|What You Need to Tell Your Lens Consultant
Bob Martin: “We, as consultants, benefit from any piece of information we can gather from the doctor—what diameter lens the patient used to wear, the pupil size, horizontal visible iris diameter, lid tension, age, lifestyle, etc.”
Jim Slightom: “Pupil size, corneal size and position of the current lenses would all be helpful clues how to fit. And, is the patient a past monovision wearer? But, when it comes to segmented lenses and translating segmented lenses, I’m a 100% believer in a fitting set. There are too many variables with how that seg height will position with the pupil, palpebral distance, lid position, etc. If you try to just describe that to me on the phone, your first lens becomes your fitting lens.”
Bob Martin, Director of Operations, Blanchard Contact Lens Laboratories:
First, even with the time, energy and research that has gone into the development of multifocal lenses over the past several years, and even though many improvements have been made, these lenses still may not work for everyone. About 20% of the population cannot translate aspheric optics. Secondly, we have made great strides over the last five years in the development and manufacturing of multifocal lenses. The tools that we have to design and manufacture these lenses have grown by leaps and bounds. Fifteen years ago, the only way we could make multifocal lenses was to actually bend the lens, cut it and release it. The process was non-reproducible and unreliable. Now, computer-generated design sequences, topography-related materials and the development of extremely sophisticated lathing processes give us the ability to focus in on much better designs—and much better results—for patients.
Where does the design process start?
We try to solve a problem. Intermediate vision is so important because of computer work. It’s always been a question in aspheric lenses: How much add power can we put into that lens without degrading both the distance vision and the intermediate vision? How can we optimize that add power, yet still make sure that, during primary gaze, the patient is still getting the best distance and intermediate vision possible? That’s a very difficult aspect of design.
How do you test possible lens designs?
We utilize ray tracing to show where the add is optimized in conjunction with other aspects of the lens—such as the back surface, where we already have the S-form. We work with practitioners who help us test this lens in patients who aren’t happy with their current lenses. We try to optimize this lens for them.
What are some of the greatest challenges you face as a lab?
As a gas-permeable lens lab, it’s the perception that GP lenses are difficult to fit, that they are uncomfortable—the perception that soft lenses are more of a ‘commodity.’ Many people don’t realize that the visual benefits of a GP lens will probably provide you with better sight than a soft lens. We have to look at the specialty end of the market. Ten to 12 years ago, Blanchard decided to make this move—to large-diameter lenses, scleral lenses and other specialty fits. How can we best provide the services that put the GP lens to the forefront?
Another challenge is the perception of increased chairtime. When it comes to a regular, spherical lens, we demonstrate about a 10% return rate—so, 90% of the time, the first lens tried is the lens dispensed. With a multifocal or keratoconus lens, the chairtime will increase dramatically. We average 1.5 lenses per eye per patient during the fitting process. At what point do you say that the patient is not a good lens candidate? After the second fit, if I don’t think we’re close, I will talk to the doctor about looking at a different design. Otherwise, the patient starts becoming frustrated and disillusioned.
|Patients Who May Not Succeed in GP Multifocal Lenses
These patients are probably not going to succeed with gas-permeable multifocal lenses. So, be very cautious when working with patients who demonstrate:
• Against-the-rule astigmatism over 0.75D.
• With-the-rule astigmatism over 2.50D.
• Oblique axis over 1.00D.
• Residual astigmatism over 1.00D.
• Steep corneas over 48.00D.
• Flat corneas under 39.50D.
• Unreasonable expectations. Know your patients well. If they have been unsuccessful in the past, find out why.
• Spectacle blur or distorted mires. The cornea must be rehabilitated before refitting this patient in a new multifocal lens. Take the patient out of lenses or refit in single vision.
• Corneal grafts.
• Emerging keratoconus.
• Signs of previousrefractive surgery, such as LASIK or radial keratotomy.
• Signs of prior lid surgery or blepharoplasty.
Other patients may require more effort during fitting, but still enjoy successful multifocal lens wear. For example, those with the following conditions may be complicated fits, but ultimately, success stories:
• High plus powers.
• Unilateral vision, vision in one eye only.
• Long-time monovision wearers.
Jim Slightom, Lens Consultant, ABB Concise:
When we recommend a design, we always start with Ks and Rx. From there, the doc asks if this patient is a candidate for a certain lens. More information can help, but if all you have is the Ks and Rx, then you can make a determination. It’s also beneficial to know the parameters of the patient’s past lenses and other history. The first thing I ask the doc about a past gas-permeable lens wearer: Did you see the lenses on the patient? I can’t tell you how many times the answer is no. It’s very important to know where and how the lens is positioning.
How do you decide whether an aspheric or translating design would be best?
I ask some lifestyle questions. What does this patient do for a living? Is he or she on the computer all day long? How important is intermediate vision? Simultaneous lenses probably provide the best intermediate vision. But, if this person is 70 years old, reads extensively and drives often, a translating lens may be the better choice. It’s not a question of if we use computers today—it’s how much. That’s a question I always ask, as well as what the patient’s hobbies are, because that enters into the design as much as Ks and Rx. A toric design is most beneficial only in certain designs when dealing with high levels of cylinder. Here’s another issue: If it is a high cylinder, and the doctor can’t provide topography, how far out does that cylinder run? Is it limbus to limbus? Is it contained in a 6mm zone? All of that enters into the lens design, and many times, we don’t have that information.
