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98 Ophthalmic Lenses

ASPHERIC LENSES FOR APHAKIC PATIENTS
Aphakic spectacle correction requires powers of + 8.00D to + 15.00D. Such
a high power correction produces a number of difficulties like:
• Magnification
• Decreased field of view
• Aberration and swimming of object in the field of view
• “Popeye” appearance of patients
• Sensitivity to exact position of the lenses.
• Lens weight and thickness.

Various design approaches have been applied to cataract lenses. There are
two main approaches the “foveal philosophy” and the “peripheral
philosophy”. Both use the aspheric curve designing. Foveal philosophy
parallels the standard lens design philosophy, trying to give the patient
the largest possible dynamic field of view, which is not possible with the
spherical curves. Therefore, aspheric curves are used. Peripheral philosophy
assumes that aphakics are more of head turner than eye turner. Therefore
it uses the front surface curves of diminishing power away from the centre
to reduce the lens thickness and weight of the lens.
Most of the lens manufacturer offer CR aspheric lens with convex
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prolate ellipsoidal surfaces. These series are available in both full aperture
and lenticular form, usually with a forty millimetre diameter. They can
also be obtained in bifocal forms with round segment and ‘D’ shapes
segment. However, the segment surface itself is not aspherical and since it
is cast on ellipsoidal distance portion, the segment tends to be oval shape
rather than circular. It is very important while dispensing these high power
aspheric lenses that they are carefully centred both vertically and
horizontally, as incorrect centration may well obviate the advantage of the
ellipsoidal surface.

Fig. 9.8: Postcataract aspheric lenses