ARTICLE
Anomalies in the Prescribing of Soft Contact Lens Power
Graeme Young, M.Phil., Ph.D., F.C.Optom., D.C.L.P., F.A.A.O., Kurt Moody, O.D., F.A.A.O.,
and Anna Sulley, B.Sc., M.C.Optom.
Objectives: To determine the proportion of prescribed soft lenses rounded to
the nearest half diopter and any variations from country to country and
between lens types.
Methods: Marketing data were obtained for soft lenses supplied during a
1-year period for lenses representing each of the following categories:
mid-water hydrogel (MWH), silicone hydrogel, daily disposable, and toric
silicone hydrogel (TSH). The data were analyzed for several countries/
regions. Spherical lenses were analyzed in the range 1.00 to 5.75 D for plus
and minus powers, and toric lenses in the range 0.50 to 5.75 D. This
ensured a similar number of lenses in full or half diopter powers were
compared with quarter and three-quarter diopter powers, and that there was
no enforced rounding due to nonavailability of powers. By comparing the
proportion of lenses from the 2 power groups, the proportion of lenses
rounded to the nearest half diopter was estimated. It was assumed that half
the difference between the totals of the 2 power groups represented those
lenses dispensed to the nearest half diopter and, therefore, dispensed inaccurately;
this was termed the “rounding rate” (RR).
Results: The power distribution curve for the sphere powers spiked in
half diopter steps, illustrated a bias toward prescribing full and half diopter
powers. With all lenses, the RR varied widely between countries. For the
MWH, this ranged from 1.7% (Canada) to 11.6% (Iberia). The RRs were
2 to 3 times higher for plus than minus power lenses, however, this also
varied by country. Overall, the RRs were lower for the silicone hydrogel
and daily disposable contact lenses compared with the MWH, in particular
for France and Iberia. The TSH results showed the greatest consistency
between countries, with RRs ranging from 3.9% (Germany) to 9.5% (Rest
of Europe). Most countries showed similar or lower RRs for TSH compared with MWH although, for some countries (e.g., United Kingdom,
Nordic), these were higher. There was less difference in RRs for TSH
lenses between plus and minus spheres.
Conclusion: A surprising proportion of soft lenses are prescribed to the
nearest half diopter, although this varies according to lens type. There are also
considerable variations between countries, presumably due to differences in
training, fitting practices, and supply routes. These findings suggest that there
is widespread room for improvement in the prescribing accuracy of soft
contact lenses.
Key Words: Soft contact lenses—Power—Prescribing.
(Eye & Contact Lens 2009;1: 11⫺14)
From Visioncare Research Ltd (G.Y., A.S.), Farnham, United Kingdom;
and Vistakon (K.M.), Division of Johnson & Johnson, Vision Care, Inc.,
Jacksonville, FL.
Address correspondence and reprint requests to Graeme Young, M.Phil.,
Ph.D., F.C.Optom., D.C.L.P., F.A.A.O., Visioncare Research Ltd., Craven
House, West Street, Farnham, Surrey GU9 7EN, United Kingdom; e-mail:
[email protected].
Accepted October 8, 2008.
DOI: 10.1097/ICL.0b013e3181909c20
Eye & Contact Lens • Volume 35, Number 1, January 2009
T
he contact lens industry has long been perplexed and bemused by
inconsistencies in the prescribing of soft contact lens sphere
powers. Full and half diopter powers are prescribed more frequently
than quarter and three-quarter diopter powers. There is also some
evidence of this in the prescribing of spectacle lenses.1,2 Clearly, this
arises through some practitioners rounding to the nearest half diopter.
This is seen by some as evidence for the acceptability of wide power
tolerances and for providing lenses in only half diopter steps. Compromises in the correction of astigmatism is a recognized feature of
soft contact lens practice; however, as far as we are aware, approximations in the prescribing of contact lens sphere powers has not
previously been discussed in the literature.
The purpose of this analysis was to gain a better insight into
current prescribing habits. In particular, to determine the proportion of prescribed lenses rounded to the nearest half diopter. Also,
whether these anomalous prescribing habits vary from country to
country and from product to product.
METHODS
Marketing data were obtained for lenses supplied over a 1-year
period by a single manufacturer. This included data for lens brands
representing 4 of the main categories of soft lens: mid-water hydrogel,
silicone hydrogel, daily disposable hydrogel, and a toric silicone
hydrogel. The data were broken down by country or region.
The spherical lenses were analyzed in the range 1.00 to 5.75 D
for plus and minus powers. The toric lenses were analyzed in the
range 0.50 to 5.75 D. This ensured a similar number of lenses in
full or half diopter powers compared with those of quarter or
three-quarter diopter powers. This power range also ensured that
there had been no enforced rounding because of nonavailability of
powers. Above ⫾6.00 D, the lenses are only available in 0.50 D
steps. Lenses supplied as diagnostic lenses were excluded from the
analysis. Although, the products are referred to as “lenses,” because they were supplied in boxes of 6 or 30 according replacement schedule, “lens units” is a more accurate description.
