Comparing Multifocals and Monovision
Comparing Multifocals and Monovision
Find out which presbyopic modality patients preferred after trying both in lenses of the same material.
By William J. Benjamin, OD, MS, PhD
Dr. Benjamin is Professor of Optometry and Vision Science, a Senior Scientist at the Vision Science Research Center, and a clinician in Contact Lens Practice and Primary Eye Care at the University of Alabama at Birmingham.
With 78 million baby boomers, the demand for presbyopic vision correction is high — especially for a generation that doesn't readily accept the concept of aging. Among this demographic, the popularity of cosmetic teeth whitening, liposuction and anti-wrinkle treatments all testify to this generation's desire to age gracefully. The use of reading or bifocal glasses may seem as negative to boomers as having yellow teeth, bulging tummies and sagging skin.
Of course with presbyopia, surgical options to restore accommodation are limited to multifocal intraocular lens implantation. Hence, presbyopic contact lens correction is a logical alternative, although one that's still under-recognized by many presbyopes and practitioners.
Presbyopic Lens Options
Monovision is a traditional strategy to convert from reading glasses to contact lenses. It's an efficient way of allowing the visual system of most individuals to alternate between distance and near vision in spite of the fact that the optical correction is simultaneously present at distance and near (Benjamin and Borish, 2006).
However, multifocal soft lenses are beginning to challenge monovision's status as practitioners' preferred presbyopic treatment. Indeed, an increasing body of literature supports that patients generally prefer some multifocal soft contact lenses over monovision when given the choice. Situ et al (2003) refit 50 successful monovision wearers into soft bifocals, and 68 percent of the subjects completing the study preferred the bifocal lens compared to 25 percent for monovision. Richdale et al (2006) had 38 subjects compare soft multifocals to spherical lenses for monovision. Seventy-six percent of their subjects preferred the quality of vision with multifocal lenses versus 24 percent with monovision.
A Comparative Study
We designed a study to compare the Proclear Multifocal (MF) (CooperVision) to monovision using Proclear (CooperVision) spherical lenses. Both modes of correction involved the material omafilcon A. The Proclear MF design is called balanced progressive technology (BPT) in that it incorporates a distance-center multifocal lens for the dominant eye and a near-center multifocal lens for the non-dominant eye (Figure 1). It's important for the 5mm-diameter optic zone to be well centered over the pupil to achieve optimal vision with these lenses.
Figure 2 shows the optic zones of a matched pair of Proclear MF lenses, revealing the method of incorporating modified monovision. The "D lens" is a distance-center concentric design that has a 2.3mm central spherical optic zone surrounded by an aspheric peripheral annulus that adds plus power out to a diameter of 5.0mm. The "N lens" is a near-center concentric design that has a smaller central spherical optic zone (1.7mm diameter) surrounded by an aspheric peripheral annulus that adds minus power out to a diameter of 5.0mm. The design presents a middle ground between the traditional concentric spherical design and the familiar concentric aspheric design (Benjamin & Borish 2006). In both lenses a progression of intermediate powers is achieved in the surrounding aspheric optical annulus, reminiscent of "modified trivision" formerly described using concentric bifocals (Pence 1987).
We enrolled 48 presbyopic subjects into a single-masked clinical crossover study in the Clinical Eye Research Facility at the University of Alabama at Birmingham, School of Optometry. Table 1 shows that participants included new lens wearers (14), previous soft lens wearers (14) and current soft lens wearers (20). Of the latter, 12 were already in a presbyopic mode of contact lens correction (six in monovision and six in soft bi/multifocals). We imposed a washout period of one week on these 12 subjects before beginning the current study. Near spectacle additions ranged from +1.00D to +2.50D (Table 2). Enrollment was limited to refractive error of +3.00D to -6.00D equivalent sphere at the corneal plane with 0.75D or less of astigmatism (Table 3). Subjects completing the study received a complimentary one-year supply of contact lenses of their preference, whether monovision or multifocal lenses.
At the initial visit, we documented baseline measurements including refraction, spectacle add power, binocular high- and low-contrast corrected visual acuities at distance and near, global and local near stereoacuity and anterior segment health. We assessed the binocular visual acuities using Bailey-Lovie charts at 4m and the Holladay Contrast Acuity Test at 40cm. Global and local stereoacuities, respectively, were measured using the Randot Preschool Stereoacuity SO-007 test and the Randot Stereotest SO-002.
We also administered three tasks binocularly with the best-spectacle correction for near: a threading task, a straw/dowel task and a computer editing task. These tasks were modified versions of those devised by Sheedy et al (1986, 1998). The first two tasks have a relationship with the degree of binocularity or stereopsis, while the latter is more likely influenced by visual acuity.
