Section TWO Rigid gas-permeable lens fitting

•Percon lens (1969)6 – fitted to give central alignment with peripheral clearance, using a spherical BOZR and a conical peripheral zone with a constant axial lift of 0.10 mm. The cone angle is almost exactly tangential to give virtually no transition. Example: 7.80:6.80/9.00. Cone angle 130°.

•Offset lens (1966)7 – with the centre of curvature of the back peripheral curve offset to the opposite side of the central axis, virtually eliminating any transition. It has been termed a continuous bicurve lens2 or contralateral offset. A homolateral offset is also possible, where the centre of curvature of the peripheral curve is displaced to the same side of the

central axis. The degree of flattening is referred to as the axial edge lift, Z value or flattening factor. Example: 7.00:6.00 AEL 0.1 at 9.00 −5.00.

9.1.2 Early aspheric designs

Nissel aspheric

The Nissel design (1967)8 was based on the US Volk lens. It had a central aspheric portion with a 7.80 mm diameter, two spherical zones at diameters of 8.40 mm and 8.80 mm, and an 0.50 mm wide bevel of radius 10.00–12.00 mm to assist tear flow. It was fitted 0.10 mm steeper than ‘K’ to give central alignment with the appearance of a multicurve.

Conflex (Wohlk, 1982)

Moulded from Anduran material (CAB + ethyl vinyl acetate), this lens consists of a spherical optic with three aspheric peripheral zones. The edge shape is termed ‘ski-tip’. The lens is fitted 2.00 mm less than the horizontal visible iris diameter (HVID) to give central alignment, or just flatter than ‘K’ with slight superior decentration.

Aspheric periphery ≡ 0.80 mm, 1.60 mm, 3.50 mm flatter than BOZR

Conflex air (Wohlk, 1989)

A fully aspheric design in a fluoropolymer material, fitted with central alignment.

Aspheric periphery e = 0.4

9.2 Current bicurve, tricurve and multicurve designs

Corneal lenses are now designed with one or more peripheral zones which are deliberately intended to lift away from the cornea. Most modern spherical lenses are based on these designs.

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Bicurve (C2)

Consists of a central radius and one flatter peripheral curve (Figure 9.1). There is a sharp transition between the two curves.

Example: 7.80:7.00/8.70:9.00 (Figure 9.2).

ØT

Ø0

r0

Sharp transition

r1

Figure 9.1  Bicurve corneal lens (ØT, total diameter; Ø0, back optic zone diameter; r0, back optic zone radius; r1, first back peripheral radius)

Figure 9.2  Tear layer profile/bicurve (C2)

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Section TWO Rigid gas-permeable lens fitting

Tricurve (C3)

Consists of a central radius and two flatter peripheral curves (Figure 9.3). It is the basic design of most modern rigid lenses, where the final curve is much flatter than first peripheral radius.

Example: 7.80:7.80/ 8.50:8.70/10.50:9.50 (Figure 9.4).

ØT

Ø1 Ø0

r0

r1

r2

Figure 9.3  Tricurve corneal lens (ØT, total diameter; Ø1, first back peripheral zone diameter; Ø0, back optic zone diameter; r0, back optic zone radius; r1, first back peripheral radius; r2, second back peripheral radius) (from Phillips and Stone, Contact Lenses, 3rd edn, ButterworthHeinemann, Oxford, by permission)

Multicurve

Consists of a central radius and three or more peripheral curves (Figure 9.5). It follows the flattening of cornea better than bicurves and tricurves and, when the transitions are well blended, behaves like a continuous curve lens.

Example: 7.80:7.50/8.40:8.20/9.00:8.90/11.50:9.50 (Figure 9.6).

Constant axial edge lift

CAEL lenses9 were developed as a further refinement of multicurve lens design to give a constant linear clearance between the edge of the lens and the cornea over the whole range of radii for a given diameter. The axial edge lift of the peripheral curves is calculated to remain constant for all BOZRs, unlike conventional lenses where the calculated AEL is greater with steeper lenses than with flatter lenses.

N.B. AEL relates to the lens design off the eye.

Average CAEL for a TD of 8.60 mm; 0.105 mm. Average CAEL for a TD of 9.20 mm; 0.11 mm. Average CAEL for a TD of 9.60 mm; 0.14 mm.

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Development of rigid lens design 9 Chapter

Figure 9.4  Tear layer profile/tricurve (C3)

ØT

r0

r1 r2

r3

Figure 9.5  Multicurve corneal lens (r3, third back peripheral radius; other symbols as in Figure 9.1)

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