A Blueprint for Multifocal & Bifocal Contact Lenses

Multifocal lenses, unlike spherical and toric lenses, are the last lens modality that has not found a single lens configuration for absolute patient success. Research and new diagnostic technology have broadened our understanding of the requirements necessary to increase multifocal success. Pupil size, line of sight, and aberrometry are some of the buzzwords that fabricators must incorporate into this new wave of multifocal lens options.1 Identifying the unique patient anatomical configurations and integrating these parameters into a multifocal lens will solve the visual complaints that have resulted in contact lens dropouts and limiting the growth of the contact lens industry.

Historically, contact lenses have shown that new lens designs are first lathe-fabricated until refinement succeeds and disposability can be offered. Customization of multifocal lenses is now possible, with multiple lens fabricators offering unlimited parameter options to meet the configurations demands of patients. Unlimited diameter size, base curve options, spherical and cylinder power selections, zone sizes choices, near-center or distance-center configurations, annular bifocal or progressive multifocal options, and decentered optics in prolate or oblate lens designs are customized to meet the needs of our patients. This article will provide a step-by-step blueprint for configuring a multifocal lens design to drive successful outcomes for your patients.

Prescribing Custom Contact Lenses

There is significantly more responsibility prescribing custom multifocal/bifocal contact lenses versus pre-designed lenses. There are minimal changes available using off-the-shelf lenses. Typically, trial-and-error changes in powers are used to increase patient satisfaction. Using the most accurate and precise vertexed sphere cylinder and axis powers are the basics of customized lens fabrication. Additionally, multifocal lens designs altering the optic zone configuration to enhance the combination of distance and near vision success add to the customization formula to create patient acceptance.

Using predesigned multifocal lenses, typically the spherical equivalent is used for patients with astigmatism rather than starting with the best sphero-cylinder power. Custom lenses allow the prescriber to use the full sphere and full cylinder power, resulting in a greater ability to attain precise functional vision.

Prescribing multifocal/bifocal custom lenses should be tackled systematically. Listen closely to the patient’s comments on the advantages, disadvantages, and visual outcomes of the lens design to modify the lens configuration. The prescriber must take a scientific approach to consider the results and, when indicated, alter the multifocal/bifocal lens design. We will now explore those methods in a systematic step-by-step approach to maximize success.

Patient Expectations

First, prescribers must interview patients and understand their needs and expectations. Patients may say they spend a significant amount of time on a computer, but the prescriber must dig deeper. Are patients on and off a single monitor all day, addressing intermediate and near tasks, and moving around a large office setting or are they stuck in a small cubicle, surrounded by three large monitors, entering critical information for 8 to 10 hours per day, and then going home to spend time on digital devices? If prescribers don’t understand exactly what patients expect, success is achieved by luck rather than design. Spend time interviewing patients to determine what they’re hoping to achieve. Make sure patients aren’t expecting a Fountain of Youth; in other words, hoping to achieve the type of vision they had in their 30s.

Second, evaluate previous experiences. Patients who have been unsuccessful with multifocal lenses in the past may achieve success more easily than patients who have never worn contact lenses. If attempts were unsuccessful in the past, find out what was attempted and consider why those designs didn’t work. Explain why you feel custom lenses have a greater likelihood of success and explain the alterations in design that you’re initiating.

If a patient has never worn contact lenses before, critically evaluate the patient. Consider performing retinoscopy rather than relying on auto-refraction to uncover latent hyperopia — even consider cycloplegic retinoscopy and refraction. Auto-refractors tend to allow the patient that has been accommodating to remain accommodating. Early presbyopes may be prescribed their maximum plus with a low progressive, aspheric add and, after an adaptation period, may see clearly at distance and near.

Other unique prescriptions should be addressed initially to alter unrealistic expectations. An example is the -2.00 myope who only needs glasses for distance and removes them for reading. They know they’ll need contact lenses for distance but often assume they’ll be able to read with the lenses on. Explain to the patient why this isn’t possible and set realistic expectations. The same thinking can occur with an emmetropic patient who uses readers. Again, education about how the aging eye functions will save time and frustration and increase your chances for success.

A unique optical correction and challenge are patients with plano sphere and two diopters of cylinder. These patients function at both distance and near, tolerating the ghost or 3-D image created by the astigmatism. These patients have two focal points. The plano portion of the prescription can be used to focus on distant objects, while patients ignore or suppress the other image. When patients read, they use the magnification from the -2.00 diopter and ignore the ghost or out-of-focus image created by the plano part of the prescription. The debate is whether a simultaneous optic multifocal or bifocal can improve upon this naturally occurring optical system.

