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Modification & Delivery
Gas Permeable Rigid Lenses


DK Value: The term DK value is used to describe the amount of oxygen which is able to transmit through a contact lens. Technically, the gas permeability or DK is a characteristic of a given lens material at a given temperature where D is equal to the diffusion of the substance or gas through the material and K is equal to the solubility of the substance within it. Another term sometimes used in this context is Oxygen Flux. Oxygen flux is defined as the amount of oxygen that can pass through a given area of the material in a given amount of time, driven by a given partial pressure difference of oxygen across the material. Therefore oxygen flux will vary with the DK of the material, the lens thickness and the pressure drop across the lens.

Thinner lenses generally permit more oxygen to pass through the lens so thickness is an important aspect of lens performance. The term oxygen permeability indicates the DK divided by the lens thickness which can be expressed in the following formula: oxygen transmissibility=DK/L where L represents the lens thickness.

Advantages of Gas Permeable Lenses

  • More Comfort: Since they can be fit tighter, there is minimal movement which results in more comfort

  • Rapid Adaptation: Lenses can generally be worn on a full time daily wear basis within one week

  • Spectacle Blur Uncommon: Hypoxia- induced spectacle blur is greatly reduced

  • Corneal Distortion: Corneal distortion or "molding" which can be an unfortunate side effect of PMMA lenses is virtually non-existant with gas permeable lens materials
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  • Can Be Made Larger: Larger lenses permit better centration resulting in greater comfort. Also larger lenses can generally cover greater amounts of corneal astigmatism, sometimes up to 5 diopters

  • Greater Safety: Long term corneal physiologic changes are less likely to occur than with PMMA.

  • Less Flare: Larger lenses permit larger optic zones which in turn minimize the probability of lens flare in dim light situations. Some optic zones can be as large as 8.5 mm, larger than the total diameter of some PMMA lenses.

Disadvantages of Gas Permeable Lenses

  • Quality Control: The larger diameters demand stringent quality control measures. Edge and peripheral curve designs become especially crucial.

  • Lenses Are More Fragile: Greater care is needed in cleaning and handling to avoid scratching and breaking

  • Deposits: Waxlike lipid deposits are more likely to form on lenses containing silicone

  • Protein Deposits: Protein deposits can form on the lens which need to be removed with special cleaners

  • Scratch Easier: Gas permeable rigid lenses will scratch easier than their PMMA predecessors

Fitting Gas Permeable Lenses:

These lenses are generally fit through the use of a trial set or by reference to a lens nomogram supplied by the manufacturer.

 

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