Article

Segmented, Translating Multifocal Lens Design: Fitting, Evaluation, and Problem-Solving

Segmented, translating lens designs are different from most multifocal gas permeable (GP) lens designs. The optics of these lenses are separated vertically, similar to progressive spectacle lenses, rather than in a concentric design like other multifocal contact lenses. Many translating GP lens designs have lined or blended lens segments dedicated to distance, intermediate, and near working distances, which can be manipulated independently of one another. This unique power layout makes it easy for contact lens practitioners to prescribe precisely based on a patient’s vision demands. Naturally, this precision makes the lens perfect for those patients who demand their very best vision at a number of working distances.

Patient Selection

Selecting the correct patient for a multifocal GP lens design is the first step toward a successful fitting experience. The visual outcomes of all multifocal designs continue to improve as the laboratory manufacturing capabilities expand, but there will always be that group demanding more from their contact lenses. Many of these patients will be great candidates for a translating GP multifocal lens design.

When considering who is a candidate for translating GP multifocal contact lenses, you should select patients who are very motivated to obtain their best vision. Translating lenses will deliver great optics, but these lenses interact with the lower eyelid more than other GP lenses. This will require some level of adaptation for new wearers. Most patients who wear GP lenses will adapt just fine, but they should be educated on what to expect, as it will feel different than a traditional lid-attached GP lens. Another consideration is the patient’s workspace. Translating lenses are a great option for patients who have multiple but consistent working distances, because of our ability to specify lens power in the segments or areas of the lens design. Examples would include a patient who works on a desktop computer and hand-held tablet or uses a sewing machine and watches television. Patients who struggled with other multifocal lens designs due to pupil size might be good candidates for translating GP multifocal lenses. Not only can the power of each lens section be customized, but the height and size of each segment is customizable, as well.

Poor candidates for translating multifocal GP lenses would include patients who seek a quick fitting experience and are not motivated to return for follow-up visits. Patients who aren’t willing to adapt to wearing a new lens design would also be poor candidates. There are also certain aspects of eyelid structure necessary for lens performance. Translating GP multifocal lenses depend on the lower eyelid for stability and translation. If the lower eyelid is very loose, or has poor apposition to the ocular surface, the lens won’t fit properly. The lens will work best when the lower eyelid is positioned at or just below the lower limbus. However, if the lower lid is 1 mm or more below the limbus, it is difficult for the lens to translate; therefore, these patients would be poor candidates.

The fitting of a translating GP multifocal can be rewarding for the practitioner and exciting for the patient. To keep the process on track, patients should be educated as with all other multifocal lens fittings. Remind them that the process can take a few follow-up visits to ensure that the segment powers, heights, and overall lens fit are appropriate. Patients also should be educated that this lens is likely their best opportunity to reduce dependence on reading glasses, but there may be certain tasks that are visually challenging, even with a perfectly prescribed lens.

Lens Selection

There are several translating lens designs. Some have a lined, small bifocal segment, while others have executive-style trifocal optics and even a blended, progressive-style lens design. Selecting a design largely depends on patient expectations and lifestyle. Keep in mind that the more visual demands a patient desires, the more complex the lens will become.

When possible, a diagnostic fitting can provide valuable information and maximize the knowledge gained at the initial patient encounter. When selecting the first lens, begin by using the manufacturer’s fitting guide, and evaluate fluorescein pattern and over-refraction. This will help you gain knowledge about base curve adjustments and if you should expect any over-refractive astigmatism or adjustment in add powers. Placing a lens on eye also can provide information about lid positioning and laxity, patient blinking patterns, lens centration, and movement. These are key factors that can determine the success of a multifocal lens. Diagnostic lenses also can help you determine if zone sizes need to be adjusted based on pupil size in different lighting situations.

In today’s busy practices, diagnostic lens fittings aren’t always possible. Thankfully, many lens manufacturers have empirical fitting formulas to design a lens based on patient parameters. A multifocal lens can be designed with just keratometry and refractive error measurements, but could be further customized with additional measurements — such as lower lid positioning, palpebral fissure width, pupil size, and corneal diameter — if needed.

Lens Evaluation

The first step to lens evaluation is to decide if the patient will need anesthetic eye drops to be comfortable during the evaluation process. Anesthetic eye drops can decrease reflex tearing and light sensitivity, and encourage normal blink patterns in new GP lens wearers, all of which allow for a better evaluation of the lens positioning and fluorescein patterns.

