Nonetheless, when properly fitted and managed, Johnson et. With these results in mind, it is not surprising that Swarbrick 2 found high levels of patient satisfaction with overnight OK in a number of qualityof- life QOL surveys. Conventional rigid lens designs involve a central alignment back optic zone which is surrounded by a series of concentric curves, each one progressively flatter than the central alignment curve.
Following this is the alignment curve for appropriate lens centration, and finally the peripheral curve for edge lift and tear exchange see Figures 1 and 2. In myopic OK, the combination of a centrally flat and mid-peripherally steep contact lens shape induces hydraulic forces in the post-lens tear film, which redistributes corneal tissue to mimic the lens shape. The central flattening, specific to the level of myopia, brings about the change in refraction and allows for clear unaided vision during waking hours. Many previous scientific works have been dedicated to the mechanisms and models occurring during OK treatment; the general consensus is that the thin tear film created beneath the centre of the lens creates shear forces which move tangentially across the corneal epithelium The change in refractive error after lens wear is described by difference maps.
The aim in myopic OK is to produce a central flattened zone, known as the treatment zone. This zone is surrounded by a mid-peripheral ring of steepening which is a result of the reversecurve zone of the reverse geometry lens. In this example, after one week of lens wear, the central corneal curvature has become flatter by 3.
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The change in corneal curvature achieved in myopic OK is achieved by central epithelial thinning and this has raised concerns about compromise of the epithelial barrier to microbial infection 2. Epithelial bacterial binding, considered a risk factor for corneal infection is more likely under hypoxic conditions, which can occur in the extended wear overnight contact lens wear situation.
It is disconcerting that patients may feel that OK is unduly risky with reports of more than cases of infectious keratitis since , 3. While this figure makes the practice OK seem rather dubious, there is more to the story. The majority of these reports of microbial keratitis originated in the East Asian region, in particular in China and Hong Kong, where contact lens fitting was previously less regulated.
Since these reports emerged, these countries have actively intervened to regulate the practice of OK and to improve safety through both practitioner and patient education. Now consider over a similar eight year period, an Australian-based study in conducted by Watt and colleagues13 found only a total of nine cases of presumed microbial keratitis associated with OK, out of which two resulted in a loss of BCVA. Ultimately, safety is more likely to be achieved with ethical and knowledgeable practitioners and educated patients. In the OK overnight-wear modality the lenses are removed during waking hours, rather than worn 24 hours per day as silicon-hydrogel lenses can be.
Above — back Surface lens in a reverse lens 1 Image courtesy of John Mountford. Above — Topography analysis showing preand post- treatment on the left. The difference map shows a change of Children and Myopia Control Another emergent yet controversial topic with OK is its usage in children. Firstly, in terms of lens handling and care there needs to be a clear understanding of associated risks between the patient, parents and the practitioner.
Clinical experience has shown those children particularly keen to wear contact lenses are motivated to comply with proper care and maintenance. The age limit varies according to practitioner confidence and patient skill. These designs are comprised of different zones that either provide treatment or stabilize the lens. This area of apparent bearing corresponds to the optic zone of the lens, commonly around 6mm in diameter.
Just outside of the optic zone is the area that generates the tear reservoir from the reverse curve RC. The RC is usually about 3. Stability and centration are critical to success when fitting ortho-k lenses. Peripheral curves play an integral role in this stabilization and also serve in closing off the lens fluid system.
Orthokeratology: Principles and Practice
Typical GP lenses have flatter peripheral curves to achieve edge lift and allow tear exchange under the lenses. Ortho-k lenses are designed to be in close alignment with the peripheral cornea just outside of the optic zone, giving this area the appropriate name of the alignment zone, which is typically about 0. The outermost zone is the peripheral curve system; these are the flattest curves, about 0.
Figure 1. Depiction of described ortho-k lens curves from inside out. Courtesy of Patrick Caroline. One theory regarding how this design works is that it creates fluid forces beneath the lens to move corneal tissue. Centrally, a positive pressure force is created from the low clearance, much like capillary action, which pushes tissue out away from the apex. Conversely, the fluid reservoir under the RC generates a negative fluid force that pulls corneal tissue into it.
The combined efforts of these two forces result in central flattening, which clinicians use to correct myopia. However, the back optic zone radius or base curve and the RC are not individually responsible for these forces, but rather it is the combination of the two that creates this effect. Together, the radii or angles and the diameters of these zones create the sagittal height sag of this central system in which all of the biomechanical forces of the procedure occur. For the treatment to be most effective, the design must account for corneal shape factors such as curvature, astigmatism, and eccentricity.
Additionally, topographical elevation data should be acquired and taken into consideration so that the lens has a similar sag to the cornea over a given chord length. Perhaps the most common issue encountered in ortho-k fitting is lens decentration, often perceived to be induced by corneal toricity Figure 2. This decentration can result in induced irregular astigmatism and subsequent decreased visual acuity, decreased image quality, and glare.
Figure 2. Superiorly decentered lens showing excessive inferior edge lift and a variable pattern. There has even been a question as to whether ortho-k lenses can induce astigmatism on clinically spherical corneas. However, research suggests that ortho-k does not cause significant refractive astigmatism in non-astigmatic patients; unanticipated subjective refractive changes may, however, result from an increase in higher-order aberrations. A majority of the treatment occurred within the central 2. Failure to achieve this seal-off allows fluid to escape along the steep meridian, thereby diminishing the negative fluid force produced by the tear reservoir Figure 3.
Figure 3. A Different corneal shape between the flat and steep meridians results in incomplete alignment along the steep axis red. Notice the greater space between the cornea and back lens surface at the smaller red arrow compared to the blue. B This results in greater fluorescein pooling and leakage. Kame stated. When you make a diameter change in the reverse-geometry lens, it dramatically tightens the lens, whereas it has very little influence on a regular lens. Or as in regular ortho-K, decreasing the optical zone size flattens the lens.
When you do that with a reverse-geometry lens, you are steepening the lens. Winkler, who uses lenses by Contex Sherman Oaks, Calif. He explained that, rather than the flatter radius of curvature in a typical RGP, the flat central portion of the reverse-geometry lens surrounded by a steeper secondary curve helps achieve results more quickly. Kame recommended beginning the process with a regular spherical lens to help the patient grow accustomed to wearing rigid lenses, build up a wearing cycle, establish a regimen and monitor the adaptation process.
Ortho-K is more effective in patients with lower myopia, said Dr. Once you get up to a certain point, into the higher prescriptions, the ortho-K effect is less successful.
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Once a patient grows accustomed to wearing such lenses, Dr. By wearing them for a solid 8 to 12 hours during a hour period early on and removing them in the evening, the patient can then begin to reduce the wearing time to realize the maximum benefit, he said. Kame suggested.
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Although overnight wear of ortho-K lenses has not been given the green light by the Food and Drug Administration FDA , it is the most efficient use of ortho-K, stated Dr. Until it receives approval, he said, the most effective way to learn about this aspect of the process is to participate in laboratory studies.