A Glossary of Ortho-K and Myopia Terms

Clinical and Treatment Terms

These are the words that describe how the treatment works, what the lens is, and what we measure. They are the terms most likely to appear in a clinical note or a research paper.

• Ortho-K. The everyday abbreviation for orthokeratology. Custom rigid lenses worn during sleep that temporarily reshape the cornea so daytime vision is clear without glasses or contacts.
• Orthokeratology. The full clinical name for the same treatment. Literally the straightening or correcting of the cornea, achieved with a lens rather than with surgery.
• Invisalens. The practice trademark for the overnight Ortho-K system used at Better Sight Overnight. It describes how the treatment is delivered here: topography-guided design for each eye, high-oxygen material, verified handling training, and a defined follow-up schedule.
• Corneal reshaping. The physical effect of an Ortho-K lens. A few microns of tissue in the outermost corneal layer are redistributed overnight, flattening the center and slightly steepening the surrounding ring. It reverses fully when lens wear stops.
• Cornea. The clear dome at the front of the eye. It performs most of the eye's focusing power, which is why changing its shape changes your prescription.
• Corneal topography. A non-contact scan that maps the curvature of your cornea point by point and produces a color-coded elevation map. It is how candidacy is determined and how each lens is designed. See Corneal Topography.
• Custom lens design. A lens designed from your own topographic map rather than selected from a set of stock parameters. Each eye is designed independently, since the two are rarely identical.
• Reverse geometry lens. The lens shape used in Ortho-K. Unlike a conventional rigid lens, its center is flatter than the cornea and the zone just outside the center is steeper, which is what produces the reshaping effect.
• Gas-permeable lens. A rigid contact lens made from a material that allows oxygen to pass through to the cornea. Ortho-K lenses are always gas-permeable, because an overnight lens that blocked oxygen would not be safe.
• High Dk material. Dk is the measure of how readily a lens material transmits oxygen. High Dk materials transmit substantially more than earlier generations of rigid lens, which is a large part of why modern overnight wear is viable.
• Treatment zone. The central area of the cornea that the lens flattens, and therefore the area providing your clear daytime vision. Its size and centration affect visual quality, particularly at night when the pupil is larger.
• Refractive error. The general term for an eye that does not focus light onto the retina correctly. Myopia, hyperopia, and astigmatism are all refractive errors.
• Myopia. Nearsightedness. Distant objects blur because light focuses in front of the retina rather than on it, usually because the eyeball is longer than its focusing power suits.
• Progressive myopia. Myopia that keeps getting stronger over time, typical in children and driven by continued elongation of the eye. It is the reason a child needs a new prescription every year. See Progressive Nearsightedness.
• Myopia control. Any treatment intended to slow the rate at which myopia worsens, as distinct from simply correcting the blur. Ortho-K, low-dose atropine, and soft multifocal lenses are the main options.
• Axial length. The physical front-to-back measurement of the eyeball, in millimeters. It is the structural driver of myopia and it does not shorten once the eye has grown.
• Axial length monitoring. Measuring axial length at baseline and remeasuring it on a schedule, then plotting it against expected age-matched growth. It is the only reliable way to see whether a myopia control strategy is working. See Axial Length Monitoring.
• Peripheral myopic defocus. The optical condition created by a reshaped cornea, in which peripheral light comes to a focus slightly in front of the peripheral retina. This signal is associated with slower axial elongation and is the mechanism behind Ortho-K myopia control.
• Astigmatism. A refractive error caused by the cornea or lens being shaped more like a rugby ball than a sphere, so light focuses at more than one point. Low to moderate regular astigmatism is frequently correctable with Ortho-K. See Ortho-K for Astigmatism.
• Diopter. The unit of refractive power, abbreviated D. A prescription of minus three diopters is moderately myopic. Roughly one diopter of myopia corresponds to about a third of a millimeter of axial elongation.
• Microbial keratitis. A corneal infection, and the principal risk of any overnight contact lens wear. It is uncommon, largely preventable through hygiene, and the reason the water rules are absolute.

Patient Experience Terms

These describe the visits, the routine, and the sequence of what happens to you as a patient. They are the terms most likely to appear on an appointment card or in a conversation at the front desk.