There are ramifications when using the spherical equivalent power of a prescription rather than the true prescription in designing multifocal contact lenses. In a spectacle prescription, even a half a diopter of astigmatism is corrected to achieve the best acuity. If the spherical equivalent is used in a multifocal contact lens prescription, the patient is immediately required to accept the blur created by this designed “off-prescription.” Custom-designed soft multifocal contact lenses allow the full prescription to achieve the best visual results. If clear vision is not achieved, then the design should be altered, rather than adjusting the power. If better distance vision is needed, increase the distance zone(s). If better near vision is desired, increase the near zone(s). The same can be done for the intermediate reading zone area.

Lens Design

Using pre-designed lenses, there is minimal reason to spend time measuring the patient optical and physical characteristics. These lens designs typically only come in one base curve and one diameter, so the prescriber can ignore corneal curvature and orbital size. The only option is to evaluate the power and use the lens design that makes the most sense for that patient. When designing custom lenses, more information is needed. Custom lens manufacturers spend time deciding how the lenses should be designed and what information is critical for success. Nomograms have been developed to assist in selecting the initial lens design. In 2006, Becherer developed the Arc Length Design method of prescribing (Figure 1).2,3 Using the radius of curvature of the cornea and the size of the cornea as measured with HVID (Horizontal Visible Iris Diameter) and simple trigonometry, a lens could be designed that essentially fits the same no matter how steep, flat, large, or small the cornea was for any particular patient.4 Hundreds of lenses were analyzed to achieve a relationship of certain materials to the curve of the eye and also how much larger than the cornea lenses should be to achieve routine success. The Arc Length of the cornea is calculated using a computer calculator (fill in the boxes) or a basic spreadsheet. The resulting lens will be slightly flatter than the flattest K (+0.3 mm, +0.4 mm, +0.5 mm depending on the rigidity of the material) and 5.0 mm larger than the arc length (or 2.5 mm. beyond the limbus). If more stability is needed, larger lenses can be designed and prescribers need to ensure increased torqueing on astigmatic lenses doesn’t occur as well as unwanted decentration using multifocal or bifocal lenses.

Figure 1a. Arc Length calculator

Figure 1b. Arc Length Spreadsheet

The custom designed lens has a B.C. of 7.8 and diameter of 14.4 in this case.

If the optics of any lens remain consistent, simply perform an over-keratometry and evaluate the mires. A lens properly aligned to the cornea yields consistent optics and a clear mire image before, during, and after a blink. If the lens curvature doesn’t align to the corneal curvature, there will be distortion of the mires, which translates to variable vision. The patient may see well momentarily, but then the quality of vision fades. A properly aligned lens eliminates the variable vision.

The mires can also be used to evaluate how quickly the lens is drying out on the eye. A lens needs to remain consistently moist between blinks or image quality will vary.

Using the Arc Length Design, the lens should center properly. If adequate vision is not achieved at distance or near, then an alteration or change in design is warranted. Using custom-designed multifocal lenses, the prescriber is attempting to enhance the distance vision with one eye and reading with the other eye. This modified monovision is better than traditional monovision because the distance eye still has some reading capabilities and the near eye has some distance vision. This creates a more visually synergistic effect, which is better than the optical competition between two eyes that comes with monovision. However, if the distance-designed eye functions well at reading or the reading eye functions well at distance, then an option is to use that lens design for both eyes. However, pay attention to the illumination. The advantage of one design over another may only occur in certain lighting situations. Always check the quality of vision in different lighting conditions to mimic real life. Using custom contact lenses allows endless options to improve functional vision.

Line-of-Sight Alignment

If a well-centered and designed lens doesn’t result in adequate vision, consider how the patient’s visual axis may affect the resultant vision. If a patient reports that turning his head at a certain angle increases vision when viewing optically “off angle,” then the Angle kappa (the angle formed between the pupillary axis and the visual axis) is causing a problem. If the lens has the near optic in the center of the lens and the patient turns to align the peripheral distance power, the patient is not viewing through the appropriate zone. However, if the head posturing doesn’t make sense, measurement of misalignment of the visual axis and pupillary axis is indicated. This can be most easily visualized and measured by performing corneal topography over the lens of interest.5,6 The measurement is made by comparing the patient’s baseline corneal topography to the image taken over the lens of interest and comparing them in the difference or subtractive display mode (Figure 2). Once the measurement is made, a new lens can be ordered displacing the center of the optics over the visual axis.7,8

