Section TWO Rigid gas-permeable lens fitting

Figure 12.1  Aspheric lens showing close fit to corneal topography

12.2 Aspheric designs

Aspheric lens designs show significant advantages in achieving true alignment but the peripheral flattening of the lens may reduce the sag compared to the equivalent spherical lens. It is necessary, therefore, to select a lens steeper than flattest ‘K’. However, in most designs, the lens geometry has been fine-tuned to permit the selection of lens BOZR equivalent to flattest ‘K’, while still providing the desired fitting relationship.

12.2.1 Fully aspheric lenses

A completely aspheric back surface can cause problems if the eccentricity chosen fits the corneal topography too closely, resulting in the lens edge pressing into the peripheral cornea. A narrow, spherical bevel or separate aspheric edge is therefore usually incorporated into the design.

If a bi-aspheric lens with fixed high eccentricity decentres, it can induce residual astigmatism because of the differential power effect of the sagittal and tangential radii towards the lens periphery. Conversely, with designs which have differential flattening, there can be a significant reduction in both positive and astigmatic aberration in the optic portion of the lens.

Quasar (No. 7 Laboratory, 1994)

The lens is designed to be a progressive eccentric aspheric, based on a corneal model with e = 0.458.The eccentricity value at the lens apex is zero and increases

152

Aspheric lenses 12 Chapter

Figure 12.2  Tear layer profile – Quasar

with the semi-diameter from the centre outwards. The back surface consists of a modified conic profile giving a central, differentially flattening aspheric geometry. An aspheric ‘ski-bevel’ peripheral band and a specially lathed edge curve are added to generate edge clearance and edge form respectively. The edge clearance of 80 µm is based on the corneal model with e = 0.458 (Figure 12.2). The central lens geometry aims for a tear layer thickness (TLT) of 6.5 µm with a fluorescein pattern showing no obvious bearing area. There is also an even band  of peripheral clearance about 0.50 mm wide, increasing in depth towards the lens edge.2

Menicon Z-Alpha (Menicon, 2002)

The design has aspheric geometry and aberration-controlled optics which are achieved by an aspheric front surface with a back aspheric design for better centration and dynamics on the eye.

Centre thickness = 0.13 mm at −3.00 D

153

Section TWO Rigid gas-permeable lens fitting

12.2.2 Mainly aspheric/part sphere

These designs consist of a mainly aspheric back surface with a spherical peripheral curve between 0.2 mm and 0.3 mm wide giving 20–60  m of edge clear-

ance. The spherical curve is needed to prevent the elliptical edge from pressing into the cornea. It also assists tears exchange and lens removal.

The common designs are the Persecon E (CIBAVision), Aquila (CIBAVision) and Asphericon (CIBAVision).

Persecon E (CIBA, 1981)

The aspheric version of an earlier spherical design, both in CAB. The back surface gradually flattens to the periphery with e = 0.40 and p = 0.84. There is a spherical edge curve to assist tears exchange between 10 mm and 12 mm in

radius and between 0.20 mm and 0.30 mm wide, to give an edge clearance of 20–60  m. It is fitted to give minimum edge clearance either with central align-

ment or slightly flat to ensure adequate movement and tears exchange.

8.800.016–0.030

9.300.020–0.041

9.800.025–0.053

10.300.033–0.069

Aquila (CIBAVision)

The Aquila and Persecon 92E designs have an aspheric back surface design which is bi-elliptic with an integrated tangential bevel. The eccentricity has an average value of e = 0.4. TDs are 9.3 mm, 9.8 mm and 10.3 mm with a minus carrier available for powers of +2.00 D or greater with the two larger diameters.

The optic zone varies with the TD, ranging from 7.50 mm to 8.00 mm with BVPs over ±8.00 D. The lens geometry is calculated using computer assisted

design (CAD).

Asphericon (CIBAVision, 1998)

The design consists of a progressive aspheric back surface with a spherical ski-tip periphery 0.40 mm wide. The lens has a standard eccentricity of e = 0.55 (p value = 0.7) which gives a constant edge clearance equivalent to an axial edge lift of 0.12 mm. There are also flatter or high (e = 0.70, p value = 0.5) and steeper or low (e = 0.40, p value = 0.84) eccentricities available. The front surface is spherical and lenticulated where necessary (Figure 12.3).

The optimum fluorescein pattern shows central alignment or slight apical clearance and gives the appearance of a well fitted multicurve. The ski-tip edge

154

Aspheric lenses 12 Chapter

Figure 12.3  Tear layer profile – Asphericon

should be 0.40 mm to 0.70 mm wide with an edge clearance between 0.10 mm and 0.12 mm. The fitting can be adjusted by selecting either the larger or smaller eccentricity. The 9.30 mm diameter is selected first, except for large corneas or tight lids which would cause lens decentration.

Centre thickness = 0.12 mm at −3.00 D

12.2.3 Mainly spherical/part asphere

These consist of a central spherical portion with an aspheric peripheral zone area and are termed polynomial aspheric designs. The aspheric zone is, in turn, surrounded by a small spherical edge bevel.

Quantum (Bausch & Lomb)

The Quantum lens has a 3.50–4.00 mm central spherical portion with an aspheric periphery. The edge has a radius of 11.25 mm, 0.3 mm wide.

Astrocon (CIBAVision)

The Astrocon HDS has a spherical back optic zone graduating into an aspheric periphery of e = 0.60. Two further eccentricities 0.45 and 0.75 are available.

155