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Monovision versus RGP translating bifocals
Journal of the British Contact Lens Association, Vol. 14, No. 4, pp 173-178, 1991
©1991 British Contact Lens Association
Printed in Great Britain
MONOVISION VERSUS RGP TRANSLATING BIFOCALS G r a h a m O. M a c a l i s t e r * a n d Craig A. Woods'~ KEY WORDS:
Presbyopia, monovision, RGP translating bifocals.
L L current bifocal designs involve some ~type of visual compromise. In simultaneous (non-translating) bifocals, vi~',ion from both distance and near is provided simultaneously through the pupil, leading to a loss of contrast, and the possibility of ghost images. Diffractive lenses have been introduced in recent years, which have the advantage over concentric and aspheric refractive designs that their performance is independent of pupil size, but the problem of contrast loss still remains. In translating bifocals the lens must move up relative to the pupil when vision through the near portion is required. It is not always possible to achieve sufficiently free translation to allow full pupil coverage. If infringement by the inappropriate portion of the lens is large enough, the quality of vision will take on some of the attributes of a simultaneous lens. Another problem is that vision appears to fluctuate if the lens is pulled up during the blink. The restricted movement characteristics of a soft lens mean that soft translating bifocals are rarely used. The success of rigid designs is very dependent on the skilful choice of parameters, and therefore these lenses are timeconsuming and expensive to fit. However, they do offer, at least in principle, the least visual compromise. The problems associated with bifocal contact lenses, and their expense, have led practitioners to favour monovision, even though it must compromise normal binocular function to some extent. A survey by Pearson 1 of British Contact Lens Association (BCLA) members showed that, while only a small proportion of presbyopes were corrected solely with contact lenses, just over half wore monovision, with t h e remainder wearing some sort of bifocal. In fact, monovision has become the yardstick against which the success of other bifocal lenses is measured. A number of clinical trials have been carried out which make a direct comparison with monovision. These include Molinari2, Papas et al. 3, Collins et al. 4, Back et al. 5, and Saunders. 6 However, only one triaV found a bifocal lens to be more acceptable than monovision. In recent years some of the technical problems
A
* BSc, FBCO, DCLP. ~ BSc, MBCO, DCLP.
associated with the production of rigid gas permeable (RGP) translating bifocals have been overcome. One particular lens, the Tangent Streak Bifocal, has been used in a number of trials in America. The success rates reported were 70%, s 82%, 9 91%, 1° and 93%.11 These success rates appear good, but it has been pbinted out that comparison with the results of other lenses is difficult because of the lack of a control group, and the lack of standardisation of subject selection and success criteria. A cross-over trial overcomes these objections. In fact, because each subject experiences all the modalities being compared, each subject effectively becomes his or her own control. The aim of this study was to see whether a RGP bifocal, which in theory should offer the best visual results of all bifocals, would challenge the position held by monovision, when a direct comparison was made in a cross-over study. Method
Lenses The Tangent Streak Bifocal is a solid one-piece design. It resembles an executive-type spectacle bifocal, in that the junction between the distance and near portions is formed by a straight line across the whole lens, giving a wide reading area. The line forms a slight ledge on the front surface, which is deepest at the edge of the lens, and reduces to zero in t h e centre, where the distance and near curves meet at a tangent. The maximum depth of this ledge would by 0.034mm for a +2.00 add, 9.4 diameter lens. The lens is stabilised by the use of prism ballast and a truncation, and has considerably more substance than an equivalent single-vision lens. A typical thickness distribution for a moderately powered lens would be 0.17mm at the top, 0.45mm in the centre, and 0.55mm at the prism base. This necessitates a high permeability material, and the standard material Fluorex 700 (nominal Dk 70 F a t t units) was used in this trial. All the lens parameters can be specified by the practitioner, so that each lens can be tailor-made for each patient. It was part of the trial protocol that every effort should be made to ensure that each lens should give an optimal performance, and there was no restriction on the number of lenses that could be ordered, or on the number of modifications. Fitting was based on a combination of the manufacturer's
173
G R A H A M O. M A C A L I S T E R AND C R A I G A. WOODS
recommendations and of information from our own fitting study. The monovision lenses were made from the same material, and their design was usually based on the single-vision lenses that were being worn prior to the trial. The dominant eye was given the distance correction, except where the subject's occupation or visual environment indicated otherwise. A full distance correction was prescribed, and the other eye was given the minimum near addition that would allow clear near vision in the range required by the subject.
