In the offices of today’s eyecare practitioners, the discussion of orthokeratology (ortho-k) commonly revolves around its use in children for myopia management. This is because research from around the world suggests that the increase in peripheral myopic defocus that results when ortho-k lenses are used to correct myopia may act as a signal to slow the progression of myopia in children (Walline, 2016).
The Mechanics of Ortho-k
The forces under an ortho-k lens act on the epithelium to mold and shape it to the contour of the back surface of the lens. That is to say, the push force in the center of the lens acts to flatten the central cornea (reducing myopia), and the pull force in the midperiphery created by the reverse curve acts to steepen the midperipheral cornea (increasing in relative plus power).
When the area of the contact lens that is the treatment curve (a.k.a. the base curve) is spherical, the corneal power change induced is equal to the amount that the lens is flatter compared to the central flat K value; conversely, the amount of steepening that occurs in the midperipheral portion of the cornea has a value that is equal to the opposite of that in the center. That is to say that if the central cornea flattens by 1.00D, then the midperipheral cornea steepens by 1.00D; and if the central cornea flattens by 3.00D, the midperipheral cornea steepens by 3.00D (Figure 1).
How Does This Relate to Presbyopia?
Another way to interpret this, especially for those patients who have presbyopia, is that an individual who has 1.00D of central flattening effectively has a 1.00D peripheral add, and a patient who has 3.00D of central flattening has 3.00D of peripheral add. This power description is essentially analogous to that of a center-distance aspheric multifocal contact lens (Figure 2). However, rather than wearing a contact lens, patients have a similar aspheric center-distance lens power profile within their corneal epithelium.
When faced with the decision-making process of what contact lens modality may be appropriate for patients who have both myopia and presbyopia, consider ortho-k. While not the first modality that you might think of for such patients, it may help to increase patient depth-of-focus at near and help them become less dependent on reading glasses. Ortho-k can also provide them with complete independence from contact lenses during the day. CLS
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