Choosing the Right GP Material for Each Patient
BY ANN LAURENZI, OD, FAAO
Matching the right material to the needs of a particular patient can be a key to the success of a GP lens fit. The material of the lens you fit can influence vision, comfort and wearing time.
I would like to review some patient types we all see in our practices and the considerations for prescribing their GP material.
Greater Oxygen Needs
High hyperopic or aphakic prescriptions result in lens designs with a greater center thickness. So a priority in considering the material for such patients should be its oxygen permeability (Dk) or oxygen transmissibility (Dk/t), the amount of oxygen that passes through the lens, which varies with thickness.
Low-Dk materials are those in the range of 25 to 50 whereas high-Dk materials have a Dk value of 50 or greater. According to Holden and Mertz (1984), a Dk/t of 24 is the critical oxygen level to avoid corneal edema for daily contact lens wear. Thicker lenses will need higher Dk values to transmit enough oxygen to the cornea to avoid a hypoxic situation. The same is true for larger overall diameter lens designs because they shield more of the cornea from atmospheric oxygen.
You should also choose materials that have high Dk values for patients who may have a greater physiological need for oxygen such as corneal transplant and extended wear patients.
Choosing the material for dry eye patients or for patients who spend hours in front of a computer, inducing dry eye, requires special consideration. These patients already have a compromised tear film and will have difficulty wetting the surface of a GP lens. Poor-wetting surfaces can cause blurred vision, reduced comfort and an increase in surface deposition.
Make it a priority to fit dry eye patients in a material with good surface wettability, which enables the blink to spread tear film mucin across the anterior surface of the lens. Fluorosilicone acrylate materials possess superior surface wettability. They also act like a non-stick surface, which helps decrease mucin and protein deposition. Allergy sufferers are less likely to deposit the increased mucin in their tear film on the surface of a fluorinated material.
Patients who have corneal astigmatism as low as 1.00D to 1.50D may experience lens flexure. Flexure is how stable the lens is over corneal toricity or the resistance to bending forces (lid blinks) while the lens is on the eye.
High-myopic lens designs are thinner centrally and can flex more easily. Also, high-Dk materials are more bendable after additional polymers are combined to increase the Dk value. Therefore, when fitting astigmatic corneas or corneas with irregularity, you should choose lower-Dk materials with good durability.
Material choice may help in the case of an inferiorly decentering lens. Each material has a specific gravity, which in simple terms is the weight of the lens and is categorized as low, medium or high. Inferiorly decentered lenses or thicker lens designs may center better with a low-specific-gravity material.
We also can choose a material to give our patients protection from ultraviolet radiation. The majority of the most popular GP materials offer UV protection, but make this a high priority for aphakic patients and for patients who may have an occupation or hobby that exposes them to large amounts of UV radiation. CLS
For references, please visit www.clspectrum.com/references.asp and click on document #160.
Dr. Laurenzi practices at the Cole Eye Institute in Cleveland, Ohio where she specializes in refractive surgery co-management, contact lenses and clinical research.