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A high resolution Hartmann-Shack wavefront sensing instrument (Optocraft UHR SHSInspect) can characterise the optical design of a multifocal contact lens by recording more than 6,000 unique measurements over the central 6mm of a contact lens. From the more than 6,000 unique measurements, a profile of how the power changes from the centre of the lens out to the periphery of the optical portion of the lens can be generated. This power profile provides great insight into how the power of multifocal contact lenses changes within the optical zone of the lens and also provides an innovative and powerful method to understand some of the key contributing factors to successfully fitting multifocal contact lenses.
Figure 1 shows power profiles of a -3.00D contact lens with a minimal amount of spherical aberration and a -3.00D contact lens with an increased amount of spherical aberration. The data are displayed as a plot of the local power as a function of radial distance. The centre of the lens is represented on the left side of the graph and the periphery of the optical zone is represented on the right side of the graph. The -3.00D contact lens with minimal spherical aberration has a power profile that remains relatively flat across the optical zone, while the -3.00D contact lens with an increased amount of spherical aberration changes across the optical zone and becomes more negative toward the periphery. As shown in the figure, the shape of a power profile changes with the addition of spherical aberration. Power profiles help discriminate spherical from aspheric designs, and provide a comparison of the different amounts of asphericity between lenses.
The power profile of a multifocal contact lens also provides insight on add power, as well as information on any variation of the profile across different regions of the optical zone and radial locations along the profile where the label power is located. This information can then be analysed across the different sphere powers that are available to determine if there is consistency across the power range.
In this study, power profiles were measured for the full power range, in all available adds, for Biofinity multifocal, Air Optix Aqua Multifocal, PureVision Multi-Focal and PureVision2 for Presbyopia. Each lens was placed in a cuvette filled with phosphate buffered saline and was measured over a 6.0mm aperture. The average of three lenses for each specific sphere power and add power was determined, and a line of best fit algorithm was applied using custom MATLAB software.
To compare the different lens designs, all lenses were examined for their variability relative to label power. Reference lines (denoted by red dashed horizontal lines in Figure 2) were drawn for each label power across the power range. Variability was then assessed at three locations: 0.5mm radial (1.0mm diameter) from the centre of the lens, the location where the power profile crosses the label power, and at a 3.0mm radial (6.0mm diameter) from the centre of the lens. The variability was determined for each lens power across the power range.
The analysis was completed as follows: At the 0.5mm location, a vertical arrow was drawn from the label power (red dashed line) to the power profile for each of the SKUs to show the separation between the label power and the power profile of the lens. The variability across the power range could then be characterised by observing the different lengths of the arrows. A similar methodology was used to compare lens designs at the 3.0mm radial location. For a design that is consistent across the power range, the lengths of the arrows would be the same for all SKUs at the 0.5mm radial location and similarly the lengths of the arrows would be the same for all SKUs at the 3.0mm location.
To assess the variability with respect to where the power profile crosses its label power, for each lens a vertical arrow was placed at the intersection of the power profile and the red dashed label power line. The radial distance location of the arrow for each power was noted and the grey shaded region on the plot represents the range of radial distances where all of the power profiles cross their label power. The grey shaded region highlights how much variability exists for a specific multifocal design. In some instances where the power profile never crossed its label power, a red circle was drawn to highlight which power profiles do not cross their label power. For a design that is consistent across the power range, the power profiles would cross their label power at a similar location and the grey shaded region would be small.
Results
Biofinity multifocal
0.5mm and 3.0mm locations
In this study, power profile analysis of Biofinity multifocal lenses shows a significant amount of variability in the design tested. At the 0.5mm radial location, the height of the arrow changed across the power range, slowly decreasing in length moving from high minus to plus powers. While the total change in power (length of arrow) is about +2.00D for the -8.00D lens design, for the +6.00D lens design, the total change in power is about +1.00D.
