Rigid lens selection and fitting 10 Chapter

GENERAL ADVICE

•As right and left eyes nearly always require the same TD, use different diameter diagnostic lenses in each eye to observe two fittings at the same time.

•Use a similar technique to evaluate two BOZRs at the same time, as they do not often differ by more than 0.05 mm for similar ‘K’ readings.

•Check that the BVP from over-refraction correlates with the spectacle Rx and astigmatism after allowing for back vertex distance. Repeat with a different diagnostic lens in case of doubt.

•Do not over-refract hypermetropes with minus diagnostic lenses and vice versa. The results are nearly always unreliable.

•If the patient was previously a PMMA wearer, order the lenses 0.02 mm thicker than normal to reduce the risk of breakage with more modern materials.

PRACTICAL ADVICE

Rigid lenses give best vision if:

•The BOZD is relatively large (>8.00 mm).

•The TD is relatively large (>9.80 mm).

•The periphery is not too wide (<1.00 mm).

•The periphery is not too flat (AEL <0.14 mm).

•Centre thickness is 0.14 mm or greater.

•Fitting is on or near flattest ‘K’.

10.7 Lens design by corneal topographers

Videokeratoscopes for corneal topography measurement often include contact lens design programmes to determine:

•The temporal keratometry reading for the initial BOZR.

•Whether the initial BOZR will allow unobstructed vertical lens movement and redesign curves if necessary.

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Section TWO Rigid gas-permeable lens fitting

•The mid-peripheral corneal curvature for superior, temporal and inferior positions.

•The degree and symmetry of any mid-peripheral corneal astigmatism and whether a toric lens is necessary.

Comprehensive computer programmes are available to custom design most types of rigid gas-permeable lens from C2 to C5. Some programmes automatically suggest the optimum lens while others allow the practitioner to insert the BOZR, diameters and AEL in order to compute the intermediate curves.

The value of videokeratoscopy is enhanced by providing simulated fluorescein patterns for the calculated lens design and demonstrating the bearing areas on the cornea. It is also possible to show the effect on the fitting when the various lens parameters are changed. An alternative approach where instruments are able to provide details of AEL and edge clearance (see Section 9.2) is to design lenses using the principle of TLT (see Section 5.4). Topographical fitting nomograms based on corneal eccentricity can select various BOZRs according to a pre-selected lens cornea fitting relationship without the need to apply a variety of diagnostic lenses.

Recommended reading

Morris J. RGP lenses Part 2 – Fitting procedures. Optician 2004;228(5976):28–35.

Szczotka LB. Computerised corneal topography applications in RGP contact lens fitting. Optometry Today 2002;February:32–4.

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11.1 Use of fluorescein

11.1.1 Instillation of fluorescein

• A fluorescein strip is moistened with saline and excess removed by shaking.

• The patient is asked to look down while the upper lid is lifted and the wet

strip gently touched onto the conjunctiva above the superior limbus, taking care not to instill excess.

•Fluorescein flows beneath the lens with the tears after two or three blinks.

•If a minim of 2% fluorescein is used, one drop only is applied with a glass rod. Some of it is allowed to wash away before inspecting the fit.

PRACTICAL ADVICE

•Never use tap water to wet fluorescein strips. Pseudomonas aeruginosa has a strong affinity for fluorescein and may be present together with other microorganisms such as Acanthamoeba.

•Do not apply the strips dry, as they can be very uncomfortable.

•It is usually more comfortable for the patient to look down to reduce lid sensation, especially with an unadapted patient.

©2010 Elsevier Ltd, Inc, BV

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

Section two Rigid gas-permeable lens fitting

•Sometimes, however, with very tight lids and squeamish patients, it is better applied in the lower outer canthus, gently pulling down the bottom lid and resting the paper flat against the lower palpebral conjunctiva.

•Insertion of the contact lens with a viscous wetting solution may encourage the fluorescein to spread across the front surface of the lens and mask the posterior fluorescein pattern. Either use saline or give the lens longer to settle.

•Do not paint the wet strip over the conjunctiva, since too much fluorescein masks the true pattern. The excess can also stain skin and clothes.

•Never reuse strips on another patient because of the risk of crossinfection. Discard after use to avoid error. Use a different strip for each eye if infection is suspected.

•If a strip is reused for the same patient, fold it as shown in Figure 11.1 to avoid contaminating the tip.

Figure 11.1  Folded fluorescein strip

11.1.2 Ultraviolet inhibitors

Some materials contain ultraviolet (UV) inhibitors, so that the fluorescein pattern with the Burton lamp shows an apparently black lens with an almost imperceptible green annulus at the periphery.

Additional fluorescein does not change the appearance and it is necessary to use the slit lamp with a blue filter to give a meaningful picture.

11.2 Examination techniques

11.2.1 Burton lamp

Blue light

The essential principles in observing fluorescein patterns are:

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