Article

A BRIGHT VIEW OF SOFT MULTIFOCAL CONTACT LENS OPTICS

Newer multifocal optics work with patients’ ocular characteristics to help improve success.

Without a doubt, practitioners and patients alike have had a tenuous, love-hate relationship with multifocal contact lenses since their inception. Studies of contact lens dropout rates in the late 1990s and early 2000s cite a lack of effective contact lens options as one of the major reasons for discontinuation among presbyopes.1,2 Some of the major hurdles experienced were increased glare and insufficient near vision performance as a result of poor optical designs.2,3 At the same time, industry reports showed that the rate of multifocal contact lens fits had “reached a plateau.”4 With more patients being fit with monovision than with multifocal lenses, there was still a way to go in providing optimal correction for the presbyopic population.5

Just like any love story with a good ending, hardships diminish and relationships improve with a little creativity and ingenuity. Multifocal contact lens success rates have improved from 50% in the early 2000s to upwards of 85% by the most recent accounts.2,6-8 Practitioners and patients alike prefer multifocal contact lenses over monovision and single-vision options by a margin of 3 to 1, and this number continues to rise. More and more contact lens practitioners are finding themselves reaching for a multifocal contact lens as their lens of choice for presbyopes. In the United States and worldwide, almost half of soft lens-wearing presbyopic patients are fit with multifocal contact lenses.9,10 Multifocal lens designs are even being considered for the non-presbyopic patient population for the purpose of myopia control.

What is it about modern contact lens designs that has given previous disbelievers pause for consideration? Why are more and more patients raving about their multifocal contact lenses? Research over the years has dramatically improved our knowledge of multifocal lens optics and has identified opportunities for improvement. Manufacturers have responded in kind by creating more effective designs that work with patients’ unique optical performance rather than against it. A look at some of the most recently available multifocal lens designs highlights innovations in multifocal lens optics. With a greater understanding of the optics of newer multifocal lens designs, practitioners can be more effective at fitting and troubleshooting these lenses.

LET’S TALK OPTICS

For GP and soft lenses, multifocal lens designs fall into one of several categories: center-near aspheric, center-distance aspheric, concentric ring, or segmented. In the soft multifocal lens category, center-near aspheric designs are the most widely utilized (Figure 1), with all four major lens manufacturers offering one or more versions of this design in their portfolios.

Figure 1. Center-near aspheric design. The red area is the most plus (the add power), and the power becomes more minus as it moves out into the periphery of the optic zone. This lens is labeled as a –3.00D distance power with a high add power.

In center-near aspheric lens designs, there is a gradual refractive power change from the most plus (the add power) in the center of the lens to the most minus (the distance power) in the periphery, and vice versa for center-distance aspheric lens designs. This creates a simultaneous vision effect whereby multiple focal points are projected into a patient’s line of sight at the same time. The object to which patients choose to draw their attention becomes the primary visual information for the brain to process. Patients can liken this to looking out of a window that has a screen. While viewing toward the window, a patient can choose to look either at the screen, at points of interest beyond the screen, or at objects in front of it.

Many of the designs available are center-near to assist with the physiological changes that the eye undergoes when viewing near objects or when accommodating.11 As a patient focuses on a near object, the eyes converge to aim at the object, pupils become smaller (miosis), and the crystalline lens becomes thicker anterior to posterior due to contraction of the ciliary muscle. Because presbyopic eyes have lost the ability to accommodate, center-near multifocal contact lens designs attempt to compensate by placing the highest add power in the visual axis as the pupils become smaller and the eyes converge during a near-vision task. In theory, this should provide a clear image.

Practitioners should understand that not all soft multifocal contact lenses are created equal, even within the same category of optical lens designs. For example, some individuals may believe that if one center-near lens design does not work well for a particular patient, then none of the other center-near designs will work. This is a false assumption. Knowledge of various nuances among lens designs can assist practitioners with understanding why multifocal lenses work, determining the lens of choice given a patient’s unique circumstances, and knowing how to troubleshoot the lenses if something should go awry.

Luckily, technology allows us to visualize the power profiles of contact lenses in detail. We used the Nimo TR1504 by Lambda-X to measure the optics of multifocal lenses in this article. The instrument produces a power profile curve (Figure 2) to visualize the power at any point on a lens. The measurements span across a specified diameter within the optic zone of the lens and plot a line that represents how quickly the power transitions over that space. This can be used to compare different lens designs and to gain knowledge of where the maximal distance and add powers are achieved. In essence, the instrument can be utilized to gain valuable information such as the effective add power, the size of the near and distance zones, and how quickly the lens transitions to the add power.

Figure 2. A multifocal lens power profile. The left y-axis reflects the lens power, and the bottom x-axis represents the distance from the center of the lens. This particular measurement was taken at a 7mm diameter chord length (3.5mm in each direction from the center of the lens). For this particular lens, the power is just under –1.75D in the center of the lens. The lens power gradually increases to –4.00D maximum at 3.5mm from the center of the lens. Keep in mind that the patient would theoretically need to have a 7.0mm diameter pupil to achieve the full –4.00D power.

