GP TRANSLATING DESIGNS
NEW DESIGNS IN GP TRANSLATING BIFOCALS AND MULTIFOCALS
Translating GP lenses have been transformed into new designs that are successful for a wider range of patients.
ROXANNA T. POTTER, OD
Much of the current discussion in GP lens technology seems focused on advancements in just a few areas, such as scleral lenses or myopia control. It may be surprising to hear that there are also great strides being made in other GP lens modalities, particularly those that are used to treat presbyopia. While perhaps not as exciting or novel, developments in presbyopic lens designs are no less innovative, and they can be greatly appreciated by patients who desire more effective solutions for their everyday visual challenges.
Today’s presbyopic patients not only have difficulty with reading, but they are also struggling to function while using a myriad of digital devices, from computers to phones to tablets and more. These tasks often require both excellent intermediate and near vision as well as the ability to switch between various working distances quickly and accurately. Even non-presbyopes have a need for near or intermediate accommodative relief. Additionally, symptoms of dry eye or allergy seem to be worsening, whether due to environmental conditions or the prevalence of digital device use, which further challenges the contact lens fitting process.
It can also be argued that spectacles are more appealing than ever. There exist virtually endless choices in style and fashion, progressive designs that are extremely advanced and effective, and options that fit into nearly any budget. Contact lenses that provide only mediocre visual acuity and comfort are simply no longer tolerated by patients given the plethora of other options for visual correction. Modern presbyopes are less willing to compromise their vision at any distance in suboptimal designs, and they have visual demands that leave little room for error in contact lens correction.
Fortunately, new solutions for demanding presbyopes who have visual needs at distance, intermediate, and near exist in the form of translating GP bifocals and multifocals. While often considered an older modality, standard translating GP lenses have undergone transformations into more modern designs that allow them to be used on more patients than ever before.
A translating lens is generally expected to provide excellent distance and near optics due to the separation of the distance and near corrective zones. Historically, however, this was only the case in patients who had optimal lid positioning (lower lid at or less than 1mm from the lower limbus). Additionally, intermediate tasks could be quite difficult without the further addition of a trifocal-like segment to the contact lens. This was challenging due to the small area available for this correction and because of image jump between each focal point, not unlike spectacle trifocal lenses. Patient complaints of poor comfort also were commonly reported in these bottom-heavy, flat-fitting, thick GP lenses.
To address these issues, new translating designs are now available from many GP lens manufacturers. While they still offer the excellent distance and near visual acuity that has always set this modality apart, they now also provide better intermediate vision, good comfort, and more flexible patient selection compared to their predecessors.
ADVANCES IN COMFORT
The easiest patient to transition to a translating GP bifocal or multifocal lens is one who is already wearing a spherical or simultaneous multifocal GP lens design; however, any patient open to a reasonable adaptation period could do well. Previously, adaptation to older translating designs challenged even the most experienced GP wearers due to the thick, heavy, and flat-fitting nature of those lenses. Marked edge awareness and bulky shifts with gaze were common patient complaints.
Advances in computer-guided lathe techniques and manufacturing technologies have allowed for improvements in comfort in many respects. Aspheric or transitional back surfaces provide thinner, lighter lenses; minimal edge thickness and more even edge profiles; and a smoother transition from straight ahead to downward gaze. More accurate placement of segment height(s), truncation, and/or beveled inferior edges and prism allows for extreme customization for optimal patient comfort and vision. Moreover, these specifications can be reproduced to replicate a well-fit and comfortable lens for any patient year after year.
A common previous cause of translating lens discomfort was the presence of harsh truncations; but fortunately, many new designs forego truncation altogether. Truncation is less important to overall lens function because of better control of posterior lens and edge performance. When still used, it is often in a modified and improved form. For example, using an inferior reverse curve that allows for lower lid catch on a smooth transitional area can create upward movement on downgaze. Figure 1 shows a lens with a unique inferior reverse curve that is just as functional, but not as harsh, as full truncation. On this patient, the lens exhibits slight nasal rotation and a slightly visible inferior reverse curve, with the posterior surface tucking just under the lower lid while the upper edge of the curve rests atop it.
Figure 1. A lens designed with a unique inferior reverse curve rather than with full truncation.
Other designs use truncation as an option only if the fit does not otherwise allow for optimal lateral stability and prism to keep the lens centered and low. In Figure 2, the patient achieves a good lens position relative to the lower lid but without any type of truncation. In fact, one of the features that is consistent with nearly all of the newer designs is the avoidance of true/full truncation, at least initially; although on many designs, it is still a troubleshooting option to improve translation if necessary.
Figure 2. Good lens position is achieved relative to the lower lid without any type of truncation.
If patients experience initial GP contact lens awareness or discomfort, the exceedingly sharp optics and vision provided by the translating design are often persuasive enough to convince them to commit to the time and wearing schedule required to adapt. In-office use of topical anesthetic may help with initial discomfort and fitting assessment, although the improved comfort of newer designs has reduced the need for this step. In the most difficult cases, prescription nonsteroidal anti-inflammatory drops may be prescribed for the first few days of wear to help reduce any persistent discomfort. This is generally not necessary for anyone other than first-time contact lens wearers who are not accustomed to any type of lens awareness.
