title: Contribution of soft lenses of various powers to the optics of a piggy-back system on regular corneas.
abstract: OBJECTIVE
  This study aims to report on the measured in vivo contribution of soft lenses of various powers to the optics of a piggyback system (PBS).
  
  
  METHODS
  This prospective, non-dispensing clinical study was conducted on regular wearers of contact lenses who showed regular corneal profiles. Subjects were masked to the products used. The study involved the use of a spherical soft lens of three different powers in a PBS, used as a carrier for a rigid gas permeable lens. Baseline data were collected and soft lenses were then fitted on both eyes of each subject. Both lenses were assessed for position and movement. Over-refraction was obtained. Soft lens power contribution to the optics (SLPC) of a PBS system was estimated by computing initial ametropia, lacrymal lens, rigid lens powers and over-refraction. A set of data on one eye was kept, for each subject, for statistical analysis.
  
  
  RESULTS
  Thirty subjects (12 males, 18 females), aged 24.4 (±4.5) years, were enrolled. The use of plus powered soft lenses enhanced initial RGP lens centration. Once optimal fit was achieved, all lenses showed normal movement. SLPC represented 21.3% of the initial soft lens power when using a -6.00 carrier, and 20.6% for a +6.00. A +0.50 did not contribute to any power induced in the system. These results are generally in accordance with theoretical model developed in the past.
  
  
  CONCLUSIONS
  On average, except for the low-powered carrier, the use of a spherical soft lens provided 20.9% of its marked power. To achieve better results, the use of a plus-powered carrier is recommended.
authors.affiliation: École d'optométrie de l'Université de Montréal (EOUM), Member British Contact Lens Association (BCLA), Canada. Electronic address: langis.michaud@umontreal.ca.
authors.forename: Langis
authors.initials: L
authors.lastname: Michaud
authors.forename: Daniel
authors.initials: D
authors.lastname: Brazeau
authors.forename: Marie-Eve
authors.initials: ME
authors.lastname: Corbeil
authors.forename: Pierre
authors.initials: P
authors.lastname: Forcier
authors.forename: Pierre-Jean
authors.initials: PJ
authors.lastname: Bernard
mesh_terms: D000293:Adolescent; D000328:Adult; D001251:Astigmatism / Q000175:diagnosis* / Q000534:rehabilitation*; D003263:Contact Lenses, Hydrophilic / Q000145:classification*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005260:Female; D006801:Humans; D008297:Male; D017755:Prosthesis Fitting / Q000295:instrumentation* / Q000379:methods*; D016896:Treatment Outcome; D014792:Visual Acuity; D055815:Young Adult
keywords: Astigmatism; Contact lens optics; Piggyback systems; Rigid gas-permeable lenses