Subjects Twenty subjects were enrolled. The following inclusion criteria were drawn up in order to avoid bias in favour of either monovision or the bifocal. All subjects were adapted RGP lens wearers, with no previous experience of bifocal contact lenses or monovision. They had equal acuities in each eye and normal binocular vision with no need of prism or orthoptic treatment. All subjects underwent a tolerance trial with the bifocal lens, to ensure that the lower lid was sufficiently taut to support the truncated edge and permit translation. The refractive error profile for the 20 subjects is shown in Figures 1 and 2.
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Distance sphere Figure 1. Subject profile, distance prescription (20 subjects, 40 eyes). 7
/ I 3
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3
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.1.75
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Figure 2. Subject profile, near addition (20 subjects
enrolled).
174
Procedure The subjects were randomly assigned into two equal groups. Group A started with the bifocal, then went on to monovision, while Group B made the comparison in the reverse order. Attempts were made to ensure the two wearing periods were equal, and the minimum trial was for 10 weeks, with a 2-week wash-out period of single-vision lens wear separating the bifocal and monovision periods. At the end of the trial, each subject was asked to state a preference between the two alternatives, and was allowed to retain these lenses. Measurements The subjects were also asked to rate the bifocal and monovision lenses for comfort, clarity of vision at distance and near, and stability of vision at distance and near. A ten-point subjective scale was used. For example, a stability score of 0 would indicate that the vision was constantly fluctuating, whereas a score of 10 meant that there was no fluctuation at all. Objective comparisons were also made. Visual acuity was measured using LogMAR charts, with 90% and 10% contrast. This was done at high luminance and at reduced luminance by using a 1 log unit neutral density filter. Distance stereopsis was measured using the Howard-Dolman test, and near stereopsis was measured with the Randot test. The Kirshner test was used to assess dynamic visual performance. This measured reading speed and accuracy, and w a s therefore more akin to a real-life situation. The task was to scan along a line of letters arranged in groups of three, and indicate the matching groups. Results and Discussion One subject was unsuccessful with both the bifocal and m0novision, and another was unsuccessful with the bifocal, but managed to wear monovision. Data f r o m the remaining 18 subjects, who wore both modalities, were used to analyse the differences in performance. Figures 3 and 4 show the mean visual acuity .scores for distance. It can be seen that the bifocal gave better acuity under all luminance and contrast conditions, but only by a small amount. Analysis of the acuity differences for each individual showed no statistically significant difference between monovision and the bifocal. The data for near visual acuity show no significant differences. It is worth noting that for low contrast letters, monovision performed slightly better than the bifocal (Figures 5 and 6). This is probably due to incomplete coverage of the pupil by the near portion. We established that our subjects found it difficult to tolerate encroachment of the near portion during distance gaze, but managed to ignore similar
MONOVISION VERSUS RGP TRANSLATING BIFOCALS
encroachment when reading. Thus, we tended to compromise near vision rather than distance vision when translation was not sufficient to clear the pupil completely. It is interesting to note that the low contrast charts do, in fact, appear to be more sensitive in detecting this type of blur.
6/6
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Figure 6. Near acuity (LogMAR) at reduced luminance. Again, the bifocal performs less well at low contrast.
6/9
03 6/12
86
One of the advantages anticipated for the bifocal lens over monovision was that there would be less Monovision Bifocal Monovision Bifocal impairment of binocular function. Figure 7 shows that 9 0 % Contrast 10% Contrast there is a statistical difference for distance stereopsis, No sig. difference No sig. difference but it is debatable whether this small improvement Figure 3. Distance acuity (LogMAR) at high luminance. Error bars are the standard error of the mean of the in stereopsis is significant in the clinical sense. The individual differences in performance between monovision difference is, in fact, greater in the Randot test of near stereopsis, but the wide variation in response and the bifocal. means that it is not statistically different. It is worth n o t i n g that McGill and Erickson 12 found that simultaneous bifocals produced at least as much reduction 105 in stereopsis as monovision. 8O
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Figure 7. Stereopsis, using the Howard-Dolman test for distance, and the Randot test for near.
6/6
.-•90
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DISTANCE Sig. Difference p =,O.O5
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6/12
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Figure 4. luminance.
