SOFT TORIC ORIENTATION
Lenses, Gravity and Other Forces
A better understanding of how toric soft lenses orientate will help you choose the best design for each patient.
By Graeme Young, MPhil, PhD, FCOptom, DCLP, FAAO
Our progress in understanding toric lenses has been lamentably slow. Because toric soft lenses have shaken off their specialist label to become a part of everyday contact lens practice, it's even more important to try to understand the mechanisms that govern their success.
Theories on Toric Orientation
many years, the predominating theory of how toric lenses orientate suggested that
pressure from the lids pushing the lens against the eye was the main influence.
Clinicians believed that prism-
ballasted lenses orientate because of the interaction of their wedge-like shape with the lids rather than from the pull of gravity (the watermelon seed principle). Although we accepted that lid forces during the blink have some effect on rotation, because the two lids move in conflicting directions, clinicians have downplayed the influence of the blink. Some felt that gravity was only a partial explanation of how prism ballasting works, others not at all. In truth, all of these forces are important influences on toric lens orientation.
Observing Toric Lens Rotation
Simple experimentation and close observation of different toric designs can throw some light on the mechanisms governing toric soft lenses. Observing what happens when prism-ballasted lens wearers lay on their side shows that gravity does indeed have some effect: The prism base swings toward the vertical but, interestingly, not through a full 90 degrees (Figure 1). Ballasting is less effective the closer the lens is to the vertical and, at about 30 degrees from the vertical, other forces from the lids usually counterbalance any effect of gravity. As expected, non-ballasted designs such as double slab-off torics show little or no rotation under these circumstances.
Observing toric soft lenses on a blink-by-blink basis as they rotate toward their settled position helps quantify the contribution of the blink. You can achieve this by measuring lens orientation from video recordings and calculating the rate of rotation during and between blinks. Normal video recordings are too slow to show the motion of the eyelids, but it's possible to observe and measure orientation position pre- and post-blink. When prism-ballasted lenses rotate 45 degrees from their normal position, they rapidly reorientate through the first 20 to 30 degrees under the influence of gravity. Once the lens rotates to a position where gravitational force is no longer significant, most rotation occurs during the blink because of the rapid motion of the upper lid as it sweeps against the contours of the lens. When crossing the visual axis, the lid moves at approximately 20cm per second. In theory, the greatest torque will be exerted furthest away from the center of rotation; therefore, designs that have no prism in the optic zone lose little in terms of rotational efficiency. Double slab-off designs as well as prism-ballasted lenses orientate through this mechanism, which can be optimized by maximizing the wedge-shaped portion of the design in the inter-blink zone (within the palpebral aperture).
With some designs, some rotation also occurs between blinks, presumably because of the watermelon seed effect described by Hanks (1983). This theory suggests that the upper lid squeezes thicker portions of the lens, causing it to rotate until a point of equilibrium is reached. The plots in Figure 2 show the re-orientation for two different toric fittings. In one case, virtually all of the rotation takes place during the blink while, in the other, a similar amount of rotation takes place between blinks as during the blink.
High speed video recording at 500 frames per second (rather than the 20 frames per second of conventional video recorders) helps us understand the influence of the lower lid. As Doane revealed, the lower lid makes a lateral movement of as much as 5mm during the blink. A relatively low bottom lid would have little or no interaction with the lens and little effect on orientation. However, a high lower lid can encourage nasal rotation, particularly if the lid is also tight. Instability can result if the upper and lower lids encourage rotation in opposing directions. With this in mind, manufacturers have designed some torics with contours that minimize any interaction with the sideways movement of the lower lid.
What are the clinical implications of these insights into toric soft lens performance? Several researchers have noted a correlation between the slope of the upper eyelid and toric lens orientation. With double slab-off designs, for instance, lenses tend to orientate parallel with the upper lid; however, because the angle of the lid can change mid-blink, this isn't a wholly reliable indicator. Instead, the trend with toric designs has been to provide lenses that orientate more consistently on a wide range of eyes, and some manufacturers now provide data on lens orientation predictability and other indicators of lens performance.
Although our increasing knowledge of toric soft lenses doesn't yet allow us to fit them without some trial and error, it does provide a better chance of understanding where problems may lie and how to overcome them. For instance, you might tackle the instability that often results from a narrow palpebral aperture by using a design with relatively thin top and bottom zones. With gravity back in vogue, you might avoid prism-ballasted soft torics for certain occupations or hobbies such as for dancers, mechanics, military personnel, etc.
A better appreciation of the mechanics of toric lenses enables us to design better lenses. The last few years have brought the introduction of new hydrogel and silicone hydrogel torics. In general, they're more comfortable, more predictable and more stable than their predecessors.
For references, please visit www.clspectrum.com/references.asp and click on document #134.
Dr. Young is director of Visioncare Research
Ltd., a United Kingdom-based company specializing in eyecare clinical research.
He's a past president of the British
Contact Lens Association, a fellow of the American Academy of Optometry and a member of the International Society for Contact Lens Research.