New research has revealed insights about how the optical designs of multifocal contact lenses can improve presbyope comfort and visual performance across all distances and under different light conditions.
The study, conducted by researchers at the Brien Holden Vision Institute on the most commonly prescribed multifocal contact lenses, has uncovered aspects of the designs that may help.1 The researchers measured the optical power profiles of thirty-eight commercially available lenses using advanced ophthalmic instrumentation.
Dr. Klaus Ehrmann, Director of Technology from Brien Holden Vision Institute and one of the study authors, says, “The design features of contact lenses are known to correlate with particular visual performances, and the measurements we conducted can inform us about things like the distribution and magnitude of relative plus with respect to the prescription power of multifocal contact lenses”.
COMMON MULTIFOCAL DESIGNS
The researchers found the most common commercially available multifocal lenses are centre-near, centre-distance, and concentriczone ring designs, using either aspheric or stepped profiles. Each design comes with its own wearer dependent advantages and disadvantages in vision quality as the design interacts with and depends on the contact lens wearer’s inherent ocular aberrations, pupil sizes, lens centration, and needs as well as expectations.
This effectively becomes a form of modified monovision whereby one eye is providing better distance and the other eye better near vision, depending on the illumination
PUPIL SIZE AND LENS ‘TYPES’
One important finding of the study relates to the effect of pupil size. “As light levels increase, the smaller pupil diameter reduces the area within a lens diameter that predominantly provide distance power for the centre-near lens types”, the authors wrote.
“It can therefore be assumed that the near performance increases at the cost of reduced distance vision. The opposite effect can be expected for the centre-distance lenses, where the near vision should improve for the larger pupil sizes at the cost of distance vision.”
Interestingly, the currently available Proclear multifocal and Biofinity multifocal come in a combination of both centre-distance and centre-near lenses, which are recommended to be fitted contralaterally, say the authors. “This effectively becomes a form of modified monovision whereby one eye is providing better distance and the other eye better near vision, depending on the illumination.”
The ‘concentric ring pattern’ design seeks to achieve visual performance irrespective of pupil size. The Acuvue Oasys for Presbyopia lens is the only one with the concentric ring patterns, a design which alternatively includes distance and near power zones with increasing pupil diameter, an attempt to neutralise the variable of pupil diameter, reported the authors.
‘ADD’ POWERS AND THE EMERGING PRESBYOPE MARKET
Regarding ‘add’ powers, the researchers suggest say that for all lens types, the visual compromises generally increase with the add power. “For early presbyopes with a considerable amount of residual accommodation, a lens with low add power will be sufficient to provide good reading performance without sacrificing distance and intermediate vision,” they say.
Most of the newly released lenses only use descriptors like ‘low’, ‘medium’, and ‘high’ add power, and make reference to the equivalent spectacle add power in the fitting guide, rather than labelling the lenses with their distance and add power. The measurements conducted by the researchers showed consistently lower add powers than the nominal add power. This indicates that the major manufacturers have realised that most patients prioritise good distance over good near vision, which can be facilely achieved by lowering the add power.
Some lenses demonstrated an increasing shift towards more positive powers of up to 1.00D across the power range from -6.00D to +6.00D, prompting the authors to recommend “careful over-refraction by eye care practitioners”, otherwise the lenses may be improperly prescribed.
TRANSITION ZONES AND GHOSTING
The nature of the transition zone between power zones varies, with manufacturers using abrupt, smooth or stair-case approaches in optical designs. In particular, the researchers detected large abrupt steps with sharp transition zones in the Acuvue Oasys for Presbyopia lenses, which they suggest could be the reason for the “often reported significant levels of ghosting associated with these lenses”. It was also found that two steps in the power profiles of the PureVision2 for Presbyopia and Biotrue Oneday for Presbyopia lenses form three distinct power rings, “effectively creating a trifocal lens design.”
The full study (published as an open-access article) can be accessed at Optometry & Vision Science: journals.lww.com/optvissci/ pages/default.aspx