To highlight similarities and differences among the most common multifocal contact lens designs on the market, we used this power-mapping instrument to evaluate a series of daily disposable multifocal contact lenses that were labeled as having –3.00D distance power and a high add (Figures 3 through 5).

Figure 3. This center-near lens exhibits a power of approximately –1.00D in the center of the lens. There is a gradual slope toward more minus in the periphery of the lens, achieving its full labeled –3.00D at about 1.5mm from the center of the lens. Therefore, at a 3mm pupil diameter, the patient would be experiencing the full minus as well as an approximate +2.00D add. Note that further out into the lens periphery, the minus power continues to increase well past the labeled –3.00D power.

Figure 4. This center-near lens has a large, approximately 2mm diameter area in the center in which the most plus power is achieved. There is a stepped approach as the lens power increases in minus, realizing the labeled –3.00D distance power at about the 4mm pupil diameter. Some experts would argue that the stepped approach (versus the sloped approach in Figures 3 and 5) causes different types of optical aberrations that may be deleterious to vision under certain circumstances, such as low-contrast scenarios.

Figure 5. This center-near lens reaches full minus at 2.5mm from the center of the lens, a point much further into the periphery compared to the lenses in Figures 3 and 4.

In contrast to aspheric designs, concentric ring lens designs have a series of distinct, alternating distance and near zones. Abrupt power changes between distance and near are arranged circularly across the optic zone. In theory, alternating both distance and near power zones makes the lenses less pupil-dependent. In other words, even with smaller pupil sizes, the retina should be receiving both near and distance focal points because both powers are located centrally within the lens and the visual axis. Some experts argue that the stepped power profile of this lens design decreases contrast sensitivity and increases unwanted aberrations.12,13

Multifocal lenses have also been used for myopia control (a.k.a., myopia management). A survey that we conducted indicated that practitioners prefer orthokeratology or soft contact lenses as their primary means of managing the progression of myopia. When using soft contact lenses for this purpose, practitioners should utilize center-distance lens designs, which are different from the more common center-near lenses that are designed specifically for presbyopes. Figure 6 shows one soft lens design marketed specifically for the purpose of myopia management. This design is not currently cleared in the United States, but it is being used in other countries around the world. The power profile (Figure 7) shows a central area of full distance correction surrounded by a ring of add power.

Figure 6. A multifocal lens used outside of the United States for myopia control. Note the concentric ring design compared to the gradual power change of the aspheric design in Figure 1.

Figure 7. Power profile (–3.00D correction) of the Figure 6 lens design. In this lens, the –3.00D power is located in the center 3mm of the lens (1.5mm from the center of the lens on each side). There is an abrupt, stepped power change toward maximum plus about 2mm from the center of the lens.

THE IMPACT OF OPTICAL AND INDUCED ABERRATIONS

Imaging and measuring the optical properties of multifocal lenses can provide some information, but other aspects of optical design dictate a patient’s success. Many people assume that induced aberrations are always an undesirable characteristic, but optical aberrations play both a positive and a negative role in the effectiveness of a multifocal lens. In fact, aspheric lens designs increase depth of focus by inducing primary spherical aberration. They allow presbyopic patients to view objects at all distances while wearing the lenses. Normal presbyopic human eyes inherently exhibit positive spherical aberration. Therefore, if a center-near lens induces an amount of negative spherical aberration that is opposite to a patient’s inherent positive spherical aberration, this may create a situation whereby a multifocal lens may be less effective for near vision. Multiple publications have theorized this as a primary reason for decreased multifocal lens performance.14-18

A clinical comparison of satisfied and dissatisfied center-near multifocal contact lens wearers revealed a statistically significant correlation between inherent spherical aberrations and the subjective and objective near visual acuity performance of center-near multifocal contact lenses. A decreased amount of inherent positive spherical aberrations was associated with increased near vision performance.19 In other words, for the patients who were the most successful with their center-near multifocal lenses, it seems as if their natural optical characteristics allowed the add power of the multifocal lens to perform better by inducing more negative spherical aberrations.

A potential new product coming down the pipeline, extended depth of focus (EDOF) lenses, utilize specific combinations of higher-order spherical aberrations to create an effective add while maximizing retinal image quality as much as possible. Short-term clinical trials comparing a prototype EDOF lens to other commercially available multifocal lenses show that patients report better clarity of vision at near and intermediate distances as well as decreased ghosting and increased overall lens satisfaction.12,13 A different EDOF design in a daily disposable lens has U.S. Food and Drug Administration clearance for multifocal correction.

DOES LENS CENTRATION MATTER?