03
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Monovision
Bifocal
10% Contrast No sig. difference
Figure 5. Near acuity (LogMAR) at high luminance. Note that at lo~v contrast the bifocal performs less well compared to monovision.
All the tests mentioned so far have been static, and have not involved scanning or tracking eye movements, which might be expected to be more difficult to co-ordinate with monovision. However, there was no significant difference in reading speed and accuracy using the Kirshner test. The objective tests have shown very little difference in performance between monovision and the bifocal. But how did the subjects themselves rate the two modalities? Figure 8 shows how each individual compared the bifocal and monovision with reference
175
GRAHAM 0. MACALISTER AND CRAIG A. WOODS
to distance vision clarity. It can be seen that four subjects found no difference, while five subjects rated monovision better by one point on the ten-point scale. However, when all the scores were analysed it was found that there was no statistically significant difference. The same applies to the subjective rating of near vision clarity (Figure 9). Thus it appears that when the group is taken as a whole, both the objective measurement of acuity and the patients' own subjective impressions show little difference between the bifocal and monovision. However, it should be remembered that some individual subjects went against this trend; for example, Figure 8 shows that two subjects scored the bifocal better for distance clarity by a margin of 4 points. We would expect vision with the bifocal to fluctuate on blinking, and Figures 10 and 11 show a strong bias in favour of monovision from the point of view of stability of vision. There is a similar preference
for monovision in terms of comfort (Figure 12). The reason for this probably lies in the increased thickness of this particular bifocal lens, and in the fact that it needs to be fitted flat in order to encourage free movement and translation. The increased lens mobility, compared to singlevision lenses, caused some patients to be afraid that the bifocal lens was going to dislodge, which sometimes did happen on extreme eye movements. A number of subjects found difficulty with night driving because of flare, induced when the pupil expanded to encompass the near portion. Another complaint was the need to make a conscious effort to adjust head or lid position when changing gaze direction, or distance. This was in contrast to monovision, which was described as effortless in this respect. Some comments regarding monovision are also worth noting. These included &wareness of not using both eyes, and a feeling Of being 'off balance',
4
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0
Monovision
Bifocal
Monovision
1
1
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Subjective score difference
Figure 8. Subjective rating on a ten-point scale of distance vision clarity. Four subjects found no difference, five subjects favoured monovision by the margin of one point, and so on. There was no statistical difference.
Bifocal
Figure 10. Subjective rating of distance fluctuation. There was a significant difference in favour of monovision, p < 0.01 (Wilcoxon's Signed Ranks Test).
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1
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3
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0
i
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ie 3
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Subjective score difference Monovision Bifocal
Figure 9. Subjective rating of near clarity. There was n o statistical difference.
176
6
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1
0
1
2
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4
5
6
Subjective score difference Monovision
Bifocal
Figure 11. Subjective rating of near fluctuation. There was a significant difference in favour of monovision, p < 0.01 (Wilcoxon 's Signed Ranks Test).
MONOVISION VERSUS RGP TRANSLATING BIFOCALS
especially in dynamic situations. Some subjects also reported difficulty in conditions of low illumination and low contrast. It is important to know how all these factors determine whether or not a lens is acceptable to a patient. If we look at success rates only, and define the criterion for success as a minimum of 10 weeks wear, with at least 8h per day, 5 days per week, we find a success with monovision of 95% (19/20) and with the bifocal of 90% (18/20). However, as previously discussed, this type of figure can be misleading, and much more information is obtained when the patient is allowed to make a direct comparison. When asked to make a choice the subject weighs up the advantages and disadvantages of each modality as they affect him personally. Thus, their personality, expectations, and tolerance to blur and to discomfort will all have an influence. So will their visual requirements at work and at leisure, including how far near tasks are below the primary position of gaze. Several subjects found it difficult to decide on an overall preference, either because the advantages and disadvantages appeared to them to be balanced, or because one lens type performed better in one situation, for example at work, while the other option was better for the remainder of the time. In fact, there was statistical evidence to suggest that at the end of the trial subjects tended to prefer the modality they had been wearing most recently, indicating that, for some at least, there was little to choose between them.
