As ophthalmic technology has improved, our patient expectations have risen as well. Surgery patients no longer accept just “good” vision. Our laser vision correction and cataract surgery patients want spectacle independence, and we strive to offer and achieve that goal. Corneal topography is a crucial piece of technology to achieve this.
Topography is used to obtain a detailed topographical “map” of the cornea, the clear tissue that is in the front of the eye and covers the iris and pupil. Topography provides keratometry readings, which show how the cornea is curved, but unlike an autorefractor or manual keratometer, the entire cornea is mapped, not just the central few millimeters. Seeing these peaks and valleys of the cornea gives us a great look at how the patient is seeing, often showing dry eye and/or complex corneal pathology before the patient is examined with the slit lamp microscope.
Corneal topography, once relegated to troubleshooting complex cases or in more niche roles (i.e. orthokeratology) is now an essential tool that can make or break our outcomes, especially in refractive surgeries.
Types of topographers
First, a quick note about terminology. Technically speaking, “topography” is the term that can be used to describe Placido disc devices, as they image the anterior shape/slope of the cornea. “Tomography” is more adept to the newer style imaging devices, as they are imaging the overall shape, thickness and contour of the cornea. However, many practitioners and technicians often use “topography” as an all-encompassing term. We will continue with that approach for this article.
Placido disc technology
The original style topographers use Placido disc technology. Circular rings are projected onto the corneal surface, and a camera takes pictures of the rings to determine how steep or flat each area is. This is excellent for managing dry eye and in contact-lens fitting applications, especially with RGP applications and orthokeratology. Placido disc technology is limited to imaging only the anterior surface of the cornea, and also is imaging the tears as opposed to the actual cornea itself.
Examples include the Nidek OPD line, Zeiss Atlas, Alcon Eye-Map, and Topcon CA-800.
Scanning slit technology
The Bausch + Lomb Orbscan combines Placido disc technology as well as scanning slit-topography to image both the anterior and posterior aspects of the cornea, the thickness of the cornea, and anterior chamber analysis.
The scanning slit works like this: two vertical scans are performed through the projection of 40 optical slits, 20 from the right and 20 from the left, and the digital video camera captures these images. The combined Placido disc rings and slit projections are combined to obtain the wanted results.
Scheimpflug technology uses a rotating beam that images the front, back, and thickness of the cornea, along with the anterior chamber of the eye. This opens up the range of applications to include more advanced contact lens fitting and multiple surgical applications (more on this below). An example is the Oculus Pentacam.
The technology employs red, green, and yellow LEDs that are each positioned in a unique relationship to four of its neighbors, giving each one a “GPS-like” coordinate. Cassini uses the ray-tracing principle to measure the relative position of each point, using the three different colors as ‘triangulation’ points. This can provide a true representation of the corneal curvature, especially taking into account the posterior corneal curvature.
Other units integrate biometry, tomography and topography in one device. By offering both Placido disc and Scheimpflug technology to image the cornea, as well as biometry, these units can obtain pre-op surgical measurements for both laser vision correction and cataract surgery. An example of this style unit is the Ziemer Galilei line.
Uses of topography
Advanced topographers measure the curvature, power and thickness of the cornea, which are of utmost importance when determining whether patients are candidates for laser vision correction surgery.
We perform topography on all of our LASIK patients prior to surgery and after surgery to determine the effectiveness of the procedure.
In the screening or pre-operative phase, we check if the corneas are either too steep or too flat and therefore may not be safe candidates for LASIK.
Patients whose corneas are deemed too thin via pachymetry have increased ectasia risk, according to a 2014 Journal of Ophthalmology study by Tatar et al. With ectasia, the cornea begins to become more curved after surgery, which is extremely difficult to manage and treat and results in poor vision. In some cases, the cornea may be too thin to safely perform LASIK; however, these patients are excellent candidates for PRK surgery.
After LASIK, we use topography to assure that the treatment was uniform and effective. If the cornea has an irregular surface, patients may complain of glare, halos and/or blurred vision despite reading 20/20 on the eye chart.
It is relatively simple to acquire measurements prior to standard cataract surgery, or surgery where patients will just have the cloudy lens replaced by a clear implant. A simple K reading and axial length can provide good outcomes, and the patient simply needs a pair of glasses after surgery.
However, a large percentage of our patients now want more freedom and independence from glasses, which they can obtain with advanced IOLs. These option include toric IOLs to correct astigmatism, multifocal IOLs and extended-depth-of-focus lenses to give patients distance and near vision. This technology require us to choose candidates with healthy, normal eyes in order to obtain satisfactory outcomes.
Topography is performed prior to surgery to let us know if the cornea is regular. It can often assist in identifying dry eye, corneal dystrophies, and irregular astigmatism even before the patient is examined at the slit lamp. Treating these conditions prior to surgery, or simply deciding not to use particular types of lenses, is often the difference between a happy and unhappy patient.
In particular, topography is extremely important in cataract surgery to identify the patients who have corneal astigmatism. Astigmatism, or the curve of the eye, can be affected by the cornea, lens, and, sometimes, the overall shape of the eyeball itself. When we remove the lens for cataract surgery, however, the cornea is the main factor for the patient’s astigmatism after surgery. Because the topographer is able to measure both the front and back surface of the cornea, it can give surgeons a very accurate idea of the patient’s astigmatism.
This allows us to use modern advancements in femtosecond laser cataract surgery and toric lenses that correct astigmatism. Laser cataract surgery uses the femtosecond laser to make corneal or limbal (the cornea area near the white of the eye)-relaxing incisions to help correct astigmatism. A small incision is made into the edge of the cornea to flatten the areas where the cornea is too steep. Toric lenses are used when the corneal astigmatism is over 1.0D.
We also perform topography after cataract surgery. The cornea can sometimes shift due to where the surgeon entered the eye, changes from the laser arcs used to manage astigmatism, and other factors. To fine-tune results after cataract surgery, our practice also offers cataract surgery packages that include laser vision correction.
Keratoconus is a condition in which the cornea begins to thin and become more cone shaped. Although this can sometimes be diagnosed with retinoscopy and slit lamp examination, the gold standard for diagnosis is a topographer. Measuring the front, back, and thickness of the cornea is all critical for making this diagnosis.
The topographer is involved in many aspects of managing keratoconus, including at regular intervals to watch for progression. Many of the patients wear scleral or RGP contact lenses. Your office may also offer corneal cross-linking to treat keratoconus. Diagnosing this condition early in its development can make a big difference in our patients lives.
Corneal topography is an integral part of our practices and is crucial for obtaining the outcomes our patients expect in 2018. It is involved pre- and postsurgery and helps guide us in providing high-quality patient care. OP
Matt Judd, OD, is a 2001 graduate of Southern College of Optometry in Memphis, TN. He practices at Loden Vision Center in Nashville, TN, specializing in pre- and postop care of cataract and laser vision correction surgeries.
Disclosures: Dr. Judd is a sub-clinical investigator for J&J Vision and Lenstec but does not receive direct financial compensation.