Advanced Multifocal Fitting and Management

David W. Hansen, O.D., F.A.A.O.
AUGUST 1999

Find out how occupations, avocations and motivation levels can help you succeed with presbyopes.

New developments in RGP and hydrophilic designs and materials now enable contact lens practitioners to offer patients better options for success while preserving acuity and stereopsis; benefits that cannot be obtained with the most popular presbyopic contact lens option, monovision.

Practitioners should be prepared to take advantage of these improvements in presbyopic designs. Although marketing the new technology for the presbyopic contact lens wearers may pose a challenge, it is important because of the huge volume of new presbyopes.

Selecting a Bifocal Contact Lens Patient

Selecting the best bifocal design for each patient is critical for success. When selecting candidates for a bifocal lens, four major considerations must be addressed: the level of motivation, occupation or avocation, ocular characteristics and the refraction.

Motivation -- Motivation is the key to success for a bifocal fit. Patients who have been rigid contact lens wearers are slightly better candidates than previous soft contact lens wearers. Those with no previous contact lens experience, or those who have never worn spectacles have a reduced prognosis for success. In these latter categories, a patient's positive outlook can overcome the lack of contact lens or eyewear experience.

Occupations and Avocations -- Investigating the occupation and avocations of a patient will assist in determining the best type of bifocal or multifocal for that individual. The amount of time spent in front of the computer should be carefully assessed. If a patient's computer time exceeds 35 percent of the day, a simultaneous or aspheric design is preferable. Such a design permits a smoother intermediate translation from other work tasks to the computer.

Patients in certain occupations, such as accounting, electrical, mechanical and plumbing, require an "arm's length" vision range. These patients are excellent candidates for aspheric or simultaneous image bifocals because of the intermediate vision they offer. Other patients who have near-to-far demands, such as farmers, teachers and truck drivers, may benefit from a translating design. In each case, discussing a patient's occupational visual requirements is essential to accurately determine the optimum bifocal design.

Ocular characteristics -- Accurate measurement of the patient's ocular characteristics enables the clinician to efficiently determine the most suitable bifocal option. Measurements must include the size and shape of the cornea, eye and adnexa, with emphasis on eyelid positioning and tonicity, and the pupil size (determined under both mesopic and scotopic conditions). A diagram of the eyelid positioning may help to determine these critical measurements. If the lower lid is below the lower limbus, a translating design is usually not recommended.

Corneal measurements using corneal topography are helpful in predicting the positioning of the diagnostic lens. Accurate measurements provide a higher probability of successfully matching a patient's occupational needs and ocular physical characteristics with the appropriate bifocal design. The apex of the cornea should be close to the center of the pupil for centering a simultaneous or aspheric contact lens.

Evaluation of the pupil is an integral part of the initial assessment. Miotic pupils will aid the simultaneous process, making aspheric lenses the design of choice. Patients with large pupils (greater than 4.5mm) under mesopic conditions may experience difficulty with most bifocal designs because of flare or ghost images. Persons who have a poor blink are poor candidates for any RGP lens, especially a translating bifocal lens. The tear meniscus must also be evaluated and tests for tear break-up time (TBUT) and tear quality are recommended. A reduced tear TBUT below 10 seconds usually indicates that a patient will not be able to wear contact lenses on an all-day basis.

You should examine the corneae, conjunctivae and the limbal junctions, especially pingueculae, corneal dellen, peripheral corneal hypertrophy, neovascularization, vascular limbal keratitis and indications of dry eyes, before attempting a trial fit. The probability of obtaining a successful result is decreased when these conditions go unnoticed.

Refraction -- Careful refraction is also important. In general, patients with a low ametropia are not good bifocal candidates, although a high motivation level can overcome this situation for certain patients.

Fitting RGP Bifocal Contact Lenses

Once the preselection criteria have been met and the appropriate observations have been obtained, the diagnostic evaluation can begin. I suggest using diagnostic contact lenses for all types of bifocal and multifocal designs, including RGP and hydrophilic materials. One potential problem that may be encountered, however, is if the patient is currently wearing PMMA materials. In this case, RGP single-vision contact lenses should be fit before proceeding with the bifocal diagnostic exam because it may take 2-3 months for the corneal and refractive findings to stabilize after the previous molding effect of the PMMA material.

