These contact lenses have a stable, nearly unchanging shape. Patients take some time to become used to them and should therefore wear them often. The goal is to achieve the best possible intimacy of fit between the posterior surface of the lens and the anterior surface of the cornea (Fig. 16.19). This allows the contact lens to float on the precorneal tear film. Every time the patient blinks, the lens is displaced superiorly and then returns to its central position. This permits circulation of the tear film.
Previously, polymethyl methacrylate (PMMA) was used as a material. However, this is practically impermeable to oxygen. The lenses were fitted in small diameters with a very shallow curvature; the central area maintained contact with the cornea while the periphery projected. This allowed excellent tear film circulation, and patients were able to wear the lenses for surprisingly long periods. Today, highly oxygen-permeable materials such as silicone
Fig. 16.19 A tear film lies between the anterior surface of the cornea and the posterior surface of the lens (visualized by fluorescein dye).
copolymers are available. This eliminates the time limit for daily wearing. These lenses may also remain in the eye overnight in special cases, such as aphakic patients with poor coordination (prolonged wearing).
Rigid contact lenses can be manufactured as spherical lenses and toric lenses. Spherical contact lenses can almost completely compensate for corneal astigmatism of less than 2.5 diopters. This is possible because the space between the posterior surface of the spherical contact lens and the anterior surface of the astigmatic cornea is filled with tear fluid that forms a "tear lens." Tear fluid has nearly the same refractive index as the cornea. More severe corneal astigmatism or internal astigmatism requires correction with toric contact lenses. Rigid contact lenses can even correct severe keratoconus.
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