Contact lenses seem relatively straightforward, but there is actually a lot that goes into the technology. This is especially the case with astigmatism contact lenses or toric contact lenses. These contact lenses have special properties to align the contact lens with astigmatism and correct vision.
Some astigmatism contacts are thicker at the bottom compared to the top. This creates additional weight in order to have gravity help align the contact lens. But not all astigmatism contacts have this design. Some are thin at both the top and bottom so allow for the eyelids to help align the lens.
These differences within the lens help to ensure that the contact lens is doing its job of not only correcting vision but also making sure that the lens is stable and doesn’t rotate away and cause fluctuation of vision.
How Do Contact Lenses Correct Astigmatism?
Within the center of a standard soft contact lens is the prescription which corrects vision. The edge of the contact lens actually just serves as a support structure to keep this contact lens in place.
For the regular contact lenses without astigmatism, this works perfectly well. As long as the center portion of the contact lens rests in the center of your eye, the contact lens works well. The prescription is uniform in all directions of the contact lens. It doesn't matter if the contact lens is upside down on not on the eye (because the contact lens doesn't have any side which is up or down).
But contact lenses that correct for astigmatism or toric contact lenses, not only need to be centered over the eye, they must remain in the same direction. These contact lenses don’t work if they aren’t lined up well. This is because astigmatism not only has a power that must be corrected, it also has a direction.
Why Does Astigmatism Have A Direction?
To best understand why astigmatism contact lenses need to be in a certain direction, we must first understand more about what astigmatism is. The easiest way to understand astigmatism is that your cornea is shaped more like a football rather than like a basketball. There is a steep part to the cornea and a flat part to the cornea. Light that enters these respective parts focus at different points on our retina causing blurriness from astigmatism.
To correct this astigmatism, a special lens can be used to redirect light onto the steep part of the cornea and flat part of the cornea in a way to negate those differences and cause the light to come into focus. But in order for this to work, this lens needs to line up with where the steep part of the cornea actually is. If not lined up, the lens loses its effectiveness. For every 10 degrees of misalignment, astigmatism lenses become 33% less effective!
Toric contact lenses must be designed so that the contact lens lines up with the astigmatism in your vision. A tall order for a circular piece of plastic.
The Eyelid Can Move Contact Lenses
Let’s say you are able to place the contact lens on the eye and line it up perfectly by manually rotating the lens until you see well (but I doubt you would even be able to do this well). This solution wouldn’t work for long.
Every blink of the eyelids pushes on the contact lens. Most of the pushing is vertical (the upper eyelid moving down and the lower eyelid moving up) and the balance between these these two eyelids cancels out and won’t cause any extra rotation of the contact lens.
But blinking actually serves a key function of washing tears out of our eye. Within the corner of our eyelid near our nose are tiny holes called punctum. Tears drain out through these holes. Each blink of the eyelids is actually designed to push the tears more towards these punctum. Thus, blinking also has some horizontal forces. This will cause the contact lens to rotate!
Astigmatism contact lenses thus must be able to resist rotation.
Design of Astigmatism Contact Lenses
Toric contact lenses must be designed not only to line up with the astigmatism but also to resist rotation as well. As we learned, if the lens isn’t lined up well with the astigmatism, this reduces the effectiveness of the contact lens. But lets say the contact lens rotates away temporarily before rotating back. This will cause the vision to temporarily become blurry before sharpening back up again - fluctuation of vision. Not ideal.
To fight this rotation, astigmatism contact lenses are slightly different from regular contact lenses. One key feature is that the contact lens is overall larger in diameter. This creates more stability for the contact lens on the eye to resist rotation.
But a larger contact lens doesn’t solve the alignment issue. There are two ways this is achieved:
The Weighted Contact Lenses
These contact lenses are thicker on the bottom and thinner on the top. The design of these contact lenses allow for gravity to assist in keeping the contact lens aligned and stable on the eye. The thinner portion on the top allows for the upper eye lid to assist in keeping the lens more stable.
The thickness can either be a smooth gradient from top to bottom or it can be more concentrated in the periphery of the lens and out of the way of the vision-correcting portion of the contact lens.
Because of the difference in thickness, these contact lenses have a dedicated top portion and a dedicated bottom portion of the lens.
“Thin-Zone" Contact Lenses
These contact lenses rely solely on the eyelids to align and keep the lens stable. Both the top portion and the bottom portion of the contact lens are thinner than the middle portion of the lens. Each blink of the eyelids pushes against the thicker middle portion of the lens to keep things lined up. There are no weights with this contact lens.
Because both the top and bottom are thinner, these lenses have have no dedicated top or bottom portion, both sides are the exact same.
The thought behind this style of contact lens is that it can allow for quicker realignment if the contact lens gets misaligned; you don't have to wait for gravity to kick in. And because this lens doesn't rely on gravity, this style may be less likely to be affected when you tilt your head.
Toric Contact Lenses Must Be Fitted
But even despite all of the technology in the contact lens, when placed on the eye, it may not fully line up the way its supposed to. This is because every eye is different. This is the reason there is more of a process fitting the eye with a toric contact lens.
After the astigmatism contact lens is placed on the eye, the eye doctor will observe the lens at the microscope. If there is consistent rotation of the lens away from the desired direction, the prescription of the contact lens can be adjusted to account for this consistent rotation.
That way you can achieve a perfect fit and perfect vision with your contact lens!
Summary
Astigmatism contact lenses are a great way to correct vision. Some of these (but not of them) will use weights to line up the contact lens properly in order that the full amount of the astigmatism is corrected.
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