continue to occur during the patient's lifetime), and it is considered generally safe for younger patients. In addition, Ortho-K is broadly not 'new' from a safety viewpoint; contact lens safety generally speaking is considered to be well understood. However it is important, as with all contact lenses, to maintain good cleaning and hygiene discipline. Rigid lenses are generally considered to be not as comfortable as soft lenses, but Ortho-K is claimed to avoid the feeling during waking hours of the eyelids moving over the lens edges while blinking. When Ortho-K lenses are worn, the eyes are closed for sleep, although the same issues that accompany wearing contact lenses during sleep (rapid eye movements and lower tear production) remain. Adaptation, success rates, and discontinuation The cornea experiences a significant degree of adaptation within hours to days, although full adaptation often requiring 2 – 3 weeks. During this initial period vision may be affected for some people. Once adapted, FDA trials showed over 65% of patients achieved 20/20 eyesight and over 90% achieved 20/40 or better (the typical US requirement for driving without glasses). The corrective effect is stable but not permanent. Left to itself, the eye will slowly lose its adjusted shape, taking approximately 3 days to return to its former vision. The Ortho-K lenses must therefore be worn regularly so the corrected corneal shape is preserved and maintained. Usually they are worn only for part of the day (typically only when asleep at night); some users may only need to use them one night out of every two or three nights. Health considerations Indications and counter-indications The US FDA overnight orthokeratology is approved up to -6.00 diopters of myopia and a maximum of 1.75 diopters of astigmatism. In the United Kingdom, the procedure is offered primarily for myopic correction up to −5.00 diopters and up to −1.50 diopters of astigmatism. Fitting evidence for the leading lens designs indicates that procedures undertaken within these parameters have the highest probability of success. Some patients with higher degrees of myopia are successfully treated by specialist practitioners with "off-label" uses of these same lenses. In South Africa, Australia and Taiwan, practitioners using the GOV orthokeratology system have achieved successful fits as high as −10.00D of myopia and +5.00D of hyperopia. Not every patient within these parameters will be suitable for the procedure and conditions such as flat or steep corneas may result in the