The single most important factor to achieving a successful result in topography based contact lens fitting and orthokeratology is the reliability and repeatability of the prefit topography.
Corneal topography is a technique that takes time to perfect, just like retinoscopy, fundoscopy etc. Forget point and shoot! Take your time, and be rewarded! The reward: Perfect fitting lenses, high first fit success, less unnecessary lens orders, less consultation time, happier patients.
The ideal baseline corneal topography:
Placido-disc corneal topographers reflect a series of concentric rings off the corneal surface back to the camera at the centre of the disk or cone. The topographer treats the cornea as a convex mirror and in reality measures the surface tear film rather than the epithelium itself
If the tear film is inconsistent, the topographer won't accurately measure and calculate the true shape of the cornea. A poor tear film due to dry eyes, disrupted epithelium or corneal scarring can cause a topographical error called 'ring jam' which occurs when the mirror surface (tear film or cornea) is inconsistent, causing the placido rings to break or intersect. Ring jam can result in analysis miscalculation or extrapolation errors. This type of assumption or guess, results in completely invalid corneal topography interpretation.
The example below show the effect of ring jam. Notice both the surface asymmetry index (SAI) showing asymmetry in four perpendicular meridians and the surface regularity index (SRI) which measures central corneal regularity show abnormal values.
The example below show the same cornea recaptured with no ring jam. Notice how both the SAI and SRI index values are now within normal limits.
Ring Jam due to dry eye can be resolved by instilling artificial tears prior to topography capture to help 'even' or 'smooth' the fluid film layer. Disrupted epithelium limits the accuracy of the instrument in reading the shape of the cornea. First treat the cause of the disrupted epithelium before attempting corneal topography capture.
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 instrument is misaligned the algorithms are invalidated and errors are introduced. 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.
In the example below, the two images on the left show pseudokeratoconus 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.
Large-cone placido disc systems, like the Oculus Keratograph, use a longer working distance and project fewer rings onto the cornea than small-cone topographers. They are prone to data loss due to varying nasal bridge anatomy and eyelash shadows.
In the example below, shadows from the eyelashes obscure the superior placido disc rings. The axial/sagittal curvature map on the right, however, does not reflect the loss of captured data. The reason is the Oculus Keratograph software uses extrapolated data to 'fill in' the missing pieces of information and warn you to the fact by superimposing small black dots over the extrapolated area.
To obtain a more realistic representation of the data captured it is better to deactivate the extrapolation function. To change the color map display go to the Setting>Miscellaneous Settings drop-down menu and in the Settings **window change the Color Map Appearance:** from 'Black Dots' to 'White Area'.
In the image below the Oculus Keratograph is in 'White Area' mode and presents the topographical map without the extrapolation data. With this setting, it is much easier to see what data is missing. In cases like this, a cotton bud or lid retractor can be used to move the eyelids out of the way to allow complete capture of the superior area.
Small-cone placido disc topographers, like the Medmont E300, project more rings on the cornea and have a shorter working distance than large-cone placido disc topographers. The advantage of the small cone devices is more complete coverage of the cornea but can struggle when measuring patients with very deep set eyes.
When dealing with deep set eyes or prominent brows, ask the patient to move their chin more forward and pivot their head so the nose points away from the cone. This will ensure the small cone fits more evenly against the orbit of the eye.
Use the tangential curvature map to verify if good corneal coverage is achieved. Below are four baseline maps took of the same patient on the same day. All have similar keratometry readings, however, the corneal coverage for each map is significantly different. The 3rd map is the best map to use for EyeSpace designs as good corneal coverage is achieved in both the horizontal and vertical meridians.
The video below explains the correct Corneal Topographical Capture technique.
An ideal baseline topography:
2.** Which of the following images displays high quality geometric optically centred placido rings?**
4 . Here are four baseline maps took of the same patient on the same day. All had similar keratometry readings, however, the corneal coverage for each map was significantly different. Which map would be best to design a contact lens using EyeSpace?
Mandell RB. A guide to videokeratography. International Contact Lens Clinic. 1996;23(6):205-228.
Kojima, R; Validating Corneal Topography Maps. Contact Lens Spectrum, July 2007.
Hubbe RE, Foulks GN. The effect of poor fixation on computer-assisted topographic corneal analysis. Pseudokeratoconus. Ophthalmology. 1994; 101: 1745-1748.