Because the true distribution of powers can be expected to
conform to a smooth curve, it was assumed that the deviation from
this was due to the rounding of powers to the nearest half diopter.
By comparing the proportion of lenses supplied from the 2 power
groups, it was possible to estimate the proportion of lenses rounded
to the nearest half diopter. Thus, it was assumed that half of the
difference between the 2 power groups represented those lenses
dispensed to the nearest half diopter and, therefore, dispensed
inaccurately. This was termed the “rounding rate” (RR) and the
formula for this is summarized below:
RR (%) ⫽ [(Nh ⫺ Nq)/N] ⫻ 0.5 ⫻ 100
11
G. Young et al.
Eye & Contact Lens • Volume 35, Number 1, January 2009
Where: Nh ⫽ no. lens units of full or half diopter power; Nq ⫽ no. lens
units of quarter or three-quarter diopter power; N ⫽ total no. of lens units.
The 95% confidence intervals were calculated for each of the RRs.
The data were analyzed for the 5 main European contact lens using
countries (France, Germany, Holland, Italy, United Kingdom) as well
as for the Nordic and Iberian regions. Other European countries were
grouped together as “Rest of Europe” (RoE). For one of the lens types
(mid-water hydrogel), data were also available for Canada, the United
States, and Japan. This lens is available in 2 base curves and the
proportion of lenses supplied in the steeper base curve were also
calculated for each country/region.
RESULTS
Figure 1 shows the breakdown of lenses by power for the
mid-water hydrogel lenses supplied in Europe. The most commonly
prescribed power was ⫺2.50 D closely followed by ⫺3.00 D. The
zigzag pattern of the distribution curve illustrates the bias toward
the prescribing of full and half diopter powers. For powers above
⫾6.00 D, the curve is smooth because of the nonavailability of
quarter and three-quarter diopter powers. However, even here, the
curve suggests a possible bias toward the prescribing of full
rather than half diopter powers as evidenced by the small peak
at ⫺9.00 D.
With all lens types, the level of rounding varied widely between
countries (Fig. 2). For the mid-water hydrogel, the RR ranged from
1.7% (confidence intervals: ⫾0.01%) for Canada to 11.6%
(⫾0.3%) for the Iberian region. The RRs were higher for plus than
minus power lenses. In most cases, the RR was 2 to 3 times higher
for plus than minus lenses. However, for Japan and Germany the
RR was notably higher for the plus power lenses: 6 and 7 times,
respectively.
Overall, the RRs were lower for the silicone hydrogel (Fig. 3)
and daily disposable lenses (Fig. 4) compared with that for the
mid-water hydrogels prescribed in Europe. With France, for instance, the RR were 4% with the silicone hydrogel and daily
disposable lenses compared with 6% for the mid-water lens.
FIG. 2. Rounding rates for mid-water hydrogel lens by country/
region split for plus and minus powers. CAN ⫽ Canada; DEU ⫽
Germany; FRA ⫽ France; IBE ⫽ Iberia; ITA ⫽ Italy; JPN ⫽ Japan;
NLD ⫽ Nederland; NOR ⫽ Nordic countries; RoE ⫽ Rest of Europe;
UK ⫽ United Kingdom; USA ⫽ the United States.
Only Germany showed notably higher RRs with the hydrogel
and daily disposable lenses.
The toric lens results showed the greatest consistency between
countries. If RoE is excluded, these ranged from 4.2% (⫾0.1%) for
Germany to 7.2 (⫾0.3%) for Italy (Fig. 5). There was also less
difference in RRs between the toric lens between the plus and
minus lenses and for some countries (France, Italy, Iberia), the RR
was actually lower for plus powers. In 2 cases (Germany, United
Kingdom), the comparative RRs for plus and minus powers were
similar to the spherical lenses. The overall toric RRs were 5.8%
(⫾0.1%) and 7.7% (⫾0.2%) for the minus and plus toric lenses,
respectively.
The 95% confidence intervals were relatively small; for the
combined plus and minus results, all of the confidence intervals
were ⱕ⫾0.5% and most were close to ⫾0.2%.
Table 1 shows the proportion lenses supplied in the steeper base
curve and also in plus powers. The flatter base curve is the most
commonly used base curve in all countries; however, there is a
disparity in usage of the steeper base curve that is relatively high
in North America (42%– 44%) and low in Europe (1%–16%).
There is also a contrast in the usage of plus powers; this is very low
in Japan compared with the other countries.