ADVANCES IN VISION
Perhaps the most important development in translating GP multifocal technology is the addition of more effective, progressive intermediate power segments. With increasing use of modern digital technology, a patient’s mid-range or intermediate vision is being utilized a great deal and cannot easily be left out of any visual correction. Proving a smooth transition without image jump to intermediate vision can be accomplished using a few different techniques; the challenge is to do so without compromising distance and near optics.
Much like with spectacles, a fully progressive/blended design transitions from distance to near with an intermediate area in between. While not a true “segment,” the size or depth of this area is often customizable to adjust for task-specific requests from the patient. By avoiding distinct junctions between areas of power, patients do not experience any distortion or image jump when multitasking back and forth between distances. The downside of this is again similar to spectacles in that the area available for each distance is limited by the size of the lens and optical zones. It does, however, avoid any visual distortion and aberration created by simultaneous aspheric or concentric multifocal designs. This makes troubleshooting patient vision complaints much easier; if vision is poor, adjustments to fit almost always correct the problem, whereas in simultaneous or monovision modalities, problems with maladaptation or aberration are often difficult to overcome.
To take advantage of stronger add power in this type of progressive design, patients must raise their chin and look in downgaze to allow for full translation and to access the near segment (Figure 3). If the head/eye positioning is done correctly, the optics will be quite clear and distinct between each area of the progressive design. If not, task-specific glasses worn over top of the contact lenses may be necessary. An example of this would be if a patient’s job requires work on a computer screen or intermediate task that is above eye level. If a slight reading prescription is required for this type of situation, it may be beneficial to include some type of high-energy visible (HEV or blue) light protection, which should not interfere with lens wear in the rest of the patient’s daily activities. Alternatively, a modified monovision approach could be pursued by adding a small amount of plus to the distance power of the nondominant eye (Tuong, 2015).
Figure 3. Viewing in downward gaze allows translation for access to the near segment.
A different approach to incorporating more intermediate power is to add an aspheric posterior surface to the translating lens. This is very similar to the non-translating simultaneous multifocals that are quite common in other contact lens modalities, including disposable and frequent replacement soft multifocals, GP multifocals, and even scleral multifocals. As experienced fitters know, these designs alone often work quite well for distance and intermediate, but they can have compromised near optics. Combining this approach with a translating segment design attempts to bring the best of both worlds together: a simultaneous multifocal for clear distance and intermediate vision with an added translating segment to provide better near focus in downgaze.
The addition of aspheric near properties eliminates the concern of available lens space for the intermediate zone. It also allows patients who do not have optimal lid positioning to potentially still benefit from a translating design, as they are less reliant on complete translation. For example, a patient whose lid is farther than 1mm below the limbus or one who has smaller pupils may still appreciate good near vision without full translation of the lens in downgaze due to the supplemental aspheric near power across the posterior surface. Indeed, these lenses may work even with a central or intrapalpebral fit, rather than in the case of inferior lens positioning atop the lower lid or when lid attachment is not present to help raise the lens. This is useful when lower lid awareness is too great or lid attachment is less than ideal in other translating designs.
FITTING NEW TRANSLATING GP DESIGNS
Patient selection for a translating GP bifocal or multifocal is now therefore more inclusive compared to many other modalities. As GP lenses naturally correct mild-to-moderate astigmatism, astigmatic patients desiring something other than monovision are easily persuaded to try a new translating design. Ideal patients for these new translating GP designs are astigmats who have moderate-to-advanced presbyopia, particularly those who have failed in simultaneous designs (Table 1).
|Astigmats who have moderate to advanced presbyopia.|
|Failures in soft, GP, or hybrid simultaneous multifocal designs.|
|Patients who have critical near visual task demands.|
|Failures in soft toric or GP monovision.|
|Those struggling with handling soft multifocals or monovision.|
|Patients who have adequate lid tension and position for at least some translation.|
While at least somewhat counterintuitive at first, especially to seasoned translating GP contact lens fitters, many new designs are fit either “on-K” or slightly steep (with some variations due to the amount of corneal toricity or corneal diameter). Figure 4 shows a somewhat steep but well-fit lens. This lens has supplemental posterior surface asphericity and achieved good fitting properties while exhibiting a slightly steep fluorescein pattern.
Figure 4. A somewhat steep, but well-fit translating GP lens.
Additionally, troubleshooting newer translating designs can be quite different as well. Previously, if a lens was rotating or translating improperly, the solution was nearly always universally the same: flatten the lens or increase prism. Now, it is not unusual to find that steepening the base curve or increasing diameter will improve fitting characteristics. In the photo series of Figure 5, the first lens was fit on-K, but the patient noted excessive inferior lid awareness. Steepening the lens by 0.50D improved patient comfort significantly and retained good fitting properties. Conversely, flattening the lens by 0.50D caused unwanted lateral decentration.
