Since their invention over 50 years ago, soft contact lenses have become a popular means of vision correction. Current estimates put the number of wearers in the United States at 40.9 million,1 and just under 90% of these are likely to be in a soft lens modality.2 Although these figures are indicative of a successful level of market penetration, it is evident that soft lens wear is not without its problems. When surveyed about the impact of their chosen mode of vision correction on their quality of life, contact lens wearers, on average, report less satisfaction than those who have had refractive surgery.3,4 A major factor in this differential is convenience, as contact lenses must necessarily be handled to some extent, and a care system, of one kind or another, is typically required.4 A second issue is that symptoms of dryness and discomfort are frequently reported, particularly toward the end of the day.5,6 Other problems include the occurrence of inflammatory responses, such as corneal infiltrates,7 which may be invisible to the wearer but are evident to the clinician.
Among the strategies that can be used to address these several concerns is the use of a daily disposable wearing schedule. Using lenses in this way releases users from the need for a care system and so enhances the convenience of contact lens wear. In addition, daily disposability appears to be protective against corneal infiltrates,8 and a recent report suggests advantages in terms of comfort.9 Fitting trends over the last few years reflect the increasing popularity of daily disposable lens types,2 and if the potential benefits mentioned previously actually do have clinically meaningful consequences, the general level of contact lens performance would also be expected to rise. The purpose of this study was to indicate how far this process has progressed, by providing an assessment of real-world performance for a range of contemporary daily disposable contact lenses.
To do this effectively, it is necessary to select an appropriate benchmark. Because the main point of contact lens wear is to give the illusion that no vision correction is being worn, the most appropriate reference in this respect would be that of no-lens wear. Hence, this analysis attempted to gauge how daily disposable contact lens wearers compare with non–contact lens–wearing emmetropes in terms of subjective responses and report the occurrence of corneal infiltrative events within these groups.
Because recent work has indicated that comfort reporting can be influenced by refractive status,10 a group of full-time spectacle wearers was also included. This provided the additional control of a non–contact lens–wearing group with a similar refractive error distribution to the contact lens wearers.
This was a retrospective analysis of seven studies. Five of these involved wear of daily disposable contact lenses (number of participants, n = 201) and had identical inclusion/exclusion criteria, one enrolled full-time spectacle wearers with no prior history of contact lens wear (n = 34), and one enrolled emmetropic non–contact lens wearers (n = 40). All studies took place at the Clinical Research and Trials Centre at the Brien Holden Vision Institute in Sydney, Australia, and were approved by a local ethics committee. Written consent was obtained from all participants, and studies were conducted in accordance with the Declaration of Helsinki. The same protocol was followed throughout, with visits at baseline, 2 weeks, 1 month, and 3 months. At all visits after baseline, subjective ratings of comfort on insertion (for daily disposable wearers) or soon after waking (for spectacle and nonwearers), as well as at the end of day, were collected. Ratings were made on a recall basis, using a numeric rating scale (1- to 10-point scale in one step, where 1 is poor and 10 is excellent). Using the same scale, participants were also asked to rate their overall impression of vision quality at the final visit.
During the baseline visit, all participants were asked to answer yes, or no, to the question, “Do you consider yourself as having dry eye?” The frequency of lubricating drop use (never, occasional, or regularly) was also ascertained.
Contact lenses trialed in the five studies included daily disposable lenses from three different manufacturers. Two of the lenses were made from hydrogel materials, and three were silicone hydrogels (Table 1). All lens types were available in the United States, Europe, Japan, and Australia at the time of writing. Participants in contact lens groups were instructed to wear the lenses for a minimum of 5 days per week and 6 hours per day, with a fresh lens being used each day. Study enrollment included myopic participants with refractive errors up to −8.50 D, all lenses had spherical refractive power only, and a minimum visual acuity of 6/12 was required during wear.
Participants in the spectacle group were required to wear their glasses full time (no part-time contact lens wearers) and have no history of contact lens wear. The nonwearer group had a prescription range of +0.50 to −0.50 D inclusive and no spectacle or contact lens wear.
Observations of the anterior eye were made with a Zeiss SL-120 biomicroscope (Carl Zeiss Meditech, Jena, Germany). Corneal infiltrative events occurring in the studies were classified according to published guidelines.11 Corneal infiltrative events regarded as significant included contact lens acute red eye, contact lens peripheral ulcer, and infiltrative keratitis, whereas asymptomatic infiltrative keratitis and asymptomatic infiltrates were categorized as nonsignificant.
