Article Date: 11/1/2005

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multifocal fitting
An "Essential" Approach to Multifocal Fitting
Two practitioners offers guidelines for how to succeed with fitting a particular family of multifocal lenses.
By Robert L. Davis, OD, FAAO, and Brad Cogswell, OD

The affirmation that "one bifocal/multifocal design cannot fit every multifocal candidate" has appeared in almost every article related to multifocal fitting over the last 30 years. However, I believe the Essential family of multifocal contact lens designs from Blanchard Contact Lens, Inc. has come closer than any lens design to dispelling this notion.

The basic lens design mixes back aspheric technology and annular configurations with a combination of many past multifocal philosophies. The Essential family offers the choice of the Essential GP multifocal lens and Essential Xtra, with the option of adding a front-surface annular component called a concentric S-form add (CSA) enhancement to either design.

To describe and understand the fitting approach is no easy task. First we'll describe the basic fitting design. Then we'll look at the construction differences between the Essential and Essential Xtra multifocal designs to appreciate the different patient opportunities for each lens. Finally, a description of the Essential CSA and Essential Xtra CSA lens designs will add to your knowledge and provide an additional method for satisfying patients' visual expectations and meeting their optical requirements.

General Design and Fitting

The Essential GP Multifocal can be a translating or simultaneous back-aspheric lens with low eccentricity to help prevent corneal molding. "S-Form Technology," a proprietary manufacturing process, produces a smooth flattening effect, creating a greater posterior power gradient and a greater near point power effect. This differs from other aspheric lens designs, which incorporate many curves cut into the lens to create the aspheric affect. Such lenses look like a notched Fresnel prism under magnification and cause additional distortion and contrast degradation as light refracts through the lens construction.

Determining Lens Diameter Fit the Essential lens designs using an alignment fitting philosophy. The total lens diameter selection follows the small/steep and large/flat paradigm. For lenses with base curves of 7.3mm and steeper, select a 9.0mm diameter. The 9.5mm diameter lens is preferred for patients who need a base curve of 7.4mm and flatter. Select a larger lens diameter for better centration or to increase sagittal depth.

Base Curve Selection Configure the initial base curve by the amount of corneal cylinder and the flat corneal meridian. Fit the lens 0.50D steeper than flat K for patients who have less than –0.75D of corneal cylinder. For patients who have –0.75D to –1.75D of corneal cylinder, fit the lens 1.00D steeper than flat K. For patients who have –1.87D or greater of corneal cylinder, make the initial lens 1.50D steeper than the flat meridian.

Troubleshooting Potential problems with this lens design directly result from the lens not positioning appropriately. A high-riding lens provides good near vision, but may be over-minused in its distance prescription. To rectify this problem and position the lens correctly, use a steeper base curve or larger diameter. A lens that rides low will create good distance vision but poor near vision. To solve this lens-positioning problem, you must change the fitting parameters. The lens will position more superior if you flatten the base curve using a minus carrier or increase the overall size of the lens.

A lens that decenters nasally will generate poor distance and near vision. Increasing the lens diameter and/or adding a lens carrier to the design will change the lens configuration to facilitate proper positioning. Let the lens position dictate the fitting parameters of the lens.

Fitting Approaches You can fit the Essential and Essential Xtra lenses with either a lid-attachment fitting approach, providing a translation style by fitting slightly flatter (manufacturer recommended), or an intrapalpebral design philosophy, providing a simultaneous methodology by fitting slightly steeper. The lid-attached fitting style will tuck underneath the upper lid and move with the blink. The intrapalpebral fitting design will center perfectly and translate very little with the blink.

Incorporating Adds The Essential and Essential Xtra lenses are available in three Series (I, II and III) of aspheric lens designs, each having a greater amount of add correction contained within a usable optic zone on the posterior surface. Each Essential/Essential Xtra lens series has a lower sagittal depth or greater eccentricity, which in turn will create a higher multifocal add effect. Use Series I for early to intermediate presbyopes to incorporate up to a +1.25D aspheric power gradient for near and intermediate vision correction. Series I features the largest portion of the optic zone intended primarily for distance vision.

The Series II add zone begins closer to the center of the lens, creating more add power as a result of increased eccentricity. The Series II power gradient generates +1.50D to +2.00D for near and intermediate vision correction.

The Series III add zone begins even closer to the center of the lens and has the greatest amount of add correction, providing +2.25D to +2.50D of add power. It has the smallest amount of distance area within the optic zone intended for distance vision. The spherical central curve provides the distance correction. As the add increases, the near portion of the lens encroaches further into the central distance area. This enables a more precise near acuity, but may adversely affect distance vision.

CSA Enhancement When a patient needs additional add power because of pupil size, near power requirement or lens positioning, you can include a CSA design onto the Essential or Essential Xtra lens design. The Essential CSA design cuts a spherical annular near power curve on the anterior surface of the lens (Figure 1) outside of the distance/intermediate optics generated on the posterior surface. It provides additional add power in +0.50D increments up to +2.00D, raising the combined posterior and anterior add power to +4.00D. You can order the diameter of the annular near power curve in 0.3mm increments depending on the patient's ocular and visual requirements. The standard distance/intermediate zone diameter of the annular near power curve is 4.3mm, but you can reduce it to 4.0mm if the patient doesn't translate into the supplemental annular add zone or increase it to 4.6mm if the standard annular add zone encroaches upon distance or intermediate vision.

