Please check out our Corneal Topography section for more information about the products mentioned in this article.
Corneal curvature is traditionally measured with a manual keratometer. The most common device is the Helmholtz type instrument, which measures two anterior corneal curvature values (maximum and minimum) separated by 90 degrees. The major limitations of the keratometer are that it only measures a small area of the cornea (2 paracentral points at the 3-4 mm zone), and it assumes that the cornea is a symmetric spherocylinder with a major and minor axis 90 degrees apart. Despite these drawbacks, for most patients the keratometer gives accurate results, and it is the standard method we use to measure astigmatism for IOL calculations and contact lens fitting. However, standard keratometry readings are frequently performed with automated devices like autorefractors and the IOLMaster because of the ease and speed of using these instruments. Furthermore, instruments such as corneal topographers and other analyzers are becoming increasingly important for the comprehensive ophthalmologist.
Corneal topography provides more data, quantitates corneal shape, and measures patterns of irregular and induced astigmatism. Corneal topography used to be considered a specialized test to diagnose corneal pathology or to help fit contact lenses, but since the introduction of laser vision correction in the mid-1990s, it has become a common test used to screen potential surgical candidates. With the refinements in cataract surgery techniques and technology, particularly advanced IOLs, corneal topography is an essential part of the preoperative workup. In addition, it is valuable for detecting subtle irregularities of the corneal epithelium (i.e., anterior basement membrane dystrophy) and tear film (dry eye).
The different topography devices can be classified according to the imaging technology used, such as placido-based (videokeratoscopy), elevation-based (rastersphotogrammetry, scanning slit), and interferometry-based (laser holography, Moiré fringes). The data is then displayed as various maps, most commonly: curvature (axial, instantaneous), power (refractive), elevation, difference, or relative. Most of the instruments also include qualitative classification systems and quantitative measures, indices, and algorithms to help in data analysis. The most familiar of these are the simulated keratometry values (Sim K; an estimate of the curvature at the 3 mm zone). Clinicians must be aware that the Sim K readings are less accurate than and should not be substituted for true manual keratometry readings. The other frequently utilized program is keratoconus screening software, which flags corneal maps suggestive of keratoconus and other corneal ectasias. Additional measurements are the surface regularity index (SRI; measures central 4.5 mm, 0 = perfectly smooth surface; this reading correlates highly with visual acuity), the surface asymmetry index (SAI; difference in each ring 180° apart, 0 = perfect sphere), and the potential corneal acuity (PC acuity; predicted vision based solely on the corneal status), and additional software modules include the Advanced Refractive Diagnostic, VisionPro (VISX Custom CAP), STARS (healing trend), MasterFit (contact lens), and Paragon CRT Lens Selection programs.
Specific features of some of the systems are as follows:
- Atlas 9000 (Carl Zeiss Meditec) - this topographer uses the cone-of-focus alignment system to ensure accurate focusing of the projectyed rings. The SmartCapture image analysis technology evaluates and automatically selects the highest quality image during alignment. PathFinder II software screens images to identify abnormal topographies (i.e., corneal ectasias), and MasterFit II software facilitates rigid gas permeable contact lens fitting which is particularly helpful for difficult and specialty fitting.
- Magellan Mapper Corneal MM-1 (Nidek) - this topographer includes the Nidek Advanced Vision Information System (NAVIS) an interface that enables easy networking and management of image information. 3D map displays are also a special feature of this device.
- Easygraph (Oculus) - this topographer is a small portable device that attaches to any slit-lamp and also has a built in keratometer. Keratoconus and contact lens fitting software is provided. OxiMap is unique software that provides a color-coded image that displays the oxygen transmissibility of contact lenses.
- TMS-4 Corneal Topographer (Tomey) - this topographer includes keratoconus screening, contact lens fitting, and 3D corneal mapping software. The Fourier refractive analysis displays refractive data for the central 6 mm of the cornea. There is also an auto shot function.
- CA-200 Corneal Analyzer (Topcon) - this topographer has a built-in touch screen as well as WiFi, which allows wireless communication with a network, printer, or computer. There is automatic selection of the best-focused image, and contact lens fitting and Zernike analysis software are also standard.
- Vista (Eyesys) - this topographer is a small hand-held placido-based device that is portable and useful for examination of patients who cannot be examined with traditional tabletop instruments. It contains the standard software found on full-feature units like the popular Eyesys System 2000.
Please see the products listed below, or check out our Corneal Topography section for more information.