A New Contact Lens Option for Presbyopic Patients
Innovation in design and metrology guided the creation of this multifocal contact lens.
By Alexis K. S. Vogt, PhD, FAAO
Technology is advancing before our eyes — both figuratively and literally. With these technological advances come not only improved methods for designing contact lenses, but also improved methods for manufacturing and measuring lenses. The ability to measure is equally as important as the ability to manufacture. Innovative designs, manufacturing techniques and metrology (measurement) methods have led to the development of PureVision®2 Multi-Focal Contact Lenses For Presbyopia.
Imagine your life without your cell phone. Perhaps hard to remember, but there was a day in the not-so-distant past when we used pen and paper to write down a grocery list. Now, we read lists off our phones or use apps that tell us in which aisle to locate the items on our grocery list. Many such technological advances create challenges for presbyopes and a large number of these patients aren’t very satisfied with their current form of vision correction. While distance vision remains important, 70% of presbyopic patients cite the need for good near and intermediate vision as a priority for their work.1 Approximately 50% of these patients say they often have difficulty with their near or intermediate vision at work.1 PureVision2 For Presbyopia was designed with a unique center-near, 3-Zone Progressive Design to improve near and intermediate vision to provide clarity where it counts — in the real world — so patients can clearly read messages on a cell phone or work on a computer, while maintaining excellent distance vision. To address these patient needs, PureVision2 For Presbyopia was designed with more add power across the center portion of the lens and a wide intermediate zone where add power gradually transitions to an accurate distance power.
PureVision2 For Presbyopia innovation began with the development of novel computer eye models of individual patients’ eyes gathered from comprehensive ocular biometry, which included pupil size, higher-order aberrations, residual accommodation, corneal topography and diameter, anterior chamber depth, axial chamber length and subjective refractions. The computer models were used to predict logMAR visual acuity scores for any number of computer-generated multifocal lens designs for each individual eye. Multifocal contact lenses were designed to optimize visual performance across a wide variety of pupil sizes, higher-order aberrations and patient biometry. What culminated from thousands of computer modeling iterations was PureVision2 For Presbyopia — a multifocal contact lens designed to provide improved near and intermediate vision, while continuing to provide excellent distance vision in the real world.*
In addition to extensive computer eye modeling, the PureVision®2 For Presbyopia design was also optimized with the use of finite element modeling — software used to predict the behavior of a contact lens on-eye. While the automobile industry uses finite element modeling to simulate collisions and determine how best to design the safest cars to withstand the greatest impact, contact lens designers at Bausch + Lomb used finite element modeling to model how contact lenses conform to corneas. When a contact lens is inserted, the first thing a patient does is blink to conform the contact lens to the cornea. Finite element modeling shows that the optical properties of a contact lens are different on-eye versus in a blister. The conformation of a contact lens onto a cornea induces changes in the optics. By understanding the deformation of a contact lens on the eye, the optical properties can be predicted to aid in designing contact lens optics that are optimized for on-eye performance. The relationship of the contact lens design and the corneal shape is what governs the fitting process and optical performance. Finite element modeling is also used to model steep and flat corneas and assess how a lens fits with particular corneal shapes. Additionally, finite element modeling evaluates the behavior of a contact lens through a blinking motion to gain insight into the effect decentration and rotation have on optical performance. Finite element modeling is an extremely powerful tool to aid in the design of new contact lenses.
PureVision2 For Presbyopia was designed to improve near and intermediate vision while continuing to provide excellent distance vision.* They were also designed with eyecare professionals in mind, because they were designed with a more predictable add across the power range for streamlined fitting.* To design a predictably fitting multifocal, a sophisticated high resolution Hartmann-Shack wavefront sensing instrument was developed to measure the power profiles of multifocal contact lenses. With more than 6,000 unique power measurements over the central 6mm of a contact lens, a profile of how the power changes from the center of a contact lens out to the periphery of the optical portion of a lens can be graphed to illustrate the distinct, discreet changes of lens power across the contact lens. The power profile shown in Figure 1 illustrates a center-near multifocal design where the lens center has the most relative plus power (for near vision), the distance power is in the periphery of the optic zone (for distant vision) and the intermediate zone (for mid-range vision) is where the power transitions from the central near power to the peripheral distance power portion of the lens optic zone.
Figure 1. Power profile of a center-near multifocal design.
