Optivision, Inc.

The Leader in 3-Dimensional Corneal Mapping

What are the Applications of Corneal Topography?

The introduction of quantitative and qualitative videokeratoscopy provided Eye care professionals with a wealth of new information at their fingertips. The explosion in corneal topographic technology has not only given us information pertaining to the curvature of the entire cornea but also to generate the wavefront information and provide us with the higher order aberration. The combination of the two allowed a better understanding of the outcome of keratorefractive procedures, the effects of degenerations, diseases, contact lenses and trauma upon corneal topography.

A clinician can now more readily treat a cornea knowing the shape factor, the eccentricity or Q value before and after any keratorefractive procedure. Corneal abnormalities such as pterygium, herpes simplex lesions, and effect of chronic dry eye can be more precisely localized. Postoperatively, it can be determined if a refractive procedure needs enhancement or penetrating keratoplasty suture requires adjustment. Diagnosis of keratoconus, contact lens induced corneal warpage, and peripheral marginal degeneration are easily rendered. Corneal topography can also be used in providing the excimer laser with the asphericity of the cornea, i.e shape factor, eccentricity and Q values can optimize the outcome of the refractive surgery by reducing the resulting spherical aberration.

Better Diagnostic Assessment and Management

Altered corneal topography is well recognized to be the major reason for routinely poor, uncorrected acuity following corneal surgical procedure. Scarring can produce clinically and visually significant topographical changes. A penetrating keratoplasty (or PKP, the removal of a diseased cornea) followed by inserting and suturing in a replacement donor cornea often leaves the patient with significant residual astigmatism and decreased acuity. The advent of Corneal Topography allows the surgeon to visualize the areas of corneal steepening that have produced the often irregular astigmatism. Tight sutures, the cause of the steepening, can then be loosened and repositioned to relax the folds on the cornea and allow it to resume its normal shape.
With penetrating keratoplasty, prior to the procedure, it is possible to map the area of irregularity that is going to be excised. Post-transplant corneal measurement is extremely helpful in determining tight sutures and the irregularity caused by the suturing technique. It becomes very helpful in determining the amount of astigmatism to reduce and which suture to alter.

Phacorefractive Surgery and IOL Calculation

Phacorefractive surgery is the systematic approach for the achievement of best corrected visual acuity through addressing all aspects of cataract surgery: the corneal contour, the resultant wound, the IOL used, and any astigmatic correction needed. Accurate topographic analysis is of value both preoperatively and postoperatively in the management of cataract patients. Intraocular lens calculation is predictably determined using the average keratometry value obtained by the corneal topographer at the 3 mm zone. Computerized videokeratography provides the surgeon with feedback regarding the topographical alterations and residual astigmatism caused by the surgical approach. In managing patients with postsurgical astigmatism, corneal topography can reveal whether a patient needs a relaxing incision, wound revision, or contact lenses or spectacle.
The measurements of the wavefront of the cornea and the determination of the higher order aberrations, allows practitioner to order a custom IOL (intra occular lens) designed specifically for his patient and correct higher order aberrations, thus improving the outcome of the surgery and visual acuity.

Surgical Keratorefractive Procedures

Surgical keratorefractive procedures, including Lasik, PRK, Lasek & Epi-Lasek are intentional alterations in the corneal curvature to correct the refractive power of the eye. Their success is dependent upon sensitive, accurate knowledge of the patientís corneal topography. The use of computerized videokeratography plays a major role in the preoperative work-ups for these procedures. The preoperative topographies screen patients for form fruste keratoconus before refractive surgery. Before surgery, the topography maps can reveal the type of astigmatisms, regular or irregular, symmetric or asymmetric orthogonal or non orthogonal. Postoperatively, corneal topography helps us to understand the effects of these procedures and / or postoperative phenomena and complications such as regression, visual fluctuation, decentration/tilt and central islands.

More Qualitative Information

The development of accurate and sensitive corneal topography greatly increases our understanding of corneal structure and biomechanics. With the continuous expansion and acceptance of keratorefractive surgery, we recognize the increasing importance of determining corneal curvature, elevation, high order aberrations and dry eye. Optivisionís sophisticated detection system gathers more qualitative information and is unrivaled by any other topographer in its category. Unlike other systems, the CornealMap EH-300 is extremely sensitive to surface and tear film irregularities as well as microirregular astigmatism. Although the anomalies may be subtle, they are often visually debilitating and frustrating. Clinicians need to be able to recognize and diagnose these conditions to provide optimal patient care.

