contact lens materials
How Much Oxygen is Enough for Safe Lens Wear?
BY N. REX GHORMLEY, OD, FAAO
So how much oxygen is really necessary for safe contact lens daily wear? Flexible wear? Continuous wear? Every month, the "oxygen battle" expands, and many clinicians find it confusing.
In the professional press and at educational meetings around the globe, industry experts debate and discuss the minimum oxygen transmission (Dk/t) needed for safe contact lens wear. In fact, I've heard it said that too much oxygen can harm the cornea!
Getting Expert Advice
Recently, I asked the following questions: "Can too much oxygen be detrimental to the cornea?" and "How much oxygen is necessary for safe contact lens wear?" to two distinguished colleagues, Dr. Dwight Cavanagh and Dr. Lyndon Jones. Here's what they had to say.
Dr. Cavanagh's Response To avoid increasing the binding of Pseudomonas aeruginosa to corneal epithelial cells, a contact lens should have a Dk/t greater than 125, irrespective of the lens type and of whether the patient wears the lens for daily or continuous wear. Only a few silicone hydrogel and GP lenses meet these criteria, and they represent the safest choices for avoiding microbial keratitis for all patients. Why would any patient want to wear or practitioner choose to fit a contact lens that increases corneal bacterial binding when lenses are available that don't?
Dr. Jones' Response The question about having "too much oxygen" has detracted from the primary issue that we should concentrate on: "What's best for our patients?" It makes sense that the more oxygen we provide to the cornea, the better its metabolism.
Many patients who wear silicone hydrogels clinically demonstrate less physiological compro mise than patients who wear conventional materials. Factors driven by chronic hypoxia such as microcysts, striae, neovascularization and limbal redness are now becoming relegated to the history books, thanks to silicone hydrogel lenses. Reduced limbal and bulbar redness, plus reduced contact lens dryness, make silicone hydrogel lenses the ideal lens of choice for many patients.
The Numbers Game
The contact lens industry has entered into a global "oxygen war," and the importance of oxygen transmission depends on the Dk/t of each company's silicone hydrogel contact lens. But you should consider the following numbers:
- LaHood calculated that to reduce corneal edema to 3.2 percent in overnight wear, a contact lens needs a Dk/t of 125.
- Harvitt & Bonanno have shown that a contact lens that has a Dk/t of 125 will eliminate stromal edema in overnight wear.
- Papas has estimated that a lens needs a Dk/t of 125 to avoid limbal hyperemia.
- Cavanagh firmly states that a contact lens must have a Dk/t of 125 to avoid Pseudomonas binding.
- The Dk/t values that manufacturers quote describe the Dk/t of a 3.00D lens at its center. It doesn't consider the oxygen transmission across the whole lens and the differences in transmission between minus, plus or toric contact lenses.
Think About It!
I've always thought that a car that gets 50 miles per gallon is much more efficient than a car that gets 10 miles per gallon. Using similar thinking, you should prescribe a contact lens that patients can safely wear on a daily wear or some type of continuous wear basis. Why? Because many of your patients will occasionally sleep in their lenses even if you fit them with a daily wear lens. I encourage you to make silicone hydrogel lenses your soft lens of choice and to carefully consider which lens is best for your patients.
Dr. Ghormley is in private practice in St. Louis, MO. He is a past president of the American Academy of Optometry and a Diplomate of its Cornea & Contact Lens Section.