TORIC LENSES

Soft Toric Lenses: A Clinical Evaluation

Mike Covey, M. Optom
June 2000

A comparative clinical performance of three types of monthly replacement soft toric lenses.

For many years soft toric contact lenses were regarded as specialty lenses requiring high levels of practitioner expertise, large fitting sets and long delivery times. Improvements in design and performance, together with advances in manufacturing technology, have made today's stock torics less expensive and simpler to fit, as well as offering improved reproducibility. The emergence of spherical disposable and planned replacement lenses led to a rapid move away from conventional modalities.

Low-cost production methods have made frequently replacing soft toric lenses economically viable. When planned replacement soft torics were first introduced, few choices of lens design and material were available; however, practitioners now have a variety of prescribing options.

Relatively few published studies have compared the performance of different monthly replacement soft torics. However, one recent comparison of two designs, CIBA Vision's Focus Toric and Bausch & Lomb's SofLens 66 Toric, found that while both were viable alternatives for correcting astigmatic patients, SofLens 66 performed better in visual acuity, reported symptoms of blurred vision, patient assessment and visual quality.

CooperVision's Frequency 55 Toric, designed for monthly planned replacement, has been reported to be comfortable, stable and reproducible. The objective of this evaluation was to assess the clinical performance of Frequency 55 Toric and to compare it with Focus Toric and SofLens 66 Toric, over one month of daily wear. Key variables of toric lens performance, such as lens stability and vision quality, are the major determining factors in patient satisfaction with toric lenses.

Lens specifications

Lens specifications for the three lenses are given in Table 1.

Study design

Subjects with a variety of previous lens-wearing experience participated in the study. Spherical refractive errors were in the range of plano to-6.00D and astigmatism of 1.00D to 2.50D, with no ocular abnormality, disease or clinically significant slit-lamp findings. Each subject was randomly assigned to one of the three test lenses, and to either a peroxide or multipurpose care system. Subjects encountering adverse reactions to one care product were changed to the other.

The baseline visit included pre-fitting assessment and enrollment. On dispensing, lenses were inserted and allowed to settle for at least 20 minutes before assessment to establish satisfactory fitting and visual performance. After dispensing, subjects attended scheduled visits at one week and one month. All subjects were requested to wear the study lenses for at least eight hours daily (once adapted), and to wear lenses to a scheduled follow-up appointment unless they were experiencing problems. They were also asked to wear lenses for a minimum of four hours prior to the one-week visits and for a minimum of one hour prior to the other follow-up visits. Subjects could discontinue or be discontinued from the study at any of the scheduled visits or between visits. Criteria for discontinuation were unacceptable subjective comfort, slit-lamp findings or fit.

The following variables were investigated:

• Lens fit: total (horizontal and vertical) centration, post-blink movement, tightness (assessed by the push-up test)
• Lens orientation: orientation position, orientation stability, re-orientation speed
• Vision: high and low-contrast visual acuity on a LOGMAR chart, subjective vision quality (%)
• Subjective acceptability: lens handleability, comfort, wearing time
• Ocular physiology: corneal staining, conjunctival staining, bulbar and limbal redness

Data Analysis

Paired analysis was used to test for differences between the Frequency 55 Toric results and those of the other two lenses: the two-sample t-test for normally distributed data and the Mann-Whitney U-test for non-normal data. In the case of visual acuity and slit-lamp findings, differences from baseline for each variable were calculated. These differences were compared between lens types at given visits. With other variables, the actual values were compared in a similar fashion.

A P-value less than or equal to 0.05 was regarded as statistically significant.

Results

A total of 50 subjects were enrolled, 14 with Frequency 55 Toric, 18 with Focus Toric and 18 with SofLens 66 Toric. During the course of the study, eight subjects discontinued lens wear. Two subjects in the Frequency 55 Toric group discontinued, one at the issue visit due to lens orientation problems and one at one week due to variable vision. Four subjects in the Focus Toric group discontinued, one due to handling problems at the issue visit, one due to lens orientation problems at the issue visit, one due to handling problems at one week and one lost to follow-up. In the SofLens 66 Toric group, two subjects were lost to follow-up. Therefore, a total of 42 subjects completed one month's wear, 12 in the Frequency 55 Toric group (22 eyes), 14 in the Focus Toric group (26 eyes) and 16 in the SofLens Toric group (31 eyes). Five subjects wore a toric lens in one eye only.

