Examining GP Biocompatibility
BY ANN LAURENZI-JONES, OD, FAAO
Bioinspiration or biomimicry is life imitating nature to solve human problems. If we take a gas permeable (GP) contact lens, for example, it solves a life problem by correcting ametropias. And it also may be the most biocompatible lens available. One article lists the properties of an ideal contact lens (Jones, 2002):
- meets or exceeds the cornea's oxygen requirements
- provides in-eye wetting
- resists deposits
- is stable and durable, physiologically inert and optically transparent
- requires minimal care
- is cost-effective to manufacture
To that list, I would add “provides optimal vision”
GP lenses provide a superior form of vision correction compared to spectacles, soft lenses and refractive surgery procedures such as LASIK. The pristine refracting surface created by a GP corrects ametropias as well as irregular astigmatism on the anterior corneal surface, which cannot be corrected with spectacles or soft lenses. Barring internal astigmatism, a GP will mask visual distortion seen on irregular, asymmetric or abnormal anterior corneal topographies.
In a paper by McMahon (2003), several studies are cited to demonstrate that the optics of GP lenses correct aberrations and contrast sensitivity better than the optics of soft contact lenses (Sorbara, 1992) do or the cornea post-surgical correction with LASIK, giving our patients the best possible vision.
GP lens materials are biocompatible because they're oxygen permeable and oxygen transmissible, much more so than a hydrogel lens material. The addition of silicone or fluorine to polymethylmethacrylate (PMMA) creates highly oxygen permeable materials that easily meet or even exceed the oxygen needs of the cornea for daily and extended GP wear. The ratio of methyl-methacrylate, silicon, fluorine and wetting agents in the GP material not only determine oxygen permeability, but its wettability, dimensional stability and deposit resistance (Jones, 2002; Tighe, 1995) — all of which contribute to on-eye biocompatibility. For example, the addition of fluorine or fluorosilicone acrylate (FSA) makes the material more wettable and deposit resistant (protein and mucin) compared to materials made of silicone acrylate. A more wettable, deposit-resistant lens improves patient comfort and provides the best visual conditions (Quinn, 2000). GP lenses made of FSA materials may provide the most biocompatible choice for a patient who is a heavy depositor or has borderline dry eye.
GP lenses allow fluid tear exchange under the lens, allowing for good debris removal and providing an additional route for oxygen to reach the corneal surface. The tear exchange under a GP can be 10 to 20 times greater compared to soft lenses. Allowing enough oxygen to the cornea may be one of the most important biocompatible considerations, as oxygen prevents corneal hypoxic conditions (Gleason, 2003). Higher rates of infectious keratitis, one of the most serious risks of contact lens wear (Key, 2007), are associated with corneas in a hypoxic state (Nilsson, 2002). GP lenses minimize oxygen deprivation, and this is one of the reasons GP lenses have the lowest rate of infection of any type of lens, adding to their biocompatibility.
GP lenses can also be lathed into many different designs to correct a myriad of complicated corneal topographies from post-penetrating keratoplasty to oblated corneas from previous RK surgery to keratoconic corneas in need of asymmetric peripheral curve systems. Their durability allows for ease of care — and they are economical value because they have a longer wear life and are replaced less often than soft lenses. So when considering what lens may be close to ideal as well as highly biocompatible, put GP lenses at the top of your list. CLS
For references, please visit www.clspectrum.com/references.asp and click on document SE2010.
Dr. Laurenzi-Jones has a staff position at NorthShore University Hospital in Glenbrook, III.