With the varied power profiles over the optical zones of multifocal contact lenses, it would make sense to assume that lens decentration in relation to the optical axis would negatively affect visual outcomes. Considering that modern soft multifocal contact lenses decenter by about 0.50mm in any given direction, this is a valid concern for contact lens fitters.8,20 Both theoretical models and in vivo studies of soft multifocal lenses indicate a definite increase in higher-order aberrations such as coma and trefoil when a multifocal contact lens decenters on eye.21 However, does decentration actually translate into worse vision for patients? How much decentration is too much?

When it comes to fitting the presbyopic patient population with the newest multifocal contact lens designs, there may be more leeway in the system than previously thought. It seems as though visual performance is affected only in situations in which large amounts of higher-order aberrations are induced.21 A 2016 study showed no clinically significant difference in the subjective performance of concentric ring contact lens designs in centered versus decentered conditions.18 Dinardo and colleagues did not find a statistically significant association between lens decentration and successful versus unsuccessful multifocal lens wearers.8 Fedtke and colleagues found that, with a variety of commercially available multifocal lenses, the correlation between multifocal lens decentration and objective and subjective visual performance was weak at best for the presbyopic patient population.

Decentration, however, was a statistically significant factor for non-presbyopic patients who exhibited larger pupils, a larger amount of lens decentration, and lower levels of inherent third-order aberrations.20 These findings have been corroborated by other in vivo studies of non-presbyopic patients and will certainly play more of a role if multifocal lenses become more widely utilized for the purpose of myopia control.

If lens decentration is a critical factor in multifocal contact lens performance for a patient, there are options. Some soft and GP lens manufacturers will allow a contact lens to be customized with decentered optics. Despite the aforementioned research to the contrary, this technique has been effective in the handful of case studies that have been reported. Regardless, more research is necessary on the impact of multifocal lens decentration and the best approach, if any, to remedy the situation.

DON’T FORGET PUPIL SIZE

Pupil size in relation to the optical zones of soft multifocal lenses plays a big factor in contact lens success. Because of the refractive power changes that occur across the optical zones of the lens, the performance of the lens hinges dramatically on a patient’s pupil size at any given moment, especially for aspheric multifocal lens designs. Therefore, patients can experience problems such as ghosting, haloes, visual fluctuations, and decreased contrast sensitivity while wearing multifocal contact lenses.

Pupil size is very much a dynamic factor because patients perform tasks in a variety of situations and illuminations. Patients’ visual experience can vary dramatically depending on environmental conditions, pupil size, and their interaction with the optical lens design. Therefore, it is critical to take the time to question patients about their daily activities and which activities are most important for them to be able to accomplish while wearing the contact lenses. A patient’s pupil size should be measured in a variety of illuminations and taken into consideration during the fitting process, particularly if a patient is experiencing problems with his or her lenses for a particular task.

Multiple studies have reported the importance of pupil size on multifocal contact lens success. Both theoretical eye models and studies of multifocal lens performance on eye suggest that smaller pupil size is an advantage in terms of depth of focus and near vision performance while wearing multifocal contact lenses.15,16,18 This is especially true for the more popular center-near aspheric lens optics utilized in most multifocal contact lenses today. For example, an analysis of successful versus unsuccessful aspheric multifocal lens wearers indicated a statistically significant difference in both scotopic and photopic pupil size between subjects who were satisfied with their multifocal lenses versus dissatisfied, with smaller pupil sizes being related to better near vision outcomes.8,19

TROUBLESHOOT WITH CUSTOMIZATION

Luckily, pupil size combined with knowledge of contact lens optics can help practitioners with patient success. Customized GP and soft lens options allow practitioners to choose lens designs that optimize distance and near vision for patients’ pupil diameter and their visual task requirements. A handful of designs allow practitioners to alter the multifocal zones by specifying the surface area within the optic zones devoted to near, intermediate, and distance optics.

Another customizable feature is the rate at which the power progresses from one power in the center of the lens to the opposite power in the periphery of the lens. This ultimately results in customized zones based on patients’ own pupil size placed over their visual axis to optimize performance during their more important visual tasks. Manufacturers are starting to understand this better and to offer contact lens designs that reflect these findings.

The most widely marketed soft multifocal lens designs are manufactured to fit a majority of the population. The inability to customize the parameters of the lenses is a major limitation of the most popular designs and can contribute to patient and practitioner struggles with the lenses. Customizing factors such as induced aberrations, optic zone sizes, or centration of the optic zones for a particular patient can assist in achieving success. Improved lens designs and customizable lens parameters have increased multifocal contact lens performance overall. As we learn more from research and patient success, we may very well see an increase in customized optics for presbyopic patients. Even more important, knowledge of power profiles can help practitioners choose a multifocal lens that works with patients’ unique visual demands, enhancing their experience while wearing the lenses when it is most important for them to see their best.

A BRIGHT FUTURE

Now more than ever before, soft multifocal lenses are an excellent option for presbyopes seeking less dependency on glasses. While there is much work to be done, we are quite possibly one step closer to closing the book on multifocal lens fitting frustrations with a “happily ever after.” CLS

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