• Candidacy evaluation. The examination that determines whether Ortho-K is appropriate for your eyes. It includes refraction, corneal topography of both eyes, tear film assessment, and corneal health evaluation, plus baseline axial length for children. Starts at $250 and is credited toward your program. See Consultation and Candidacy.
• Insertion and removal training. The hands-on teaching session at the dispensing visit where you or your child learns to put lenses in, take them out, and clean them. Lenses do not go home until this is independent. See Insertion and Removal Training.
• Dispensing visit. The appointment where your finished lenses are fitted, evaluated on the eye, and released to you along with the training and starter care kit.
• Adaptation period. The first one to two weeks of wear, during which lens awareness fades and vision progressively sharpens and stabilizes. Higher prescriptions take longer. See Adaptation Guides.
• Morning-after check. The appointment on the first morning after your first night of wear, done within a few hours of removing the lenses, so we can measure the corneal response while it is fresh.
• End-of-day regression. The common early pattern where vision is sharp in the morning and softens by evening. It usually resolves as the cornea stabilizes, and is a design question if it persists past the first month.
• Lens care and hygiene. The nightly and morning routine of cleaning, rinsing, disinfecting, and storing your lenses, plus the rules about water and case replacement. See Lens Care and Hygiene.
• Follow-up care. The scheduled appointments after dispensing, typically the morning-after check, one week, and one month, then every three to six months. Included in the program for the first month. See Follow-Up Care.
• Long-term maintenance. The ongoing phase after stabilization: routine reviews, corneal health checks, axial length remeasurement for children, and periodic lens replacement. See Long-Term Maintenance.
• Daytime freedom. The practical outcome of the treatment. Nothing worn during waking hours, so glasses, daytime contacts, sports goggles, dust, chlorine, and dry air stop being a consideration.
• Backup glasses. A current pair of glasses kept for nights the routine cannot happen, for a lost or damaged lens, or for any period we ask you to stop wearing. Part of the program rather than an optional extra.
• Starter care kit. The cleaner, rinse, disinfecting solution, case, and handling tools provided at dispensing so you begin with exactly what the routine requires.

Comparison and Decision Terms

These come up when you are weighing Ortho-K against something else, or reading about what a treatment can and cannot promise.

• Reversibility. The property that defines Ortho-K. Because no tissue is cut or removed, stopping lens wear allows the cornea to return to its original shape over days to weeks, with no lasting change. See Reversibility.
• Non-surgical vision correction. Correction achieved without incision, laser ablation, or implant. Glasses, contacts, and Ortho-K all qualify. LASIK, PRK, SMILE, and implantable lenses do not. See Clear Vision Without Surgery.
• LASIK. A refractive surgery that permanently reshapes the cornea with a laser. Adults only, requires a stable prescription and adequate corneal thickness, and cannot be undone. Compared honestly in Ortho-K vs LASIK.
• Low-dose atropine. A heavily diluted eye drop used nightly to slow myopia progression pharmacologically. It slows eye growth but does not correct vision, so glasses are still required. See Ortho-K vs Atropine.
• Soft multifocal lenses. Daytime soft contact lenses designed with concentric zones that create peripheral defocus for myopia control. A legitimate alternative for a child who will not tolerate a rigid lens.
• Single-vision correction. Standard glasses or contacts with one power throughout. They correct blur and have no effect on myopia progression, which is the distinction that matters for a growing child.
• Combination therapy. Using two myopia control treatments together, most commonly Ortho-K plus low-dose atropine, for a child who continues to progress on one alone. A clinical decision driven by measured axial length, not a default.
• Realistic expectations. The honest framing of outcomes. Most patients reach functional distance vision without correction and many reach 20/20, but some retain a small residual prescription and results vary. See Realistic Expectations.
• Residual prescription. The small amount of uncorrected refractive error some patients retain after treatment stabilizes. It may be negligible, or it may mean occasional glasses for night driving.
• Elective service. How most vision plans classify Ortho-K, which is why it is frequently not covered in full. Flexible spending and health savings accounts can often be applied instead. See Insurance.
• Starting-at pricing. The way figures are published on this site. A complete program starts at $1,800 and a consultation starts at $250, credited toward your program. Your exact figure depends on prescription and corneal complexity, and you receive it in writing before you commit.

If a term you encountered is missing, email hello@bettersightovernight.com and we will add it. For the questions behind the vocabulary, go to the FAQ or the Parent Guide, or browse everything in Resources.