Researching GP Multifocals
BY EDWARD S. BENNETT, OD, MSED
Recently, two students from the University of Missouri-St. Louis, College of Optometry, Allyson Conn, OD, and Tyler Kramer, OD, performed a pilot study to evaluate the following three questions:
1. How does the initial comfort of GP bifocal and multifocal lens designs compare to a spherical GP design?
2. Does adding truncation to a prism-ballasted, segmented bifocal lens benefit translation?
3. How does base curve radius affect the rotation of a segmented, translating bifocal design?
Our research involved a multi-purpose, double-masked, single-center study, which included 12 myopic subjects whom we selected on the basis of good eye health and no previous GP lens wear. We asked participants to make three separate visits to the clinic, where we fit them with various GP contact lenses.
Question 1: Initial Comfort On each of three visits separated by approximately one week, we randomly fit subjects with either a spherical; aspheric multifocal; or segmented, prism-ballasted, translating lens design, all fit according to the manufacturers' recommended fitting guide. Subjects then completed a comfort questionnaire within five minutes and again after 30 minutes of lens application. They ranked lens comfort on a scale from 0 to 100.
After wearing the lenses for 30 minutes, four out of 12 subjects preferred the comfort of sphericals, three preferred aspherics and two preferred translating. One subject found translating and spherical equally comfortable, one found spherical and aspheric equally comfortable and one deemed aspheric and translating designs equal in comfort.
Although clinicians have theorized that spherical lenses would provide more initial comfort, the limited movement associated with aspheric multifocals and segmented bifocals should translate into less initial awareness. Our results indicate equal comfort between the three GP lens designs.
Question 2: Prism Ballasted vs. Prism Ballasted/Truncated for Lens Translation
At a separate visit, the examiners evaluated the translation of both a prism-ballasted, truncated translating bifocal contact lens and a prism-ballasted, non-truncated translating bifocal contact lens. We used a scale from 1 to 5 to quantify the translation. We found that, contrary to common belief, truncation improved the amount of translation in only two of the 12 cases, but decreased translation in five of the 12 cases. In the remaining five cases, truncation had no effect on translation. Because our subjects were myopic, it's possible that truncating the lens actually reduced the effective ballast by decreasing edge thickness.
Question 3: The Effect of Base Curve Radius on Lens Rotation We compared the effects of different base curvatures (0.50D flatter than K, on K and 0.50D steeper than K) of one segmented, prism-ballasted design on segment rotation. We fit one eye of each subject randomly with each base curve at one visit, evaluating each lens after 15 minutes of wear. We recorded rotation in degrees and ranked the stability of rotation on a scale of 1 to 5.
On six of the 12 patients, the flatter lens demonstrated the least rotation. On three of the 12, the on-K lens demonstrated the least rotation. On three of the 12, the steeper lens demonstrated the least rotation. You could theorize that steeper lenses attempt to center on the cornea and so are more subject to the torquing effects of the upper lid, whereas flat, heavy lenses tend to fall quickly to the lower lid.
Just the Beginning
Although this study represented a small study population and somewhat variable results, we hope that these efforts will encourage further investigation of GP multifocal lenses and further advocate their usefulness in presbyopic correction.
Dr. Bennett is an associate professor of optometry at the University of Missouri-St. Louis and is executive director of the GP Lens Institute.
Contact Lens Spectrum, Issue: February 2005