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Acknowledgements
1
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Subjective score difference Monovision
presents the practitioner with a dilemma. If he chooses to fit all his patients with monovision simply because it is less time-consuming and less expensive, he will be denying half his patients the benefits of the bifocal lens that they may have preferred. This does not mean that these patients would not have adapted to monovision if they had been given it, just that they would have preferred the compromises inherent in the bifocal to those in monovision. In fact, most of our subjects did not take long to adapt to monovision. The result of this trial shows that both the RGP translating bifocal and monovision have a role to play in the correction of presbyopia with contact lenses. On balance, they performed equally on the aspects of vision tested, though some subjects favoured one or the other. Practitioners should choose fitting parameters that will minimise fluctuation of vision, without having an adverse effect on comfort. In order to decide which option to fit, practitioners should undertake a trial fitting with the bifocal lens. This is the only way to anticipate problems with lens dynamics. They should then examine the individual patient's visual requirements, attitudes and expectations, in order to evaluate how the anticipated problems will affect that particular patient. It is more difficult to predict the outcome of monovision. Unfortunately, there is no instrument available for use in the consulting room that can evaluate a patient's interocular suppresion characteristics. However, a tolerance trial might be simpler, and just as effective. The main conclusion from this trial is that, while monovision and RGP translating bifocals are equally effective, they have differing advantages and disadvantages. Success lies in matching these to the needs of the individual patient.
Bifocal
Figure 12. Subjective rating of comfort. There was a significant difference in favour of monovision, p < 0.01 (Wilcoxon 's Signed Ranks Test).
When the 18 subjects who completed the trial were asked to state a preference, the results were m o n o v i s i o n 10 and bifocal 8. These figures were analysed using Fisher's Exact Test, and no significant difference was found. This means that when a subject is given the choice, the chances of opting for a rigid translating bifocal are equal to the chances of opting for monovision. This
The authors thank Fused Kontacts of Chicago and the No. 7 Contact Lens Laboratory of London, for supplying and modifying the lenses used in this trial; Chris Wilson of Barts Medical College for advice on the statistical methods used; Eric Papas and Geoff Roberson for their advice and support; a n d Eva Macalister for co-ordinating the trial, which was carried out at the Institute of Optometry, London.
Address for correspondence Graham Macalister, 'Fingellan', Whines Lane, Over, Cambridgeshire, United Kingdom. REFERENCES 1 Pearson, R. Contact lens trends in the UK in 1991. J. Br. Contact Lens Assoc., submitted for publication. 2 Molinari, J.F. A clinical comparison of subjective effectiveness of monovision aperture dependent and independent bifocal hydrogel lens fittings. J. Br. Contact Lens Assoc., 11, 58-59 (1988) Papas, E., Young, G., and Hearn, K. Monovision versus diffractive bifocals. J. Br. Contact Lens Assoc., 12, 75-76 (1989).
177
GRAHAM 0. MACALISTER AND CRAIG A. WOODS 4 Collins, M.J., Brown, B., and Bowman, K.J. Short term responses to soft lens correction for presbyopia. Clin. Exp. Optom., 2(2), 40-45 (1989). 5 Back, A.P., Holden, B.A., and Hine, N.A. Correction of presbyopia with contact lenses: comparative success rates with three systems. Optom. Vis. Sci., 66(8), 518-525 (1989). 6 Saunders, B.D. Optical performance of bifocal contact lenses in vivo. J. Br. Contact Lens Assooc., 12, 71-74 (1989). 7 Josephson, J.E. and Caffery, B.E. Monovision versus bifocal contact lenses: a crossover study. J. Am. Optom. Assoc., 58, 652-654 (1987). s Jos'el~hson, J . E . , Wong, M., and Caffery, B . E . Clinical experi-
178
ence with the Tangent Streak rigid bifocal contact lens. J. Am. Optom. Assoc., 60, 166-170 (1989). Remba, M.J. The Tangent Streak rigid gas permeable bifocal contact lens. J. Am. Optom. Assoc., 59, 212-216 (1988). 10 Kirman, S.T., and Kirman, G.S. The Tangent Streak bifocal lens, Contact Lens Forum 1988. 11 Van Meter, W.S., Gussler, J.R., and Litteral, G. Clinical evaluation of three bifocal contact lenses. CLAO J., 16, 203-207 (1990). 12 McGill, E., and Erickson, P. Stereopsis in presbyopes wearing monovision and simultaneous vision bifocal contact lenses. Am. J. Optom. Physiol. Opt., 65, 612-626 (1988).
