Section TWO Rigid gas-permeable lens fitting

Figure 9.6  Tear layer profile/multicurve

9.3 Current aspheric lenses

Aspheric lenses have one or both surfaces of a non-spherical construction. Aspherics usually take the form of a parabola, ellipse or hyperbola and are defined by eccentricity.

Definitions

Eccentricity (e): Defines mathematically the departure of an aspheric curve from a circle. Used to describe both a lens form and the curvature of the cornea.

P value: Defines the rate of flattening with eccentricity: p = 1 − e.2

The closest mathematical approximation to the topography of the human cornea is an ellipse. Mean eccentricity = 0.45; p = 0.8.

Circle: Completely symmetrical. Eccentricity = 0; p = 1.

Ellipse: Symmetrical about two axes but has two diameters – one long and one short. Eccentricity = 0 < e <1; p = <1.

Parabola: Symmetrical about one axis. Eccentricity = 1; p = 0. Hyperbola: Eccentricity >1; p = <0.

All aspheric curves can be defined by two peripheral radii – the sagittal radius which is the steeper and the tangential radius which is the flatter. The relation-

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Development of rigid lens design 9 Chapter

Figure 9.7A, B  Representations of aspheric surfaces

ship between the vertex and peripheral radii determines the eccentricity value and consequently the lens shape.

Two subgroups of aspheric surfaces commonly used in the contact lens industry are: (1) conicoids and (2) higher order curves, termed polynomials. The conicoid is a curve derived by taking a section through a cone (Figure 9.7A and B). As the section is made more oblique, the curve becomes increasingly elliptical, then parabolic and finally changes to a hyperbola. The hyperbola produces the greatest peripheral flattening and so is used to produce the peripheral zone of bi-aspheric designs. A polynomial is a progressive eccentric curve increasing from the apex outwards. It is described as a differentially flattening aspheric curve, departing only slightly from a sphere centrally but with a rapid increase of axial edge lift in the periphery.

9.4 Reverse geometry lenses

Reverse geometry lenses differ from conventional designs in that the intermediate curve is steeper than the base curve. Such lenses are used with corneal flattening procedures such as orthokeratology (see Chapter 14) and for some therapeutic applications (see Section 32.7).

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Section TWO Rigid gas-permeable lens fitting

References

1.Dickenson F, Hall KGC. An Introduction to the Prescribing and Fitting of Contact Lenses.

London: Hammond and Hammond; 1946.

2.Bennett AG. Aspherical contact lens surfaces. Ophthalmic Optician 1968;8:1037–40, 1297–300, 1311, 9:222–30.

3.Bier N. The contour lens. Journal of the American Optometric Association

1957;28:394–6.

4.Bayshore CA. Report on 276 patients fitted with microcorneal lenses apical clearance and central ventilation. American Journal of Optometry 1962;39:552–3.

5.Thomas PF. Conoid Contact Lenses. Australia: Corneal Lens Corporation; 1967.

6.Stek AW. The Percon contact lens – design and fitting technique. Contact Lens 1969;2:12–4.

7.Ruben M. Use of conoidal curves in corneal contact lenses. British Journal of Ophthalmology 1966;50:642–5.

8.Nissel G. Aspheric contact lenses. Ophthalmic Optician 1967;7:1007–10.

9.Stone J. Corneal lenses with constant axial edge lift. Ophthalmic Optician 1975;15:818–24.

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10.1 Introduction

Opinions differ as to what constitutes a satisfactory rigid lens fitting, but the alignment method is the most commonly accepted concept. Alignment means:

•The majority of the back surface of the lens is made to align with the cornea.

•The weight is distributed over as large an area as possible.

•The alignment must be consistent with tears interchange behind the lens, largely promoted by lens movement on blinking.

•Alignment must be consistent with satisfactory vision.

The following information and measurements are required:

•Accurate refraction. The ocular refraction should be calculated as this enables a subsequent check on the liquid lens power (see Section 5.4).

•Accurate keratometry readings. These give the corneal astigmatism and govern the choice of BOZR (see Section 5.4).

•Horizontal visible iris diameter (HVID). This is approximately the corneal diameter and governs the choice of TD.

•Vertical palpebral aperture. This also governs the TD and should be considered in conjunction with the size of the eye (small, medium or large) to give optimum centration and movement.

•Pupil size in average and low illumination. This influences the BOZD.

©2010 Elsevier Ltd, Inc, BV

DOI: 10.1016/B978-0-7506-7590-1.00011-X