Types of RGP bifocal designs

There are two basic types of bifocal designs: alternating (translating) and simultaneous image. RGP materials are available in both designs, but most soft lenses utilize the simultaneous image because they are not fit to move excessively. However, all bifocal and multifocal contact lens designs need to move in order to be effective and to allow for sharp acuity.

Translating Lens Designs

There are five types of RGP translating designs: 1) one-piece crescent; 2) Tangent Streak; 3) decentered de Carle (Fig. 9); 4) fused crescent; and 5) Solitaire.

Most translating designs are fit with similar methods, including vertical movement of the lens so that each area of prescription will be positioned in front of the pupil for the appropriate viewing distance.

Fitting translating bifocal lenses -- It's important to know each lens design and where to position the height of the bifocal segment. Crescent lenses are fit with the bifocal segment near the lower pupillary margin or 1mm below the pupil edge under mesopic conditions. The Tangent Streak translating design is fit with the bifocal segment into the pupillary zone .3mm (Fig. 1). These are just two examples of the specificity of these designs. Most designs utilize a prism ballasting effect to help stabilize the lens and prevent unnecessary rotation.

Fitting translating bifocals requires knowledge of fitting single-vision contact lenses (Fig. 5). The same concept is used when selecting the base curve to provide a good lens-to-cornea relationship and fit on "K" or taking into consideration the corneal astigmatism. The inferior eyelid acts as a support system to move the lens vertically, allowing the patient to utilize the bifocal segment.

Troubleshooting translating bifocals -- Even though each design has its own idiosyncrasies, the following guidelines will assist you in managing these problems:

* Visual acuity -- Compare the overrefraction findings with the expected lens calculations. Hyperopic corrections will require relatively less prism than myopic prescriptions.

* Lens position -- If the distance segment is too high and the contact lens is resting on the lower lid, then truncate the lens to lower the distance segment. If the contact lens is not resting on the lower lid and is being captured by the upper lid, then steepen the lens base curve, increase the prism or reduce the lens diameter. If the distance segment is too low, then decrease the prism or order a non-truncated diameter. If the lens lags, then steepen the lens (usually by 0.50D) or increase the diameter. If the lens rides high, then decrease the diameter or steepen the base curve (usually by 0.50D). If the lens slips beneath the lower lid, then use a thicker lens with more prism or redesign the fit (Fig. 3).

* Lens rotation -- If the lens rotates nasally, then flatten the base curve (usually by 0.50D). If the lens rotates temporally, then steepen the base curve (usually by 0.50D). The Fluoroperm ST bifocal (Paragon Vision Sciences) usually does not rotate excessively and the prism is not offset like most prism ballast lenses. The prism is usually positioned on the 90 degree vertical orientation (Fig. 7).

Simultaneous Image RGP Designs

There are three basic RGP simultaneous designs: 1) concentric (annular or target design) 2) de Carle design 3) aspheric multifocal.

The first two options are nonaspheric and have no prismatic ballasting system for orientation. The aspheric design can be manufactured with a single posterior aspheric base curve with a spherical front surface, multiple back aspheric curves and a spherical front power cut or with a single posterior aspheric radius with an aspheric anterior surface. Each of these designs offers different options.

Fitting simultaneous RGP bifocal & multifocal lenses -- The concentric annular, or target, and the de Carle design are fit with a parallel lens-cornea base curve relationship that is similar to single-vision RGP lenses (Fig. 2). The "fitting curve" of the de Carle design is the peripheral most curve, and the "bifocal zone" is proportionately steeper to accommodate the difference in distance and in bifocal power. The fitting curve is calculated to match the topographical shape of the cornea. Most of these lenses are fit on "K" or slightly steeper than "K" like single-vision lenses to compensate for corneal astigmatism (Fig. 4).