Figure 5. This first lens was fit on-K, but the patient noted excessive inferior lid awareness (A). Steepening the lens by 0.50D improved patient comfort significantly and retained good fitting properties (B). Conversely, flattening the lens by 0.50D caused unwanted lateral decentration (C).
To a novice translating GP fitter, the newer designs are simpler to fit as they are more like standard spherical GP fits compared to the flatter designs of the past. Newer designs tend to be on the larger size, with overall diameters that are often 10.0mm or larger depending on lid fissure and corneal size. Prism is a standard, laboratory-specified amount to start with each design, but it is adjustable in both amount and orientation as needed.
The initial assessment after first lens application is as basic as any spherical GP lens fit. Observe the lens centration, relationship to the upper lid (lid attachment or intrapalpebral), and fluorescein pattern. After this, note the position of any visible segment height in relation to the pupil, rotation of the lens, and lower lid interaction along with the amount of translation achieved with downgaze. If all of the fitting elements are correct and the over-refraction is complete with accurate powers, the patient will have good visual acuity at the onset of lens wear and will only need to adapt to lens awareness/comfort.
Rotational stability is critical in any translating design. Some mild nasal rotation is usually acceptable or even helpful with the natural inferior and nasal convergence of the eyes while reading. Excessive nasal or any temporal rotation is corrected first by flattening or steepening the base curve (based on fluorescein pattern observations) and then by either increasing or changing the position of the prism per design recommendations. Figure 6 shows slight nasal rotation and an excessively steep fluorescein pattern along with poor lower lid apposition. Flattening this lens by 0.50D should improve positioning and rotation. Lastly, truncation may be added or increased if undesirable rotation persists. Design-specific troubleshooting is best done by referencing the brand’s fitting guide or by contacting the consultation team of the manufacturing laboratory, as the sophistication of each design does not lend itself to blanket troubleshooting recommendations in all cases.
Figure 6. Slight nasal rotation and an excessively steep fluorescein pattern along with poor lower lid apposition. Flattening this lens by 0.50D should improve positioning and rotation.
Being mindful of pupil size and position in relation to the lower limbus is helpful, as smaller pupils may require higher segment heights or greater translation effect to access the full near power (Figure 7). Lid tonicity also contributes either positively or negatively to translating designs (Benoit, 2016), impacting either the upper lid with superior lid attachment or inferiorly with lens lift during translation. Significant lid chalasis, laxity, entropion, or ectropion may require a translating design with supplemental posterior asphericity or the use of a different modality altogether. Making note of corneal diameter or horizontal visible iris diameter (HVID) can be useful in choosing an initial overall diameter.
Figure 7. Smaller pupils may require higher segment heights or greater translation effect to access the full near power.
Some new designs even allow for the addition of a front-surface toric correction to help with residual cylinder, if present. Incidentally, this may be less difficult to add to a translating design compared to many other modalities because lens rotation is already strictly controlled by virtue of the design using prism and, occasionally, truncation. Because successful translation is so important to fitting success, use of a diagnostic fitting set is advisable, at least until you are able to accurately assess all of the required elements that contribute to proper rotation and stability and can predict any necessary initial modifications for empirical ordering (Bennett, 2015).
Despite being able to reduce both edge and overall lens thickness and using other design properties to stabilize and create translation, newer translating GPs may still not be as thin or as lightweight as other multifocal modalities. This is beneficial in that flexure is rarely an issue, but detrimental if corneal molding occurs, which can possibly cause difficulty in the highest of refractive corrections. Use of high-Dk materials is advisable unless issues with wetting or depositing require otherwise.
Although fitting a translating GP lens may require more attention to positioning of the lids and gaze centration, the resulting crisper, clearer optics are certainly worth the time invested. Luckily, after a few exposures, fitting these lenses becomes quite straightforward and intuitive; and in difficult cases, the laboratories that manufacture these designs are extremely helpful in problem-solving. The level of customization available in translating GP bifocal and multifocal designs is unmatched by any other presbyopic contact lens modality on the market today.
If you abandoned older translating GP lenses due to issues with comfort or intermediate blur, newer translating designs should be attempted, as the improvements in technology and performance are significant. Mediocre vision in simultaneous design soft or GP multifocals can be improved with minimal additional chair time and patient adaptation. Refitting a soft toric monovision or soft aspheric or concentric multifocal patient into a GP lens that doesn’t compromise vision at any distance can result in both happier, more loyal patients and improved practice revenues. An “oldie but a goodie,” the translating GP bifocal has found new life and new applications in today’s digital society. Practitioners should keep this modality in mind as a great option when fitting or refitting presbyopic patients. CLS
For references, please visit www.clspectrum.com/references and click on document #253.
Dr. Potter owns a private practice in Sylvania, OH. She enjoys fitting, writing, and lecturing on specialty contact lenses and can be reached at firstname.lastname@example.org.