7%
DISCUSSION
6%
This analysis has highlighted the habit of some practitioners to
round to the nearest half diopter when prescribing contact lenses
5%
4%
3%
2%
1%
-8
.0
0
-6
.0
0
-5
.0
0
-4
.0
0
-3
.0
0
-2
.0
0
-1
.0
0
+0
.0
0
+1
.0
0
+2
.0
0
+3
.0
0
+4
.0
0
+5
.0
0
+6
.0
0
+8
.0
0
0%
BVP (D)
FIG. 1. Distribution of powers for mid-water hydrogel sphere contact lenses supplied in Europe (solid line). Historic data2 for spectacle
lenses prescribed in the United Kingdom are shown for comparison
(dashed line).
12
FIG. 3. Rounding rates for silicone hydrogel sphere by country/
region split for plus and minus powers.
Eye & Contact Lens • Volume 35, Number 1, 2009
Eye & Contact Lens • Volume 35, Number 1, January 2009
Anomalies in Soft Contact Lens Power
TABLE 1.
Proportion of steeper base curve and plus power lenses
dispensed for mid-water hydrogel
Canada
France
Germany
Iberia
Italy
Japan
Netherlands
Norway
United Kingdom
United States
RoE
All
FIG. 4. Rounding rates for daily disposable contact lens by country/
region split for plus and minus powers.
and how this varies from country to country. The clinical significance of inaccurate prescribing of lens power is complex and
difficult to interpret. On the one hand, the popularity of monovision shows that some patients are able to tolerate large amounts of
anisometropia but, on the other, some patients appear sensitive to
small amounts of refractive error. Furthermore, rounding of this
sort can cause varying types of refractive error: over-minusing,
over-plusing, with or without anisometropia. However, incorrect
prescribing of sphere power is the most common reason for the
dissatisfaction with spectacle lens prescriptions. Hrynchak3 found
that 59% of cases of failure of spectacle lens acceptance were due
to incorrect power of which 40% were due to being over-minused
or over-plused for distance.
Several studies have examined the effect of inaccurate prescribing of sphere power and most conclude that rounding to the nearest
half diopter should be avoided. Miller et al.4 found that the
addition of ⫹0.25 D binocularly had only a small effect on visual
acuity but that 45% of subjects found this unacceptable. Atchison
et al.5 challenged subjects with an asymmetric 0.25 D error (i.e.,
⫹0.25D and ⫺0.25D in contralateral eyes); 34% of subjects found
this unacceptable and some subjects reported mild headache,
distortion, and depth perception problems. Other researchers have
evaluated visual thresholds and found that defocus is just noticeable at between 0.15 and 0.23 D.6 – 8
FIG. 5. Rounding rates for toric silicone hydrogel lens by country/
region split for plus and minus powers.
© 2009 Lippincott Williams & Wilkins
Steep base curve (%)
Plus powers (%)
43.9
6.9
9.9
8.0
0.9
6.8
13.1
13.6
17.8
41.6
16.0
29.0
6.5
8.8
7.0
11.3
6.7
0.8
15.2
10.2
15.9
10.0
5.2
6.6
The ISO tolerance for soft contact lens sphere power is wider
than for spectacle lens power but probably represents expediency
rather than better tolerance of error with contact lens wear.9,10 Soft
lenses are more difficult to accurately manufacture and measure
than spectacle lenses.11 However, if as seems the case, the manufacturing accuracy of soft lenses is relatively low, then, in fact, the
desirability of accurate prescribing is increased. The relatively
high proportion of patients discontinuing contact lens wear may
partly be explained by prescribing inaccuracy. In a UK study of
lapsed contact lens wearers, the second most common reason given
for discontinuation was “problems with vision,” which accounted
for 13% of dropouts.12 A US study found a similar result in
discontinued wearers but an even higher level of visual problems
(26%) amongst dissatisfied contact lens wearers.13
The wide variation in RR between countries was surprising and
probably arises for a number of reasons. First, training in contact
lens practice varies widely within as well as between countries. In
Holland, for instance, contact lens fitting is undertaken by 3 types
of professionally qualified practitioners but also by nonqualified
fitters. Even within a given profession, the level of training can
vary according to when the training took place; in Spain and Italy,
for instance, the dominant profession fitting contact lenses is
optometry and, in both countries, the training has moved in recent
years towards university based courses. Aside from duration of
training, the type of training is probably significant and whether
the course emphasizes the optical aspects of contact lens practice
as well as the medical aspects. In France, most contact lens
prescribing is undertaken by ophthalmologists, however, their
9-year training includes only a relatively short course in refraction.