©1991 British Contact Lens Association
Printed in Great Britain
MONOVISION VERSUS RGP TRANSLATING BIFOCALS G r a h a m O. M a c a l i s t e r * a n d Craig A. Woods'~ KEY WORDS:
Presbyopia, monovision, RGP translating bifocals.
L L current bifocal designs involve some ~type of visual compromise. In simultaneous (non-translating) bifocals, vi~',ion from both distance and near is provided simultaneously through the pupil, leading to a loss of contrast, and the possibility of ghost images. Diffractive lenses have been introduced in recent years, which have the advantage over concentric and aspheric refractive designs that their performance is independent of pupil size, but the problem of contrast loss still remains. In translating bifocals the lens must move up relative to the pupil when vision through the near portion is required. It is not always possible to achieve sufficiently free translation to allow full pupil coverage. If infringement by the inappropriate portion of the lens is large enough, the quality of vision will take on some of the attributes of a simultaneous lens. Another problem is that vision appears to fluctuate if the lens is pulled up during the blink. The restricted movement characteristics of a soft lens mean that soft translating bifocals are rarely used. The success of rigid designs is very dependent on the skilful choice of parameters, and therefore these lenses are timeconsuming and expensive to fit. However, they do offer, at least in principle, the least visual compromise. The problems associated with bifocal contact lenses, and their expense, have led practitioners to favour monovision, even though it must compromise normal binocular function to some extent. A survey by Pearson 1 of British Contact Lens Association (BCLA) members showed that, while only a small proportion of presbyopes were corrected solely with contact lenses, just over half wore monovision, with t h e remainder wearing some sort of bifocal. In fact, monovision has become the yardstick against which the success of other bifocal lenses is measured. A number of clinical trials have been carried out which make a direct comparison with monovision. These include Molinari2, Papas et al. 3, Collins et al. 4, Back et al. 5, and Saunders. 6 However, only one triaV found a bifocal lens to be more acceptable than monovision. In recent years some of the technical problems
A
* BSc, FBCO, DCLP. ~ BSc, MBCO, DCLP.
associated with the production of rigid gas permeable (RGP) translating bifocals have been overcome. One particular lens, the Tangent Streak Bifocal, has been used in a number of trials in America. The success rates reported were 70%, s 82%, 9 91%, 1° and 93%.11 These success rates appear good, but it has been pbinted out that comparison with the results of other lenses is difficult because of the lack of a control group, and the lack of standardisation of subject selection and success criteria. A cross-over trial overcomes these objections. In fact, because each subject experiences all the modalities being compared, each subject effectively becomes his or her own control. The aim of this study was to see whether a RGP bifocal, which in theory should offer the best visual results of all bifocals, would challenge the position held by monovision, when a direct comparison was made in a cross-over study. Method
Lenses The Tangent Streak Bifocal is a solid one-piece design. It resembles an executive-type spectacle bifocal, in that the junction between the distance and near portions is formed by a straight line across the whole lens, giving a wide reading area. The line forms a slight ledge on the front surface, which is deepest at the edge of the lens, and reduces to zero in t h e centre, where the distance and near curves meet at a tangent. The maximum depth of this ledge would by 0.034mm for a +2.00 add, 9.4 diameter lens. The lens is stabilised by the use of prism ballast and a truncation, and has considerably more substance than an equivalent single-vision lens. A typical thickness distribution for a moderately powered lens would be 0.17mm at the top, 0.45mm in the centre, and 0.55mm at the prism base. This necessitates a high permeability material, and the standard material Fluorex 700 (nominal Dk 70 F a t t units) was used in this trial. All the lens parameters can be specified by the practitioner, so that each lens can be tailor-made for each patient. It was part of the trial protocol that every effort should be made to ensure that each lens should give an optimal performance, and there was no restriction on the number of lenses that could be ordered, or on the number of modifications. Fitting was based on a combination of the manufacturer's
173
G R A H A M O. M A C A L I S T E R AND C R A I G A. WOODS
recommendations and of information from our own fitting study. The monovision lenses were made from the same material, and their design was usually based on the single-vision lenses that were being worn prior to the trial. The dominant eye was given the distance correction, except where the subject's occupation or visual environment indicated otherwise. A full distance correction was prescribed, and the other eye was given the minimum near addition that would allow clear near vision in the range required by the subject.
