Pupil size and the ablation zone in laser refractive surgery

Letters to the editor generally contain questions or complaints from readers about recently published articles. In this letter, I ask the forbearance of the journal and readers because instead of that, I wish to expand on several statements in an article I coauthored regarding the size of the optical zone in photorefractive surgical procedures.¹ Due to increasing medico-legal pressures within the field of refractive surgery related to the optical zone diameter and ablation centration, I felt that this belated clarification and correction to the article was important.

The article states that “the center of the pupil has no fixed relationship to the line of sight (LOS, the line connecting the point of fixation to the fovea,” and the text implies that the laser ablation should be centered on the center of the physical pupil (PP) rather than on the LOS. However, the definition, we gave for the LOS is not, in fact, the correct general definition. The LOS is defined by the path of the principal ray in object space (ie, before refraction by the cornea). The principal ray is that unique ray arising from the fixation object that will intercept the center of the PP after refraction. The entire bundle of light rays that traverses the PP and contributes to the foveal image is evenly dispersed about the principal ray. The point where the principal ray intercepts the cornea is the corneal sighting center (CSC).² Another commonly used definition of the LOS is a straight line drawn from the fixation object to the center of the entrance pupil (EP), which is the magnified and anteriorly displaced virtual image of the PP. The portion of this line between the fixation object and the corneal surface is identical to the path of the principal ray. Because the eye is not a perfectly centered optical system, the CSC is usually slightly nasal to the geometrical center of the cornea.

It is impossible to center an ablation over the center of the PP because we do not “see” the PP but, rather, the EP. The perceived location of the EP is dependent on the location of the observer relative to the patient's LOS. When an ablation is centered on the EP, it is centered on the CSC as long as the surgeon and patient are sighting coaxially.³ In this case, the surgeon is looking at the same EP that the patients uses to look out. It is more reasonable to think of centering the ablation on a point than on a line, and the CSC is the correct corneal locus for centration.⁴ Since the EP is the image of the PP, by definition the locations of the LOS and CSC are dependent on the location of the center of the PP. It is true, however, than an ablation centered on the CSC will not be exactly centered on the PP in most patients.

Sandra M. Brown MD

Lubbock, Texas, USA