CYLINDRICAL PRISM STABILIZED LENSES

Spherical hydrogel lenses do not correct the spectacle astigmatism.  If a patient has significant astigmatism, usually over 0.75 D, if it is to be corrected with hydrogel lenses a toric lens is required.

If cylinder is generated in the lens, the lens 
must not rotate on the eye in order to keep 
the cylinder in the proper axis.  One way to 
prevent this rotation is to put prism in the 
lens.  Due to the thin apex as compared to 
the thicker base, the rotation of the lens is 
minimized.  The force of the upper lid against 
the thicker edges of the lens keeps the thin 
apex superiorly.

The amount of prism used is in the range of 1.0 
to 1.5   (the    is the symbol for prism diopter).  
The prism adds thickness to the lens which can 
cause corneal edema.