There are different systems that compute the curvature of the cornea. The Orbscan combines optical sectioning with Placido reflection and the Atlas uses Placido reflection. The Pentacam utilizes Scheimpflug imaging and the Galilei uses a combinataion of dual Scheimpflug imaging and Placido ring reflection.
The major limitation of keratometry is the assumption that the cornea has an sphero-cylindrical surface, with a minor axis separated from its minor axis by 90 degrees.
With keratoscopy, it was possible to evaluate 70% of the cornea surface; however, only gross abnormalities can be seen with this technique. Astigmatism less then 3 diopters is not detected.
In videokeratoscopy the information is digitalized from thousands of points of the corneal surface to produce detailed color-coded maps.
Placido-based videokeratoscopes: all systems contain a transilluminated cone acting as a modified Placido ring. These can be divided into near and distant designs, with near having greater corneal coverage but more susceptibility to focusing error, and distant design requiring more illumination. All Placido-generated maps are based on a 2 dimensional reflection from the corneal surface; elevation maps can be generated from this data.
Elevation-based topography: Pentacam, Orbscan and Galilei, these devices directly measures the x, y, and z coordinates. The Orbscan and Galilei employ a Placido disk to augment the anterior corneal measurements. An advantage of Pentacam is that the cross-sectional images of the cornea are meridional and have a central common point for image registration.
- To determine whether the patient´s corneal shape will allow surgery
- Operative assessment to determine surgical parameters
- Aid in the selection of IOLs for patients with significant corneal astigmatism
- Postoperative evaluation
- Aid in the calculation of IOLs for patients who have undergone refractive procedure.
The most common usage of corneal topography is in the routine evaluation of the potential refractive surgical patient.
First we need to look for contraindications to surgery, the average adult cornea has a central radius of curvature 7.8 mm and a central corneal power of 43.5. Preoperative pachymetry and preservation of an adequate residual stromal bed are important elements to consider in corneal refractive surgery. In general, there is approximately .7 D of central corneal flattening for every 1.0 D of refractive effect for myopic corneal refractive procedures.
Preoperative topography is routinely obtained before refractive or cataract surgery in order to identify conditions such as keratoconus, irregular astigmatism, contact lens induced warpage, and occult ectatic disorders.
Identification of patients with keratoconus is important before refractive surgery or cataract surgery to manage patient expectations, safely choose candidates for surgery, and aid in intraocular lens calculations if indicated. Forme fruste keratoconus and early keratoconus are contraindications to corneal refractive surgery. Examination of curvature-based topography reveals an area of localized steepening usually in the inferotemporal quadrant. The following artifacts can also be confused with keratoconus: a prominent tear meniscus, misalignment when obtaining the topography, and accidental external pressure on the globe. Rabinowitz and McDonnell were the first to publish guidelines for the diagnosis of keratoconus that included the following: maximum simulated keratometry reading > 48.7 D, absolute simulated keratometry difference between the 2 eyes >.5 D, and the I-S value greater than 1.7 D. Also the creation of a pachymetric map allows for the identification of the true thinnest point and can contrast the thinnest point to the geometric center of the cornea.
Is useful for assessing the quality of the surgery and uniformity of laser in corneal refractive procedures. The best time to take this study is 30 days after PRK and at least 1 week after LASIK. Postoperative corneal ectasia may also be detected, but this often occurs later in the postoperative period. Topography may be useful not only for detecting ectasia but monitoring progression.
Post-keratoplasty suture removal and modification
The most common usage of corneal topography post-kereatoplasty in a corneal practice is to assist with suture removal or adjustment in corneal graft patients.
As more and more people undergo refractive surgery and the patients are getting older, cataract surgery is becoming more complex. Manual keratometry is known to be inaccurate in postrefractive surgical patients, since the keratometer overestimate the effective corneal power. Numerous methods to compute IOL power in these cases exist and include:
- Hard contact lens over-refraction.
- Modified IOL power computations based on derived keratometry.
- Refraction derived keratometry based on the prerefactive surgical keratometry and knowledge of the pre and postoperative refractions.
- Modified IOL power computations based on topographically derived corneal power estimations.
Many investigators and most corneal topography companies are working on determining the effective corneal power in the postrefractive surgical patient.
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