CUSTOM SOFT LENSES

Personalizing Vision with Custom Soft Lenses

Prescribing custom soft lenses can help you optimize vision, comfort, and ocular health in even the most difficult patients.

By Robert L. Davis, OD, FAAO; S. Barry Eiden, OD, FAAO; & Jeffrey Sonsino, OD, FAAO

Custom soft lens designs give practitioners the freedom to develop distinct power profiles and unique fitting characteristics to better serve patients. Today, we can offer lens designs that truly provide superb, personalized vision correction for each patient and that can be worn with exceptional comfort.

Prefabricated contact lenses are appropriate in many cases, but having the ability to prescribe exact specifications for patients who are not optimally served by these lenses allows contact lens practitioners to provide true contact lens service. Many custom lens design alternatives have been developed to help improve the contact lens outcomes of our most difficult patients. By applying the tools available in the fitting of these lenses, we can optimize our patients’ vision, comfort, and physiological response.

Overview of Custom Soft Lenses

Specialty contact lenses represent a $1.8 billion market that is growing faster than the overall $7.1 billion contact lens market (Falkner, 2013). Product designs are constantly being refined, and the resulting improved fitting success rates will help drive this increasing market. Patients will no longer be satisfied with limited wearing times and with only adequate vision; practitioners will gravitate toward these products to improve their outcomes and profitability and to help counteract the commoditization of contact lenses. Custom soft lens patients become legacy patients who are loyal to the practice and return yearly because they value this service.

Customizing soft lenses allows practitioners to:

• Refine lens powers in 0.1D increments for both sphere and cylinder powers.

• Prescribe cylinder axes in 1º increments.

• Choose from unlimited base curve options to better match the cornea.

• Select a secondary curve that can enhance the cornea-to-lens relationship either in a reverse design or with a unique fitting profile.

• Specify diameters in 0.1mm increments.

• Choose central zone thickness options that can successfully mask astigmatism and corneal irregularity.

• Select optic zone sizes to reduce aberrations from large pupils during scotopic vision.

• Choose zone sizes to appropriately fit multifocal lens modalities.

• Prescribe custom tinted/opaque cosmetic lenses.

Custom Designs for Presbyopia

The primary demographic driver for custom design multifocal contact lenses is presbyopia and the resulting decline of visual performance at intermediate and near working distances. An easy remedy for presbyopes is spectacle lenses, either in a multifocal format or reading glasses. However, many of the world’s almost 1.5 billion presbyopes have become accustomed to wearing contact lenses and are reluctant to give up this convenience. This target market for custom soft multifocal contact lenses offers much opportunity for practitioners who can successfully resolve deteriorating near vision from aging.

Custom soft lenses for presbyopia are primarily available in two different designs, both of which use simultaneous-image correction. An annular bifocal design consists of distinct concentric areas of distance and near correction, whereas an aspheric design features a smooth transition in power between distance and near corrections (Figure 1). In both cases, the retina receives both in-focus and out-of-focus images, and the brain must learn to select the in-focus stimulus while suppressing the out-of-focus stimuli to achieve a successful visual outcome (Benjamin, 1993). One potential hurdle is that the contrast of the desired in-focus image is reduced by the superimposed out-of-focus image(s) (Charman and Saunders, 1990; Charman and Walsh, 1988).

Figure 1. Multifocal zone options: annular design (left images) and aspheric design (right images).

Aspheric multifocal contact lens designs provide a progressive gradation of power from the center to the edge of the optical zone that is achieved by the use of one or more aspheric surfaces (Figure 2). The multifocal effect in soft aspheric contact lenses is accomplished by controlling spherical aberration. Although the “best” retinal image is degraded by the induced spherical aberration, it is countered by the increase in the vergence range over which there is no apparent deterioration in retinal image quality because depth of focus is increased (Bakaraju et al, 2010; Montés-Micó et al, 2011; Rajagopalan et al, 2006; Richdale et al, 2006). The one factor that cannot be controlled is an individual patient’s tolerance to blur (Woods et al, 2010), which may be attributed to inherent optical factors such as pupil size (Bradley et al, 1993; Bakaraju et al, 2012), higher-order ocular aberrations (Ruiz-Alcocer et al, 2012), and binocular summation (Plainis et al, 2011).

