Calculated tear oxygen tension under contact lenses offering resistance in series: Piggyback and scleral lenses☆

Abstract 

Background

Models of oxygen delivery to the cornea with contact lens wear have considered the customary situation of one lens being worn on the cornea. However, two lenses, such as a rigid lens piggybacked on a soft lens, may be worn by patients with special ocular and visual needs. This paper extends existing single lens models to circumstances where two lenses offer resistance to oxygen in series.

Methods

Theoretical oxygen tensions are calculated for tear layers trapped beneath piggyback contact lens systems (and scleral lenses) and the anterior corneal surface with a simple single chamber corneal model using a personal computer software spreadsheet.

Results

Acceptable values are anticipated for contact lens materials of modern oxygen permeability values of approximately 60–100Fatt Dk units under open eye conditions.

Conclusions

The results of this spreadsheet model of oxygen supply to the anterior corneal surface offer a useful guide to clinicians concerned about providing their patients with physiologically acceptable piggyback lens systems.

Keywords: Piggyback, Contact lenses, Oxygen transmissibility, Oxygen permeability, Corneal physiology