Results for the variables investigated follow, and are summarized in Table 2.

Vision

Subjects reported Frequency 55 Toric as significantly better than Focus Toric and SofLens 66 Toric for high and low contrast acuity after one month of wear. Compared to baseline spectacle findings, Frequency 55 Toric patients had a decrease of only one letter on the acuity chart, while Focus Toric and SofLens 66 Toric patients had a decrease of five and four letters respectively. Corrected low contrast acuity was also significantly better with Frequency 55 Toric than Focus Toric and SofLens 66 Toric (see Graph 1). Visual quality after one month of wear was also rated significantly higher with Frequency 55 Toric than with SofLens 66 Toric (see Graph 2).

Lens orientation

Mislocation: No difference was observed between the three lens types for the amount of mislocation. Frequency 55 Toric had a slight tendency to stabilize in an inferior-temporal position, whereas SofLens 66 Toric and Focus Toric settled inferior-nasally.

Stability: No difference was observed between the three lens types for the degree of stability of the lens. This result was measured by observing the movement of the mislocation indicator from the resting position after each blink by the patient.

Reorientation Speed: Frequency 55 Toric relocated to the correct location faster than Focus Toric. This variable was measured by intentional rotation of the lens and measuring the speed of return to the original resting position.

Lens fit

Post-Blink Movement: Subjects reported Frequency 55 Toric to have significantly greater movement on blink compared to Focus Toric. SofLens 66 Toric had the greatest level of movement but not a statistically significant level compared to Frequency 55 Toric.

Tightness on Push Up: Frequency 55 Toric was found to be significantly looser than Focus Toric and significantly tighter than SofLens 66 Toric.

Total Decentration: Frequency 55 Toric had the best centration of all three lens types and was significantly better than SofLens 66 Toric. Total decentration is a combination of the sum of the vertical and horizontal decentration.

Subjective acceptability

Comfort: Subjective comfort on average graded high (>=9.0, 0-10 scale) with all lens designs at each visit. Subjects reported no differences in comfort between lens designs.

Wearing time: After both one week and one month of lens wear, subjects wearing Frequency 55 Toric wore lenses on average for longer each day than those subjects wearing Focus Toric or SofLens 66 Toric (see Graph 3).

Handleability: Handleability assessed by the practitioner, was found to be better with Frequency 55 Toric than with Focus Toric. Subjects reported no difference between Frequency 55 Toric and SofLens 66 Toric (see Graph 4).

Ocular physiology

When differences in slit-lamp findings from baseline findings after one month wear were analyzed, no differences were found between the three lens types for corneal staining, bulbar or limbal redness. However, SofLens 66 Toric induced more conjunctival staining (one grade higher on average) than Frequency 55 Toric.

Discussion

Around 45 percent of the population requires a cylindrical correction of 0.75D or greater, the level of astigmatism at which toric designs are usually considered. Soft toric lenses are an increasingly popular prescribing option for astigmatic patients as an alternative to rigid gas permeable designs. In more recent times, new generation front surface aspheric designs also offer improved vision compared to spherical lenses for low astigmats as well.

The latest generation of soft torics can not only be fitted easily from stock rather than custom made, but also offer the clinical benefits of planned replacement, such as fewer deposit-related problems. Soft torics can now be more reliably reproduced such that differences between lenses on replacement are no longer the problem they once were. The majority of these planned replacement soft torics are back surface, prism-ballasted designs, but differences in manufacturing processes, materials and other design features can lead to differences in clinical performance.

In the current study, all three test lenses performed well, but some significant differences existed between the designs, particularly in relation to visual performance, wearing times and lens fitting characteristics after one month of wear. Frequency 55 Toric outperformed both Focus Toric and SofLens 66 Toric for high and low contrast vision. These results were matched in a higher vision quality rating by patients wearing Frequency 55 Toric compared to the other two lens types. Visual performance is the most crucial variable in soft toric lens performance. Interestingly, there was no trend between lens mislocation and visual performance. Further investigation is required to determine the exact reason. However, it is most likely linked to the optical quality and/or correct power and axis labelling resulting from the manufacturing process.

Frequency 55 Toric was found to have optimum fitting characteristics and outperformed SofLens 66 Toric in a number of categories with regards to tightness and centration. Patients wearing Frequency 55 Toric also wore lenses on average for longer each day than those subjects wearing Focus Toric or SofLens 66 Toric. 

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TABLE 1: Lens Specifications