Figure 2. Example of a custom aspheric multifocal lens design.

Designing Custom Soft Lenses for Presbyopia The first step in designing a custom multifocal contact lens is investigating patients’ visual requirements. A candid discussion will uncover the visual priorities required for a successful outcome. Patients tend to emphasize the quality of either near or distance visual activities. Using this information will direct you toward a general lens design profile.

Multifocal lens designs incorporate either a distance-center or a near-center lens design. Near-center lenses provide enhanced near outcomes, whereas distance-center lenses create an improved distance vision result. The goal during the fitting process is to determine the relationship between the distance and near zones that will provide a positive outcome. As the near zone size increases, distance vision deteriorates. The converse is also true so that when the distance zone size increases, the near vision will relatively deteriorate. This is controlled by the image relationship entering in through the pupil.

After choosing the zone size, you need to determine the fitting parameters. The one constant in selecting the correct fitting parameters for multifocal lens designs is that the lens must center. Select a base curve and diameter that result in 0.75mm to 1mm of movement in primary gaze.

If a lens is too tight, it will demonstrate inadequate or no movement. For multifocal toric designs, a tight fit will not allow the lens to return to its proper position after the blink, causing blurred vision. Bubbles may also develop beneath a lens if it is too steep, and the patient will experience clear vision after the blink followed by poor vision as the lens lifts off the cornea. A lens that is too flat will exhibit greater than 1mm of movement; it will drop down toward the limbus upon upward gaze, and fluting may occur toward the edge of the lens. Vision will fluctuate and may be worse after the blink. For multifocal toric designs, a flat fit will cause the lens to continually rotate without a clear and distinct resting point.

Once the lens is properly fit, it’s time to determine the lens power. Rather than being limited to the low, medium, or high adds that are typical of mass-produced multifocal designs, custom soft lenses can be ordered in any multifocal power. You can select the add power that is true to your patients’ refraction.

Changing the size or power of the center zone alters the multifocal optical characteristics. Decreasing or increasing the center zone size not only impacts the near/distance optical balance, it also changes the rate at which the power progresses toward the periphery in multi-zone aspheric lens designs. In such designs, the power changes between the center and the peripheral zone. Increasing the progressive intermediate zone diameter will enhance the intermediate vision, and a more gradual power shift will occur within the zone, thus minimizing any negative influences on distance vision that often occur when rapid power changes are created with smaller progressive intermediate zone sizes. These influences are relative to the patient’s pupil size and required add power. Any decision to change the eccentricity of the progressive intermediate zone must be made in relation to pupil size assessment.

An additional strategy to improve the balance between distance, intermediate, and near vision is to use a distance-center lens design on one eye and a near-center lens design on the other eye. This approach, especially for mature presbyopes, reduces the degree of interocular visual asymmetry that is experienced with monovision. This modified monovision, or perhaps more appropriately termed “modified multifocal” approach, has an advantage of improved intermediate vision and relative binocular balance when compared to traditional monovision.

Aberration Control Lens Designs

GP lens laboratories have nearly all entered the custom soft lens market. These fabricators offer very exciting technology, in particular to control for spherical aberration. Spherical aberration is created when light refracts from the periphery of a lens at a greater rate than from the optical axis. The first attempt to compensate for spherical aberration came in the form of apochromatic lenses (three photographic lenses sandwiched together). Next came aspheric lenses, which have a smooth flattening in the radius of curvature from the center of the lens to the periphery (mimicking the shape of the cornea).

