Rigid gas permeable contact lenses, as the name suggests, are stiff and no where near as flexible as soft lenses. They are sometimes called hard lenses, reflecting the physical properties of the lens. This gives them both advantages and disadvantages. Their ability to hold shape without support means that they can correct corneal irregularities, such as keratoconus and irregular astigmatism, giving better vision than either glasses or soft lenses. Rigid lenses can also be used to manipulate the shape of the cornea as seen in orthokeratology to correct focusing errors. In this technique, lenses are usually worn overnight to reshape the eye and then removed during the day to give clear eyesight without glasses. Whilst the rigid nature of the lenses is a key benefit, it also means that the lenses can be a little more noticeable on the eye than soft lenses, and will require a longer adaptation period. The lens also tends to be a little smaller on the eye and a little more mobile, meaning that activities involving rigorous movement may cause the lens to decentre. The practitioner fitting the lenses also has to be skilled in ensuring a correct fit. Recent advances in corneal topography have greatly improved outcomes.
