Section THREE Hydrogel and silicone hydrogel fitting

Table 15.3  Parameters available for Omniflex

lenses

Typical specification

8.40:14.30  −3.00

Related lenses

•Omniflex toric: a back surface toric with prism free front optic and stabilization zones.

•Omniflex Softints: a range of four colour enhancers in 10% and 20% densities.

GENERAL ADVICE

•Select the flatter of two possible fittings where there is a choice because:

(a)It is easier to see the movement of a lens which is too loose rather than the relative lack of movement of a steep fitting.

(b)A sharper end-point is obtained with over-refraction.

(c)Soft lenses become steeper and therefore tighter after initial settling.

•Steep corneas require lens selection to be relatively much flatter than flat corneas where the optimum result is likely to be much closer to ‘K’. This applies to both corneal and semi-scleral lenses. Thus a radius of 8.10 mm may be necessary for a 13.00 mm lens on a 7.40 mm cornea, whereas an 8.40 mm cornea might well require a much closer radius of 8.70 mm or perhaps 8.50 mm.

•It is better to achieve a more stable fitting by increasing the size. A steeper radius is likely to give worse acuity.

•It is essential to ensure adequate lens movement to allow proper exchange of tears and the removal of debris.

•It is also important to avoid fitting too tightly because:

(a)Corneal oedema is possible with most lenses except silicone hydrogels with their very high Dks.

(b)Limbal or conjunctival compression can occur.

Reference

1.Young G. How to fit soft contact lenses. Journal of the British Contact Lens Association

1992;15(4):179–80.

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Section

Hydrogel and silicone hydrogel fitting THREE

Soft lens fitting 16CHAPTER characteristics

Every make of lens has, to some extent, its own fitting characteristics. Most of the more general points, however, are summarized below, although the rather greater differences between corneal and semi-scleral lenses have been covered in Chapter 15.

16.1 Characteristics of a correct fitting

The general principles for a good fitting are complete corneal coverage, correct lens movement, a satisfactory push-up test, good vision and comfort.

•Good centration.

•Complete corneal coverage.

•Approximately 0.25–0.5 mm of vertical movement on blinking in primary position.

•Lag of up to 1 mm on upwards gaze or lateral eye movements.

•Smooth recovery on vertical displacement (push-up test).

•Comfortable in all directions of gaze.

•Good visual acuity.

•Retinoscopy reflex crisp and sharp before and after blinking.

•Vision remains stable on blinking.

•Refraction gives a precise end-point.

•Refraction correlates with spectacle BVP.

•Keratometer mires stable and undistorted.

•No irritation of limbal vessels.

•No compression of bulbar conjunctiva.

©2010 Elsevier Ltd, Inc, BV

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

Section THREE Hydrogel and silicone hydrogel fitting

16.2 Characteristics of a tight fitting

The obvious feature of a tight lens is insufficient movement. Initial comfort is therefore good and sometimes better than a correct fitting. Centration is also usually good, although a corneal diameter lens may sometimes assume a decentred position. This is easily differentiated from a decentred flat fitting because of the lack of movement. A steep lens vaults the corneal apex, but is momentarily pressed onto the eye by blinking to give a transient improvement in vision.

•Little or no movement on blinking (less than 0.2 mm).

•Little or no movement on upwards or lateral gaze (less than 0.25 mm).

•Good centration.

•Slow recovery with push-up test.

•Good initial comfort.

•Subsequent symptoms of discomfort such as heat or stinging.

•Poor visual acuity.

•Retinoscopy reflex shows irregular distortion.

•Unstable vision with temporary improvement on blinking.

•Refraction difficult with poor end-point.

•More negative power required than anticipated because of flexure or possible liquid lens.

•Keratometer mires give irregular distortion.

•Irritation of limbal or conjunctival vessels.

•Compression ring in bulbar conjunctiva (scleral indentation), often seen

after the lens is removed.

16.3 Characteristics of a loose fitting

The obvious feature of a loose lens is excessive movement. On looking upwards it may catch against the top lid and cause noticeable discomfort. In the primary position, lower lid sensation is experienced if the lens sags and the discomfort is accentuated if the fitting is so flat that the periphery buckles to give edge standoff (Figure 16.1). This is not uncommon on steep corneas with single fitting and disposable lenses with a limited range of parameters. The more rigid silicone hydrogels with a high modulus are particularly prone to edge buckling.

Figure 16.1  Very loose fitting lens see buckling of lens edge

200

Soft lens fitting characteristics 16 Chapter

Table 16.1  Fitting characteristics of soft lenses

201