SOFT TORIC ORIENTATION
Lenses, Gravity and Other Forces
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,
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).
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.
Contact Lens Spectrum, Issue: January 2007