Figure 2. Shown here is a soft multifocal: 8.7/14.4 -3.00 (+2.50 Add) N

Common Sense Approach

If the multifocal/bifocal design is successful but the patient needs a visual enhancement for certain conditions, consider all options. In a multifocal/bifocal lens design where distance, intermediate, and near zones are present, increasing or decreasing the amount of area devoted to a certain activity improves the overall function for the patient.9,10 For example, the patient is satisfied with the overall vision, but wishes the distance was slightly better at night, increase the distance area minimally and evaluate results. This can be accomplished in one lens or both depending on the prescriber’s objectives. Increasing a distance-center zone by 0.4 mm adds 2 square mm of distance area; therefore, simply increasing the distance area from 3.0 square mm to 3.4 square mm increases the distance by 25%. Small changes have large impacts on results. If the patient complains of inadequate intermediate vision, reducing the zone size of either the distant area or near area or both will increase the intermediate area. Pupil size in photopic and mesopic conditions is critical in selecting the appropriate zone sizes. Aberrometry can measure very accurately these different lighting conditions using different illuminations within the instrument.11 Knowing patient pupil size measurements allows the prescriber to design a multifocal/bifocal lens design according to the patient’s anatomy. Under photopic lighting conditions, a patient with a 2.0-mm pupil will see very differently than a patient with a 4.0-mm pupil in a near-center lens design that has a 2.0-mm central zone. The 2.0-mm pupil size will never be able to read because the pupil size doesn’t allow vision through the near zone. Customizing zone sizes is the one method to attain successful multifocal/bifocal designs for all patients.

Charts have been developed to assist the prescriber in choosing the proper design and zone sizes (Table 1). These designs are a starting point that can be refined to meet patient needs.

Table 1. Multifocal Near Center Zone
Photopic Pupil Size 3.0mm 3.5mm 4.0mm 4.5mm 5.0mm 5.5mm 6.0mm
Multifocal zones 1.8/2.5 1.8/3.0 2.0/3.5 2.0/4.0 2.3/4.5 2.5/5.0 2.8/5.5

Multifocal Versus Bifocal

Custom designing lenses allows the prescriber to choose between a multifocal design and a bifocal design.12 The size of the pupil and active accommodation may warrant one design over another, as well as tasks performed with the lenses. A young presbyope may perform well with either design. A young presbyope will have an active amount of accommodation; therefore, if a bifocal is prescribed, the range of focus will come from the patient’s own accommodative ability. A multifocal design will have a built-in range of focus due to the aspheric transition between powers built within the design and, as the young presbyope will typically have larger pupils, the different areas of powers should be attainable.

A more mature presbyope may have little, if any, accommodative ability and might benefit from the aspheric multifocal design as long as the pupils are large enough to allow usage of the different power areas. However, if the pupils are small, less than 3.0 mm, a bifocal may be indicated so that both the distance and near areas can be crowded into a small usable area within their pupil.

Small pupils, less than 2.5 mm, may need a distance center in one eye and a near center in the other eye, so under extreme conditions, the lenses could offer a monovision type arrangement. If it is bright, the pupils would constrict and the patient could use one eye for distance and one for reading.

Custom Multifocal Versus Monovision

For many prescribers, monovision is the lens of choice, and they don’t want to spend time refining multifocal or bifocal lenses. For those practitioners, consider a modified monovision approach. Take the distance eye and prescribe the full distance power with a multifocal/bifocal lens.13 A simulator has been developed to help determine what zones would offer clear distance vision plus good intermediate vision and some near vision. The lens for the other eye design should offer clear near vision plus good intermediate vision and some distance vision. The result is functional distance vision and functional near vision similar to monovison, but an enhanced intermediate vision. In addition to the clearer intermediate vision, there is less competition between the eyes. The distance eye is somewhat blurred at near point, but not as blurred as with monovision. The near eye is also somewhat blurred at distance, but less than monovision. The advantage to custom lenses is being able to correct fully for astigmatism, while designing a simultaneous progressive aspheric multifocal.

A Customized Approach

Custom lenses allow for the prescriber to design lenses that align to the shape, size, and the line of sight of the eye, and multiple pupil configurations. Full powers can be prescribed without settling for available powers, which, ultimately, decrease the opportunity for success. Custom lenses allow minor adjustments, improving results not available in pre-designed lenses. No matter what the power, shape, or size needed, the patient can be prescribed custom lenses as they mature into presbyopia. The design can change over time, keeping a successful contact lens patient happy throughout their stages of life. Successful custom design lenses avoid contact lens dropouts and provide an opportunity for referrals to our practices. ■


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