title: Apical clearance rigid contact lenses induce corneal steepening.
abstract: OBJECTIVE
  Anecdotal reports suggest that steeply fitted rigid contact lenses can induce corneal steepening and myopic shifts in refraction. This study investigated changes in corneal topography, corneal thickness, and refraction after short-term wear of apical clearance lenses.
  
  
  METHODS
  : Ten young adults participated in the study in which apical clearance lenses (Conoid, Hydron Pty. Ltd., Frenchs Forest, NSW, Australia), fitted approximately 0.3 mm steeper than the flattest keratometric reading, were worn for 4 h. A polymethyl methacrylate (PMMA) lens was worn in one eye, and a Boston XO lens (Bausch & Lomb, Rochester, NY) was worn in the other. Lenses were nonfenestrated in the first trial, and a single midperipheral fenestration was incorporated for a second trial. Corneal topography was monitored using the EyeSys topographic analysis system (EyeSys Vision, Inc., Houston, TX); central corneal thickness was quantified using ultrasonic pachometry; and refraction and anterior ocular response were also monitored.
  
  
  RESULTS
  Significant central corneal edema (8.2 +/- 2.2%) was demonstrated in the PMMA lens-wearing eye only. Fenestration did not reduce the edema response. We found significant corneal steepening over a 5- to 6-mm central zone, which was more pronounced in the PMMA lens-wearing eye and was modified by lens fenestration. The curvature changes altered central corneal shape from prolate (Q = -0.06 +/- 0.10) to oblate (Q = -0.26 +/- 0.31). A slight myopic shift in refraction (-0.38 +/- 0.60 D) was found with the PMMA lenses only.
  
  
  CONCLUSIONS
  : Apical clearance lenses induce corneal steepening after short-term wear. Although corneal molding is implicated as the primary factor driving the shape change, corneal edema and postlens tear film pressures can modify this response.
authors.affiliation: School of Optometry and Vision Science, University of New South Wales, Sydney, Australia. h.swabrick@unsw.edu.au
authors.forename: Helen A
authors.initials: HA
authors.lastname: Swarbrick
authors.forename: Ross
authors.initials: R
authors.lastname: Hiew
authors.forename: Ai Vy
authors.initials: AV
authors.lastname: Kee
authors.forename: Sophia
authors.initials: S
authors.lastname: Peterson
authors.forename: Nina
authors.initials: N
authors.lastname: Tahhan
mesh_terms: D000328:Adult; D003261:Contact Lenses / Q000009:adverse effects*; D003315:Cornea / Q000473:pathology*; D015715:Corneal Edema / Q000175:diagnosis / Q000209:etiology*; D019781:Corneal Topography; D004311:Double-Blind Method; D005260:Female; D006801:Humans; D008297:Male; D009216:Myopia / Q000175:diagnosis / Q000209:etiology*; D019904:Polymethyl Methacrylate / Q000009:adverse effects*; D017755:Prosthesis Fitting; D012029:Refraction, Ocular
chemical_list: D019904:Polymethyl Methacrylate