title: A piggyback contact lens for the correction of irregular astigmatism in keratoconus.
abstract: OBJECTIVE
  Although the combination of a hard contact lens "piggybacked" on a soft lens base for the correction of severe astigmatism in keratoconus has existed for two decades, little is known about its effect on the corneal epithelium and endothelium. This study involves two parts: the measure of oxygen pressure in rabbits, and the long-term effects on patients with keratoconus wearing these hybrid lenses.
  
  
  METHODS
  A polarographic sensor was used to measure the oxygen pressure on rabbit corneas under two types of piggyback lenses: oxygen-permeable hard lenses on high water-content soft lenses, and polymethylmethacrylate (PMMA) lenses on low water-content soft lenses. In the clinical evaluation, 11 patients with keratoconus (8 men, 3 women; 25.1 +/- 4.9 years of age) who could not wear hard contact lenses due to constant pain or inadequate lens fitting were given oxygen-permeable piggyback contact lenses and were observed for at least 9 months.
  
  
  RESULTS
  In the animal study, the oxygen pressure under piggybacked oxygen-permeable hard contact lenses was 95 +/- 14 mmHg after 5 minutes wear, but it was only 34 +/- 14 mmHg when PMMA and low water-content lenses were used. Ten of 11 patients were successfully treated using this technique, whereas one patient had persistent contact lens loss, requiring penetrating keratoplasty. Specular microscopic observation of the corneal epithelium and endothelium did not show any changes during the follow-up period.
  
  
  CONCLUSIONS
  These results suggest the efficacy and safety of the piggyback combination of oxygen-permeable hard and soft contact lenses for the correction of astigmatism in patients with keratoconus.
authors.affiliation: Department of Ophthalmology, Tokyo Dental College, Chiba, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Tsubota
authors.forename: Y
authors.initials: Y
authors.lastname: Mashima
authors.forename: H
authors.initials: H
authors.lastname: Murata
authors.forename: M
authors.initials: M
authors.lastname: Yamada
mesh_terms: D000293:Adolescent; D000328:Adult; D000818:Animals; D001251:Astigmatism / Q000209:etiology / Q000473:pathology / Q000628:therapy*; D003263:Contact Lenses, Hydrophilic; D003315:Cornea / Q000378:metabolism / Q000473:pathology; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007640:Keratoconus / Q000150:complications / Q000473:pathology / Q000628:therapy*; D008297:Male; D020366:Methylmethacrylate; D008768:Methylmethacrylates; D008875:Middle Aged; D010100:Oxygen / Q000378:metabolism; D011048:Polarography; D011817:Rabbits
chemical_list: D008768:Methylmethacrylates; D020366:Methylmethacrylate; D010100:Oxygen