title: Qualitative identification of rigid gas permeable contact lens materials by densitometry.
abstract: OBJECTIVE
  We describe a practical method to qualitatively identify polymethylmethacrylate (PMMA) and rigid gas permeable (RGP) contact lens materials.
  
  
  METHODS
  By progressive dilution of a saturated saline solution made with distilled or tap water and sodium chloride, we recorded comparative densitometry of rigid contact lens materials using a small hydrometer or by liquid displacement.
  
  
  RESULTS
  The method was sensitive enough to separate the polymethylmethacrylate, all silicon-methacrylates, and all but two fluorine-containing silicon-methacrylates. The hydrometer had a precision of three decimals rounded to the nearest 0.005. There was only one RGP product that could have been confused with the PMMA material. Most silicon-methacrylates had lower densities than fluorine containing silicon-methacrylates. Only four of 25 products under 1.117 gm/cm3 contained fluorine.
  
  
  CONCLUSIONS
  Densitometry with a hydrometer is an effective non-destructive method to identify RGP materials and to verify their quality. The method is easier when lens blanks are tested, but in spite of differences in shape, size, and weight, densitometry may also be used with new or used contact lenses. Its simplicity and low cost makes densitometry feasible for any contact lens laboratory or clinic to use on a routine basis. Only silicon-methacrylates had an inverse relationship between density and oxygen permeability. As the silicon content of the contact lens increases, the Dk increases and the density decreases.
authors.affiliation: Arce Oculistas & Asociados, Lima, Peru.
authors.forename: C G
authors.initials: CG
authors.lastname: Arce
authors.forename: P D
authors.initials: PD
authors.lastname: Schuman
authors.forename: W P
authors.initials: WP
authors.lastname: Schuman
mesh_terms: D001697:Biomedical and Dental Materials; D003261:Contact Lenses; D003720:Densitometry; D005461:Fluorine / Q000032:analysis; D005740:Gases; D006801:Humans; D008689:Methacrylates / Q000737:chemistry; D010539:Permeability; D019904:Polymethyl Methacrylate; D012828:Silicones
chemical_list: D001697:Biomedical and Dental Materials; D005740:Gases; D008689:Methacrylates; D012828:Silicones; D005461:Fluorine; D019904:Polymethyl Methacrylate