BIASPHERIC MULTIFOCAL LENS
Enhancing Near Acuity with Biaspheric Multifocal Lenses
By Marc D.
Are your presbyopic patients complaining of poor reading acuity? Give these gas permeable lenses a try.
One of our greatest challenges in contact lenses is managing the presbyopic patient. With today's emphasis on vanity and superior visual function, patients want a better option for maintaining sharp distance vision balanced with clear reading ability. Many patients are currently wearing or have worn rigid lenses, both PMMA lenses of the past and fluoronated gas permeable lenses of the present. The biaspheric gas permeable multifocal can provide many of these patients with proper far and near acuity. These lenses possess outstanding optical qualities, so it is not unreasonable to consider patients with no previous rigid lens experience as candidates, provided they have patience for lens adaptation and possess a positive attitude towards their ultimate success.
The Lifestyle Design
While there are a number of outstanding lens designs in this lens modality, my personal choice is the Lifestyle GP biaspheric multifocal, mostly due to the large array of parameters available. These lenses are different because they possess a low E-value, or more gradual change in the sloping of the inside curve. This generally allows for excellent lens centration, a crucial foundation needed to ultimately gain comfort and the proper vision.
Perfect centration is not always needed to succeed in this lens as it would be in other multifocal designs. These lenses incorporate a unique back surface design that utilizes a central aspheric base curve along with a slightly flatter aspheric midperipheral curve, known as the EQ or equivalent base curve, as well as a standard blended peripheral edge design. Other designs are often fit much steeper in comparison and may not translate well to a nearpoint position. They may also display a poor tear film exchange and demonstrate signs of peripheral corneal desiccation.
I Can't Get No
Like many practitioners, I have experienced frustration with all multifocal lenses, regardless of the manufacturer, in attaining satisfactory near vision. Acceptable distance vision and comfort are far less of a problem because these lenses often surpass the comfort achieved in traditional gas permeable lenses. I can partially attribute my receding hair line and greater consumption of antacids to poor satisfaction in reading, but the frustration and aggravation rapidly vanish when that 56-year-old patient whom I've just completed fitting says, "Thank you for helping me feel 20 years old again."
In the past, few practitioners would advocate these lenses for more mature presbyopes with greater than a +1.75D add in their refraction. Instead, the alternating vision bifocal was the lens of choice. This is still true in some cases, but because of new options and better comfort associated with superior corneal positioning and reduced thickness, I have greater optimism for these individuals in a biaspheric multifocal design.
Lost in the Translation
The problem in attaining an acceptable level in reading is generally due more to the lack of accessing the near zone in this lens design than in having ordered the lens too short in the add power. It is often a problem with lens translation, or the inability of the pupil to access the more paracentral and midperipheral aspect of the lens to facilitate proper near vision. The Lifestyle GP is generally designed with a 4.5mm central distance optical zone. The reading zone is contained within the remaining surrounding portion of the lens.
Allow the first set of lenses a minimum of 20 minutes to settle, then check vision and perform an over-refraction for distance and near with handheld trial lenses. Be sure that the myope is corrected with the least amount of minus and the hyperope is accepting of as much plus as possible. Reorder the lens if the distance needs to be adjusted, but only after assessing the near add need and making sure that there is an optimal lens-to-cornea fitting relationship.
Adding plus to near as the only parameter change in a lens reorder has rarely resulted in a totally acceptable effect at near. If the over-refraction is equal to or greater than +0.75D of additional add power, consider adding the plus at near on the front surface of the lens while reducing the distance optical zone (DOZ), assuming that the fitting is optimal. Proper lens fitting will demonstrate good lateral centration and superior positioning on the cornea where there is minimal central pooling with paracentral and midperipheral alignment. The DOZ is available from 3.0mm to 6.0mm in its central diameter, in 0.5mm steps.
TABLE : Guidelines for Improving Near
Vision After Initial Lens Order
1 Add additional plus to the front surface of the lens at
near with reduction of distance minus in the myope or the addition of plus in the hyperope whenever possible.2
Reduce the DOZ (distance optical zone).
3 Lens modification: flatten or widen the peripheral curve.
Lens modification: reduce the overall lens diameter.
5 Switch to a smaller diameter while maintaining the
same EQ (equivalent base curve).
6 Flatten the EQ
7 Switch to the New Dimension design.
Upon delivery of the next lens, if the reading is nearly ideal but not good enough, consider modifying the lenses just as you would a standard gas permeable lens. These lenses are aspheric, but peripherally they have the same type of edge design as a standard gas permeable lens. Work can be done peripherally to all of the lenses.
