SINGLE-USE MULTI FOCALS
A New Era for Single-Use Multifocal Lenses
The presbyopic population can greatly benefit from a vision correction option in a daily disposable modality.
|Bradley Rogers is a UK-based contact lens optician. In addition to his work in practice, he offers professional support to practitioners in the north of England and Scotland through his role as a CooperVision Clinical Specialist.|
By Bradley Rogers FBDO (Hons) CL, SLD
The year 2012 was a pivotal one for presbyopic contact lens correction. A plethora of multifocal options in reusable modalities are now available, which in 2012 were supplemented by the introduction of two new single-use multifocal contact lenses to the U.K. market. This acceleration in product release comes 12 years after the launch of the first (and until recently only) single-use multifocal contact lens.
Given the wide choice of contact lens modalities and designs for correcting presbyopia, why choose a single-use multifocal? This article will examine the opportunities and challenges that influence the efficacy of presbyopic correction in a single-use, daily disposable modality. It will concentrate specifically on those aspects that relate to vision, rather than universal attributes of single-use disposables per se, such as material properties.
Why Choose Single-Use Multifocal Lenses?
This is a fundamental question. Because the product options have only just recently expanded, why should we consider multifocal correction in a single-use lens when there are many more choices in other modalities?
Reason 1: The Potential is Huge Following the baby boom of the latter part of the 20th century, it has been estimated that there are now 1 billion presbyopes globally, projected to increase an additional 35 percent by 2020 and projected to reach 1.8 billion by 2050 (Holden et al, 2008) (Figure 1). This exponential increase will bring proportionally more new patients who have never received an eye examination before as well as existing patients who now require a new near vision correction. The ocular and lifestyle needs of these patients are interrelated and may be explained in terms of their generational experience as the so-called baby boomers and generation X.
Generation X is the term for those people born from the mid-1960s to the late-1970s and includes all of our new and emerging presbyopes. These patients have grown up during a technological revolution and will have a greater motor-visual demand resulting from their use of mobile technology such as phones, tablets, and laptops as well as computers. This visual demand will also vary with environment, the technology being used, its duration, and working distance. Generation X patients will tend to lead a more active lifestyle and are strongly motivated to maintain both visual function and a youthful appearance. These are potent drivers to multifocal contact lens prescribing, which are facilitated by a greater accommodative and adaptive ability (Studebaker, 2009).
Figure 1. The global presbyopic population is predicted to increase to 1.8 billion by 2050 (Holden et al, 2008).
The baby boomer generation includes people born between the mid-1940s and the early 1960s, and so these patients are likely to have started lens wear at a later stage in life. They matured with comparatively less frequent and diverse use of modern technology and are characterized by a need for comfortable near vision for relatively longer periods of time. As many baby boomers will have reached retirement, their lifestyle will be different compared to their younger counterparts. They are strongly driven to preserve visual function in favor of improving cosmesis (Studebaker, 2009).
Not only are there increasing numbers of people requiring near correction, there is also the question of how we deal with our existing contact lens wearers. A recent study found that more than 60 percent of contact lens wearers aged 35 to 49 years and 80 percent aged over 50 years said that they were interested in multifocal contact lenses (Gallup Study, 2008). With this in mind, there is a huge potential for providing patients with a viable alternative to spectacle wear, one that will offer wearers the flexibility to wear on an occasional or part-time basis.
There is also great potential in dealing with the reasons for patients who have ceased lens wear: this is most acute after the age of 40, when the principal explanations cited for dropout are related to comfort, vision, and convenience (Akerman, 2010). Paradoxically, this comes at a point when there is a significant increase in the number of people requiring sight correction. Issues of comfort are usually attributable to symptoms of dryness, which become greater in the aging eye as does the visual problem related to presbyopia. It follows that a multifocal contact lens that can be worn comfortably will result in more patients remaining in contact lens wear, which would be further enhanced through the convenience of a single-use modality.
Reason 2: The Single-Use Modality is the “Lifestyle Modality” Since their introduction in the mid-1990s, the prevalence of single-use daily disposable contact lenses has grown steadily and now represents 45 percent of new soft lens fits and 39 percent of soft lens refits in the United Kingdom (Morgan, 2012). Factors influencing this popularity are associated with increased convenience, optimum comfort, and, from a practitioner perspective, increased compliance (Dumbleton et al, 2009) and hygiene.
Proclear 1 Day Multifocal lens selection guide.
Figure 2. Distribution of time spent by practitioners advising presbyopes on their vision correction options.