Lens Design Specifics

Essential Now that you know the basics, we'll differentiate between the different lens designs. The original Essential GP Aspheric Multifocal consists of three Series of individual lens designs that differ in eccentricity and resulting distance optic zone areas. As the Series increases, the multifocal addition increases and the useable distance area on the lens becomes smaller (Table 1). The estimated distance zone size for Series I is 2.5mm, for Series II 2.0mm and for Series III 1.7mm (according to the manufacturer), assuming that approximately +0.25D doesn't affect the distance zone in this aspheric configuration. You can modify the quality of distance acuity by selecting the Series that provides optimal near vision. You need to find a balance between the distance and near zones within the optic zone to achieve an optimal fit and symptom-free patients.

Essential Xtra The Essential Xtra GP Aspheric Multifocal performs the same as the original Essential design by changing the eccentricity of the contact lens in relationship to the cornea. The increased rate of flattening of the contact lens relative to the corneal curvature results in the "add" power. Therefore, by increasing the eccentricity of the lens, you can increase the "add" power. This is how the three Series are differentiated in both the Essential and Essential Xtra lens designs. The result is more "add" power closer to the center of the lens in Series III compared to II and in Series II compared to Series I. The eccentricity of the posterior aspheric power gradient creates the configuration differences between the Essential and the Essential Xtra. The Xtra's posterior power curve begins farther away from the center of the lens and changes more rapidly compared with the Essential's design. This results in a larger area in the center of the lens for distance vision that is unhindered by add power. The more rapid change in the aspheric geometry allows the Xtra to develop enough "add" power within a smaller radius of the lens (Table 2, estimated values according to the manufacturer). The Essential Xtra lens has greater eccentricity with a larger distance optical zone to provide better distance vision with less distortion from the posterior aspheric curve as well as more power in the near zone for Series III. It's constructed with less intermediate area because of the rapid power delivered by the aspheric curve, although this doesn't subjectively compromise intermediate vision.
 

Fitting Guidelines

The Essential lens design has many parameter choices, so taking the first step may feel overwhelming. Understanding the lens design differences is key in intuitively manipulating parameters according to patients' visual requirements. Once you understand the differences among the Essential family of lenses and appreciate their nuances, choosing lens parameters becomes a simple task.

Pupil size and visual demands are factors that you must consider. Regarding pupil size, patients who have larger pupils will generally do better with the Essential Xtra because the posterior aspheric power curve begins further from the center of the lens. Patients who have large pupils may also do better with a lower Series lens as the near power starts further away from the center of the lens. For example, a Series I lens has the same near power at 3mm from the center as a Series III has at 2mm from the center. The Series I would ultimately limit the total near power available, but incorporating a CSA design can increase the near power to the desired amount.

You need to explore visual demands with each patient to determine which lens to select first. It's important to reveal what the patient does most during the day (computer use, sewing, sports, etc.) so you can give priority to these activities from a visual standpoint. A patient may tell you that she sews, but her real concern is being able to see the flight of a golf ball. This information may lead you to the optimal lens design and Series to select.

The Essential Xtra might be the logical choice for a patient whose priority is distance vision. Its larger central distance area provides a less interrupted view from the near power. You could use a Series I lens in an early presbyope or in someone who has few near demands. A patient who has more near demands would benefit from a Series II lens design.

Figure 1. The CSA design provides up to +2.00D more add power when needed.

If a patient's visual priority is with intermediate vision, then the Essential would theoretically be the lens of choice because its more gradual eccentricity results in a larger intermediate area. The most advantageous Series for patients who have high intermediate demands depends on the stage of presbyopia and other visual tasks that they regularly perform.

In patients who have high near demands, the pupil size may direct your choice. Even though the near power begins further away from the center of the lens in the Xtra design, its near vision can be equal to or greater than the Essential because of the steeper eccentricity. But, the near power in the Xtra may not be as effective in patients who have small pupils or if the lens isn't translating properly. These patients may be unable to utilize the near power effectively. Consider a Series III for most of these patients unless they're early presbyopes or their distance vision is overly compromised; in such cases use Series II Essential or Essential Xtra CSA.

Consider a CSA enhancement to either design for patients who don't obtain adequate near vision from just the posterior aspheric curvature. This typically occurs because of small pupils or poor lens translation. By placing add power on the front surface of the lens, you can alleviate some of the limitations in the aspheric lens design. Some patients cognitively prefer an annular design modality. A Series I Essential or Essential Xtra CSA lens design will solve these problematic lens adaptation issues.

The Essential Multifocal family of lenses offers a complete distance and near range of powers to satisfy all visual acuity requirements. Manipulating the Essential parameter options requires a learning curve to satisfy the majority of presbyopic visual requirements. Understanding the parameter alternatives available with the Essential Multifocal will enable fitting changes during follow-up appointments to meet patients' expectations. This along with listening to and understanding patients' needs will facilitate successful presbyopic patient lens dispensing.

Dr. Davis has an eyecare specialty practice outside Chicago. He is a diplomate of the Cornea and Contact Lens Section of the AAO and a past chair of the Cornea and Contact Lens Section of the AOA. He is also an inductee to the National Academy Practice in Optometry.
 

Dr. Cogswell is in practice in Oak Lawn, IL concentrating on problematic contact lens cases. He has been involved with the research and development side of vision care and has limited his interest to specialty contact lens
designs.

 

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Contact Lens Spectrum, Issue: November 2005

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