Glass lenses were never comfortable enough to gain widespread popularity. The first lenses to do so were lenses made from polymethyl methacrylate (PMMA or Perspex/Plexiglas). PMMA lenses are commonly referred to as “hard” lenses. A disadvantage of these lenses is that they do not allow oxygen to pass through to the cornea, which can cause a number of adverse clinical events.
Starting in the late 1970s, improved rigid materials which were oxygen-permeable were developed. Lenses made from these materials are called rigid gas permeable or ‘RGP’ lenses.
A rigid lens is able to replace the natural shape of the cornea with a new refracting surface. This means that a spherical rigid contact lens can correct for astigmatism. Rigid lenses can also be made as a front-toric, back-toric, or bitoric. This is different from a spherical lens in that one or both surfaces of the lens deliver a toric correction. Rigid lenses can also correct for corneal irregularities, such as keratoconus. In most cases, patients with keratoconus see better through rigid contact lenses than through glasses. Rigid lenses are more chemically inert, allowing them to be worn in more challenging environments than soft lenses.