A new technology to control spherical aberration takes advantage of our recent understanding of wavefront optics. The Intelliwave (Art Optical) lens is particularly noteworthy because it incorporates wavefront technology to reduce spherical aberration. The posterior shape of the lens was constructed based on the topographic shape of the cornea, offset by an average tear thickness value to represent the power of the tear film. The anterior surface lens shape was then calculated using wavefront optics to counter the spherical aberration of the human eye. The result is a wavefront optimized contact lens. This lens is available in aspheric, aspheric toric, multifocal, and multifocal toric designs, with a distance-center design coming soon.

Custom Soft Toric Contact Lenses

Unlimited base curve, diameter, and power options have provided a framework for custom toric hydrophilic contact lenses to flourish. This niche is fueled by the lack of parameter availability in stock toric lenses. Astigmats can achieve visual acuity comparable to that with spectacles when fitted with toric soft lenses (Sulley et al, 2013). Custom design options offer practitioners the ability to address patients’ distinctive vision and comfort problems. Refining the prescription without parameter limitations allows prescribers to develop their own fitting philosophy and to improve outcomes.

Designing Custom Soft Torics Now that we can fabricate lenses in virtually any power, the main challenge in fitting custom soft toric lenses is lens stability. The eyelids play a critical role in contact lens rotational position and stability. Fluctuating vision with the blink can result from either a poor tear film or lens interactions with the lid. A successful toric design allows the lid to glide over the lens surface without causing friction that results in lens rotation. Tight lids exert greater force and torque on the surface of the lens compared to those that are not as tight. Giant papillary conjunctivitis and chalazion formation will reduce the space between the lid and contact lens, exacerbating rotational instability and unpredictability. Smaller palpebral apertures improve rotational stability because the lens is tucked beneath the upper and lower lids, reducing blink cycle influence.

The tear film and the lens fit will control how quickly the lens returns to its rotational position. A tight-fitting lens or an inadequate tear film will reduce lens movement on the eye and delay the lens recovery, creating fluctuating vision. A loosely fit lens will float on the ocular surface with no stable endpoint for recovery. Lens instability is one problem that custom hydrophilic toric lenses can overcome because of the unrestricted parameters.

Extreme versions and postural positions can also affect toric soft lens orientation and visual acuity. (Young et al, 2002; McIlraith et al, 2010). Rotational stability needs to be achieved with each blink and in various positions of gaze. This is important for patients who read in downward gaze for long periods of time or change fixation frequently.

Figure 3. This prosthetic soft lens can be built layer by layer in the presence of patients.

The shape of the eyelids also influence the rotation of the lens with each blink. If the outer canthus is higher compared to the inner canthus, lens rotation will occur temporally. Nasal rotation will occur when the inner canthus is higher than the outer canthus (Young et al, 2002).

Patients’ individual sensitivity to blur is the subjective component of fitting custom soft toric lenses. The way in which patients react to rotational changes in cylindrical power and axis is a good predictor of patient acceptability with this lens modality. All hydrophilic toric lenses should move on the eye to exchange tears under the lens. Some degree of rotational instability and lens movement-induced vision fluctuation will occur. Patient acceptance of this fluctuation is not universal. A patient who does not notice blur until you rotate the axis 15° or more will not be sensitive to small lens rotations on the eye. A patient who notices blur with less axis rotation may report poor or fluctuating vision.

It is difficult to achieve success with patients whose cylindrical power is greater than the spherical power because of third-order or monochromatic aberration.

Custom Cosmetic Soft Lenses

Custom cosmetic contact lenses are useful for conditions that cause photophobia or polyopia, such as oculocutaneous or ocular albinism, polycoria, surgical iris trauma, colobomas, aniridia, injuries, or traumatic anisocoria. Although cost can be high, such lenses can be a life-altering strategy for patients who have relatively few options.

Custom cosmetic lenses can be color-matched to patients’ iris colors in a variety of ways. Trial sets are available for some lenses, and with some companies you can send photographs of the unaffected eye that an artist can use to hand paint a lens.