Subjects Twenty subjects were enrolled. The following inclusion criteria were drawn up in order to avoid bias in favour of either monovision or the bifocal. All subjects were adapted RGP lens wearers, with no previous experience of bifocal contact lenses or monovision. They had equal acuities in each eye and normal binocular vision with no need of prism or orthoptic treatment. All subjects underwent a tolerance trial with the bifocal lens, to ensure that the lower lid was sufficiently taut to support the truncated edge and permit translation. The refractive error profile for the 20 subjects is shown in Figures 1 and 2.
6
6 5
i ,,il,i,
i,,i: i
-10-9
-8
-7
-6
-5
-4
-3
-2
-1
0
+1 +2 +3 +4 +5 +6 +7 +8
Distance sphere Figure 1. Subject profile, distance prescription (20 subjects, 40 eyes). 7
/ I 3
.1.00
3
*1.25
/
|
÷1.50
.1.75
*2.00
*2.25
Near add
Figure 2. Subject profile, near addition (20 subjects
enrolled).
174
Procedure The subjects were randomly assigned into two equal groups. Group A started with the bifocal, then went on to monovision, while Group B made the comparison in the reverse order. Attempts were made to ensure the two wearing periods were equal, and the minimum trial was for 10 weeks, with a 2-week wash-out period of single-vision lens wear separating the bifocal and monovision periods. At the end of the trial, each subject was asked to state a preference between the two alternatives, and was allowed to retain these lenses. Measurements The subjects were also asked to rate the bifocal and monovision lenses for comfort, clarity of vision at distance and near, and stability of vision at distance and near. A ten-point subjective scale was used. For example, a stability score of 0 would indicate that the vision was constantly fluctuating, whereas a score of 10 meant that there was no fluctuation at all. Objective comparisons were also made. Visual acuity was measured using LogMAR charts, with 90% and 10% contrast. This was done at high luminance and at reduced luminance by using a 1 log unit neutral density filter. Distance stereopsis was measured using the Howard-Dolman test, and near stereopsis was measured with the Randot test. The Kirshner test was used to assess dynamic visual performance. This measured reading speed and accuracy, and w a s therefore more akin to a real-life situation. The task was to scan along a line of letters arranged in groups of three, and indicate the matching groups. Results and Discussion One subject was unsuccessful with both the bifocal and m0novision, and another was unsuccessful with the bifocal, but managed to wear monovision. Data f r o m the remaining 18 subjects, who wore both modalities, were used to analyse the differences in performance. Figures 3 and 4 show the mean visual acuity .scores for distance. It can be seen that the bifocal gave better acuity under all luminance and contrast conditions, but only by a small amount. Analysis of the acuity differences for each individual showed no statistically significant difference between monovision and the bifocal. The data for near visual acuity show no significant differences. It is worth noting that for low contrast letters, monovision performed slightly better than the bifocal (Figures 5 and 6). This is probably due to incomplete coverage of the pupil by the near portion. We established that our subjects found it difficult to tolerate encroachment of the near portion during distance gaze, but managed to ignore similar
MONOVISION VERSUS RGP TRANSLATING BIFOCALS
encroachment when reading. Thus, we tended to compromise near vision rather than distance vision when translation was not sufficient to clear the pupil completely. It is interesting to note that the low contrast charts do, in fact, appear to be more sensitive in detecting this type of blur.
6/6
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t
9O
6/9 I
t
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6o
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105
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6/6
100
Bifocal
Monovision
9 0 % Contrast No sig. difference q)
Bifocal
10% Contrast No sig. difference
Figure 6. Near acuity (LogMAR) at reduced luminance. Again, the bifocal performs less well at low contrast.
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86
One of the advantages anticipated for the bifocal lens over monovision was that there would be less Monovision Bifocal Monovision Bifocal impairment of binocular function. Figure 7 shows that 9 0 % Contrast 10% Contrast there is a statistical difference for distance stereopsis, No sig. difference No sig. difference but it is debatable whether this small improvement Figure 3. Distance acuity (LogMAR) at high luminance. Error bars are the standard error of the mean of the in stereopsis is significant in the clinical sense. The individual differences in performance between monovision difference is, in fact, greater in the Randot test of near stereopsis, but the wide variation in response and the bifocal. means that it is not statistically different. It is worth n o t i n g that McGill and Erickson 12 found that simultaneous bifocals produced at least as much reduction 105 in stereopsis as monovision. 8O
6/6
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80
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Distance
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at
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Monoviaion
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t
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75
Monovision
Bifocal
NEAR No Sig. Diff
Figure 7. Stereopsis, using the Howard-Dolman test for distance, and the Randot test for near.
