The intention is that the fitting pattern should be similar over the vast majority of different cases presenting in the consulting room, from early keratoconus to well-advanced disease. Whilst the surface is truly aspheric itís polynomial nature is such that it enables you to combine the comfort of a conventional elliptical lens with the visual advantages of a spherical lens without the attendant disadvantage described above.

Computer modelling and consistent results have validated this approach and demonstrated its superiority in terms of the visual result and the improvement of patient comfort.

The optimally fitting lens in keratoconus is only loosely linked to the keratometry readings. Therefore, careful assessment of lens fit with fluorescein is crucial to success. However, an appropriate starting point in early keratoconus will be made by choosing a B.O.Z.R. close to the flattest keratometry reading. As the disease progresses, keratometry readings become increasingly meaningless, but where readings are attainable, a starting point will be the average of the flattest and steepest reading.

It must be emphasised that there is not a substitute for careful lens observation and several trial lenses will probably be required until the practitioner is entirely happy with the fitting.

The table below is for guidance only:

Desirable Fitting Patterns

Always assess the fitting with the lens centred on the cornea.

Central Fit:

Always allow sufficient time for the lens to equilibrate on the eye. Instil fluorescein and ensure that the patient blinks sufficiently for it to penetrate under the central portion of the lens. A well-fitted Quasar K should show 2-3mm of light apical touch. This should be complemented by mid-peripheral touch, as shown in Fig 1. In-between the two areas of touch there will be an annulus of fluorescein, which should be free of bubbles. If no central touch is seen, as seen in Fig 2, then the B.O.Z.R. should be flattened until the correct appearance is seen. Conversely, if there is excessive central touch, as shown in Fig 3, then the B.O.Z.R. should be steepened until only a light touch is visualised.

 Figure 1 – Mid-peripheral Touch

 Figure 2 – No Central Touch

 Figure 3 – Excessive Central Touch

Peripheral Fit:

Ideally Quasar K should show the same amount of peripheral clearance as a conventional Quasar lens on a normal eye (equivalent to 60 microns of corneal edge clearance over a width of 0.50mm). However, because of the asymmetric, irregular nature of the keratoconic eye, this is rarely achieved. If any part of the edge-band appears grossly narrow (<0.3mm) or if the dark zone of the mid peripheral touch extends towards the edge, then a lens with greater edge lift should be ordered. This will typically occur in the superior part of the lens edge.

If any part of the lens edge shows stand off, where there is air rather than tears under the edge, then a steeper B.O.Z.R. should be used. This will typically occur under the inferior portion of the edge.

Centration is particularly difficult to control in keratoconus. About 75% of cones are displaced down with regard to the pupil centre and therefore a well-fitted lens will usually centre slightly downwards. A high-riding lens is often too flat or has too great an edge clearance. To improve the optical result with a low-riding lens, it is best to increase the diameter if in other respects the fitting is optimal.

Over-Refraction:

The trial lenses are increasingly negatively powered as the B.O.Z.R. increases. This is to help reduce the magnitude of the over-refraction in more advanced cases. There will, in many cases, be a small residual cylinder, but wherever possible try and optimise the vision using spherical lenses only. Ensure the over-refraction takes place using a trial lens of the same B.O.Z.R. as the final lens as small changes to the B.O.Z.R. can dramatically affect the visual acuity and over-refraction. Ensure that the initial lacrimation has subsided. Obviously, the vertex distance should be taken in account when the over-refraction is above 4.00D.

Where there is a marked residual cylinder (>0.75) try to dispense a spherical lens and see if the patient will adapt to the visual shortfall. If not, consider use of spectacles over the lenses for detailed work, or contact our technical services personnel for details of our front surface torics.

• Junctionless back surface
• Improved centration
• Improved comfort
• Comprehensive trial sets
• Unparalleled technical support
• Choice of lens materials

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