The popularity of daily disposables does appear to be somewhat attributable to their use for occasional or part-time wear. A study published in 2009 found a bimodal pattern of use for wearers of one-day disposable contact lenses, with 40 percent wearing them full-time (four to seven days per week) whereas 60 percent wore them part-time (one to three days per week) (Efron and Morgan, 2009). The study suggested that daily disposables were particularly popular for part-time use because of their increased convenience and hygiene; whereas the lower proportion of full-time wearers could be attributable to other factors—including perceived value for money—in comparison to part-time use.
When we consider that the needs of our presbyopes may vary considerably depending on their prescription/add, expectations, and the type of visual tasks that they undertake, it is logical to conclude that a single-use modality offers wearers the greatest degree of flexibility of use. This need is reinforced by recent research finding that the majority of lens-wearing presbyopes (78 percent) preferred to habitually correct their vision with a combination of multifocal contact lenses and progressive spectacles (Neadle, 2008). This study found that spectacles were preferred for solitary or stationary activities such as reading a book, watching TV, or using a computer, while multifocal contact lenses were preferred for active and social activities such as going out to a social event, to work, or to the gym. In addition, there was some overlap in preference for some uses such as driving or reading a menu.
We also know that gender plays a part, too, as almost twice as many women wear contact lenses compared to their male counterparts, and women are increasingly wearing their lenses in their late 30s and early presbyopia, reflecting a desire for maintaining both cosmesis and a good standard of vision (Morgan and Efron, 2008).
Given the lifestyle characteristics of generation X presbyopes, today’s increased choice in single-use multifocal correction is timely.
Reason 3: Multifocals are More Popular Compared to Monovision According to a recent survey of U.K. contact lens prescribing, the fitting of multifocal contact lenses now exceeds that of monovision (Morgan, 2012) and accounts for one-third of all contact lenses prescribed to wearers older than 45 years of age (Morgan et al, 2012).
It could be speculated that practitioner preference for multifocal contact lenses may possibly be due to an increasing need for effective correction, increasing practitioner confidence in fitting multifocals, an improvement in multifocal lens designs, and/or an increased level of patient satisfaction with multifocals over monovision.
Monovision, although in decline, offers correction with minimal chair time while maintaining a broad choice of parameter availability; it is less dependent on pupil size compared to multifocals and has quoted success rates of between 59 percent and 67 percent (Evans, 2007). In this sense it lends itself well to daily disposables because there is a great deal of choice in materials and prescription availability. However, while it can work well in early presbyopia, several aspects of visual function can become impaired as the add increases, which are not necessarily seen to the same degree in multifocal wear.
Figure 3. A high-add multifocal design requires a dramatic change of power within a finite pupil area, unlike a low-add design, which facilitates a smoother progression.
It has long been established that unilateral blur reduces stereopsis (Levy and Glick, 1974; Donzis et al, 1983), which has also been shown to be the case in wearers of monovision (Sheedy et al, 1993). One study found a reduction of stereoacuity for monovision wear in the range of 58 to 96 seconds that was proportional to the add (Koetting, 1970). Chapman et al (2010) concluded that changes to the gait of adapted monovision wearers were likely to be associated with reduced stereoacuity, with a range of reduction between 17 to 87 seconds of arc. On the other hand, multifocal contact lens correction has been found to provide better stereoacuity compared to monovision (Gupta et al, 2009; Richdale et al, 2006).
Figure 4. A high-add, center-near, aspheric multifocal design produces an increased level of ghosting and haloes, but is predisposed to good near vision.
Figure 5. The low area of diffuse focus with a low/mid-add, center-near aspheric design produces higher image quality for distance and intermediate vision.
The suppression of the blurred eye in monovision is more effective in photopic conditions (Schor et al, 1987), so comfortable viewing of a low-contrast target in low light, such as a restaurant menu, may be impaired. Some visual disturbance of small, high-contrast targets at night may occur for the same reason, giving rise to haloes around street lights, and this will become more noticeable as the dioptric difference between the eyes increases.
Another example of the challenge faced by monovision wearers who require an increasing add is that it becomes more difficult for practitioners to provide an adequate intermediate correction without compromising distance or near acuity. This has been shown to be significant above a near addition of +2.00D (Erickson, 1988). On the other hand, modern simultaneous vision multifocal designs are predisposed to providing better binocularity throughout the add ranges because they employ a binocular reading addition, and many utilize aspheric surfaces that offer more effective intermediate correction and a better range of near vision compared to monovision (Gupta et al, 2009).
However, there does appear to be a perception that monovision is a quicker form of correction to fit compared to multifocals. A study by Woods et al (2010) challenged this view by demonstrating that there was no significant difference in the number of lenses required to fit a patient with a newly developed multifocal compared to monovision.