Aspheric multifocal options require a well-centered design to produce a "simultaneous image" effect, but flare and glare are always a potential problem. Most aspheric designs have a high-eccentricity posterior and are fit according to the manufacturer's guidelines (Fig. 8). Some are fit close to on "K," while others are fit 1.50D -6.00D "steeper than K" to produce the graduated intermediate and near vision zones. The higher the eccentricity value, the more variable power range effect is generated on the back and sometimes the front surfaces. The only aspheric lens which is fit "flatter than K" is the Lifestyle RGP design. This multi-aspheric design uses a special nomenclature to effectively fit an average of 0.1mm flatter than the flattest curvature measurement, allowing for a superior lens positioning (Korb fit) (Fig. 6).

Troubleshooting simultaneous image bifocals and multifocals -- The following helpful hints will assist you when modifications are required:

* Visual acuity -- If the near acuity is poor and the contact lens slips under the lower lid, then design the lens to increase the add power (i.e., steepen the base curve or add the plus power to the front surface). If the distance visual acuity is poor and the contact lens is not centering, then modify the lens by adding more minus power, otherwise residual astigmatism in excess of 0.75D will result. The principal reason for unsuccessful aspheric fits is the failure to fit the lens steep enough to obtain maximum lens centration.

* Lens position -- If the lens rides high, then steepen the base curve or add prism, if necessary. If the lens rides low, then use a flatter base curve or use a larger diameter lens. The eyelid configuration or a displaced corneal apex usually creates lateral decentration. Therefore, if the lens rides nasally, then flatten the base curve. If the lens rides temporally, then steepen the base curve (Fig. 10).

Hydrogel Multifocal Contact Lenses

For years, the soft lens industry has been diligently trying to develop multifocal and bifocal designs to provide the crisp visual acuity of RGP lenses and to overcome the lack of lens movement needed to provide the far-to-near translation that is currently available in RGP designs. The perceived comfort of hydrogel materials, combined with their disposability options, provides an expanded array of materials and design modalities.

Simultaneous image designs comprise the majority of soft lens multifocals because it is difficult for them to generate the movement needed for translation and therefore, somewhat limits the astigmatism correction. However, a few laboratories have mastered the concept.

Simultaneous aspheric multifocals have either posterior or anterior surface designs. The back surface aspheric options usually have near reading limitations, but typically provide sharper distance visual acuity, while anterior aspheric lenses feature extended reading clarity, but may compromise distance visual demands.

Simultaneous non-aspheric lenses can be manufactured in a traditional concentric bifocal where the distance prescription is either in the center of the lens or the periphery. Conversely, the near power may be designed for the geometric center or positioned on the outside of the lens.

Fitting soft bifocal contact lenses -- To achieve a successful soft multifocal fit, follow the instructions provided by the individual manufacturers. They usually provide fitting guidelines that follow a nomogram and a flow chart. The companies have utilized practitioners who have completed field studies to aid in developing these fitting procedures.

Troubleshooting soft bifocal contact lenses -- Centration is the key factor to success with any of the hydrogel multifocal designs. The patient must be able to use the advantage of the individual designs for both the distance and near areas. Many patients need to experience different soft contact lens options before deciding which design is best for them. "Modified monovision" may overcome deficiencies of the soft lens modality. By overcorrecting one eye by +0.50D to +0.75D to achieve near prescription, you can turn a marginally satisfied patient into an extremely satisfied patient. Establishing "eye dominance" or "controlling eye preference" is advisable when utilizing modified monovision techniques. If the patient consistently "prefers" a controlling eye for distance, there is a high probability that they will adapt to this option.

A Final Note

Because of new technology, we can now offer an expanded range of prescription options to satisfy more visual demands. Aspheric curves can be adjusted to accommodate for distance, intermediate and near vision. The aspheric modality can be considered a primary lens of choice. Those patients with residual astigmatism or wide ranges of near visual demands may need to have a translating bifocal.

Computer usage is the key factor when deciding on an initial multifocal design. Emerging presbyopes can benefit with aspheric lenses with no additional modification to the front surface and usually do extremely well.

Previously, RGP bifocal and multifocal designs were considered more successful by practitioners (three-to-one), but success rates with new hydrogel designs are improving. Even though presbyopes continue to challenge contact lens practitioners' skills, new manufacturing technology offers new materials and designs which may contribute to achieving success with hydrogel lenses.

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