept of the centre of gravity. This has the property that the object acts as though all of its weight were concentrated at that one point. For a corneal lens, the position of the centre of gravity is near the back surface or actually behind the lens. The further the centre of gravity moves behind the lens, the greater the area of support above it. As the centre of gravity moves towards the front surface of the lens, there is less support for the lens, and it tends to drop or ‘lag’ more readily under the effect of gravity.

The position of the centre of gravity is affected by the lens total diameter (TD), back vertex power, thickness and back optic zone radius (BOZR) ( Fig. 9.3 ). Thus the effects of gravity are lessened for lenses with negative powers, minimal centre thickness, steep corneal curvature and larger TD.

summarised the relative effects of various changes in lens parameters and the effect that these have on the movement of the lens centre of gravity. These are shown in Table 9.4 . This shows the markedly superior effect achieved by increasing the TD in comparison to other design changes.

For example, if a practitioner wishes to stabilise a −3.00 D lens of 9.00 mm TD, 7.40 mm back optic zone diameter (BOZD) and 7.80 mm BOZR, the following options are realistically available:

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  Increase the diameter by 0.10 mm steps to shift the centre of gravity back by 0.018 mm.

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  Steepen the BOZR by 0.05 mm steps to shift the centre of gravity back by 0.004 mm.

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  Reduce the centre thickness by 0.01 mm steps to shift the centre of gravity back by 0.006 mm.

In conclusion, it is therefore apparent that changes in lens thickness, TD and power are the parameter changes most likely to affect lens position on the eye.

Specific gravity

Table of RGP Materials available at https://expertconsult.inkling.com lists the range of specific gravities in most common examples of RGP materials. In a study on a small group of wearers exhibiting poor lens centration, found no major effect by changing to materials of differing specific gravity but made the following recommendations:

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  If a patient consistently exhibits a high-riding lens, material changes to manipulate specific gravity will probably not significantly improve centration. However, refabricating a lens in a thinner design may provide some benefit.

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  If a patient consistently exhibits a low-riding lens, selecting a low-specific-gravity material and/or thinning the lens should prove beneficial.

Tear meniscus/edge tension force

The existence of a tear meniscus under the edge of a corneal lens produces an edge tension force (ETF) which is essential for lens centration ( , , ). This force acts to hold the lens against the cornea whenever the lids do not cover the edge. For any given lens, the greater the circumference of the meniscus, the better the lens centration. If the peripheral curve of the lens is too close to the cornea, the tear meniscus will be ineffective in holding the lens on the eye and will reduce tear interchange. If the clearance is too great, the meniscus will be inverted and will reduce the adhesion, causing possible 3 and 9 o'clock staining (see p. 196 ) or bubbles under the edge.

The ETF depends on the radius of the tear meniscus: The smaller the radius, the stronger the force ( , ). Altering the edge clearance and edge thickness can vary this (see p. 186 ).

The ETF has a secondary effect on mechanical performance but plays a key role in maintaining a continuous tear film at the edge of the lens (Guillon 1994). Here again, the shorter the radius, the more likely the tear film is to remain contiguous.

Eyelid force (ELF) and position

The eyelids exert the principal role in RGP lens mechanical performance.

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  During blinking, the lens may displace 2–3 mm.

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  The lens may be supported by the lower lid.

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  Between blinks the ELF affects extrapalpebral fitting lenses by acting normally to the contact lens to hold it against the cornea. This results in negative pressure, which keeps the lens centred or riding high, counteracting gravity which would force the lens to ride low.

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  Where TFSP and ETF are inadequate, gravity forces excessive or the lens edge too thick, the lids may push the lens down on the cornea where it may ‘bind’ (see ‘Lens Adhesion Phenomenon’ p. 195 ).

Selection of BOZR and BOZD in spherical or near-spherical corneas

Because the cornea is aspherical in shape, as stated earlier, a small layer of tear liquid must exist between the cornea and spherical back optic zone of the lens. This volume will tend to increase when the lens is decentred and thereby tend to recentre the lens after lens movement by the negative pressure thus induced. In the case of an aspheric back surface lens, negative pressure can be induced only if the lens fit creates a positive tear layer between the lens centre and the point of contact with the cornea.

Thus, although practitioners commonly refer to ‘lens alignment’ of the cornea, in reality there will always be a very thin layer of tears between the lens and cornea as discussed earlier. This is shown diagrammatically in Fig. 9.6 .

Sag of the contact

( r = BOZR and y = BOZD/2) and the sag ( x ) of the cornea (assumed elliptical) can be determined from

(see Fig. 9.1 )

where y = BOZD/2, r ₀ = apical corneal radius and p = asphericity.

Also, r ₀ = b ² / a and p = b ² / a ² where a and b are the semi-major and semi-minor axes of the corneal ellipse ( , ).

