Basic principles of rigid lens design

8.1Basic principles of rigid lens design

•Lenses may be spherical, aspheric or a combination of both.

•Most corneal lenses have a central zone which is fitted just apically clear or in alignment with the central cornea, combined with a much flatter peripheral zone which is designed to lift away from the cornea.

•Central alignment gives optimum acuity (see Section 10.2).

•Peripheral clearance is necessary for adequate tears exchange.

•The transition between the central portion and the periphery is sharp for a spherical bicurve design, becoming smoother as additional curves are added.

•Aspheric lenses have a much smoother transition and, for some designs, can be compared with very well-blended spherical multicurves (see   Chapter 12).

8.2Forces controlling design

Corneal lenses of all materials are affected by a variety of forces when placed on the eye. These factors are both ocular (see Chapter 5) and physical in nature.1

8.2.1 Centre of gravity

•The centre of gravity of a lens lies somewhere behind the back surface (Figure 8.1).

©2010 Elsevier Ltd, Inc, BV

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

Section TWO Rigid gas-permeable lens fitting

C C

A B

Figure 8.1  Centre of gravity (C) in (A) minus and (B) plus lenses

•It is affected by radius, diameter, thickness and power.

•Steep lenses have the centre of gravity further back than flat lenses and therefore give better centration (Figure 8.2A).

•Flat lenses have the centre of gravity further forward and give worse centration (Figure 8.2B).

•Large diameter lenses have the centre of gravity further back than small lenses and give better centration.

•Small lenses have the centre of gravity further forward and give worse centration.

C C

A B

Figure 8.2  Centre of gravity (C) in (A) steep and (B) flat lenses

8.2.2 Frictional forces

The viscosity of the tear film maintains the lens in a stationary position by means of frictional forces. Thinning of the tear film or an increase in its aqueous content (e.g. during adaptation) reduces the centration ability of these forces.

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8.2.3 Capillary attraction

•The closer the lens matches the shape of the cornea, the greater the capillary attraction and stability.

•Since a rigid lens cannot ever exactly match the shape of the cornea, a balance has to be found between sufficient capillary attraction for lens stability and sufficient movement for tears exchange.

•An excessively flat fitting gives less capillary attraction and greater movement.

•A steep fitting can create a negative pressure or suction effect.

•The tears meniscus at the edge of the lens also provides forces for centration. The greater the meniscus, the better the adhesion.

8.2.4 Specific gravity

•The clinical significance is demonstrated when two lenses of the same design (and volume) but different specific gravity behave differently on the eye. The lens with the lower specific gravity has less weight (Table 8.1; see also Tables 7.3 and 7.4).

•A lens that drops because gravitational forces are greater than fluid forces may achieve better centration by using a material of lower specific gravity and vice versa.

•With prism ballast, a high specific gravity material is advantageous as it gives a greater difference in weight between the apex and base of the lens.

Table 8.1  Typical specific gravity values

8.2.5 Thickness and lenticulation

Thickness depends on back vertex power (BVP), design and material. Centre thickness (tc) and edge thickness (te) are both important (Tables 8.2 and 8.3).

•BVPs greater than −6.00 D or +4.00 D should be lenticulated to reduce excess thickness and mass.

•Lenticulation reduces thickness away from the centre for plus and towards the edge for minus lenses by making the front optic zone diameter (FOZD) smaller (Figure 8.3).

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Section TWO Rigid gas-permeable lens fitting

•The FOZD should be approximately 0.50 mm larger than the back optic zone diameter (BOZD).

•The carrier portion of a lenticulated lens can be plano, negative or positive in shape (Figure 8.4). The choice depends upon the intended effect (e.g. lid attachment techniques) (see Section 8.5).

Table 8.2  Typical thickness values assuming a constant TD = 9.60 and

BOZD = 7.80 mm

Table 8.3  Suggested minimum thicknesses for different materials (BVP

−3.00 D)

Optical zone

Negative carrier

Figure 8.3  Lenticulation of plus lens

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A B C

Figure 8.4  Carrier portion shapes of a lenticulated lens: (A) plano; (B) positive; (C) negative

8.2.6 Refractive index of materials

The following are typical examples of refractive index (see also Tables 7.3 and 7.4):

•The higher the refractive index, the thinner the lens can be made.

•Modern rigid lenses have a lower refractive index than PMMA and are therefore thicker.

•High refractive index plastics are used for bifocal segments. They can incorporate fluorescent dye to assist fitting.

•The refractive index is important in toric lens fitting (see Chapter 22).

PRACTICAL ADVICE

Using a material with a high refractive index (e.g. 1.513 compared with 1.455) means that both high plus and minus lenses can be produced with reduced weight to improve their fitting characteristics.2

8.2.7 Edge shape

•Extremely important for comfort.

•Must be smooth and well finished.

•Should blend into the final peripheral curve.

•Can help lens removal.

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