Article Date: 10/1/2002

GP BIFOCAL
A New Concept in GP Bifocal Contact Lenses
Compare aspheric multifocal contact lenses to a new seamless bifocal contact lens design.
By Robert B. Mandell, OD, PhD

The trend in bifocal contact lenses over the last two decades has been toward aspheric designs. The popular belief was that an aspheric lens provided a multiplicity of powers that allowed the wearer to see over a range of distances. This was an attempt to duplicate the successful design of spectacle multifocals, or "progressives." The problem is that progressive spectacle lenses perform differently on the eye than aspheric contact lenses.

Bifocal Basics

Figure 1. One power in an aspheric lens is in focus and the remaining powers are out of focus.

An aspheric bifocal lens provides a range of powers to allow vision at different distances, but as a result, it also presents a range of powers for the light that enters the eye. Only one power from the range is correct for the distance viewed and the remaining powers are out of focus (Figure 1).

When we use lower power adds, the power range is small, but for higher power adds the power range is large and degrades the retinal image. This isn't a problem in spectacle lenses because the power range is spread over a larger lens area. Compare that to contact lenses where only the part of the lens that is directly in front of the pupil is used at any given time. Typically the power range of light that enters the eye for a spectacle lens with a +2.50D add is only about 0.25D, whereas the same add in an aspheric bifocal contact lens presents the eye with a power range of about 1.50D (Figure 2).

Figure 2. The power range for a +2.50D add spectacle lens is about 0.25D; the same add in an aspheric bifocal contact lens presents the eye with a power range of about 1.50D.

The same principle explains why aspheric multifocal contact lenses are more successful in lower than higher add powers­because the range of powers before the pupil at any given time is less. However, even with lower adds, the power range may cause a decrement in the patient's vision. So if aspheric bifocal contact lenses can't provide us with the ultimate bifocal then what's the answer?

Basic Bifocal Options

We can choose from two classic approaches. We can design a contact lens to function according to the principle of simultaneous vision or the principle of alternating vision.

Simultaneous vision. The term "simultaneous vision" applies to any bifocal contact lens design in which part or all of both the far- and near-power zones position in front of the pupil at the same time so that each contributes to the retinal image. This has the advantage of requiring little or no movement of the lens on the eye.

Simultaneous vision does have a drawback. The wearer sees two images at once, and at least one of the two retinal images formed by light from the distance and near zones is always out of focus. Consequently, the wearer's vision is always compromised. It makes no difference how a simultaneous vision bifocal is designed ­ two zones, multiple zones or diffraction elements ­ the lens will never provide a visual result that will satisfy more than a small proportion of patients.

Alternating vision. We can also use a bifocal contact lens designed according to the principle of "alternating vision," in which a vertical shifting action or translation of the contact lens occurs to place the distance or near zone in front of the pupil of the eye at the appropriate time. Up until now, the only lens able to carry out this function was the prism ballast bifocal in rigid lens form, which has been available for 40 years. But it's the most uncomfortable rigid lens design that has been made, and it's the most difficult lens to fit properly.

 

Base Curve and Diameter Selection for the Seamless Bifocal

BASE CURVE:
Fit larger lenses on K or slightly flatter than K
 
FOR ASTIGMATISM OF: FIT THE BASE CURVE:
None 0.25D flatter than K
0.25D to 0.50D on K
0.75D and greater K plus 1/3 delta K
DIAMETER SELECTION:  
For average eye 9.8mm diameter with 3.4 distance zone
For large eye 10.2mm diameter with 3.4 distance zone
For large pupil 10.2mm diameter with 3.8 distance zone
For high riding lens 10.2mm diameter with 3.8 distance zone

 

The Seamless Bifocal

One solution to the bifocal problem is to produce a concentric-type bifocal contact lens that provides comfort and shifts on the eye according to the alternating lens principle. The lens ideally has true spherical optics for both distance and near to provide optimal vision.

The Seamless Bifocal (Con-Cise Contact Lens Company) is a front surface concentric lens with the distance portion in the center. This is the opposite construction of present day GP bifocal contact lenses in which the central zone is for near vision correction and contains aspheric optics. In addition, the distance and near portions of the Seamless Bifocal have true spherical powers and provides the maximum vision for each distance. The two zones are connected by a new curve relationship that allows a smooth and easy transition, without the occurrence of annoying image jump. There's no vision compromise as with the various aspheric contact lenses.

Figure 3. Patients use the Seamless Bifocal contact lens much the same way they use progressive spectacle lenses.

The Seamless Bifocal is an alternating vision lens controlled by lid action imparted to a proprietary front surface design that provides the lens movement needed for alternating vision. The lens contains a series of carefully engineered slopes on the front surface that allow lens shifting while retaining comfort. When the patient looks down, the lower lid makes contact with the slop of the lens periphery, moving the lens up. When the patient looks up, the upper lid pushes the lens down. This allows the patient to use the lens in much the same way he would a spectacle bifocal (Figure 3).

Because the bifocal optics of the Seamless Bifocal are formed on the front surface of the lens, the back surface is available for any possible design. You can fit patients who've previously worn GP contact lenses in exactly the same way as you would for their single vision lenses. However, certain modifications are usually recommended to enhance the functioning of the bifocal lens, as will be described in the fitting section. There's no need to fit the lens steep, as with aspheric lenses, so you also avoid corneal distortion caused by aspheric bifocals.

When fitting the Seamless Bifocal, you must fulfill these requirements: The lens must achieve good distance vision and the patient must learn to posture his head as in wearing bifocal spectacle lenses.