First look carefully at the periphery. Is there adequate peripheral edge lift? Does the peripheral bevel width appear narrow? While the lens may look good centrally, it may not translate well to near because peripherally the lens is tight. This is where lens modification can produce immediate results.
Estimate the tools needed to open the bevel based on the EQ value. Starting with your flattest velveteen-covered tools, apply adequate amounts of Alox (aluminum oxide) or like finishing compound to flatten the lens periphery slowly, then proceed on to the steeper tooling. For example, with an EQ of 7.60mm, start with an 11.50mm tool first, then proceed on to a 10.50mm, 9.50mm, and then an 8.75mm tool as the last. The amount of pressure you put on the tool and the amount of finishing compound on the tool will dictate the rate of change to the lens. I recommend Alox 721 over Xpal as the finishing solution of choice. Alox, while more abrasive, doesn't leave scratches on the lens surface. Xpal dries much more quickly on your lens and tooling and may also leave the lens surfaces scratched.
Confidence in lens modification comes with experience; however, this is an easy modification to do. The SGP II plastic, a mid-Dk silicone-acrylate gas permeable material, is very easy to work with. A note of caution: do less to the lens than more. It is easy to reapply the lens to an eye after a minor modification and get a subjective response from the patient on the near vision. You can always go back and do more. If the EQ is flatter, 8.3mm, for example, consider starting with a flatter tool like a 12.25mm covered tool and work into a 9.50mm tool, thereby being at least 1.00mm flatter than the EQ.
|The Lifestyle GP design utilizes an EQ, or an inside curvature, instead of a base curve measurement seen in traditional gas permeable
In some cases, you may want to consider reducing down the overall diameter. I have found that taking down the lens size by 0.1mm or 0.2mm has at times helped aid lens translation to the near zone. Moderation is very important here because taking the lens too far down in size can have a negative effect on lens centration and may ultimately ruin the vision and comfort. If the EQ still looks good but the lens does not translate well, consider reordering to the next smallest diameter available while leaving the EQ unchanged. This may help facilitate translation.
Try flattening the EQ if the reading is still unsatisfactory. By flattening the lens curve, we hope to aid access to the near zone. Generally going 0.50D or 0.1mm flatter should help. The combination of raising the add, reducing the DOZ and flattening the EQ generally will bring about the effect needed. Monitor the distance acuity to make sure that it is not lost by all of these adjustments. Likewise, guard against corneal apical flattening from an unexpected orthokeratological change. In spite of our best intentions, apical flattening is always a possibility. It is a good idea to repeat keratometric readings and/or corneal topography to monitor corneal stability.
If All Else Fails...
After adding plus at near, decreasing the distance optical zone and flattening the EQ, little remains to improve near vision short of undercorrecting the distance vision in one eye and setting up what is effectively a modified monovision in multifocal lenses. This may be a good option for some patients if they do not focus on the difference between the two eyes.
A newer design called the New Dimension lens features a larger reading zone which is easier to access. This lens allows for a greater amount of plus at near over the original Lifestyle design. The top add in the original design is +3.00D while this new lens, which comes with +0.75D on the back surface, will allow for an additional +3.25D on the front for a total add power of +4.00D. It is best to go steeper with this design to aid centration, which is critical for maintaining good distance vision because the standard distance optical zone is smaller in this lens than in the original design. Going larger can aid the stability of this lens similarly but in a lesser sense than steepening the EQ. This design is available in 9.0, 9.5, 10.0 and 10.5mm diameters. The larger two diameters are generally selected because they help promote the central positioning and stability that makes this design successful. The New Dimension design with near vision often helps reading, but patients may notice a degradation of the distance because of the smaller DOZ. Therefore, I think this lens is best left as a last resort in lens selection.
While I have found these guidelines helpful in lens fittings with problematic near vision result, I acknowledge that patient needs are not the same. These lenses may not be right for every patient. These fitting tips have contributed significantly towards gaining an overall success rate greater than 85 percent last year in our office. The key is communication with patients, and educating them on the time frame required to properly fit them while understanding their expectations as well as their visual needs. If you have discontinued fitting these lenses because you have run into similar problems, I ask you to try these lenses again. With more options now available, there is an infinitely greater chance of succeeding with these lenses.
Herwitz is a graduate of The Illinois College of Optometry and is a member of
the AOA. He is a private primary care practitioner in Chicago where he
specializes in presbyopic contact lenses.
Contact Lens Spectrum, Issue: August 2000