CORNEAL TOPOGRAPHY - AN OVERVIEW
Dr Srinivas K Rao, DO DNB FRCSEd
Dr Ramya Subramaniam, DNB
Darshan Eye Clinic
T 80, Anna Nagar, Chennai
srinikrao@gmail.com
WHAT IS CORNEAL TOPOGRAPHY?
Corneal topography is the study of the shape of the corneal surface. Traditionally, such measurements were limited to the near spherical central portion of the anterior corneal surface. With the advent of corneal refractive procedures, the necessity to study the more peripheral parts of the cornea and to understand better the optics of both the anterior and posterior corneal surfaces, has spawned a number of new devices that allow the clinician to better understand corneal shape, power and optical performance.
EVOLUTION OF CORNEAL TOPOGRAPHY KERATOSCOPY:
It is the evaluation of the corneal surface using circular mires reflected from its surface. The earliest device designed to perform this function was the PLACIDO’S DISC, developed by Antonio Placido. It consists of equally spaced alternating black and white rings with a hole in the centre to observe the patient’s cornea. The central opening houses a convex lens for magnification and to aid the examiner’s accommodation.
Given below are examples of images obtained from the reflection of the mires from the cornea.
Keratoscopes are devices with a self illuminated set of concentric rings, like the Placido disc, and documentation of the obtained mire reflection can be done using a camera (PHOTOKERATOSCOPY) or a videocamera (VIDEOKERATOSCOPY).
With advances in computing power, the addition of computers to the above devices has increased the amount of information that can be obtained and quantified from the reflected images. Apart from the qualitative assessment that is possible from the mire reflections, the use of algorithms to analyze the images results in large amounts of quantitative data that can be obtained virtually real-time, with the powerful computers available today.
KERATOMETRY The keratometer was invented by Helmholtz. It measures the radius of curvature of the anterior corneal surface from four points approximately 3 mm apart.
However, its limitations are that the corneal apex and peripheral cornea are not taken into account, it assumes corneal symmetry, measures a variable area and is less accurate in very steep and very flat corneas, and treats the cornea as a spherocylindrical structure with two principal meridia separated by 90 degrees, resulting in errors in astigmatic axis measurement.
RASTERSTEREOGRAPHY projects a calibrated grid of horizontal and vertical lines 0.2 mm apart onto a fluorescein stained tear film and takes photograph and analyses the pictures for displacement of the grid pattern.
INTERFEROMETRY uses the phenomenon of light wave interference. The fringes produced can cover the entire anterior ocular surface and not just the cornea.
ELEVATION BASED TOPOGRAPHY
Slit scanning devices – These work on the Projection principle and measure surface elevation directly by slit scan triangulation. Forty scanning slit beams (20 from the left and 20 from the right with up to 240 data points per slit) are used to scan the cornea from limbus to limbus and to measure independently the x, y, and z locations of several thousand points on each surface. In the newer version of the Orbscan® system, a placido disk has been mounted to this device in order to improve the accuracy of the curvature measurements.
Scheimpflug Imaging - The Pentacam images the anterior segment of the eye using a rotating Scheimpflug camera and pictures in three dimensions of the anterior segment are shown by this rotating process. The images captured are then used to construct the anterior corneal surface, posterior corneal surface, and anterior iris and anterior lens surfaces.
WORKING PRINCIPLES OF COMMONLY USED SYSTEMS
All devices in use today comprise of 3 basic units or modules. There is an image acquisition device - which can either be a Placido disc based tool, a slit-scanning instrument, or a Scheimpflug camera system. In some devices, more than one of these tools are utilized to obtain information - as mentioned later. The devices also have a camera to capture the images from the patient’s eye. Often, some kind of a video display is built-in to monitor the patient’s eye prior to capture. The entire module is usually integrated along with a trigger for initiating image capture and the unit is placed on a stand that can be moved up and down to allow the patient to place his or her chin comfortably in the machine rest. There is also a processing computer that will analyze the data from the input device.
PROJECTION DEVICE:
Three types of projection devices are currently used. They all measure and analyze more than 8000 points.
1. PLACIDO –BASED SYSTEMS
These project a transilluminated disc or cone (modified Placido disc) onto the cornea surface and analyses the image reflected. Between 8- 32 concentric rings can be projected.
2. ELEVATION BASED SYSTEMS
3. SCHEIMPFLUG DEVICES
VIDEOCAMERA:
It captures the reflected image. Proper acquisition of data depends on focussing, decentration, good tear film quality.
COMPUTER:
It generates the topographic maps based on the information collected.
