Lenses For Astigmatism
Astigmatism is a type of focusing error that
is usually caused by the cornea being non-spherical. Rather than the
surface being completely round like a football, it is a bit ‘pointy’
more like a rugby ball.
If the astigmatism is over a certain amount it will need to be corrected
to give good vision. Small degrees of astigmatism usually don’t
cause any problems, and often people will not bother having it corrected.
Astigmatism can be corrected by either soft or gas permeable lenses.
Nowadays the soft ones are more popular.
Soft Lenses For Astigmatism
These are called Soft Toric lenses.
Such a lens has two different powers eg one power running along one
meridian and then a different one along another one.
The lens must sit on the cornea with these powers running along specific
directions. To ensure that the lenses stay in the required positions
and don’t rotate, the lenses incorporate some form of stabilisation.
Lenses by different manufacturers have different stabilisation designs
and some brands work better in some eyes than others. It is sometimes
necessary to try a few different makes to find a good fit.
Soft toric lenses can be in disposable or non-disposable
form:
Toric Disposables are
now available to correct a wide range of astigmatism strengths. There
are daily, two weekly and monthly disposable options. The monthly’s
have the highest range of power choices and are thus the mostly prescribed
option.
Toric Non Disposable these are usually
only required when disposable lenses are out of the required power
or fitting range.
Gas Permeable Lenses For Astigmatism
These lenses work by artificially making the
corneal surface spherical again and thus ‘eliminating’
the astigmatism. The lenses themselves are spherical and they float
on the surface of the cornea, trapping a tear layer beneath it. The
tear layer acts like a liquid lens, and the combination of lens and
tears provides the required correction.
Gas Permeable Toric Lens
In very high degrees of astigmatism the corneal surface is very pointy.
In such cases a lens with a spherical curvature will not sit stably
but will rock back and forth against the surface. This will give poor
vision and will be uncomfortable.
In these eyes, a lens that is moulded so that it matches the shape
of the cornea is required. This type of lens is known as a Toric Gas
Permeable.
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