Another factor that may play a part in the success of multifocals is the importance of assessing visual satisfaction in “real-world” situations. Another study by Woods et al (2009) compared objective and subjective results for a monovision modality to those obtained for a low-add, center-near aspheric multifocal. Subjective results showed that multifocal correction was preferred over monovision for real-world tasks such as driving during daytime and nighttime, watching television, and when changing focus from distance to near.
Given the ease of adaptation with low-add presbyopes, the speed with which multifocals can now be fitted, and the advantages of better binocularity, there is now more reason than ever for practitioners to use multifocal contact lenses as the first choice for presbyopic correction.
How Can We Best Correct Presbyopes With a Daily Disposable?
From a practitioner perspective, correcting presbyopes’ vision should be a straightforward process, obtaining the desired result with minimal chair time. A 2010 CooperVision-sponsored study found that U.K. practitioners spend an average of nearly 11 minutes advising presbyopic patients on their vision correction options (Figure 2).
Given the time pressure on practitioners, it could be argued that for a daily disposable multifocal to work effectively, this form of correction must achieve a good standard of vision as quickly as possible. This means using an easy-to-use lens selection (or fitting) guide, a manageable trial lens inventory, and a multifocal design philosophy that consistently delivers acceptable vision. Naturally, this is a challenging balancing act, which will ultimately involve an element of “trade-off,” but we can consider how these aims may be achieved.
Distance Vision is Key The most important rated visual factor for success in a simultaneous vision contact lens is distance vision. This was established by Back et al (1992), who analyzed the vision ratings in groups of successful and failed diffractive bifocal contact lens wearers. Discriminant analysis of the results demonstrated that for wear to be deemed successful, the minimum subjective ratings had to be in excess of 77 percent for distance vision, 53 percent for night vision and 46 percent for near vision. The conclusion that near vision had a lower rating might be because wearers can exercise some control on factors that affect the quality of their near vision, such as changing working distance or increasing illumination. Factors affecting distance vision rating, on the other hand, cannot be controlled by wearers and are attributable to the effects of lens design and fluctuating pupil size.
The Significance of Profile Design To appropriately balance the visual compromise, there is no escaping the importance of the power profile in multifocal lens design. Many factors affect the optical performance of simultaneous vision contact lenses, including lens centration, pupil size, optic zone diameter, and the combined spherical aberration of the eye and contact lens (Bakaraju et al, 2010). Because ocular aberrations and pupil size cannot be independently controlled, the single most influential feature must be the lens design itself (Figure 3).
It is no coincidence that all current daily disposable multifocal contact lens designs include an element of center-near asphericity; a gradual change of curvature from center to periphery allows a wide focusing range to be accommodated. An aspheric surface also lends itself well to the high-quality mass production required for single-use disposable lens supply. Center-near designs perform well for near tasks, aided by the reduction in pupil size during accommodation.
However, aspheric, simultaneous vision multifocal designs are subject to the effects of longitudinal spherical aberration, which increases proportionately with the power of the add. The principal reason for this is that the central portion of the lens focuses paraxial light at a different point compared to the marginal area of the lens. In this sense, center-near aspherics produce negative spherical aberration, which has the potential to become troublesome depending on how the profile is configured.
A high-addition, center-near aspheric lens produces a necessarily more dramatic transition between the distance and near, producing a large area of diffuse focus and a high level of spherical aberration that gives rise to ghosting, image doubling, and the perception of blur by the patient. However, the close vision is generally better because the center of focus is conjugate with near vergence (Figure 4).
Conversely, a low/mid-addition aspheric surface produces a smoother transition between distance and near with a shorter area of diffuse focus and thus a lower level of spherical aberration. This generally provides a higher quality image with lower levels of ghosting and doubling, though in itself will not truly bring near objects to a clear focus (Figure 5).
In reusable modalities, an optimum standard of vision is achieved by the use of up to four power profiles, each being engineered for a specific add range (Kerr and Rushton, 2012). For one-day multifocal fitting, however, the need to provide an acceptable standard of vision has to be balanced by the desire for a convenient, straightforward fitting approach. Each manufacturer seeks to achieve this balance in a different way.