The fluorescein pattern of the lens-to-cornea fitting relationship should exhibit an alignment-to-slightly flat pattern to decrease lens binding and facilitate adequate translation in down gaze. The lens should be centered horizontally on the cornea, resting on the lower eyelid. Next, evaluate the segment or zone size and heights. The intermediate and near segment zones should be in the lower 1/4 to 1/3 of the pupil area, so that any upward movement on blink doesn’t interfere with distance vision. A comfortable lens will have quick, limited movements on blink, with a peripheral design that results in an average edge clearance. Many of these lens designs will be made with prism, to keep the lens in its proper orientation between blinks, and they will also have a truncation or reverse curve to interact with the lower lid during translation. On down gaze, the patient’s line of sight should be looking through the lower half or lower third of the lens (Figure 1).

Figure 1. The position of a properly translating segmented trifocal design on down gaze.
Image courtesy of TruForm Optics

Troubleshooting

Like most custom lens designs, there are some hurdles to clear from time to time. Thankfully, we are able to address most of these troubleshooting aspects with reliable and effective strategies.

When working to improve a patient’s vision quality, we must ensure the patient is looking through the correct area of the lens before we can determine lens power. Evaluate the lens positioning in the slit lamp to ensure it is sitting in the desired position of slightly low, resting on the lower lid. If the lens is moving significantly with blink, the over-refraction should be performed with loose lenses to ensure the patient responds when the lens is in the appropriate position. If the over-refraction must be performed with the phoropter, educate patients on the correct lens position, so they’re aware of when to respond. Once the correct distance power has been determined, evaluate the lens translation, and use loose lenses to adjust the add power, if necessary.

If very poor near vision is noted, it’s likely that the lens isn’t translating appropriately. When evaluating translation, have the patient look down while in the slit lamp, and then lift the upper eyelid to evaluate lens positioning. The upper edge of the lens should be near or crossing the upper limbus. If the lens isn’t translating, it will dive below the lower lid on down gaze. It is important to evaluate the eyelids in their natural state; do not create false lid ‘tightness’ by pulling on the upper lid too much or lifting it up before the patient is in downgaze. There are a few reasons for poor translation. First, the lens may be too tight, either in the base curve or peripheral curve systems. This would limit the interaction with the lower eyelid. The lens may also be rotating, preventing the truncation or reverse curve from interacting with the lower lid. Translation can be improved by flattening the base curve and/or peripheral curves. Increasing the lens mass by increasing lens diameter may also aid in translation as well. To improve the performance of a rotated lens, rule out a flat fitting lens by evaluating the lens-to-cornea fitting relationship, then consider adding some prism to aid in lens orientation.

Patients may also comment on reduced vision quality, even when the lens power is correct. In this case, evaluate the lens in the slit lamp and encourage patients to blink and move their eyes normally. Watch for excessive upward movement of the lens on blink or lens rotation with side to side eye movements. Patients experiencing a high-riding lens will often comment on moments of blurry vision, followed by clear vision. Vision becomes blurry after each blink and clears as the lens settles. Watch for pupil placement within the distance portion of the lens optics. If the intermediate segment is interfering with distance vision, the lens can be adjusted by lowering the segment height, decreasing the lens diameter, or increasing prism to keep the lens in the low position. If the pupil is encroaching the superior edge of the optical zone, it is likely that patients will complain of flare or glare when in dim conditions. Increasing the size of the optical zone or lens diameter can help move the pupil into the center of the distance optics to improve vision quality in dim conditions. Consider flattening the base curve radius if making a significant change to the lens size or optical zone to maintain the corneal relationship. In all of these cases, laboratory consultants are only an email or phone call away and their experience is invaluable when troubleshooting segmented, translating GP lenses. If you can send photos or video — which is simple today with an iPhone adaptor on the slit lamp — consultants can observe the specific problem and likely recommend a change that will result in a more successful fit.

Troubleshooting comfort is similar to other corneal GP lenses. Lens awareness can be improved by steepening the peripheral curves, increasing diameter, and steepening the base curve, if the fit allows. Be cautious not to eliminate lens movement by creating a lens that is too tight. Also, don’t forget to encourage the patient to continue adapting to this new lens design.

Summary

Translating GP multifocal lenses have received a less than optimal reputation for being complex and uncomfortable for far too long. We can now embrace these lenses, thanks to advanced designs that address patient comfort concerns, and really deliver an amazing product to our presbyopic patients. ■