December 19, 2018
In this blog, we are continuing our series on taking good topography maps. Next, we will look at how vital capturing geometrically centred topography maps are.
The figure below shows a diagram of a placido disk topography system. The rings are distributed on the inner side of the cone. The optical imaging system collects the reflected rays that pass through the equivalent nodal point. Placido disc topography systems do not measure elevation; instead, they obtain anterior corneal elevation data by reconstructing actual anterior curvature measurements via sophisticated algorithms.
The algorithms used to construct the topographical power maps assume the instrument is aligned to the geographical centre of the cornea. If the device is misaligned, the algorithms are invalidated and will introduce errors. A study by Hubbe and Foulks found eccentric fixation of aspheric surfaces, with corneal topography, produce power maps with irregular astigmatism, similarly seen in keratoconus. They concluded that improper alignment induces errors in computer-assisted topographic analysis(1).
In the example below, the two images on the left show pseudo-keratoconus when the misalignment occurs below the instrument axis. The two images on the right show the same cornea when vertically aligned with the instrument axis.
As the optical axis and the visual axis are not coincident the patient needs to shift their line of sight away from the instrument axis. In most cases to achieve a geometric optically centred corneal topography, the patient must fixate 1-2 rings nasally in the topographer cone.
In the example below, the image on the left shows the placido rings horizontally decentred and underneath the resultant topographical map. This happens when the patient fixates at the centre of the placido rings and the corneal topography is measured along the visual axis. The image on the right shows geometrically centred placido rings with its topographical map underneath. See how the outer placido rings are equidistant to the horizontal limbal zone.
The image below show horizontally decentred placido rings with ring jam captured from an Oculus Keratograph.
Use the image below as a chairside guide for directing your patient's gaze when capturing topography maps with the Medmont topographer. Please feel free to download this and laminate it as a capture aid.
Tip: For Orthokeratology, both the pre-fit and post-fit follow maps should be captured with geometrically centred placido rings. If the post-fit follow-up map is captured with eccentric fixation, the difference map can be erroneous in showing a decentred treatment zone.