Does material matter?
Dry eye is always an issue when you get older. Different materials work differently, and you need to consider that. If you have a compromised cornea, you may want higher oxygen permeability. If the patient is going to sleep in their lenses, then you want a material with higher oxygen permeability that’s approved for extended wear. Across the board, most materials are very good. The higher the silicone content, however, as a rule of thumb, the less wettability of the lens.
What style of lens do you prefer?
The biggest issue we have with simultaneous vision lenses is centration. So, a reverse-geometry lens, in which the secondary curve is slightly steeper than the base curve, will center better horizontally or vertically. Sometimes, reverse geometry is successful with lenses that are upper-lid attached to bring them closer to center for translation. If the lens is locked under the upper lid, then the lens won’t translate and you have less add effect. You still want the lens to translate, so it’s just a very slight amount. Reverse geometry can be beneficial in simultaneous vision lenses. Sometimes, when there needs to be more add for near, I also utilize a D lens and an N lens, just like in soft lenses. Though some patients can’t accept this, if they are able to visually accept monovision, they’ll most likely accept the D and N lenses.
How do you develop a multifocal concept and bring it to the marketplace?
I start with what I already know, which may be wrong—for example, I started with a great amount of reverse geometry on a lens for an irregular cornea, which helped centration. But, in the case of multifocal vision, the lens needs to translate somewhat. I discovered that an eye not compromised in shape needs much less of a reverse curve to bring the lens to center. The process was research and development. I work with practitioners who can send me sodium fluorescein images and video of what the lens looks like on the eye; hopefully, then we can redesign it and make it work. From concept through research, it usually takes a year, at minimum, before you can really ‘give it a name’ and do something with the lens. Then, the lens is named and trademarked.
What’s the most important part of the process?
Troubleshooting. In our experience, patients’ number-one complaint is a lack of reading vision. Where does the lens position upon reading gaze? Instill fluorescein in the patient’s eye, seat him or her in the chair, turn the room lights out, raise the chair to its highest level, put your stand light on a near vision card, and have the patient read. View the eye with a cobalt blue filter from directly beneath the patient’s face. You can see exactly where that lens is positioning. What’s the biggest mistake that practitioners make? Communication with the patient. Often, the patient’s expectations are too high. Simultaneous vision lenses can never provide vision as clear and sharp as single vision at distance. If that hasn’t been properly communicated to the patient, the patient may expect to see as well as with single-vision lenses for distance and near. That doesn’t happen—there is going to be compromise.
Where Multifocal Lenses Are Going in the Future
Ed Sarver, Lens Consultant, Medlens:
In my consulting company, we design primarily for IOLs. We’ve designed toric lenses and aspheric lenses, and we determined power complications. Then, we looked at a specific design for an IOL that would correct for chromatic aberration, and we discussed how we could also provide a multifocal IOL design, which led us to the applications of the design. We believed our multifocal design would also be good as a multifocal contact lens and an ablation pattern in refractive surgery. How do you correct for chromatic aberration in a multifocal lens? We wanted to take advantage of the binocular suppression that occurs with binocular vision. So, we came up with this idea of using a non-symmetrical point-spread function (PSF) on purpose; that way, one eye will have clear vision in certain areas and through certain parts of an object, and the other eye sees other parts in sharp focus. The stray light is directed elsewhere. You’ll get a clear binocular view because the stray light will be in different parts of the features you’re trying to view.
The benefits of an asymmetric PSF include:
• The ability to control the amount of energy in optics by changing the ratio of top and bottom half—to see better near or far. We can also add additional zones, but we have to be careful how we direct the stray light.
• Being able to control astigmatism by adjusting the back surface of the contact lens.
• The ability to manipulate stray light to fall in different directions so that we enhance the binocular vision using suppression.
When it comes to chromatic correction, the index of refraction for the elements of the eye is a function of the wavelength of light being refracted. It’s an inverse relation: As the wavelength increases, the index of refraction decreases. This difference leads to a chromatic aberration in the eye. So, blue and green and red focus differently—if you’re focused at green, then the difference between red and blue is roughly 1.50D for a 6mm pupil. One problem, however: You eliminate true color vision as you do with blue blockers, because you block middle light.
What is your method for correcting chromatic aberration?
At the retina, white light is spread in a blur circle due to chromatic aberration. Green is in focus, red has a myopic shift, and blue has a hyperopic shift. For the same amount of longitudinal chromatic aberration, rays further from the axis cause more blur than rays in the middle. We want to block chromatic rays in the periphery that lead to large amounts of blur, but we want to pass the rays in the center through so that color information can be received. We’ve come up with a chromatic filter that’s clear in the center with an annulus of a green-pass filter for contrast and clarity. This is how we reduce chromatic aberration. The size of that innermost circle is such that we limit the chromatic aberration to one-eighth of a diopter. Each of the two zones is individually corrected for spherical aberration, and then we layer this chromatic aberration “donut” on top of it. This is a soft lens—it’s large, a little over 14mm for stability. It needs ballast so that it stays in place. Bottom line: If you remove chromatic aberration, the vision is improved.
Dr. Sindt is the Co-Chief Clinical Editor of Review of Optometry. She is also Director, Contact Lens Service, and an associate professor of clinical ophthalmology at the University of Iowa.