Overnight orthokeratology causes rapid central corneal epithelial thinning and midperipheral stromal thickening. The consequent change in corneal sagittal height is the primary factor underlying the refractive effect of orthokeratology.

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Orthokeratology (also called OK, ortho-k, corneal reshaping, corneal refractive therapy or CRT, and vision shaping treatment or VST) is a clinical technique that uses specially designed and fitted rigid contact lenses to reshape the corneal contour to temporarily modify or eliminate refractive error. Today, the most common clinical application of orthokeratology (OK) is for the reduction of myopia through corneal flattening, although new lens designs targeting astigmatism, hyperopia and presbyopia are under development.OK is not a new procedure but has experienced a resurgence in clinical and research interest over the past decade. In 1971, during the heyday of 'traditional' OK, the International Orthokeratology Section of the National Eye Research Foundation defined OK as 'the reduction, modification or elimination of refractive Orthokeratology (OK) is a clinical technique that uses specially designed rigid contact lenses to reshape the cornea to temporarily reduce or eliminate refractive error. This article reviews the history of traditional daily-wear OK (1960s to 1980s) and discusses the reasons for the recent resurgence in interest in the new modality of overnight OK, using reverse-geometry lens designs (1990s to the present). The clinical efficacy of the current procedure is examined and outcomes from clinical studies in terms of refractive error change and unaided visual acuity are summarised. Onset of the effects of overnight OK lens wear is rapid, with most change after the first night of lens wear and stability of refractive change after seven to 10 days. Mean reductions in myopic refractive error of between 1.75 and 3.33 D and individual reductions of up to 5.00 D have been reported. There appear to be slight reductions or minimal changes in astigmatism with the use of reverse-geometry lenses and most patients are reported to achieve 6/6 unaided vision or better. The induction of higher order aberrations, in particular, spherical aberration, has been reported and this may affect subjective vision under conditions of low contrast and pupil dilation. Patient satisfaction with overnight OK has been reported as similar to or better than with other popular modalities of contact lens wear. Available evidence suggests that the corneal changes induced by overnight OK are fully reversible. The refractive effect in OK is achieved by central epithelial thinning and this has raised concerns about compromise of the epithelial barrier to microbial infection. Recent reports of microbial keratitis in the modality are reviewed and the overall safety of the procedure is examined critically. Recent research on stromal contributions to the OK effect, particularly relating to overnight oedema, is summarised. Emerging issues in OK, including myopic control, correction of other refractive errors and permanency of the OK effect, are discussed. INVITED REVIEW

OK significantly reduced myopia in the central 20° VF in myopic children, converting relative peripheral hyperopia measured at baseline to relative peripheral myopia. These changes in children are similar to changes reported in myopic adults wearing OK lenses. No change in either central or peripheral refraction was found after 3 mo of daily GP lens wear. OK lenses can be used to induce myopic defocus in the periphery in myopic children and may thus provide a potential mechanism for myopia control.

The high frequency of MK in overnight OK in young Asian patients is likely to reflect the demographics of the OK lens-wearing population. The high frequency of Acanthamoeba infection strongly suggests that tap water rinsing should be eliminated from the lens care regimen for overnight OK. This study does not reveal the absolute incidence or relative risk of MK in overnight OK, and it is therefore premature to ascribe increased risk to this lens-wearing modality compared with other contact lens modalities.

The cornea responds rapidly to the application of reverse-geometry lenses for orthokeratology, with significant central corneal flattening and improvement in visual acuity after just 10 min of lens wear. This suggests that the corneal epithelium is able to be molded or redistributed very rapidly in response to the tear film forces generated behind reverse-geometry lenses.

Simple SummaryIn Australia, management of the unowned urban cat population is a continuing challenge. This is because the numbers of cats culled in trap-and-kill programs are inadequate to balance the breeding rate of the remaining cats, and also because of immigration of sexually active cats from surrounding areas in a “vacuum” effect into areas where culling has been applied. In contrast, programs based on management of cat reproduction, such as trap-neuter-return (TNR), supplemented by rehoming of socializable cats and kittens, have been shown to produce significant reductions in free-living cat populations. However, evidence is lacking that these approaches are effective in an Australian context. In this paper, we document a nine-year TNR program on an Australian university campus, supplemented by rehoming, that reduced a free-living cat population from 69 to 15 cats, while also rehoming 19 campus-born kittens and managing a further 34 immigrant cats that either joined the resident colony (n = 16), were rehomed (n = 15), or died/disappeared (n = 3). Subsequent institutional support for the program was strong because of a reduction in complaints from campus staff and students, the minimal institutional costs, and the improved health status of the remaining cats, all of which are desexed, microchipped, registered and fed on a daily basis.AbstractIn August 2008, the University of New South Wales (UNSW) in Sydney, Australia, commenced a trap-neuter-return (TNR) program to manage the population of approximately 69 free-roaming unowned urban cats on its Kensington campus. The goals of the program included an ongoing audit of cats on campus, stabilization of cat numbers through TNR, and a subsequent reduction in cat numbers over time while maintaining the health of remaining campus cats. Continuation of the TNR program over nine years resulted in a current population, as of September 2017, of 15 cats, all desexed (78% reduction). Regular monitoring of the cats through a daily feeding program identified a further 34 cats that immigrated on to campus since initiation of the program; these comprised 28 adult cats (16 unsocialized, 12 socialized) and six solitary kittens. In addition, 19 kittens were born on campus, 14 of which were born to immigrant pregnant females. Unsocialized adult immigrants were absorbed into the resident campus population. Where possible, socialized adult immigrants, solitary kittens, and campus-born kittens were removed from campus through rehoming. Overall, reasons for reductions in the cat population (original residents, immigrants, campus-born kittens; n = 122) included rehoming or return to owner (30%), death/euthanasia (30%) and disappearance (29%). This successful animal management program received some initial funding from the university to support desexing, but was subsequently funded through donations, and continues with the university’s approval and support.