Daily Disposable Multifocal Lens Options
Three single-use disposable multifocal lenses are currently available in the United Kingdom:
• Alcon’s Focus Dailies Progressives All Day Comfort
• Sauflon’s Clariti 1Day Multifocal
• CooperVision’s Proclear 1 Day Multifocal
|Principal Parameters for Current Single-Use Multifocal Lenses|
|MANUFACTURER||LENS NAME||LENS DESIGN||EFFECTIVE NEAR CORRECTION||BOZR (MM)||DIAMETER (MM)||POWER RANGE|
|Alcon||Focus Dailies Progressives All Day Comfort||CN Aspheric||Up to +3.00D||8.6||13.8||+5.00D to −6.00D|
|CooperVision||Proclear 1 Day Multifocal||CN Aspheric||Up to +2.50D||8.7||14.2||+6.00D to −10.00D|
|Sauflon||Clariti 1Day Multifocal||CN/BS Aspheric||Low: Up to +2.25D
High: +2.25D to +3.00D
|8.6||14.1||+5.00D to −6.00D|
Table 1 summarizes the principal parameters for these contact lenses from manufacturer and industry literature.
Focus Dailies Progressives All Day Comfort The world’s first daily disposable multifocal, this lens was originally launched in 2000. According to the company, the optical design incorporates a front central optic zone diameter of 7.80mm into which a high-add, center-near aspheric profile carries a reading addition, effective up to +3.00D. This lens design was examined by Hough (2002), who found the dimensions of the power peak at approximately 2mm in diameter and +3.00D high.
The fitting philosophy is as a paired system in which the initial trial lens is selected using best vision sphere (corrected for vertex distance) plus half the add. The power is initially adjusted binocularly to optimize vision, and further improvement may be achieved through adjustments to the power of either or both lenses, according to the company.
Clariti 1Day Multifocal According to the company, this lens entered the market in April 2012 and employs a choice of two center-near profiles (Low and High), which, as outlined above, permits an element of control to be exercised in maintaining an optimal standard of vision.
Having assessed ocular dominance, the chosen distance power is based on highest-plus best vision sphere (adjusted for vertex distance), supplemented by additional plus power dependent upon the add and refractive state as detailed in the fitting guide. The Low profile is used for adds up to +2.25D, with the High profile in the non-dominant eye for adds over +2.25D, and further enhancements may be achieved through adjusting the power of either or both lenses, according to the company.
Proclear 1 Day Multifocal This lens is designed to make fitting presbyopes convenient and effective through appropriate management of visual compromise. To achieve this, Proclear 1 Day Multifocal has been designed with a single-profile, aspheric, center-near design (Figure 6), in combination with a novel fitting approach. This has enabled CooperVision to reduce the need for complex fitting guides and to isolate a clear recipe for success, according to the company.
Figure 7. For additions greater than +1.00D, Proclear 1 Day Multifocal uses different powers in each eye to improve near vision, while minimizing distance vision compromise.
The low/mid-add center-near aspheric surface used in the Proclear 1 Day Multifocal contact lens is designed to produce a lower range of diffuse focus and at the same time corrects up to +1.00D of near add, so for very early presbyopes it is designed to be fitted in the same way as a pair of single-vision contact lenses.
Correcting higher-add presbyopes is achieved through using modified monovision in which a “dialed-in,” controlled amount of plus power (a “near boost” of up to +1.00D) is added to the nondominant eye. Through this use of a low/mid-add profile, each eye receives separate, uncompromised signals in which the dominant eye is biased to distance and intermediate, and the nondominant eye is biased to intermediate and near (Figure 7). Adding a predetermined amount of plus power is designed to improve the near vision while binocular distance vision remains unchanged (Figure 8). It is interesting to note that in clinical studies, the average stereoacuity found with Proclear 1 Day Multifocal corrected in this manner was 61 seconds of arc, which compares favorably with the mean stereoacuity measured with monovision by Richdale et al (2006).
According to the company, the combination of profile design and novel fitting philosophy results in a system that delivers excellent vision at all distances; it improves near vision without disturbing distance vision, produces overall a more natural visual experience, and allows ease of adaptation through different stages of presbyopia. The simplicity and flexibility of the fitting approach means that the lens can be fitted very quickly; this is further aided by a straightforward lens selection guide (See Proclear 1 Day Multifocal lens selection guide on p. 39). The visual results can also be assessed and optimized within a few minutes of the lens being applied.
Figure 8. With Proclear 1 Day Multifocal, the addition of a predetermined amount of plus power to the nondominant eye can improve near vision while binocular distance vision remains unchanged.
There is clearly great potential for daily disposable multifocals in today’s market. The expanding population of over-40s is marked by diverse lifestyle needs that can benefit from one-day disposable lens use. Daily disposables may also meet the needs of patients who have dropped out of lens wear due to vision-related issues. Given the shifting preference for multifocals over monovision, the increased choice of single-use multifocal lens options means that we now have a greater opportunity than ever to meet those needs. CLS
A similar article appeared in the October 5, 2012 issue of Optician.
For references, please visit www.clspectrum.com/references.asp and click on document #208.