Abstract
Background/Purpose
Although scleral contact lenses are prescribed with increasing frequency, little is
known about their long-term effects on ocular physiology. The main goal of this paper
is to predict values of oxygen transmissibility of scleral lens systems by applying
the concept of resistors in series to parameters characteristic of current scleral
lenses. A second aim is to find the maximal lens and post-lens tear layer thickness
combinations above which hypoxia-induced corneal swelling would be found.
Methods
Theoretical calculations were used to predict the oxygen transmissibility of scleral
lens systems, considering several material permeabilities (Dks 100–170), varying lens thicknesses (250–500 μm), the known tear permeability (Dk of 80) and expected post-lens tear layer thicknesses (100–400 μm). The Holden–Mertz Dk/t criteria of 24 Fatt units for the central cornea and the Harvitt–Bonanno criteria
of 35 Fatt units for the limbal area were used as reference points.
Results
Our calculations of oxygen transmissibility, with varying tear layer and lens thicknesses,
ranged from 10 to 36.7 at the scleral lens centers and from 17.4 to 62.6 at the peripheries.
Our calculations of maximum central lens thicknesses show a practical range of 250–495 μm, in conjunction with a post-lens tear layer thickness of 100–250 μm.
Conclusion
Our computations show that most modern scleral lenses, with recommended fitting techniques,
should lead to some level of hypoxia-induced corneal swelling. Recommendations are
made to minimize hypoxia-induced corneal swelling: highest Dk available (>150) lens with a maximal central thickness of 250 μm and fitted with a clearance that does not exceed 200 μm.
Keywords
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Article info
Publication history
Published online: August 09, 2012
Accepted:
July 20,
2012
Received in revised form:
July 12,
2012
Received:
February 29,
2012
Identification
Copyright
© 2012 British Contact Lens Association. Published by Elsevier Inc. All rights reserved.