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.-•90
70
0
DISTANCE Sig. Difference p =,O.O5
100 95
t
Bifocal
No sig. difference
acuity
00
6/12
10% Contrast
No sig. difference
Figure 4. luminance.
03
Bifocal
9 0 % Contrast No sig. difference
Monovision
Bifocal
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Figure 5. Near acuity (LogMAR) at high luminance. Note that at lo~v contrast the bifocal performs less well compared to monovision.
All the tests mentioned so far have been static, and have not involved scanning or tracking eye movements, which might be expected to be more difficult to co-ordinate with monovision. However, there was no significant difference in reading speed and accuracy using the Kirshner test. The objective tests have shown very little difference in performance between monovision and the bifocal. But how did the subjects themselves rate the two modalities? Figure 8 shows how each individual compared the bifocal and monovision with reference
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to distance vision clarity. It can be seen that four subjects found no difference, while five subjects rated monovision better by one point on the ten-point scale. However, when all the scores were analysed it was found that there was no statistically significant difference. The same applies to the subjective rating of near vision clarity (Figure 9). Thus it appears that when the group is taken as a whole, both the objective measurement of acuity and the patients' own subjective impressions show little difference between the bifocal and monovision. However, it should be remembered that some individual subjects went against this trend; for example, Figure 8 shows that two subjects scored the bifocal better for distance clarity by a margin of 4 points. We would expect vision with the bifocal to fluctuate on blinking, and Figures 10 and 11 show a strong bias in favour of monovision from the point of view of stability of vision. There is a similar preference
for monovision in terms of comfort (Figure 12). The reason for this probably lies in the increased thickness of this particular bifocal lens, and in the fact that it needs to be fitted flat in order to encourage free movement and translation. The increased lens mobility, compared to singlevision lenses, caused some patients to be afraid that the bifocal lens was going to dislodge, which sometimes did happen on extreme eye movements. A number of subjects found difficulty with night driving because of flare, induced when the pupil expanded to encompass the near portion. Another complaint was the need to make a conscious effort to adjust head or lid position when changing gaze direction, or distance. This was in contrast to monovision, which was described as effortless in this respect. Some comments regarding monovision are also worth noting. These included &wareness of not using both eyes, and a feeling Of being 'off balance',
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Figure 8. Subjective rating on a ten-point scale of distance vision clarity. Four subjects found no difference, five subjects favoured monovision by the margin of one point, and so on. There was no statistical difference.
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Figure 10. Subjective rating of distance fluctuation. There was a significant difference in favour of monovision, p < 0.01 (Wilcoxon's Signed Ranks Test).
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Figure 9. Subjective rating of near clarity. There was n o statistical difference.
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Figure 11. Subjective rating of near fluctuation. There was a significant difference in favour of monovision, p < 0.01 (Wilcoxon 's Signed Ranks Test).
MONOVISION VERSUS RGP TRANSLATING BIFOCALS
especially in dynamic situations. Some subjects also reported difficulty in conditions of low illumination and low contrast. It is important to know how all these factors determine whether or not a lens is acceptable to a patient. If we look at success rates only, and define the criterion for success as a minimum of 10 weeks wear, with at least 8h per day, 5 days per week, we find a success with monovision of 95% (19/20) and with the bifocal of 90% (18/20). However, as previously discussed, this type of figure can be misleading, and much more information is obtained when the patient is allowed to make a direct comparison. When asked to make a choice the subject weighs up the advantages and disadvantages of each modality as they affect him personally. Thus, their personality, expectations, and tolerance to blur and to discomfort will all have an influence. So will their visual requirements at work and at leisure, including how far near tasks are below the primary position of gaze. Several subjects found it difficult to decide on an overall preference, either because the advantages and disadvantages appeared to them to be balanced, or because one lens type performed better in one situation, for example at work, while the other option was better for the remainder of the time. In fact, there was statistical evidence to suggest that at the end of the trial subjects tended to prefer the modality they had been wearing most recently, indicating that, for some at least, there was little to choose between them.