The sample size of 40 participants for each individual trial was originally estimated to permit detection of differences in subjective ratings of 1 ± 1.5 points, on a 1- to 10-point numerical rating scale,10 at 95% confidence and 80% power. For this analysis, retrospective power evaluation showed that the assembled samples achieved more than 90% power to detect a grouped difference of 1 ± 1.5 points. In terms of adverse events, the available samples permitted differences of 7.5% between nonwearers and daily disposables or 14% between nonwearers and spectacles, to be detected with 80% power.
Demographic factors were compared between trials to ensure comparability. These included age, sex, ethnicity, baseline Rx, and self-reported dryness at baseline. Those factors that were significant between trials were then included in the statistical model as possible confounders.10
Participants who did not complete the trial were included in the analysis data set based on an intent-to-treat methodology.
Subjective responses from three visits (2 weeks, 1 month, and 3 months) were included in the analysis and compared between study groups using a linear mixed model, with subject random intercepts, to account for any repeated visits and enrollment. The statistical model included study group, visits, possible confounding factors, and the interaction of group with visit. If the interaction was significant, the effect of group was estimated at each visit. Estimated values of the subjective response, along with 95% confidence limits, were then determined based on the model. Post hoc multiple comparisons were adjusted using Bonferroni correction.
First event and adverse event incidences are reported as a percentage of participants. Incidence rates by participants were compared using χ2 test. The level of statistical significance was set at P = .05. Statistical analysis was performed using IBM SPSS Statistics for Windows, version 21.0 (IBM Corp., Armonk, NY, released 2012).
Table 1 summarizes demographic information, and Table 2 indicates the ethnicity of the participants. There were no differences in age or ethnicity between any of the groups (P > .3). Because sex was significant (P = .04), and as might have been expected, spectacle wearers had slightly greater amounts of astigmatism (P = .001), these factors were included in the statistical model.
Participants were also asked to report their self-perceived dry-eye status, and it is evident from Table 1 that more spectacle wearers considered themselves to be affected by this problem than the other groups (P = .02, χ2). Accordingly, dry-eye status was also included among the confounding factors in the statistical model.
The overall dropout rate in the analysis sample over the 3-month period was 7.6%. There was no difference between the groups (P = .91), with the emmetropes having 7.5% (n = 3/40) and spectacle wearers having 5.9% (n = 2/34). The dropout rate among the contact lens wearers was 8% (n = 16/201), with a range among the lens types of 5 to 12.5% (P = .67).
Fig. 1 shows the estimated mean comfort ratings at insertion for the contact lens wearers and on awakening for nonwearers. The only significant difference was between lens type B (mean ± 95% confidence interval, 7.2 ± 0.4) and lens C (8.0 ± 0.4, P = .007). None of the lens-wearing groups were significantly different from emmetropes or spectacle wearers (P > .05).
Fig. 2 illustrates comfort data collected for the end of the day, by which time lens B (5.7 ± 0.6) had become significantly less comfortable than lens E (6.7 ± 0.5, P = .005), while remaining less comfortable than lens C (6.6 ± 0.6, P = .007). In addition, wearers of both lens B and lens D (6.2 ± 0.5) were now reporting lower comfort ratings than emmetropes (7.3 ± 0.6, P < .05). Spectacle wearers (6.2 ± 0.6) were not different from any of the other groups (P > .09).
To see how comfort altered over the wearing day and minimize the influence of possible intergroup variation in ratings criteria, a within-subject, daytime, comfort change variable (ΔC) was defined as the difference between the comfort scores at the end of day and those on insertion/waking. Negative values of ΔC thus indicate declining comfort over the measurement period. Estimated means for this new variable are shown in Fig. 3, from which it can be seen that all groups showed statistically significant comfort decrements over the day (P < .05), although the between-group effect was not significant (P = .11). Note that this result indicates that even non–lens wearers experience some increased discomfort during the day.
Fig. 4 shows the visual quality response ratings. Emmetropes (8.6 ± 0.5) reported significantly better vision than lens B (7.5 ± 0.4, P < .001), lens A (7.6 ± 0.4, P = .003), and spectacle wearers (7.8 ± 0.5, P = .04). Lens types C, D, and E were also significantly better than lens B (all P < .01), and every lens type differed from at least one other lens type (P < .05).
