Have you ever wondered why certain contact lenses feel like a second skin, while others leave your eyes craving relief?

The answer is found in a little-known factor called the DK value. We’ll discuss this important feature of contact lenses and learn how it affects your eyesight and comfort.

What Is DK Value, Anyway?

Simply stated, the DK value measures how much oxygen can pass through a contact lens material to reach your eye. Consider it the "breathability" score of the lens. The greater the DK value, the more oxygen flows through your eyes, thus making them healthier and more comfortable.

Why Does Oxygen Matter for Your Eyes?

Your cornea, the transparent front surface of your eye, has no blood vessels for blood supply. Thus, it relies on oxygen from the air to stay healthy. Wearing contact lenses for a long time can block some of this much-needed oxygen. When there is a lack of oxygen it can result in issues such as:

Dryness and Discomfort

Your eyes can become dry and irritated when they don’t get enough oxygen. Such dryness can make your eyes feel:

  • Gritty
  • Red
  • Less sensitive to light

All of these will compromise your comfort. 

Corneal Swelling (Edema)

Decreased oxygen could make the eyes swollen, which will blur the vision. This swelling occurs as the cornea retains fluid that can cause haziness and distort your vision if left untreated.

Infections

Eyes with low oxygen are more susceptible to infection. The hypoxic condition depletes the cornea’s immune system and puts it at greater risk of bacteria or fungal infections, therefore leading to severe complications.

Corneal Neovascularization

In severe cases, blood vessels may enlarge into the cornea and impair vision. This abnormal growth is a result of chronic lack of oxygen in the eye, causing blurred vision and possibly blindness.

Decoding DK and DK/t: What's the Difference?

You'll often see these two terms: DK and DK/t. Here's what they mean:

DK (Oxygen Permeability)

DK indicates the oxygen permeability of the contact lens material, or the amount of oxygen that can pass through the lens to the cornea. This property is inherent to the material itself and is measured in units called Fatt Dk units, named after Dr. Irving Fatt, a pioneer in corneal physiology research.

The higher the DK value, the more oxygen will enter the material, subsequently helping to keep the cornea in good condition and prevent hypoxia.

DK/t (Oxygen Transmissibility)

DK/t, oxygen transmissibility describes both the oxygen permeability (DK) of the material and the thickness (t) of the lens. It is calculated by dividing the DK value by the lens thickness in centimeters. This metric provides a more reliable indicator of the actual oxygen that actually reaches the cornea during lens wear.

For example, even if a lens material is good in DK value, a thicker lens will inhibit oxygen flow to result in a lower DK/t value. Therefore, DK/t plays an important role in lens design, particularly with higher prescription and thicker lenses.

How Do Different Lens Materials Stack Up?

Contact lenses come in various materials, each with its own DK value:

Hydrogel Lenses

Traditional hydrogel lenses have Dk values ranging from 25 to 50. For example, Nelfilcon A has a Dk value of 26, and Omafilcon A has a Dk of 25. These lenses rely on their water content to transport oxygen to the cornea.

Silicone Hydrogel Lenses

Silicone hydrogel lenses, like the Acuvue Oasys contacts, typically come with much higher Dk values, often above 100. For example, Lotrafilcon B has a Dk of 110, while Comfilcon A has a Dk of 128. This permeability opens up more space for oxygen to get to the cornea and allows for extended wear.

Choosing the Right DK Value for Your Needs

Selecting a lens with an appropriate DK value depends on several factors:

Wearing Schedule

For everyday use, lenses with a Dk/t value of 25 to 30 should be sufficient to prevent corneal hypoxia. However, for extended or overnight wear, higher Dk/t values are recommended to keep eyes healthy.

Eye Health

Patients with pre-existing eye conditions or increased oxygen needs might need lenses with higher Dk values to keep them oxygenated and reduce the risk of complications. 

Lifestyle

Active lifestyles or environments with low humidity can impair lens performance. Selecting lenses with the right Dk values and moisture retention capacity can increase comfort and visual health.

FAQs About DK Value in Contact Lenses

Can I wear high DK lenses overnight?

Lenses with high DK/t values, like silicone hydrogel lenses, are designed to allow more oxygen to the cornea, making them suitable for extended or overnight wear. However, always consult your eye doctor before changing your wearing schedule.

Do thicker lenses have lower DK/t values?

Yes, even when the lens has a high DK value, greater thickness will decrease the DK/t value, so less oxygen can get to the eye.

Are there risks associated with low DK lenses?

Lenses with low DK values can limit oxygen to the cornea, and could lead to discomfort, dryness, and more serious eye health issues over time.

How do I know the DK value of my lenses?

DK values are often listed in the product specifications. If you are unsure, ask your eye care professional or check the manufacturer's instructions.

Is a higher DK value always better?

Higher DK values provide greater oxygen transmission, but the best lens for you will vary based on comfort, fit, and your specific eye health needs.

Prioritize Your Eye Health: Choose Contact Lenses with the Right DK Value

Knowing DK value is key to making an informed purchase of your contact lenses. Investing in lenses that have a proper level of oxygen permeability will help you feel comfortable and protect your vision. Always discuss your options with an eye care specialist about what would work best for you and your lifestyle.

Your eyes are your windows to the world—keep them healthy by staying informed!