CHAPTER 22 Corneal topography and wavefront analysis
Corneal topography
Historical overview and types of topographers
The corneal first surface is the part with the highest dioptric power of the eye. The simplest device to measure corneal curvature is the manual keratometer. It measures the distance between the first Purkinje images of two test marks along two orthogonal meridians. The distance measured is a function of corneal curvature and usually is converted into a dioptric power value by multiplying the inverse of curvature (radius) with the corneal refractive index. Corneal astigmatism is determined by measurement of two orthogonal meridians. In contrast, keratoscopy visualizes the corneal curvature and irregularities across the entire cornea: the mirrored image of a disk with concentric rings (Placido disk) on the cornea is inspected. The advent of personal computers, video cameras, and image analysis software allowed a combination of both techniques: a digital still image of the reflection on the cornea is captured and a software program calculates corneal curvature at each corneal point based on the distance of the mires in the corneal reflection image6,8. Computerized videokeratoskopy, as the novel technique was called, enabled measurement of corneal curvature across the entire cornea. In analogy to keratometry, curvature is still converted into power values and expressed as ‘keratometric diopters’. On a color contour map, curvature is plotted as a function of location, hence the more common term ‘corneal topography’.
Placido-based corneal topographers are the topographers first constructed and still most widely used. Placido topographers measure with good reliability at a high dynamic range8. One disadvantage is the sparing of the corneal center because of the location of the camera front lens. Scheimpflug photography produces optical sections of the cornea with anterior and posterior surface in focus, enabling the measurement of both anterior and posterior corneal surfaces. From raw images, corneal height, curvature and refractive power as well topographic maps of corneal thickness can be calculated. Some manufacturers (Bausch & Lomb, Ziemer) have equipped their Scheimpflug topographers with a Placido disk for anterior surface imaging, while others (Oculus, iVis) rely on the improved resolution of new generation CCD cameras and offer a Scheimpflug-only solution. Apart from corneal back surface imaging, rotating Scheimpflug cameras have the advantage of high density imaging of the central cornea, because this region is imaged in each frame. Other techniques not or not yet commercially available include raster stereography, interference fringe polarimetry, and Hartmann–Shack-based topography (iZone, Abbott Medical Optics)8.
