Expanded Parameters of a Soft Toric Lens
SOFT TORIC LENSES
Expanded Parameters of a Soft Toric Lens
A study investigates the rotation, fit, and comfort of expanded parameters in a daily disposable lens.
By Joachim Nick, Dipl. Ing. (FH), Mary Fahmy, OD, & Peter Bergenske, OD, FAAO
Joachim Nick is currently director, Clinical Affairs, Ciba Vision, Germany.
Dr. Fahmy is a senior principal clinical scientist at Ciba Vision.
Dr. Bergenske is director, Professional Support, Ciba Vision North America.
Ciba Vision originally launched the Focus Dailies Toric lens in 2002 with just two axes and one cylinder power. Despite these limited parameters, this first daily disposable toric lens provided excellent vision and comfort for a surprisingly wide range of refractive errors. A second cylinder power was eventually made available, and most recently the parameter availability has expanded significantly to include four new axes (Table 1).
With only two axes, it was reasonable that the lens did not have scribe markings to determine rotation, as correcting for rotation was not an issue. With the expansion of the parameters, however, it became desirable to add rotation markers so that fitters could be guided when fine-tuning correction with these lenses.
These lenses are made using the Lightstream technology, which utilizes glass and quartz molds, so adding conventional scribe engravings, as is done with conventionally molded lenses, was not practical. Instead, Ciba employed a printing method similar to that used for its FreshLook One-Day lenses.
The ink is based on nelfilcon A and contains phthalocyanine green, a color additive commonly used for coloring soft contact lenses. In addition to the printed scribe marks (four dots at either side of the horizontal axis), an “OK” inversion indicator is printed so that the vertical line of the “K” is in line with the vertical axis, and as such, can be used as an additional rotation indicator (Figure 1).
Figure 1. Inversion and orientation printed markings on the Focus Dailies Toric contact lens.
Because this lens utilizes a back-surface cylinder with a dual-thin-zone stabilization system, unlike prism-ballast designs it may orient in either of two positions. Depending on how the lens is applied, the “OK” may be seen either at the 6 o'clock or the 12 o'clock position. On some eyes the inversion mark may be visible on the eye when in the 6 o'clock position. This is easily remedied by teaching the patient to position the lens so the marking is concealed by the upper lid.
Clinical testing was done to confirm that the new parameters and the markings would perform as intended. For any toric soft contact lens, a predictable orientation of the lens axis is required for success, and the markings on the lens provide an easy way to assess the orientation. With a daily disposable contact lens in particular, it was important to assure that the orientation would be consistent from lens to lens on the same eye. The markings provide useful functions for both fitters and patients, but could not compromise patient comfort and the appearance of the printed marking had to be acceptable when the lens was worn.
We designed a nondispensing observational study to explore these questions. The purpose of the study was to evaluate the rotation and orientation performance for predictability, consistency, and repeatability. We also assessed comfort and the cosmetic acceptability of the markings.
The protocol for this trial was reviewed and approved by the freiburger ethik-kommission (Freiburg, Germany). The trial was conducted in accordance with the tenets of the Declaration of Helsinki. Habitual soft toric contact lens wearers were recruited for this non-dispensing trial. After signing an informed consent, each subject was fit sequentially with three pairs of Focus Dailies Toric lenses having the same parameters. Each pair was allowed to equilibrate on eye for five minutes.
Each of the lenses was assessed for fit, centration, orientation, and oscillation with blink. Fit was assessed by observing both movement with blink and with a push-up test for tightness. Fit was graded overall on a five-point scale consisting of ratings of unacceptably loose, acceptably loose, optimal, acceptably tight, and unacceptably tight.
Centration was graded as optimal, slight, mild, moderate, or severe decentration. Rotational orientation of the horizontal scribe marking was measured in absolute values with direction indicated as “nasal up” or “nasal down.” Oscillation with blink was measured by observing the position of the 3 o'clock and 9 o'clock lens marking using a thin slit beam (5X to 8X magnification) over a number of blinks. The degree to which the lens rotated with blinking was graded as the lens oscillation according to the following scale: none (no rotation with blink), <3 degrees of rotation with blink, 3 degrees to 5 degrees of rotation with blink, >5 degrees of rotation with blink.
We asked subjects to rate each pair of lenses for comfort on a 1 (worst) to 10 (best) scale. After completion of these assessments, a second pair of the same parameters was applied and allowed to settle for five minutes. The assessments were repeated for this pair and then for a third pair of lenses. Visual acuity was not assessed as correct prescription lenses for each subject were not available at the time the trial was conducted.
Descriptive statistics were provided for each of the efficacy endpoints. No inferential testing was carried out. Two-sided confidence intervals were computed on the percent of lenses within 5 degrees and 10 degrees from the horizontal position for the scribe marks. Confidence intervals assume independence and account for within-subject correlation.
A total of 54 subjects were enrolled and participated in the study. The subject demographics are shown in Table 2.
Comfort After Settling The average of the comfort ratings was 9.1 ± 1.3, with 96 percent of subjects rating it 7 or higher (Figure 2). Sixty-five percent of subjects rated all three pairs equally and 94 percent of subjects rated all three pairs within one point of each other on a 10-point scale.
Figure 2. Subject comfort ratings. Each pair was evaluated on a 1 to 10 scale with 10 being the best comfort.
Lens Orientation After five minutes of wear, the direction of orientation was measured in absolute values documented in positive or negative degrees for nasal and temporal rotation, respectively. The average orientation was 7.0 degrees ± 6.6 degrees. This trend toward nasal upward rotation is typical for thin-zone-stabilized toric contact lenses. We found that 48 percent of lenses were within 5 degrees from horizontal (Correlated 95 percent confidence interval [39.2 percent; 56.0 percent]) and 86 percent were within 10 degrees from horizontal (Correlated 95 percent confidence interval [79.7 percent; 91.3 percent]).
Additional analysis examined the consistency of orientation from lens to lens within each of the three lens groups. For 75 percent of the eyes, the standard deviation of the orientation within the three lenses worn on the same eye was less than or equal to 3 degrees.
Oscillation With Blink Oscillation with blink was measured for each contact lens. The average oscillation was 1.7 ± 1.2 degrees, ranging from 0 to 5 degrees. Ninety-nine percent of all lenses had less than 5 degrees oscillation with blink; 83 percent had 2 degrees or less.
Lens Fit After five minutes of wear, we graded overall lens fit and centration. Most lenses evaluated—321 (99 percent) of the 324—were judged to have optimal or acceptable fit (Figure 3). Similarly, we found centration to be optimal or to have only mild decentration for 318 (98 percent) of 324 lenses.
Figure 3. Overall lens fit, evaluated at five minutes.
Acceptability of On-Eye Appearance While wearing the lenses with the OK mark in a 6 o'clock position, subjects were allowed to view the lenses in a mirror and asked if the markings were visible. Sixty-five percent responded that they could not see the marking. Of those who could see the marking, only one subject found this objectionable.
The expanded parameters of the Focus Dailies Toric contact lens performed well, with minimal and consistent rotation, excellent fit, and excellent initial comfort in this study. Currently, toric fits represent a disproportionately small percentage of the daily disposable category compared to the weekly/monthly replacement category. Now with parameters that will allow practitioners to fit a large majority of astigmatic patients with a daily disposable lens, we expect that this modality will continue to grow so astigmatic patients can enjoy the convenience and comfort of a true daily disposable contactlens. CLS
Contact Lens Spectrum, Issue: February 2010