Nearsightedness (myopia) is a common refractive error where distant objects appear blurred, but close-up vision remains clear. This occurs because the eye is elongated or the cornea is too curved, causing light to focus in front of the retina instead of directly on its surface. The retina is the light-sensitive tissue at the back of the eye. Corrective lenses shift the focal point backward, ensuring the image lands precisely on the retina to restore clear vision.
How Corrective Lenses Function
Myopia correction uses the principle of light divergence to adjust focus. Since the myopic eye focuses light too strongly, the corrective lens must spread the light rays before they enter the eye. This is achieved using a concave lens, which is thinner in the center and thicker toward the edges.
This shape provides negative optical power, often called “minus” power. The concave lens acts as a diverging element, pushing light rays outward to delay convergence inside the eye. This controlled divergence ensures the final image projects sharply onto the retina, restoring clear distance vision. The greater the degree of nearsightedness, the stronger the minus power needed.
Eyeglasses: Materials and Design Options
Eyeglasses are a popular method for correcting nearsightedness, with materials evolving for better aesthetics and comfort. Conventional lenses are often made from standard plastic (CR-39), which is affordable and provides good clarity. For active lifestyles or children, impact-resistant options like polycarbonate and Trivex are recommended.
Polycarbonate lenses are lighter, thinner, and highly shatter-resistant. Trivex is a strong, lightweight material that offers superior optical clarity compared to polycarbonate.
For stronger prescriptions, high-index plastic lenses are used. These are designed to be much thinner and lighter while providing the same corrective power. High-index material bends light more efficiently, reducing the bulky appearance associated with thick lenses. Coatings, such as anti-reflective treatments and scratch-resistant layers, can further enhance durability and reduce glare.
Contact Lens Modalities
Contact lenses offer an alternative to spectacles, providing a wider field of view and freedom from frames. Standard myopia correction primarily uses soft lenses, made from flexible polymers that conform to the eye’s shape. These are categorized into traditional hydrogel and newer silicone hydrogel materials.
Silicone hydrogel lenses are widely prescribed because they transmit significantly more oxygen to the cornea than standard hydrogel lenses, promoting better eye health. Lenses are available in various replacement schedules.
Replacement schedules include:
- Daily disposables, which are discarded after a single use for maximum hygiene.
- Bi-weekly replacement lenses.
- Monthly replacement lenses, both of which require daily cleaning and disinfection.
- Extended-wear lenses, approved for continuous wear for a set number of days, including overnight.
Lenses for Myopia Progression Management
A distinct category of lenses is designed to actively slow the progression of nearsightedness, particularly in children. One specialized approach is Orthokeratology (Ortho-K), which uses custom Rigid Gas Permeable (RGP) lenses worn overnight. These lenses temporarily reshape the cornea while the patient sleeps, allowing for clear vision throughout the day without correction.
Another method uses specialized soft contact lenses, such as dual-focus designs. These lenses correct central vision while creating a zone of myopic defocus in the peripheral visual field. This optical signal is believed to slow the excessive elongation of the eyeball, which causes myopia progression. Specialized spectacle lenses, utilizing Defocus Incorporated Multiple Segments (DIMS) technology, also create this peripheral defocus signal.