Fortunately, these calculations can be done easily using suitable lens design programs such as the one available at https://expertconsult.inkling.com . The examples listed below can thus be reproduced, and neophyte practitioners are urged to do this.

Clinical experience has shown that the ideal central TLT to give apparent alignment is between 10 and 25 microns with a typical value of around 20 microns ( ). Increasing the TLT beyond this will show increasing lens clearance ( Fig. 9.7 ).

As the BOZD increases, the BOZR should be flattened on the normal prolate cornea. Thus the choice of BOZR will depend not only on the apical radius and the degree of flattening ( e or p value) but also on the BOZD selected which, in turn, will depend on the lens TD. The practitioner must look not at one parameter in isolation but at the overall lens design and fitting philosophy. However, it is logical to examine the selection of BOZR first and to determine how typical changes in corneal and lens parameters will affect this.

Keratometry typically measures the corneal curvature approximately 1.50–1.75 mm from the corneal apex, i.e. a diameter of around 3.0–3.5 mm across. Fortuitously this gives an approximate average mid-value curvature for the 7.00 mm BOZD which historically was a common value in early fitting set lenses. To create a TLT of 15–20 microns, practitioners then simply fitted lenses 0.05 mm steeper than the flattest K reading. This is shown in Fig. 9.8 for a typical cornea with K readings of 7.80 mm at 180° and 7.60 mm at 90°, an e value of 0.5 and BOZR of 7.75 mm.

Small diameter lenses are rarely used nowadays except in some keratoconus fittings. RGP lenses are usually fitted with larger TDs which allows:

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  larger BOZDs for reduced flare and better vision

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  greater capillary attraction and edge tension force to improve centration and reduce lens loss

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  greater comfort by commonly tucking the upper lens edge under the upper lid so that there is reduced lens edge sensation.

Fig. 9.9 shows the change in parameters necessary to produce a central TLT similar to that in Fig. 9.8 with the BOZD now increased to 8.30 mm (and the TD increased to 9.80 mm).

So far we have seen what effect changing the BOZD has and how this alters the BOZR for an equivalent fit. A rough rule of thumb is:

However, using the program
available at https://expertconsult.inkling.com allows a much more accurate approach, for example when a suitable-diameter trial lens is not available for a specific patient (see also ‘Effects of Variations in the BOZD’, p. 200 ).

Next we need to examine the effect of varying corneal eccentricities and how these affect the choice of BOZR.

In the example given in Fig. 9.9 , we have assumed an average e value of 0.5. Let us now examine the typical extreme values of 0.3 and 0.7. Figs 9.10 and 9.11 give the TLTs of the lenses necessary to produce a ‘ideal’ fit for these values.

We can therefore see that for this cornea of 7.80 mm by 7.60 mm and this particular BOZD, the ideal BOZR can vary from 7.75 to 7.95 mm. Table 9.5 shows how the BOZR varies according to the flattest K reading ( K _F ) and e value.

Although a corneal topographic device is extremely helpful in selecting this ‘ideal’ BOZR, an experienced clinician can visually estimate the fluorescein pattern and judge an acceptable fit. However, neophyte practitioners should determine the first BOZR to show mild central fluorescence and then fit 0.05 mm flatter since a borderline flat fit can be difficult to determine. Even topographical devices have limitations (see Chapter 8 ), and two BOZR both may produce central TLTs that are within the acceptable range.

In conclusion, the initial BOZR for a nearly spherical cornea is chosen as the flattest K reading, but it may need to be modified depending on:

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  The BOZD chosen (which in turn may be governed by the TD selected).

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  The patient's corneal eccentricity. The higher the e value, the flatter the compromise spherical BOZR should be.

Thus the final BOZR selected typically will vary by 0.10 mm steeper than the flattest K reading to 0.20 mm flatter, and possibly 0.25 mm flatter for patients at the steep end of corneal curvature and with high e values.

Selection of the BOZR in astigmatic corneas

The majority of contact lens patients will show some degree of corneal astigmatism. If this is under 1.00 D, most can be fitted with a spherical BOZR. As the astigmatism increases, a steeper (0.05 mm) BOZR may centre better. If a topographical device is available, corneas showing central astigmatism only (as opposed to a limbus-to-limbus astigmatic pattern) are more likely to be able to be fitted with a spherical BOZR. For corneal astigmatism of 1.50 D and above, a toric BOZR becomes more essential for a good fit.

showed that, in an astigmatic cornea, the steeper corneal meridian has a slightly lower e value than the flatter meridian; therefore toric trial lenses are advantageous in most cases. Fitting sets with meridional differences of 0.4–0.6 mm are extremely useful.

The decision to use an astigmatic lens will depend on:

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  lens centration, i.e. stable vision

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  patient comfort

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  acceptable corneal and limbal physiology.