Comparing these two powers indicates there is a +1.00D difference across the power range. Similarly, at the 3.0mm radial location, the power profile is about 0.50D more negative than label power for the -8.00D power, while for the +6.00D power, the power profile is actually slightly more positive in power than the label power. The inconsistencies at the 0.5mm and 3.0mm radial locations for the -8.00D and +6.00D power profiles indicate a large amount of variability in the Biofinity multifocal design.
Location of label power crossing
Based on Figure 2, the location of where (and how) the Biofinity multifocal +1.50 N lenses (near at the centre) cross the labelled power of the lens has a large amount of variability across the power range. For the +6.00D and +3.00D lens designs, the power profiles actually cross the label power twice, and for the +1.00D and -1.00D lens designs, the power profiles do not in fact cross the label power at all. Excluding the +1.00D and -1.00D lenses that do not even cross the label power, there is still over 1.0mm in radial distance (2.0mm diameter) variability of where the remaining power profiles intersect the label power across the power range.
Air Optix Aqua (AOA) Multifocal
0.5mm and 3.0mm locations
AOA multifocal lenses also demonstrated some variability. Figure 3 illustrates the AOA multifocal high add power profiles across the power range. The AOA multifocal high add lenses are fairly consistent at the 0.5mm radial distance from the centre of the lens; as the length of the arrows across the power range is fairly consistent. At the 3.0mm radial location, there is less variability across the power range, and the power profiles are overall relatively consistent across the power range.
Location of label power crossing
The location of where the AOA multifocal power profiles cross their label power does have some variability across the power range. In Figure 3, the red arrows in the central portion of the plot indicate where each power profile crosses its label power. The separation of the red arrows is shown by the grey shaded box, which has a width of approximately 0.75mm. The grey shaded box indicates the total variation of where these AOA high add multifocal power profiles cross label power is around 0.75mm radially from the +6.00D to the -9.00D, equating to 1.50mm diametric variation.
PureVision Multi-Focal (PVMF)
0.5mm and 3.0mm locations
Power profiles of PureVision Multi-Focal high add lenses are illustrated in Figure 4. At the 0.5mm radial location, the PVMF high add lenses are relatively consistent across the power range. There is more variability at the 3.0mm location, where, similar to Biofinity multifocal, the power profile is in some cases more minus, and other cases, more plus, indicating greater variability across the power range.
Location of label power crossing
The PureVision Multi-Focal high add power profiles also demonstrate some variability relative to where the power profiles cross their label power, such as in the +6.00D and +3.00D designs, where the power profiles never cross label power. Variability across the rest of the powers is less than 1mm radially.
PureVision 2 for presbyopia
The results described in the preceding paragraphs characterise the inconsistencies that exist among currently marketed multifocal products and highlight the opportunity and need for a novel multifocal design that is consistent across the power range. PureVision 2 for Presbyopia was designed to specifically address this issue.0.5mm and 3.0mm locations.
As shown in Figure 5, the study found that the PureVision 2 for Presbyopia high add design is more consistent across the power range at the 0.5mm and 3.0mm locations than the other currently marketed products, with extremely small differences at these locations across the entire power range.
Location of label power crossing
The location of where each power profile intersects its label power across the power range for the PureVision 2 for Presbyopia designs was also found to be extremely consistent, with only 0.1mm variation from the +6.00D to the -10.00D lens (shown by the grey shaded region).
Conclusion
A high resolution Hartmann-Shack wavefront sensing instrument can record more than 6,000 individual measurements over the central 6.0mm of a contact lens to characterise the optical design. Using this technique, illustrations of how the power of the contact lens changes from the centre to the edge of the optical zone can be created to provide insight into the consistency of multifocal contact lens designs. Power profile consistency across the dioptric range helps to ensure that, regardless of which power is fit on-eye, the eye care practitioner can expect a more predictable fitting experience.
The findings of this research show that there are opportunities to develop multifocal lens designs with more consistency. PureVision 2 for Presbyopia was designed to make multifocal fitting a quicker, easier and more satisfying experience for both clinicians and patients by improving the add power consistency, label power consistency and general profile shape consistency across the entire range of available parameters.
? Hovinga, KR and Vogt, AKS, were formally researchers at Bausch+Lomb, Rochester NY