TREATMENT CENTRATIONS

The optics of the eye may be described in terms of numerous axes. However, the question of which axis is most important to visual performance when planning cataract and refractive procedures is being debated, said Raymond Applegate OD, PhD, College of Optometry, University of Houston, Texas, US.

“There are two key factors to consider when selecting the treatment axis that will provide the best acuity. They are the light passing through the pupil and the visual image quality at the fovea,” he told the XXXII Congress of the ESCRS in London.

Dr Applegate said that on that basis he would recommend using the line of sight as the axis for centring refractive surgery. The line of sight starts at the fixation point and goes to the centre of the entrance pupil and from the centre of the exit pupil to the foveola. The angle formed by the intersection of the line of sight with the pupillary axis is the angle Lambda. However, it is often mistakenly referred to as angle Kappa, which more correctly refers to the angle formed by the intersection of the visual axis and the pupillary axis.

He noted that the pupillary axis is the line that passes through the pupil’s centre and is perpendicular to the plane tangential to the point where the axis intersects the corneal surface. The visual axis is a theoretical construct and is typically defined as the line extending from the fixation target to the first nodal point of the eye, then from the second nodal point of the eye to the fovea. Importantly, there is no constraint that requires the visual axis to go through the eye’s pupil.

MODEL EYES

The optical axis is a theoretical construct used more appropriate for centred model eyes since it is the line that connects the centres of curvature of all the optical surfaces. However, unlike the visual axis or the line of sight, the optical axis is conceived of as a line passing through a common centre of curvature of the eye’s refractive surfaces. It would be a rare eye for the refractive surfaces centres of curvature to fall on a straight line. The optical surfaces of the eye are typically tipped with respect to each other.

Dr Applegate cautioned that treatment axes based on the pupil centre raise some additional considerations. The relationship of the pupil centre to the videokeratometric (VK) axis – the line that passes through the centre of the Purkinje image reflected from the corneal surface and is perpendicular to the plane tangential to that point – varies considerably among patients.

He also noted that, although centring refractive procedures on the line of sight makes good sense in that in a well corrected eye it focuses the image on the most sensitive part of the retina, many surgeons have reported that they obtain better results by centring their treatment somewhere between the pupil centre and the VK axis.

There could be several possible explanations for that finding. It could result in part from the curvature optimisation that is built into the laser refractive surgery platforms are optimised of corneal topography data as opposed to the centre of the pupil.

Raymond Applegate: rapplegate@uh.edu