Soft lens fitting and design 15 Chapter 
•The radius is selected to be at least 0.60 mm flatter than ‘K’ for 14.00 mm lenses and 0.30 mm flatter than ‘K’ for 13.00 mm lenses.
•Lens movement should be 0.25–0.50 mm.
•The relative stiffness of the material gives an advantage in correcting low to medium degrees of astigmatism (0.75–1.00 D).
•A low rate of lens dehydration is claimed to assist or avoid dry eye problems.
•All lenses are manufactured with a UV inhibitor.
Typical lens specification
8.00:13.00 −3.00 8.90:14.00 −3.00
Related lenses
•Standard front surface toric.
•Toric Rx TDI, a back surface toric.
•Lunelle Therapeutic, plano lenses for bandage use.
15.3 Semi-scleral lenses
The majority of lathed semi-sclerals are significantly larger and thicker than corneal soft lenses, giving better stability of both vision and fitting. In order to provide good physiological response, they are now mostly manufactured from silicone hydrogel or medium to high water content hydrogel materials with good Dk values.
Indications
•Most straightforward cases.
•Large corneas.
•Large palpebral apertures.
•Sensitive lid margin.
•Sensitive limbus.
•Hyperopes and high powers, if high Dk.
•Moderate degrees of astigmatism (0.75–1.00 D).
Contraindications
•Very small corneas.
•Small palpebral apertures and tight lids if handling difficult.
•Corneas prone to oedema, if low to medium water content.
•Where cosmetic appearance is important.
Fitting
Radius
•Radius selection is based on keratometry.
•Most radii are fitted between 8.30 and 9.20 mm.
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Section THREE Hydrogel and silicone hydrogel fitting
•Silicone hydrogel radii are generally between 8.3 and 8.7 mm.
•High water content lenses are fitted between 0.30 and 1.00 mm flatter than ‘K’.
•Low water content lenses are fitted flatter, between 0.70 and 1.30 mm flatter than ‘K’.
•Fitting steps are usually 0.30 or 0.40 mm.
•Most semi-scleral lenses are of bicurve construction, with a relatively flat but narrow peripheral curve.
Total diameter
•Lenses are fitted significantly larger than the visible iris diameter, to give deliberate apical touch with further support beyond the limbus where they overlap onto the sclera. This type of three-point touch is shown in Figure 15.2.
•The total diameter, where there is a choice, is selected to be 2.00–3.00 mm larger than the horizontal visible iris diameter. However, many semi-scleral lenses are manufactured in only one size.
•The majority of semi-scleral lenses are fitted with total diameters of 14.20–14.80 mm; the possible range is from 13.50 to 16.00 mm.
•High and low water content lenses are both fitted with very similar diameters.
•Fitting steps are usually 0.50 mm.
Power
Mainly because of flexure effects, the power of a correctly fitting lens often shows approximately 0.25–0.50 D less minus than the spectacle Rx, after allowing for any vertex distance considerations. With more rigid low water content lenses, this difference can be as great as 0.75 D, although this does not necessarily apply with silicone hydrogels (see Chapter 19).
Fitting appearance and lens movement
Fitting characteristics are mainly as described in Chapter 16 and Table 16.1. It is essential that a correctly fitting lens should be sufficiently large to span the limbus and not interfere with the blood vessels in this region.
Figure 15.3 (A–C) shows diagrammatically the limits of acceptable movement and position for semi-scleral lenses with the eye in the primary position and in lateral and upward gaze. Figure 15.4 (A–C) indicates the lack of movement with a tight lens, whereas Figure 15.5 (A–C) shows the excessive mobility of a loose fitting.
Figure 15.2 Semi-scleral lens giving three-point touch
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Soft lens fitting and design 15 Chapter 
A B
C
Figure 15.3 Appearance of a correctly fitting semi-scleral lens: (A) primary position; (B) lateral gaze; (C) upward gaze
A B
C
Figure 15.4 Appearance of a tightly fitting semi-scleral lens: (A) primary position; (B) lateral gaze; (C) upward gaze
Clinical equivalents and altering the fitting
The principle of clinical equivalents also applies so that two lenses of different but related specification behave in the same way on the eye.
Examples: |
8.10:13.50 ≡ 8.40:14.00 |
|
8.70:14.50 ≡ 9.00:15.00 |
Clinical equivalents have approximately the same ratio of sagittal depth to total diameter. They do not have the same sagittal depth.
RULE OF THUMB
A change in radius of 0.30 mm ≡ a change of diameter of 0.50 mm.
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Section THREE Hydrogel and silicone hydrogel fitting
A B
C
Figure 15.5 Appearance of a loose fitting semi-scleral lens: (A) primary position; (B) lateral gaze; (C) upward gaze
To improve a loose fitting
•Select a steeper radius.
•Select a larger total diameter.
•Use a more rigid or lower water content material.
•Use a different lens thickness.
A lens of specification 8.70:14.00 may be progressively tightened with the following steps: 8.70:14.50; 8.40:14.00; 8.40:14.50.
To improve a tight fitting
•Select a flatter radius.
•Select a smaller total diameter.
•Use a less rigid or higher water content material.
•Use a different lens thickness.
A lens of specification 8.70:14.50 may be progressively loosened with the following steps: 8.70:14.00; 9.00:14.50; 9.00:14.00.
15.3.1 Examples of semi-scleral lenses
Durasoft 3 Lite Tint (CIBAVision)
A medium water content semi-scleral lens manufactured by lathing.
Material properties
Chemical nature: Copolymer of HEMA and 2-ethoxyethyl methacrylate.
|
Non-ionic. |
Water content |
55% |
Dk |
15 × 10−11 at 35°C |
Refractive index |
1.412 |
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Soft lens fitting and design 15 Chapter 
Table 15.2 Parameters available for Durasoft
3 Lite Tint lenses
Radius (mm) |
8.30, 8.60, 9.00 |
|
|
Diameter (mm) |
14.50 |
|
|
Power (D) |
±20.00 to −20.00 |
|
|
Lens geometry
•Back surface is of monocurve construction.
•Front surface is lenticulated.
•Centre thickness is 0.05 mm for a lens of BVP −3.00 D.
Parameters available
See Table 15.2.
Fitting method
•The 8.60 radius (designated median) is used in the great majority of cases and is selected first unless the cornea is very steep or flat.
•The total diameter should be 1.5–2.0 mm larger than the HVID.
Typical specification
8.60:14.50 −3.00
Related lenses
•Durasoft 3 Optifit Toric: a back surface toric stabilized with front surface thin zones.
Omniflex (CooperVision)
A high water content, semi-scleral lens for daily wear, manufactured by lathing.
Material properties
Chemical nature: |
A copolymer of methyl methacrylate and vinyl pyrrolidone. |
|
Non-ionic. |
Water content |
70% |
Dk |
32 × 10−11 at 35°C |
Refractive index |
1.39 |
Geometry
Spherical bicurve back surface; spherical lenticulated front surface. Centre thickness 0.12 mm for −3.00 D and 0.23 mm for +3.00 D.
Parameters available
See Table 15.3.
Fitting method
•The total diameter is constant at 14.30 mm.
•The 8.40 mm radius fits approximately 80% of eyes.
•The 8.80 mm fitting is required for flat or small diameter corneas.
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