Fluorescein patterns and fitting 11 Chapter

•The method gives a dynamic assessment of lens fitting.

•Dark blue represents corneal touch.

•Green represents corneal clearance.

•Compared with slit lamp observation, the patient is more relaxed with the Burton lamp, so that both head and eyelids maintain a normal position.

•The Burton lamp uses a UV filter.

PRACTICAL ADVICE

•Exercise care with nervous patients. They sometimes feel faint and the UV lamp is often the trigger mechanism. If in doubt, delay fluorescein examination until they are more at ease.

•Despite this, they nearly always make very good contact lens wearers.

White light

White light with low magnification allows initial investigation of lens position and movement prior to fluorescein assessment, but care is required with a photophobic patient.

11.2.2 Slit lamp

•The cobalt blue filter of the slit lamp permits evaluation of the fluorescein pattern with relatively low magnification (6 to 10).

•A wide beam of 3–5 mm is used for general assessment.

•A narrow slit beam with higher magnification gives a qualitative indication of tear layer thickness and corneal clearance.

•The head and lids are not in a relaxed position on the instrument, so that the assessment of lens centration is not completely reliable.

•The fluorescein pattern appears normal even if the lens material contains a UV inhibitor.

•Contrast is enhanced using a yellow filter, Wratten 6 or 12.

11.3Fitting

•The central and peripheral fit are independent variables and should always be assessed separately.1

•The fitting should be evaluated with the lens both centred on the apex of the cornea and in the habitual position which may be decentred.

•Lens movement during and after a blink should be noted.

•Lens position after a blink is important.

•Lens movement on eye excursions is also significant.

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Section two Rigid gas-permeable lens fitting

11.4 Correct fitting

11.4.1 Assessment with white light

Excursion lag

With the lids in a normal position, the lens periphery should not extend beyond the limbal area even with wide excursions of the eye.

Static lag

If the lids are held apart and the lens pushed upwards, it should drop slowly of its own accord.

11.4.2 Assessment with fluorescein

Alignment fitting (e.g. with modern multicurve designs)

Appearance

The ideal fluorescein pattern should show three fitting areas (Figure 11.2):

•Alignment or the merest hint of apical clearance over the central 7.00 mm.

•Mid-peripheral alignment over about 1.50 mm.

•Edge clearance about 0.4 mm wide.

Figure 11.2  Alignment fitting

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Fluorescein patterns and fitting 11 Chapter

Apically clear fitting (e.g. with high Rx)

Appearance

•Fluorescein pooling over the central 5.00 mm.

•Mid-peripheral alignment.

•Edge clearance about 0.70 mm wide.

Flatter than ‘K’ fitting (e.g. most aspherics)

Appearance

•Alignment or light touch over the central 5.00 mm.

•Mid-peripheral alignment.

•Narrow edge clearance just under 0.2 mm wide.

11.5 Flat fitting

Appearance (Figure 11.3)

• The fitting pattern gives a dense central area of dark blue touch surrounded by fluorescein to the edge of the lens.

•The area of touch is small with an indistinct as opposed to a sharply demarcated border.

•Fluorescein encroaches beneath the periphery of the central portion where

alignment would be expected with a correct fit.

Figure 11.3  Tear layer profile/flat fit

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Section two Rigid gas-permeable lens fitting

•The lens is unstable and decentres.

•If the lens drops, it may show inferior arcuate fluorescein pooling. This is not an indication of steepness, since the lens is overhanging the lower peripheral cornea and the shape of the pooling indicates flatness. If the lens is repositioned, central touch is observed.

•The entire periphery of the lens shows clearance as a wide area of fluorescein.

•Blinking gives excessive and rapid lens movement which may well be uncomfortable.

•Arcuate movement occurs when dropping between blinks or with static lag.

PRACTICAL ADVICE

•Use a steeper BOZR for greatest effect on the fluorescein pattern.

•All other remedies have a lesser effect and the choice of action depends on the degree of flatness.

•Carefully evaluate the TD. If the lens is already large enough, further increase might not be feasible.

•Increasing the BOZD has a greater effect on the fluorescein pattern than making the lens larger.

•Use tighter or narrower peripheries to help the lens centre better.

To correct a flat fit

•Use a steeper BOZR to improve centration.

•Increase the TD to stabilize the lens.

•Increase the BOZD to give a larger sag and steepen the fit.

•Use tighter peripheral curves to reduce dynamic forces and the effect of the lids.

•Use a thinner lens to reduce mobility.

11.6 Steep fitting

Appearance (Figure 11.4)

•The fluorescein pattern gives central pooling.

•An air bubble is sometimes present with excessive central clearance.

•Heavy bearing is seen at the transition as an area of dark blue touch beyond the central pooling.

•The smaller the area of central pooling, the greater the degree of   steepness.

•The periphery gives only a thin annulus of fluorescein around the lens edge.

•There is little lens movement on blinking.

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Fluorescein patterns and fitting 11 Chapter

Figure 11.4  Tear layer profile/steep fit

To correct a steep fit

•Use a flatter BOZR.

•Decrease the TD to increase lens mobility.

•Decrease the BOZD to give a smaller sag.

•Use flatter peripheral curves to increase the dynamic forces on the lens.

•Use a thicker lens.

•Fenestrate (see Section 25.3).

PRACTICAL ADVICE

•Use a flatter BOZR to give the greatest effect on the fluorescein pattern.

•Carefully evaluate corneal and pupil sizes before reducing the TD and BOZD.

•Central fenestration is generally a last resort, but encourages tear flow beneath the lens apex and increases mobility.

Clinical equivalents

Example 1 (spherical lens BOZR:BOZD): 7.70:7.00 ≡ 7.75:7.50 ≡ 7.80:8.00

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