The German route toward qualification as a contact lens fitter
incorporates a large optical syllabus and this may explain their
apparent high levels of accuracy. In some European countries,
contact lenses are also supplied over-the-counter through pharmacies and supermarkets (Italy, Holland, respectively) without a
written prescription which seems likely to contribute to the error
rate. It may also be tempting to interpret the findings in terms of
national personality characteristics; however, there is little scientific information on which to base this.
With toric contact lenses, one might expect greater prescribing
accuracy for 2 reasons. First, because the cylinder powers and axes
are limited, there is greater incentive to optimize the sphere power.
Second, those eyecare practitioners who fit more toric soft lenses
are less likely to approximate the prescription. Curiously, in
several cases (Holland, Nordic, United Kingdom), the RR was
higher with torics than with the spherical equivalent. However,
13
G. Young et al.
there was greater consistency between countries toric lenses than
with the spherical options.
The lower accuracy in prescribing plus power lenses can be
explained by the ability of prepresbyopic hyperopes to compensate
for under-correction by accommodating. Many practitioners deliberately under-correct hyperopes and, apparently in doing so, round
to the nearest half diopter. With spherical lenses, the RR is
universally higher with the plus powers than minus powers but,
again, this practice varies from country to country. Overall the RR
was ⬃2 and a half times higher with plus lenses than minus lenses,
however, with Japan and Germany, this was close to 6 times.
Bennett1 and Jalie’s2 statistics for spectacle lens prescribing in
the United Kingdom in the 1960s and 1970s, respectively, indicate
a similar rounding of powers. Both studies analyzed ⬃20,000
prescriptions of which approximately one third were for spherical
lenses. The prescribing trends were similar for the 2 studies and, by
our calculation, the RR for minus spherical lenses (⫺1.00 to ⫺5.75 D)
was 16% in both cases. This is substantially higher than the RR for
mid-water hydrogel contact lenses which, for the United Kingdom,
was 2.3%. Assuming that the prescribing habits for spectacle
lenses have not changed significantly, this suggests that power is
prescribed in the United Kingdom with greater accuracy for soft
contact lenses than for spectacle lenses.
For this analysis of contact lenses, only a limited number of
brands were sampled and it is possible that there is some variation
in prescribing habits between brands. The lenses analyzed in this
study tend to be priced at a premium rate. It is possible that those
practitioners who select lenses based on cost might also be more
prepared to compromise the prescription and that the RRs might be
higher with other products.
In conclusion, a surprisingly high proportion of soft lenses are
prescribed to the nearest half diopter. This trend is more evident
14
Eye & Contact Lens • Volume 35, Number 1, January 2009
with plus than minus powers and more evident with mid-water
lenses than daily disposable or silicone hydrogel. There are considerable variations between countries, presumably due to differences in training, fitting practices, and supply routes. These findings suggest that there is widespread room for improvement in the
prescribing accuracy of soft contact lenses.
REFERENCES
1. Bennett AG. Lens usage in the supplementary ophthalmic service. Optician
1965;149:131–137.
2. Jalie M. Lens usage in the UK. Optician 1980;149:27–33.
3. Hrynchak P. Prescribing spectacles: reasons for failure of spectacle lens
acceptance. Ophthalmic Physiol Opt 2006;26:111–115.
4. Miller AD, Kris MJ, Griffiths AC. Effect of small focal errors on vision.
Optom Vis Sci 1997;74:521–526.
5. Atchison DA, Schmid KL, Edwards KP, et al. The effect of under and over
refractive correction on visual performance and spectacle lens acceptance
Ophthal Physiol Opt 2001;21:255–261.
6. Legras R, Chateau N, Charman WN. Assessment of just-noticeable differences for refractive errors and spherical aberration using visual simulation.
Optom Vis Sci 2004;81:718 –728.
7. Burton GJ, Haig ND. Effects of Seidel aberrations on visual target discrimination. J Optom Soc Am 1984;1:373–385.
8. Haig ND, Burton GJ. Effects of wavefront aberrations on visual instrument
performance, and a consequential test technique. Appl Optics 1987;25:492–
500.
9. ISO 18369-2. Optics and Optical Instruments—Contact Lenses—Part 2:
Tolerance. 2006.
10. ISO 8980-1. Ophthalmic Optics. Uncut Finished Spectacle Lenses. Specifications For Single-Vision and Multifocal Lenses. 2004.
11. Young G, Lewis Y, Coleman S, et al. Process capability measurement of
frequent replacement spherical soft contact lenses. CLAE 1999;22:127–135.
12. Young G, Veys J, Coleman S. A multicentre study of lapsed contact lens
wearers. Optom Physiol Opt 2002;22:516 –527.
13. Richdale K, Sinnott LT, Skadahl E, et al. Frequency of and factors
associated with contact lens dissatisfaction and discontinuation. Cornea
2007;26:168 –174.
Eye & Contact Lens • Volume 35, Number 1, 2009