contact lens materials
Clinical Pearls and Greater Hyperpermeability
BY WILLIAM J. BENJAMIN, OD, MS, PHD
Material hyperpermeability may subdue the clinical impact of hypoxia. Indeed, it's now difficult to clearly distinguish clinically relevant differences between lenses made of one hyperpermeable gel material versus another on the basis of hypoxia.
Using hyperpermeable materials, we achieve an unprecedented level of "oxygen independence" in material and design decisions and can then emphasize other aspects of contact lenses. Hypertransmissible contact lens designs are proliferating to better accommodate an extended range of spherical powers, additions, cylinders, modes of wear and ocular conditions. Lenses can be customized to better fit individual patients without worry about the former oxygen disadvantages.
Over time, manufacturers will better distinguish between hypertransmissible materials and designs. Surface and bulk properties will be, no doubt, intentionally formulated in the future to even further differentiate the hypertransmissible lines.
Always Room at the Top
The surfaces of silicone hydrogel lenses and the bulk properties of the materials may become so refined that the added oxygen benefit of greater permeability becomes less obscured. We look forward to that day in which clinicians and patients will recognize tangible benefits of oxygen capability in the upper end of the hypertransmissible range.
Though debatable now, perhaps the future of silicone hydrogels is in "greater hypertransmissibility," the definition depending not on the harmonic thickness but on the thickest portion of the lens (denoted MaxRT in Table 1). It's the maximum thickness, after all, that lies under many upper eyelids in the openeye condition and that has the potential for the greatest hypoxic, mechanical and inflammatory stress of the troubled superior cornea in the large majority of lens wearers (myopes).
Greater hyperpermeability infers that hypertransmissibility is attained at every point on each spherical lens of a series including that area covering the superior cornea. A material Dk as high as 175 to 190 Fatt Dk units would be necessary. The silicone elastomer, currently available only for aphakia, is the only greater hyperpermeable. In other words, there is still room at the top if we intend to achieve nearly complete oxygen independence with a silicone hydrogel. CLS
For references, please visit http://www.clspectrum.com/references.asp and click on document #157.
Dr. Benjamin is Professor of Optometry and Vision Science, a Senior Scientist at the Vision Science Research Center, and a clinician in Contact Lens Practice and Primary Eye Care at the University of Alabama at Birmingham.