Corneal analysis systems continue to become more sophisticated. The devices range from basic corneal topographers to multifunction units with additional capabilities including OCT, Scheimpflug imaging, wavefront aberrometry, pachymetry and more.
The ability to accurately measure and analyze the anterior corneal surface is necessary for most ophthalmologists and is indispensable for surgeons performing cataract, refractive and anterior segment surgery. It is also becoming increasingly important to be able to measure other components that contribute to the refractive power of the eye, particularly the posterior corneal surface.
Corneal videokeratography (CVK), the first type of computerized corneal analysis system, was developed in the early 1990s. CVK made keratorefractive surgery safer by enabling us to detect irregular astigmatism and measure changes in corneal shape after surgery. Corneal analysis is now routinely used for planning both corneal and cataract surgery. It is essential for screening patients for corneal abnormalities such as contact lens-related warpage, ectasias, anterior basement membrane dystrophy, ocular surface disease, and previous refractive surgery, as well as for choosing the most appropriate intraocular lens implant, and for determining the best method of astigmatism correction.
Numerous devices exist, but corneal topographers can be classified by their imaging technology: placido (videokeratoscopy), elevation (rastersphotogrammetry, scanning slit), and interferometry (laser holography, Moiré fringes). Once the corneal image is captured, the data is analyzed and can be displayed as different types of maps. The most common are curvature (axial, instantaneous), power (refractive), elevation, difference, or relative maps.
Multifunction systems can provide additional information such as anterior chamber depth, angle measurement, corneal thickness, lens densitometry, and high order aberration values. Devices usually include qualitative classification systems and quantitative measures, indices, and algorithms to facilitate interpretation of the data. Examples include simulated keratometry values (Sim K; an estimate of the curvature at the 3 mm zone), keratoconus screening, the Holladay Diagnostic Summary, Advanced Refractive Diagnostic, VisionPro (VISX Custom CAP), STARS (healing trend), MasterFit (contact lens), and Paragon CRT Lens Selection programs.
Purchasing a system may initially seem confusing because of the multitude of options. However, the process is relatively straightforward once you decide on the features and price range that are best for your practice. If you only require an instrument for screening, then a basic corneal topography machine is all you need. It will provide the standard maps as well as software for keratoconus detection and contact lens fitting. However, if you desire a system that can perform multiple functions, then a system that combines other technologies may be a better choice.
Here is an update on some of the most popular corneal analysis systems:
AKRCornea 500 from Stereo Optical Company
This instrument is an auto-kerato-refractometer-topographer that provides refraction and corneal topography. Unique rotary prism technology provides precise and reliable objective refraction measurements, even in patients with small pupils. Placido ring technology generates a large selection of topography graphical representations to perform display and quality analysis. The corneal topography software allows screening for corneal problems. The device also can be connected to the APH500 for direct and automatic transfer of the refraction data.
ATLAS 9000 from Zeiss
This Placido disc topographer uses the cone-of-focus alignment system to ensure accurate focus and SmartCapture technology to analyze and automatically select the best image. In addition to the standard topographic map displays, additional software includes PathFinder II to identify abnormal topographies and MasterFit II for contact lens fitting of rigid gas permeable lenses.
Keratograph 5M from OCULUS
This advanced topography unit contains a keratometer and a color camera for capturing external images. The latter allows evaluation of the meibomian glands, tear film break-up time, tear meniscus height, and lipid layer. The device uses different illumination sources for different measurements: white placido ring to map the whole surface of the cornea, infrared ring for analysis of the tear film, and light emitting diodes: white for the tear film dynamics, blue for fluo-images, and infrared for meibography.
Pentacam from OCULUS
This sophisticated camera utilizes Scheimpflug imaging to capture data from the entire anterior segment and then displays the results with numerous maps and tables. Features include topography, elevation, and refractive maps of the anterior and posterior corneal surfaces, corneal ring display, true net power, comparative and differential analysis of two examinations, keratoconus detection and classification, Zernike analysis and corneal wavefront, corneal optical densitometry, corneal pachymetry maps, absolute and relative overview of all captured Scheimpflug images, anterior segment tomography, 3D anterior chamber analysis, 3D cataract analysis, contact lens fitting, Belin/Ambrosio display, and Holladay report.
CA-200F Corneal Analyzer from Topcon Medical Systems
This placido-based topography system, with 24 rings measuring over 10,000 data points, produces accurate, high resolution images of the anterior cornea. There are also eight blue LED lights for fluorescein images to aid in contact lens simulation. The unit is easy to operate and the 10-inch color touch screen and high speed CCD camera makes acquisition and evaluation of the cornea fast and easy. Special features of the camera are auto-capture, best image capture, and automatic pupil recognition.
KR-1W Wavefront Analyzer from Topcon Medical Systems
This device is a 5-in-1 combination instrument that measures wavefront aberrometry, topography, keratometry, pupillometry, and autorefraction. The auto-alignment and large color touch screen make this system very easy to use. It also provides simulated visual acuity assessment, and it is fully networkable in various configurations.
iTrace Visual Function Analyzer from Tracey Technologies
This unique ray tracing device is a 5-in-1 system that measures auto-refraction, corneal topography, ray tracing aberrometry, pupillometry and auto-keratometry, It also measures quality of vision and visual function using a fundamental thin beam principle of optical ray tracing that sequentially projects 256 near-infrared laser beams into the eye to measure forward aberrations, processing data point-by-point. The integration of wavefront aberrometry and corneal topography enables the system to calculate the internal ocular aberrations by subtracting the corneal from the total aberrations.
GALILEI G2 from Ziemer Group
This device uses dual Scheimpflug and placido 3D imaging technology for corneal topography and three dimensional analysis of the anterior segment. It performs a scan in less than one second and produces more than 40 different customizable maps, reports and displays. Features include high resolution Scheimpflug images, 3D pachymetry, corneal topography, corneal wavefront, total corneal power, 3D anterior chamber analysis, lens thickness, corneal and lens densitometry, and pupillometry.
OPD-Scan III from NIDEK
This multifunction corneal analyzer has various features such as autorefraction, keratometry, placido disc topography, wavefront aberrometry, and pupillometry. Automatic alignment and tracking make this device easy to use. The software produces numerous map, wavefront data, and measurement displays including lenticular residual astigmatism, angle kappa, pre/post toric IOL measurements, and IOL tilt/decentration. It is fully integratable with EMR and computer networks.