Performance Relative to Emmetropes
To better show relationships between the various groups, graphical representations were constructed in which performance was related directly to the emmetropic control. Emmetropes were chosen as the primary referent rather than spectacles, because their performance was generally superior across the categories assessed. Fig. 5 shows the plot for comfort. This was arranged so that relative initial comfort (i.e., the difference in initial comfort between each lens and emmetropes) appears on the abscissa and relative end-of-day comfort (i.e., difference in end-of-day comfort between each lens and emmetropes), on the ordinate axis. This construction locates the emmetropic benchmark at the origin and ensures any outcome that is numerically worse than this appears as a negative value. Hence, the position of lens C just to the right of the ordinate axis indicates that its initial comfort rating was, on average, slightly better than no-lens wear. This was a unique occurrence. All the other groups had numerically worse performance than emmetropes on both measures and so fell into negative plot space. A view of combined comfort performance can be obtained by noting the distance of any given group from the origin.
Extending this approach to overall performance (i.e., comfort and vision together) produces the arrangement in Fig. 6. For this plot, a combined comfort rating was obtained by adding together, in quadrature, the components of initial and end-of-day comfort from Fig. 5. Again, distance from the origin offers an indication of overall performance in terms of both vision and comfort.
Statistically significant differences relative to emmetropes are shown against the appropriate axis in both Figs. 5 and 6. Note that these values may differ slightly from those in Figs. 1, 2, and 4, because they are derived from direct comparison with emmetropes alone, rather than between all groups simultaneously.
Corneal Infiltrative Events
Table 3 shows the proportion of participants who experienced corneal infiltrative events during the study. Two incidents of infiltrative keratitis and one of contact lens peripheral ulcer occurred among contact lens wearers, and there was one case of asymptomatic infiltrates in the spectacle-wearing cohort. However, for significant corneal infiltrative events, nonsignificant corneal infiltrative events, and all events, there was no statistically significant difference between the groups (P > .2).
When referring to Fig. 5 or 6, it is easy to see that the further a given group is from the origin, the greater its performance differential relative to emmetropes. What is not immediately obvious is where such distances become meaningful. For example, in Fig. 5, the three groups furthest from emmetropes in comfort terms were lenses B, D, and spectacle wearers. Each of these groups had at least one component of their comfort decrement that was statistically significant, but would this be noticeable to a wearer? Answering this question requires knowledge of the minimum distance from the origin that would be discernible in the general population, a quantity that might be thought of as the clinically significant distance. Previous literature suggests that a value of approximately 0.75 units for each component of comfort would be a reasonable estimate for this,12 and adding this criterion to Fig. 5 (dotted line) gives an indication of where the approximate boundary of clinical significance lies.
With this modification, it is now evident that wearers of lenses B, D, and spectacles would have been noticeably less comfortable than emmetropes, given their positions well outside the boundary and in negative plot space. Conversely, there are no grounds to suppose that the comfort of lens A, C, or E materially diverged from emmetropes, as there were no statistically significant differences, and all were located either inside or very close to the boundary.
Although the foregoing treatment provides a means of discriminating between modalities on their comfort performance, it is evident that further insight might be possible by introducing additional dimensions. Accordingly, a similar methodology has been used to include vision. In this case, the boundary of clinical significance is a composite, created by using the same criterion as before for each contribution to overall comfort (i.e., √(0.752 + 0.752) = 1.06), together with a value for vision of 1.46.13 The difference in magnitude between the comfort and vision criteria produces an elliptical profile, as is shown by the dotted line in Fig. 6.
This addition suggests that there were three categories of behavior. In category I are those lenses such as C and E, whose performance cannot be distinguished from emmetropes under the conditions of the study. This is because they are placed well within the boundary of clinical significance and show no statistically significant differences.
Category II contains groups such as lenses A, D, and spectacles, which have at least one statistically significant component (i.e., related either to an aspect of comfort or vision) and are close to the boundary but lie on the imperceptible side. Although more equivocal than for category I, the balance of evidence currently falls on the side of these lenses being inseparable from emmetropes. This interpretation does, of course, depend critically on the location of the boundary, and it should be realized that the vision criterion in particular may not be very strict. Should an improved evidence base (or personal preference) precipitate more demanding criteria, the boundary will shrink toward the origin. The view that a given lens is imperceptibly different may then be altered if the boundary moves past its position.