Selecting the Right Patients

As with all bifocal contact lens fitting, patient preparation is the key to success. The high degree of success with the Seamless Bifocal makes it easier to present a positive approach to patients.

Previous wearers of GP contact lenses who are beginning presbyopia are the obvious first choice for fitting this lens. Because the lens uses no prism ballast, patients usually experience an uneventful transition. There's also no limit to the power range that you can fit.

Fitting Considerations

Aim for fitting a lens that rides centrally or only slightly high when the patient views distance objects. The lens should move in the same manner and speed as a single vision lens.

Figure 4. A well-fitted lens rides centrally or slightly high.

Fit the average patient with a lens diameter of 9.8mm. Lenses as large as 10.2mm are common and may seem excessively large, depending on the fitter's previous experience. Nevertheless, it's important that the lens have a large diameter for two reasons. It helps the lens to center, and, together with the front curve design, it helps the lens shift for alternating vision. The larger lens diameter also makes the lenses more comfortable than smaller diameter lenses. The special design of the front surface that allows the lens to produce alternating vision will cause a small diameter lens to ride high, along with some blurring of distance vision. You may need lenses of 10.4mm diameter or bigger for larger eyes (Figure 4). Lenses smaller than 9.6mm diameter are rarely needed.

Fortunately, with modern gas permeable materials, larger lenses are no longer a fitting problem as far as oxygen deprivation is concerned.

The Fitting Process

If a trial set is available, start with the 9.8mm diameter/3.4mm distance zone lens with the base curve described on P. 36. Observe whether the lens is centering well. If the lens rides high, choose a lens that's 0.50D steeper and/or 0.4mm larger. If no trial lens is available, order a 9.8mm/3.4mm lens and evaluate as follows.

Cover the patient's eye that you're not testing. Perform an over-refraction using spectacle trial lenses, not a phoropter. Test the patient's monocular visual acuity. Make sure the distance visual acuity is equivalent to that obtained with single-vision lenses. If not, the patient may experience poor lens centering or end up with a distance zone that's too small. To avoid this, use a larger lens and/or a larger central distance zone.

Give the patient something to read and have him cover one eye. Make sure he holds his head in the straight-ahead position while observing at near. This is exactly what a patient does with spectacle lenses and is just as easy to learn with contact lenses. Let the patient hold the reading material while you hold the spectacle trial lens with the distance over-refraction in front of his eye. The patient can then move the reading material in and out to find the position of best vision. At this point, if you need to, you can determine any adjustments for the final add.

If the patient has good distance vision but poor near vision, reduce the distance zone size to 3.0mm for the final lens order. Before doing this, check to make sure that the lens is shifting. If not, the patient simply may not be holding his head up during near vision. Also be alert if the patient seems to require an unusually high add, which is another sign that the lens isn't shifting. This is the step that most fitters fail to emphasize to the patient. That's why you should demonstrate to the patient several times during the dispensing visit. Tell him, "If you have trouble seeing the reading material, just raise your chin."

If the patient has good near vision but poor distance vision, check to see if the lens is centered. If the lens is decentered (usually high) then you should order a larger and/or steeper lens diameter (10.2mm/3.4mm). If the lens is centered, then you need to use a larger distance zone (9.8mm/3.8mm).

Figure 5. Number of lenses used per successful and unsuccessful patients.

Success Rate

One of the most difficult things to determine for a new contact lens is its rate of success. Frequently in the past, an elite group of investigators are given the lens and are monitored closely while they fit about 20 patients each. This almost always produces a high success rate because the patients are so carefully screened and receive so much attention and training during the fitting. After these trials, when average practitioners fit the same lens, they usually find that the success rate is only a fraction of that claimed by the manufacturer.

Con-Cise Contact Lens Company tested the success rate of the Seamless Bifocal contact lens by analyzing the 300 most recent contact lens orders from regular customers. Practitioners ordered the lenses by the "case method," which allows three pairs of lenses on a money-back basis. Success was measured by the number of completed, unreturned lens fittings that a practitioner performed, compared to the total number of patients for which there was an original order. Using this criterion, it was found that overall, 66 percent of 100 randomly selected patients from the group were successful in keeping their contact lenses

The company made a further analysis of the results to determine the number of lenses that practitioners used to fit each patient in the success and failure groups. Figure 5 shows the number of lenses that the fitter used either to successfully fit the patient, or before the fitter concluded the patient was unsuccessful. Note that while there were about twice as many successful patients as unsuccessful patients, the number of lenses used per patient was nearly the same. In each case, most patients were fitted or rejected based on one or two pairs of lenses. It may be concluded that the chair time involved in fitting the Seamless Bifocal is only slightly greater than that needed for single vision lenses.

The single most common mistake found in fitting the Seamless Bifocal was using a distance optical zone that was too small. However, nearly every patient was extremely happy with his near vision. The next most common fitting mistake was not teaching the patient to hold his head up when reading, the same way spectacle bifocal patients hold their heads. This was especially important in converting patients from using aspheric bifocal contact lenses to the Seamless Bifocal contact lens.

We may conclude that the Seamless Bifocal contact lens is easy to fit, can provide optimal vision for both distance and near and has a high rate of patient success.

Dr. Mandell is Professor Emeritus of the School of Optometry, Univ. of California, Berkeley. He is the author of Contact Lens Practice and over 150 scientific publications. He is currently Director of Research for Con-cise Contact Lens Company.

 


Contact Lens Spectrum, Issue: October 2002