Low_resolution_Thesis_CDD_221009_public - Visual Optics and ...

CHAPTER 1
1.9.5. Multifocal contact lenses
Presbyopic contact lens correction is still a challenge for both patients and
practitioners. Despite the number of potential patients as the population gets older, the
use of multifocal contact lenses accounts for only a small proportion of contact lens
wearers (Section 1.9.6). As a result, a number of different fitting strategies and designs
continue to be developed for the presbyopic subject. Most of the more successful
current types are concentric and based on the principle of simultaneous vision. Soft
lenses are frequently the only choice, as the larger total diameter enables the
practitioner to achieve better control of centration and lens movement. Figure 1.22
shows some examples of simultanoeus vision multifocal designs. In this thesis only
lenses with progressive transitions are considered, to avoid problems in the
topographic and aberration measurements.
Fig. 1.22. Some examples of multifocal designs by simultaneous vision. The far and
near zones (different tones of blue) are arranged at different radial distances.
As already mentioned in Section 1.5.2.5, the adaptation to the new visual experience
these lenses provide is often difficult. Moreover, no adjustments are possible with
hydrophilic lenses and the fitting procedure in quite inefficient, as it is based on trial
and error with different lenses.
Depth of focus and tolerance to blur, pupil diameter and optical aberrations of the
eye are additional issues in understanding the fitting of multifocal contact lenses, to be
considered in combination with other fitting effects already mentioned affecting
monofocal contact lenses.
1.9.6. Prescribing trends
Contact lenses are a popular way to correct refractive errors of eyes. Contact lens
wearers are a significant proportion of the population in western countries, and their
use is increasing: about 3 in 10 contact lens fits around the world are prescribed as
“new fits” (Morgan et al., 2009). Soft lenses account for 90 percent of fits worldwide.
The use of rigid lenses is declining (14 per cent of all fits in 2002 to 9 per cent in
2008), but the proportion of ortho-K lenses has risen from 0 to 11 per cent of rigid
fittings. Multifocal and monovision lenses represent 5 per cent of the rigid fittings,
and 7 per cent of the soft fittings (1% and 15 % respectively, in Spain).
1.10. FITTING CONTACT LENSES AND OPTICAL ABERRATIONS
Wavefront aberrometry offers exciting new opportunities to advance our
understanding of the ways in which contact lenses interact with both the normal and
abnormal eye, and will likely become a common tool in the contactology practice.
This in turn should allow significant improvements in the optical design of lenses and
in our ability to select lens designs which produce the optimal overall visual
performance for the individual eye (Charman, 2005).
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