Finally, in category III is lens B, whose performance was inferior to emmetropes. This lens' performance had multiple statistically significant components and fell on the perceptible side of the boundary.
The relatively poor performance of spectacles deserves some further comment. So far as ocular comfort is concerned (Fig. 4), the outcome is consistent with a previous report of significant dryness symptoms among spectacle wearers.14 The authors of that work pointed to the presence of contact lens failures among their spectacle-wearing cohort as a possible explanation. Although there were no such individuals in the current spectacle group, 17% still self-identified as having dry eye (Table 1). This may mean that dry eye symptoms are reasonably common among spectacle wearers.
Perhaps more surprising was the relatively poor subjective vision reported by spectacle wearers (Figs. 4, 6). Although this group had more astigmatism (Table 1), the nature of spectacle prescribing would suggest that, even if differences existed between the prescription being worn and the true refraction, the in situ corrections should have been reasonably complete, relative to those of either the emmetropes or the contact lens wearers. Thus, spectacle wearers would generally have had most of their refractive error corrected and so should not have had inferior vision. One possible explanation for this unexpected result is that the data are reflecting vision problems associated with dry eye,15–18 although this is difficult to verify for the current study. A further alternative is that the subjective responses to spectacle wear have a different psychological and/or physiological basis to those emerging during contact lens wear.
Previous research has shown a greater occurrence of adverse events in a daily disposable–wearing population compared with spectacles,19 but the current study did not have sufficient power to be able to confirm this finding. Despite the incidence of corneal infiltrative event ranging from 0 to 7.5% across the study groups (Table 3), the differences were not statistically significant at the sample size available. None of the events occurring during the study were serious, but it was noteworthy that the most severe episodes (two infiltrative keratitis and one contact lens peripheral ulcer) were experienced by daily disposable wearers, and all five lens types had an event of some kind. Although non–lens wearers are not expected to be entirely free of corneal infiltrative events,19,20 current evidence suggests that contact lenses impart some additional risk. The magnitude of the problem evidently depends on the wearing schedule however, as reusable lenses are associated with around four times higher risk of corneal infiltrative events than daily disposables.8 The safest currently available option for contact lens wear thus seems to be daily disposability, provided compliance with the wearing schedule is adhered to.21,22
Several limitations accompany this analysis, and these must be considered when interpreting the presented outcomes. In addition to the possible distinctiveness of spectacle wearer responses mentioned earlier, it is likely that additional variability was introduced into the assessment because of the need to use comfort on insertion for the lens-wearing groups, as opposed to comfort on waking for nonwearers. This probably led to the relevant assessments being conducted at slightly different times during the early part of the day. It is also possible that this approach did not accurately sample the comfort peak for nonwearers, as this may be delayed by as much as an hour after waking.23 These issues might be avoidable in future studies by using real-time rating methods24; however, these were not available for the whole period of data collection in the present work.
Study enrollment was on a volunteer basis; hence, no effort was made to either include or exclude those who may have been symptomatic during contact lens wear. Although it is anticipated that this approach will reflect normal clinical practice, different outcomes might result from individuals, or samples, with extreme symptomatic characteristics.
Care should also be taken when interpreting the clinical significance boundary estimates in Figs. 5 and 6. Data in these areas are sparse, and precisely locating such indicators is challenging owing to the subjective nature of the underlying characteristic and the necessity of combining several such measures into a single estimator. Use of these boundaries as a guide to lens behavior should be undertaken in the light of their underlying derivation and the understanding that their location may change significantly should these assumptions alter or as better knowledge accumulates. For example, the dashed line in Fig. 6 represents the boundary of clinical significance resulting from applying stricter criteria for comfort and vision of 0.5 units12 and 0.12 units,13 respectively. Under these circumstances, none of the lenses would be considered clinically similar to the emmetropic condition.
Some contemporary daily disposable contact lenses offer performance in terms of comfort and vision that was indistinguishable from nonwearing emmetropes, under the conditions of this study. Although this result may attest to the improvements in contact lens design, chemistry, and manufacture made in recent years, it must be viewed in the context of the experimental and interpretational uncertainties associated with acquiring and presenting the data. Although corneal infiltrative responses were similar for the lens types, emmetropes, and spectacle wearers, the available sample size was insufficient to support robust conclusions in this respect. A larger study is required to confirm the relative rates for infiltrative events.
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