What is Dk/t Value?
The Dk/t value is used to measure oxygen transmissibility of a contact lens. Contact lenses reduce the amount of oxygen entering the eye, specifically the cornea. The oxygen transmissibility value, Dk/t, quantifies the extent to which oxygen is allowed through a specific contact lens material, based on its thickness. Contact lenses with a lower Dk/t value allow less oxygen into the eye. When the cornea does not get enough oxygen, it can cause complications such as corneal neovascularization, corneal edema (swelling), blurry vision, and painful red eyes.
- Dk/t is a property that describes the oxygen transmissibility of a contact lens.
- Oxygen transmissibility (Dk/t) is calculated based on the material’s oxygen permeability (Dk) and center thickness (t).
- A low Dk/t value indicates that less oxygen is transmitted to the cornea through the contact lens. Low oxygenation of the cornea can put the eyes at risk for complications such as corneal swelling, inflammation, neovascularization, and redness.
Understanding the Dk/t Value
Oxygen transmissibility (Dk/t) depends on permeability (Dk) and thickness (t) of the contact lens. Permeability (Dk) is a property inherent to the material of the contact lens and how much oxygen can pass through the material. Thickness (t) of the contact lens is based on the contact lens prescription. Higher contact lens prescriptions have greater central thickness (t) and lower oxygen transmissibility (Dk/t).
The human cornea is avascular, meaning it does not have any blood vessels to receive oxygen-rich blood. The cornea largely receives oxygen from the atmosphere. Wearing contact lenses reduces oxygen availability, since the material forms a barrier between the oxygenated air and the cornea. It is critical that the contact lenses worn have enough oxygen transmissibility (Dk/t) for the cornea to carry out essential functions.
A higher Dk/t value is good, because it means that the contact lens allows a high amount of oxygen through the contact lens into the cornea. Conversely, contact lenses with a low Dk/t value can be harmful to the cornea, because the cornea is deprived of oxygen. When the cornea lacks oxygen, it is said to have corneal hypoxia.
Corneal hypoxia from a low Dk/t value may cause complications such as:
- Corneal neovascularization: new blood vessels form to compensate for oxygen deprivation. These new blood vessels grow into the clear cornea, which can make vision blurry.
- Keratitis: inflammation of the cornea can cause redness and pain in the eyes.
- Corneal edema: the corneal layers can swell, which makes the cornea lose its transparency and causes blurry vision.
- Prescription changes: decreased oxygen can make the glasses or contact lens prescription more nearsighted.
In general, the average Dk/t value required to prevent corneal hypoxia is:
- 30 or greater for daily disposable lenses
- 125 or greater for extended/overnight contact lens wear
Dk Vs. Dk/t
The ability of oxygen to pass through a contact lens is measured by two properties: permeability (Dk) and transmissibility (Dk/t). The Dk value refers to permeability. The material used to make the contact lens determines its permeability (Dk). Each contact lens material has a different permeability.
The Dk/t value refers to transmissibility. Oxygen transmissibility (Dk/t) is based both on the permeability (Dk) and central thickness of the contact lens (t). The oxygen transmissibility value (Dk/t) varies based on the contact lens power, since the power of the contact lens alters its central thickness. Contact lenses with high farsighted prescriptions (e.g. +10.00) have a high central thickness. Contact lenses with high nearsighted prescriptions (e.g. -10.00) have low central thickness. Inherently, contact lenses with higher prescriptions, such as those that correct high nearsightedness or farsightedness, have lower oxygen transmissibility (Dk/t) values and allow less oxygen into the cornea.
Dk/t Value in Specialty Lenses
Specialty contact lenses, such as Ortho-K lenses, hard gas permeable lenses, and scleral contact lenses all have variable Dk/t values. Hard gas permeable (GP) lenses have high Dk/t values and allow for a larger amount of oxygen transmissibility than soft contact lenses, and thus are the most oxygenated contact lens option. Ortho-K lenses, which are worn overnight and removed in the daytime to provide clear vision, have high Dk/t values as well because it is critical for contact lenses worn overnight to provide healthy amounts of oxygen. Scleral lenses have similar Dk/t values as soft contact lenses.