Corneal Topography and Standard Methods of detecting and diagnosing Corneal Topographic Disorders

Once a diagnosis has been made, a Corneal Topographer is extremely useful in the monitoring and management of such conditions. When pathology is in its early stages, slit lamp corneal changes may be too subtle or not yet apparent. In this instance, when keratometry readings and retinoscopy may also be normal, the topographical map is a key component in making a diagnosis. The greater capabilities of the CornealMap EH-300 are required to detect nonspecific steepening as well as subtle corneal and tear film abnormalities. Fourier analysis and Zernike polynomials are very helpful in this regard. Visually debilitating anomalies, including micro-irregular astigmatism or loss of contrast sensitivity from flap complications and central island after Lasik, are not discernible through other methods. It is incumbent upon the clinician, therefore, to accurately detect, diagnose, and manage such evasive conditions if he or she is to provide the best patient care and reassurance.
Corneal topography is an important component of preoperative evaluations of refractive surgery patients. The evaluations are necessary to screen for corneal disorders, such as keratoconus. Astigmatic keratotomy, and Lasik are currently contraindicated in patients with keratoconus. Other topographic disorders include keratoglobus, CL warpage, distortion caused by pterygium, corneal dystrophies, degenarations, and severe dry eyes. Patients with any of these preexisting conditions are not good candidates for refractive surgery.

Detect small Aberrations and Details

Small aberrations and important details, which can be left out by other systems, are highlighted by the CornealMap EH-300 system. For example, with the CornalMap EH-300 system, it is very easy to demonstrate in the RK patient the exact locations of the RK incisions because of the high concentration and sensitivity of the data rings. Other systems have fewer rings, spread widely apart. In order to fill in the data between the rings, other systems extrapolate the information causing a smoothing effect on the cornea, and meaningful data is lost. ( The CornealMap EH-300 has the capability to emulate the competitors maps by activating the smoothing filter to smooth the data).
Screening a patient prior to refractive surgery will determine the amount and direction of the corneal astigmatism. Additionally, screening for conditions that would preclude surgery is important. As with any refractive procedure, the accuracy of the data collected will be a determining factor in ultimate success of the outcome. If the information is missed by the detection system, then no amount of software programming can reinstate that which was not measured. To detect small details, set the smoothing filter to off, select the relative scale, and choose the smallest steps (1/4 or 1/2).

Contact lens warpage

Typically, candidates for refractive procedures wear contact lenses. Most patients exhibit some amount of corneal warpage due to the contact lenses. It is recommended by most physicians that patients with soft contact lenses should discontinue wear for 2-3 weeks prior to any refractive procedure; patients with RGP (rigid gas permeable), 6-10 weeks; patients wearing PMMA lenses, 12-16 weeks; to ensure that the warpage caused by contact lenses is resolved.

To follow a preoperative refractive patient who has discontinued lens wearing, capture a corneal topography image of the patient at the first visit. Note the time the patient removed the lenses prior to the exam. Follow the patient at predetermined intervals (based upon type of contact lens) and capture an image at each additional visit. The evaluation of the reduction of corneal warpage due to contact lenses can be visualized using serial displays of exams. As the cornea regains its normal shape, the serial exam will demonstrate fewer and fewer changes in the difference map.

A comparison of primary astigmatism axes from serial exams can alert the surgeon to shifts in astigmatism, averting problems that would result from a premature operation. If the surgeon were to operate on a patient without the cornea being stabilized, the outcome would not be ideal. The resulting fluctuations in a patientís vision would be a result of the cornea returning to its natural state. With its ability to measure and track contact lens warpage, the CornealMap EH-300 can help surgeons determine the optimal time for surgery.

The CornealMap EH-300 system is also helpful with patients who are photophobic and hard to manage. The Optivision CornealMap EH-300 system has a manual image-capture override that allows the operator to capture the image precisely, without hesitation. The instantaneous manual capture of the corneal image makes these patients easy to manage.

How does corneal topography facilitate contact lens fitting?

Easy and difficult fits can be expedited and the number of changes required minimized with a good corneal topographer system. The Optivision CornealMap EH-300 system has software to assist in fitting contact lenses. Its Contact Lens Program uses the topography to suggest a custom RGP design. It allows for interactive design changes and evaluations with their corresponding simulated fluorescein pattern.

All parameters, including base curve radius, optical zone, peripheral curves, and overall diameter, are determined and can be altered and evaluated via fluorescein evaluations. The steep areas of pooling are shown in bright green and the flat areas of touch are dark green or black. The initial RGP suggestion is a computer-generated, best-average alignment fit with three peripheral curves. The initial values for these parameters are determined by the doctorís default nomogram. The Optivision CornealMap EH-300 system has both predefined nomograms and the ability to save physician-customized nomograms.

Software provides a selection of specific contact lenses

The lens software also provides a selection of specific contact lens nomograms from different manufacturers. By choosing different diagnostic lenses, altering their parameters, and evaluating the fluorescein patterns, you can quickly arrive at your final lens selection. The rapid fitting process benefits patients and practitioner: Chair time, lens costs, shipping, and handling are all decreased; efficiency is increased.