Section TWO Rigid gas-permeable lens fitting

A B C D E

Figure 8.5  Edge shapes of lenses: (A) posterior; (B) central; (C) anterior;

(D) blunt; (E) sharp

PRACTICAL ADVICE

•Do not order centre thickness less than 0.14 mm with most modern materials because of lens flexure, particularly with toric corneas and tight lids.

•Edge thickness should be a minimum of 0.12 mm. A ‘knife edge’ causes discomfort and is fragile, especially with plus lenses.

•Minus lenses usually give a natural lid attachment.

•A negative carrier helps give lid attachment with a low-riding or plus lens.

•A positive carrier helps reduce a high-riding tendency.

•Varying degrees of taper and roundness are used, depending on fitting philosophy (see Section 8.5) and lid sensitivity. Edges are described as (a) posterior, (b) central, (c) anterior, (d) blunt, and (e) sharp3 (Figure 8.5).

•Edge thickness depends on BVP (see Section 8.2.5).

8.3Concept of edge lift

The concept of edge lift is related to the lens design off the eye. It embodies the series of curves that lead into the edge shape. Edge lift can be specified in an axial or a radial direction.

Axial edge lift is defined as the distance between a point on the back surface of a lens at a specified diameter and the continuation of the back central optic zone, measured parallel to the lens axis4 (Figure 8.6). The flatter the back optic zone radius (BOZR), the greater the degree of peripheral curve flattening that is required to maintain a particular edge lift.

Current lens designs are usually defined in respect of axial edge lift (AEL). Lenses are sometimes designed by deciding on the AEL required and calculating the peripheral curves needed. The edge lift of each individual curve contributes to the total figure (Table 8.4).

Historically, the concept of edge lift has used a variety of terms:

•Z value, or axial edge lift.

•Constant axial edge lift (CAEL) (see Section 9.2).

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Principles of rigid lens design 8 Chapter

•Z factor, or radial edge lift.

•Flattening factor.

TD

AEL

REL

Figure 8.6  Axial edge lift (AEL) and radial edge lift (REL) in a rigid lens design (TD, total diameter)

Table 8.4  Average CAEL values (mm)

Figures courtesy of No. 7 CL Laboratory.

The usual value of axial edge lift varies between 0.09 mm and 0.15 mm. If the same increase is given to the peripheral curves of both steep and flat lenses, the steep lens has, relatively, a greater edge lift.5

CAEL lenses4 are multicurves which, for a given total diameter (TD), are designed to give the same AEL throughout the range of radii. The clinical appearance and performance are therefore consistent.

The Z factor6 or radial edge lift (REL) is defined as the distance between a point on the back surface of the lens at a specified diameter and the continuation  of the back central optic zone, measured along the radius of the latter (see  Figure 8.6).

The ratio of axial to radial edge lift is approximately 5 : 4.

Example:

7.40:7.00/8.10:7.80/9.30:8.60/10.50:9.00

Taken from 0.15 mm CAEL trial set with a TD of 9.00 mm.7 AEL at 7.80 = 0.025; AEL at 8.60 = 0.095; AEL at 9.00 = 0.15 The designated value for the final peripheral curve

REL at 9.00 = 0.12 Ratio REL/AEL = 0.80

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Section TWO Rigid gas-permeable lens fitting

The flattening factor (ff) defines the extent to which the peripheral curve flattens in relation to the central radius in an offset lens (see Section 9.2).

There are two approaches to calculating the contribution of each peripheral curve of a given lens design:

Band width method

The band width method considers the edge lift at the edge of the intermediate curve.8

Step-by-step method

The step-by-step method calculates the AEL of the mid-curve as the AEL produced as if the mid curve were extended out to the total diameter of the lens.

Compared with the band width approach, the step-by-step method produces a larger AEL for the mid-peripheral curve and a smaller AEL for the third curve.

For a tricurve, the mid-curve produces two-thirds of the AEL, with the third curve contributing one-third. For a tetracurve lens, the first peripheral curve provides half of the AEL, the second one-third, and the fourth one-sixth. The contribution of each curve is easily calculated with the aid of a computer programme.

8.3.1 Concept of edge clearance

The term edge clearance relates to the lens on the eye and is estimated by the fluorescein pattern. The lens periphery must be fitted flatter than the cornea to:

•Provide tears exchange beneath the lens for the maintenance of corneal metabolism.

•Give a tears meniscus so that capillary attraction and lens centration forces can function (see Section 8.2).

•Assist lens removal by the lids.

•Avoid pressure and corneal insult at the lens edge.

•Avoid lens adhesion.

Too little edge clearance gives:

•Inadequate tears exchange.

•Poor lens movement.

•Pressure at the lens edge and arcuate staining.

•Difficulty with lens removal.

•Lens adhesion.

Too much edge clearance gives:

•Excessive lens movement.

•Bubbles under the lens periphery which can cause frothing or dimpling.

•Poor centration.

•Lens displacement off the cornea.

•3 and 9 o’clock staining because of tear film disruption.

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Principles of rigid lens design 8 Chapter

PRACTICAL ADVICE

•AEL relates to the lens design off the eye.

•Edge clearance relates to the lens on the eye.

8.4Tear layer thickness

•Tear layer thickness (TLT) is the clearance between the back surface of the lens and the cornea, usually in respect of the central area (typical example, Figure 8.7).

•The fitting technique and lens design govern the values for apical (and edge) clearance.

•TLT is expressed in microns ( m) (1  m = 0.001 mm), whereas edge

lift is given in millimetres and relates only to the physical dimensions of   the lens.

•Fluorescein with a TLT of <20  m cannot be seen with a Burton lamp.

Figure 8.7  Tear layer profile

Typical values9

Tear layer thickness = 5–10  m.

Edge clearance (EC) = 75–80  m.

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