Most soft multifocal contact lenses are symmetrical, making it only necessary to view one side of a power profile to appreciate the change in power from center to periphery of a lens optic zone. This allows for comparison of measured add power, distance power, and the power of the intermediate zone. The consistency of the measured powers across a range of labeled powers can also be compared. Figure 2 illustrates the +3D, +1D, -1D, -3D and -6D power profiles of a multifocal contact lens design. The vertical axis of the graph is the lens power and the horizontal axis represents the different regions (near, intermediate and distance) across the optical zone of the contact lens. The power profile shown in the example illustrates a center near multifocal design.
Figure 2. Power profiles of a +3D, +1D, -1D, -3D and -6D multifocal contact lens. The center portion of the lens is on the left side of the plot and the distance portion of the lens is on the right side of the plot. The vertical axis is lens power.
Ideally, the power profile of a lens will have the same shape for all labeled lens powers across the available prescription range. Inconsistencies that are found across the available prescription range, more with some multifocal designs than others, may contribute to the challenges in effectively and efficiently achieving successful outcomes in the fitting of multifocal lenses. The profiles in figure 2 can be overlaid to easily compare the relative power available in the near, intermediate or distance zones in different labeled lens powers. In Figure 3, the +3D, +1D, -1D, -3D and -6D power profiles of PureVision2 For Presbyopia were normalized to a -3D power. The normalization provides a means to observe similarities and differences between the power profiles of lenses across the power range.
Figure 3. The +3D, +1D, -1D, -3D and -6D power profiles of figure 2 (PureVision2 Multi-Focal Contact Lenses For Presbyopia) normalized to a -3D power to show consistency of the multifocal design across the power range.
Normalized power profiles show inconsistencies for many currently marketed multifocal contact lenses. A comparison of the normalized power profiles of PureVision®2 For Presbyopia, PureVision Multi-Focal, Biofinity Multifocal (CooperVision) and Air Optix Aqua Multifocal (Alcon) shows that PureVision2 For Presbyopia has the highest degree of consistency (using F-test for variance, p<0.001) across the prescription range.
PureVision Multi-Focal had a variance of 0.50D at the near portion of the lens and a variance of 1.07D at the distance portion of the lens across the -6D, -3D, -1D, +1D and +3D range (Figure 4). Across the -6D, -3D, -1D, +1D and +3D range, Biofinity Multifocal had a variance of 1.24D at the near portion of the lens and a variance of 0.6D at the distance portion of the lens (Figure 5). Air Optix Aqua Multifocal contact lenses had a variance of 0.78D at the near portion of the lens and a variance of 0.20D at the distance portion of the lens across the -6D, -3D, -1D, +1D and +3D range (Figure 6).
Figure 4. PureVision Multi-Focal normalized power profiles. The dashed horizontal line indicates the location of the labeled distance power.
Figure 5. Biofinity (CooperVision) Multifocal normalized power profiles.
Figure 6. Air Optix Aqua (Alcon) Multifocal normalized power profiles.
A comparison of the normalized power profiles shows PureVision®2 For Presbyopia has less variance at near, less variance at distance and a wide intermediate zone that naturally transitions from near to distance. Across the -6D, -3D, -1D, +1D and +3D range, PureVision®2 For Presbyopia had a variance of 0.33D at the near portion of the lens, a variance of 0.12D at the distance portion of the lens and a wide intermediate zone where add power gradually transitions to an accurate distance power (Figure 3). PureVision2 For Presbyopia has the highest degree of consistency of power at near or distance across the prescription range compared to PureVision Multi-Focal (p<0.001), Biofinity Multifocal (p<0.001) and Air Optix Aqua Multifocal (p=0.001).
With unique optical modeling and power profile measurements, PureVision2 For Presbyopia lenses were designed to provide improved near and intermediate vision, while continuing to provide excellent distance vision in the real word; and to provide eye care professionals with a predictable fitting experience for effective patient management, all with just two adds. ●
1. Merchea M, Vogt AKS, Raj S. Eye care professional and patient experiences with contact lenses for presbyopia. Poster presented at the American Optometric Association Annual Meeting. June 26-30, 2013. San Diego, CA.
*Analysis based on use of a Hartmann-Shack wavefront sensing instrument to map lens power across contact lenses. More than 6000 unique measurements over the central 6mm of a contact lens were plotted to determine local power measurement as a function of radial distance from the center of the lens.