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presents the practitioner with a dilemma. If he chooses to fit all his patients with monovision simply because it is less time-consuming and less expensive, he will be denying half his patients the benefits of the bifocal lens that they may have preferred. This does not mean that these patients would not have adapted to monovision if they had been given it, just that they would have preferred the compromises inherent in the bifocal to those in monovision. In fact, most of our subjects did not take long to adapt to monovision. The result of this trial shows that both the RGP translating bifocal and monovision have a role to play in the correction of presbyopia with contact lenses. On balance, they performed equally on the aspects of vision tested, though some subjects favoured one or the other. Practitioners should choose fitting parameters that will minimise fluctuation of vision, without having an adverse effect on comfort. In order to decide which option to fit, practitioners should undertake a trial fitting with the bifocal lens. This is the only way to anticipate problems with lens dynamics. They should then examine the individual patient's visual requirements, attitudes and expectations, in order to evaluate how the anticipated problems will affect that particular patient. It is more difficult to predict the outcome of monovision. Unfortunately, there is no instrument available for use in the consulting room that can evaluate a patient's interocular suppresion characteristics. However, a tolerance trial might be simpler, and just as effective. The main conclusion from this trial is that, while monovision and RGP translating bifocals are equally effective, they have differing advantages and disadvantages. Success lies in matching these to the needs of the individual patient.
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Figure 12. Subjective rating of comfort. There was a significant difference in favour of monovision, p < 0.01 (Wilcoxon 's Signed Ranks Test).
When the 18 subjects who completed the trial were asked to state a preference, the results were m o n o v i s i o n 10 and bifocal 8. These figures were analysed using Fisher's Exact Test, and no significant difference was found. This means that when a subject is given the choice, the chances of opting for a rigid translating bifocal are equal to the chances of opting for monovision. This
The authors thank Fused Kontacts of Chicago and the No. 7 Contact Lens Laboratory of London, for supplying and modifying the lenses used in this trial; Chris Wilson of Barts Medical College for advice on the statistical methods used; Eric Papas and Geoff Roberson for their advice and support; a n d Eva Macalister for co-ordinating the trial, which was carried out at the Institute of Optometry, London.
Address for correspondence Graham Macalister, 'Fingellan', Whines Lane, Over, Cambridgeshire, United Kingdom. REFERENCES 1 Pearson, R. Contact lens trends in the UK in 1991. J. Br. Contact Lens Assoc., submitted for publication. 2 Molinari, J.F. A clinical comparison of subjective effectiveness of monovision aperture dependent and independent bifocal hydrogel lens fittings. J. Br. Contact Lens Assoc., 11, 58-59 (1988) Papas, E., Young, G., and Hearn, K. Monovision versus diffractive bifocals. J. Br. Contact Lens Assoc., 12, 75-76 (1989).
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GRAHAM 0. MACALISTER AND CRAIG A. WOODS 4 Collins, M.J., Brown, B., and Bowman, K.J. Short term responses to soft lens correction for presbyopia. Clin. Exp. Optom., 2(2), 40-45 (1989). 5 Back, A.P., Holden, B.A., and Hine, N.A. Correction of presbyopia with contact lenses: comparative success rates with three systems. Optom. Vis. Sci., 66(8), 518-525 (1989). 6 Saunders, B.D. Optical performance of bifocal contact lenses in vivo. J. Br. Contact Lens Assooc., 12, 71-74 (1989). 7 Josephson, J.E. and Caffery, B.E. Monovision versus bifocal contact lenses: a crossover study. J. Am. Optom. Assoc., 58, 652-654 (1987). s Jos'el~hson, J . E . , Wong, M., and Caffery, B . E . Clinical experi-
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ence with the Tangent Streak rigid bifocal contact lens. J. Am. Optom. Assoc., 60, 166-170 (1989). Remba, M.J. The Tangent Streak rigid gas permeable bifocal contact lens. J. Am. Optom. Assoc., 59, 212-216 (1988). 10 Kirman, S.T., and Kirman, G.S. The Tangent Streak bifocal lens, Contact Lens Forum 1988. 11 Van Meter, W.S., Gussler, J.R., and Litteral, G. Clinical evaluation of three bifocal contact lenses. CLAO J., 16, 203-207 (1990). 12 McGill, E., and Erickson, P. Stereopsis in presbyopes wearing monovision and simultaneous vision bifocal contact lenses. Am. J. Optom. Physiol. Opt., 65, 612-626 (1988).