Section THREE Hydrogel and silicone hydrogel fitting

17.4 Thin lenses

Lenses with a centre thickness less than 0.10 mm may be regarded as thin (see Section 7.3.2). Lenses in the range 0.05–0.07 mm are ultrathin and those less than 0.05 mm have been termed either superthin or hyperthin.

The fitting characteristics of thin and standard lenses differ even if having otherwise identical specification. Lens thickness may therefore be regarded as an additional fitting variable. In practical terms, thin lenses generally:

•Possess greater flexibility.

•Have fewer fitting steps.

•Drape the cornea more completely and give less mobility.

•Give less lid sensation and are more comfortable.

•Have better transmissibility (Dk/t).

•Possess different fitting characteristics and sometimes permit better centration.

•Give less satisfactory acuity on toric corneas.

•Dehydrate to a greater extent on the eye after settling.

•Prove more difficult to handle.

Zero 6 (CooperVision)

Thin HEMA, semi-scleral lenses for daily wear; manufactured by lathing.

Material properties

HEMA 38.6% (see Section 15.1).

Lens geometry

•Centre thickness is 0.06 mm for all minus lenses of power −3.00 D or greater.

•The mid-periphery is deliberately thickened to make handling easier.

•Zero 6 plus lenses have an average thickness of of 0.10 mm.

•The back surface is a bicurve with a constant BOZD of 13.28 mm and peripheral curve width of 0.36 mm (13.28 mm + 0.36 mm + 0.36 mm = 14.00 mm).

•The front surface is lenticulated with BOZDs of 6.70 mm at −10.00 D and 8.30 mm at +10.00 D.

Parameters available

See Table 17.1.

Fitting technique

•Approximately 70% of minus lenses are fitted with the 8.70 mm radius.

•The most common radius for plus lenses is 9.00 mm.

•There is no very firm relationship between ‘K’ and radius because of greater lens flexibility.

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Other soft lens fitting considerations 17 Chapter

Table 17.1  Parameters available for Hydron

Zero 6 and Z Plus lenses

•14.00 mm is the standard diameter. TDs of 13.5 mm, 14.50 mm and 15.00 mm, and radii of 8.10 mm and 9.60 mm are available to special order.

Typical specification

Zero 6: 8.70:14.00  −3.00

Related lenses

•Zero 6T: a front surface toric with prism-free optic zone.

•A range of five colour enhancer tints with densities of 10% and 20%.

17.5 Aspheric lenses

Back surface aspherics

A minority of soft lenses have an aspheric back surface designed to match the aspheric nature of the cornea. A correctly fitting lens behaves in the main as described in Section 16.1, but compared with spherical lenses:

•Aspherics do not have a true radius but are designated in some other way such as by ‘fitting value’ or posterior apical radius (PAR).

•Changing the total diameter does not necessarily alter the fitting characteristics.

•Lens mobility of between 0.25 mm and 0.75 mm can be acceptable.

•With some corneal geometries, proper centration cannot be achieved and a spherical lens is required, although the reverse is also true.

•Centre thickness of a −3.00 D lens is 0.09 mm.

•All lenses have an elliptical back surface with a flat (FL) or steep (ST) fitting value instead of a radius.

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Section THREE Hydrogel and silicone hydrogel fitting

•Radius and eccentricity are varied to give a consistent performance throughout the power range.

•Lenses feature a tangential bevel to give a continuous transition between back surface and ski-shaped edge.

Parameters available

See Table 17.2.

Table 17.2  Parameters available for Weicon CE

lenses

Fitting technique

•The 13.80 mm diameter is most commonly used.

•The FL fitting value is generally selected.

•The ST fitting value is tried only if the FL is excessively mobile after complete settling.

•The 13.00 mm fitting can be used as a high water content corneal diameter lens.

Typical specification

Weicon CE: FL 13.80  −3.00

Related lenses

•The Weicon CE Toric: a high water content back surface toric stabilized by dynamic stabilization.

•The Weicon 38E: the original HEMA version of the Weicon CE.

Front surface aspherics

A front surface aspheric (e.g. Cantor + Nissel EV38) can sometimes give improved acuity in cases of low to medium astigmatism (0.75–1.25 D) by the correction of optical aberrations. Essentially, by reducing the size of the blur circle on the retina, better quality of vision is obtained, although over-refraction usually reveals the same level of astigmatism.

Soflens daily disposable (59%) (Bausch & Lomb)

A daily disposable soft lens with an aspheric anterior surface.

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Other soft lens fitting considerations 17 Chapter

Lens geometry

•Centre thickness of a −3.00 D lens is 0.09 mm.

•All lenses have an equivalent base curve (BOZR) of 8.6 mm and TD of 14.2 mm.

Parameters available

See Table 17.3.

Table 17.3  Parameters available for Soflens daily

disposables

Fitting technique

• A single fitting lens.

Typical specification

8.60:14.20  −3.00

Related lenses

• The Soflens daily disposable toric stabilized by refined prism ballasting.

17.6 Spun-cast lenses

Spun cast HEMA lenses from Bausch & Lomb (the original Soflens) were the first to obtain FDA approval in 1971 and are now mainly of historical importance. Both corneal and semi-scleral lenses were possible, manufactured by spin-  casting in open moulds. They have been almost entirely superseded by more modern designs, materials and methods of manufacture. A few general points are worth mentioning.

Advantages of spin-casting

•Excellent surface quality and edge shape.

•Mass production ensured good reproducibility and consistency of manufacture.

Disadvantages of spin-casting

•Limitations on the variety of back surface forms which could be conveniently obtained.

•The full power range was not obtainable for each series of lens.

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