To conclude the initial visit, half of the subjects (24) received monovision with Proclear spheres while the remaining half (24) were first prescribed Proclear MF lenses. We instructed almost all patients to use Opti-Free Express (Alcon) except for a few patients whom we permitted to use their habitual lens care system. All patients were directed to remove the Proclear sphere and Proclear MF lenses nightly for daily wear.
At the second visit one week later, we made any necessary refractive power changes to optimize each patient's presbyopic vision. We instructed subjects to return three weeks later for their third visit at one month of wear.
Figure 1. The Balanced Progressive Technology or "BPT" designs of Proclear Multifocal lenses. The central optical zone of 5.0mm should be well centered in front of the pupil to achieve the best vision with these lenses.
At the third visit, participants subjectively rated their presbyopic vision correction for distance, near and overall vision during the wearing period on a scale of 0 (not satisfied) to 100 (completely satisfied). A masked investigator measured global and local stereoacuity, binocular visual acuity at distance (4m) and near (40cm) with high- and low-contrast charts, and administered the threading, straw/dowel and computer editing tasks. Measurements were performed with the patient's presbyopic contact lens correction without an over-refraction. Although the investigator was masked at this visit, it wasn't possible to mask the method of presbyopic correction from the subjects.
At the fourth visit, subjects were crossed over from monovision to multifocal contact lenses or vice versa, depending on which group they started in. At the fifth visit one week later, we made any necessary refractive power changes to optimize each patient's new form of presbyopic vision. At the sixth and final visit after another month of wear, the investigator administered the same measurements performed at visit three, but for the alternative presbyopic correction. Each subject then disclosed his or her preferred presbyopic modality, whether monovision or multifocal contact lenses.
Of the original 48 subjects, 46 completed the study. One office worker who started the study in the multifocal group discontinued because of dry eye discomfort. The other dropout was a person who started in the monovision group, but discontinued because of an inability to make the study visits when two family members were hospitalized.
Of the 46 wearers completing this study that compared two modes of presbyopic correction in the same lens material, 14 chose monovision while 32 chose vision with Proclear MFs. Thus, subjects preferred multifocals to monovision in a 2.28-to-1 ratio — almost a 70 percent preference. In a small group of six previously successful monovision wearers, only two (33 percent) indicated an overall preference for multifocals; but for a group of six previously successful bifocal soft lens wearers, only two (33 percent) preferred monovision. In a larger group of 34 participants with no prior or recent presbyopic contact lens wear, 26 (more than 76 percent, a 3.25-to-1 ratio) selected Proclear MFs over monovision.
Subjective visual impressions of the participants were consistent with their final lens preference. Overall, subjective assessment of vision averaged 3.7 percent to 5.8 percent higher for multifocals for distance, near and overall vision. These differences weren't statistically significant (p>0.10). When we grouped subjects into those preferring multifocals (32 individuals), the subjective visual assessments averaged 14.5 percent to 15.0 percent higher in favor of multifocals and were statistically significant (p<0.05). When we grouped subjects into those choosing monovision (14 individuals), the subjective visual assessments averaged 12.3 percent to 16.6 percent in favor of monovision with statistical significance (p<0.05).
Although subjective visual assessments clearly followed overall preference for multifocals or monovision, there was relative parity between the two modes of presbyopic correction in terms of binocular distance and near visual acuity, local and global near stereoacuity and the ability to perform the three visual tasks. We found no statistically significant differences when we compared multifocals and monovision in terms of binocular high-contrast visual acuity (at 4m and 40cm), low-contrast visual acuity (at 4m and 40cm), stereoacuity (global and local), and the three visual tasks (threading, straw/dowel and computer editing). In other words, measurements of visual function at distance and near didn't seem to predict the subjective preference of multifocals over monovision.
While stereoacuity wasn't statistically distinguishable between multifocals and monovision, the number of subjects achieving finer levels of stereopsis was greater among the multifocal wearers. The number of subjects achieving better than 40" of global stereoacuity was 12 in multifocals versus 8 in monovision. The number achieving better than 40" of local stereoacuity was 15 in multifocals versus 8 in monovision. These trends tend to corroborate studies such as Richdale et al (2006), which found slightly better stereoacuity with multifocal contact lenses than with monovision.
It's perhaps surprising that this study didn't find a significant difference in low-contrast binocular acuity between monovision and multifocals. Peyre et al (2005) demonstrated that soft multifocal lenses increased higher-order aberrations. These aberrations were expected to reduce low-contrast visual acuity in particular. Richdale et al (2006) reported a loss of three letters to four letters of low-contrast acuity for multifocal lenses compared to monovision. Situ et al (2003) found that multifocal correction decreased low-contrast acuity compared to monovision.