A newer option from Orion Vision Group allows prosthetic lenses to be built using a series of add-on layers. A variety of opaque and transparent colored lenses are available in a V series for light-colored eyes or an X series for darker-colored eyes. To this colored lens you can add a star burst and/or a limbal ring. A cylinder stand is included in the set so you can build the lens right in front of the patient (Figure 2). These custom soft cosmetic lenses are available in diameters from 11.5mm to 22.00mm, any spherical powers, and cylinder corrections up to –10.00D. Pupils can be either clear or black from 3.0mm to 9.0mm. Opaque backing colors are available in black, light brown, dark brown, light gray, or dark gray. We’ve found that building custom cosmetic soft lenses with patients is a partnership that is appreciated by the patients and improves their quality of life.

Custom Soft Lenses for Irregular Corneas

Irregular corneas can result from a number of factors including disease, trauma, corneal transplants, and complications from refractive surgery. The commonality among these conditions is distortion of the anterior corneal surface requiring an optical correction that will neutralize the corneal irregularity. The reduced vision from these conditions is not adequately addressed by either spectacle lenses or standard soft contact lenses.

Traditionally, patients who have irregular corneas have been limited to corneal GP lenses, which often resulted in reduced wearing time because of discomfort caused by friction. Today, many contact lens options are available for correcting irregular astigmatism; specialty GP lens designs, scleral lens designs, and hybrid lenses have increased our ability to produce successful outcomes.

Soft lens designs for irregular corneas can offer improved comfort and, in many cases, vision equal to that of GP lenses. Such designs use lens thickness to mask the corneal irregularities that produce astigmatism. These hydrogel lens options provide an opportunity for success when GP options have not been successful or desired.

Designing the Lenses Generally, these lenses feature aspheric front surfaces to minimize the spherical aberration that often results from corneal irregularity. Toric designs are available to address any residual regular astigmatism. Increasing the central lens thickness will improve its ability to mask astigmatism.

For ectatic conditions, the more decentered the cone, the more lens thickness is required to neutralize the corneal distortion from meridional differences. Typically, these hydrophilic lenses are 0.35mm to 0.65mm thick. Employing a plus lenticular design allows for a relatively thin lens periphery, which will improve oxygen transmission to the critically sensitive peripheral cornea where the limbal cells are found. You should typically fit these contact lenses with significantly greater movement compared to that of custom soft lenses for regular corneal surfaces. The greater movement and tear exchange helps maximize corneal oxygenation.

Centration is desirable to optimize visual outcomes; however, perfect centration is often difficult to achieve in highly irregular corneas. Even in such cases, optical outcomes are often impressive with these types of custom soft lenses.

As with other custom soft lens designs, excessive lens movement or edge lift indicates a flat-fitting lens, which can be addressed with a steeper base curve and/or use of a larger diameter. Conversely, a steep lens will demonstrate inadequate movement and visual fluctuations with the blink due to deformation within the central optic zone.

On occasion, irregular corneas have peripheral meridional asymmetries that require custom lens peripheries to properly align to these corneas. For example, the inferior cornea is significantly steeper compared to the superior cornea in many cases of keratoconus and pellucid marginal degeneration. Many custom soft lens designs can be fabricated with unique peripheral curve parameters to better match these asymmetric corneal topographies.

Plan to Get on Board

Significant improvements in contact lens engineering have brought us custom hydrophilic lens products to help our patients succeed with contact lenses when more standard options have failed. As these categories expand and consumers continue to demand improvements in their vision and comfort with contact lenses, customization will becomes more standard.

Contact lens fitting is about troubleshooting patient complaints to avoid contact lens dropouts. Patients can either accept contact lens design limitations—or discover a practitioner who will personalize their vision correction. CLS

For references, please visit www.clspectrum.com/references and click on document #225.