The optical implications of astigmatic lenses are discussed in Chapter 11 and under ‘Assessment of Fit’, below.

Selection of the BOZD

This should be at least 1.50 mm larger than the pupil diameter in average room illumination and larger still if the lenses are to be used frequently for activities such as night driving.

As already mentioned, if the BOZD is changed, a corresponding alteration to the BOZR should be made to maintain the same lens sag.

Selection of the edge curve (see also ‘Tear Meniscus’, p. 181 )

The purpose of the edge curve is to:

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  prevent indentation from the lens edge on lens movement

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  assist in lens centration by providing a tear meniscus

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  Improve comfort

It should:

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  be adequate enough to encourage good retro-lens tear flow

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  be adequate enough to allow easy lens removal

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  not be so excessive as to allow lens movement onto the perilimbal area

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  not be so excessive as to cause edge awareness.

The axial edge clearance (AEC) will be dictated by the radii and width of peripheral curve(s). The optimal AEC generally ranges from 60 to 90 microns. However, in patients with 3 and 9 o'clock staining, a small (40–60 microns) AEC may be preferable ( , ), providing that the lens remains mobile and does not bind to the cornea. By keeping the lens edge closer to the corneal surface, better contact is achieved between the superior palpebral conjunctival surface and the cornea and limbal conjunctiva, reducing desiccation. A small AEC will produce greater lens comfort, as a lens edge that is close to the cornea causes less irritation of the upper lid margin during blinking.

However, if the AEC is too small, the periphery may cause discomfort when it moves to a flatter part of the cornea, as it will exert greater pressure on the corneal surface, also making it more difficult to remove.

Conversely, excessive AEC can result in:

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  thinning of the corneal tear layer adjacent to the lens edge, resulting in punctate staining

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  destabilising the lens fitting, resulting in excessive movement. This is partly due to a reduction in the surface tension force caused by a change in the tear meniscus around the edge of the lens and partly due to the effect of the lid action on blinking

• ▪

  possible formation of bubbles under or adjacent to the lens edge.

Lens movement is largely controlled by the peripheral zone width and the AEC.

However, a narrow peripheral zone combined with an average AEC may cause a problem when the lens moves towards the flatter corneal periphery as fluorescein will disappear from the peripheral zone (see Fig. 9.7b ), resulting in the edge indenting the cornea and causing discomfort. Conversely, if the edge curve is too wide, it may move too far to the corneal periphery and accumulate large amounts of debris in the tear reservoir which can be a source of foreign body irritation.

The ideal peripheral curve width was shown by to be approximately 0.50–0.60 mm. If the peripheral curve is smaller, a large change in radius is necessary to change the AEC significantly, but a small error in diameter during manufacture would change the edge clearance considerably. Similarly, with large peripheral curve widths, a small error in radius would be significant but a large error in diameter less significant. An example of a curve change is given below and can be calculated using the lens design program material available at https://expertconsult.inkling.com .

It is necessary, therefore, to assess the performance of a trial lens with a known peripheral curve radius and width, and then to modify the lens design to ensure that the peripheral curve is neither inadequate nor excessive. A typical edge curve fluorescein pattern is shown in Fig. 9.12 .

Selection of the TD

In general nowadays, larger-TD lenses made of RGP materials are fitted such that the superior lens edge is positioned under the upper lid. This can provide better comfort for the patient as the interaction between the lens edge and the lid margin will be reduced. Nevertheless, each case should be judged on its own merits.

Too large a TD may allow limbal bumping on ocular excursion and, in the case of moderately astigmatic corneas, may precipitate the need for an astigmatic fitting lens due to excess edge clearance along the steeper corneal meridian.

Where the upper lid margin lies above the lens, factors dictating comfort will be the design and shape of the edge of the lens. Lindsay and Bruce (personal communication, 2004) devised a schema for the selection of the lens TD depending on the relative position of the two eyelids. This is shown in Fig. 9.15 . As most eyes show ‘low’ upper lids (i.e. covering the upper part of the cornea) and lower lids level with or slightly covering the lower limbal area, a relatively large TD is possible for the majority of patients. Thus most corneal lenses now have TDs of 9.50–10.50 mm.

Selection of the intermediate curve(s)

These curve(s) provide a transition between the BOZR and edge curves. Each curve should have a minimum width of 0.25 mm (i.e. 0.5 mm between the BOZD and BPZD), with a radius that is between the BOZR and edge curve and showing a very small edge lift. Commonly only one intermediate curve is necessary, but if there is a relatively large distance between the edge of the BOZ and edge curve, more than one curve may be necessary to follow the corneal contour. Examples of intermediate curves have been given above in Figs. 9.8–9.10 , so for example, the intermediate curve in Fig. 9.8 has a radius of 8.10 mm and a diameter of 7.60 mm.