Yet Peyre et al (2005) showed that Proclear MF lenses increased higher-order aberrations by the greatest amount of several multifocals tested with wavefront aberrometry. Perhaps presbyopic simultaneous-vision multifocal contact lenses can attain a magnitude of aberration that lends itself to visual preference without causing significant low-contrast acuity loss. This concept would be counter-intuitive for those believing that elimination of higher-order aberrations is theoretically desirable. It's also possible that monochromatic aberrometry doesn't reveal the entire picture occurring with the full visible spectrum (Thibos et al 2006).
As for high-contrast binocular acuity, we again found no statistically significant differences at distance or near between Proclear MF lenses and monovision. Binocular visual acuity was impressive in both modalities; 43 of 46 patients (93.5 percent) achieved vision better than 20/20 at distance and at least 28 of 46 (60.9 percent) achieved better than equivalent 20/20 at near. Only one patient failed to achieve at least 20/20 at distance and only 18 patients did so at near. In no case was binocular acuity worse than 20/25 at distance or near with either modality.
We attribute the difference in binocular acuity at distance versus near to the emphasis on the non-dominant eye at near. Realize that most patients would continue to emphasize the dominant eye at distance and near if the non-dominant eye wasn't forced into dominance. Emphasis of non-dominant eyes at near with the BPT design isn't as intense. However, the BPT approach still emphasizes the non-dominant eye for near vision by placing the spherical near center over its central pupil (whereas it places the spherical distance center there on the dominant eye) and because the spherical near center of the N lens is smaller in diameter than the distance center of the D lens (1.7mm vs. 2.3mm).
Figure 2. The optical zones of Proclear Multifocal (BPT) lenses. In the more central portions of the aspheric optical annuli, the intermediate power is approximately the same in both lenses, which we believe provides better intermediate vision.
Why can BPT be Better than Monovision?
Westin et al (2000) surveyed contact lens practitioners and confirmed that monovision was more popular among them than multifocals for correcting presbyopia. Yet more recent data, including that of this study, indicates that most patients prefer certain multifocal contact lenses over monovision when able to directly compare them. Although monovision may always have a place in clinical practice, the accumulating evidence is that patients may prefer multifocal lenses. Practitioners who successfully respond to this need garner an advantage in an increasingly competitive marketplace, while also servicing their patients' visual requirements.
Although not specifically tested in this study, many participants commented about improved intermediate vision in the range of 1m to 3m with Proclear MF lenses over monovision. Indeed, patients in the multifocal segment of the study were able to read the inscriptions on the sides of our fitting sets located against the wall of the exam room. The inscriptions were at a distance of 2.0m from the patient sitting in the exam chair. The patients couldn't read the inscriptions when wearing monovision. We ascribe this response to the balanced progressive nature of the Proclear MF design, which allowed a form of modified trivision (Pence 1987) in that both eyes had an annular range of intermediate power in the optic zones of the midperiphery of the dominant and non-dominant eyes. This aspect of the design allowed better binocular vision at distances of 1m to 3m, into which many activities fall. In concert with distance and near vision on par with that of monovision, this created the positive patient responses for the Proclear MF reported here. Thus, most patients felt that the vision provided by the Proclear MF lenses was more "useable" than was the vision provided by monovision. A logical next step would be to investigate intermediate vision in a detailed manner, comparing multifocals to monovision in a subsequent study that's now in its initial planning phase.
Another interesting finding of this evaluation was that 38 percent of eyes completing this study (35/92) were within almost 1.00D of emmetropia. Twelve patients required only a single lens in monovision because they were either virtually emmetropic (requiring a lens for only the near eye) or ametropic by the amount of the spectacle addition (requiring a lens for only the distance eye). Nine of these 12 patients preferred to wear Proclear MF and continued to do so after the study — in spite of the fact that they each then had to wear lenses in both eyes! Such successes in this clinical evaluation argue that there's an expanded patient population for presbyopic contact lens correction not normally seen in practitioners' offices, including even those who could get away with wearing only a single lens in monovision. CLS
Dr. Benjamin's special thanks go to Ms. Maria Voce of the Clinical Eye Research Facility for her diligence and enthusiasm in coordinating this study. Dr. Kent Daum, Dr. Robert Rutstein and Dr. James Sheedy are gratefully acknowledged for contributing specific aspects of the testing methodology. The project was funded with a grant from CooperVision Corporation to the University of Alabama at Birmingham in the name of Dr. Benjamin.
For references, please visit www.clspectrum.com/references.asp and click on document #140.
Contact Lens Spectrum, Issue: July 2007