title: 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 60-100 Fatt 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.
authors.affiliation: Jules Stein Eye Institute and Department of Ophthalmology, David Geffen School of Medicine at UCLA, Los Angeles, CA 90095-7003, USA. weissman@jsei.ucla.edu
authors.forename: Barry A
authors.initials: BA
authors.lastname: Weissman
authors.forename: Ping
authors.initials: P
authors.lastname: Ye
mesh_terms: D003261:Contact Lenses; D003315:Cornea / Q000378:metabolism; D004867:Equipment Design; D006801:Humans; D008962:Models, Theoretical; D010100:Oxygen / Q000032:analysis* / Q000378:metabolism; D010101:Oxygen Consumption / Q000502:physiology; D013499:Surface Properties; D013666:Tears / Q000737:chemistry* / Q000378:metabolism
chemical_list: D010100:Oxygen