A toric contact lens is one in which
either or both of the optical surfaces have the effect
of a cylindrical lens, usually in combination with the
effect of a spherical lens. Myopic (nearsighted) and
hypermetropic (farsighted) people who also have astigmatism
and who have been told they are not suitable for regular
contact lenses may be able to use toric lenses. If one
eye has astigmatism and the other doesn't, the patient
may be told to use a spherical lens in one eye and a
toric lens in the other. Toric lenses are made from
the same materials as regular contact lenses but have
a couple of extra characteristics:
• They correct for both spherical
and cylindrical aberration.
• They have a specific top and
bottom, as they are not symmetrical around their centre
and must not be rotated. Lenses must be designed to
maintain their orientation regardless of eye movement.
Often lenses are weighted more at the bottom and are
marked by tiny striations so the wearer can insert them
in the correct orientation, or they are designed so
that the correct orientation is restored when the user
blinks. Some do both.