title: The subject-fixated coaxially sighted corneal light reflex: a clinical marker for centration of refractive treatments and devices.
abstract: OBJECTIVE
  To describe the inconsistencies in definition, application, and usage of the ocular reference axes (optical axis, visual axis, line of sight, pupillary axis, and topographic axis) and angles (angle kappa, lambda, and alpha) and to propose a precise, reproducible, clinically defined reference marker and axis for centration of refractive treatments and devices.
  
  
  METHODS
  Perspective.
  
  
  METHODS
  Literature review of papers dealing with ocular reference axes, angles, and centration.
  
  
  RESULTS
  The inconsistent definitions and usage of the current ocular axes, as derived from eye models, limit their clinical utility. With a clear understanding of Purkinje images and a defined alignment of the observer, light source/fixation target, and subject eye, the subject-fixated coaxially sighted corneal light reflex can be a clinically useful reference marker. The axis formed by connecting the subject-fixated coaxially sighted corneal light reflex and the fixation point, the subject-fixated coaxially sighted corneal light reflex axis, is independent of pupillary dilation and phakic status of the eye. The relationship of the subject-fixated coaxially sighted corneal light reflex axis to a refined definition of the visual axis without reference to nodal points, the foveal-fixation axis, is discussed. The displacement between the subject-fixated coaxially sighted corneal light reflex and pupil center is described not by an angle, but by a chord, here termed chord mu. The application of the subject-fixated coaxially sighted corneal light reflex to the surgical centration of refractive treatments and devices is discussed.
  
  
  CONCLUSIONS
  As a clinically defined reference marker, the subject-fixated coaxially sighted corneal light reflex avoids the shortcomings of current ocular axes for clinical application and may contribute to better consensus in the literature and improved patient outcomes.
authors.affiliation: Empire Eye and Laser Center, Bakersfield, California. Electronic address: dchang@empireeyeandlaser.com.
authors.forename: Daniel H
authors.initials: DH
authors.lastname: Chang
authors.affiliation: Medical University of South Carolina, Storm Eye Institute, Charleston, South Carolina; Department of Bioengineering and Science, Clemson University, Clemson, South Carolina.
authors.forename: George O
authors.initials: GO
authors.lastname: Waring
mesh_terms: D059925:Anatomic Landmarks; D003315:Cornea / Q000033:anatomy & histology* / Q000502:physiology / Q000601:surgery; D019781:Corneal Topography; D004867:Equipment Design; D006801:Humans; D008027:Light; D012018:Reflex / Q000502:physiology*; D012029:Refraction, Ocular; D012030:Refractive Errors / Q000503:physiopathology*; D054140:Refractive Surgical Procedures / Q000295:instrumentation*